HomeMy WebLinkAboutPoint Hope Wind-Diesel Project Geotechnical Review and Feasibility Study for Wind Turbines - Jan 2012 - REF Grant 7040026
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GEOTECHNICAL REVIEW AND
FEASIBILITY STUDIES FOR
WIND TURBINES
Point Hope, Point Lay, and Wainwright, Alaska
Submitted To: Messrs. Jay Hermanson and Ross Klooster
WHPacific
300 West 31st Avenue
Anchorage, AK 99503
Submitted By: Golder Associates Inc.
2121 Abbott Road, Suite 100
Anchorage, AK 99507 USA
Distribution:
1 PDF - WHPacific
2 Copies Golder Associates Inc.
January 27, 2012 113-95573
REPORT
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Table of Contents
1.0 INTRODUCTION ........................................................................................................................ 1
2.0 EXISTING GEOTECHNICAL DATA REVIEW.............................................................................. 2
2.1 Point Hope Relevant Geotechnical Data ............................................................................... 2
2.2 Point Lay Relevant Geotechnical Data.................................................................................. 2
2.3 Wainwright Relevant Geotechnical Data ............................................................................... 3
3.0 POINT HOPE EXISTING INFORMATION ................................................................................... 4
3.1 Point Hope Regional Climate Information ................................................................................ 4
3.2 Anticipated Subsurface Conditions in Point Hope .................................................................... 5
3.2.1 Point Hope Wind Turbine Site Conditions ............................................................................ 6
4.0 POINT LAY EXISTING INFORMATION ...................................................................................... 7
4.1 Point Lay Regional Climate Information................................................................................... 7
4.2 Anticipated Subsurface Conditions in Point Lay....................................................................... 8
4.2.1 Point Lay Wind Turbine Site Conditions............................................................................... 8
5.0 WAINWRIGHT EXISTING INFORMATION ............................................................................... 10
5.1 Wainwright Regional Climate Information .............................................................................. 10
5.2 Anticipated Subsurface Conditions in Wainwright .................................................................. 10
5.2.1 Wainwright Wind Turbine Site Conditions .......................................................................... 11
6.0 FOUNDATION CONCEPTS AND CONSTRUCTABILITY CONSIDERATIONS .......................... 12
6.1 Point Hope Foundation Concepts .......................................................................................... 12
6.2 Point Lay and Wainwright Foundation Concepts .................................................................... 13
7.0 USE OF REPORT ..................................................................................................................... 15
8.0 CLOSING ................................................................................................................................. 16
9.0 REFERENCES ......................................................................................................................... 17
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List of Tables
Table 1 Engineering Climate Indices for Point Hope, Alaska
Table 2 Engineering Climate Indices for Point Lay, Alaska
Table 3 Engineering Climate Indices for Wainwright, Alaska
List of Figures
Figure 1 Vicinity Map
Figure 2 Point Hope Location and Site Map
Figure 3 Point Lay Location and Site Map
Figure 4 Wainwright Location and Site Map
List of Appendices
Appendix A Point Hope Existing Geotechnical Data
Appendix B Point Lay Existing Geotechnical Data
Appendix C Wainwright Existing Geotechnical Data
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1.0 INTRODUCTION
Golder Associates Inc. (Golder) is pleased to present our geotechnical review to assist with the feasibility
studies of wind power projects in Point Hope, Point Lay, and Wainwright, all in the North Slope Borough
(Figure 1). The purpose of our geotechnical review is to identify potential geotechnical hazards and to
provide conceptual foundation recommendations for the proposed wind tower sites in these three Alaskan
communities. Our work has been conducted in general accordance with our proposal to you, dated
January 7, 2011.
Point Hope: We understand that the proposed wind turbine sites are east and west of the
developed village in relatively undisturbed areas. The proposed Point Hope wind turbine
sites are shown on Figure 2
village).
Point Lay: There are two proposed wind turbine sites at Point Lay: to the north of
the community, and southwest of the community, located on a bluff adjacent to
the lagoon.
bluff overlooking the lagoon. The proposed Point Lay wind turbine sites are shown on
Figure 3.
Wainwright: There are two proposed wind turbine sites at Wainwright, both northeast of
the village:
further from the village near an existing access road. Both sites appear to be located in
relatively undisturbed tundra. The proposed wind turbine sites are shown on Figure 4.
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2.0 EXISTING GEOTECHNICAL DATA REVIEW
Historical reports were reviewed to provide a general understanding of the subsurface conditions near the
proposed tower sites. Select borehole and test pit logs are attached as appendices to this report.
2.1 Point Hope Relevant Geotechnical Data
The approximate locations of select borings and test pits on shown on Figure 2. Relevant data can be
found in Appendix A.
September 1994: Duane Miller Associates (DMA) drilled ten boreholes to depths ranging
from 10 to 25 feet below ground surface (bgs) and excavated 11 test pits between 5 and
6.5 feet bgs for water and sewer improvements. The active layer was observed between
5 and 6 feet deep in the village area. Observed soil conditions consisted of sand and
gravel with little to no fines (material passing US no. 200 sieve), except in thin deposits at
the base of the swales. Considerable visible ice was observed beneath the active layer;
however ice was typically noted as filling the pore spaces.
May 1994: DMA drilled two boreholes to 35.5 feet and 30 feet deep for a proposed power
plant addition near the center of the village. Gravel and sand deposits were observed
from the surface to borehole exploration depths. Gravel size was typically smaller than
1.5-inch diameter, however, a minor amount of gravel was observed larger than this size.
Sand content was observed to increase somewhat with depth.
2.2 Point Lay Relevant Geotechnical Data
The approximate locations of select borings and test pits are shown on Figure 3. Relevant data can be
found in Appendix B.
October 2008: DMA excavated eight test trenches for new roads and road upgrades in
Point Lay. The trenching equipment was limited to 8 feet in depth. Two of the test
trenches were terminated in massive ice. Massive ice was not observed in the other test
tranches, however the silt and organic soils were ice-rich with high moisture contents in
the natural soils ranging between 20 and 240 percent, and averaging 150%.
December 1994 and May 1995: DMA drilled fifteen test boreholes to depths between 4
feet and 21.5 feet bgs. A surficial organic mat of live moss and peat 1 foot to 5 feet thick
is underlain by organic silt with high ice contents ranging between 2 feet and 12 feet
thick. The organic silt is interlayered with gray mineral silt. Massive ice with silt inclusions
is common and associated with the organic silt. Sand and gravel deposits were observed
around 15 feet. Salinity contents were low in the peat and organic silts, but were elevated
in the gravel and some silt samples, ranging between 5 and 23 parts per thousand (ppt).
May 1996: Shiltec Alaska Ltd. Measured ground temperatures in 6 of the boreholes
drilled by DMA for the water and sewer project. The average ground temperature for the
6 boreholes was 15 ºF at 15 feet bgs. Ground temperatures measured in May would
reflect a relatively cool temperature profile from near surface to 15 feet deep.
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2.3 Wainwright Relevant Geotechnical Data
The approximate locations of select borings and test pits are shown on Figure 4. Relevant data can be
found in Appendix C.
May and June 1994: DMA conducted a field investigation in support of a buried water
and sewer project throughout Wainwright by drilling and sampling 64 boreholes with a
local highway auger. The subsurface soils generally consisted of an organic layer of peat
and organic silt (tundra), underlain by icy silt, sandy silt and silty sand. Massive ice was
observed in many of the test borings throughout the village and averaged about 5 feet
thick, although was observed as a 19.5-foot-thick layer in one borehole. Ground
temperatures in the boreholes were measured in the summer, with average shallow
permafrost temperatures between 22 ºF and 27 °F at about 10 feet below existing grade.
Pore water salinity was measured in recovered soil samples and ranged from 0 to 8 ppt.
1994 and 1995: Shiltec Alaska Ltd. measured a series of ground temperatures in 10 of
the boreholes drilled by DMA for the water and sewer project. The average ground
temperature for the 10 boreholes was 17 ºF at 15 feet deep and below.
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3.0 POINT HOPE EXISTING INFORMATION
Point Hope is located on the west coast of northern Alaska, along the Chukchi Sea. They all are in the
Arctic climate zone, with temperatures ranging between 30 ºF and 50 ºF in the summer, and 0 ºF and -10
ºF in the winter, although temperatures have been recorded between -78 ºF and -50 ºF. For much of the
year, the Arctic Ocean has little or no moderating effect on the climate when it is covered by sea ice.
Precipitation is light, about 5 inches to 12 inches annually, with about 21 inches to 36 inches of snowfall.
The village is located on a gravel spit extending to the west about 6 miles from the tundra mainland. The
spit is bordered by the Chukchi Sea and has been formed by long shore currents moving along the
shoreline. The gravel spit is about 4000 feet wide and consists of a series of beach ridges and intervening
swales that parallel the axis of the spit to about elevation 14 feet on the north side. The swales between
the beach lines are 2 feet to 4 feet lower than the tops of the ridges.
The spit is actively eroding from the west and aggrading on south. North of the village, near the border
with Marryat Inlet, alluvial deposits of stratified silt and sand with peat and tundra vegetation are typical.
Polygonal ice wedge terrain is evident from aerial photography along the southern shore of Marryat Inlet.
In the 1970s, erosion along the northwest beach of the spit, measured at 8.8 feet per year, prompted the
village to move. By the late 1970s, the majority of the village has moved eastward on the spit
approximately 2 miles, bringing it to its current location. The village is currently located near the center of
the spit in an area that has been leveled by filling the beach troughs with fill material. The village area is
unvegetated, but the troughs of the old beach lines reportedly had thin organic deposits before that were
covered by fill.
3.1 Point Hope Regional Climate Information
Design climate data including average thawing and freezing indices for the Point Hope area are
presented in Table 1. The indices are calculated from data available by the University of Alaska Fairbanks
(UAF) Scenarios Network for Alaska Planning (SNAP). Design indices are based on the average of the
three coldest winters (freezing index) or the three warmest summers (thawing index) observed during the
analysis period. Included in Table 1 are projected climate data for year 2012 to 2042 based on the UAF
SNAP data.
SNAP data was prepared by Rupp et al. (2009), and is distributed as two separate products. Historical
records were calculated using the PRISM model by combining climate data from multiple meteorological
records across the state of Alaska from 1901 to 2009, and modeled across the state in a manner that
Forward-looking projections were prepared from 2012 to 2042 utilizing the ECHAM5 global climate model,
assuming the mid-range A1B carbon emission scenario.
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Table 1: Engineering Climate Indices for Point Hope, Alaska
1948 1978 1979 2009 2012 2042
(estimated)1
Average Air Temperature 17.8 °F 19.4 °F 22.7 °F
Average Freezing Index 6515 °F-days 6100 °F-days 5025 °F-days
Average Thawing Index 1330 °F-days 1500 °F-days 1635 °F-days
Design Freezing Index 7440 °F-days 7180 °F-days 6520 °F-days
Design Thawing Index 1565 °F-days 1900 °F-days 2230 °F-days
Notes: 1) Projected by UAF SNAP, Global Climate Model ECHAM5, Emission Scenario A1B
3.2 Anticipated Subsurface Conditions in Point Hope
Point Hope is underlain by relatively uniform subsurface conditions, consisting primarily of gravel and
sand deposits with low amounts of fines and permafrost beneath the active layer. Within the developed
village area, DMA measured active layer depths ranging between 5 to 6 feet in 1994. The active layer
depths observed in 1994 may have been influenced by surface disturbance and fill placement within the
village, and also possibly the effect of warming trends that have affected the region. The warming trends
have been continuing, and deeper active layer depths could be expected, in the range of 5 to 8 feet.
Average moisture contents measured in the upper 5 feet were 5 percent for unfrozen gravel, and 15
percent for the frozen gravels. Below 5 feet, most soils tested were frozen, with average moisture content
of 13 percent.
salinity content of soil testing from the 1995 DMA report in Point Hope. Moisture contents tend to vary
considerably in the gravel and sand below 5 feet, generally ranging between 5 and 25 percent. Just below
the active layer, a slight elevation in the moisture contents is apparent, and is likely due to an increase in
ice content in the zone near the base of the active layer, which will provide a surface for groundwater to
perch on and will fluctuate with time depending partially on annual seasonal air temperatures.
Average pore water salinity in samples throughout the village is 4 parts per thousand (ppt), and tends to
range between 0 and 3 ppt in the upper 10 feet. There is a wide range of salinity values near the 20 foot
depth, ranging between 2 and 20 ppt. A pore water salinity content of 10 ppt will lower the freezing point
of water by approximately 1 ºF.
In general, the material that has been observed in Point Hope has little fines content. In the upper 5 feet,
the fines content is below 5 percent. Below 5 feet, the fines content becomes more variable and can
range between 3 and 12 percent. The observed gravel and sand underlying the Point Hope is generally
non-frost susceptible.
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3.2.1 Point Hope Wind Turbine Site Conditions
Both proposed wind turbine sites are located outside the existing village infrastructure in relatively
undisturbed areas. Boreholes advanced in the developed portion of the village indicate the area is
underlain by sands and gravels with little fines content. Some surface or near surface deposits of silt or
organic material may be present, with non-frost susceptible gravel and sand deposits underlying the area.
Active layer depths are likely within the range of 4 feet to 8 feet deep; however, snow drifting and site
disturbance may influence the thermal conditions sites. The insulating effect of deep snow in particular
could reduce the winter cooling, which could result in a deeper active layer and warmer ground
temperatures, compared to windswept areas with little winter snow accumulation.
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4.0 POINT LAY EXISTING INFORMATION
Point Lay is located on the west coast of northern Alaska, along the Chukchi Sea. They all are in the
Arctic climate zone, with temperatures ranging between 30 ºF and 50 ºF in the summer, and 0 ºF and -10
ºF in the winter, although temperatures have been recorded between -78 ºF and -50 ºF. For much of the
year, the Arctic Ocean has little or no moderating effect on the climate when it is covered by sea ice.
Precipitation is light, about 5 inches to 12 inches annually, with about 21 inches to 36 inches of snowfall.
Point Lay is on the coast of the Chukchi Sea, situated on ice-rich soils between a beach ridge and a
lagoon. The lagoon and barrier beach protect the village from direct ocean current erosion, but some
bank deterioration, aided by thermal erosion, is occurring. Point Lay lies within the Arctic Coastal Plain
physiographic province, which is typified by gently topography, ice-bonded permafrost soils, wet tundra,
oriented thaw lakes, and meandering stream channels.
The tundra plain in the Point Lay area has little relief, and surficial drainage is poorly defined. A low hill
near the northern end of the village provides minimal surface drainage, but water ponding is common
between the gravel pads in several parts of town. Drifting snow is a continuous problem throughout the
winter months and snow storage at the edge of gravel pads contributes to the standing water in the spring
and early summer.
The village has been built directly on the tundra. Some smaller structures are built on at-grade sills on a
gravel fill pad but most buildings are pile supported. A 2 foot to 4 foot gravel overlay is commonly used for
roadways, parking and staging areas.
4.1 Point Lay Regional Climate Information
Design climate data, including average thawing and freezing indices for the Point Lay area are presented
in Table 2. Parameters were derived based on the methods discussed in Section 3.1.
Table 2: Engineering Climate Indices for Point Lay, Alaska
1948 1978 1979 2009 2012 2042
(estimated)1
Average Air Temperature 12.1 °F 14.1 °F 16.5 °F
Average Freezing Index 8400 °F-days 7850 °F-days 6985 °F-days
Average Thawing Index 1120 °F-days 1310 °F-days 1340 °F-days
Design Freezing Index 9360 °F-days 8845 °F-days 8300 °F-days
Design Thawing Index 1370 °F-days 1710 °F-days 1700 °F-days
Notes: 1) Projected by UAF SNAP, Global Climate Model ECHAM5, Emission Scenario A1B
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4.2 Anticipated Subsurface Conditions in Point Lay
The soils underlying Point Lay are very icy. The surficial organic mat of live moss and peat is underlain by
organic silt with high ice contents. Below about 10 feet the brown organic silt is interbedded with gray silt
with sand and fine gravel, probably of estuarine origin. Old beach deposits of sand and sandy gravel are
present at depths below an average of about 15 feet, but as shallow as about 10 feet in some areas. The
coarse granular material is well rounded and may contain saline pore water.
Massive ice with silt inclusions is common in association with the organic silt, and generally is observed in
the upper 10 feet. The coarse-grained deposits contain some interstitial ice; however, massive ice is
uncommon.
Average moisture contents measured in the upper 10 feet were 160 percent for the surficial organic
samples and 103 percent for silt and sandy silt, reflecting the high ice content in the soil in the soils
closest to the surface. Below 10 feet, the average moisture content for the silts and sandy silts was 73
and 57 percent, respectively. Coarse material consisting of sand, gravel and silty sand are typically
observed below about 15 feet deep. The average moisture content of the coarse grained material is 20
percent. Two plots of moisture contents are shown on .
Average pore water salinity of the surface organic samples is low, ranging between 0 ppt and 3 ppt in the
upper ten feet (organic and fine grained silt deposits). Below 10 feet, pore water salinity varies more, and
ranges between 1 ppt and 25 ppt however salinity concentrations of 100 ppt have been reported in
portions of the village. The elevated pore water salinities were generally measured in samples that were
classified as sandy silt that was typically observed above the coarse granular beach deposits. A pore
water salinity content of 10 ppt will lower the freezing point of water by approximately 1 ºF. A plot of pore
water salinity content with depth is shown
There is limited ground temperature data for the Point Lay area. Ground temperatures measured by
Shiltec in May 1996 indicate an average temperature of 15 ºF at 15 feet below ground surface. The
reviewed geotechnical explorations were conducted during winter when the soil profile was fully frozen,
and the active layer was not observed. Based on the average annual air temperature data and the
general near surface soil conditions, the active layer is expected to range between 1 and 3 feet deep.
4.2.1 Point Lay Wind Turbine Site Conditions
Two sites for the wind turbine have been identified in Point Lay. is on the north end of the
community, while the Site is on a bluff to the southwest of the community. Both sites appear to be
located on relatively undisturbed tundra. Subsurface conditions are similar in most areas of the village,
and are typified by icy soils with massive ice underlying much of the area. Beneath icy fine-grained soils,
coarse grained beach deposits are observed generally from 15 feet below ground surface. Elevated pore
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water salinity contents have been measured in samples near the 20 foot depth, however, typically range
between 1 and 10 ppt. However pore water salinities on the order of 100 ppt have been reported in the
village. Active layer thickness is likely within the range of 1 foot to 3 feet. The proximity of the wind
turbine sites to landforms and topography that may encourage snow drifting may increase the thickness
of the active layer and may also result in relatively warmer ground temperatures beneath the sites.
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5.0 WAINWRIGHT EXISTING INFORMATION
Wainwright is located on the west coast of northern Alaska, along the Chukchi Sea. They all are in the
Arctic climate zone, with temperatures ranging between 30 F and 50F in the summer, and 0 ºF and -10 ºF
in the winter, although temperatures have been recorded between -78 ºF and -50 ºF. For much of the
year, the Arctic Ocean has little or no moderating effect on the climate when it is covered by sea ice.
Precipitation is light, about 5 inches to 12 inches annually, with about 21 inches to 36 inches of snowfall.
the Kuk River
Estuary.
gentle topography, ice-bonded permafrost soils, wet tundra, oriented thaw lakes and meandering stream
channels. Wainwright is in a zone of cold continuous permafrost. The terrain has little relief, although the
polygonal patterned ground from ice-wedge development is evident on the terrain.
5.1 Wainwright Regional Climate Information
Design climate data, including average thawing and freezing indices for the Wainwright area are
presented in Table 3. Parameters were derived based on the methods discussed in Section 3.1.
Table 3: Engineering Climate Indices for Wainwright, Alaska
1948 1978 1979 2009 2012 2042
(estimated)1
Average Air Temperature 10.2 °F 12.4 °F 14.4 °F
Average Freezing Index 8745 °F-days 8130 °F-days 7380 °F-days
Average Thawing Index 765 °F-days 970 °F-days 945 °F-days
Design Freezing Index 9780 °F-days 9095 °F-days 8570 °F-days
Design Thawing Index 1015 °F-days 1360 °F-days 1235 °F-days
Notes: 1) Projected by UAF SNAP, Global Climate Model ECHAM5, Emission Scenario A1B
5.2 Anticipated Subsurface Conditions in Wainwright
The subsurface soil conditions in Wainwright appear to be similar throughout the village. Subsurface soils
typically consist of a thin live organic tundra mat, underlain by ice-rich organic soils and ice rich silts and
sandy silts. Silty sand, sand and gravelly sand generally underlay the area, and have been observed at
depths ranging from about 15 feet to 20 feet deep, although coarse grained deposits may be deeper in
some locations.
The area is underlain by continuous permafrost, although shallow zones of unfrozen soil have been
observed associated with drained lake beds. The ice content of the soils varies widely. Polygonal ground
is present throughout the area. Massive ice is common in Wainwright and is typically observed at an
average of 3 feet below the natural ground surface.
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Average moisture contents measured in the upper 10 feet were 170 percent for the surficial organic
samples and 83 percent for silt and sandy silt, reflecting the high ice content in the soil in the soils closest
to the surface. Below 10 feet, the average moisture content for the silts and silty sands was 83 and 34
percent, respectively. Coarse material consisting of sand, gravel and silty sand are typically observed
below about 15 feet deep. The average moisture content of the coarse grained material is 36 percent. A
endix C.
Average pore water salinity in samples the near surface organic samples are low, ranging between 0 and
less than 1 ppt in the upper ten feet (organic and fine grained silt deposits). In the silts and silty sands in
the upper 10 feet, pore water salinity ranges between 0 and 8 ppt, and average 1.2 ppt. In the silts and
silty sands below 10 feet, pore water salinity varies more, and ranges between 1 and 11 ppt, and average
2.7. The elevated pore water salinities were generally measured in samples that were classified as silty
sand that was typically observed above the coarse granular beach deposits. A pore water salinity content
of 10 ppt will lower the freezing point of water by approximately 1 ºF. A plot of pore water salinity content
with depth is
The ground temperature data that was measured by Shiltec throughout 1994 and 1995 provide a range of
seasonal temperatures. The average temperature at a depth of 18.8 ºF at 15 feet deep, and ranges
seasonally between about 16.1 ºF and 21.5 ºF. At a depth near 24 feet deep, the average temperature is
16 ºF, and ranges seasonally between about 14 ºF and 22 ºF at depths between 15 and 25 feet deep.
Based on the average annual air temperature data and the general near surface soil conditions, the active
layer is expected to range between 1 and 2.5 feet deep. Active layer depths may be relatively deeper in
areas with relatively thicker snow drifting in the winter.
5.2.1 Wainwright Wind Turbine Site Conditions
A proposed wind turbine sites in Wainwright are located northeasterly of the village on relatively
undisturbed tundra. Reviewed areal imagery shows that both sites are characterized by polygonal
patterned ground. Thaw lake and drained lake beds do not appear to be present at the sites, although
some localized ponding may be present or nearby.
Subsurface conditions are expected to be similar to that observed elsewhere in the Wainwright area,
consisting of a thin surficial organic mat, underlain by 1 to 5 feet of organic silt, and further underlain by
deposits of silt, sandy silt and silty sand. Coarse grained deposits of sand and gravel may underlie the
fine-grained deposits, and could be encountered as shallow as 15 feet deep. The soils are expected to be
icy, with massive wedge-ice common and moisture contents in excess of thawed state saturation in the
fine-grained deposits. Pore water salinities are not expected to affect the thermal state of the soils.
Ground temperatures at the site are expected to be typical of the Wainwright area, ranging between about
14 ºF and 22 ºF.
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6.0 FOUNDATION CONCEPTS AND CONSTRUCTABILITY CONSIDERATIONS
6.1 Point Hope Foundation Concepts
The permafrost conditions are unique at the proposed Northwind 100 tower location in Point Hope. While
permafrost is present, it is typically ice poor - a condition where ice is not present in concentrations
exceeding thawed state saturation and individual sand and gravel particles have grain-to-grain contact in
the permafrost. If permitted to thaw, the permafrost will generally experience thaw strains based on ice to
water phase change volumes, but thaw strains in ice-poor materials generally do not exceed the
volumetric change due to ice/water phase change. Seasonal frost heave is generally low in this area,
primarily related to the coarse-grained nature of the in-place soil. In general, the granular soils at Point
Hope are considered non-frost susceptible.
Accordingly, many structures in Point Hope are founded on post and pad or at-grade foundation with or
without passive subgrade cooling and rigid insulation.
A site-specific geotechnical exploration is required for turbine foundation system. However, based on
nearby geotechnical data, it may be possible to use a concrete or steel frame mat foundation system for
this site. The mat foundation can be founded on the prepared in-place granular soils but a structural fill
section between the foundation and the in-place soil should be considered.
Structural fill should be Alaska Department of Transportation and Public Facilities (ADOT&PF) Subbase A
or similar material. Rigid insulation under the mat foundation should also be considered. If used, the rigid
insulation should have a compressive strength suitable for the design loads, both sustained and transient.
Structural fill should be placed and compacted in a thawed state. Structural fill should be placed in
nominal one foot thick lifts compacted to at least 95 percent of maximum dry density as determined by the
modified Proctor test method.
A mat foundation system will rely solely on gravity to resist overturn and uplift loads, which may be
considerable with the Northwind 100 turbine systems. The civil and structural design will determine the
depth of excavation and fill requirements above the mat foundation. Mat foundation embedment depths
on the order of 10 to 12 feet may be necessary to develop adequate overturning and uplift resistance.
Passive subgrade cooling may be required under the mat foundation. If so, Arctic Foundations, Inc. (AFI)
Flat Loop passive subgrade cooling may be considered. The AFI Flat Loop passive subgrade cooling
system may be installed within the structural fill.
Subgrade cooling will provide several engineering advantages. Thaw into the underlying soil will be
limited reducing the volumetric ice to water thaw strain discussed above. If frozen throughout the project
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design life, the underlying soil will have a greater allowable bearing capacity and stiffness relative to thaw
state conditions.
Site access for construction and long-term operations and maintenance should be included with the civil
design.
6.2 Point Lay and Wainwright Foundation Concepts
The soils at Point Lay are ice rich silt to massive ice underlying an organic mat of several feet thick. Active
layer depth (seasonal thaw depth) is expected to be in the range of three feet, but may be deeper in areas
with disturbed surface vegetation or along the margins of the patterned (polygonal) ground.
Large thaw strains will occur if the underlying permafrost is disturbed. Accordingly, the civil engineering
design for this site must not allow the underlying permafrost to thaw or warm to levels where accelerated
creep will occur.
Pore water salinity is present, but generally at concentrations that will not significantly impact the tower
foundation performance provided the ground temperatures are maintained at their current levels
However, pore water salinity concentrations in excess of 100 ppt
have been reported in Point Lay. Elevated pore water salinity concentrations at either site may impact
foundation performance and creep rates under sustained loads. It is important the site-specific
geotechnical efforts verify pore water salinity concentrations throughout the expected foundation
embedment depths at the planned tower sites as part of the geotechnical engineering work.
While a site-specific geotechnical exploration and engineering assessment is required for the tower
foundation, several conceptual-level foundation design elements should be considered at the feasibility
stage.
The tower site subsurface conditions will most likely consist of very icy silt to massive ice under the
tundra. The tundra mat must be protected during the tower construction and for operations and
maintenance access. A gravel pad should be included with the project for construction and regular
maintenance. The gravel pad should be 4 to 5 feet thick but a thinner section may be feasible if rigid
insulation is placed within the pad fill.
An adfreeze pile foundation system should be used for the tower foundation with an above grade pile
cap/tower base system. Cast-in-place concrete, pre-cast concrete and steel frame pile cap/tower base
systems have been used in permafrost regions.
A clear space between the concrete or steel tower base is necessary. However, the tower systems are
not heated thus an 18 to 24 inch clear space may be suitable if a gravel pad is used. The tower base
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should not rest directly on the pad or tundra in order to allow for season frost related pad movements.
Also, access for welding and mechanical connections between the pad or tundra and the tower base
typically requires 18 to 24 inches.
The foundations should be steel pipe piles installed with drill and slurry methods. The pipe pile may
include a 2-inch helix around the bottom portion of the pile to develop additional capacity. The pile
boreholes should be dry augered without use of drill muds or added heat sufficient to provide a nominal 3
inch radial annulus between the pile (or helix if used) and the borehole sidewall. The slurry should be
clean sand and gravel mixed with potable water, placed and densified in the annular space.
The pipe pile dimensions will depend on the structural loads, but pipe piles in the range of 16 to 24 inch
diameter have been used for Northwind100 turbine units in similar conditions. Six piles supporting a
continuous concrete or steel base frame have been used for Northwind 100 turbine systems in Alaska.
Pile embedment depth will depend on structural loads, ground temperatures, and adfreeze bond capacity
under sustained and transient load conditions, but pile embedment depths in the range of 30 feet are
anticipated.
Passive subgrade cooling may be required to achieve and maintain the required ground temperatures to
develop adfreeze capacity throughout the pr . If passive subgrade cooling is required,
AFI Thermoprobes installed in the slurry backfill at each pile are typically used.
In general, settlements will be controlled by long-term creep in ice rich permafrost and can be significantly
impacted by pore water salinity. The foundation system should be designed to maintain long-term creep
rates in the primary or secondary phase. If properly designed and installed, creep movement in the range
of 1 to 2 inches over a 20 year design life can be established, but a site-specific geotechnical assessment
is required.
Lateral capacity will depend on the pile and cap material and connection (free or fixed head), gravel pad
thickness and seasonal thaw depths. In general, the point of fixity is considered about one foot below the
base of maximum seasonal thaw.
Exterior appurtenances such as electric conduits should be design for total and differential movements at
the tower system.
January 2012 15 113-95573
WHPacific NSB Wind Feasibility
7.0 USE OF REPORT
This report has been prepared for use by WHPacific for the wind turbine feasibility assessments proposed
in Point Hope, Point Lay and Wainwright, Alaska. The geotechnical engineering concepts presented
herein are based on the assumption Northwind 100 wind turbine systems will be used and are not
developed for design or construction. The subsurface conditions are based on existing geotechnical data,
and the surface and subsurface conditions at the proposed wind turbine sites may vary from the
descriptions and soil index data present in this submittal. In our opinion, site-specific geotechnical
investigations and engineering analyses are required at each turbine location. If there are significant
changes in the nature, design, or location of the proposed improvements, we should be notified so that
we may review our feasibility findings and engineering concepts presented in this submittal in light of the
proposed changes and provide a written modification or verification of the changes.
The work program followed the standard of care expected of professionals undertaking similar work in
Alaska under similar conditions. No warranty expressed or implied is made.
January 2012 17 113-95573
WHPacific NSB Wind Feasibility
9.0 REFERENCES
CRREL. 1978. Fresh water supply for a village surrounded by salt water, Point Hope, Alaska. dated April.
(Project No. 78-7). Hanover, New Hampshire: CRREL
Duane Miller Associates. 2009. Geotechnical Exploration and Recommendations, IRR Roads. Prepared
for WHPacific, dated May 14. (Project No. 4033.14). Anchorage, AK: DMA.
Duane Miller Associates. 1995. Geotechnical Exploration, Water and Sewer Improvements, Wainwright,
Alaska. Prepared for North Slope Borough, dated April 1995. (Project No. 4120.003). Anchorage, AK:
DMA
Duane Miller Associates. 1995. Geotechnical Exploration, Water and Sewer Improvements, Pt. Hope, AK.
Prepared for the North Slope Borough, dated May 12. (Project No. 4120.004). Anchorage, AK: DMA
Duane Miller Associates.1995. Geotechnical Exploration, Water and Sewer Improvements, Pt. Lay,
Alaska. Prepared for the North Slope Borough, dated December 1. (Project No. 4120.005).
Anchorage, AK: DMA.
Duane Miller Associates. 1994. Geotechnical Investigation, Power Plant Upgrade, Pt. Hope, Alaska.
Prepared for RSA Engineering, Inc., dated June 1. (Project No. 4091.002). Anchorage, AK: DMA
Rupp, S., Duffy, P., Olson, M., Springsteen, A., Schmidt, J., and Fresco, N., July 2009. Scenarios
Network for Alaska Planning. University of Alaska Fairbanks. http://snap.uaf.edu/. Accessed May
2011.
FIGURES
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PLZ WIND TURBINE SITE APLZ WIND TURBINE SITE BPIZ-3PIZ-4PL-10PL-13J:\2011 Jobs\113-95573 WHPacific NSB Wind Feasibility\CAD\Vicinity Map.dwg | 1/27/2012 11:20 AM | AGarrigus | Anchorage, AK---- ----DBC 1/27/12RAM 1/26/12RAM 1/26/120 ----CHECKREVIEWDESIGNCADDSCALEFILE No.PROJECT No.TITLEAS SHOWNREV.FIG.113-95573Vicinity Map.dwgPOINT LAY LOCATION MAP0APPROXIMATE SCALEFEET600600LEGENDPLZ-1GOLDER BOREHOLE NAME AND APPROXIMATELOCATION
MET STATIONA1A2AIN WIND TURBINE SITE AAIN WIND TURBINE SITE BFS44J:\2011 Jobs\113-95573 WHPacific NSB Wind Feasibility\CAD\Vicinity Map.dwg | 1/27/2012 11:16 AM | AGarrigus | Anchorage, AK---- ----DBC 1/27/12RAM 1/26/12RAM 1/26/120 ----CHECKREVIEWDESIGNCADDSCALEFILE No.PROJECT No.TITLEAS SHOWNREV.FIG.113-95573Vicinity Map.dwgWAINWRIGHT LOCATION MAP0APPROXIMATE SCALEFEET600600LEGENDA12011 GOLDER BOREHOLE NAME AND APPROXIMATELOCATION
APPENDIX A
POINT HOPE EXISTING GEOTECHNICAL DATA
APPENDIX B
POINT LAY EXISTING GEOTECHNICAL DATA
APPENDIX C
WAINWRIGHT EXISTING GEOTECHNICAL DATA
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Anchorage, AK 99507 USA
Tel: (907) 344-6001
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