HomeMy WebLinkAboutBradley Lake Geology Report Vol 1 1991A Iaska Porer Authority
FINAL
CONSTRUCTION GEOLOGY REPORT
BRADLEY LAKE
HYDROELECTRIC PROJECT
Homer, Alaska
VOLUME 1
GEOLOGY REPORT
AND APPENDIX A
PRBPARED BY:
BECHTEL CORPORATION
SAN FRANCISCOJ CALIFORNIA
ALASKA ENERGY AUTHORITY
Anchorage, Alaska
BRADLEY LAKE HYDROELECTRIC PROJECT
FINAL CONSTRUCTION GEOLOGY REPORT
May, 1991
Prepared by:
BECHTEL CORPORATION
San Francisco, CA
Section
1.0
2.0
2. 1
2.2
2.3
2.3.1
2.3.2
2.4
2. 4. 1
2.4.2
2. 4. 3
3.0
3. 1
3.2
3.2.1
3.2.2
3.3
3. 3.1
3.3.2
3.4
3. 4. 1
3.4.2
3.4.3
4.0
4. 1
4.2
T A B L E 0 F
INTRODUCTION
POWERHOUSE
Site Description
Vicinity Geology
Geology of Excavation
Rock Weathering
Ground Water
C 0 N T E N T S
Excavation of Slopes and Foundations
Controlled Blasting and Overbreak
Rock Reinforcement
Foundation Conditions
POWER TUNNEL MANIFOLD, PENSTOCK TUNNEL
AND ACCESS ADIT
General Description
Access Adit
Excavation and Support
Geology
Penstock Tunnels
Excavation and Support
Geology
Power Tunnel Manifold
Excavation and Support
Geology
Tunnel Lining and Grouting
LOWER POWER TUNNEL
General Description
Tunnel Boring Machine (TBM)
1-1
2-1
2-1
2-1
2-2
2-3
2-4
2-4
2-5
2-8
2-9
3-1
3-1
3-1
3-1
3-2
3-2
3-2
3-3
3-5
3-5
3-7
3-9
4-1
4-1
4-2
Section
4.3
4. 3. 1
4. 3. 2
4.3.3
4.4
4. 4. 1
4.4.2
4.4.3
4.4.4
4.5
4. 5. 1
4. 5. 2
4. 5. 3
4. 5. 4
4. 5. 5
5.0
5.1
5.2
5. 2. 1
5.2.2
5.2.3
5. 2. 4
5.3
5. 3. 1
5.3.2
5. 3. 3
5.4
5.4.1
5.4.2
5.4.3
6.0
6.1
T A B L E 0 F C 0 N T E N T S
Tunnel Sta 5+20 to Sta 14+10
Excavation and Support
Geology
Tunnel Lining and Grouting
Tunnel Sta 14+10 to Sta 31+60
Excavation and Support
Geology
Design Modifications
Tunnel Lining and Grouting
Tunnel Sta 31+60 to Sta 177+51
Excavation and Support
Geology
Continuous or Skip Lining
Tunnel Lining and Grouting
High Pressure Compaction Grouting
UPPER POWER TUNNEL AND GATE SHAFT
INTAKE CHANNEL, AND POWER TUNNEL SHAFT
General Description
Upper Power Tunnel and Gate Shaft
Excavation and Support
Geology
Concrete Lining and Grouting
Seepage Barrier Grout Curtain
Intake Channel
Excavation and Wall Reinforcement
Geology
Removal of Rock Plug
Power Tunnel Shaft
Excavation and Support
Geology
Concrete Lining and Grouting
DAM, SPILLWAY, AND DIVERSION TUNNEL
GATE SHAFT
General Description
4-4
4-4
4-6
4-9
4-10
4-10
4-12
4-15
4-15
4-16
4-16
4-19
4-26
4-26
4-29
5-1
5-1
5-2
5-2
5-3
5-4
5-6
5-7
5-7
5-9
5-10
5-12
5-12
5-13
5-15
6-1
6-1
Section
6.2
6. 2. 1
6. 2. 2
6.2.3
6.2.4
6. 2. 5
6.3
6. 3. 1
6.3.2
6.3.3
6.4
6. 4. 1
6. 4. 2
6. 4. 3
6. 4. 4
7.0
7. 1
7.2
T A B L E 0 F
Dam and Cofferdam
Foundation Excavation
C 0 N T E N T S
Plinth Excavation and Foundation Treatment
Foundation Geology
Rockfill Materials
Foundation Grouting
Spillway
Foundation Excavation and Treatment
Geology
Foundation Grouting
Diversion Tunnel Gate Shaft
Excavation and Support
Geology
Concrete Lining and Grouting
Seepage Barrier Grout Curtain
MIDDLE FORK AND NUKA DIVERSION
General Description
Excavation
6-2
6-3
6-4
6-6
6-8
6-10
6-11
6-11
6-12
6-13
6-14
6-14
6-15
6-16
6-16
7-1
7-1
7-1
Section
1.0
2.0
2.1
2.2
2.3
2.3.1
2.3.2
2.4
2.4.1
2.4.2
2.4.3
3.0
3.1
3.2
3. 2. 1
3.2.2
3.3.
3.3.1
3.3.2
3.4
3.4.1
3.4.2
3.4.3
4.0
4. 1
4.2
T A B L E 0 F
INTRODUCTION
POWERHOUSE
Site Description
Vicinity Geology
Geology of Excavation
Rock Weathering
Ground Water
C 0 N T E N T S
Excavation of Slopes and Foundations
Controlled Blasting and Overbreak
Rock Reinforcement
Foundation Conditions
POWER TUNNEL MANIFOLD, PENSTOCK TUNNEL
AND ACCESS ADIT
General Description
Access Adit
Excavation and Support
Geology
Penstock Tunnels
Excavation and Support
Geology
Power Tunnel Manifold
Excavation and Support
Geology
Tunnel Lining and Grouting
LOWER POWER TUNNEL
General Description
Tunnel Boring Machine (TBM)
Section
4.3
4. 3. 1
4.3.2
4.3.3
4.4
4. 4.1
4.4.2
4.4.3
4.4.4
4.5
4. 5. 1
4. 5. 2
4. 5. 3
4. 5. 4
4. 5. 5
5.0
5. 1
5.2
5. 2. 1
5.2.2
5.2.3
5.2.4
5.3
5. 3. 1
5. 3. 2
5.3.3
5.4
5. 4. 1
5.4.2
5.4.3
6.0
6. 1
T A B L E 0 F C 0 N T E N T S
Tunnel Sta. 5+20 to Sta. 14+10
Excavation and Support
Geology
Tunnel Lining and Grouting
Tunnel Sta. 14+10 to Sta. 31+60
Excavation and Support
Geology
Design Modifications
Tunnel Lining and Grouting
Tunnel Sta. 31+60 to Sta. 177+51
Excavation and Support
Geology
Continuous or Skip Lining
Tunnel Lining and Grouting
High Pressure Compaction Grouting
UPPER POWER TUNNEL AND GATE SHAFT
INTAKE CHANNEL, AND POWER TUNNEL
SHAFT
General Description
Upper Power Tunnel and Gate Shaft
Excavation and Support
Geology
Concrete Lining and Grouting
Seepage Barrier Grout Curtain
Intake Channel
Excavation and Wall Reinforcement
Geology
Removal of Rock Plug
Power Tunnel Shaft
Excavation and Support
Geology
Concrete Lining and Grouting
DAM, SPILLWAY, AND DIVERSION TUNNEL
GATE SHAFT
General Description
Section
6.2
6. 2. 1
6. 2. 2
6. 2. 3
6.2.4
6. 2. 5
6.3
6. 3. 1
6.3.2
6.3.3
6.4
6. 4. 1
6.4.2
6.4.3
6.4.4
7.0
7. 1
7.2
T A B L E 0 F
Dam and Cofferdam
Foundation Excavation
C 0 N T E N T S
Plinth Excavation and Foundation Treatment
Foundation Geology
Rockfill Materials
Foundation Grouting
Spillway
Foundation Excavation and Treatment
Geology
Foundation Grouting
Diversion Tunnel Gate Shaft
Excavation and Support
Geology
Concrete Lining and Grouting
Seepage Barrier Grout Curtain
MIDDLE FORK AND NUKA DIVERS!ON
General Description
Excavation
Volume
1
2
2
2
2
2
A P P E N D I C E S
Final Geology and Supporting Data
Appendix
A
B
c
D
E
F
Title
Project Photographs
Powerhouse
Power Tunnel Manifold, Penstock
Tunnels, and South Access Adit
Lower Power Tunnel
Upper Power Tunnel & Gate Shaft,
Intake Channel, and Power Tunnel Shaft
Main Dam, Spillway, Diversion Tunnel
Gate Shaft, and Middle Fork and Nuka
Diversions
The above appendices of figures, plates, and tables are
referenced in the text of this report. Most of the plates
have been adapted from Stone & Webster Engineering's project
design drawings by the deletion of data considered non-
essential for the purposes of this geologic report. Figures
are intended to reflect the as-built geologic conditions of
the Bradley Lake Hydroelectric Project; however, the figures
do not show in every detail the constructed conditions.
Adapted plates are included to show the primary project
features, as well as to indicate the extent of excavation
and rock reinforcement.
Appendix
A
B
c
D
LIS'!' OF FIGURES, PLATES, AND 'lABJ:&CJ
Plate No.
2-1
2-2
4-1
4-2
Figure No.
2-1
Title
Project Photographs
Powerhouse Rock Reinforcement
Plan and Section
Powerhouse Excavation & Rock
Reinforcement and Sections
Geologic Maps,
Powerhouse Excavation Walls
- 8 sheets
2-2 Geologic Maps,
Powerhouse Foundations - 2 sheets
3-1 Geologic Face Maps,
South Access Adits -20 sheets
3-2 Geologic Maps,
Power Penstock Trench and Portal
-3 sheets
3-3 Geologic Maps, Unit No. 1
Penstock Tunnel and Thrustblock
- 7 sheets
3-4 Geologic Maps, Units No. 2
Penstock Tunnel and Thrustblock
-10 sheets
3-5 Geologic Maps, Unit No. 3
Penstock Tunnel and Thrustblock
-21 sheets
3-6 Geologic Tunnel Logs & Maps,
Power (Manifold) Tunnel -30 sheets
3-7 Geologic Map,
Power (Manifold) Tunnel Portal &
Bridge Abutment
Power Tunnel, Bid Option B
Plan & Profile
Power Tunnel Intersection with
South Access Adit
LIST OF FIGURES, PlATES, .ARD TABLBS -(continued)
Appendix Plate No.
4-3
4-4
E
5-1
5-2
Figure No.
4-1
4-2
Table 4-1
Table 4-2
Table 4-3
5-1
5-2
Title
Geologic Log and Maps,
Lower Power Tunnel Sta 5+20 to 14+10
-40 sheets
Power tunnel Geologic Plan & Profile
-4 sheets
Power Tunnel Alignment & Major Faults
Geologic Tunnel Logs,
Lower Power Tunnel Sta 14+10
to Sta 177+50 -82 sheets
TBM Daily Progress
Lower Power Tunnel
Tape Extensometer Data
Lower Power Tunnel -19 sheets
Strain Gage Readings
Lower Power Tunnel - 3 sheets
Pilot Hole,
Power Tunnel Vertical Shaft
- 4 sheets
Geology Map,
Upper Power Tunnel Gate Shaft
Collar Excavation
Power Tunnel Gate Excavation,
Sections
Power Tunnel Gate Shaft & Chamber,
High pressure Grouting
5-3 Geologic Tunnel Logs
- 6 sheets
5-4 Geologic Face Maps,
Upper Power Tunnel -13 sheets
5-5 Geologic Map,
Power Tunnel Gate Shaft
LIS'!' OF FIGURES, PLA'l'ES, AND TABLES -(continued)
Appendix Plate No.
5-3
5-4
5-5
5-6
F 6-1
6-2
6-3
Figure No.
5-6
5-7
Title
Power Tunnel Intake Channel
Excavation Plan
Power Tunnel Intake Portal
Excavation Plan & Section
Power Tunnel Intake Channel
Excavation Sections & Details
Geologic Wall Haps,
Power Tunnel Intake - 3 sheets
Power Tunnel Shaft Sections
Geologic Wall Haps,
Power Tunnel Vertical Shaft
- 8 sheets
Main Dam Rock Excavation
Plan & Profile
6-1 Geologic Hap,
6-2
6-3
6-4
6-5
6-6
Main Dam Plinth Foundation
- 5 sheets
Geologic Data Summary,
Main Dam Toe Plinth Foundation
- 2 sheets
Concrete Face Rockfill Dam Sections
Main Dam, Drilling and Grouting,
Plan and Profile
Spillway Plan and Section
Geology of Spillway Foundation
-3 sheets
Main Dam Spillway Drilling & Grouting
Main Dam Spillway Drilling
& Grouting Profile
LIST OF FIGURES, PLATES, ARD TABLES -(continued)
Appendix Plate No. Figure No.
6-4
6-7
7-1
7-2
Title
Main Dam Diversion,
Gate Shaft Excavation Sections
Geologic Maps,
Main Dam Diversion Gate Shaft
- 2 sheets
Middle Fork Diversion,
Intake & Upper Channel
Profile & Sections
Nuka Diversion,
Nuka River Outlet Structure Plan
REFERENCES:
1. conformed Copy
General Civil Construction Contract
Contract No. 2890060
Bradley Lake Hydroelectric Project
Volume 3
Volume 5
Volume 6
Civil, Structural and Architectural
Technical Requirements
General Civil Contract
Design Drawings
Geotechnical Interpretive Report
Supporting Documents
Prepared by Stone & Webster Engineering Corporation
2. Bradley Lake Hydroelectric Project
Main Dam and Spillway Grout Curtain
October, 1989
Prepared by Bechtel Civil, Inc.
3. Bradley Lake Hydroelectric Project
Project (Jobsite) Files
File Retainer Bechtel Civil, Inc.
Construction Manager
4. Bradley lake Hydroelectric Project
Lower Power Tunnel
High Pressure Compaction Grouting
Sta 65+00 t Sta 31+60
Prepared by Stone & Webster Engineering Corporation
with Inspection Records by Bechtel Corporation,
Construction Manager
1.0 INTRODUCTION
The Owner of the Bradley Lake Hydroelectric Project is the
Alaska Energy Authority (AEA), previously known as the
Alaska Power Authority (APA). Stone & Webster Engineering
Corporation is the project design Engineer and Bechtel
Civil, Inc. is the Construction Manager. Construction began
July 1, 1988, and by November, 1989 most of the final
foundation and underground excavation had been completed.
At the end of 1990 construction of the power facilities. was
essentially completed. The Bradley Lake Hydroelectric
Project is located on the east side of Kachemak Bay about 27
miles northeast of Homer, Alaska. The principal project
features consist of a 145 ft high rock-fill concrete face
dam, a 3.6 mile long, 15.1 ft excavated diameter power
tunnel, and an outdoor two unit 70 MW powerhouse operating
under a maximum head of approximately 1,000 feet. The
general location of project features are shown on Plate 1-1
at the end of this section.
The primary intent of this report is to document the "as-
built" engineering geology for the constructed facilities.
Documentation developed during and at the conclusion of
construction is intended to serve a number of purposes which
include those given below:
1. The report is a requirement of FERC regulations for
project licensing.
2. The project contract documents specified the
ments for geologic mapping of excavations for
features.
1 - 1
require-
principal
3. During construction, mapping to identify and record
geologic features, e.g. faults, shear seams, weak rock,
water in flows, that may require monitoring and/or remedial
measures prior to covering with permanent concrete.
4. As an adjunct
logging of open and
mentation of rock
performance which
to (3.) above, geologic mapping and
underground excavations provides docu-
characteristics, ground quality and
can be useful in settling contractor
claims of changed conditions.
5. The report provides a geologic record of the excava-
tions for the Owner. This information can be useful in the
development of remedial measures in the event a geological
related problem arises after the facility goes into opera-
tion.
In fulfilling the above requirements, geologic mapping was
done for all the principal project features, tunnels,
shafts, dam, spillway, and the powerhouse foundations
including interior and exterior walls. Geologic maps, logs,
and data sheets completed during construction of the project
features are included in the Appendices. The report also
includes a discussion of the project geology and rock
conditions encountered during construction. Selected photo-
graphs of selected excavations and geologic features of
project features and rock conditions during various stages
of construction are given in Appendix A. With few excep-
tions, which are discussed subsequently in the report, the
geology and rock conditions encountered during construction
are in substantial agreement with the pre-construction
Geotechnical Interpretive Report (GIR) with Supplements,
1 - 2
prepared by the Stone & Webster Engineering Corporation
(SWEC).
Included in the responsibilities of the project Construction
Manager were monitoring, mapping, and recording of
geotechnical aspects of the Bradley Lake Hydroelectric
Project throughout the construction period, July, 1987
through November, 1990. These geotechnical activities
included:
o Geologic mapping of tunnels, shafts, foundations, and
open excavations
o Monitoring near surface and underground excavations and
providing geotechnical assessments of any geological related
concerns to engineering and construction managers.
o Inspection and direction on final foundation prepara-
tion for dam and powerhouse
o Monitoring and inspection of pressure and contact
grouting of dam foundations and grouting
o Blasting vibration monitoring
This report is organized into five main sections:
Introduction
Powerhouse
Manifold and Penstock Tunnels
Main Dam and Spillway
Upper and Lower Power Tunnel
1 - 3
For background project geology, reference should be made to
the previously mentioned Geotechnical Interpretive Report
(GIR). This report with supplements provides a detailed
comprehensive description of the geology and the expected
rock conditions based on pre-construction geotechnical
studies and field investigations.
The project geologist, Dale L. Roberts, who prepared this
report was assigned to the Bradley Lake Hydroelectric
Project full time. Other Bechtel geologists; Hike Beathard,
John Sollo, Peter Yen, and Jacque Lord on temporary assign-
ments monitored the foundation grouting of the main dam and
spillway.
Doug Isler formerly of (SWEC) performed most of the geologic
field mapping and prepared the draft report write up for the
Lower Power Tunnel from Sta. 14+10 to Sta. 177+51. To avoid
constraints on the Bechtel project geologist, arrangements
were made for a geologist from the Engineer to assist the
Construction Manager with tunnel geologic mapping activi-
ties.
1 - 4
' I BRADLEY JUNCTION
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.APAPTED FROM FIG. 2
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TRANSMISSION
LINE ROUTE
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REV1SED 8 /3188
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SCALE fil MI.ES
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···:-·-~---·· ........ .. ..
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llGHTlL
SAN FRANCISCO
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ALASKA ENERGY AUTHORITY
. A~loret•• Al .. lle'
BRADLEY LAKE HYOROELE
PROJECT FEATURES
----
'• •,
17707 PLATE 1-'f
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-·
2.0 POWERHOUSE
2.1 Site Description
The outdoor powerhouse is located near the head of Kachemak
Bay approximately one mile south of the Bradley River
entrance. The site is benched into the hillside above the
line of high tide. The powerhouse tailrace extends westward
from the top of the hillside onto the tidal flats. Water is
conveyed to the powerhouse from the raised Bradley Lake at
El 1070 via a vertical shaft and a 3.6 mile power tunnel.
At the lower end of power tunnel three short penstock
tunnels branch-off into the powerhouse from the manifold
section of the power tunnel.
During an earlier stage of site preparation construction a
large rock bench was excavated to El 40 at the base of the
mountain front. The rock surface of this level bench was
the starting point for the powerhouse excavation. The
lowest excavated foundation level, the invert of the
turbine-generator pits, is El -9.
2.2 Vicinity Geology
Rocks in the surrounding vicinity of the powerhouse consist
of Cretaceous age assemblage of folded and sheared
graywacke, argillite and chert. These lithologies occur
both separately and intermixed with transitional contacts
for the most part. The most significant geologic structural
feature was a shear seam (i.e. minor fault) exposed in a
ravine south of the powerhouse and subsequently exposed
along the full length of the back (east) wall of the power-
house excavation. This shear seam had been delineated
during earlier site investigations and described as Z2 shear
zone in the GIR (see bottom of page 1-3). The project
2 - 1
facilities are in a seismically active region of southern
Alaska, but no Recent age displacements along faults at or
near the site have been recorded (GIR). The design basis
earthquake is 0.35g (GIR). Bedrock at the powerhouse
vicinity consist mainly of graywacke with minor areas of
argillite with and without chest.
2.3 Geology of the Excavation
Geologic mapping of the powerhouse excavation was conducted
as the excavation progressed downward. With few exceptions
both the wall and final foundation surfaces were mapped at a
scale of 1"=10'. Geologic maps with notes were prepared for
most of the final excavated surfaces. These maps, Figure
2-1 (8 sheets) and Figure 2-2 (2 sheets) are included in
Appendix B.
Graywacke, the predominant rock type exposed in the power-
house excavation, is moderately fractured and moderately
jointed. Many of the joint surfaces are iron stained. The
dominant joints trend N60-85W and typically dip 80 to 85
degrees south. The graywacke hardness is relatively greater
than that of argillite and the jointing in graywacke is more
distinct. Argillite, a much darker gray color than
graywacke, is less well jointed but has strongly developed
foliation with considerable fracturing of the foliated
layers. Whitish chert frequently occurs within the dark
gray argillite as thin, discontinuous, elongated lenses and
nodules. The chert content in argillite ranges generally
from 10 to 20 percent.
The argillitic shear seam identified as Z2 during the
pre-construction investigations (GIR) is prominently exposed
on the back (east) wall of the powerhouse excavation for the
2 - 2
full length of the powerhouse as well as through the switch-
yard. The dark grey argillitic seam within largely
graywacke rock mass is generally 12 to 18 inches wide, but
on the south side of Penstock 3 portal excavation the seam
width is as much as 50 inches. The dip of the shear seam is
about 25 to 35 degrees east into the hillside. The shear
seam, which could also be classified as a fault, is strongly
foliated and cross fractured with numerous silkensided
surfaces. Locally discontinuous thin layers of plastic clay
and soft decomposed argillitic rock are prevalent. With-
standing the weak character of this shear seam, its location
mainly along the back wall of the powerhouse excavation had
little effect on the wall stability or foundation bearing
strength.
A near vertical dacite dike intersects the northeast corner
of the powerhouse excavation. It is about 20 feet wide,
highly weathered and very closely jointed and fractured.
Fresh dacite is gray, whereas in the weathered outcrop at
the powerhouse excavation the color is noticeably uniform
light buff. The altered rock contact transition between the
dacite and graywacke is less than a foot wide. There is
very little difference in the slope stability characteris-
tics of the two rock types.
2.3.1 Rock Weathering
The most prevalent unanticipated rock condition in the
powerhouse excavation area is the degree and extent of rock
weathering. The starting grade of the powerhouse site,
which had been previously excavated, was El 40. Over the
northern two-thirds of the site, highly weathered and
closely fractured rock extended downward an additional 20 to
30 feet. Over the southern third of the site the depth of
2 - 3
the weathered rock condition is somewhat less, ranging from
5 to 15 feet. Also, the degree of weathering is relatively
more intense at the front (i.e. bay side) half of the
powerhouse excavation. Generally, the back (east) half of
the powerhouse excavation below El 2 where rock cover was
greater, the extent of weathering is less and was classified
as moderately weathered. There are a few small areas of
slightly weathered rock in the excavation below El 2. The
effects and consequences of rock weathering during excava-
tion of the powerhouse are discussed in Section 2.4.
2.3.2 Ground Water
Although water accumulated in the powerhouse excavation
which required light pumping for control, most of the water
was from sources other than groundwater. Much of the water
entering the powerhouse excavation came from underground
drainage of the manifold tunnel through the connecting
penstock tunnels. Ground water from the foundation was only
a small portion of the flow; perhaps 1 or 2 gal/min emanated
from joints and fractures at a few isolated locations in the
east wall of Units 1, 2, and 3. Another suspected source of
water entering the deeper powerhouse excavations may have
been seawater from the bay seeping beneath the base of the
cellular cofferdam at times of high tide. The combined flow
from these sources probably was no more than 5 to 10
gal/min.
2.4 Excavation of Slopes and Foundations
The blasted rock surface at El 40, previously excavated
under another contract was the starting level of the power-
house foundation excavation.
tion from El 40 to El -9 was
The entire powerhouse excava-
completed by drill and blast
2 - 4
methods. Blasted rock was removed by backhoe and front-end
loader.
The contract specifications included a number of provisions
defining the method of excavation to be used by the Contrac-
tor. The maximum excavation lift was limited to 11 feet.
Controlled blasting was specified with the intent of mini-
mizing overbreak. Extensive rock reinforcement of the cut
slopes was also detailed on the design drawings. These and
other provisions for completing the powerhouse excavation
are discussed separately below.
2. 4. 1 Controlled Blasting and Overbreak
The contract documents contained a number of specific
requirements for conducting control blasting of final cut
slopes in the powerhouse excavation. Requirements for
controlled blasting included the following:
1) Cushion blast method be utilized for all final cut
slopes.
2) Maximum blast hole spacing of 2 feet.
3) Maximum explosive charge of 1/4 lb/ft. of hole.
4) Maximum lift height of 11 feet.
5) Maximum overbreak allowance of 6 inches.
6 ) A $400/cu-yd penalty for overbreak beyond 6 inches.
Starting with the above criteria, prior to production
excavation, the Contractor performed the specified blasting
tests for the purpose of establishing ·a hole charge weight
that would minimize overbreak beyond the 6-inch tolerance.
None of the cushion blast tests achieved the desired re-
sults. The specified Trimtex explosive (1/4 lb/ft.) with
several pound bottom charge were used in most of the final
excavation line holes above El 18. In one section of the
2 - 5
east wall of Penstock 3, the presplit method was tried with
good results, but this method was not permitted by specifi-
cation. Below El 18 the Contractor tried modifying his hole
charges; 200 grain primacord was used in place of the
1/4 lb/ft. Trimtex. Results were mixed using smaller hole
charges, but generally there was less overbreak below El 18
than above. There were several causes for excessive
overbreak of final cut slopes particularly above El 18.
These included: (1) the weathered and fractured condition
of the rock, (2) poor execution of the control blasting, and
(3) unfavorable orientation of geologic joint~ng relative to
the direction of the cut slope. As previously described in
2.3.1, Rock Weathering, the graywacke rock, with local
exceptions, from El 40 to El 18 in Unit 3, and from El 40 to
El 2 in Units 1 and 2 is highly weathered. As a consequence
this highly weathered, weak, fractured rock was susceptible
to readily breaking and parting along joints and fractures
even though small explosive charges were used in cushion
blasting. After scaling of the loosened rock beyond the
blasted surface, the overbreak above El 18 along the final
cut slopes ranged from 12 to 30 inches for most of the cut
slopes. Locally, sections of the east wall north of Unit 2
were overbroke as much as 3 to 5 feet. The extent of
excessive overbreak was strongly influenced by
weathered condition of the graywacke rock.
highly
Although the weathered condition of the rock had a strong
influence on the limited success of controlling overbreak by
cushion blasting, other factors such as, poor execution of
cushion blasting and unfavorable geologic structure contrib-
uted significantly to the excessive overbreak. Overbreak of
the final slope of the south wall of the powerhouse excava-
tion was largely due to the above two factors. At the south
2 - 6
wall the moderately weathered and hard graywacke was strong-
ly jointed. Joints spaced at 12 to 24 inches intersected
the final face at a highly acute angle with the cut slope
face. Cushion blasting of the final cut slope was done by
loading every other hole (4ft. spacing). Owing to the near
vertical attitude of jointing, upon blasting large blocks of
rock either fell out or were considerably loosened resulting
in excessive overbreak after scaling. With better execution
of the cushion blasting such as lightly loading every hole
on 2-foot spacing, or perhaps reduced spacing to 18 inches,
may have improved the chances of reducing the overbreak.
Excessive overbreak of several feet, due to the high degree
of rock weathering and unfavorable joint orientation, also
occurred along the east wall above El 28 and along the walls
and corners of the turbine discharge pits below El 18.
The geologic conditions at the powerhouse site reported in
the GIR (Volume 6 of the contract bid documents) were not
significantly different from those encountered in the
powerhouse excavation. However, the expectation that
cushion blasting method, given the predicted rock condi-
tions, could control overbreak to the specified 6-inch
maximum was overly optimistic. While specifying a
of $400/cu-yd for overbreak quantities beyond the
penalty
6-inch
tolerance made the Contractor put more effort in to minimiz-
ing overbreak, it did not necessarily assure the desired
results: The highly weathered rock conditions were not
adequately taken into consideration when specifying results
to be achieved by control blasting. The Contractor's claim
for overbreak in the powerhouse excavation was largely based
on unreasonableness of the 6 inch overbreak tolerance for
the encountered rock conditions. Settlement of the
2 - 7
$400/cu-yd penalty was resolved by mutual agreement between
the Contractor and the Owner.
2.4.2 Rock Reinforcement
Extensive rock reinforcement of cut slopes was included in
the design of the powerhouse excavation (Appendix B, Plates
2-1 and 2-2). All final cut slopes excavated by cushion
blasting methods were reinforced with pattern rock bolts,
dowels and chain link mesh. Additionally, large diameter
rock anchors were installed horizontally in the rock wall at
the back of the powerhouse to maintain long term stability.
The bar diameter of the rock anchors varied from 1 to 1-3/8
inches, and the length ranged from 3 to 60 feet. Anchorage
and encapsulation was provided by resin cartridges. Wall
reinforcement was installed progressively downward as each
11-foot lift was excavated.
There were five different types of rock anchorages used the
powerhouse excavation. These types were designated A-1,
A-2, A-4, A-6, and A-9. All were tensioned with exception
of the A-6 dowels. The A-1 types were 10 feet long, 5 kip
tension vertical dowels installed one foot back at 6-foot
spacing along every cut slope wall. A-1 type rock bolts
were installed horizontally typically on 6 x 6 foot pattern
on every cut slope face. These resin anchored, pattern rock
bolts 10 to 25 feet long were tensioned to 30 kips. A-4
rock anchors 1-3/8 inches in diameter were installed in
clusters of 4 to 6 anchors at several locations on the east
wall, over the penstock tunnel portal and in the foundation
at El 18 and El 2. Clusters of A-4 anchors were installed
at 15 locations. A-9 rock anchors, 1-3/8 inches in diameter
with corrosion protection were installed vertically in
2 -8
clusters of 9 at the spherical valve locations for Units 1
and 2 and as rock anchors in the penstock thrust block
foundations. The A-9 anchors, 50 feet in length, were
tensioned and locked-off at 165 kips.
Problems with drilling and installing the A-1 tensioned rock
bolts were few. In a small percentage of the holes, prima-
rily those in the highly weathered rock, hole caving and
sloughing slowed down drilling, but only a few holes had to
be redrilled. Tensioning of the A-1 rock bolts to 30 kips
also was accomplished with only a few anchorage failures out
of the more than 1500 A-1 rock bolts installed. Hole
sloughing was more of a problem in the horizontal holes
drilled for the 1-3/8 inches A-4 anchors on the east wall.
These bolts were up to 40 feet long; maintaining a stable
hole for inserting the resin cartridges and installing the
bolt was critical. Although considerable time and effort
was expended by the Contractor in A-4 bolt installation,
only one had to be abandoned and all met the tensioned 180
kip proof load and 165 kip lock-off.
2.4.3 Foundation Conditions
At Units 1, 2, and future Unit 3 the three main foundation
levels in the powerhouse excavation are El 18, El 2, and
El -9 (Appendix B, Figure 2-2, sheet 4). The lowest level
in the powerhouse is the foundation of the turbine/generator
pit. Dimensions of the powerhouse foundation area for the
three units are 90 x 254 feet.
The foundation rock for all three units consists primarily
of graywacke with smaller areas less than 10 percent of
foliated argillite with minor chert. For the most part the
2 - 9
foundation rock is moderately to slightly weathered. The
foundation rock surface at El 18 is relatively more
weathered than at El 2. Rock in the turbine discharge
channels in all three units is moderately to locally highly
weathered. Although the foundation rock is closely jointed
(6" to 2") spacing and fractured, the joints and fractures
are reasonably tight. The rock is uniformly hard with few
shear seams or large areas of weak rock. Occasional thin
shear seams of less than a few inches in width were observed
in the rock foundation of all three units. Rock material in
these steeply dipping shear seams is finely fractured, in
part foliated, and lightly interspersed with discontinuous
clay. Apart from the relatively minor rock seams there was
one significant shear seam which traversed the entire length
of the powerhouse and the switchyard as well. It is located
on the east wall of the powerhouse excavation at Units 2 and
3 (Appendix B, Figure 2-1, Sheets 1 and 2) and crosses the
foundation at El 2 in the northeast corner of Unit 1. This
prominent shear seam is designated as Z-2 in the GIR. The
characterization of this argillite, foliated seam has been
previously described in Section 2.3. The width of the Z-2
shear seam in the Unit 1 foundation ranges from 6 to 24
inches.
The foundation surface of the powerhouse after preparing and
cleaning for concrete placement was typically irregular and
below grade, but no special treatment or dental excavation
was needed. A levelling mat of concrete was placed over the
inverts of Units 1 and 2 to facilitate rebar placement. The
foundation surface of the future Unit 3 was not completely
cleaned up; the specifications called for backfilling the
Unit 3 excavation to El 18, which was done.
2 -10
The foundation rock of the substation consists of highly
weathered, but reasonably competent intermixed graywacke and
argillite. A series of prominently displayed, closely
spaced joints are prevalent over the north half of the
foundation. The argillitic, closely foliated Z2 shear seam
previously mentioned crosses the center of the foundation.
A geologic map of the substation foundation is included in
Appendix B, Figure 2-1 (Sheet 8).
2 -11
3.0 POWER TUNNEL MANIFOLD, PENSTOCK TUNNELS
AND ACCESS ADIT
3.1 General Description
Water conveyed from Bradley Lake through the upper and lower
power tunnels to the powerhouse Units 1 and 2 and future
Unit 3 consist is distributed through a reach of manifold
tunnel and three short wye-branch penstock tunnels. These
lower tunnel reaches are lined with steel penstock pipe
encase with pumped backfill concrete. To allow construction
of the manifold-penstock tunnel system to proceed concur-
rently with the lower power tunnel excavation a separate
temporary bypass access adit was excavated just south of the
powerhouse. Further details of the tunnels are described
below.
3.2 Access Adit
3.2.1 Excavation and Support
Excavation of a 250-ft long approach trench was needed to
provide access to the temporary adit portal. Final slopes
of the trench were excavated by the presplit method with
reasonably good success considering the weathered and
fractured condition of the graywacke rock. The cut slopes
were reinforced with pattern 10-ft long resin anchored and
tensioned (30 kips) rock bolts on a 6 x 6 foot pattern;
chain link mesh was also installed. The south access adit
intersects the lower power tunnel about 175 feet in from the
portal. Drill and blast method was used to excavate the
access adit; the excavated diameter is 16.5 feet. Horseshoe
shaped steel sets on 4-ft centers and patterned rock bolt
array were installed from the portal to the wye intersection
with the manifold section of the lower power tunnel.
3 - 1
3.2.2 Geology
Geologic mapping of the walls and crown similar to that
completed for the other tunnels was not done for the tempo-
rary access adit due to time constraint.
wood lagging, particularly in the crown,
Steel sets and
obscured much of
the rock surface which greatly reduced the rock exposure for
mapping. However, individual geologic tunnel face maps at
1" c 10' scale were completed after most the daily advances.
Face maps of the access adit are included in Appendix c,
Figure 3-1 (20 sheets). Rock types encountered in the
access adit to the intersection with the manifold tunnel
(Sta. 5+22) consisted of slightly weathered to fresh
argillite with 10 percent chert, graywacke and intermixed
argillite and graywacke; estimates of their respective
percentages were 30, 50, and 20. The blocky rock was
closely jointed and fractured from the blasting consequently
there was need for the installation of of pattern rock
bolts. At the Contractor's option WF 6 x 20 steel arch sets
were also installed on 4-ft centers. The tightness of the
rock jointing improved beyond the first 100 feet in from the
portal. It appeared that rock bolts in the crown could have
provided the adequate safe supports, but the Contractor
elected to continue with steel sets.
3.3 Penstock Tunnels
3.3.1 Excavation and Support
The three short penstock tunnels for Units 1, 2, and future
Unit 3 turbine-generators exit the rock slope behind the
east wall of the powerhouse. A short, 35-ft long open
trench leads to each portal. The penstock tunnels are
parallel; penstock tunnels 1 and 2 are separated by about
40-ft rock pillar and the pillar between penstock tunnels 2
3 - 2
and 3 is about 85 feet. The lengths of penstock tunnels 1,
2, and 3 including the thrust block chambers are 50, 85, and
140 feet, respectively. Each tunnel intersects the manifold
tunnel at about 50 degree angle. All have the same excavat-
ed diameter of 14 feet to the thrust block chamber and a
10-ft diameter to the intersection with the manifold tunnel.
The penstock tunnels through the thrust blocks are enlarged
to approximately 20 x 22 feet chambers. Inverts of the
thrust block chambers are approximately 5 feet below the
invert of the penstock tunnels. The plan and sections of
penstock tunnel excavation is included in Appendix C,
Plate 3-1 (Sheet 1).
The penstock tunnels were excavated by drill and blast
method. Pattern rock bolts and W 6 x 20 and 4W x 13 steel
arch sets on 4-ft centers were installed. Steel sets were
shown on the drawings for the thrust block chambers. Due to
better than predicted rock conditions, steel sets were
deleted from the thrust block chambers for penstocks 2 and
3. Steel sets were installed in penstock 1 thrust block
chamber not because of lower rock quality but to provide
added wall support to resist possible rock disturbance from
nearby blasting of the manifold tunnel. Each of the thrust
block chambers roofs was reinforced with a webbing of steel
straps and pattern rock bolts. The penstock tunnels includ-
ing the thrust block chambers during and after excavation
were essentially dry.
3.2.2 Geology
Geologic conditions of the penstock trench walls are reason-
ably stable considering rock weathering ranges from moder-
ately to highly weathered. Shear seams intersect the walls
and some joints are filled with soft clay. Geologic maps
3 - 3
including notes of the penstock trench walls and the
penstock tunnel portal are given in Appendix C, Figure 3-2
(Sheets 1, 2, and 3). Geologic conditions in each of the
penstock tunnels were ideal for stability. Although steel
arch sets on 4-ft centers, in addition to pattern rock bolts
were installed in the tunnels, the steel sets could have
been deleted without any concern for loss of stability. As
previously mentioned, the steel arch sets were deleted from
the thrust block chambers of penstock tunnels 2 and 3. The
geology of the penstock tunnels was mapped at 1" = 20' scale
and the thrust blocks at 1" = 20' Appendix C, Figures 3-3,
3-4, and 3-5. Separate geologic face maps of each penstock
tunnel excavation were also prepared daily, and are also
included in Appendix C as noted above. Intermixed graywacke
and argillite occur in penstock tunnel 1 where as the
principal rock type encountered in penstock tunnel 2 is
graywacke. Three rock types are present in penstock tunnel
3: graywacke, intermixed graywacke/argillite with 10
percent chert. With the exception of the prominent shear
seam intersecting the thrust block chamber of penstock
tunnel 3, rock in the three tunnels beyond approximately 20
feet in from the portals, is generally fresh and hard. The
rock is closely jointed. The joint traces are slightly
undulating the joints are tight with smooth, clean surfaces.
The shear seam exposed mid-height on the penstock 3 thrust
block chamber walls is 12 to 18 inches wide. The shear seam
is the same one that crosses the east wall of the powerhouse
excavation previously described in
and designated at Z-2 in the GIR.
Section 2.0, Par. 2.4.3
The dip of the seam is
about 35 degrees northeast; it disappears beneath the tunnel
invert about 30 feet upstream of the thrust block excava-
tion. Material within the seam is foliated, argillitic and
3 - 4
although closely fractured, fractures are reasonably tight.
Some slickensides were observed in the seam as well as thin
clay coating along some fracture surfaces. Rock exposed in
the roofs of the three thrust block chambers is slightly
weathered. Joint spacing ranges from 4 to 12 inches with
6-inch spacing predominant; beyond the blast damage zone
(approximately 12 inches) jointing is tight. Joints typi-
cally dip steeply north with an east-west strike (see
Appendix, Figures 3-3, 3-4, and 3-5).
3.4 Power Tunnel Manifold
3.4.1 Excavation and Support
The curved reach of the lower power tunnel downstream of the
wye intersection with the south access adit is referred to
as the manifold tunnel. The manifold section of the power
tunnel is about 420 feet
is 16 feet. The three
in length; its excavated
penstock tunnels branch
diameter
off the
manifold tunnel at about a 50 degree angle (see Appendix C,
Plate 3-1). Like the penstock tunnels the drill and blast
method was used to excavate the manifold tunnel.
Similar to the penstock tunnels there is also a tunnel
enlargement for a thrust block in the manifold tunnel. It
is located about 45 feet upstream from the main portal.
Differing from the penstock thrust block chamber of all
around enlargement, in the manifold tunnel thrust block only
the invert was lowered by 3 feet and lengthened 10 feet.
This design change was deemed necessary because of the low
rock cover above the thrust block chamber combined with the
moderately weathered rock conditions in the roof. To
facilitate installation of wye sections of steel penstock at
each of the penstock tunnel intersections, an additional 3
3 - 5
feet of rock was excavated from the crown of the manifold
tunnel. At the wyes of penstock tunnels 2 and 3, the higher
roofs were extensively reinforced with pattern rock bolts.
At the wye section of penstock 1 tunnel a number of steel
sets, supported on one side by a broad arch steel
were installed, Appendix C, Figure 3-1, (sheet 2).
spandrel,
At this
location the steel sets were not deleted as was done at the
penstock 2 and 3 intersections. Because of the close
proximity and geometry of the intersection there was concern
that the stability of the manifold tunnel might possibly
lessen with time (over one year) before the steel liner was
installed and backfilled around with concrete.
With exception of a few localized areas, rock conditions in
the manifold tunnel upstream of the thrust block excavation
(Sta 1+68 to 5+20) were reasonably stable. One minor water
inflow of less than 1 gpm was encountered; otherwise, the
tunnel was essentially dry. The original design specified
W 6 x 20 steel sets on 4-ft centers, as well as pattern
(6x6ft.) rock bolts in the crown for the full length of the
manifold tunnel. Additionally, structural steel support
members at the penstock intersection were indicated. Owing
to better than expected rock conditions, only three short
sections of steel horseshoe shaped sets, two of which were
specified by design were installed. Tunnel support loca-
tions in the manifold tunnel are indicated on the geologic
tunnel logs of the manifold tunnel, Appendix C, Figure 3-6,
(Sheets 1 and 2). At the main portal, owing to the acute
angle of the portal face with the tunnel centerline, as well
as the highly weathered rock conditions, the initial 40 feet
of tunnel had to be supported on 2-ft centers. Withstanding
the adverse portal geometry and poor rock condition, tunnel
excavation progressed without major delays through this
3 - 6
section. With exception of one short section of tunnel
shotcrete was applied over the rock surface above springline
from the main portal to 10 feet upstream of the penstock 3
tunnel intersection.
As mentioned above adverse portal geometry also affected
slope stability of the approach cut leading to the power
(manifold) tunnel portal. Because these side slopes also
serve as abutments for a permanent bridge across the front
of the tunnel portal, extensive rock reinforcement was
included in the design. An indication of the extent and
type of rock reinforcement is shown in Appendix C, Plate
3-2. After installation of the rock bolts, the bridge abut-
ment cut slope and the area around the portal entrance was
shotcreted in entirety.
3. 4. 2 Geology
With the exception of a
conditions in the manifold
few localized areas, geologic
tunnel upstream of the thrust
block excavation were favorable for maintaining the tunnel
opening with minor support. Most of the rock types de-
scribed in the GIR for the powerhouse area were mapped in
the manifold section of the lower power tunnel. These
included intermixed graywacke, argillite, argillite with
chert and dacite. After the rock was cleaned, intervals
where argillite with chert occurred foliated structure was
prominently displayed in the roof, wall, and in the invert.
Well developed, near vertical jointing stood-out on the
tunnel walls in sections where graywacke and intermixed
graywacke and argillite occurred. Several shallow dipping
shear seams were mapped. Locally these seams were as wide
as 12 inches but more generally seam widths were less than 6
inches. Typically, the seams material is foliated and
3 - 7
closely fractured argillite with very little clay. Differ-
ent type seams were encountered at the wye intersection of
the manifold tunnel and the temporary access adit. These
shear seams of only a few inches in width are filled with
soft, whitish clay and brecciated rock. They are inclined
downstream about 50 degrees intersect the tunnel roof about
15 feet upstream of the pillar intersection. Rock fallout
and destabilized roof conditions occurred as a result of
these adverse seams. The intersection area is further
described in Section 4.3.2. At another shear seam location
immediately downstream of the intersection, a half of dozen
steel sets were installed because of the associated
blockiness of the rock. Withstanding a few argillic shear
seams the rock downstream of steel sets to the thrust block
is fresh and hard. Joint spacing is mainly 12 to 18 inches
and the joints are tight. Overall, the rock conditions for
this section of tunnel provide to a stable opening. Geolog-
ic maps and logs of the power (manifold) tunnel from Sta
1+07 to 5+40 are included in Appendix C, Figure 3-6 (Sheets
1 through 30).
Rock in the thrust block below the manifold tunnel invert
(i.e. from El 17.5 to El 10) is fresh, sound argillite.
Above the tunnel invert in the walls and crown the argillite
is slightly weathered and somewhat blocky, but still rela-
tively competent. Owing to marginal rock cover above the
thrust block, design changes in the thrust block chamber
were made. These modifications are briefly discussed in
Par. 3.4.1, and in addition to the geologic mapping, are
further explained in Appendix c, Figure 3-6 (Sheets 3 of
30).
3 - 8
Geology of the power (manifold) tunnel portal and bridge
abutment is given in Appendix C, Figure 3-7. Initially both
highly weathered intermixed graywacke and argillite and
dacite were exposed in the portal face. Approximately 30
feet in from the portal the tunnel face consisted mainly of
moderately weathered argillite. As indicated on the geology
maps, closely spaced, near vertical joints are prevalent on
both rock face above the portal and on the bridge abutment
cut slopes.
3.4.3 Tunnel Lining and Grouting
Permanent steel liner encased in pumped concrete were
installed in the three short wye penstock tunnels and in the
manifold section of the lower power tunnel. Additionally
the massive concrete thrust blocks were cast-in-place around
the steel penstocks in the four tunnel sections.
Prior to placing the steel liners and the backfill surround-
ing concrete in the tunnel sections, a system of l-inch
diameter grouting pipe loops were installed in the tunnel
crown including separate loops for the thrust blocks. Grout
piping included riser pipe into high points in the tunnel
crown for air venting.
Contact grouting of the three penstock thrust blocks was
done August 22 and 23, 1989. A water-cement ratio of 2:1
was used initially which was thickened to 1:1 when the
pressure started to increase (i.e. after grout pipe system
filled and return valve was shut.) The Contact grouting
pressure was 30 psi. The total grout takes in penstocks
thrust block of 1, 2, and 3 were 16, 38, and 16 sacks,
respectively. The crown manifold tunnel thrust block
including a short section of the manifold tunnel downstream
3 - 9
of the thrust block and a short section of penstock 1 tunnel
upstream of the thrust block were contact grouted though a
preset pipe loop. A total of 94 sacks of cement was placed
on October 6, 1990.
In mid-July, 1990, the portion of tunnel upstream of the
thrust block of penstock tunnel 2 and 3 was contact grouted
through a preset pipe loop with 1:1 grout at 50 psi.
Penstock tunnels 2 and 3 took 71, and 143 sacks, respective-
ly. The last manifold tunnel section to be contact grouted
extended several hundred feet downstream and upstream, from
the wye intersection with the south access tunnel Sta. 5 x
40). Grouting was done through a continuous grout and vent
pipe installed in the tunnel crown upstream and downstream
from a bulkhead at the wye intersection. Contact grouting
was done on April 4, 1991. The total sacks of cement place
was 170.
3 -10
4.0 LOWER POWER TUNNEL
4.1 General Description
The lower power tunnel extends from the downstream portal
near the north end of the powerhouse to the base of the
725-ft power tunnel vertical shaft. The downstream portal
is at Sta 1+00 and the base of the shaft is at Sta 177+05, a
distance of 17,605 feet. The downstream 420 feet of the
power tunnel is also designated as the manifold tunnel
section which branches off into three penstock tunnels. The
manifold portion of the power tunnel is intersected by the
temporary south access adit at Sta 5+20. Excavation and
concrete lining of the lower power tunnel was done through
the access adit. The manifold section of the power tunnel
has been previously described in Section 3.0. Section 4.0
describes the lower power tunnel upstream of Sta 5+20.
The design alignment of the lower power tunnel upstream of
the wye intersection (Sta 5+20) is straight except for a
small directional change of 7 degrees near the half way
point. The slope of the tunnel is 1.667 percent. A plan
and profile of the power tunnel is presented in Appendix D
(Plate 4-1). From the portal Sta 1+00 to Sta 31+60 the
permanent tunnel lining consist of pumped concrete around an
11-ft diameter steel liner. From the end of steel liner the
tunnel is lined with reinforced concrete to Sta 37+60.
Upstream from Sta 37+60 to the lower elbow of the power
tunnel shaft, the concrete lining is unreinforced except
through three fault zones where the lining is reinforced.
The Contractor elected the TBM option for excavating the
lower power tunnel. Until the TBM arrived on site and was
4 -1
assembled ready for use, the drill and blast method of
tunneling had progressed to Sta 14+10. The TBM operation
commenced at Sta 14+10 and continued to the base of the
power tunnel vertical shaft, a distance of 16,295 feet.
Geologic mapping was done for the entire lower power tunnel.
Mapping of the tunnel side walls and plan view at springline
was done on 8.5 x 11 inch sheets at a scale of 1" = 20'
scale. Other data entered on the map sheets includes rock
type, geologic description, excavation progress, support
method, special condition and comments and water inflow.
Geologic face maps of the drill and blast tunnel headings
were also prepared. Geologic log sheets for the lower power
tunnel for both the drill and blast reach to Sta 14+10, as
well as that excavated by TBM are included in Appendix D.
4.2 Tunnel Boring Machine (TBM)
Withstanding several wide fault zones described in the GIR
other conditions such as rock quality, rock hardness, and
tunnel length were favorable for excavating the lower power
tunnel with a tunnel boring machine (TBM). A used (1979)
Robbin TBM Model 148-213/3 was purchased in Norway by
Enserch Constructors, J.V. and shipped to Robbins Equipment,
Inc. in Seattle, Washington for reconditioning. After the
reconditioning, the TBM with trailing gear and supporting
equipment was shipped by barge to the jobsite, arriving
February 9, 1989. The diameter of the TBM cutter head is
15.1 feet which is larger than needed for the finished 11-ft
I.D. A mutually beneficial arrangement for accepting the
larger tunnel diameter was agreed to by the Engineer and the
Owner.
4 - 2
The front section of the TBM which includes the cutter head
with thirty-five 17-inch diameter cutters and opposing
gripper pads is 35-ft long. The rear section attached to
the front section by the main support beam is about 20-ft
long; the operators console and hydraulic power units are
located on this section. The attached trailing gear deck
sections are about 400 feet in length. Approximately the
first 60 feet or so of the trailing gear houses electrical
control cabinets, transformers power cable reel, air scrub-
ber, and the bridge conveyor overhead. The deck sections of
about 320 feet behind the electrical gear section includes a
double track with California switch which accommodates two
10-car trains with 6 cu-yd muck cars. Located overhead on
T-support columns is the muck conveyor for loading cars and
the twin ventilation pipes which extend forward to the front
section of the TBM. A photograph of the TBM and trailing
gear decks taken outside prior to moving into the tunnel is
included in Appendix A.
Considerable preparatory work was necessary prior to
the TBM in to the tunnel. A 120-ft long starting
with circular concrete walls was constructed. The
moving
chamber
tunnel
invert downstream to the access adit portal was cleaned to
rock and a leveling concrete mud mat was placed. Air
ventilation pipe and other obstructions in the tunnel were
removed to provide clearance for the TBM. Rail track had to
be lain from the tunnel heading to the marshalling yard
outside.
After weeks of preparation in the marshalling yard, the TBM
and trailing gear were moved into the tunnel to the starting
chamber (Sta 14+10). Two weeks of final assembly including
electrical, mechanical, and hydraulic work and welding of
4 - 3
the outside cutter and bucket assemblies on to the cutter
head was done. Approximately 160 hours of welding was
involved. Assembly and electrical and hydraulic checkout
was completed by March 4, 1989 and the first trial run of
the TBM was made. By March 8, most of the minor operational
and equipment problems were corrected to where the TBM was
operational more than 12 hours per day. The Contractor's
work schedule for TBM operation was 7 days a week, 3 shifts,
with maintenance done on day shift. The TBM check-out
period lasted about one week, after which the daily advance
for the first month of operation (March, 1989) averaged 115
feet.
The TBM completed excavation of the lower power tunnel (Sta
172+29) September 6, 1989. Overall TBM monthly progress
averaged 104 feet a day. Best daily progress was 275 feet
which was reportedly a worlds record for this diameter
tunnel in hard rock. Further description of the TBM tunnel-
ing excavation are described in Section 4.5.1. Daily TBM
progress is given in Appendix D, Table 4-1.
4.3 Tunnel Sta 5+20 to Sta 14+10
4.3.1 Excavation and Support
The south access tunnel excavation continued from Sta 5+20
to Sta 6+40 where the alignment coincides with the power
tunnel alignment. The right wall of the access adit (look-
ing downstream) was then widened by "slashing" to achieve a
curve to the right in the power (manifold) tunnel alignment.
At Sta 5+20 a full tunnel face was developed on the manifold
tunnel alignment. The roof span of 39 ft at the fork of the
manifold and access tunnels (Sta 5+20) transitions to 14.5
ft at Sta 6+40. Upstream of the wye intersection in the
4 - 4
widened tunnel section between Sta 5+20 and Sta 6+40 the
specified 10 and 15-ft long pattern, tensioned (30 kips)
rock bolts were installed in the crown. A plan of the
intersection of the south access adit with the power tunnel
including the rock bolt pattern is shown in Appendix D,
(Plate 4-2).
Upstream of Sta 6+40 to Sta 14+10 the excavated tunnel
diameter is approximately 14.5 feet. The last 120 feet was
enlarged and a 50-ft section of concrete lining was in-
stalled on the walls to form a starting chamber for the TBM.
The drill and blast heading upstream of Sta 6+39 advanced
without difficulty at an average rate of 35 feet per day (3
shifts). No steel tunnel support sets were installed. Rock
bolts were installed as needed. In general, the Contractor
installed four 6-ft long tensioned (5 kip) bolts on a radial
pattern in the arch at 4 to 5-ft spacing. Where shear seams
were encountered, supplemental rock bolts, as required, to
assure stability were installed.
Adverse excavation conditions were encountered upstream from
the wye intersection between Sta 5+55 and Sta 5+20. During
widening (downstream advance) of the power tunnel into the
right turn of the manifold tunnel, several subparallel clay
seams up to 12-inches wide were encountered causing fall-out
and unstable roof conditions. The shear seams were not
unexpected because one had been encountered during the
excavation of the access tunnel. However, the consequences
of the clay shear seam geometry became more apparent during
tunnel widening (slashing). With widened roof spans of (30
to 39 feet) localized rock failure occurred adjacent to the
clay seams. Withstanding the adverse clay seam orientation
instability of the clay seam material and adjacent altered,
4 - 5
fracture rock was controlled. Careful advance was made by
drilling and blasting shorter (4 feet) rounds closely
followed by installation of 10 and 15-ft long tensioned rock
bolts on centers as close as 3 feet. Six-inch wide steel
straps were also used between pairs of rock bolts. Welded
wire mesh was pinned to the roof area above springline
between Sta 5+65 and Sta 5+20 and a minimum 3-inch thickness
of shotcrete was applied over the entire roof area. No
further instability was observed for the treated area.
4.3.2 Geology
Geologic logs of the walls and crown of the lower power
tunnel between Sta 5+20 and Sta 14+10 were made continuously
during excavation. Also, geologic face maps of the tunnel
heading were completed daily. Geologic mapping (40 sheets)
for this reach of tunnel is included in Appendix D, Figure
4-1. The heading was advanced typically at 35 feet per day,
consequently tunnel face maps can be as much as 35 feet
apart. Mapped rock units encountered consisted of
argillite, graywacke, and intermixed graywacke and
argillite. Rock types for this reach of tunnel change
moderately frequent; ranging from 40 to 120 feet. Although
not easily delineated, short (10-50 feet) transition zones
of intermixed graywacke and argillite frequently occur at
the contact between graywacke and argillite.
Joint frequency is variable but mostly spaced at 12 to 18
inches. Perhaps 25 percent of the jointing is outside this
range but rarely exceeds 24 inches on the high side but on
the low side 12 to 6 inches is not uncommon. Fracturing
appears to be less than the core logs of exploration holes
indicated. Overall, the rock along this tunnel reach would
be described as moderately jointed and fractured.
4 - 6
For tunnel intervals delineated as argillite, the rock is
tightly foliated, in part with chert stringers and lenses
paralleling the foliation. Foliation is more strongly
developed in some areas than others; one such tunnel inter-
val Sta 12+00 to Sta 12+50, foliation is particularly well
displayed. Argillite that is foliated is not necessarily
less stable than graywacke except that shear seams appear to
occur more frequently in argillite. Foliated argillite
layers although mostly less than 0.5 inch thick are tight in
fresh unweathered condition; the rock mass is moderately
hard and fractured. Graywacke on the other hand being as
much as 50 percent harder than argillite breaks out (drill
and blast tunneling) more blocky. The jointing on the
tunnel walls is relatively more developed in graywacke. The
irregular wall surface gives the appearance of excessive
overbreak. A survey of overbreak for this reach of tunnel
revealed local areas of excessive overbreak, but for the
most part overbreak beyond the allowed tolerance of 12
inches was minor.
As previously mentioned brecciated shear seams, predominant-
ly 3 to 12 inches wide, occur more frequently in the
argillite or argillite with chert. Frequency of shear seams
is variable but a 50-ft interval of tunnel without inter-
secting at least one would be unusual. Most shear
encountered consist of closely foliated and closely
tured, platy, argillitic rock with minor clay coating
seams
frac-
along
some fracture surfaces. Relatively, few shear seams are
brecciated truly clay gouge seams. Argillitic shear seams
of less than 12 inches width generally did not cause any
unusual stability problems. In a couple of instances
argillitic seams as much as 18 to 24 inches wide were
4 - 7
intersected in the tunnel without significantly effecting
tunnel advance.
Shear seams with intercalated clay gouge generally cause
some localized instability and require some type of tunnel
support to retain fallout from the roof. This was the case
for a short interval of tunnel immediately upstream of the
wye intersection of access and manifold tunnels between Sta
5+20 and Sta 5+55. Within this 35 feet interval two
subparallel clay seams or small faults ranging from 6 to 12
inches in width obliquely crossed the tunnel intersection.
A third seam with less inclination (dip) intersected one of
the two seams in a roughly diagonal direction. Clay gouge
was several inches wide in the two parallel seams and the
adjacent highly altered, closely fracture and partly
brecciated rock zone was up to 29 inches wide. The inter-
section by the third seam although relatively narrower with
minor clay added to the instability. This adverse geologic
structure conditions resulted in several feet of fallout
from the tunnel roof in two localized areas. Continuous
light water dripping from the clay seams also caused inter-
mittent sloughing of clayey material. As previously de-
scribed in 4.3.1, this unstable zone was carefully excavated
in short advances and the wide roof spans 30 to 39 feet were
progressively stabilized with extensive rock bolting and
shotcreting.
rock bolting,
(Plate 4-2).
Design support for these wide spans relied
no steel sets were specified, Appendix
on
D,
Ground water inflow for this reach of tunnel (Sta 5+20 to
Sta 14+10) was minor; there was only one location of flowing
water. High on the left wall looking (upstream) at Sta
13+8~_!nJ!-ial water inflow estimated at 8 gpm emanated from
4 - 8
a joint fracture.
of water. Within
stopped.
There were several locations of dripping
a month these water sources essentially
4.3.3 Tunnel Lining and Grouting
The manifold section of the lower power tunnel
downstream from Sta 5+20 to the main portal. This
of the steel penstock was installed and encased with
concrete from Sta 5+20 to the main portal in fall of
extends
portion
pumped
1989.
Because of the continued need for tunnel access for person-
nel and equipment, the liner from Sta 31+55 downstream to
Sta 14+10 continuing to Sta 6+55 could not be started until
after the concrete lining of the lower power tunnel was
completed August 25, 1990. In fact, due to need for access
to supply equipment and materials for finishing and contact
grouting of the tunnel concrete lining, installation of the
first section of steel liner at Sta 31+11 did not commence
until November 6, 1990. The steel lining installation and
welding work was completed to Sta 14+10 on December 8, 1990
and progressed to the closure section at Sta 5+55 on Decem-
ber 16, 1990. The 40-ft long liner sections were welded
together outside the tunnel into 120-ft long sections before
transporting into the tunnel on rail. Typically three
120-ft sections were installed in the tunnel and welded
together each week. Concrete encasement of the steel liner
was done after each 120-ft section was added and welded;
however, the advancing concrete face was kept about 120 feet
upstream of the last liner joint being welded. Installation
of the closure sections of steel liner at Sta 6+55 including
concrete placement around the liner was completed the end of
March, 1991.
4 - 9
Contact grouting along the crown of the tunnel through the
pre-tapped holes in the steel liner from Sta 31+55 to Sta
14+10 started December 27, 1990 and was completed January 9,
1991. Grouting was placed at 30 psi using a 1:1 grout mix.
From Sta 14+10 to 6+55 the tunnel was excavated by drill and
blast method and grout takes were somewhat larger than the
TBM bored tunnel upstream of Sta 14+10. Contact grout takes
for this section averaged approximately 0.60 sacks per
lineal foot.
Void or
concrete
"skin" grouting
backfill was
between the steel liner and
also performed. Twenty-two
the
( 22)
locations of voids were identified by sounding of the liner.
These locations were drilled and tapped for contact grout.
In the reach between Sta 14+10 and Sta 6+55 the total void
grout injected between the steel liner and the concrete was
nominal.
4.4 Tunnel Sta 14+10 -Sta 31+60
4.4.1 Excavation and Support
The lower power tunnel including the penstock and manifold
tunnel sections from portals up tunnel to Sta 31+60 will be
lined with steel varying in thickness from 3/4 inch to two
inches. Between the steel liner and the excavated rock
surface, unreinforced concrete with a 28-day compressive
strength of 2,500 psi will be placed. The lower power
tunnel between Sta 14+10 (end of drill and shoot tunnel) and
31+60 (end of steel liner) was excavated by tunnel boring
machine (TBM). TBM progress, support requirements, and
water inflow data are included on the geologic tunnel logs
in Appendix D, figure 4-2 (82 sheets).
4 -10
The tunnel advance through this section averaged
mately 106 feet per day after a normal four day
approxi-
breaking
period during which time minor modifications were completed
and crew responsibilities were clarified. The TBM started
mining on March 4, 1989 and completed this section on
March 22, 1989.
Rock support within this section was predicted by the
Geotechnical Interpretive Report (GIR) to be equivalent to
that required in a generally highly fractured rock mass.
The actual rock mass encountered was more closely repre-
sented by the moderately fractured to sound classification
of rock mass quality. The difference between actual and
predicted rock mass quality is difficult to access quantita-
tively because the GIR tunnel ground classifications are
based upon a drill and blast excavation techniques. The use
of a TBM results in reduced support requirement, a fact
which was readily observable near the beginning of the TBM
bore. Just downstream of the TBM bore, in the drill and
blast section, the jointing and fracturing is clearly
exposed and support bolts are installed (usually coincident
with overbreak) where intersecting planes required ground
support. Upstream, within the TBH bored section, the
jointing and
bolting is
occurred.
fracturing
infrequent,
are
and
less
very
clearly
little
exposed, rock
over excavation
With exception of two relatively short zones described
below, normal rock treatment within this 1,750-ft long
section was accomplished with "spot bolts" installed only at
locations which were thought to have potential for insta-
bility and/or fall out. These bolts and the decisions to
install them were made based upon the condition of the rock
4 -11
exposed between the TBM cutter head and grippers, a distance
of roughly 20 feet. On occasion, additional spot bolts and
8 inch wide x 14-gage steel strap was installed from off of
the trailing gear deck or from a work platform installed on
a railroad flat car behind the TBM and trailing gear. Two
short sections within this tunnel reach required significant
support within localized zones of highly fractured rock.
These zones occurred between Sta 23+95 to Sta 24+20 and
Sta 26+20 to Sta 26+40. Both areas consisted of closely
jointed and fractured, blocky rock in the left rib (left
sidewall looking up station, i.e., upstream) above spring-
line and the left crown. These zones were supported with
rock bolts and steel strap and were significant enough to
slow TBM boring advance. Thin shear zones evidenced by some
clayey coatings infilling joints and fractures may have
influenced weathering and frequency of fracturing within
these zones. Subsequent ground stability has not changed
over time; no further fall out or deterioration of sta-
bility has occurred.
4. 4. 2 Geology
Lithology within this tunnel reach is interpreted in the GIR
to be mostly graywacke with lesser quantities of argillite
with chert, argillite, intermixed graywacke and argillite
and dacite. A comparison of actual lithology with predicted
lithology in this limited zone (10 percent of the entire
lower power tunnel) would lead to inconclusive results. A
more reasonable comparison is contained in this report as
Appendix D, Plate 4-3 which summarizes the entire power
tunnel graphically giving estimated and as-built percentag-
es. Notwithstanding, relatively more intermixed argillite
and graywacke was encountered within this section than
graywacke as predicted. Dacite was not encountered, but a
4 -12
small amount of greenstone,
the total tunnel was found.
tively equivalent. More
quantify lithologies, is
lithology on advance rate.
the only greenstone located in
Other lithologies were rela-
important than an
to determine the
attempt
affect
to
of
Throughout this section, advance
was rapid regardless of the variable lithologies, but slowed
in the aforementioned zones requiring moderate support to
maintain stability. However, the difference in rock
strength of the principal lithologies was not great.
Advance rate in the power tunnel is mostly influenced by
structural integrity of the rock mass, more so than the rock
type. This fact is further developed in Section 4.5 for the
remainder of the lower power tunnel TBM excavation.
Jointing and fracturing within the TBM
less well exposed than they would be in
bored section are
a drill and blast
tunnel. Joints and fractures intercepted by the tunnel
bore that trend normal to the tunnel alignment are very
difficult to distinguish unless they are open or closely
spaced and locally plucked out along the joint trace by the
boring action of the TBM. Joints and fractures trending
sub-parallel to the tunnel alignments are more readily
visible due to break away of thin wedges formed by the
actual angle of intersection between tunnel wall and joint/
fracture trend. The resulting "kerf" typically 1/2 to 2
inches deep varies in shape from a vertical line for verti-
cal joints cutting the tunnel perpendicular to its axis
to truncated ellipses for vertical and/or steeply dipping
joints intersecting the tunnel acutely to the alignment.
Kerfs occur on both walls but are more prevalent on the left
wall (looking upstream) and rarely occur on the roof. The
development of kerfs is possibly due to several causes.
Cutting action of the clockwise rotation of the TBM cutter
4 -13
head bucket teeth can cause strong abrasive force acting on
thin joint intersections. The gage cutter wheels exert
tangential forces as well as forward loads during their
travel path across joints and seams. This action can result
in break out of small rock slivers along discontinued ties.
Another mechanism for localized failure is due to pressure
exerted by the TBM gripper pads.
Jointing and fracturing within this reach (Sta 14+10 to
31+60) was stabilized where necessary by spot bolts and
limited use of steel straps unless fall out was progressive
as in the two areas described in Section 4.4.1. These areas
of highly fractured rock were stabilized by extensive use of
6-ft long rock bolts and steel straps approximating a 4-ft
by 4-ft pattern.
No known fault was predicted to intercept this reach of
tunnel. However, three surface lineaments were identified
on aerial photographs. In the tunnel three shear zones were
mapped for this reach, however, the shears did not line up
with the surface lineaments. This apparent lack of correla-
tion is not an unusual condition, as the lessor surface
lineaments identified on aerial photographs do not necessar-
ily imply a corresponding linear feature at tunnel depth.
The shears were thin, closely fractured, generally 6 inches
to 12 inches wide with minor associated jointing and frac-
turing. One shear, however, at Sta 23+95 to Sta 24+20 had a
larger zone of associated jointing and fracturing approxi-
mately 5-ft wide (discussed in Section 4.4.1). Minor rock
support was installed except as described in Section 4.1.1
at Sta 23+95 to Sta 24+20.
4 -14
Point source groundwater inflows intercepted in this reach
were minor, less than 10 gpm. The aggregate flow however
approached 600 to 700 gpm.
4.4.3 Design Modifications
From Sta 14+10 to Sta 31+60 the power tunnel included a
steel liner as designed. Hydrofracture testing of rock was
performed within the transition zone from the end of the
steel liner through the reinforced concrete lined section
and to unreinforced
Sta 37+60). Although
concrete lining (Sta 31+60 to
the test results were inconclusive,
the Federal Energy Regulatory Commission, (FERC)
recommended not to extend the steel liner beyond Sta
Board
31+60.
The Board's recommendation was based upon favorable quality
of rock exposed. However, based on later design decisions,
the concrete lining was reinforced to Sta 38+60 and in 17
separate sections upstream. High pressure compaction grout
holes initially planned to extend to Sta 38+00 on earlier
recommendations by the FERC Board. Subsequently the compac-
tion grouting was extended upstream to Sta 65+00 by SWEC,
with the FERC Boards concurrence, because of the potential
for tunnel leakage through open joints/fractures with large
water inflows.
4.4.4 Tunnel Lining and Grouting
The sequence of the steel liner installation and the contact
grouting procedures were presented in Section 4.3.3. There
it was mentioned that the first 120-ft section of steel
liner went in the tunnel to Sta 31+77 on November 6, 1990.
The upstream end of the 10-ft diameter steel liner connected
into a concrete transition to the 15-ft ID concrete tunnel
lining. Working downstream installation of the 120-ft
sections of steel liner was completed to Sta 14+10
4 -15
mid-December. The placement of concrete backfill around the
steel liner was completed shortly thereafter.
Contact grouting of the crown through pre-tapped holes in
the liner from Sta 31+77 to 14+10 was completed in early
January, 1991. Void grouting between the liner and the
concrete was done more or less concurrently with contact
grouting. Grout takes for contact grouting averaged 0.60
sacks/LF. Void grouting grout takes were minimal.
4.5 Tunnel Sta 31+60 -Sta 177+51
The lower power tunnel from Sta 31+60 (end of steel liner)
to Sta 177+51 (vertical shaft) was excavated by TBM. This
segment of tunnel was designed to be lined with a twelve
inch minimum thickness concrete lining with a 28-day com-
pressive strength of 3,000 psi. Steel reinforcement was
incorporated into the concrete lining in a number of sec-
tions after evaluating the excavated tunnel ground condi-
tions.
4.5.1 Excavation and Support
Excavation of this segment began on March 23, 1989 and was
completed on September 6, 1989. The tunnel advance through
this section averaged approximately 100 feet per day. TBM
progress support method and water inflow data are included
on the geologic tunnel logs in Appendix D1 Figure 4-1 (82
sheets).
Rock support within this section was predicted by the GIR to
range from none in sound rock to circular heavy steel sets
in poor rock within fault zones. This segment of the lower
4 -16
power tunnel is discussed in the GIR as six separate sec-
tions divided upon the basis of two known large scale fault
zones, an inclined shaft option, and intervening sections.
A lengthy comparison of predicted and as-built support
requirements is beyond the scope of this report. Only a
summary of actual support placed and the structures and
lithologies encountered are presented. The predominate
(over 90 percent) form of rock support installed within the
TBM bored tunnel consisted of non-patterned 6-ft long A-2
type spot rock bolts. The majority of these type of bolts
were installed from drill locations between the TBM cutter
head and the TBM gripper pads. Rock bolts were installed in
holes drill with jack-leg rock drills using epoxy resin
cartridges. This type of support was installed where
individual blocks of rock, localized shear zones or closely
spaced joints and fractures were considered to have failure/
fall out potential. In zones where the frequency of frac-
turing and/or fall out was deemed excessive for single spot
bolts to achieve support, supplemental support was used in
conjunction with A-2 type rock bolts. Supplemental support
included steel straps (8 inches wide x variable length x 14
gage steel), rolled steel channel (C 10 x 20 formed arch),
rolled steel "mine ties", steel channel "lagging" (MC
10 x 6.5 x 2), welded wire fabric (WWF 4 x 4), and various
sizes of wood timber blocking and wedges.
Also available to the Contractor was a selection of 4 types
of steel sets including an arch set (240 degrees
arch) and 3 full ring (full circular section)
variable weight/strengths and placement frequency.
2-piece
sets of
Although
the need for steel sets was predicted and an allowance of
1500 full circle sets was provided in the Contract and were
purchased by the Contractor only one installation of steel
4 -17
sets was made within the TBM bore. Full circle steel sets
on (W6 x 25, 60ksi) on 4-ft centers were installed within
the Bull Moose Fault Zone between Sta 68+00 and 72+00.
Although deemed necessary by the Contractor the placement of
these 100 sets was not done until after the TBM had cleared
the fault zone. The Construction Manager was not in agree-
ment with the Contractors determination that sets were
needed. Installation of the sets was made under the "Con-
tractor's Option" clause of the specifications.
Throughout this section
177+51), the majority of
of tunnel (i.e., Sta 31+60 to
tunnel support beyond spot rock
bolts was installed in fractured rock associated with shears
and faults. The most significant zones of support were
usually in areas of multiple shears intersecting each other
with overlapping highly fractured zones. The intersection
of low dip angle and high dip angle shears caused wedge
shaped blocks of fall out until stable arching was achieved
or cribbing and support steel was placed. In two locations
on the left wall in cavities caused by raveling and fall
out, Sta 103+20 and Sta 103+60, the Contractor chose to
place backfill concrete behind the TBM gripper pads to
provide necessary bearing to the gripper thrust for advanc-
ing the TBM. The three known faults penetrated by the TBM
within this section were substantially less difficult to
bore through than was expected from the most conservative
interpretation of the GIR discussion of tunnel ground
classification. The maximum support required approximated a
4 x 4-ft pattern of rock bolts from springline to springline
with alternating rolled steel channel and rolled steel mine
ties. After this initial treatment, welded wire fabric
(WWF) was placed with additional rock bolts where necessary.
4 -18
Within this section the excavated tunnel bore will be lined
with unreinforced concrete lining a minimum of 1-ft thick.
Reinforced concrete lining in limited sections will be
placed in accordance with specifications and drawings. An
option to "skip line" the lower power tunnel between
Sta 84+00 and Sta 177+05 was developed by the Engineer at
the Owner's request. The outcome of the tunnel lining
option is described in Section 4.5.3.
4.5.2 Geology
The lithology description within this section is interpreted
in the GIR to cover the full range of rock types recognized
for the project area. Appendix D, Plate 4-3 in conjunction
with the continuous geologic tunnel log, Appendix D, Figure
4-2 best describes the actual range and percentages of rock
types that were excavated. The predicted and as-built rock
type percentages compare favorably. The relative hardness
of the various rock types cannot be shown conclusively to
have significantly affected the TBM excavation rate.
Rather, the rate of penetration was more influenced by
condition of the cutters, the capacity of the belt conveyor
and the muck train cycle time. Structural geology of
shears, faults, and joint/fracture splays locally affected
the TBM advance rate, but in the overall tunnel length the
effect of geologic conditions encountered in the lower power
tunnel on advance rate was not significant. To further
amplify the lack of harness control on penetration rate, two
examples are given below. From Sta 33+54 through Sta 40+62
average advance rate of 187 feet per day was achieved in
nearly predominantly graywacke which has a relatively higher
strength than argillite. In this interval a world record
run of 275 feet per day as verified by the TBM manufacturer
was also achieved. The final five days of TBM bore averaged
4 -19
74 feet per day in a massive graywacke. At this
haulage was at its maximum, and cutter wear
extended to the maximum possible to conserve
time,muck
was being
change out
time. Daily TBM advance rates from March 4 to
1989 (Sta 14+10 to 117+51) are presented
Appendix D.
September 6,
in Table 4-1,
Three significant geologic lineaments identified on
ground surface above the tunnel were recognized as
scale fault traces from earliest investigations (GIR).
were colloquially named the Bull Moose, Bradley River,
Bear Cubs Faults. Locations of these fault traces
the
large
They
and
with
respect to the tunnel alignment are included in Appendix D,
Plate 4-4. The GIR includes detailed descriptions of
the anticipated tunneling conditions and hazards associated
with these faults based upon deep angle borings which
penetrated the Bull Moose and Bradley River fault zones
(Bear Cubs Fault was not drilled). The Bull Moose Fault
(Sta 65+80 to Sta 72+00) was the first to be encountered by
the TBM. It was approached with extreme caution by the
Contractor with a 2-inch diameter "feeler" probe hole being
advanced approximately 75 feet in advance of the cutter head
prior to subsequent TBM advance. The probe hole provided
several pieces of advance knowledge about the boring condi-
tions ahead. It provided an indication of on rock hardness,
soundness, the presence or lack of ground water and the
relative pressures. Withstanding the great care and caution
exercised, the Bull Moose Fault System was penetrated at a
surprisingly high average penetration rate of 138 feet per
day. Four mapped zones of faulted, sheared and brecciated
rock were identified as coincident with those represented by
the GIR along with additional relatively minor mapped
gouge/breccia shear zones. Three faulted zones were
4 -20
associated with dacite dikes. The
near Sta 66+90 displayed duplication
first zone encountered
of a thin dacite dike
giving an inferred right lateral senses of offset. Minor
delay in TBM advance to install channel, strap, and rock
bolts to support fractured rock associated with this dacite
and a thick dacite dike at Sta 71+00. Advance rates were
nonetheless high throughout the fault system.
The Bradley River Fault was next encountered from Sta 145+00
to Sta 148+40. The first fault splay was thin and easily
penetrated with no support needed. The second and third
splays were each approximately 30-ft wide. Both zones were
highly deformed, sheared and faulted, the entire zones being
considerably brecciated with clayey gouge seams to 3-inches
thick. Withstanding the advance geologic conditions these
zones were bored relatively quickly. However, installation
of a large number of channel sections with cribbing, and
rock bolts were necessary to stabilize the zones. Upon
completion of the TBM bore through the zone, additional WWF
and rock bolts were added from the trailing gear deck to
further stabilize the rock mass. Owing to the absence of
ground water within the faulted sheared zones, the wall rock
retained a fair degree of intactness. Fall out of up to 3
feet occurred near the TBM gripper pads at several locations
where lateral thrusts were large enough to deform and
displace the gouge/breccia, giving rise to rebound slough-
ing.
The Bear Cubs Fault (Sta 164+90 to Sta 167+20) occurred
within a zone of numerous low angle to high angle shears
requiring extensive support using rock bolts, channel,
lagging, and welded wire fabric. The TBM bore was slowed
through this area due to frequent localized blocky rock fall
4 -21
out which was supported behind the cutter head with bolts,
channel, and lagging. With better access, WWF steel chan-
nels were added from the trailing gear deck to further
complete the support. The shears were not unusually exten-
sive, however, they were intercepted at a low angle which,
with associated jointing and fracturing, caused numerous
small wedge failure blocks. The stability of these blocks
was further affected by TBM cutting action and gripper pad
pressures.
To summarize the impact of the three major faults on TBM
tunnelling, a number of statements can be made. The three
faults were located within the intervals predicted in the
GIR. Each fault caused only minor delays to TBM progress.
The faults were relatively dry and without significant
weathering of gouge and breccia zones. The absence of
serious adverse ground conditions such as running or squeez-
ing ground allowed a significant acceleration in tunnel
schedule. The widths of gouge/brecciation (especially
within the Bradley River Fault) were consistent with GIR
predictions. The predicted severity of the gouge/
brecciation although reasonably accurate failed to impact
support requirements to the degree implied in the GIR. Had
the local stress fields, ground water regime or weathering
influences been more adverse, the full range of GIR predict-
ed problem ground conditions may have been encountered.
Shear zones, in which movement has occurred on a large scale
so as to partially crush and brecciate the rock, occurred
frequently throughout the TBM bore. More than 100 such
zones were mapped within the lower power tunnel. Shear
zones were distinguished from fault zones based upon width
and position with respect to projected surface fault
4 -22
locations. Whereas shears may be associated with faults on
the basis of similar genetic origin, age and may be actual
splays of recognized faults, they are so designated to
segregate them from the three known faults. In addition, a
lack of marker beds and general mixing of lithologic types
during tectonic accretion has destroyed sedimentary struc-
tures which may have been used for reconstruction of fault/
shear geometry. Typically, shears were identified based
upon the presence of clayey gouge, brecciation, and local-
ized joint, fracture, and calcite rehealed tension fracture
frequencies. They varied in size from several inches to
several feet of gouge/breccia thickness with associated
fracturing ranging from inches to tens of feet wide. The
impact of shears upon TBM advance ranged from none to
considerable. The majority of shears encountered were
either not in need of support or were supported with spot
rock bolts. In general, low dip angle shears and areas of
several shears intersection within or near the perimeter of
the tunnel bore and shears having soft, weathered, and
easily erodable gouge/breccia caused TBM delays. The worst
case was located near the halfway curve where two zones of
intersecting shears combined with eccentric thrust of the
TBM caused heavy fall out. To provide adequate gripper
thrust for tunnel advance, two placements of fill concrete
were made in raveled out cavities. Also, heavy use of
blocking, channels, and rock bolts to stabilize the zones.
Several days were expended in these zones with reduced
advance rate between the supported zones. Most support of
less significant shear zones was accomplished immediately
behind the TBM cutter head causing little or no advance
delays.
4 -23
Joints and fractures were encountered throughout most of the
TBM excavated tunnel bore. The frequency and orientation of
joints and fractures contributes strongly to overall rock
mass quality. Jointing and fracturing that was not clearly
associated with faulting and shearing was for the most part
widely spaced. Three localized zones of closely spaced
joints were encountered that required moderate rock support
with channels, rock bolts, steel strap, and limited WWF
welded wire mesh. They occurred at Sta 26+25, Sta 44+40,
and Sta 169+60. Normally, where joints and fractures were
closely spaced support consisted of either spot rock bolts
or spot rock bolts in association with short channel sec-
tions. Except as noted above, jointing and fracturing had a
minor impact on TBM advance, needed support being installed
concurrently with advance.
Ground water inflows within this section of TBM bore was
encountered in quantities close to those predicted in the
GIR. The primary source of ground water seepage into the
tunnel was through joints and fractures and occasionally
shear zones. The faults were relatively dry. The largest
sustained zone of ground water inflows occurred from numer-
ous joints oriented near parallel with the tunnel alignment
between Sta 80+50 and Sta 89+00. Total aggregate flow from
the tunnel approached 5,000 gpm peak when the heading
advance had reached Sta 84+00 during steady boring through a
zone of competent intermixed graywacke and argillite which
required only minimal rock support. Inflow occurred inter-
mittently along joint traces suggesting that the slightly
undulating joint surfaces had been moved differentially a
few inches or less thereby dilating the joints. Dilation of
joints could open considerable space for percolating ground
water to fill. Beyond the area of high inflows (up tunnel
4 -24
from Sta 89+00) ground water inflows decreased steadily.
The remainder of the tunnel bore saw only occasional in-
creases of high ground water inflow. Two highs of note
occurred at Sta 104+50 (associated with intense shearing at
the half way curve) and at Sta 157+50, a localized zone of
open, travertine coated joints. Ground water was handled
effectively by the Contractor through the use of pump
stations. Collection sumps, seven in all were installed in
the left rib near invert. Water was pumped into a 24-inch
diameter discharge pipe extending to the portal and on to
settlement ponds. The affect of ground water inflows on
excavation was notable only within two zones, those at
Sta 82+00 through Sta 89+00 and Sta 157+50. At these
locations cuttings ranging from clay sized to fine gravel
sized particles were washed out from under the cutter head
and accumulated in the tunnel invert. The washout necessi-
tated slusher and hand mucking of accumulated rock cuttings
from the invert to allow installation of the railroad ties
and rails. This type of mucking, along with difficult
access and working conditions for disc cutter changes in
front of the cutter head resulted in advance delays.
Tunnel instrumentation consisting of tape extensometer and
vibrating wire strain
ments. Extensometer
gages provided for in the bid
pin sets were installed in the
docu-
Bull
Moose Fault and the Bradley River Fault. Vibrating wire
strain gages were installed on the steel sets within the
Bull Moose Fault. The instrumentation installed within the
Bull Moose Fault gave no indication of ground movements of
any significance. Tape extensometer measurement sheets are
given in Table 4-2. Strain gage readings are presented in
Table 4-3. Both tables are included in Appendix D.
4 -25
The extensometer pin sets installed within the Bradley River
Fault gave no indication of significant movement at the
first two pin set locations (Sta 140+31 and Sta 144+40). At
locations Sta 147+76 and Sta 148+15, inward movements of
0.243 inches maximum were recorded. The mechanism for these
minor inward movements was not clearly understood. Possible
causative factors include load relief from gripper pad
deformations, squeezing ground, swelling ground, or stress
relaxation due to excavation related reorientation of insitu
stresses. Regardless of cause, the reinforced concrete
liner specified for this zone is designed for the load
transfer which may occur long term.
4.5.3 Continuous or Skip Lining
SWEC at the request of the Owner, identified large segments
of the lower power tunnel upstream of Sta 84+00 having
requisite qualifications to remain unlined. A process of
evaluation was developed to define the "skip lining" propos-
al including unreinforced concrete liner, reinforced con-
crete liner, reinforced shotcrete "patch lining", additional
spot rock bolts (with and without channel) additional
scaling, and no treatment. The proposal was reviewed by the
FERC Technical Review Board and found to be lacking in
justifiable economic cost/benefit as well as requiring
addition conservatism with regard to treatments in unlined
segments. The Contractor's cost evaluation provided to the
Owner was not considered a justifiable savings by the Owner
and the decision was made not to partially line but to
continuously line the entire tunnel.
4.5.4 Tunnel Lining and Grouting
After completing the concrete lining of the power tunnel
vertical shaft including the lower elbow concrete, lining of
4 -26
lower power tunnel was started at the beginning of March,
1990. From the end of the transition of the lower elbow
(Sta 177+04) 93 consecutive placement were completed down-
stream ending at Sta 31+45 on August 25, 1990. Most of the
lining placements were 150 to 180-ft long, some were 210 to
240-ft in length. Placement concrete volumes ranged from
typically 250 to 375 cu-yd. There were numerous concrete
placement problems particularly the upstream half of the
tunnel which resulted in numerous but limited areas of
segregated concrete exposed in the concrete lining. Con-
crete lining repairs took months to complete. Handling
numerous large water inflows during concrete placement was a
persistent problem frequently caused localized degradation
of concrete. Rebar reinforcement in the concrete lining was
installed at 19 locations. Tunnel lining sections with
rebar ranged from 20 to 700 feet, but most ranged from 140
to 230 feet. The total footage of reinforced lining was
2,305 feet.
Contact grouting of the lower power tunnel started early
May, 1990 and was completed in late September, 1990. During
this period grouting work was interrupted by the Contractor
for his purposes several times; the actual time spent
grouting was perhaps two months. The contact grouting work
was performed by the general civil contractor (i.e.
Enserch). Due to other ongoing construction activities, the
Contractor tended to skip some reaches, but for the most
part contact grouting was performed from downstream end to
the upstream. The design drawings called for two lines of
grout holes in the crown 20 degrees left and right of
vertical. The hole spacing is staggered so as to be on
10-ft spacing. However, the double grout hole line layout
was modified shortly after starting grouting to a single
4 -27
grout line of holes spaced 10 feet apart down the center of
the crown. After grouting several hundred feet of double
line with minimal take, a single line down the center of the
crown was tried and felt to be more effective in filling the
shrinkage voids. The single line of grout holes was uti-
lized for contact grouting the concrete lining for the
remainder of the tunnel length.
For the most part the Contractor's performance of the
grouting went well. In zones where water flowed from
contact grout holes grout at 30 psi plus water inflow
pressure was pumped until excessive pressure build up
occurred. In many cases the grout and water advanced to the
next hole to be grouted. The concrete lining construction
joints were generally tight against the crown which limited
the grout travel to the length of lining placements.
Contact grouting was done with water-cement ratio of 1:1.
Quantifying the grout takes for filling the voids in the
crown from those required to seal off water inflow below the
concrete lining invert for many reaches of the tunnel can
only be approximated. Grout take records indicate that in
those reaches with little or no water inflow, grout takes in
the crown were around a quarter of a sack of cement per
foot. In those reaches of concrete lining through signifi-
cant water inflows total contact grouting quantities includ-
ing grouting water drains in the invert ranged from a half
of sack to locally as much as two sacks of cement per foot.
Approximately 8,200 bags of cement were used in contact
grouting of the tunnel concrete lining (14,500 ft). Contact
grouting was not a separate pay quantity; the Contractor had
to include these cost in the concrete lining.
4 -28
4.5.5 High Pressure Compaction Grouting
Recognized in the design was the need for compaction
grouting of a reach of tunnel upstream from the end (Sta
31+55) of the steel liner to minimize the possibility of
excessive water loss during tunnel operation. For a reach
of tunnel at the downstream end with low rock cover opera-
tional water head is considerably greater than static ground
water head outside the tunnel. Without treatment of a rock
zone (20 ft + or -) around the tunnel to seal open water
passages and to improve the rock mass modulus, excessive
water losses from the tunnel through opened shrinkage cracks
and construction joints in the lining may occur. The need
and extent of rock treatment was observed in more detail
after excavation of the tunnel was completed. Geologic
mapping of rock fractures and joints and monitoring of water
inflow in this low cover reach of tunnel provided data for
developing a treatment plan.
After studying and field checking the geologic data SWEC
developed a comprehensive plan for high pressure compaction
grouting to seal and consolidate the open fractures and
joints in the near field around the tunnel. Their grouting
program was expanded and modified beyond that indicated in
the contract documents. The SWEC grouting plan called for
compaction grouting coverage to continue upstream to tunnel
Sta 65+00 from the required high pressure ring grouting near
the end of the steel liner, Sta 31+55. The SWEC work plan
(Option C) for compaction grouting which included hole
locations and all the details of sequencing and carrying out
the work under the direction of the Construction Manager is
included as an attachment to the SWEC compaction grouting
report. A copy of Option C work plan is also retained in
the project files.
4 -29
For this final geology report the basic scope of the
grouting plan is briefly described below and a summary of
compaction grouting results is given in Table 1. Details on
the results of compaction grouting between Sta 31+45 and Sta
65+00 can be read in the SWEC final compaction grouting
report retained in the project files. The report is cited
in the referenced list of this report.
Compaction grouting was done utilizing four
arrangements at different tunnel intervals.
pattern data is summarized below:
grout hole
The grouting
1) From Sta 31+47 to 31+78 six-ring grout curtain of eight
holes each, 60-ft long, grouted in three stages at 165,
500, and 500 psi.
2) From Sta 31+90 to Sta 34+60, eight holes 30-ft deep,
spaced 15-ft apart, grouted in two stages at a maximum
pressure of 165 psi.
3) From Sta 34+60 to Sta 65+00 both a six and two-hole
pattern of holes 20-ft deep, grouted at 240 psi in one
stage. Two-hole pattern on 40-ft centers and six-hole
pattern on 20-ft centers.
Stationing of the six-hole and two-hole pattern grouting is
given below:
Six-hole Two-hole
35+60 to 38+60 39+00 to 43+60
43+60 to 45+20 45+60 to 46+00
4 -30
46+00 to 47+86 48+20 to 50+50
50+50 to 52+30 52+70 to 61+20
61+20 to 62+20 62+60 to 65+00
After starting a hole with 2:1 water-cement mix initially
most of the grouting was then completed using 1:1 mix.
Total grout injection was limited to a maximum of 120 sacks
of cement per hole. The primary intent of the compaction
grouting was to fill the open joints and fractures for one
tunnel diameter around the tunnel opening thus sealing off
the water and improving the rock mass modulus. A higher
rock mass modulus is expected to result in a relative
reduction in water loss through the concrete lining due to
minimizing crack opening in the concrete lining during
tunnel operation.
Drilling of the compaction grout holes, using a single boom
jumbo with rotary percussion drill was started October 31,
1990 at Sta 65+00, drilling progressed downstream. High
pressure grouting started soon afterward on November 7,
1990. Typically 450 feet of drilling was completed each day
(3 shifts). Grouting progress depended on grout takes of
individual holes. Many of the holes made water and these
holes usually took significantly more cement than low inflow
or dry holes. Table 6-1 taken from the SWEC compaction
grouting report summarizes grout hole connection, drilling
and grout (sacks of cement) quantities placed within station
intervals. SWEC's data was obtained from daily drilling and
grouting records kept by full time Bechtel shift grouting
inspectors. Compaction grouting under Option C work plan,
4 -31
was completed February 1, 1991. The Contractor's grouting
operation was performed seven days a week, three shifts/day
with a 30-day shutdown during the month of December, 1990.
4 -32
TABLE 1
Summary of High Pressure Compaction Grouting
Lower Power Tunnel
Drilling Redrilling No. of No. of Grout Sand
Station Interval Description (LF} (LF l Holes Connections (Sacks) (Sacks)
31+47.5 to 31+77.5 60 ft, 3 stage 2,880 1,920 47 141 0
31+90 to 34+60 30 ft, 2 stage 4,740 2,280 158 310 1,595 120
34+60 to 35+60 2-hole ring 80 0 4 4 191 0
35+60 to 38+60 6-hole ring 1,920 0 96 96 594 0
38+60 to 43+60 2-hole 480 0 24 24 69 0
43+60 to 45+20 6-hole 1,140 0 57 57 158 0
45+20 to 45+60 2-hole 40 0 2 2 0 0
45+80 to 47+80 6-hole 1,280 0 60 64 1,170 80
48+20 to 50+20 2-hole 240 0 12 12 94 0
50+ 50 to 52+50 6-hole 1,520 220 78 92 2,106 0
52+60 to 60+70 2-hole 1,020 100 49 56 1,480 0
61+20 to 62+20 6-hole 760 0 38 39 422 0
62+60 to 65+00 2-hole 280 0 14 _J& __JA _Q
TOTALS 16,380 4,520 639 911 200
5.0 UPPER POWER TUNNEL AND GATE SHAFT, INTAKE PORTAL,
AND POWER TUNNEL SHAFT
5.1 General Description
The power tunnel is segmented into two sectors largely by
elevation. The upper power tunnel at El 1029 is separated
from the lower power tunnel (El 301) by an interconnecting
720 foot vertical shaft. The length of the upper power
tunnel from the inlet portal to the power tunnel vertical
shaft is 738 feet. The portal of the upper power tunnel is
at the downstream end of a deep approach channel which opens
into Bradley Lake. Approximately 520 feet downstream from
the portal the upper power tunnel is intersected by a gate
shaft. A profile of the upper power tunnel, gate shaft and
the power tunnel shaft is included in Appendix E, Figure
5-1.
As was done at the main dam, construction activities at the
upper power tunnel work were suspended during the colder
winter months of 1988-89. During the winter months of
1989-90 limited construction activity, mainly concrete
placement, was continued. Completion of the lower power
tunnel excavation in September, 1989 allowed access to start
the raise boring of the vertical shaft. The raise bore
drilling was completed the first week of November, 1989.
Concrete lining of the power tunnel vertical shaft exclusive
of the upper and lower elbows, did not start until early
January, 1990; slip form lining took less than six weeks to
complete.
5 -1
5.2 Upper Power Tunnel and Gate Shaft
5.2.1 Excavation and Support
Drill and blast method of excavation was utilized for both
the upper power tunnel and power tunnel gate shaft. Exca-
vated diameter of the gate shaft is 29 feet. Elevation at
the top and bottom of the shaft is 1194 and 1029, respec-
tively. Initially 4-foot and later 6-foot drill rounds were
utilized to sink the 150 deep foot gate shaft. A large
crane was used for lowering and hoisting workers, materials,
and equipment; it was also used to hoist shot rock. In
place rock suitable for setting the shaft collar was encoun-
tered 16 feet below the ground surface El 1194. Owing to
weak materials around the top of the shaft, excavation for
the shaft collar foundation had to be laid back at 1:1
Appendix E, Figure 5-2. After the shaft collar and lining
was completed back to ground surface compacted backfill was
placed around the shaft lining. Below the shaft collar (El
1178) excavation of 6-foot rounds progressed to invert of
the upper power tunnel without incident. Installation of 6
x 6 foot pattern rock bolts around the wall perimeter
closely followed shaft excavation Appendix E, Plate 5-1. A
full excavation cycle drilling, shooting, mucking, and
installing rock bolts usually took more than two shifts.
Concrete lining in 10-foot lengths was placed from the
ground surface downward keeping 20 to 30 feet above the
blasted surface. Light {est. total 2 gpm) water inflows oc-
curred from several point sources on the shaft walls. After
concrete lining of the shaft inflow discharged from horizon-
tal joints resulting in persistent, heavy dripping at the
base of the shaft concrete lining. Grouting behind the
lining was done subsequently to stop the dripping.
5 - 2
After the gate shaft (which provided personnel and equipment
access to the upper power tunnel) was completed to tunnel
level, excavation of the 13-foot excavated diameter upper
power tunnel was initiated. The downstream heading toward
the power tunnel vertical shaft was completed first followed
by excavation of the upstream heading toward the entrance
portal. A temporary rock plug was left in the upstream
heading over the 1989-90 winter to keep water in the intake
channel from entering the tunnel. Rock stability in the
upper power tunnel was very good; only widely scattered
"spot" rock bolts were installed. The specified smooth wall
blasting method was utilized with moderate results, it was
not always well executed. Water inflow into the upper power
tunnel was negligible. Initial flows of 1000 gpm and
sustained inflows of 500 gpm were predicted in the GIR.
5.2.2 Geology
Similar to the rock types exposed in the dam foundation
nearby, graywacke, argillite and intermixed graywacke and
argillite also occur in the upper power tunnel and gate
shaft. Geologic mapping of the gate shaft revealed primari-
ly graywacke for the full depth. Graywacke also occurred in
the upper tunnel downstream of the gate shaft. Upstream
from the gate shaft all three rock types were present.
Approaching the tunnel intake argillite predominated.
Geologic tunnel logs of the upper power tunnel are given in
Appendix E, Figures 5-3 and 5-4.
Excellent geologic conditions for maintaining a stable
opening with minimal rock reinforcement prevailed throughout
the upper power tunnel. Similarly good geologic conditions
were encountered in the gate shaft excavation; support
consisted of the specified pattern rock bolts. The
5 - 3
graywacke rock mass quality is sound and exhibits moderately
high hardness. Both the argillite and intermixed graywacke
and argillite was fresh and moderately hard. Strong joint-
ing generally spaced 12-24 inches is prevalent on the walls
of perhaps 50 percent of the upper power tunnel, as well as
the gate shaft. Elsewhere jointing is not well developed.
In the intervals of argillite the rock is more massive in
that the common foliated structure is absent for the most
part. Similar primary joint trends of N 60 to 85 W with 85
N dips were observed in both the gate shaft and upper power
tunnel. Several near vertical, iron stained joints with
minor clay were mapped. Two of the more prominent seams
extended from El 1178 to 1140. Minor thin (less than
1/4-inch} discontinuous inclusions of soft clay were ob-
served along these two shear seams. Below El 1140 in the
gate shaft and most everywhere in the upper power tunnel
joints were unweathered and very tight. Geologic mapping of
the gate shaft walls is presented in Appendix E, Figure 5-5.
The several shear seam encountered in the upper power tunnel
were relatively minor and did not significantly affect the
tunnel stability.
5.2.3 Concrete Lining and Grouting
Reinforced 12-inch thick concrete lining was placed through-
out the upper power tunnel and in the shaft upper power
tunnel gate. The length of concrete placement forms used in
lining the shaft and upper tunnel were 10 and 50 feet,
respectively. In the shaft lining a flexible membrane with
one end embedded in the previous pour and the other against
the wall were used for water stops at each construction
joint. The membrane water stop was not entirely effective.
Leakage occurred through several of the lining construction
joints and low pressure grouting to seal off the leakage was
5 -4
done with good success. Concrete placement in the shaft was
done in 10-foot lifts from the top down staying 20 to 30
feet above the blasting for shaft excavation. Ten feet of
lining was generally completed every other day.
Concrete lining of the 13-ft excavated diameter circular
upper tunnel upstream of the gate shaft chamber was done
between October 21 and December 1, 1989. The portion of
tunnel lining downstream between the gate chamber and the
upper elbow of the vertical power tunnel shaft was done six
months later, from July 1 to July 12, 1990. The upper
tunnel lining was reinforced with rebar and the lining was
completed in two stages, the invert first followed by the
arch.
Contact grouting of the crown of the upper power tunnel was
completed through two lines of staggered holes spaced 10 ft
apart. Contact grouting using 1:1 mix at 30 psi injection
pressure was completed July 23 and 24, 1991. A total of 125
bags of cement was injected above the crown lining in the
tunnel section upstream of the gate shaft. In the tunnel
section downstream of the gate shaft grout take was consid-
erably higher. A total of 282 sacks of cement were used in
contact grouting of the crown.
The crown of the power tunnel intake lining was contact
grouted mid-July 1990. Two rows of grout holes of 16 each
in the crown of the transition structure were grouted using
2:1 mix at 30 psi. The total grout injected was 410 sacks.
These relatively large grout takes would indicate consider-
able void space behind the lining.
Grouting adjacent to the horizontal construction joints in
the gate shaft concrete lining was also done although none
5 - 5
was originally intended. Water leakage occurred around the
water stops along many of the construction joints (i.e.
every 20 feet). Most grout takes of 1:1 water-cement mix
occurred in relatively few holes. Grouting was effective in
stopping most of the leakage.
5.2.4 Seepage Barrier Grout Curtain
To provide a seepage barrier upstream of the upper power
tunnel gate shaft and gate chamber three radial high pres-
sure grout rings were completed. Each ring consisted of 15
to 18 staggered holes located immediately upstream of the
gate chamber. Staggered 2-inch diameter holes in three
rings 30 to 60 ft long, radially 20 degrees apart and were
inclined upstream at 10 degrees. The rings designated as
GSG1, GSG2, and GSG3 shown in Appendix E, Plate 5-2 are
spaced 6 feet apart, at Sta 186+60, 186+66, and 186+72,
respectively.
These three rings, drilled and grouted through the tunnel
concrete lining were completed working 3-shifts per day from
August 27 to September 6, 1990. On days, when downtime was
minimal, typically 100 to 200 feet of drilling per shift was
completed. Drilling was done with drills mounted on a two
boom jumbo.
Ring grouting was done shortly after completing the drilling
of the three ring holes. Grouting started with GSG1 ring
followed by GSG3 and finally the intermediate ring GSG2.
The holes in each ring were grouted single stage at 160 psi
using a water-cement ratio of 2:1.
Grout takes in the three rings were relatively low, most
holes took less than one sack. Total cement injected into
5 - 6
rings GSG1, GSG2, and GSG3 were 21, 18, and 23 sacks,
respectively.
Void skin grouting of the gate chamber steel liner was also
done. A total of 38 holes were drilled and grouted. Grout
take was nil, only 2-1/2 sacks of 2:1 grout in total. Only
14 of the 38 holes took grout.
5.3 Intake Channel
5.3.1 Excavation and Wall Reinforcement
The open channel from Bradley Lake to the intake portal of
the upper power tunnel is approximately 350 ft in length.
Excavation of the intake channel started at El 1230 descend-
ing to El 1090, side slopes cut 35-ft high vertically were
separated by progressively wider benches 20 to 40-ft wide.
The overlying rock quarried from the intake channel was used
for rockfill material in the main dam construction. Start-
ing at a base level of El 1090 (after the overlying rock had
been removed) the intake channel was excavated by drill and
blast method in several lifts down to portal invert eleva-
tion of 1026. A temporary access/haul road to the channel
bottom was constructed along the east wall. To allow
excavation to be done in the dry a temporary rock plug,
which was later removed, was left across the channel near
the shoreline of Bradley Lake. The width of the channel at
the top and bottom is 80 and 60 feet, respectively. The
side walls with a 10-ft wide bench at El 1055 are vertical.
A plan of the power tunnel intake channel excavation is
shown on Appendix E, Plate 5-3. The intake tunnel portal
cut slope is also vertical, without a bench (i.e. 64-ft high
cut slope). Near the entrance to the intake a 10-ft deep
rock trap was excavated in the bottom of the intake channel.
5 -7
Perimeter blasting along the wall lines was done by the
specified cushion blasting method. Overbreak caused by
blasting resulted in irregular bench surfaces particularly
on the outside corners. However execution of cushion
blasting of the vertical walls of the intake channel was
well done. Even wall surfaces without significant overbreak
were achieved. The specified design reinforcement of the
cut slopes/walls was extensive. Pattern A-1 type (l-inch
diameter) tension (5 kips) rock bolts were installed on the
channel wall and on the vertical face above the intake
portal Appendix E, Plate 5-4. Most A-1 rock bolts range in
length from 15 to 35 feet. Around the arch of the intake
portal and in three rows along both intake channel walls A-4
type (1-3/8 inch diameter) tension (165 kips) rock bolts
were installed. Rock bolt length varied, the A-4 type
installed at El 1085, 1065, and 1045 are 35, 22, and 18 feet
long, respectively. A-4 rock bolts above the arch of the
tunnel intake are 30 feet in length. Vertical dowels, A-2
type (l-inch diameter) were installed before excavation
around the perimeter of the intake channel (El 1090) and at
El 1030 before excavation at the downstream edge of the rock
trap excavation. Locations of the rock reinforcement
installed in the intake channel are shown on Appendix E,
Plate 5-5.
Excavation of the cut slopes and benches of the intake
channel excavation and rock quarry above El 1090 were
previously mentioned. One year later the adequacy of the
scaling and cleanup of the slopes and rock benches above the
channel walls came into question. There was concern that
detached rock could be further loosened and dislodged by ice
movement down the slope causing damage to the intake gate
structure. After considerable debate with the Contractor, a
5 - 8
change order was issued to the Contractor for scaling the
slopes and benches above the top of the intake channel (El
1090). The slopes were scaled and the benches cleaned back
3 feet from the outside edge and the accumulated rock debris
was disposed. This work was started in late September and
completed in October, 1990.
5.3.2 Geology
The predominant
El 1090) consist
rock types in
of argillite,
the intake channel (below
intermixed argillite and
graywacke. Graywacke was also exposed in a short section of
wall and the rock plug on the east side of the channel.
Graywacke rock was predicted in the GIR for the tunnel
intake area. In the excavation argillite and intermixed
graywacke and argillite were exposed. Geologic mapping of
the intake channel wall was performed at a scale of 1" =
10'. Geologic map sheets are included in Appendix E, Figure
5-6 (3 sheets).
For the most part cushion blasted rocks forming the vertical
walls of the intake channel are sound, widely jointed, and
massive appearing. Many of the more visible joints were
slightly opened by the blasting effects. Above the intake
portal for approximately 20-ft joint spacing of 2 to 5 feet
is relatively closer than the 5 to 10-ft spacing on the
walls. Primary jointing strikes N 70 -80 W and the joint
dips typically exceed 70 degrees to the north. A secondary
joint set that appears as an undulating, roughly horizontal
trace strike N 40 -50 W on the west wall and N 40 -50 E on
the east wall. Dips for these secondary joints are low,
approximately 10 degrees south.
5 - 9
For the most part joint attitudes did not adversely influ-
ence wall stability. As previously mentioned graywacke is
exposed in the channel excavation upstream in the vicinity
of the rock plug. The graywacke while moderately fresh and
hard is strongly jointed and blocky. Considerable overbreak
along the joints and shear seams occurred on both the
outside corner of the El 1090 bench and on vertical cut
the
more
slopes. The occurrence of shear seams elsewhere on
channel walls were few and mostly minor. One of the
visible shear seam which had N 70 E strike and 75 degree
(west) south dip extended from El 1090 down along the left
side of the intake tunnel portal to El 1026. The shear seam
was 6 to 12 inches wide with minor intercalated discontinu-
ous clay in finely fractured, platy rock. Because of its
location Appendix E, Figure 5-5 (sheets 1 of 3) and attitude
the seam had no adverse effect on the stability of the high
vertical face above the intake portal. Overall, rock
stability of the intake channel cut slopes would have to be
assessed as very good.
5.3.3 Removal of Rock Plug
As previously mentioned in 5.3.1 above, a rock plug approxi-
mately 75 feet wide at the base (El 1055) and 29-feet wise
at the crest (El 1090) was left in place across the entrance
to the intake channel temporarily to allow excavation and
concrete work in the intake area to be done in the dry.
Upon completion of the intake channel excavation,
tion of the intake structure, insertion of the
trash racks and the completion and closure of
construe-
gate and
the upper
power tunnel gate valve the plug was no longer needed it was
removed by the Contractor.
5 -10
Preparation and execution of the rock plug
September and October, 1990 involved a number
removal in
of steps.
Briefly summarized below were the main considerations and
work activities included in the Contractor's approved work
plan
1. A determination of expected over pressure on the intake
structure 240 feet away from the blast zone.
2. A blasting vibration limit of 5 inches per second at
the structure resulting in a maximum explosive charge
per delay of 780 pounds.
3. Pre-grouting of a row of holes across the top of the
rock plug on the upstream (i.e. lake side) side to
consolidate and minimize leakage prior to starting plug
excavation.
4. Removal of a slice of rock by careful drilling and
blasting from both the upstream and downstream side of
the plug to minimize the rock quantity for final
removal.
5. Drilling for final plug removal six rows, 45-feet deep
blast holes inclined upstream and downstream from the
vertical center row.
6. Established a maximum explosive charge per hole 113
pounds with 6 holes per delay for total of 678 pounds
per delay. Overall powder factor up to 1.4 pounds per
cubic yard.
5 -11
7. Flood intake channel to lake level prior to blasting.
8.
During blasting operate a "bubbler" air cushion at the
front of the intake structure.
Utilize the services of an outside blasting
to develop and implement the drilling and
plan.
specialist
blasting
The objectives of the rock plug removal by well planned
blasting were achieved with good success. Breakage of the
rock plug met expectations. Throw rock into the intake
channel was minimal. The heaved rock was leveled off above
water level (El 1073). Excavation shot rock from the intake
channel was done mostly with the Cat 245 backhoe working on
the muck pile and to a lessor extent with the Manitowoc 4000
crane using a 4 cu-yd clam bucket.
5.4 Power Tunnel Vertical Shaft
The 720-ft vertical shaft with 90 degree elbows top and
bottom completes the connection of the upper and lower
reaches of the power tunnel. The upper and lower power
tunnels were previously described in Sections 5.0 and 4.0.
5.4.1 Excavation and Support
Excavation of the power tunnel vertical shaft was done by
raise boring. First, to provide a center guide for the
raise bore head, a 14-inch diameter pilot hole was drilled
from the ground surface (El 1250) to the invert grade of
lower power tunnel (El 301) refer to Appendix E, Figure 5-1.
Drilling of the pilot hole took 15 days (24 hr days), which
was longer than initially scheduled. Soon after TBM excava-
tion of the lower power tunnel had reached the pilot hole,
the raise bore equipment was mobilized and drilling was
5 -12
started October 2, 1989. Similar to the pilot hole drilling
raise bore drilling also was slower than expected. The
raise bore was holed through at the invert of the upper
power tunnel (El 1029) November 4, 1989. Several days of
down time occurred during the drilling period due to a
generator outage. Progress summary sheets of pilot hole and
of the raise drilling is included in Appendix E, Figure 5-l.
After completing the raise bore drilling and shaft outfit-
ting, a preliminary inspection of the rock stability condi-
tions in the shaft was conducted. For the most part rock
conditions in the shaft were good. Several localized
fall-out zones and break-out occurred along a number of
vertical joint-seams that definitely required rock bolt
support and wire mesh. Pattern rock bolt support for the
entire length of shaft was specified on the design drawings
Appendix E, Plate 5-6. However, based on reasonably good
rock conditions, a design change was made not to rock bolt
and mesh the entire shaft. Instead rock bolts and wire mesh
were installed in areas of poor wall rock stability and
elsewhere including the upper and lower elbows to assure
safe working conditions. Approximately 1,300 six-foot long
resin encapsulated rock bolts were installed in the straight
portion (647 ft) of the shaft. Approximately 100 resin rock
bolts were 10-ft rock bolts. Welded wire fabric and/or wire
mesh and limited mine straps were placed over approximately
30 percent of the shaft walls.
Total water inflow into the shaft, most of which enters near
the top of the shaft, appears to be less than 15 gpm.
5.4.2 Geology
The slow penetration rate while drilling both the pilot hole
and the shaft indicate graywacke was the predominant rock
5 -13
type encountered. Logs of a nearby boring drilled during
the pre-construction investigations also indicated a majori-
ty of graywacke rock. After completion of excavation and
installation of rock bolts geologic mapping of the shaft was
done from a steel 3-deck work platform which was lowered and
raised by a large drum hoist located at the top rim of the
shaft, within the upper tunnel.
Geologic mapping of the shaft walls confirmed the occurrence
of graywacke as indicated by the pilot hole drilling. In
the upper 80 ft of the shaft the graywacke is intermixed
with perhaps 30 percent argillite. Slight to locally high
weathering occurs along some shear seams and joints.
Elsewhere the rock is fresh, sound, and moderately jointed.
For the most part, jointing is tight, many of the joints are
re~ealed with calcite. Completed geologic mapping of the
vertical shaft is attached.
There are three main geologic structural conditions which
resulted in localized rock instability in the shaft walls.
1) Steeply included shear seams and joint seams at several
locations both with and without soft clay infill~
oriented roughly normal to the shaft walls.
2) Steeply inclined joints subparallel to the shaft walls;
approximately eight locations.
3) Localized fractured rock areas associated with shear
seams subparallel to the shaft walls; two locations.
Instability at these locations resulted in rock fallout from
the shaft walls which in most cases left potentially
5 -14
unstable blocks of various sizes around the edges of the
fallout areas. Generally, fallout along joints and near
vertical seams is less than 6 inches (i.e. into the shaft
wall) and as much as 12 inches for steeply inclined joints.
The most significant unstable area in the shaft is
on the northeast wall at a depth of 300 ft. This
located
fallout
area is approximately 15 ft wide at the top and narrows to 3
ft in width approximately 50 ft below. Generally from 1 to
2 ft of fallout, locally as much as 3 ft has occurred from
this area. Rock around the irregular edges of the fallout
zone is moderately weathered and closely fractured. The
back face of the fallout area appears to be a joint surface
subparallel to the shaft wall.
The geologic mapping of the power tunnel vertical shaft
including geologic notes are presented in Appendix E, Figure
5-7. Mapping of the circular shaft is represented on
flattened quadrants at a horizontal and vertical scale of 1"
= 20'.
5.4.3 Concrete Lining and Grouting
After completing excavation and rock bolting/wire mesh
support in the power tunnel vertical shaft, the Contractor
prepared the lower elbow of the shaft for concreting. The
concrete lining of the lower elbow started just prior to the
Christmas break and was completed at the end of January,
1990. Upon completion of the lower elbow preparations were
made for concrete lining of the straight portion (647-ft) of
the shaft. Steel forms 20 feet in height were used to
successively concrete line the shaft from the bottom
Concrete was hauled (approximately 8 miles) from the
plant near the powerhouse in transit mix trucks
5 -15
upward
batch
to the
collar of the gate shaft where it was dropped through a
vertical pipe to the based of the gate shaft. From here it
was pumped 200 ft to the vertical shaft where it was dropped
again through a pipe down the shaft into remix pot at the
top of the concrete forms. Concrete shaft lining work
started February 5 and was completed March 13, 1990.
There was no contact grouting behind the concrete lining of
the straight portion of the shaft. However, contact
grouting in the crown of both the upper and lower shaft
elbows was done. A single line of eleven contact grout
holes around the crown of the lower elbow lining were
drilled and grouted at the end of August, 1990. Two holes
took 8 and 11 sacks of 1:1 mix at 30 psi; the remainder of
holes were tight. For the upper elbow, prior to concrete
placement, a looped grout pipe had been preset in the dome
above the upper elbow. The concrete rock contact above the
upper elbow was grouted July 5, 1991; eighteen sacks of
cement were placed.
On this same date the remainder (197') of the vertical shaft
pilot hole (14") was also grouted. A pipe was sealed in the
bottom of the pilot hole (i.e. at the top of the dome above
the upper elbow). Grout (1:1) was pumped in from the bottom
until grout overflowed from the top of the hole. A total of
150 sacks of cement was used to backfill the pilot hole.
5 -16
6.0 DAM, SPILLWAY, AND DIVERSION TUNNEL GATE SHAFT
6.1 General Description
The Bradley Lake Hydroelectric Project facilities are
separated into two areas, the upper area at general eleva-
tion of 1100 to 1200 and the lower area tens of feet above
sea level. Facilities of the two areas are connected by the
3.6 mile lower power tunnel. Road access between the two
areas is 8 miles long. The upper facilities described in
this section include the dam and cofferdam, spillway, and
the diversion tunnel gate shaft. They are all located in
the vicinity of the Bradley Lake outlet. Most of the upper
facilities were under construction during the summer season
of 1989. Excavation of the diversion tunnel not described
here was completed in 1987 under a previous contract. The
main dam including the concrete face was essentially com-
pleted by September, 1989. By this same date spillway
construction was approximately 45 percent complete and was
finished by July, 1990. The cofferdam was constructed the
summer before (1988) as was the initial dam foundation
stripping and quarry development. Owing to snow accumula-
tion and extreme low temperatures at the upper facilities
area, the Contractor elected to shut-down construction
operations on the dam and spillway from mid-November,
to March, 1990.
1989
In March, 1990 the Contractor resumed construction opera-
tions at the upper works. During early spring and summer
the spillway structure was completed, the diversion tunnel
gate shaft excavated and lined, finishing work on the dam
was completed. A temporary fish-water bypass and other
required rock and concrete work was in progress in the
6 - 1
diversion tunnel through the winter of 1990-1991. Permanent
equipment was also installed during this time. By mid-April,
. 1991 the upper works were essentially completed.
6.2 Dam and Cofferdam
The main dam was constructed near the outlet of Bradley Lake
between a massive rock knob forming the right abutment and a
broad rock nose at the end of a sloping ridge. The upstream
toe of the dam is less than 200 feet from shoreline of
Bradley Lake. The cofferdam across Bradley Lake outlet was
constructed within this 200-ft wide zone. Flow into the
Bradley River was diverted through the diversion tunnel.
The crest length and height of the main dam is approximately
600 feet and 140 feet, respectively. The upstream concrete
face and the downstream slope of the rockfill embankment
have similar side slopes of 1.6H:V. The concrete face is
joined to a concrete toe plinth founded on sound bedrock. A
foundation seepage barrier is provided by a single line
grout curtain along the length of the plinth. The power
tunnel inlet portal is slightly upstream and to the west of
the left abutment of the dam. Rock quarried from a rock
mass above the tunnel intake channel was used as rockfill in
the dam. The cofferdam, constructed across the outlet
channel of Bradley Lake was constructed with a central core
founded on bedrock to minimize leakage. Geomembrane sheet-
ing was installed the length of the core from bedrock to the
crest of the cofferdam to further reduce leakage. The crest
length and maximum height of the cofferdam was approximately
100 feet and 25 feet, respectively.
Bedrock exposed in the cofferdam core trench foundation
including abutments consist mainly weathered graywacke rock
overlain primarily with previous, dense, sandy, silty
6 - 2
gravels and boulders. Because of continual leakage from
Bradley Lake through the upstream side of core trench, the
cofferdam foundation was never completely dewatered before
the start of core material placement. Consequently, compac-
tion at the rock contact was less than desired due to
saturation of core materials. Leakage through the cofferdam
foundation near the right abutment continued throughout
foundation excavation and construction of the main dam. It
was necessary for Contractor to operate a dewatering pump
full time to remove water (est. 125 gpm) leaking through the
cofferdam.
6.2.1 Foundation Excavation
The base of the dam embankment is approximately 550 feet
wide upstream to downstream. Its breath across the river
bottom is approximately 280 feet. Foundation excavation in
the river channel consisted of alluvial deposits and local-
ized areas of glacial till. On the lower half of the left
abutment slope excavated materials consisted of colluvium
and talus deposits. Whereas on the right abutment bedrock
was exposed for the most part. A plan and profile of the
rock excavation for the main dam is shown on Plate 6-1,
Appendix F.
The thickness of the excavated alluvial deposits was vari-
able ranging from as much as 15 feet in one deep bedrock
depression near the upstream toe on the west side of the
river channel to more generally one to three feet downstream
of the dam axis. Excavated talus/colluvium deposits which
extended from the river channel approximately 200 feet up
the slope of left abutment were as much as 10 feet thick
locally, but more generally one to three feet thick. The
quantity of talus excavated from the lower right abutment
6 - 3
was relatively minor. Excavation of most of the unsuitable
foundation materials was done by backhoe and dozer. The
stripped bedrock foundation surface in the river channel
area, with the exception of the large depression near the
plinth line was highly irregular, pock marked with large and
small potholes and elongated shallow, narrow erosion chan-
nels. Foundation preparation for the river channel portion
of the embankment foundation consisted machine cleaning of
the bedrock, as allowed in the specifications. Rock excava-
tion and rock surface preparation beneath the embankment
portion of the dam was minimal. On the upper two-thirds of
right abutment slope bedrock was widely exposed as was the
upper third to a lessor extent on left abutment.
6. 2. 2 Plinth Excavation and Foundation Treatment
Requirements for excavation and rock surface preparation of
the plinth foundation were considerably more stringent than
for the embankment foundation. Foundation elevations for
the various plinth segments were specified on the contract
drawings. The depth of acceptable foundation rock varied,
but in general rock excavation ranged from a few feet to as
much as 10 feet below the original rock surface. Foundation
excavation removed all the weathered unsound rock as well as
providing for a more uniform slope transition from one
plinth segment to another. Careful blasting was done to
avoid excessive fracturing of the plinth foundation.
However, over excavation occurred in portions of some plinth
segments. Preparation of the foundation for concrete
included scaling of all rock fragments loosened by blasting
and cleaning-washing with high pressure water jets. The
actual foundation width prepared for detail cleanup was
approximately 15 feet. An a additional 10 to 20 feet of
natural rock surface upstream of the plinth foundation was
6 - 4
also sufficiently cleaned to permit inspection and delinea-
tion of any geologic features which could affect foundation
treatment including curtain grouting. Minor quantities of
lean concrete were placed locally in a number of plinth
blocks to level out sharp irregularities in the plinth
foundation. A dozen or so thin shear seams 6 to 12 inches
wide crossed the plinth foundation. Several of the wider
seams in plinth foundation segments B, C, and A required
minor foundation dental treatment, i.e. excavation to twice
the width and filled with concrete. The locations of these
shear seams and their character are further described in
6.2.3 Foundation Geology.
Owing to weathered condition of the rock above an open joint
seam near the up-slope end of Segment A, a considerable
quantity (approximately 60 cu-yd) of rock was removed and
subsequently replaced with concrete. At final excavation
depth the aperture of this particular joint was 1 to 3
inches. Holes for washing the joint were drilled further up
the slope. Return water from the open joint did not appear
for a couple of hours; the joint seam was only weakly
responsive to air-water jetting. Special attention was
given to treating the open joint during foundation grouting.
Details are given in a separate dam foundation grouting
report listed in the references.
A prominent erosion channel that crossed the plinth founda-
tion at the west end of Segment D at the base of a high rock
face on the right abutment was found to be deeper than
expected. At approximately El 1038 the width of the rock
channel narrowed from 8 to less than 4 feet and was filled
with densely compacted boulders and gravel. Exploratory
rotary percussion holes revealed infill bottomed at El 1034.
6 -5
To facilitate infill removal additional rock excavation by
drilling and blasting was done. After excavation and
cleanup, the shear seam along the bottom (El 1032) of the
erosion channel was found to be tight and less than 6 inches
wide. Concrete backfill was placed in over excavated
erosion channel back to base of the plinth, El 1052.
For the most part the final plinth foundation on the right
abutment was excavated and constructed along a narrow 4 to 5
feet, upward sloping (30°) bench which had been drilled and
blasted from a high precipitous rock face. With one excep-
tion, the condition of the rock both in slope stability and
soundness was very good. Approximately two-thirds up the
right abutment plinth slope additional rock excavation was
required in a 30-foot wide zone. It was necessary to remove
highly weathered, fractured and blocky rock associated with
an open joint seam along one side of a huge, massive rock
wedge. At plinth foundation grade, the open joint seam
pinched-out and was tight. However, outside (upstream) the
joint, 6 to 10 inches wide, was infilled with silty gravelly
sand up to the ground surface. Dental treatment of this
open joint seam consisted of excavation of the infill
material down one to two feet all along it surface exposure
(approximately 50 feet) and backfilling with small aggregate
concrete.
6.2.3 Foundation Geology
Geologic mapping was limited primarily to the plinth founda-
tion where the excavated rock surface had been cleaned and
prepared for concrete placement. Machine cleaning of the
foundation surface beneath the rock fill embankment limited
mappable exposures to gross geologic features. However,
close inspection of the machine cleaned foundation surface
6 - 6
did not reveal any significant geologic features such as
faults or soft materials. Several narrow erosion/scour
channels and a number of large, shallow potholes were
present. The most prominent, undulating scour channel of 1
to 3 feet in depth, is located along the base of the right
abutment rock knob. It extends perhaps 50 feet upstream and
downstream of the dam axis. Some of the scour channels were
filled with dense clay till, others were infilled with
densified sandy gravel. The orientation of the erosion
channels appear to be controlled by north trending joints.
The portion of embankment foundation in the river channel
and on the right abutment is almost entirely graywacke.
Rock quality is good; weathering is minor. Primary joint
spacing where it could be seen appears to be spaced several
feet apart. Although the surface of the massive rock knob
forming the right abutment had been discolored by weather-
ing, the penetration depth
than 12 inches. Above the
jointing on the precipitous
of weathering is generally less
channel on the right abutment
rock wall is widely spaced,
ranging from a few feet to as wide as 10 feet or more.
Foundation preparation for the rock fill embankment against
this right abutment area was minimal. A thin veneer of
colluvium and talus was removed from isolated erosional rock
pockets and from narrow, elongated depressions along some
prominent joints where erosion had occurred in the past.
On the left abutment slope above the river channel graywacke
predominates, but localized areas of argillite within the
graywacke mass occur also. Several small bedrock areas on
the left abutment were overlain by thin deposits of
colluvium. Colluvium was also present in narrow ravines
located between small knobs partly isolated from the main
6 -7
rock mass of the left abutment. Exposed foundation rock
after stripping was moderate to locally highly weathered.
Geologic mapping of the plinth foundation at a scale of 1" =
20' was completed as each segment of the plinth was being
prepared for concrete placement. Geology maps (five sheets)
of the plinth foundation segments including brief geologic
description are given in Appendix F, Figure 6-1, signifi-
cant geologic features occurring in each segment of plinth
foundation have been summarized and are included in Appendix
F, Figure 6-2.
In summary, the dam plinth
weathered graywacke. The
closely jointed, without
is founded on fresh to slightly
foundation rock is sound, but
major geologic defects. Minor
defects were encountered in the form of shear seams 6 to 12
inches in width, most with intercalated 1 to 3 inches wide,
discontinuous clay layers. These thin seams were treated as
provided for in the specification. In plinth Segment A and
the right abutment plinth open joint seams were uncovered.
These too were treated by cleaning and grouting in Segment A
and partially excavating and filling with concrete on the
right abutment.
For further geologic discussion of the plinth foundation the
reader should refer to Appendix F for the "as-built" mapping
and descriptions of the plinth foundation geology.
6.2.4 Rockfill Material
The primary source of embankment rockfill materials come
from the rock ridge from which the power tunnel intake
channel was excavated. Rockfill is composed primarily of
graywacke with perhaps one-third of the quarry rock
6 -8
consisting of argillite and intermixed argillite and
graywacke. The intake channel extends approximately 320
feet from the water's edge of Bradley Lake to the tunnel
portal. Rock cover over the intake channel before excava-
tion was about 200 feet. Rockfill (quarry) development work
starting at the top of the ridge was done during the summer
and fall season of 1988. Large quantities of stripped
materials requiring drilling and blasting had to be wasted
to eliminate unsuitable materials, i.e. soil, vegetation and
excessively weathered rock. In this stripping operation the
ridge top was roughly leveled-off to El 1200. Useable
portion of quarried rock above El 1200 was dozed over the
rock cliff where it was loaded and hauled to the darn founda-
tion for placement and compaction. Late spring of 1989,
after the winter shut down, quarrying operations and darn
embankment construction resumed. There were five construc-
tion material zones in the darn embankment, Appendix F, Plate
6-2. Zones 1 and 5 consisted of upstream and downstream
slope protection materials. Zone 2 consisted of a horizon-
tal layer of rockfill materials placed directly on the
machine cleaned foundation surface. Rockfill in Zone 3
upstream of centerline and Zone 4 downstream formed the bulk
of the embankment. Zones 2, 3, and 4 were obtained directly
from angular shot rock developed in the quarry. Zone 5
riprap was developed by selectively separating oversize shot
rock 36 to 48 inch diameter. Tunnel muck from the TBM
operation at the lower power tunnel was hauled to the darn
for use as Zone 1 materials. After testing for gradation
and permeability, the tunnel muck was found acceptable and
was substituted for the specified Martin River aggregates.
Ready availability of the tunnel muck strongly influenced
the Contractor's request to change material sources. Com-
pacted tunnel muck provided an excellent bedding material
6 - 9
for the concrete facing of the dam. Dam embankment materi-
als were compacted with 6 passes of a smooth drum 10-ton
vibratory roller. Dam embankment construction was completed
by mid-summer and the placement of the concrete face slabs
was completed early September, 1989.
6. 2. 5 Foundation Grouting
A single line grout curtain was completed through the
concrete plinth for its entire length. A change order by
the Engineer extended the left and right abutment grout
curtains 200 and 120 feet, respectively beyond the ends of
the abutments. Grouting of the plinth foundation was
completed the summer of 1989. The grout curtain extensions
were partially done in 1989, but due to low winter tempera-
tures, grouting was temporarily terminated. Grouting was
restarted and completed in the spring of 1990.
Details of the dam foundation grouting are given in a
separate Bechtel report, Main Dam and Spillway Grout Cur-
tain, October, 1989. For this report a generalized a plan
and profile of the main dam drilling and grouting program is
shown in Appendix F, Figure 6-3. Also, included on the
profile and in the notes is a summary of grout takes by
plinth segment. An approximate total of 680 sacks of cement
were injected in the dam plinth foundation. Of this total
approximately 85 percent of the sacks were injected in three
holes. The majority of the grout holes were tight, taking
no grout or a nominal amount. Grout takes in the left and
right abutment extensions were approximately 195 and 21.5
sacks, respectively. One hole on the right abutment exten-
sion took 850 sacks. Details of the abutment extension
grouting is included in above-mentioned separate grouting
report.
6 -10
6.3 Spillway
The concrete spillway structure was constructed across a
narrow rock saddle between the massive rock knob forming the
right dam abutment and the precipitous rock cliff rising
from the base of the mountain mass to the east. Crest
elevation at the abutments of the spillway is El 1090 and
the overflow section is El 1080. The length of the spillway
structure at the crest is about 275 feet. Curtain grouting
was done from the foundation rock surface. Drainage holes
located in a gallery at the base of the spillway structures
were drilled prior to completion of gallery roof construc-
tion. A plan and section of the spillway is shown in
Appendix F, Plate 6-3.
6.3.1 Foundation Excavation and Treatment
Excavation of the spillway foundation lasted over several
months starting in Hay, 1989 and completed August, 1989.
With exception of deep erosional feature at the base of the
right abutment, the remainder of the foundation consisted of
variously weathered graywacke bedrock. The erosional
feature consisted of a deep, enclosed depression 15 to 25-ft
wide incised upstream by a deeper, narrower channel 2 to
5-ft wide. Until excavated the channel had been filled with
dense alluvial deposits. The lowest elevation of the
excavated incised channel at the upstream toe of the
spillway was approximately 35 feet lower than the bedrock
surface at the downstream toe.
West of the erosion channel to the end of the left abutment
the stripped rock surface was highly uneven, in part due to
past glaciation and in part to differential weathering along
open, repetitious jointing. Drilling and blasting was
6 -11
necessary to remove excessively weathered, blocky rock and
to satisfy design foundation elevations. In several local
areas "over" excavation of up to 10 feet was necessary to
reach suitable foundation rock conditions. A high (El 1200)
precipitous rock face existed on the right abutment side of
the spillway. Although the rock was massive and of good
quality the design called for a thin slice of rock to be
removed to develop a bench at the base at El 1145. The rock
slice was removed by pre-split blasting resulting in a
continuous, even surface. Foundation excavation in the
spillway was done by backhoe and dozer. Most of the rock
excavation was pushed or cast upstream as permitted by the
specification. Final foundation preparation included detail
scaling of loose rock materials and cleaning of the founda-
tion surface with pressurized air and water jetting.
Special seam treatment was done in the incised channel and
in several local areas where inverted V-shape, elongated
rock projections occurred in the foundation. Lean concrete
was applied to fill and even out these and other near
surface irregularities.
6.3.2 Geology
Foundation rock in the spillway consist entirely of
graywacke. At foundation grade the rock is generally fresh
to slightly weathered for an 100-ft wide section east of the
right abutment including the deep erosion channel and the
precipitous rock face. Over the next 175 feet of foundation
to the end of the left abutment rock weathering is slight to
moderate. The dominant geologic structure is the strong
repetitious northwest trending joint system. This joint
system prevails across the entire spillway foundation. Over
most of the foundation area this
reasonably tight. Withstanding
persistent jointing was
the generally tight
6 -12
jointing, there were a number of short sections (less than 5
feet) of individual joints that were open 1/2 to 2 inches in
otherwise sound rock. The extent of the openness of these
joints was further demonstrated by the relatively high grout
takes during foundation curtain grouting. A brief summary
of the spillway foundation grouting is given in Section
6.3.3 of this report.
Several shear seams, 3 to 12 inches wide were mapped in the
final spillway foundation. Two of the seams traversed the
full width of the foundation. The seam material consists
mainly of finely fractured rock materials discontinuously
interspersed with minor clay. As part of the foundation
preparation these seams were scaled and washed with air and
water jets to a tight surface. A further description of the
spillway foundation geology is given on three geology map
sheets included in Appendix F, Figure 6-4.
6.3.3 Foundation Grouting
A separate grouting report was prepared for main dam and
spillway. The report, Main Darn and Spillway Grout Curtain
Construction Report, gives a detailed description of curtain
grouting and includes grouting records and drawings. In
this report a location map of grout and drain holes and a
summary of the grout takes including, large individual hole
takes is provided in Appendix F, Figures 6-5 and 6-6. As
indicated on Figure 6-6 most of the grout take was consumed
in seven holes; takes ranged from 93 to 356 sacks. These
large grout takes closely corresponded to locations of open
joints in the spillway foundation which are indicated on the
spillway foundation mapping in appendix F, Figure 6-4. In
other sections of the spillway foundation contact grouting
was also done along the grout curtain line after one or more
6 -13
lifts were placedi grout takes were low. Drain holes
located in the spillway gallery (indicated on Appendix F,
Figure 6-5) were drilled by rotary percussion methods.
Drilling was done through preset nipples after completion of
curtain grouting, but prior to construction of the gallery
roof.
Previously mentioned in 6.2.5 was the curtain grout line
extensions of 160 and 120 feet on the left and right abut-
ment of the main dam. The right abutment extension ties
into the left abutment spillway curtain grout line. A
description of the extension grout curtain results is
included in the above cited grouting report.
6.4 Diversion Tunnel Gate Shaft
The short (400 feet) diversion tunnel, excavated during a
previous contract, passes through the rock knob forming the
main dam right abutment. Approximately 110 feet upstream of
the diversion tunnel outlet portal the 20-foot diameter 110
feet deep vertical gate shaft was excavated and lined with
concrete to the crown of the diversion tunnel below.
6.4.1 Excavation and Support
The shaft collar preparation was initiated March 14, 1990.
Sinking by the drill and blast method was started March 22
and was excavated through the crown of the diversion tunnel
a little more than a month later, April 27, 1990. Similar
to the power tunnel gate shaft, sinking blast rounds were 5
feet deep. One-half of the shaft bottom was drilled, shot,
and mucked at a time (i.e. alternating benches). Except
near the bottom where minor water inflow was encountered the
shaft wall excavation was dry. Wire mesh and rock bolts
6 -14
were installed, over the entire shaft surface. A ring of
fifteen 10-ft long rock bolts were spaced at an average of
6-ft apart every 5 ft in elevation. Three-foot long rock
bolts were installed between the rings as well as between
bolts in the rings where it was required to pin the wire
mesh to the rock surface. Shaft excavation outlines and
design support is shown in Appendix F, Plate 6-4.
6.4.2 Geology
The rock in the shaft walls consists primarily of graywacke
with occasional lenses of argillite. The rock is generally
fresh, hard, and massive. Slight to moderate weathering and
staining occurs locally on some joint and shear planes.
This weathering is more prevalent near the surface and
become negligible with depth. Jointing is generally tight.
Many joints are filled with calcite and calcite veinlets
occur frequently on the walls of the shaft.
Only one significant geologic condition was noted in the
shaft, which resulted in an overbreak fallout of 6 ft in the
southeast wall at a depth of 86 ft (El 1110}. The fallout
occurred along the footwall of a 6-in thick clay/silt shear
plane. This plane may be connected to a similar shear noted
in the mapping of the diversion tunnel. The shaft was
generally dry except for minor seepage of surface water in
the upper part of the shaft. However, it was noted that
water drained out of the shaft when it was in contact with
the shear seam mentioned above. After the excavation
crossed the shear plane the seepage no longer occurred. It
appears that the shear seam is connected to either the
diversion tunnel or the surface excavation of the right
abutment of the dam or possibly the left abutment wall of
the spillway.
6 -15
The mapping was done from either
platform suspended into the shaft
a steel basket or a
by a crane. Rock
steel
debris
had collected behind the wire mesh in some areas. Prior to
mapping, the walls were washed with a high pressure hose to
remove the powder cover from the blasting operation. The
walls were moderately clean with only limited areas where
the dust cover was still present.
As a result of the conditions in the shaft the strike and
dip of joints and shear sets were not measured directly,
except at the bottom of the shaft. The strike was approxi-
mated by extrapolating from known north, east, south, and
west plumb lines dropped from reference points on the
surface at the top of the shaft. Dips were approximated by
measurements perpendicular to the estimated strike direc-
tion. Geology mapping of the diversion tunnel gate shaft is
shown in Appendix F, Figure 6-7.
6.4.3 Concrete Lining and Grouting
Different from the power tunnel gate shaft where concrete
lining was placed from top to bottom as excavation pro-
gressed, the lining in the diversion tunnel gate shaft was
placed upward from the bottom of the shaft after excavation.
Lining was done using 10-ft high forms. Water stops were
included at each construction joint. Concrete lining
commenced May 4, 1990 and finished two weeks later on
May 17, 1990. No contact grouting was done behind the
concrete lining in the diversion tunnel gate shaft.
6.4.4. Seepage Barrier Grout Curtain
Upstream of the main dam diversion tunnel gate chamber, two
sets of triple rows of high pressure grout rings, similar to
those upstream of the power tunnel gate shaft, were
6 -16
completed. One triple ring of holes was located approxi-
mately 20-ft upstream of the gate shaft centerline. The
second triple ring was approximately 115-ft upstream of the
shaft centerline. Hole depths were 40 feet in the upstream
ring set and 30 feet in the rings closest to the shaft;
spacing between individual rings was 5 feet. In the up-
stream ring set grout holes above the tunnel springline were
inclined downstream somewhat so as to overlap the lower ends
of the spillway curtain grout holes which terminated about
15-ft above the diversion tunnel.
Pressure grouting (110 psi) of the triple ring of holes
nearest the diversion tunnel gate shaft was done mid-April,
1991. The 30-ft long holes, 15 to 17 holes in each ring,
were grouted in one stage with a 2:1 water-cement grout mix.
There was no grout take, other than a nominal amount used to
backfill the hole, in any one of the holes in the three
rings. Apparently all the open rock fractures/joints had
been sealed by the previously completed spillway curtain
grouting done from the surface.
The drilling and grouting of the three rings of holes
upstream of the gate shaft chamber, Sta 2+35, Sta 2+40, and
Sta 2+45 was done during early March, 1991. Eight of the
radially-oriented holes (those above springline) were
extended in depth from 40 to 60 feet. Holes were inclined
15 degrees downstream to meet the curtain grout holes
drilled from the surface. The upstream ring was grouted
first, the downstream ring second, and the middle ring last.
Holes were grouted with 2:1 water-cement mix in one stage at
100 psi. Grout takes in the 15 hole ring patterns at Sta
2+35, Sta 2+40, and Sta 2+45 were 22, 1, and 41 sacks,
respectively. Locations and hole orientation of the grout
6 -17
rings in the main darn diversion tunnel are shown in Appendix
F, Plate 6-4.
By way of Change Order No. 127, twelve 60-ft deep, radially
oriented 2-inch diameter drain holes inclined 15 degrees
upstream were drilled at Sta 0+82 in the diversion tunnel.
The holes started 15 degrees below springline and were
spaced 20 degrees apart. These drain holes were installed
to intercept any leakage by passing the spillway grout
curtain in the vicinity of the gate shaft chamber.
6 -18
7.0 MIDDLE FORK AND NUKA DIVERSION
7.1 General Description
The Bradley Lake Hydroelectric Project also includes the
supplemental construction of two facilities for diverting
offstream water into the Bradley lake watershed. One of the
works consists of two small, low earthen dikes and a diver-
sion channel with a concrete outlet weir. The other facili-
ty diverts the Middle Fork Bradley River on north side into
an existing drainage leading to Bradley Lake.
Construction work for the Middle Fork and Nuka diversion was
done under a separate contract to Alaska Energy Authority.
The facilities were designed by SWEC and the construction
managed by Bechtel. The work was contracted to low bidder,
Wilder Construction Co., Inc., Anchorage, Alaska. Total
contract cost was $1,729,095. The construction started
June 10, 1990 and was completed August 7, 1991. Due to
remoteness of the construction sites and the stringent ac-
cess/environmental restrictions all construction personnel,
materials, and equipment were airlifted by helicopter to and
from the jobsite.
7.2 Excavation
Construction involved excavation of soil and rock mainly for
diversion channels and fill placement for low earthen dikes.
The channel for the Middle Fork diversion was 900 feet long.
Although the metagraywacke and argillite rock was fractured
and weathered the deeper portions of channel excavation at
both sites required drilling and blasting. A total of
43,718 cu-yd of undifferentiated soil and rock excavation
was done.
7 - 1
Location and cross sections of the Middle Fork and Nuka
diversions are shown in Appendix F, Plates 7-1 and 7-2.
7 - 2
/ ~~ ;7/'C:"}Pk
Vp?k~e /
A
h//c:'~e-e;5 B __ C: P, E t F
c2re /)1 0 h/7/e z-
+
\
Photo Selection
Final Geology Report
PoHerhouse & Penst.ocks & l!anifold Tunnels
Book No.
(Jile,9=n4'Ve rl~!s.
Page,No. ~Photo Nc. Date
~34/ d,_/1 4 9/08/88 1
2 ~·/!If 1 9/24/88
2 v{621 g'A 9 10/13/88
5 /682/ 5 6 2/12/89
4 ~97 / 4 5 1/06/89
6 /817/5 _§. 3/18/89
6 photos
Additional Photos for Cover
Book No. Page No. Photo No. Date
3 /~84 / <P 8 11/20/88
9 vi2ss/ ;e>li : 5/16/89
9 /1:;94 / t2A-12 6/14/89
2 V~85'/ VI _] 10/20/88
* 4 photos
Photos to be printed 3" x 3" size.
Book No. Page No. Photo No. Date
11 /1593/ r 8 7/27/89
~
Portion of East wall of
Pm·1erhouse cushion blasting
El 4C -18
South wall of Powerhouse
El 40 -El 18
Dark color -shear seam
along the East wall of P.H.
Pmverhouse Unit, proof load-
ing 1-3/8 in corrosion
resistant anchors at
spherical valve foundation
Graywacke rock jointing
Penstock 1 tunnel wye
Pmverhouse Units 1 & 2
excavation El 2 winter
conditions
Subject
Winter scene drilling
toe plinth -main dam
at
TBH in tunnel large water
inflm1
Drilling in upper power
tunnel gate shaft
East \vall of Pm1erhouse
excavation, Penstocks 1 & 2
Subject
Dam site area, dam nearly
topped out, shaft collar &
quarry intake -taken from
shaft pilot hole pad
Photo Selection
Final Geology Report
Dam & Spillway, Quarry
Book No. Page No. Photo No. Date
1 ~24 / (,,+ 6 / 9/02/88
7 ~023 / 7A 9 4/10/89
8 vl106 ,/'' ;2-13 4/25/89
8 vC121 2-2 4129/89
8 v1~94 / 7 8 5/06/89
/
9 ;;_354 / 2A 4 6/01/89
9 /1384/ 4 4 6/05/89
11 7/27/89
8 photos
Subject
Main Dam Foundation
Excavation Lcoking
at Right Abutment
Main Dam -Toe Plinth
excavation, lmv-er left
abutment
Main Dam -Toe Plinth
foundation Block C-3,
Note thin clay seam
Main Dam -Toe Plinth
foundation Block B2
Main Dam excavation of
infilled erosion channel
noted shear seam base of
vertical rock wall
Main Dam Right Abutment
plinth -note rock cut slope
Main Dam -Rock Wedge along
Plinth Foundation -Note
infilled joint
Main Dam -Quarry and PoHer
Tunnel intake excavation
Photo Selection
Final Geology Report
Lower Po\-rer Tunnel and Manifold
Book No.
3
3
3
5
5
6
6
6
7
Page No. Photo No.
~76'/)
~4./3A /s;
/?23
916/ q
1
4
1
5
1
5
8
9 photos
Date
11/18/88
11/14/88
11/17/88
2115/89
2/25/89
3/20/89
10/29/89
3/30/89
4/30/89
Subject
Lower Power Tunnel,
shotcreting at wye
intersection
Lower Tunnel Drill & Blast,
loading holes
Rock bolting shear seam
Sta 5+60 LoHer Power Tunnel
TBM arrival at tunnel yard
TBM Cutting Head in Lower
Tunnel Starting Chamber
Typical rock bolts & straps
power tunnel
Water inflow from tunnel
face and accumulation of TBM
rock cuttings in invert
Typical formed steel channel
support in arch rock bolted
Bad ground Sta 44+60 >·lOod
blocking in shear seam
behind gripper pad
doe5 /Jt:JI-yo /~~
/,1/::;>pe~Ce_s . . -.L-r!
:Jce3 ah L-~~ e~c/
oP ~c /u &' --P-
7-//e re?dr~ ;i;-if
Portion of East wall ot
Powerhouse cushion blasting
El 40 -18
South t'lall of Powerhouse
El 40 -El 18
Dark color • shear seam tr along the East wall of P.H.
Powerhou~e Unit, proof load-
ing 1 ~3/8 ~ corrosion
resistant anchors at
spherical valvf foundation
East wal.l. of Pouernouse
excavation, Penstocks 1 & 2
Pouerhouse Units 1 & 2
excavation tl 2 winter
conditions
Lower Tunnel Drill & Bl ast,
loading holes
lbck bolting shear Se am
Sta 5+60 Lower Powe t Tunne l
TBH arrival at tunnel yard
TBM Cutting Head in Lower
Tunnel Starting Chamset
tad ground Sta 44+60 wood
blook1n<J in ·shear seam
behind gripper pAd
Typical rock bolts & straps
power tunnel
Water inflow from tunnel
face and accumulation of ~M
~ock cuttings in invert
'i'BH :Ln tunne 1 la
i nflo\-1
Typical formed st~ channel
5uppdft in arch rock bolted
Drilling in upper pouer
tunnel gate shaft
Main Dam Foundation
IXeavation Looking
a~ Right Abutment
Winter scene drilling
toe pl inth -main dam
Main Dam -Toe Pl inth
excavation, lower left
abutment
Main Daa -Right Abutaent
Rock wedge adjacent to
Plinth foundation
Main Dam -Toe Plinth
foundation Block C-3,
Note thin clay seam
Main Dam -Toe Plinth
f~dation Block 82
Main Dam excavation of
infilled erosion channel
noted shear seam base of
vertical rock wall
Hain Dam Right Abutment
plinth -note rock cut slope
Dam site area, dam ne~rly
~opped out, shaf t collar &
quarry intake -taken from
shaft ~i10 ~ hole pad
Main Dam -Quarry and Po\·ter
Tunnel intake excava~ion