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HomeMy WebLinkAboutUtility-Scale Railbelt Wind Final Design and Permitting Project - Preliminary Geotechnical Information for Little Mount Susitna 2022 - REF Grant 7015021Project No. 108246-001- Preliminary Geotechnical Information - West Susitna Wind Energy Project 5430 Fairbanks Street Suite 3 Anchorage, Alaska 99518-1263 907 561-2120 Fax 206 695-6777 www.shannonwilson.com July 14, 2022 Mr. Matthew Perkins Alaska Renewables, LLC 2595 Allen Adale Road Fairbanks, Alaska 99709 RE: PRELIMINARY GEOTECHNICAL INFORMATION, WEST SUSITNA WIND ENERGY PROJECT, BELUGA, ALASKA Dear Mr. Perkins: At your request, we have developed a preliminary geotechnical framework for the potential West Susitna Wind Energy project on Little Mount Susitna, north of Beluga, Alaska. The information contained herein is intended to be used by you to provide a preliminary understanding of the conditions that may be encountered within the project limits. It is also intended to be used by you to form an understanding of potential geotechnical considerations that will need to be addressed during design and construction of the project. Note that the included narrative is based on our generalized understanding of the project and regional conditions, our experience in the area with past projects, and limited data research. It is likely that other conditions exist and/or other design and construction challenges could be encountered if the project progresses. While this document provides a preliminary description of likely conditions, subsequent phases and final design will be based on site specific geotechnical explorations and analysis. Shannon & Wilson makes no warranties on the accuracy of the information contained herein and this document should not be interpreted as a guarantee of the conditions that will be encountered for this project. Those who rely on this information do so at their own risk. PROJECT DESCRIPTION The land around and on Little Mount Susitna is being considered as a site for wind power generation. At full build-out the project would include more than 60 wind turbines on Little Mount Susitna along with required power storage and transmission infrastructure required to deliver the generated power to the existing Chugach Electric Associations (CEA) grid. The additional infrastructure will likely include battery banks, access roads, transmission lines, substations and other small buildings needed to maintain and run the new facility. The connection point is located on the existing CEA high voltage transmission lines that connect Beluga Power Plant to Point MacKenzie, approximately 10 miles south of Little Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 2 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 Mount Susitna and about 11 miles northwest of the Beluga Power Plant. The project lies on the south end of the Susitna Lowlands characterized by transitional topography between mountainous regions to the north and alluvial and coastal lowlands to the south and east. REGIONAL GEOLOGY The Susitna Lowlands are bound on the north and west by the Alaska Range and bound on the east by the Talkeetna Mountains. The topography is generally flat to rolling hills and gains relief near the foothills of the Alaska Range. Little Mount Susitna, Mount Susitna (to the east) and Beluga Mountain (to the north) form a triangular-shaped area of upland/hilly terrain on the west side of the Susitna River flats. Evidence indicates that at least five glaciations played a part in the landscape and deposits that form the lowlands. Glacially carved bedrock, moraines, drumlins, and kettle lakes are a few of the landforms found and continuous erosional processes are constantly reshaping the land. The following sections discuss the bedrock geology, soil stratigraphy, tectonics, seismicity, and permafrost conditions that can be expected in the region and at the project site. Bedrock Geology Beneath the Quaternary surficial deposits, the bedrock geology consists primarily of Tertiary deposits of the Kenai Group overlying a pre-Tertiary basement complex. The Kenai group represents clastic forearc basin deposits of early and late Cenozoic tectonic cycles and the rocks are characteristic of a fluvial system. The Kenai Group is dominated by sedimentary rocks including sandstone, claystone, siltstone, conglomerate, and coal beds; with the thickest coal beds in the Beluga and Tyonek formations. These sedimentary rocks likely dominate the low-lying areas within the project limits. Little Mount Susitna (as well as Beluga Mountain and Mount Susitna) is known to consist of Paleocene magmatic intrusive rocks. Rock exposed on Little Mount Susitna has been mapped as granitic plutons. Quaternary Deposits Potentially thick sequences of Quaternary sediments derived from glacial and erosional processes are likely present in some portions of the project area with the thickest deposits located in the lower elevations. Glacially derived soil (glacial drift) materials may consist of till, outwash, and glaciolacustrine sediments. Glacial till is typically randomly sorted and consists of relatively equal fractions of silt, sand, and gravel, along with some cobble- and boulder-sized particles. Outwash materials generally consist of cleaner sand and gravels Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 3 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 that may be well or poorly graded. More recent deposits of sand and gravel are likely present in localized areas near active streams. Steep slopes may be covered or skirted by talus as frost wedging pries the bedrock apart. In addition, ash layers have been observed up to 3 feet thick in some places and vegetation suggests that it may have fallen more than 100 to 200 years ago. Within this project area, geologic mapping indicates that bedrock is exposed at or near the ground surface for the higher elevations of Little Mount Susitna indicating that there is likely very little soil overburden in the area where most of the proposed towers are located. It is likely that very little soil overburden exists in steeply sloping areas. Surficial geology maps indicate glacial soils (likely glacial till) cover bedrock at lower elevations of Little Mount Susitna on its flanking slopes and in other low-lying areas. The thickness of the soil overburden is likely highly variable, with gradually increasing thickness at lower elevations. Tectonics and Seismicity The project region is one of the most seismically active areas in the U.S. and historically subjected to relatively large earthquakes. According to the Alaska Earthquake Information Center, three large (greater than magnitude 7) earthquakes have occurred within or near the project area in 1933, 1943, and 2018. Several hundred smaller earthquakes have also been recorded since 1899. On a large scale, the tectonics and active seismicity of the region are the result of ongoing north-northwest movement of the Pacific Plate relative to the North American Plate. The relative movement results in a region of right lateral strike-slip faulting along the eastern margin of the Gulf of Alaska and subduction along the central and western margins of the gulf. Along the eastern margin of the Gulf of Alaska (+ 250 miles southeast of the project area), the relative right-lateral movement between the plates is accommodated primarily by northwest-striking high-angle strike-slip faults (i.e., Fairweather and Queen Charlotte Faults). The right-lateral movement is translated northwest of the gulf into the interior of Alaska along the right lateral Denali Fault system (DFS), which extends through the southern Alaska Range to the north of the project area. In terms of engineering significance, three broad seismic sources may present hazards in the project area. Nearest the project area, the DFS was responsible for the 2002, magnitude 7.9 Denali Fault earthquake. Associated surface rupture was documented for hundreds of Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 4 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 kilometers along the fault trace. The Alaska-Aleutian Subduction Zone, a mega-thrust source at the interface between the North American and Pacific Plates was the source of the 1964, magnitude 9.2 Great Alaska Earthquake and the magnitude 7.1 Anchorage Earthquake. The Castle Mountain Fault is a mapped, active shallow crustal fault that traverses the south end of the Susitna Lowlands (trending east-northeast to west-southwest) approximately midway between the proposed windfarm development area and the existing CEA transmission lines. It is postulated that the Castle Mountain fault is capable of producing earthquake magnitudes up to 7.5 and displacement along the Castle Mountain Fault is visible in the landforms across the Susitna Flats. Given the various sources of ground motions that are associated with this part of Alaska, the types of shaking during seismic events can vary significantly. In addition to the magnitude of the events, the peak ground accelerations as well as duration of shaking can have a significant impact on design of the proposed facilities. Seismic events along the Benioff zone of the Alaska-Aleutian Subduction Zone that lie directly beneath the project area are generated from very deep sources (typically greater than 120 miles below the ground surface). This shaking is caused by large scale rupture along the subducting and overriding plates and is typically characterized by relatively low peak ground acceleration, low frequency, and long duration seismic events. Such long duration shaking typically effect very large regions and can have significant impacts on soils sensitive to strength loss and liquefiable soils. Shallow-source earthquakes such as the Castle Mountain Fault tend to create high frequency, high acceleration, and relatively short duration shaking events. Such events can impart very high stresses of structures, can result in significant displacements along the surface expression of the fault, and can also result in localized soil strength loss and liquefaction. Lateral extent of shaking from these sources is typically limited to areas relatively close to the portion of the fault that ruptures. Permafrost Permafrost is defined as soil or rock beneath the ground surface where a temperature below 32 degrees Fahrenheit has existed for two or more years. Most of the area within the project limits is likely generally free from permafrost. Isolated zones and discontinuous permafrost could be found in localized areas of high elevation or on north facing slopes. The thickness of the active layer (the near-surface ground that undergoes an annual freeze-thaw cycle) is Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 5 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 largely dependent on soil type, ground cover, and snow depth. In general, the active layer across the project is likely within the upper 10 feet below the ground surface. GEOTECHNICAL CONSIDERATIONS Planning and design of these facilities will need to account for near-surface geological conditions and other environmental effects like climate and seismicity. Major components of this work will include foundations for turbine towers on Little Mount Susitna, foundations for support structures and substations at other locations, support for transmission line poles, and support for access roads. The information below is intended to be used for general project planning and should not be used for final design. Design level geotechnical recommendations for this project should be based on site specific geotechnical explorations. Foundations Based on the available information turbine towers will be located in areas where bedrock is either exposed at the ground surface or will be very near the surface. Where present, soil overburden in these areas likely consists of relatively thin layers of surface organics and glacial till soils. As such, we believe that subsurface conditions for the proposed towers will be favorable for development of shallow foundations. In zones of shallow bedrock, foundations may need to be anchored to bedrock to resist uplift and overturning. In areas where glacial soils are thicker than approximately 10 feet, we anticipate those soils to be relatively dense and capable of carrying tower foundation loads, however, they are likely frost susceptible. As such, shallow foundations bearing on soil will likely need to incorporate mitigation for seasonal freezing and thawing of support soils. Because of the likely dense soil and/or bedrock foundation support for the towers, we do not anticipate significant risk of seismically induced hazards such as liquefaction, lateral spreading, or settlement. Additionally, given the distance to known faulting, we do not anticipate ground rupture to be a risk at the proposed tower locations. Slopes on the north and west side of Little Mount Susitna appear to be relatively steep in some areas and slope stability will likely need to be considered when developing in those areas. However, given the anticipated favorable soil and bedrock conditions, it is likely that if slope issues exist, they will be readily accommodated through moderate slope setbacks. Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 6 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 Foundations for other project features such as buildings and transmission poles will need to account for variable conditions depending on where they are located. We anticipate that foundation conditions will generally improve with increasing elevation, but that even in low-lying areas, we believe that project features will be able to be located or aligned such that shallow foundations will be appropriate. Isolated areas may need to accommodate seismic hazards such as liquefaction or settlement, but we do not anticipate that these conditions will be to the magnitude such that deep foundations such as driven piles or drilled shafts will be needed. Access Development Conditions that will support roadways will be variable throughout the project site. Roads in upland areas near the wind farm will have good support on bedrock or dense, well-drained, glacial soils. Lower elevations will have variable support conditions and roadway design will need to consider isolated areas of poorly drained soils, potential boggy areas with thick surface organics, and frost susceptible soils. While these conditions can provide challenges during design and construction, we do not anticipate that they will be extraordinarily difficult in comparison to what industrial road developments typically encounter in Southcentral Alaska. Given that the gravel roadways servicing the area will need to support heavily loaded vehicles, robust structural sections will be needed. We believe that the anticipated subsurface conditions will be capable of supporting such roadways reasonably. Geotextiles or geogrid can be used in areas to improve stability of the roadway and embankments in isolated areas of poor subgrade support. Wintertime work can also be used for initiating embankment construction in areas of poor support to improve stability conditions and reduce environmental impact. Geosynthetics may also be useful in reducing overall structural section thickness and reduce cost through reduction in fill quantities. Bridge and/or large diameter culvert structures will likely be needed at several locations along the access road. We believe that in general, support conditions for these types of structures will be relatively good and shallow foundations will likely be sufficient to support the features. If shallow foundations are used, revetment will be needed to ensure scour does not undermine foundation elements. If near surface and scour conditions are not amenable to development of shallow foundations, driven pile foundations will be appropriate for structure support. Based on the available information, we do not anticipate that pile foundations would need to be driven to extraordinary depths as we anticipate Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 7 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 relatively dense alluvial and/or glacial soils likely exist within the upper 20 to 40 feet in most of the low-lying areas. A barge landing will likely be needed at the tidewater terminus of the access road. Due to the relatively shallow beach slope, the landing will likely need to be developed such that barges go dry during low tide, but that they can still be unloaded during this time. We anticipate that the beach conditions should be capable of supporting a beached barge and minimal work will be needed to develop the ramp and beach erosion protection to ensure the long-term viability of the asset. Borrow Materials Quality fill material availability will be a key component of this project. It is our opinion that the wind tower site and access road alignment is situated on and traverses land that will yield, on average, high quality construction materials. Because of the granitic bedrock geology of Little Mount Susitna, there will be a plentiful source of high quality, relatively low cost, durable aggregate to be used for construction. This material will be useful as embankable fill, high quality structural section and surface aggregate, and likely as large diameter armor stone that can be used as revetment at stream crossings and the barge landing. The low-lying portions of the access road traverses ground that is expected to range from glacial till to outwash sand and gravel. It is likely that much of the mineral soils encountered during this project will be embankable and much of the outwash material may be of sufficient quality for structural section development. We anticipate that multiple borrow sources will be able to be identified along the access road route such that hauling costs should be manageable during construction. CLOSURE AND LIMITATIONS This letter was prepared for the exclusive use of our client and their representatives for evaluating the site as it relates to the geotechnical aspects discussed herein. The conclusions contained in this report are based on assumed conditions formed through area experience and limited data research. Additional work will be needed to refine our conclusions and recommendations. Unanticipated conditions are commonly encountered. Such unexpected conditions frequently require that additional expenditures be made to attain a properly designed and constructed project. Therefore, some contingency fund is recommended to accommodate Mr. Matthew Perkins Alaska Renewables, LLC July 14, 2022 Page 8 of 8 Preliminary Geotechnical Information - West Susitna Wind Energy Project Project No. 108246-001 such potential extra costs. Shannon & Wilson has prepared the attachment Important Information About Your Geotechnical/Environmental Report to assist you and others in understanding the use and limitations of the reports. Copies of documents that may be relied upon by our client are limited to the printed copies (also known as hard copies) that are signed or sealed by Shannon & Wilson with a wet, blue ink signature. Files provided in electronic media format are furnished solely for the convenience of the client. Any conclusion or information obtained or derived from this document shall be at the user’s sole risk. If there is a discrepancy between the electronic files and the hard copies, or you question the authenticity of the report please contact the undersigned. We appreciate this opportunity to be of service. Please contact the undersigned at (907) 561- 2120 with questions or comments concerning the contents of this report. Sincerely, SHANNON & WILSON Kyle Brennan, PE Vice President Enc. Important Information about your Geotechnical/Environmental Report Page 1 of 2 3/2004 SHANNON & WILSON, INC. Geotechnical and Environmental Consultants Attachment to 108246-001 Date: July 2022 To: Alaska Renewables, LLC Re: West Susitna Wind Energy Important Information About Your Geotechnical/Environmental Report CONSULTING SERVICES ARE PERFORMED FOR SPECIFIC PURPOSES AND FOR SPECIFIC CLIENTS. Consultants prepare reports to meet the specific needs of specific individuals. A report prepared for a civil engineer may not be adequate for a construction contractor or even another civil engineer. Unless indicated otherwise, your consultant prepared your report expressly for you and expressly for the purposes you indicated. No one other than you should apply this report for its intended purpose without first conferring with the consultant. No party should apply this report for any purpose other than that originally contemplated without first conferring with the consultant. THE CONSULTANT'S REPORT IS BASED ON PROJECT-SPECIFIC FACTORS. A geotechnical/environmental report is based on a subsurface exploration plan designed to consider a unique set of project-specific factors. Depending on the project, these may include: the general nature of the structure and property involved; its size and configuration; its historical use and practice; the location of the structure on the site and its orientation; other improvements such as access roads, parking lots, and underground utilities; and the additional risk created by scope-of-service limitations imposed by the client. To help avoid costly problems, ask the consultant to evaluate how any factors that change subsequent to the date of the report may affect the recommendations. Unless your consultant indicates otherwise, your report should not be used: (1) when the nature of the proposed project is changed (for example, if an office building will be erected instead of a parking garage, or if a refrigerated warehouse will be built instead of an unrefrigerated one, or chemicals are discovered on or near the site); (2) when the size, elevation, or configuration of the proposed project is altered; (3) when the location or orientation of the proposed project is modified; (4) when there is a change of ownership; or (5) for application to an adjacent site. Consultants cannot accept responsibility for problems that may occur if they are not consulted after factors, which were considered in the development of the report, have changed. SUBSURFACE CONDITIONS CAN CHANGE. Subsurface conditions may be affected as a result of natural processes or human activity. Because a geotechnical/environmental report is based on conditions that existed at the time of subsurface exploration, construction decisions should not be based on a report whose adequacy may have been affected by time. Ask the consultant to advise if additional tests are desirable before construction starts; for example, groundwater conditions commonly vary seasonally. Construction operations at or adjacent to the site and natural events such as floods, earthquakes, or groundwater fluctuations may also affect subsurface conditions and, thus, the continuing adequacy of a geotechnical/environmental report. The consultant should be kept apprised of any such events, and should be consulted to determine if additional tests are necessary. MOST RECOMMENDATIONS ARE PROFESSIONAL JUDGMENTS. Site exploration and testing identifies actual surface and subsurface conditions only at those points where samples are taken. The data were extrapolated by your consultant, who then applied judgment to render an opinion about overall subsurface conditions. The actual interface between materials may be far more gradual or abrupt than your report indicates. Actual conditions in areas not sampled may differ from those predicted in your report. While nothing can be done to prevent such situations, you and your consultant can work together to help reduce their impacts. Retaining your consultant to observe subsurface construction operations can be particularly beneficial in this respect. A REPORT'S CONCLUSIONS ARE PRELIMINARY. Page 2 of 2 3/2004 The conclusions contained in your consultant's report are preliminary because they must be based on the assumption that conditions revealed through selective exploratory sampling are indicative of actual conditions throughout a site. Actual subsurface conditions can be discerned only during earthwork; therefore, you should retain your consultant to observe actual conditions and to provide conclusions. Only the consultant who prepared the report is fully familiar with the background information needed to determine whether or not the report's recommendations based on those conclusions are valid and whether or not the contractor is abiding by applicable recommendations. The consultant who developed your report cannot assume responsibility or liability for the adequacy of the report's recommendations if another party is retained to observe construction. THE CONSULTANT'S REPORT IS SUBJECT TO MISINTERPRETATION. Costly problems can occur when other design professionals develop their plans based on misinterpretation of a geotechnical/environmental report. To help avoid these problems, the consultant should be retained to work with other project design professionals to explain relevant geotechnical, geological, hydrogeological, and environmental findings, and to review the adequacy of their plans and specifications relative to these issues. BORING LOGS AND/OR MONITORING WELL DATA SHOULD NOT BE SEPARATED FROM THE REPORT. Final boring logs developed by the consultant are based upon interpretation of field logs (assembled by site personnel), field test results, and laboratory and/or office evaluation of field samples and data. Only final boring logs and data are customarily included in geotechnical/environmental reports. These final logs should not, under any circumstances, be redrawn for inclusion in architectural or other design drawings, because drafters may commit errors or omissions in the transfer process. To reduce the likelihood of boring log or monitoring well misinterpretation, contractors should be given ready access to the complete geotechnical engineering/environmental report prepared or authorized for their use. If access is provided only to the report prepared for you, you should advise contractors of the report's limitations, assuming that a contractor was not one of the specific persons for whom the report was prepared, and that developing construction cost estimates was not one of the specific purposes for which it was prepared. While a contractor may gain important knowledge from a report prepared for another party, the contractor should discuss the report with your consultant and perform the additional or alternative work believed necessary to obtain the data specifically appropriate for construction cost estimating purposes. Some clients hold the mistaken impression that simply disclaiming responsibility for the accuracy of subsurface information always insulates them from attendant liability. Providing the best available information to contractors helps prevent costly construction problems and the adversarial attitudes that aggravate them to a disproportionate scale. READ RESPONSIBILITY CLAUSES CLOSELY. Because geotechnical/environmental engineering is based extensively on judgment and opinion, it is far less exact than other design disciplines. This situation has resulted in wholly unwarranted claims being lodged against consultants. To help prevent this problem, consultants have developed a number of clauses for use in their contracts, reports and other documents. These responsibility clauses are not exculpatory clauses designed to transfer the consultant's liabilities to other parties; rather, they are definitive clauses that identify where the consultant's responsibilities begin and end. Their use helps all parties involved recognize their individual responsibilities and take appropriate action. Some of these definitive clauses are likely to appear in your report, and you are encouraged to read them closely. Your consultant will be pleased to give full and frank answers to your questions. The preceding paragraphs are based on information provided by the ASFE/Association of Engineering Firms Practicing in the Geosciences, Silver Spring, Maryland