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HomeMy WebLinkAboutKotzebue Wind Farm Expansion Project Geotechnical Report - Mar 2011 - REF Grant 2195427Wind Turbines - Kotzebue Golder Associates Inc. 2121 Abbott Road, Suite 100 Anchorage, AK 99507 USA Tel: (907) 344-6001 Fax: (907) 344-6011 www.golder.com Golder Associates: Operations in Africa, Asia, Australasia, Europe, North America and South America Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation DRAFT March 15, 2011 103-95444 Mr. Brad Reeve Kotzebue Electric Association (KEA) P.O. Box 44 Kotzebue, AK 99752 RE: GEOTECHNICAL EXPLORATION AND FOUNDATION DESIGN, KEA WIND TURBINES, KOTZEBUE, ALASKA Dear Mr. Reeve: 1.0 INTRODUCTION Golder Associates Inc. (Golder) is pleased to present the results of our geotechnical exploration, laboratory testing, and geotechnical foundation recommendations for the proposed wind turbines planned at Kotze services have been conducted in general accordance with our proposal to KEA dated April 23, 2010 and subsequent change order, dated November 26, 2010. We understand KEA will be installing two new wind turbines at the wind generation site south of the city of Kotzebue (Figure 1). The proposed turbines will be EWT Directwind 900 units. The turbine units use a tubular steel monotower. Foundation design loads were provided to us, reference Directwind 54x900 HH75, EWT Document Number 1003240, July4, 2005. EWT has developed base reactions for a concrete gravity foundation system. However, permafrost is present at the proposed project area near Kotzebue. The permafrost underlying the planned project site area is not considered conducive to an at-grade gravity foundation system to the provided design loads. Based on the tower geometry and provided tower base reactions, the project structural engineer, BBFM, Inc., developed pile capacity loads assuming 14 adfreeze-type piles supporting a reinforced concrete tower base. Pile geometry will consist of piles arranged in a 12.0-foot radius with piles equally spaced along the concrete base perimeter. Pile caps will be cast into the reinforced concrete base. Per pile unfactored design loads provided to us are 255-kips axial compression and 169-kips axial tension. These unfactored values are considered short-term transient state load conditions. Sustained dead load per pile is estimated to be approximately 50-kips under axial compression. Base shear is expected to be less than 10-kips per pile at the base of the concrete. 2.0 REGIONAL SETTING 2.1 General Conditions and Geologic Setting Kotzebue is located in the Kobuk-Selawik Lowland physiographic province, which is characterized by the drainages of the Noatak, Kobuk and Selawik Rivers. The city is located at the northwest tip of the Baldwin Peninsula, about 25 miles north of the Arctic Circle with Kotzebue Sound to the north, west and south. The city of Kotzebue lies on a narrow sand and gravel spit about one-half mile wide by several Golder Associates: Operations in Africa, Asia, Australasia, Europe, North America and South America Golder Associates: Operations in Africa, Asia, Australasia, Europe, North America and South America Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation Golder, Golder Associates and the GA globe design are trademarks of Golder Associates Corporation Kotzebue is located in the Kobuk-Selawik Lowland physiographic province, which is characterized by the Kotzebue is located in the Kobuk-Selawik Lowland physiographic province, which is characterized by the drainages of the Noatak, Kobuk and Selawik Rivers. The city is located at the northwest tip of the drainages of the Noatak, Kobuk and Selawik Rivers. The city is located at the northwest tip of the Baldwin Peninsula, about 25 miles north of the Arctic Circle with Kotzebue Sound to the north, west and Baldwin Peninsula, about 25 miles north of the Arctic Circle with Kotzebue Sound to the north, west and south. The city of Kotzebue lies on a narrow sand and gravel spit about one-half mile wide by several south. The city of Kotzebue lies on a narrow sand and gravel spit about one-half mile wide by several Brad Reeve March 11, 2011 Kotzebue Electric Association 2 103-95544 Wind Turbines - Kotzebue miles long that is separated from the main part of Baldwin Peninsula by a brackish water lagoon. The Kotzebue area has been mapped within the continuous permafrost zone. The Baldwin peninsula is a moraine complex formed in the middle Pleistocene, composed of marine, glacio-marine and glacio-terrestrial deposits. The peninsula formed as a result of three coalescing ice lobes from glaciers originating in the Kobuk, Selawik and Noatak River valleys. The bulk of the peninsula consists of glacial and glacio-marine sediments. Sediments exposed in bluffs on the southwestern coast of the peninsula at Cape Blossom and further south, show fine grained glacial marine deposits of silt and clayey silt, with fine interbeds of coarse silt and fine grained sand and poorly sorted glacial till in some areas.1 The city of Kotzebue is located on a sand and gravel spit, overlying marine deposited silt that has formed at the northwest tip of the peninsula. 2.2 Climate Kotzebue is in a transitional climate zone, with a sub-arctic climate that has a maritime influence when Kotzebue Sound is ice-free, usually from early July to early October. The area receives 9 inches of precipitation annually with about 40 inches of snowfall. Temperatures can range from -52 to 85 °F, although the average low temperature in winter is -12 ºF and the average high temperature in the summer is 58 ºF.2 The following table summarizes our recommended engineering design air temperature data for 3 analysis of air temperature records prior to 1978 to our analysis of Kotzebue air temperature records from 1980 to 2004. Table 1: Recommended Engineering Design Indices and Air Temperature Data (Kotzebue) Design Index H&J 1978 1980-2004 Average Air Temperature 21 ºF 22.9 ºF Average Freezing Index 5,900 ºF-days 5,500 ºF-days Design Freezing Index 6,500 ºF-days 6,300 ºF-days Average Thawing Index 1,600 ºF-days 2,150 ºF-days Design Thawing Index 2,400 ºF-days 2,650 ºF-days Based on our revised climatic data, permafrost in the Kotzebue area should be considered to be warming, particularly in lower lying micro-relief areas. 3.0 SITE CONDITIONS The KEA wind generation facility site is south of the city of Kotzebue, on the uplands portion of the Baldwin Peninsula. The terrain is characterized by a gently undulating tundra plain with the Bering Sea coast to the west. The wind site is in an area with polygonal patterned ground and gently undulating terrain. The site is wet in summer, and typically windswept in winter, with drifts forming at access road borders and on the lee side of structures. The terrain appears to be poorly drained, with relatively low elevation areas that are reportedly wet in the summer months. The proposed wind turbine sites are on slightly elevated ground relative to the surrounding area. Two new wind turbines are planned at the site. The northern turbine location is about 400 feet south of the access road to the wind power generation site. The southern turbine location is about 1,200 feet south of the north-south oriented access road at the wind site (along the middle row of wind turbines). At 1 Huston, M. M, and Brigham-Grette, J. Paleogeographic significance of middle Pleistocene glaciomarine deposits on Baldwin Peninsula, northwest Alaska. Annals of Glaciology. 14, p 111-114.2 Alaska Department of Community, Commerce, and Economic Development, Community Profiles, available online at: http://www.dced.state.ak.us/dca/commdb/cf_comdb.htm3Hartman & Johnson. 1978. Environmental Atlas of Alaska. Institute of Water Resources. University of Alaska. the access road to the wind power generation site. The southern turbine location is about 1,200 feet the access road to the wind power generation site. The southern turbine location is about 1,200 feet south of the north-south oriented access road at the wind site (along the middle row of wind turbines). At south of the north-south oriented access road at the wind site (along the middle row of wind turbines). At Huston, M. M, and Brigham-Grette, J. Paleogeographic significance of middle Pleistocene glaciomarine deposits on Baldwin Huston, M. M, and Brigham-Grette, J. Paleogeographic significance of middle Pleistocene glaciomarine deposits on Baldwin Community ProfilesCommunity Profiles, available online at: , available online at: University of Alaska. University of Alaska. Brad Reeve March 11, 2011 Kotzebue Electric Association 3 103-95544 Wind Turbines - Kotzebue both proposed wind turbine locations, the surrounding terrain is similar, consisting of a generally flat tundra surface. 4.0 EXISTING GEOTECHNICAL INFORMATION The following geotechnical report and ground temperature information was reviewed to provide a general understanding of subsurface conditions near the proposed Kotzebue wind turbine site. The approximate locations of select historical boreholes are shown in Figure 2. In 2001 Duane Miller Associates explored the subsurface conditions in the general vicinity of the proposed wind turbine sites by advancing five boreholes to about 25 feet deep with a disk-auger drill. The subsurface conditions observed consisted of a thin layer of peat overlying organic silt that extends from the surface organics to between 2 and 12 feet deep. The organic silt was underlain by mineral silt with some organic silt interbeds to borehole exploration depths. Massive ice was observed between 4 and 8 feet deep in organic silt deposits, in three of the five boreholes drilled. The mineral silt layer was composed of non-plastic silt, finely interbedded with plastic silt. Visible ice was observed in the mineral silt layer at 5 to 15 percent by volume. Borehole KW-5 was drilled near the southernmost proposed wind turbine, massive ice was observed between 4.5 and 7 feet deep, within the organic silt deposit. 4 5.0 GEOTECHNICAL INVESTIGATION Due to tower location adjustments, two field explorations and a site visit were conducted under this scope of work. In May 2010, a geotechnical exploration was conducted to explore sites initially under consideration for the proposed new wind turbines. A site visit was conducted in September 2010 to obtain ground temperatures at boreholes drilled in May, and to conduct end of season thaw-probing at the KEA wind site. KEA refined the proposed wind turbine locations and selected sites outside of areas previously explored during the May 2010 field exploration. In December 2010, a geotechnical field exploration was conducted to explore the subsurface conditions at the revised wind turbine locations. Mr. Matt Bergen assisted Golder personnel during each of the field explorations and site visits. 5.1 May 2010 Field Exploration The first geotechnical field exploration was conducted on May 13 and 14, 2010. Five boreholes (G10-01 through G10-05) were advanced to between 25 and 27-feet deep in areas under consideration for the proposed wind turbines. The planned borehole locations were identified in the field by Mr. Matt Bergen, a representative for the client. The boreholes were drilled with a Texoma drill owned and operated by KIC Construction, LLC. The drill was equipped with a 16-inch diameter disk auger. The boreholes were logged and sampled by Golder staff engineer Jeff Kenzie. Soil samples were obtained by collecting soil directly from the disk auger. 5.2 September 2010 Site Visit On September 16, 2010, Jeremiah Drage, PE of Golder Associates traveled to Kotzebue to conduct a site reconnaissance at the wind turbine site. The depth to permafrost was probed in the areas of the proposed wind turbines, and at select locations to characterize general surface and seasonal thaw conditions around the wind turbine site. The depth to frozen ground was determined at select locations by pushing a steel t-probe into the ground until hard resistance was encountered, which is considered the depth to permafrost at this site. Ground temperatures were measured at boreholes drilled in May 2010. 4 DMA. 2001. Foundation Design Report, Kotzebue Wind Farm, Kotzebue, Alaska.Prepared for Kotzebue Electric Association, Inc. by pushing a steel t-probe into the ground until hard resistance was encountered, which is considered the by pushing a steel t-probe into the ground until hard resistance was encountered, which is considered the depth to permafrost at this site. Ground temperatures were measured at boreholes drilled in May 2010. depth to permafrost at this site. Ground temperatures were measured at boreholes drilled in May 2010. Prepared for Kotzebue Electric Association, Inc.Prepared for Kotzebue Electric Association, Inc. Brad Reeve March 11, 2011 Kotzebue Electric Association 4 103-95544 Wind Turbines - Kotzebue 5.3 December 2010 Field Exploration The second geotechnical field exploration was conducted to explore the subsurface conditions at the revised turbine locations on December 1 and 6, 2010 by Golder staff scientist Melanie Hess. Two boreholes (G10-KEA-01 and G10-KEA-02) were advanced to between 30.5 and 36-feet below the ground surface, one borehole at each of the proposed wind turbine locations. Mr. Bergen was onsite during the exploration and identified the planned location of the boreholes using a handheld GPS. The boreholes were drilled with a skid-mounted CME-45, owned and operated by Denali Drilling of Anchorage, Alaska. The drill rig equipped with hollow stem augers and standard penetration test (SPT) equipment. Drilling operations, including drill mobilization, were supported by KEA equipment and personnel. Subsurface conditions were logged and representative soil samples were collected during the exploration. Soil samples were obtained by driving a 1.4- with a 140-pound manually operated hammer falling 30 inches. Soil samples were also obtained by collecting material directly from the auger flights. 5.4 Field exploration closure One-inch, schedule-40 PVC was installed in all boreholes to facilitate ground temperature measurement. Soil cuttings were used to backfill all boreholes after PVC installation. Geographic coordinates of the borehole locations were recorded with a hand-held GPS instrument. All soil samples collected were visually classified in the field by the Golder representative conducting the field exploration. Retained soil samples were sealed in polyethylene bags to preserve their natural moisture content and were delivered to our Anchorage laboratory for additional soil classification and index property testing. Stabilized ground temperatures were measured in the boreholes after at least one month following completion of drilling, to allow any drilling induced temperature disturbance to dissipate from the boreholes. Client representative Matt Bergen measured ground temperatures in the boreholes that were drilled in May 2010 on June 11 and 12, 2010. Ground temperatures in the May 2010 boreholes were measured again during the September site visit. A return trip was completed by Golder on January 4, 2011 to measure stabilized ground temperatures in the boreholes drilled in December 2010. The soils have been classified according to the Unified Soils Classification System (USCS) consistent with ASTM standard D-2487-05 as described in Figure 3. Visual ice in recovered samples has been classified as described in Figure 4. Borehole logs drilled during the December exploration are presented in Appendix A, Figures A-1 and A-2 and borehole logs from the May 2010 exploration are presented in Appendix B, Figures B-1 to B-5. Graphs of measured ground temperatures are presented in Figure A-3 and B-6. 5.5 Laboratory Testing In the laboratory, recovered samples were re-examined to verify field classifications and to select samples for geotechnical index testing, including natural moisture content, pore water salinity, grain size distribution and plasticity testing. Laboratory test results are shown graphically on the borehole logs and are tabulated in the Sample Summary, Figure A-4 and B-7. Grain size distribution plots are shown in Figure B-8. Plasticity Index test results are shown in Figure A-5 and B-9. 6.0 SUBSURFACE CONDITIONS 6.1 General Subsurface Soil and Thermal Conditions Subsurface conditions observed during the May and December field explorations were similar and generally consisted of a surface organic mat of fibrous peat, typically underlain by organic silt, which is present to between 6 and 16 feet deep. Beneath the organic soils, mineral silt was observed, in most boreholes, to exploration depth. A deeper layer of organic silt was observed in one borehole at the site. Subsurface conditions observed during the May and December field explorations were similar and Subsurface conditions observed during the May and December field explorations were similar and generally consisted of a surface organic mat of fibrous peat, typically underlain by organic silt, which is generally consisted of a surface organic mat of fibrous peat, typically underlain by organic silt, which is present to between 6 and 16 feet deep. Beneath the organic soils, mineral silt was observed, in most present to between 6 and 16 feet deep. Beneath the organic soils, mineral silt was observed, in most boreholes, to exploration depth. A deeper layer of organic silt was observed in one borehole at the site. boreholes, to exploration depth. A deeper layer of organic silt was observed in one borehole at the site. Brad Reeve March 11, 2011 Kotzebue Electric Association 5 103-95544 Wind Turbines - Kotzebue The upper ten feet of the soil profile was observed to be ice-rich, with visible ice ranging from 10 to 60 percent (by volume), with corresponding moisture contents (by dry weight) generally above 200 percent. Stabilized ground temperatures measured at the boreholes drilled at the wind site show cold permafrost temperatures at depth, with an average temperature of 26 ºF below the 20 foot depth. The active layer depth was observed in early fall with an average depth of about 1.5 to 2.0 feet in undisturbed tundra areas. Deeper thaw was observed near roads and at existing wind turbine bases, between 2.5 and 4.0 feet deep. 6.1.1 Borehole G10-KEA-01 Proposed wind turbine location In borehole G10-KEA-01, thin surficial peat overlies an ice-rich organic silt layer observed to about 8 feet deep, with visible ice contents up to 50 percent by volume. Less icy organic silt was observed to about 13 feet deep, with visible ice content at about 5 to 10 percent by volume. Frozen silt was observed beneath the organic silt to borehole exploration depths, with visible ice contents ranging between 5 and 10 percent. Ground temperatures measured at Borehole G10-KEA-02 showed ground temperatures at about 29 ºF at 10 feet deep, decreasing to about 26.5 ºF at borehole termination depth. 6.1.2 Borehole G10-KEA-02 Proposed wind turbine Location In borehole G10-KEA-02, a thin layer of surficial peat was observed overlying an ice rich organic silt deposit to about 10.5 feet deep. The ice rich organic silt had visible ice contents ranging between 40 and 60 percent by volume. Less icy organic silt with low to medium plasticity was observed between about 10.5 and 16 feet deep, with 25 to 30 percent visible ice content. Mineral silt was observed between 16 and 27 feet deep, with a visible ice content ranging between 10 and 15 percent. Frozen, nonplastic, organic silt was observed beneath the silt to borehole exploration depth, 30.5 feet below the surface, with visible ice content between 10 and 15 percent. Ground temperatures measured at Borehole G10-KEA-02 showed ground temperatures at 28.5 ºF at about 10 feet deep, decreasing to about 26.0 ºF at borehole termination depth. 6.2 Laboratory Results Measured soil moisture contents in soils at the wind turbine site were in excess of frozen state saturation levels, particularly in the upper 10 feet of the soil column, and generally corresponds to the concentrations of visible ice observed in the recovered soil samples. Mineral silt samples generally had moisture contents near saturation levels. Moisture content generally decreased with depth. The graph to the right shows soil moisture contents with depth and by general soil type, as measured in samples recovered from both the May and December investigations conducted in 2010. Note the soil moistures greater than 200 percent in the graph are plotted at 200 percent for presentation purposes. The specific soil moisture contents are provided in the sample summary in Appendix B. Laboratory findings indicate pore water salinities in the range of 0 to 1 part per thousand (ppt) were present throughout borehole exploration depths. Pore water salinity contents are considered low at the wind turbine site and significant freezing point temperature depression due to pore water salinity is not expected. 0 ft 5 ft 10 ft 15 ft 20 ft 25 ft 30 ft 35 ft 40 ft 0% 50% 100% 150% 200% Soil Moisture as a percent of dry sample weight KEA Wind Turbine Site 2010 Investigations Soil Moisture Peat, Organic Silt Silt ppt) were t) were present throughout borehole exploration depths. Pore water salinity contents are considered low at the present throughout borehole exploration depths. Pore water salinity contents are considered low at the wind turbine site and significant freezing point temperature depression due to pore water salinity is not wind turbine site and significant freezing point temperature depression due to pore water salinity is not Brad Reeve March 11, 2011 Kotzebue Electric Association 6 103-95544 Wind Turbines - Kotzebue 7.0 DISCUSSION Based on discussions with BBFM and the project construction manager/contractor, STG. Inc., we understand each tower foundation system will consist of 14 passively cooled steel pipe piles seated into a cast-in-place reinforced concrete base. The wind turbine towers will be bolted onto anchor bolts cast into the concrete. The anticipated construction sequencing is to install the piles this spring when the ground is frozen then return in the summer to pour the reinforced concrete base. The concrete will be mixed on-site with an aggregate source to be determined by the design and construction team. After the concrete is cured, the towers will be installed and the turbine units set. Per pile design loads for short-term and sustained loading conditions were discussed previously. KEA has requested the design team use existing, but unused, Arctic Foundation, Inc. (AFI) Thermo-helix piles fabricated and delivered to Kotzebue for another project. AFI has provided the construction schematics for these piles and KEA has verified the construction schematics represent the piles available in Kotzebue. AFI will verify the Thermo-helix piles are functional and meet their design criteria prior to installation. Based on data provided to us, the Thermo-helix piles in Kotzebue are 16-inch diameter pipe piles with a 2-inch wide steel helix welded along the bottom 17 feet of the pile starting approximately one foot from the bottom of the pile. The pressurized section of each pile is approximately 26.5 feet long with a non- pressurized riser section ranging from approximately 9 to 11 feet long atop the pressurized section. A short, 0.5-foot long non-pressurized pipe section is provided at the base of each pile below the pressurized section. Each pile has an external valve and pressure relief system with a small metal rock for details and verification of pile geometry. We understand that boreholes G10-KEA-01 and -02 were advanced at planned wind turbine locations. The proposed site is currently undeveloped, thus a granular fill access roadway and pad will be needed for construction and longer-term access to the towers. Based on our site exploration and laboratory analysis on recovered soil samples, the subsurface soils are considered suitable to support the wind towers using drilled and slurried adfreeze steel piling coupled with a passive subgrade cooling system. Specific site preparation, passive subgrade cooling and pile embedment recommendations are provided for this project, particularly for the valve and pressure relief system. 8.0 CONCLUSIONS AND RECOMMENDATIONS Ice-rich silty permafrost soils are present beneath both proposed towers. Maintaining the thermal integrity of the permafrost under the proposed tower foundations is essential to the long-term foundation performance of the structures. A nominal 16-inch diameter, AFI Thermo-helix pipe pile with a 2-inch helix installed along the basal 17 feet is recommended for the tower foundation. Based on design loads developed by the structural engineer, 14 piles per tower will be needed. An insulated structural fill pad will be required at each site as well as granular access roads to the tower sites. Based on discussions with STG, some site preparation may be required at the tower sites. Recommendations for site preparation, structural fill pads, passively cooled foundation piles, and constructability considerations are provided. At this time, we understand the concrete base may be cast directly over a rigid insulation section but a structural fill section of undetermined thickness may be placed over the insulation to facilitate construction activity and concrete placement. When site layout and civil design is completed, we should review the site plans for the leveling course, insulation, and structural fill geometry prior to initiating construction. The site preparation, leveling course and insulation placement, structural fill placement and placement of the Thermo-helix piles will require careful attention to sequencing and site control. If construction scheduling results in warmed permafrost or thawed soil conditions, tower foundation performance and pile capacities may be compromised. Accordingly, we should review the civil design and construction At this time, we understand the concrete base may be cast directly over a rigid insulation section but a At this time, we understand the concrete base may be cast directly over a rigid insulation section but a structural fill section of undetermined thickness may be placed over the insulation to facilitate construction structural fill section of undetermined thickness may be placed over the insulation to facilitate construction activity and concrete placement. When site layout and civil design is completed, we should review the activity and concrete placement. When site layout and civil design is completed, we should review the site plans for the leveling course, insulation, and structural fill geometry prior to initiating construction. site plans for the leveling course, insulation, and structural fill geometry prior to initiating construction. The site preparation, leveling course and insulation placement, structural fill placement and placement of The site preparation, leveling course and insulation placement, structural fill placement and placement of the Thermo-helix piles will require careful attention to sequencing and site control. If construction the Thermo-helix piles will require careful attention to sequencing and site control. If construction scheduling results in warmed permafrost or thawed soil conditions, tower foundation performance and pile scheduling results in warmed permafrost or thawed soil conditions, tower foundation performance and pile capacities may be compromised. Accordingly, we should review the civil design and construction capacities may be compromised. Accordingly, we should review the civil design and construction Brad Reeve March 11, 2011 Kotzebue Electric Association 7 103-95544 Wind Turbines - Kotzebue scheduling for this project. In addition, if the contractor is considering site grading that would remove appreciable portions of the organic mat, we should be advised of the proposed grading plan prior to commencing site work. In this manner adjustment to our recommended site preparation, structural fill and insulation thickness, and pile capacities can be reviewed and modified, if necessary, prior to site work. 8.1 Site Preparation Site preparation, rigid insulation, and structural fill under the concrete base will require specific consideration. Based on site grades, the contractor has determined some site grading may be necessary under the concrete tower base for uniform pile embedment depths and to provide for a uniform structural fill section. The area under the insulation footprint should be cleared of snow, surface ice, debris and deleterious material prior to geotextile or fill placement. Complete removal of the organic mat at the tower sites is not recommended. However, some isolated removal of hummocks may be conducted. The selective removal of the organic mat may result in decreasing the thermal resistance to the underlying permafrost. If isolated organic mat removal is conducted, it should be limited in areal extent and conducted only during winter or early spring. The prepared area must be thermally protected with rigid insulation to prevent thawing and warming of the underlying ice-rich permafrost. Surface albedo changes resulting from even minor surface grading can result in warming and thaw during periods with sub-freezing air temperatures. Once the site preparation is completed, a leveling course of structural fill and rigid insulation must be installed to maintain the ground temperatures below freezing. If massive site grading is under consideration that would remove large sections of the organic mat or would extend outside the proposed concrete base footprint, we must be notified prior to commencing site work. Large scale site grading may require revision to our geotechnical recommendations. 8.2 Insulated Structural Fill Pad and Access Roadways An insulated structural fill pad is required under and around the concrete tower base. The structural fill pad should be constructed with locally available, well-graded sand and gravel structural fill aggregate that meets a non-frost susceptible (NFS) classification. If NFS material is not used for structural fill, material mee reduce surface water or wind erosion. If aggregate with a frost classification other than NFS is used for structural fill, increased seasonal frost related differential movements may occur. A basal geotextile, such as Geotex 601, should be installed on prepared site surface prior to fill placement. A leveling course of structural fill should be placed over the geotextile to provide a uniform, level surface for installation of the rigid insulation. Six (6) inches of 40-psi compressive strength extruded or expanded polystyrene insulation is recommended at the base of the fill section at each tower location. At each tower location, the rigid insulation should extend at least eight (8) feet laterally from the edge of the concrete base footprint. The insulation should be installed as three layers of 2-inch thick insulation with overlapping and offset vertical joints. The insulation must be trimmed to a tight fit around the Thermo-helix piles, passive subgrade cooling systems, and temperature access standpipes discussed below. If site preparation removes appreciable organic mat or construction scheduling results in softening or thaw of the in-place soil, we recommend an additional two-inch thick layer of low density (10-psi compressive strength) insulation be installed under the thicker, higher density insulation discussed above. The lower density insulation will tend to crush and will dampen differential movements as the underlying soil freezes. The fill should be compacted to 95 percent of maximum dry density as determined by ASTM-D 1557, modified Proctor. Final grades and structural fill sections will be developed by the civil engineer or the contractor and should be reviewed by us and the structural engineer prior to commencing site work. At this time, we understand a nominal 12-inch layer of structural fill will be used for a leveling course with the appreciable organic mat or construction scheduling results in softening or thaw of the in-place soil, we appreciable organic mat or construction scheduling results in softening or thaw of the in-place soil, we recommend an additional two-inch thick layer of low density (10-psi compressive strength) insulation be recommend an additional two-inch thick layer of low density (10-psi compressive strength) insulation be installed under the thicker, higher density insulation discussed above. The lower density insulation will installed under the thicker, higher density insulation discussed above. The lower density insulation will The fill should be compacted to 95 percent of maximum dry density as determined by ASTM-D 1557, The fill should be compacted to 95 percent of maximum dry density as determined by ASTM-D 1557, modified Proctor. Final grades and structural fill sections will be developed by the civil engineer or the modified Proctor. Final grades and structural fill sections will be developed by the civil engineer or the contractor and should be reviewed by us and the structural engineer prior to commencing site work. At contractor and should be reviewed by us and the structural engineer prior to commencing site work. At this time, we understand a nominal 12-inch layer of structural fill will be used for a leveling course with the this time, we understand a nominal 12-inch layer of structural fill will be used for a leveling course with the Brad Reeve March 11, 2011 Kotzebue Electric Association 8 103-95544 Wind Turbines - Kotzebue rigid insulation placed atop the leveling layer. If additional structural fill is required, subsequent fill lifts should be placed and compacted in nominal 12-inch lifts to final grade. Compaction should be conducted with a vibratory roller compactor. If hand operated or light energy vibratory compaction equipment is planned, thinner lifts are advised. All structural fill under and near the concrete tower base must be placed and compacted in a fully thawed state. If construction sequencing does not permit placement and compaction of fully thawed fill concurrent with insulation placement, temporary covers may be installed over the insulation and the structural fill placed during warmer air temperature periods. If this option is being considered, we should be contacted to provide additional geotechnical recommendations. Current conceptual site plans include a thin fill section atop the rigid insulation to hold the insulation in place until concrete placement. Alternatively, a geotextile anchoring system has been proposed by the contractor to hold the insulation in place until concrete placement. Since the rigid insulation will extend outside the perimeter of the concrete base, additional protective measures such as thicker fill or rig mats may be required to maintain the integrity of the insulation during construction. Side slopes should be no steeper than 3H:1V (horizontal to vertical), unless a flatter slope is required for access or safety reasons. Side slopes should be compacted as discussed for structural fill. Final grades should direct surface water away from the tower foundation. Access roads and pads to the tower sites may be constructed of granular fill but a thicker fill section will be required if rigid insulation is not used within the access roadway prism. A geotextile similar to the material recommended under the towers should be installed over the tundra surface prior to fill placement. If an uninsulated fill section is planned, a minimum three foot thick fill prism is recommended. Provisions for regrading and additional fill should be included for an uninsulated access roadway fill section. If an insulated fill section is planned, we recommend four inches of rigid insulation be used, placed as discussed above with a two foot fill section above the insulation. Side slopes should be 2H:1V or flatter. Drainage ditches and culverts within the roadway and access pads should direct surface water away from the tower foundations. 8.3 Passively Cooled Foundation Piles Each tower will be founded on a series of AFI Thermo-helix piles with 16-inch pipe diameter and 2-inch helix coupled to a cast-in-place reinforced concrete tower base. For conventional application, AFI Thermo-helix piles requires at least three feet of pressurized section open to cold ambient temperatures and wind to activate the passive cooling system with reasonable efficiency. The current tower foundation and concrete base design has the concrete mat seated on an insulated structural fill pad and structurally tied to the AFI Thermo-helix piles along the non-pressurized pile section. Thus the conventional above grade pressurized section necessary for passive cooling activation is not possible with the proposed tower foundation and concrete base design and a specialized subgrade cooling system is needed. 8.3.1 Passive subgrade cooling system The current reinforced concrete tower base design will require a specialized subgrade cooling system to develop and maintain the recommended design ground temperatures. To stimulate the Thermo-helix pile passive cooling, each Thermo-helix pile will be coupled with a standalone AFI Thermoprobe specifically designed to mate with the pressurized Thermo-helix pile and extend above the top of the reinforced concrete base. AFI is developing the specialized Thermoprobe system in conjunction with the foundation design. The AFI Thermoprobe/Thermo-helix pile is a passive cooling system that relies on seasonal cold ambient air temperature to achieve and maintain design ground temperatures. The passive subgrade cooling will need to maintain the existing ground temperature at a sustained temperature of 26°F or colder below the base of the insulation. Pile foundation design includes a several foot deep layer beneath the insulation for seasonal temperature variation, but all material below the insulation must be maintained in a frozen state. Maintaining the material below the insulation in a frozen designed to mate with the pressurized Thermo-helix pile and extend above the top of the reinforced designed to mate with the pressurized Thermo-helix pile and extend above the top of the reinforced concrete base. AFI is developing the specialized Thermoprobe system in conjunction with the foundation concrete base. AFI is developing the specialized Thermoprobe system in conjunction with the foundation design. The AFI Thermoprobe/Thermo-helix pile is a passive cooling system that relies on seasonal cold design. The AFI Thermoprobe/Thermo-helix pile is a passive cooling system that relies on seasonal cold at a sustained sustained Pile foundation design includes a several Pile foundation design includes a several foot deep layer beneath the insulation for seasonal temperature variation, but all material below the foot deep layer beneath the insulation for seasonal temperature variation, but all material below the insulation must be maintained in a frozen state. Maintaining the material below the insulation in a frozen insulation must be maintained in a frozen state. Maintaining the material below the insulation in a frozen Brad Reeve March 11, 2011 Kotzebue Electric Association 9 103-95544 Wind Turbines - Kotzebue state will require preserving the 2010/2011 winter-cooled ground temperatures prior to loading the piles or will require artificial cooling if ground temperatures exceeding our design temperatures develop over the summer. 8.3.2 Adfreeze Piles To achieve the requested design loads, 14 Thermo-helix piles are recommended under each tower. Each pile should be installed at least 26 feet below the base of the rigid insulation. The pile embedment depth may require final adjustment to position the existing Thermo-helix pile valve system within the pad insulation layer. Based on discussions with AFI, the valve assembly should not be installed within the slurry or structural fill to reduce the potential for damage to the valve assembly. AFI is providing additional recommendations regarding the valve and pressure relief system. Owing to concerns related to the valve and pressure relief system, the civil plans showing final grading, pad, and insulation elevations and details for the non-pressurized pile sections extending into the reinforced concrete, should be provided prior to site work. A minimum pile-to-pile (centerline) spacing of three (3) pile diameters (with helix) is recommended. The pile with the helix should be installed in an augered hole and backfilled with a slurry made with a clean, well-graded sand and gravel. The bore holes for the piles should be drilled with a dry auger or air rotary drill system. Drilling mud or other fluids should not be used. Thawing of the permafrost with steam, water or other means should not be allowed. If unfrozen soil or free water is encountered that causes caving of the sidewall, we must be notified and the piles must not be installed until we have reviewed the site conditions. Inflow of surface water must also be prevented. The cuttings from the bore holes should be properly disposed off-site. The finish diameter of the pile bore hole should be 6 to 8 inches larger than the outside diameter of the pile and helix. The pile should be protected from corrosion with a fusion- bonded epoxy coating over an aluminum coating that extends approximately three feet below the insulation layer. All piles should be plumb and checked for horizontal and vertical position prior to the placement of the slurry. Piles should be positioned so they have at least 2-inch clearance between the helix and the edge of the hole. The piles should be held in position with wedges or other devices until slurried. To permit temperature measurements, a closed end, 1-inch diameter schedule 40 steel pipe should be installed in each pile bore hole to the full length of the pile. The temperature probe access pipe should be configured to allow for ground temperature measuring strings to be installed from the surface. We recommend the steel temperature access standpipes be welded along all buried connections and end caps. Galvanized pipe sections should be used through the concrete base and for all above grade sections to reduce the potential for surface corrosion and water infiltration into the concrete base. A threaded above grade end cap is recommended. 8.3.3 Pile Slurry After placing and aligning each pile, the hole should be backfilled with a pre-mixed slurry made with a well-graded sand and gravel aggregate and potable water. The material should meet the gradation as discussed below. A representative portion of the slurry aggregate should be submitted to us for testing to assure the aggregate has proper gradation and that freezing point depression materials are not present. The slurry aggregate must be fully thawed prior to mixing and placement. The temperature of the mixed slurry should be at 40°F ± 5°F at time of placement. The slurry should be saturated and have a consistency equivalent to a concrete slump of 5 inches ± 1-inch. The slurry aggregate should contain less than 10 percent material (dry weight basis) finer than the U.S. No. 200 sieve size and should not contain gravel larger than 1-inch size. The slurry should be placed in lifts of approximately 3 feet with each lift being densified with a concrete vibrator as the slurry is placed. Densification is required to assure the slurry completely encases the helix and is in full contact with the pile. The piles should be accurately installed and plumb. The piles should be installed so that the centerline point of the pile is within ± 1/2-inch of the horizontal design location, or °F ± 5°F at time of placement. The slurry should be saturated and have a °F ± 5°F at time of placement. The slurry should be saturated and have a consistency equivalent to a concrete slump of 5 inches ± 1-inch. The slurry aggregate should contain consistency equivalent to a concrete slump of 5 inches ± 1-inch. The slurry aggregate should contain less than 10 percent material (dry weight basis) finer than the U.S. No. 200 sieve size and should not less than 10 percent material (dry weight basis) finer than the U.S. No. 200 sieve size and should not The slurry should be placed in lifts of approximately 3 feet with each lift being densified with a concrete The slurry should be placed in lifts of approximately 3 feet with each lift being densified with a concrete vibrator as the slurry is placed. Densification is required to assure the slurry completely encases the helix vibrator as the slurry is placed. Densification is required to assure the slurry completely encases the helix and is in full contact with the pile. The piles should be accurately installed and plumb. The piles should and is in full contact with the pile. The piles should be accurately installed and plumb. The piles should be installed so that the centerline point of the pile is within ± 1/2-inch of the horizontal design location, or be installed so that the centerline point of the pile is within ± 1/2-inch of the horizontal design location, or Brad Reeve March 11, 2011 Kotzebue Electric Association 10 103-95544 Wind Turbines - Kotzebue as required by the structural engineer. The piles should not be loaded until the sand-water slurry is fully frozen which can be confirmed using a thermistor string in the access pipe. Unfilled bore holes with piles that are placed but not fully slurried must be protected from infilling with snow, water and other deleterious matter. The slurry around the piles must be below 31°F and frozen prior to installing the pile caps and concrete base to allow for minor pile movement during freezeback. 8.3.4 Estimated Settlement For design purposes, we have estimated a total settlement in the range of 1 inch in 20 years. However, this settlement will depend on subsurface conditions, soil temperatures, and the pile installation being consistent with the geotechnical recommendations. 8.3.5 Lateral Capacity The lateral loads will be resisted by passive pressures against the piles, predominantly along the frozen soil portion of the pile. The deflection also depends on whether the piles are fixed or pinned at their top. For the pile foundation recommended above (nominal 16-inch diameter, standard schedule pipe pile (0.375-inch wall)) that is fixed against rotation at its top and laterally loaded to 10 kips at the bottom of the concrete base, the movement at the ground surface will be less than 0.25 inches. If larger lateral loads, smaller pile diameter, or a thinner pile wall section is being considered by the design team, lateral movement may be greater. The bending moment imposed on the piles depends on the lateral load imposed at the top of the pile and the height of the top of the pile above the point of fixity. For the permafrost condition, the pile can be assumed to be a cantilever above the point of fixity, which is considered at the maximum depth of thaw. The depth of thaw depends on the site fill thickness and insulation, design summer thawing index, the soil type(s) from finish grade, and the surface albedo (n-factor). For this site, the point of fixity is considered to be 0.5-foot below the pad insulation. 8.3.6 Pile Installation Considerations It is essential that construction planning for the pile foundations include adequate time after installation to allow the slurry backfill to freeze and the winter cooled ground temperatures around the pile should be thermally protected over the summer to maintain the design ground temperatures by the time the piles are subjected to their design loads. If installation schedules do not provide for this constraint, artificial cooling during the warmer periods may be needed to develop the required ground temperatures prior to fully loading the piles. If conditions restrict the installation depth of any pile to less than recommended design embedment depth, we must be notified and allowed to verify the allowable axial capacity of the pile in question. Any pile installed to a depth of less than the recommended embedment may require colder ground temperatures to achieve the design loads. 8.3.7 Seismic Design Criteria Based on site conditions observed, the proposed project locations are in permafrost and should meet seismic site B B defined as essentially rock with average shear wave velocities between 2,500 and 5,000 feet/second. However, if the permafrost below the insulation warms toward phase change or thaws, a reduced site classification will be required. The criteria are based on mapped spectral response acceleration for short periods (Ss) of 0.43g and mapped spectral response accelerations for a 1 second period (S1) of 0.13g. Site coefficient factors Fa and Fv of 1.0 and 1.0, respectively, are considered appropriate to determine B Based on these values, the design spectral response However, if the permafrost below the insulation warms toward phase change or thaws, a reduced site However, if the permafrost below the insulation warms toward phase change or thaws, a reduced site ) of 0.43g and ) of 0.43g and of 1.0 and 1.0, respectively, are considered appropriate to determine of 1.0 and 1.0, respectively, are considered appropriate to determine Based on these values, the design spectral response Based on these values, the design spectral response Brad Reeve March 11, 2011 Kotzebue Electric Association 11 103-95544 Wind Turbines - Kotzebue acceleration for short period and 1-second period for B equations: SDs =2/3 FaSs and SD1 = 2/3 FvS1 SDs = 0.28g and SD1 = 0.09g Liquefaction of saturated fine-grained soil may occur during seismic events. However, based on our site findings and our general knowledge of the area geology, the risk of liquefaction is considered low. 9.0 CONSTRUCTION CONSIDERATIONS Construction scheduling is a critical element in the installation and performance of the tower system. It is essential that construction scheduling and practices do not result in thawed material below the insulation layer. In addition, the design ground temperatures must be achieved prior to fully loading the foundation piles. We should review the c commencement of site work. Our geotechnical recommendations should be coordinated with AFI and the civil and structural engineering effort as the project design develops and construction scheduling is finalized. A critical element with this foundation system is review and coordination with AFI engineers to verify the existing on-site Thermo-helix piles meet the recommended design requirements and the modified passive cooling system is fully compatible with the recommendations presented herein. Construction sequencing is considered an important element to the foundation performance. The performance of the foundation piles requires the soil along the pile section develop and maintain the recommended design temperatures prior to pile loading. Careful attention to site preparation, pile installation, and fill and insulation placement is advised. Also, the access to the site may be restricted or impossible during breakup. Vehicle access to the site should be confirmed for thawed ground conditions. Delays in fill and/or insulation placement may adversely impact the foundation performance and may require artificial cooling of the Thermo-helix piles. Based on discussions with the design and construction team, we understand the site preparation work and pile installation will occur during the spring of 2011 prior to the initiation of spring thaw. It is essential the site soil does not experience thaw or significant heat gain prior to fill and insulation placement. schedule should provide for means and methods to retain the winter cooled ground temperatures. We have assumed the rigid insulation and frozen aggregate fill will be placed on the site within days of pile and slurry placement. Frozen fill placed above the insulation can be re-worked and compacted after it thaws and prior to concrete placement. If frozen aggregate is placed, compaction as discussed under Section 8.2 is not possible. Accordingly, we recommend the contractor use a performance based compaction method to densify the leveling course if a frozen aggregate is used. This assumes a leveling course of 6 inches or thinner (total thickness) is placed. If so, three passes with a vibratory roller compactor is generally adequate to uniformly densify a 6 inch thick frozen granular fill layer. The frozen aggregate must be mechanically processed into 3-inch minus material prior to placement and compaction. If compaction of a frozen leveling course is planned, on-site observation by the civil or geotechnical engineer is recommended to determine the aggregate is adequately processed and the compaction effort is adequate for the intended performance. If a leveling course thicker than 6-inches is planned, we should be notified well in advance of fill placement to review and discuss options with the owner, design team, and contractor. Alternatively, the contractor may elect to place and compact a thawed leveling course or allow surface albedo to thaw in place the material over a prepared frozen base. It is important that the in-place thawing procedure not result in surface thaw or significant heat gain impact the in-place soils. If a temporary processed into 3-inch minus material prior to placement and compaction. If compaction of a frozen processed into 3-inch minus material prior to placement and compaction. If compaction of a frozen leveling course is planned, on-site observation by the civil or geotechnical engineer is recommended to leveling course is planned, on-site observation by the civil or geotechnical engineer is recommended to determine the aggregate is adequately processed and the compaction effort is adequate for the intended determine the aggregate is adequately processed and the compaction effort is adequate for the intended performance. If a leveling course thicker than 6-inches is planned, we should be notified well in advance performance. If a leveling course thicker than 6-inches is planned, we should be notified well in advance Alternatively, the contractor may elect to place and compact a thawed leveling course or allow surface Alternatively, the contractor may elect to place and compact a thawed leveling course or allow surface albedo to thaw in place the material over a prepared frozen base. It is important that the in-place thawing albedo to thaw in place the material over a prepared frozen base. It is important that the in-place thawing procedure not result in surface thaw or significant heat gain impact the in-place soils. If a temporary procedure not result in surface thaw or significant heat gain impact the in-place soils. If a temporary Brad Reeve March 11, 2011 Kotzebue Electric Association 12 103-95544 Wind Turbines - Kotzebue insulation cover is planned with the intention to remove the insulation and place additional fill after breakup, we should be notified in order to review and comment on the proposed construction sequencing and methodology. We also recommend rigid insulation be installed to retain the winter cooled ground temperatures. This may require air cargo transporting insulation to Kotzebue, if the recommended insulation is not locally available. We should be contacted if the recommended insulation is not expected to be placed and secured from damage this spring. 10.0 USE OF REPORT This report has been prepared for the use of KEA for the proposed wind turbine projects in Kotzebue, Alaska as discussed in this report. If there are significant changes in the nature, design, or location of the facilities, we should be notified so that we may review our conclusions and recommendations in light of the proposed changes and provide a written modification or verification of the changes. There are possible variations in subsurface conditions and ground temperatures between explorations and also with time. Therefore, observations and testing by a qualified geotechnical professional should be included during construction to provide corrective recommendations adapted to the conditions revealed during the work. Unanticipated soil conditions are commonly encountered and cannot fully be determined by a limited number of explorations or soil samples. Such unexpected conditions may result in additional project costs in order to construct the project as designed. Therefore, a contingency for unanticipated conditions should be included in the construction budget and schedule. The work program followed the standard of care expected of professionals undertaking similar work in the State of Alaska under similar conditions. No warranty expressed or implied is made. 11.0 CLOSING It has been a pleasure to work with you on this project. Please feel free to contact us with any further questions. GOLDER ASSOCIATES INC. Melanie M. Hess Richard A. Mitchells, PE Staff Scientist Senior Geotechnical Consultant Attachments: Figure 1 Project Vicinity Map Figure 2 Site Map Figure 3 Soil Classification / Legend Figure 4 Frozen Soil Classification / Legend Appendix A December 2010 Exploration Appendix B May 2010 Exploration MMH/RAM/mlp FIGURES KEA/KEA WIND TURBINES / AK 1 PROJECT VICINITY MAP KEA WIND TURBINES KOTZEBUE, ALASKA REFERENCE USGS TOPOGRAPHIC MAPS, KOTZEBUE (D-5 AND D-2), AK, 1951 MAP GENERATED USING NATIONAL GEOGRAPHIC TOPO! SOFTWARE PROJECT LOCATION PROJECT LOCATION DRAFT KEA/KEA WIND TURBINES / AKKEA/KEA WIND TURBINES / AK PROJECT VICINITY MAPPROJECT VICINITY MAP KEA WIND TURBINESKEA WIND TURBINES KOTZEBUE, ALASKAKOTZEBUE, ALASKA DRAFTDRAFT K10-01 K10-02 G10-KEA-01 G10-KEA-02 KW-1 KW-2 KW-3 KW-4 KW-5 ROAD TO KOTZEBUE CONTROL BUILDING K10-03 K10-04 K10-05 CADD DATE REV. DATE CHECK SCALE FILE No. PROJECT No. TITLE KEA / KEA WIND TURBINES / AK 2 SITE MAP KEA WIND TURBINES KOTZEBUE, AK BOREHOLE_NAD83.dwg 103-95444 AS SHOWN DBC/APG 3/11/2011 0 FIG. DRAFT 0600 600 SCALE FEET NOTES LEGEND REFERENCE BASE IMAGE BY DIGITALGLOBE DATED 05-31-2007 FROM GOOGLE EARTH BOREHOLE LOCATIONS DETERMINED IN THE FIELD WITH A HANDHELD GPS KW-5 G-KEA10-01 DECEMBER 2010 GOLDER BOREHOLES NAME AND APPROXIMATE LOCATIONS 2001 DMA BOREHOLES NAME AND APPROXIMATE LOCATION K10-01 MAY 2010 GOLDER BOREHOLES NAME AND APPROXIMATE LOCATIONS KEA / KEA WIND TURBINES / AKKEA / KEA WIND TURBINES / AK SITE MAPSITE MAP KEA WIND TURBINESKEA WIND TURBINES KOTZEBUE, AKKOTZEBUE, AK DRAFTDRAFT APPENDIX A DECEMBER 2010 EXPLORATION APPENDIX B MAY 2010 EXPLORATION