HomeMy WebLinkAboutUnalakleet Wind Project Geotechnical Report - May 2009 - REF Grant 2195401Duane Miller Associates LLC Arctic & Geotechnical Engineering
5821 Arctic Bouievard, Suite www.alaskagoo.com
Anchorage, AK 99518-1654
(907) 644-3200 Fax 644-0507
May 26, 2009
STG, Inc.
11820 South Garnbell St.
Anchorage, AK 99515
Attention: Clinton White
Subject: Geotechnical Exploration and Recommendations
Unalakleet Wind Towers
Unalakleet, Alaska
DMA Job No. 4179.010
The letter summarizes our geotechnical field exploration and foundation
recommendations for the proposed wind towers in Unalakleet, Alaska. Based on
discussions with STG and BBFM, Inc., the project structural engineer, we
understand six new Northwind 100 wind turbines will be installed in Unalakleet.
Each tower will be approximately 120 feet tall with the following preliminary
foundation design loads:
Axial download: 44-kips
Base shear: 28-kips
Maximum moment: 2,471-kips-ft
Torsional Moment: 23-kips-ft
The wind turbine power generation units will be seated on steel monotube
towers bolted to a reinforced concrete foundation. It is desired to use a gravity
concrete foundation system without rock or soil anchors. Preliminary tower
foundation dimensions are a nominal 21.5-ft by 21.5-ft by 3-ft thick primary
subbase with an integral 8-ft diameter by 4-ft thick concrete tower pier. The
concrete foundation base will be approximately 7 feet below grade and compact
granular fill will be required above the primary concrete base to final grade.
A field exploration program was developed to estimate the depth of soil
overburden, weathered rock thickness, and the inferred depth to competent rock
at the tower locations. A local air track drill rig was used to advance the test
holes with geologic, thermal, and groundwater conditions inferred from the
STG Unalakleet Wind Tower Project
May 26, 2009
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Duane Miller Associates LLC
drilling action and returned chip samples. Subsurface conditions would require
verification by split barrel soil sampling or rock coring, but these exploration
methods were not conducted under this scope of services.
Geotechnical Exploration
Duane Miller Associates (DMA) conducted a geotechnical exploration near
the proposed new wind turbine locations. Joseph Marshall of DMA traveled to
Unalakleet April 29 through May 1, 2009 to explore the subsurface conditions at
the proposed tower sites. In the field DMA was assisted by Brennan Walsh of
STG. The proposed tower site is on the east side of the Landfill Road near
Borrow Pit 2. Prior to mobilization for the geotechnical exploration effort, two
proposed turbine foundation orientations were developed for this area by STG.
Paul Farmer, PLS of STG surveyed and marked the proposed tower lease lot,
adjacent allotment parcel, and the proposed tower foundation centers for the
original two alignments then left Unalakleet. After the surveyor left the area, a
third tower alignment was provided to the field team. Project maps showing the
proposed third tower alignment provided by STG were used for the geotechnical
field exploration. The third tower foundation orientation and the test hole
locations advanced for this exploration are summarized on the attached Site and
Test Hole Location Plan, Plate 1.
A total of 12 test borings were advanced in the project area. On April 29 two
test holes were drilled along portions of the original tower alignments, DMA test
holes Unk-01 and Unk-02. Upon receiving the revised tower alignment plan on
April 30, the six revised tower locations were field located by the field team.
Since the STG surveyor had departed Unalakleet, the revised tower foundations
were located with a hand-held Garmin 60Csx GPS instrument based on the
revised tower alignment latitude / longitude coordinates provided to the field
team. The revised foundation coordinates were provided in text format with
truncated latitude / longitude precision relative to the original tower location
coordinates. Some field location error was incurred due to the truncated
coordinates supplied for the revised tower locations. In addition, nominal 4 to 8
foot high alder brush and snow drifts near the proposed tower locations
restricted drilling equipment access.
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May 26, 2009
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Six test holes were advanced near the revised tower foundations, labelled
DMA test holes Unk-03 through Unk-08. Four additional test holes were
advanced along portions of the original tower alignments, labelled DMA test
holes Unk-09 through Unk-12. These additional test holes were advanced to
define the general subsurface conditions along the ridge crest (Unk-09 and
Unk-10). DMA test holes Unk-11 and Unk-12 were advanced along the Landfill
Road right of way and the edge of the alder and snow drift area to compliment
the revised tower alignment field data.
The test holes were advanced with an air track drill rig was equipped with a
3-inch diameter bit. The drill is owned by Quality Asphalt and Paving, Inc. but
was operated by James Saunders of STG. Subsurface conditions were inferred by
drilling action, returned cuttings, and the field geologist's interpretation of
subsurface conditions. Highly disturbed soil and rock samples were field logged
by our geologist during the exploration by collecting returned cuttings at the top
of the test hole. The samples were visually field classified with portions of the
recovered samples sealed in polyethylene bags. Temperature monitoring pipes,
closed end, solid 1-inch PVC, were installed in test holes Unk-03 and Unk-05.
Slotted 1-inch PVC for groundwater measurement was installed in Unk-08. All
test borings were backfilled with cuttings prior to departing the site. GPS
coordinates of the test boring locations were recorded with the hand-held
instrument.
Inferred Subsurface Conditions
The inferred subsurface conditions encountered near the revised alignment
consisted of a surficial brown sandy silt with four test holes encountering 1 to 2
feet of peat over the silt layer. The silt was underlain by a reddish -brown
weathered limestone rock with silt interbeds. Beneath the silts and weathered
limestone a hard gray siltstone was consistently encountered to the test hole
termination depths. Test hole depths varied depending on the inferred hard
siltstone bedrock contact depth with most holes advanced 13 to 16 feet below
grade. Test holes were advanced a minimum 5 feet into hard rock.
Test holes Unk-01, Unk-03 and Unk-12, encountered approximately 8 feet of
unconsolidated sediment over 7 feet of weathered and fractured rock. In these
three test holes changes in drilling action and cuttings indicated hard rock at
STG Unalakleet Wind Tower Project Duane Miller Associates LLC
May 26, 2009
Page 4
approximately 15 to 17 feet below existing grade. Test holes Unk-02, Unk-04,
Unk-06, and Unk-07 encountered 2 to 3 feet of unconsolidated sediment
overlying 3 to 4 feet of weathered and oxidized rock. With the exception of
Unk-02 where hard bedrock was observed at 4 feet below grade, hard rock was
generally encountered 5 to 7 feet below existing grade. Test holes Unk-05 and
Unk-08 through Unk-11, inclusive, encountered 5 to 6 feet of unconsolidated
sediment over 2 to 3 feet of weathered rock. A hard, gray rock was encountered
below the weathered rock in each of these test holes.
Frozen soils were observed in test holes Unk-03, Unk-05, and Unk-08 to less
than 10 feet below grade. The frozen soil is inferred to be seasonal frost. The
remaining holes did not exhibit frozen ground conditions. Groundwater of any
significance was not encountered in any of the test holes advanced at this site.
However, the field exploration program was conducted prior to spring thaw but
seepage or groundwater may be present after thaw. Soil, rock, and frost
conditions observed in the test holes advanced for this project are summarized
below.
Test
Hole
Total
Depth
Soil
Horizon
Weathered Rock
Horizon
Depth to
Harder Rock
Contact
Frozen
Horizon
Unk-01
22.0-ft
0.0-ft
to
6.0-ft
6.0-ft
to
15.0-ft
15.0-ft
------________
None Encountered
------_
Unk-02
-------
13.5-ft
--------
0.0-ft
------------
to
2.0-ft
2.0-ft
-------------
to
4.0-ft
4.0-ft
--------
None Encountered
-------------
Unk-03
22.5-ft
0.0-ft
to
8.0-ft
8.0-ft
to
16.0-ft
16.0-ft
0.0-€t to 10.5-ft
Unk-04
11.5-ft
0.0-ft
to
2.0-ft
2.0-ft
to
5.0-ft
5.0--ft
None Encountered
_ _U_nk_-_05_
-
- 1_6.3_-ft
-��14.3-ft
0.0_-€t_
-0.0-ft
to _
_8.0-ft
-None Encountered
!_6.0-ft
----- _8._0-ft
_0.0-ft _ta_ 8_.0-_ft
Unk-06
_
to
-
2.5_-ft
_2_.5_-£t_-to_
_
6.0-ft
None Encountered
Unk-07
I3.6-ft
0.0-ft
to
3.3-ft
3.3-£t
to
6.0-ft
6.0-ft
None Encountered
Unk-08 r
16.3-f_t
---r16.0-ft
_0.0_-ft
to
-4.0--ft
_5._7-ft
_5.7-f t
_t_o
7.0-ft
r _ _ _ -7_._0-_ft
2._0-_ft_ _to_ _ _5.5_-f t
Unk-09
0.0-ft-to
-0.04t
4.0_-ft
to_y�9.0-ft
9.0-ft
None Encountered
Unk-10
16.5-ft
to
5.0-ft
5.0-ft
to
7.04t
^ ^ 7.0-ft
-None _Enc_o_unt_er_ed
_
Unk-I1
--- I_
18.3-ft
. -------------------
0.0-ft
to
7.0-ft
7.0-ft
------------
to
9.0-ft
9.0-ft
--------
1.5-ft-to 7.5-ft
------------
Unk-12
22.0-ft
0.0-ft
to
8.0-ft
8.0-ft
to
14.5-ft
14.5�-ft
1.0-ft to 6.5-ft
Inferred geologic cross sections based on the field findings are summarized
on Plates 2 (section A -A' roughly along the revised tower alignment) and Plate 3
(section B-B' roughly parallel to the Landfill Road right-of-way). Test hole
elevations noted on the cross sections are inferred from the US Geological Survey
topographic map based on a 50 foot contour interval and STG surveyed
sTG Unalakleet Wind Tower Project
May 26, 2009
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Duane Miller Associates LLC
elevations at select locations within the investigation area. The test hole
elevations presented on Plate 2 and 3 should be considered approximate and not
used for civil engineering design until verified with a site survey.
As noted on the cross sections on Plate 2 and 3, the exploration area has
considerable topographic relief, on the order of 15 to 25 feet with a swale noted
along section A -A'. This swale may act as a drainage area and had frozen soil
above rock.
Discussion and Recommendations
The inferred subsurface conditions along the revised tower alignment
encountered hard rock at depths considered suitable for the proposed concrete
gravity tower foundations. Varying thickness of unconsolidated soil and
weathered rock are present over harder rock. The unconsolidated soil and
weathered rock are not considered suitable as bearing material for the proposed
tower foundations.
Frost was encountered in several test holes at the time of our field work.
The tower foundations must not bear on frozen soil or weathered rock to avoid
thaw consolidation. Also, the topography at the proposed revised alignment
indicates swales or drainage areas may be present along the alignment. The
potential for shallow surface water seepage or groundwater to enter the tower
foundation excavations exists in this area. Construction planning should include
methods to control water seepage or groundwater that may enter the tower
foundation excavations.
We understand the proposed tower foundations will be constructed during
the summer/early fall of 2009 and a winter construction program is not expected
at this time_ If a winter or freezing air temperature construction schedule is
planned, we should be contacted to augment our recommendations for cold
weather construction practices. Tower foundation footprints will be excavated
with a track excavator with sufficient breakout capacity to remove frozen and
unfrozen weathered and fractured rock. We understand a Cat 345 or equivalent
track excavator is scheduled for tower foundation excavation work.
Tower foundations must bear on hard, competent rock or compact fill as
discussed below. All soil and weathered or fractured rock must be removed
STG Unalakleet Wind Tower Project
May 26, 2009
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Duane Miller Associates LLC
under the tower foundations. The foundation bearing surface must also be free
of discontinuities (joints, bedding planes, etc.) with orientations having soil
infilling that could consolidate under the foundation loads. This is particularly
important if frozen infilling is encountered.
If hard, competent rock is not encountered at the tower foundation
elevation, the tower foundation can bear on clean, well -graded fill placed and
compacted from the hard, competent rock surface to the tower foundation
elevation. For this project, fill material meeting Alaska Department of
Transportation and Public Facilities (ADOT&PF) Subbase "A" is recommended.
Subbase "A" has the following material material properties and gradation.
I ADOT&PF Subbase "A" I
Sieve
% Passing by
Weight
4-inch
100
- - - 2-inch-
- - 85 100 -
---- --
1-inch
-----------
----- inch
----------
------_-----------------
No. 4
20 - 55
---- 16 ---
-----------
No 200 T -
- - 10 max. - -
Subbase "A" must consist of hard, durable particles of fragments of stone or
gravel. The particles must not break up when alternately frozen and thawed or
wetted and dried. Muck, frozen material, roots, sod, or other deleterious matter
is not permitted. Subbase "A" must have a maximum L.A. abrasion of 50 (test
method AASHTO T-96) and a degradation of at least 40 (test method ATM 313).
We have assumed Subbase "A" can be produced from Borrow Pit 2 near
the site with some material processing. If fill is required under the tower
foundations, it should be placed in a fully thawed state in nominal 12-inch lifts
and compacted to 95%of maximum dry density as determined by test method
ASTM D-1557.
The tower foundation footprint excavation should extend at least three feet
beyond the tower foundation perimeter and sufficiently wide to permit safe
STG Unalakleet Wind Tower Project
May 26, 2009
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access for equipment and labor. If fill is required under the tower foundation, the
fill must extend at least three feet laterally in all directions from the tower
foundation perimeter then slope at a 2H:1V (horizontal:vertical), or shallower,
from the top of the fill section to competent, hard rock. A level rock surface is
recommended under the tower foundation. However, some relief of the rock
surface is permitted, but not to exceed 1 foot vertical in 10 feet horizontal under
the tower foundation or fill section.
Assuming a compact fill section is necessary under the tower foundation, an
allowable bearing capacity of 4,000 pounds per square foot (psf) is recommended
for sustained load states. An allowable bearing capacity of 5,500-psf is
recommended for short term, transient load states. A significantly greater
allowable bearing capacity can be developed for tower foundations cast directly
on hard, competent rock.
Estimated settlements for tower foundations bearing on compact fill are
expected to be less than 1-inch total with less than 0.5-inch differential, provided
fill sections less than 4 feet thick are needed. Thicker fill sections may experience
greater total and differential settlements. The majority of the settlements are
expected to occur concurrent with loading. Total and differential settlements for
tower foundations cast directly on hard, competent rock will be negligible,
provided discontinuity infilling that would consolidate is not present under the
tower foundations.
Lateral resistance can be developed by frictional resistance between the
concrete tower foundation and the underlying bearing surface. If the tower
foundations are cast directly on hard, competent rock, an allowable frictional
resistance of 0.4 times the design dead weight is recommended. If the tower
foundations are cast on compact fill, a allowable frictional resistance of 0.35 times
the tower dead weight is recommended.
Fill will be required above the primary (square) concrete tower foundation
section to surrounding grade at each tower site. Fill placed above the primary
foundation should be ADOT&PF Subbase "A", compacted to 901yo of maximum
dry density as determined by ASTM D-1557.
STG Unalakleet Wind Tower Project
May 26, 2009
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Duarte Miller Associates LLC
The site is located in an area of moderate seismic activity and may be
subject to strong ground motion. The spectral response acceleration for short
period (SS) is 0.34g and 0.10g for a 1-second period (SI), both based on Site Class
"B". Assuming the tower foundations bear on compact granular fill over
competent rock, the proposed site should be considered site Class "C". Site
coefficient factors FQ and F, are 1.2 and 1.7, for Site Class "C" respectively. Based
on these values, the design spectral response acceleration for the short and 1-
second periods can be determined as follows:
SDs = 2/3 F.Ss and SD1 = 2/3 FvSl
SDs = 0.27g and SD1= 0.11g
However, it is assumed wind load would control the foundation design
load and the base shear and maximum moment summarized earlier are based on
the design wind load conditions for the proposed tower.
Fill placed under the tower foundations should be field tested to verify
compaction. At least three field compaction tests per lift are recommended for
fill placed and compacted under the tower foundations. For the fill placed above
the primary tower foundation, either field compaction testing or a compaction
performance requirement can be used. If field tested, one compaction test per lift
is recommended. Alternatively, a performance requirement of three passes with
a vibratory compactor that develops at least 5,000 pounds of impact energy is
recommended for fill placed above the primary tower foundation section. If a
performance specification is used, a maximum 8-inch lift thickness is
recommended.
The rock surface under the tower foundations should be observed by a
trained, experienced engineer or geologist to verify the rock bearing surface is in
accordance with our recommendations. We should also review the project plans
and specifications as they are developed to conformance with the intent of our
geotechnical recommendations.
We have not reviewed local borrow sites for suitability as concrete
aggregate. If local materials are being considered for concrete aggregate and
historic concrete performance data are not available that would satisfy the
structural engineers' concrete design requirements, an experienced concrete
STG Unalakleet Wind Tower Project
May 26, 2009
Page 9
Duane Miller Associates LLC
materials engineer or technician should be consulted to confirm the suitability of
local material being considered for concrete aggregate.
If you have any questions please feel free to contact us.
Respectfully submitted
Duane Miller Associates LLC
Joe Marshall
Geologist
Attachments:
Plate 1: Site and Test Hole Location Plan
Plate 2: Generalized Cross Section, A -A'
Plate 3: Generalized Cross Section, B-B'
/
.
19wk
Richard Mitchells, P.E.
Geotechnical Engineer
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