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Geotechnical Data Report Moose Creek Dam Safety Modification (CHL023) Chena River Lakes Flood Control Project, North Pole, Alaska Alaska District, Pacific Ocean Division

February 2021 Status: Final

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A rm y C o rp s o f E n g in ee rs

A la sk a

D is tr ic t

RFP MOOSE CREEK DAM SAFETY MODIFICATION

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CHL023

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DEPARTMENT OF THE ARMY

U.S. ARMY CORPS OF ENGINEERS, ALASKA DISTRICT

P.O. BOX 6898

JBER, AK 99506-0898

CEPOA-EC-G-GM February 2021

MEMORANDUM FOR CEPOA-CW-PM, Steve Howard

SUBJECT: Geotechnical Data Report for Moose Creek Dam (US Dam ID: AK00085), Geotechnical Investigations 2010 through 2020, North Pole, Alaska

1. Enclosed is the Moose Creek Dam Safety Modification Geotechnical Data Report for recent field investigations at Moose Creek Dam, located in North Pole, Alaska. Included with this report are the results of field investigations from 2010 through 2020, project maps and drawings, exploration logs, and soil laboratory testing results.

2. Questions should be addressed to Coleman Chalup (907) 753-2686 or John Rajek (907) 753- 5695.

COLEMAN CHALUP, P.E.

Dam Safety Program Manager Lead Engineer, Moose Creek Dam Mod.

CEPOA-EC-G-GM

JOHN J. RAJEK, P.E.

Chief, Geotechnical and Materials Section

CEPOA-EC-G-GM

DOUGLAS A. BLISS, P.E., P.G.

Chief, Geotechnical and Engineering Services Branch

CEPOA-EC-G

//S//

//S//

//S//

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Moose Creek Dam Modification, North Pole, Alaska i

U.S. Army Corps of Engineers

TABLE OF CONTENTS

1. INTRODUCTION

2. LOCATION AND PROJECT DESCRIPTION

3. PREVIOUS GEOTECHNICAL INVESTIGATIONS

4. REGIONAL GEOLOGY

5. GEOTECHNICAL FIELD EXPLORATIONS

5.1 Geotechnical Exploration 2010

5.2 Piezometer & Thermistor Installation 2010/2011

5.3 Piezometer Replacement 2012

5.4 Floodway Tree Island Test Pits 2012

5.5 Piezometer Replacements 2013

5.6 Geophysical and Geotechnical Explorations 2017

5.7 Geotechnical Exploration 2020

5.8 Test Boring Locations

6. LABORATORY TESTING AND SOIL CLASSIFICATION

6.1 Barrier Wall Mix Design

6.2 Laboratory Data Report

6.3 Chemical Data Report

7. SITE CONDITIONS

7.1 Surface Conditions

7.2 Subsurface Conditions

7.3 Moose Creek Dam Fill Soils

7.4 Moose Creek Dam Foundation Soils

7.5 Bedrock

7.6 Groundwater

7.7 Ground Temperatures

7.8 Seasonally Frozen Soils

7.9 Permafrost Soils

APPENDIX A – LOCATION MAPS

Project Location and Vicinity Map .................................................................................... Sheet A-1

Test Boring Location Map ...................................................................... Sheets B-401 to B-403

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Moose Creek Dam Modification, North Pole, Alaska ii

U.S. Army Corps of Engineers

APPENDIX B – EXPLORATION LOGS

Key to Exploration Logs ............................................................................................... 1 Sheet

2010 Deep Exploration Logs (AP-2885 to AP-2893) 2010/2011 Exploration Logs (AP-2894-P to AP-2920-P) 2012 Exploration Logs (AP-2921-P to AP-2927-P) 2013 Exploration Logs (AP-2928-P to AP-2933-P) 2012 Exploration Logs (TP-2934 to TP-2941) 2017 Exploration Logs (AP-2943 to AP-3014) 2020 Exploration Logs (AP-3027 to AP-3062)

APPENDIX C – LABORATORY TESTING RESULTS

Test Boring Naming Cross-Reference .................................................................................... 1 Page 2010/2011 Laboratory Data (AP-2894-P to AP-2920-P) ................................................. 348 Pages 2012 Laboratory Data (AP-2921-P to AP-2927-P) ............................................................ 61 Pages 2013 Laboratory Data (AP-2928-P to AP-2933-P) ............................................................ 50 Pages 2012 Laboratory Data for Tree Islands (TP-2934 to TP-2941) .......................................... 17 Pages 2017 Laboratory Data (AP-2943 to AP-2993 and AP-3013 to AP-3014) ........................ 641 Pages

2017 Laboratory Data (List 1) ................................................................................ 37 Pages 2017 Laboratory Data (List 2-3) .......................................................................... 106 Pages 2017 Laboratory Data (List 3-1) .......................................................................... 147 Pages 2017 Laboratory Data (List 4) ................................................................................ 48 Pages 2017 Laboratory Data (List 5) .............................................................................. 143 Pages 2017 Laboratory Data (List 6-1) .......................................................................... 170 Pages 2017 Laboratory Data (List 7-3) ............................................................................ 28 Pages

2020 Laboratory Data (AP-3027 to AP-3062) ................................................................ 216 Sheets

ANNEX

Annex A - Seismic Hazard Assessment CHL023 Moose Creek Dam Modification, 2020 Annex B - Geophysical Investigation of Subsurface Features at the Moose Creek Dam, July 2018 Annex C - Moose Creek Dam Weir Geotechnical Data Report, Golder Associates Inc., 2017 Annex D - Geophysical Survey Report Chena River Lakes Flood Control Project, Barrier Wall

Modification, Golder Associates Inc., November 2020 Annex E - Moose Creek Dam Mix Design Report, Golder Associates Inc., 2018 Annex F - Laboratory Data Report, CHL023 Moose Creek Dam Modification, R&M, September

Annex G - Chemical Data Quality Review Moose Creek Dam Borrow Pit Sampling, USACE 2020 Annex H - TR-19-26-2020 Web-Based Monitoring of Piezometers for USACE Moose Creek

Dam, November 2020 Annex I - Letter Report No. 21 Permafrost Effects on Under Seepage of Moose Creek Dam, 1987

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Moose Creek Dam Modification, North Pole, Alaska 1

U.S. Army Corps of Engineers

1. Introduction The purpose of this report is to identify surface and subsurface conditions and address geotechnical issues associated with the proposed Moose Creek Dam Modification project (CHL023) for the installation of a mix-in-place barrier wall located at the Chena Flood Control Project, North Pole, Alaska. This report presents data from multiple geotechnical investigations performed from 2010 through 2020 and a summary of the findings based on site observations and results of the field exploration and laboratory testing.

2. Location and Project Description The 7.5-mile-long Moose Creek Dam is located on the Chena Flood Control Project just east of North Pole, Alaska. The Moose Creek Dam consists of an earth-filled embankment and concrete control structure with four gated bays used to regulate high flows on the Chena River. In non-operational mode, the Dam does not impound water and the Chena River flows unregulated through the control structure. During operations, the control structure is used to regulate the discharge of the Chena River and water is stored in the floodway upstream of the Dam.

The proposed modification project includes the construction of a centerline barrier wall within the Moose Creek Dam starting just south of the Chena Project Office near Dam station 235+00 and extending north to station 490+00 at the North Abutment. The purpose of the barrier wall is to reduce the risk of potential backward erosion and piping through the Moose Creek Dam’s foundation soils. The barrier wall will consist of a mix-in-place Portland Cement Concrete and bentonite barrier wall, located on the Moose Creek Dam crest slightly upstream of the centerline.

Depth of the barrier wall will vary from 61 to 67 feet below the crest of the Dam. These features are illustrated in the project vicinity map located on sheet A-1 in Appendix A.

3. Previous Geotechnical Investigations The U.S. Army Corps of Engineers began conducting geotechnical investigations of multiple different sites for a potential Flood Control project on April 4, 1962. Since then, subsurface investigations have been conducted for several different projects or subprojects of the Moose Creek Dam. As of 2020, approximately 3,000 locations have been drilled or excavated by test pits.

The drilling methods varied from hand auguring to the use of drill rigs employing varying drilling methods and techniques to perform geotechnical test holes. Excavation of test pits has been performed with either heavy equipment or manual digging with hand tools. The depths of these test borings and pits vary in depth from between one to 618 feet below the ground surface. The test borings were designated permanent numbers with the prefix AH (auger hole) and DH (drill hole), which was later standardized to the prefix AP (auger/probe). Instrumentation was installed after some of the test borings were completed. When instrumentation was installed, the permanent AP number is designated with a suffix. The suffixes are either P for Piezometer or TH for Thermistor. (Examples, AP-xxxx-P, AP-xxxx-TH). This labeling convention is used on the exploration logs for applicable borings in Appendix B.

4. Regional Geology The Moose Creek Dam is located on the boundary between the Tanana-Kuskokwim lowland and the Yukon-Tanana Upland sections of the Northern Plateaus province of the Intermontane Plateaus physiographic division. The Moose Creek Dam is approximately 16 miles southeast of Fairbanks

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Moose Creek Dam Modification, North Pole, Alaska 2 in the essentially flat Tanana River basin. The basin transcends to the lowermost foothills of the Yukon-Tanana Upland to the north-northeast. These foothills consist of a series of low rounded hills and ridges. The Upland is a maturely eroded plateau generally underlain by metamorphosed schistose bedrock of Pre-Cambrian age.

During the Pleistocene and Holocene periods, cycles of deposition and erosion occurred in the Tanana basin as a result of changes in climate condition. Deposition occurred during times of glacial advance and erosion of fine material during glacial recession. As a result, the basin aggraded with alluvial deposits (fluvial and glaciofluvial) of gravel, sand, and silt.

These deposits create a complex stratigraphic mosaic that extends to bedrock and ranges from a few feet in thickness near the foothills to more than 600 feet near the Tanana River (maximum depth not established). The surficial alluvial deposits are Holocene age, while the deeper deposits are primarily of Pleistocene age. The Tanana River and its southern glacier-fed tributaries from the Alaska Range formed these deposits during the aggradation of the basin. This aggradation, migrating northward, effectively pushed the Tanana River against and near the Yukon-Tanana Upland.

During the periods of glacial advance, vast amounts of silt and silty fine sand were wind borne from the flood plain and deposited as a blanket of loess on the upland ridges. This blanket ranges from a few inches in thickness on ridge summits to more than 160 feet on lower and mid slopes.

Some of these fines have since been re-transported both by wind and fluvial processes to the creek bottoms and the basin floor. Currently, in areas not subject to erosion, a mantle of silt or silty sand overlies the alluvial deposits of the basin to depths ranging from about three to greater than 20 feet thick.

Bedrock at the Moose Creek Dam site has been mapped as the Keevy Peak Formation. The age of the formation ranges from early Mississippian to the Devonian and consists of a siliceous and carbonaceous assemblage of phyllite, meta-argillite, quartzite and possible metachert that has undergone lower to upper greenschist facies metamorphism. At the project site the Keevy Peak Formation daylights at the surface in outcrops on the north abutment of the Moose Creek Dam, and near the south end on the upstream side of the Dam.

Permafrost is defined as ground, soils or bedrock that has remained at or below the freezing point 32°F/0°C continuously for a minimum of two years. This definition of permafrost is based solely on temperature and is not dependent on moisture content, or if visible ice is present. Alaska is generally divided into three zones of permafrost: continuous, discontinuous, and sporadic. The continuous zone is generally located in northern portions of the state. The discontinuous zone covers central Alaska, which includes Moose Creek Dam. The presence of permafrost is widespread throughout the interior with variable occurrence. The sporadic zone contains little to no permafrost and generally encompasses the southern portion of the state and the Aleutian chain.

Where present in the discontinuous zone, permafrost is encountered below the active layer up to depths of approximately 450 feet below the ground surface in the Fairbanks North Star Borough area. This zone frequently occurs in multiple layers of varying thickness separated by taliks. At the Moose Creek Dam project site when permafrost is present in coarse-grained soils, segregated

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Moose Creek Dam Modification, North Pole, Alaska 3 ice or large masses of ice are uncommon. Ice content in the surficial silts and silty sands is generally much higher than that of the coarse-grained soils. Frequently in fine-grained soils ice lenses form that vary from under an inch to several feet in thickness.

The highly permeable sands and gravels of the Tanana basin alluvial plain form a prolific aquifer.

The extent of the alluvium deposit is vast, this results in enormous water storage capacity with transmissivity of the alluvium to be several hundred thousand gallons per day per foot. The maximum thickness of saturated sediments has not been completely defined, however may extend as deep as 700 feet below the ground surface in some areas. The maximum depth penetrated by a drill near the Moose Creek Dam was 616 feet below the ground surface. In general, the ground water is unconfined and is under static water table conditions; however, there are several silt layers that may partially confine ground water in sections along the Moose Creek Dam and artesian flow may be encountered at the Dam site. Ground water levels throughout the alluvial plain are generally less than 10 feet below the existing ground surface and are greatly influenced by the Chena River and Tanana River in the project area. Groundwater generally flows in a west-northwestward direction in the Moose Creek Dam project area, similar to the flow direction of the Chena and Tanana Rivers. However, permafrost bodies and streams do influence flow direction on a localized basis.

A site-specific seismic hazard assessment was performed by Golder Associates Inc. in 2020 for the purpose of identifying the seismic setting of the region surrounding the Moose Creek Dam.

This information was used as the basis to identify major geologic sources of earthquakes that may affect the Moose Creek Dam. Results of this assessment are provided in Annex A as the “Seismic Hazard Assessment CHL023 Moose Creek Dam Modification”.

5. Geotechnical Field Explorations Between 2010 and 2020 several separate geotechnical field investigations have occurred at the Moose Creek Dam to further characterize the site subsurface conditions. The investigations performed included a total of 139 geotechnical borings that varied in depths between 15 and 204.5 feet below the ground surface and 8 test pits that varied between 2 and 11 feet below the ground surface. Piezometers were installed in 39 of the geotechnical borings to collect groundwater level data. The types and extent of the different explorations are discussed below. Project location maps that include the boring locations for geotechnical investigations documented in this report are provided in Appendix A.

Drilling equipment and methods are indicated on the exploration logs and field exploration or soil sampling methods consisted of one or more of the following:

ASTM D1586 “Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils”. Soil samples were generally collected at the surface, at 2.5 feet, 5 feet, 7.5 feet, 10 feet, and at 5-foot intervals thereafter to the limit of the exploration. Samples were obtained using a 2.0-inch outside diameter by 1.375-inch open shoe diameter split barrel sampler driven with a 140-pound automatic trip hammer falling 30-inches on drill rods. The number of blows required to drive each six-inch increment is recorded on the exploration logs for 18 or 24-inch intervals. The purpose of sampling beyond 18-inches was to collect additional sample volume. It should be noted that blow counts obtained in frozen soils do not represent

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Moose Creek Dam Modification, North Pole, Alaska 4 the penetration of those same soils in a thawed state.

ASTM D3550 “Thick Wall, Ring-lined, Split Barrel, Drive Sampling of Soils”. Commonly referenced to as a Large Penetration Test (LPT) this method was used to increase sample size in coarse grained soils. Soil samples were generally collected at the surface, at 2.5 feet, 5 feet, 7.5 feet, 10 feet, and at 5-foot intervals thereafter to the limit of the exploration.

Samples were obtained using a 3.0-inch outside diameter by 2.5-inch inside diameter split barrel sampler driven with a 340-pound automatic trip hammer falling 30-inches on drill rods. The number of blows required to drive each 6-inch increment is recorded on the exploration logs for 18 or 24-inch intervals. Penetration resistance obtained with the 2.5-inche inside diameter split barrel sampler can be factored to approximate the equivalent SPT “N” value by a hammer potential energy to sampler end-area ratio adjustment.

ASTM D6914 “Sonic Drilling for Site Characterization and the Installation of Subsurface Monitoring Devices”. Continuous samples were collected using an unlined 10-foot tube sampler. The sampler consists of a 6.0-inch outside diameter and 5.25-inch inside diameter tube and typically advanced 10 feet into the undisturbed soils ahead of the casing.

ASTM D6282 “Standard Guide for Direct Push Soil Sampling for Environmental Site Characterizations”. The direct push sampler used consisted of either the Geoprobe Macro- Core tube soil sampling system which had an outside diameter of 5.25-inches, an inside diameter of 4.5-inches, a length of 60.0-inches, and fitted with clear PVC liners or the Geoprobe Macro-Core which had an outside diameter of 2.375-inches, an inside diameter of 1.5-inches, a length of 60.0-inches, and fitted with clear PVC liners. Macro-core samplers were advanced with the Geoprobe percussion hammer in five-foot intervals.

Test Pits were excavated and grab samples were taken from the excavator bucket at visually identifiable soil lithology changes, or on regular depth intervals when lithology changes were not visually apparent. Excavations were performed to the depth determined by field staff, or maximum depth achievable by the excavator.

Engineers with the Alaska District supervised the drilling and logged samples collected from the test borings in accordance with ASTM D2488, “Description and Identification of Soils (Visual – Manual Procedure).” Soil classifications and descriptions provided on the exploration logs are in accordance with ASTM D2488 or ASTM 2487 “Classification of Soils for Engineering Purposes (Unified Soil Classification System)”. It should be noted that the size of the gravel and or cobble particles in the samples obtained is limited by the size of the opening of the sampler, therefore the sample is not necessarily representative of the coarse-grained fraction. If frozen soil was encountered each sample was visual examined and assigned a group symbol following ASTM D 4083 “Description of Frozen Soils (Visual-Manual Procedure)”.

Soil samples collected within 15 feet of the ground surface were also assigned frost classifications based on the Unified Facilities Criteria (UFC) 3-250-01 Pavement Design for Roads and Parking Areas, Table 19-2 Frost Design Soil Classification. Estimated frost classifications are followed by an asterisk (*) on the exploration logs and are based on ASTM soil classifications. Select soil samples collected above the ground water elevation were scanned for volatile organic compounds by Alaska District engineers using a photoionization detector (PID). Two types of readings were taken on recovered soil samples. The first reading was taken while the material was still on the

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Moose Creek Dam Modification, North Pole, Alaska 5 sampling barrel and was designated as a “cold” reading. The second reading was taken when the sample was placed in a sample collection bag and exposed to some form of heat such as heating the bag with the heater of the service truck. This second reading was designated as “hot”. The PID readings are shown on the exploration logs, presented in Appendix B.

Soil descriptions and classifications contained in this report and presented on the final exploration logs are the project engineer’s interpretation of the field logs and results of the laboratory testing program. Solid and dashed stratification lines shown on the exploration logs represent approximate boundaries between soil and/or rock types. Transitions are often gradual or not discernible by drill action.

5.1 Geotechnical Exploration 2010

U.S. Army Corps of Engineers Geotechnical and Materials Section personnel performed a geotechnical investigation from 12 May to 1 October 2010. A total of nine geotechnical borings were completed at various depths from 40 to 204.5 feet below the ground surface. Drilling was performed by the U.S. Army Corps of Engineers drill crew using a CME 850 track mounted drill rig. The test borings were advanced with the use of 4-inch inside diameter steel casing driven by a 340-lb automatic trip hammer. The casing was cleaned with a 3.75-inch diameter tri-cone bit. A mud slurry consisting of bentonite and water was used to displace the cuttings and the heaving sand normally encountered in the area. Two-inch inside diameter polyvinyl chloride (PVC) pipes were installed in all borings for future instrumentation use. Test borings were backfilled with cuttings and were capped with bentonite pellets. Standard Penetration Test (SPT) and Large Penetration Test (LPT) methods were used for soil sampling and blow count data collection, with methods used for each sample indicated on boring logs AP-2885 to AP-2893 located in Appendix B.

5.2 Piezometer & Thermistor Installation 2010/2011

Twenty-five new piezometers and two thermistors were installed from October 2010 through April 2011 at various depths from 26.5 to 66.5 feet below the ground surface and are indicated on exploration logs AP-2894-P through AP-2920-P. Drilling was performed by the U.S. Army Corps of Engineers drill crew with a CME 850 track mounted drill rig. Borings were performed using 6.25-inch inside diameter hollow stem auger (HSA) using the Large Penetration Test (LPT) sampling method with CME auto hammer. Field work was supervised, soils samples collected, and logged by U.S. Army Corps of Engineers Geotechnical and Materials Section personnel.

Thermistors were installed using 2-inch inside diameter PVC pipes, backfilled with auger cuttings and capped with a bentonite seal. No fluid was added to the PVC during installation. Four-inch by four-inch steel casing with locking cap were installed to protect the thermistor from damage. The two test borings with thermistors installed were AP-2901-TH and AP-2903-TH. Piezometers were installed using 2-inch PVC risers with a 10-foot pre-packed slotted screen interval. The slotted interval was filled using clean 10-20 silica sand, and the remaining hole was backfilled with auger cuttings and capped with a bentonite seal. Six-inch by six-inch or four-inch by four-inch above ground locking steel monument were installed to protect the piezometers from damage.

Piezometers at the crest of the Dam were installed with 4-inch ID circular casing with screw in cover and were cut flushed to the ground.

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5.3 Piezometer Replacement 2012

Seven piezometers were replaced in July 2012. The piezometers varied in depth from 31 to 34 feet and are indicated on exploration logs AP-2921-P through AP-2927-P. Drilling and replacement of piezometers was performed by the U.S. Army Corps of Engineers drill crew. Test borings were drilled using 6.25-inch inside diameter hollow stem auger using the Large Penetration Test (LPT) sampling method with an automatic trip hammer. Soil sampling, logging and piezometer installation was supervised by U.S. Army Corps of Engineers Geotechnical and Materials Section personnel. Piezometers were installed using 2-inch inside diameter schedule 40 PVC with a 10-foot schedule 40 slotted screen interval. The slotted interval was backfilled using clean 10-20 silica sand, and the remaining boring was backfilled with auger cuttings and capped with a bentonite seal. An above ground locking steel monument was installed to protect the piezometer from damage.

5.4 Floodway Tree Island Test Pits 2012

Eight test pits were excavated to depths of seven to 12 feet to determine the extent of silt blanket thickness in a tree island upstream of the Moose Creek Dam and are indicated on exploration logs TP-2934 through TP-2941. The operations were conducted in July 2012 with the application of a Caterpillar 416E Backhoe operated by a staff member from the Moose Creek Dam Flood Control project. U.S. Army Corp of Engineers Geotechnical and Materials Section personnel supervised the operations and collected and logged soil samples.

5.5 Piezometer Replacements 2013

Six piezometers were replaced in May 2013. The piezometers varied in depth from 31 to 39 feet and are indicated on exploration logs AP-2928-P through AP-2933-P. Drilling and replacement of piezometers was performed by the U.S. Army Corps of Engineers drill crew. Test borings were drilled using 6.25-inch inside diameter hollow stem auger using the Large Penetration Test (LPT) sampling method with an automatic trip hammer. Soil sampling, logging and piezometer installation was supervised by U.S. Army Corps of Engineers Geotechnical and Materials Section personnel. Piezometers were installed using 2-inch inside diameter schedule 40 PVC with a 10-foot schedule 40 slotted screen interval. The slotted interval was backfilled using clean 10-20 silica sand, remaining boring was backfilled with auger cuttings and capped with a bentonite seal. An above ground locking steel monument was installed to protect the piezometer from damage.

5.6 Geophysical and Geotechnical Explorations 2017

During the spring of 2017, U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) performed a geophysical investigation of subsurface features at the Chena Flood Control Project to help delineate areas of seasonally frozen soils, permafrost, and changes in sediment type below the Moose Creek Dam. CRREL geophysical survey methods included the use of capacitive-coupled resistivity (CCR), ground-penetrating radar (GPR), and electrical resistivity tomography (ERT). Information was collected on a total of 18 transects located along the downstream toe, upstream toe, and on the crest of the Moose Creek Dam. Information collected during this effort is presented in Annex B as the “Geophysical Investigation of Subsurface Features at the Moose Creek Dam, North Pole, Alaska, July 2018”.

U.S. Army Corps of Engineers Geotechnical and Materials Section personnel also performed a geotechnical investigation from 12 July through 31 August 2017 and 14 November through 8

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December 2017. A total of 52 geotechnical borings were completed at depths ranging from 60 to 130 feet below the ground surface in various locations along the Moose Creek Dam crest and upstream and downstream areas. These borings are indicated on exploration logs AP-2943 through AP-2993 and AP-3013 through AP-3014. Drilling from 12 July through 31 August 2017 was performed by the U.S. Army Corps of Engineers Savannah District drill crew using a track mounted Terra Sonic International TSi 150cc Sonic drill. Continuous samples were obtained using an unlined 10-foot long tube sampler. The sampler consists of a six-inch outside diameter by 5.25-inch inside diameter tube advanced ten feet into the undisturbed soils ahead of the casing.

Continuous samples were collected in 10-foot intervals from the surface to the limits of exploration. Test borings through the centerline of the Moose Creek Dam were backfilled in stages with sands and gravels from an onsite stockpile and consolidated as the sonic drill casing was removed from the ground to an elevation one foot above the water table. From one foot above the water table, a Type II Portland cement/bentonite grout mix was added to the test boring to one foot below the ground surface. The grout mix was comprised of 3 to 5 pounds of bentonite to every 94-pound bag of Portland cement mixed with six to eight gallons of water. Sands and gravels were then placed to the surface.

Test borings located upstream and downstream of the Moose Creek Dam embankment were filled in stages with sands and gravels from an onsite stockpile and consolidated as the sonic drill casing was removed from the ground. From one foot above the water table, 3/8-inch bentonite chips were placed and hydrated to one foot below the ground surface. Excess drill cuttings were then placed to the surface.

Three new piezometers were installed during this program, AP-2962-P through AP-2964-P. The installed piezometers were completed with 10-foot schedule 40 prepacked slotted screen intervals and 2-inch ID schedule 40 PVC riser. The test borings were backfilled with pea gravel or drill cuttings to various distances below the slotted screen interval, 10-20 silica sand to a point above the interval and plugged with either bentonite or grout mix. Six-inch by six-inch steel casing with locking cap were installed to protect the two piezometers installed downstream of the Dam. Four-inch ID circular steel casing with screw-in cap was installed to protect the piezometer installed at the crest of the embankment.

Five geotechnical borings were completed from 14 November through 8 December 2017 and ranged in depth from 87 to 103.3 feet. Discovery Drilling Inc. under contract with the U.S. Army Corps of Engineers performed the drilling with a track mounted CME-75 drill rig. Three of the borings were advanced with 4.5-inch inside diameter by 8-inch outside diameter hollow stem auger with spade bit. The rest of the borings were advanced with the 3.5-inch Tricone bit and mud slurry mixture of bentonite and water. Samples were collected from the surface and at 2.5-foot intervals utilizing the Large Penetration Test (LPT) method of sampling. Personnel from U.S. Army Corps of Engineers Geotechnical and Materials Section supervised the drilling operations and collected and logged the soil samples. Complete geotechnical borings and piezometer installations logs are included in Appendix B.

A geotechnical investigation was also performed by Golder Associates Inc. in 2017 for proposed weirs along the South Seepage Collector Channel. That report is presented in Annex C as the “Weir Geotechnical Investigation for the Moose Creek Dam Safety Modification Study”.

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U.S. Army Corps of Engineers

5.7 Geotechnical Exploration 2020

U.S. Army Corps of Engineers Geotechnical and Materials personnel performed a geotechnical investigation from 23 June to 1 July 2020. A total of 36 geotechnical borings were completed at various depths from 15 to 31.5 feet below the ground surface. Drilling was performed by Discovery Drilling using a track mounted Geoprobe 7822DT with percussion and automatic trip hammers.

Test borings were advanced by using either a direct push system or hollow stem augers. Soil sampling was conducted with a Geoprobe Macro-Core and Large Penetration Test (LPT) methods.

During this effort several test borings had to use the Geoprobe percussion hammer to advance the Large Penetration Test sample tubes because the automatic trip hammer was having mechanical difficulties. Individual drilling and sampling methods used for each test boring are indicated on the exploration logs AP-3027 through AP-3062 located in Appendix B.

A geophysical investigation was also performed by Golder Associates Inc. in 2020 for proposed Borrow areas on the Moose Creek Dam project. That report is presented in Annex D as the “Geophysical Survey Report Chena River Lakes Flood Control Project, Barrier Wall Modification”.

5.8 Test Boring Locations

Test boring locations were determined using standard survey techniques. Vertical control from the soil boring locations are referenced to North American Vertical Datum (NAVD88) in U.S. survey feet. Horizontal coordinates are based on Alaska State Plane Coordinates Zone 3, North American Datum (NAD) 83 (1992) in U.S. survey feet. The Test Boring Location Map is in Appendix A as Sheets B-401 through B-403.

6. Laboratory Testing and Soil Classification A laboratory testing program was established to classify and determine the physical and engineering properties of encountered soils. The program consisted of particle-size analysis, moisture content, engineering classification, and frost classification. Terra Firma, HDL Engineering, Golder, and R&M under contract with the Alaska District, performed the laboratory tests using the latest version of the test standards listed in Table 1.

Laboratory test results are provided in Appendix C.

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Table 1. Soils Laboratory Test Standards Test Designation Test Description

ASTM D 7928 Standard Test Method for Particle-Size Analysis of Soils (Sieve and

Hydrometer) ASTM D 422 Standard Test Method for Particle-Size Analysis of Soils

ASTM D 2216 Standard Test Methods for Laboratory Determination of Water (Moisture) Content of Soil and Rock by Mass

ASTM D 2487 Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System)

ASTM D 6913 Standard Test Methods for Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis

UFC 3-250-01 Pavement Design for Roads and Parking Areas Table 19-2 Frost Design Soil Classification

6.1 Barrier Wall Mix Design

During the 2017 geotechnical exploration, soil samples collected from the sonic drilling effort within the Moose Creek Dam embankment and foundation soils were transported to Golder Associates to conduct a Bentonite-Portland cement mix design for the proposed mix-in-place barrier wall. For reference this report is provided in Annex E as: “Weir Geotechnical Investigation and Mix Design for the Moose Creek Dam Safety Modification Study”.

6.2 Laboratory Data Report

Additional laboratory testing for material characteristics related to the suitability of Moose Creek Dam embankment and foundation soils for use in a mix-in-place barrier wall are presented in Annex F as the “Laboratory Data Report, CHL023 Moose Creek Dam Modification” dated 30 September 2020.

6.3 Chemical Data Report

A Chemical investigation was also performed by the U.S. Army Corps of Engineers Environmental Engineering Branch during the 2020 Geotechnical exploration to determine the presence or absence of polyfluoroalkyl substances (PFAS). Currently the Moose Creek Dam Barrier Wall project is located outside areas of know contamination; however, due to PFAS contaminated groundwater from Eielson Air Force Base, sampling and chemical analysis were completed in several test borings to screen against Alaska Department of Environmental Conservation soil cleanup levels. All samples collected and tested were reported as “analyte not detected” and results from this effort are presented in Annex G as the “Chemical Data Quality Review Moose Creek Dam Borrow Pit Sampling”.

7. Site Conditions

7.1 Surface Conditions

The Moose Creek Dam embankment crest is approximately 24 feet wide, level, with a gravel surface, an upstream Dam embankment slope of 2.5 horizontal to 1 vertical (2.5Hto1V) and a

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Moose Creek Dam Modification, North Pole, Alaska 10 downstream embankment slope of 2Hto1V. Within the project limits south of the Moose Creek Dam control works an underground electric utility is located near the embankment crest, adjacent to the downstream edge of the Dam crest. Figure 1 provides the typical surface conditions on the crest of the Moose Creek Dam and shows the general location of underground electric utilities near station 251+00. No utilities are located along the Moose Creek Dam embankment crest north of the control works. Within the actively maintained project area of the Moose Creek Dam, the site is generally cleared of trees and consists of mowed grass or non-vegetated zones. Vegetated areas not maintained by the Moose Creek Dam maintenance staff consist of thick brush with trees such as alders, willow, spruce and birch. The densely wooded areas typically can be found downstream of the stability berm and east of the Moose Creek Dam floodway.

Figure 1. Typical surface conditions of the Moose Creek Dam embankment crest looking south near station 251+00. Underground electric utilities are marked along the downstream side of the Dam crest.

Within the floodway the original upstream silt blanket was designed to supplement the natural stratum of silty soil and effectively lengthen the seepage path beneath the Moose Creek Dam. The silt used to construct the blanket was excavated during the embankment foundation preparation.

This silt was less uniform than originally estimated, resulting in problem seepage areas during the July 1981 test filling. To enhance the effectiveness of the silt blanket, additional silt was added to form a 1000-foot wide continuous section with a minimum thickness of 3 feet along the length of the Moose Creek Dam embankment.

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7.2 Subsurface Conditions

Details of the subsurface conditions encountered by the test borings are shown on the Exploration Log sheets in Appendix B. The general subsurface conditions encountered and their pertinent characteristics are described in the following paragraphs.

7.3 Moose Creek Dam Fill Soils

Test borings drilled at the crest of the Moose Creek Dam indicate an embankment thickness of up to 52.5 feet. The soils used to build the embankment came from a local source and are very similar to the foundation soils. The extent of engineered embankment fill material placed by controlled earthwork procedures was determined by drill action, from existing construction documents, and by visual inspection of soil samples. The fill soil unit in general was relatively clean sand and gravel classified as Poorly to Well Graded Gravel with Sand (GW, GP), Poorly to Well Graded Gravel with Silt and Sand (GW-GM, GP-GM), Silty Gravel with Sand (GM), Poorly Graded Sand with Gravel (SP), Poorly Graded Sand with Silt and Gravel (SP-SM), and Silty Sand (SM) with one instance of fine soil unit classified as Sandy Silt with Gravel (ML). Cobbles were encountered sporadically throughout the Moose Creek Dam embankment. Figure 2 illustrates the typical fill soils in the Moose Creek Dam embankment consisting of Poorly Graded Gravel with Sand (GP).

Fill material encountered on the Moose Creek Dam stability berm extended up to 17.5 feet below ground surface. The fill soil was relatively clean and was mostly classified as Poorly to Well Graded Gravel with Silt and Sand (GP-GM, GW-GM) and Poorly Graded Sand with Silt and Gravel (SP-SM). Some soil samples had higher fine soil contents and were classified as Silty Gravel with Sand (GM) and Silty Sand with Gravel (SM). Clean soils classifying as Poorly to Well Graded Gravel with Sand (GP, GW) were also seldom encountered. Cobbles were encountered sporadically throughout the Moose Creek Dam stability berm embankment.

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Figure 2. Typical embankment fill soil encountered in AP-2962-P at a depth of one foot below the Moose Creek Dam crest.

7.4 Moose Creek Dam Foundation Soils

Below the Moose Creek Dam embankment, foundation soils were generally coarse-grained soils comprised of sands and gravel classified as Well to Poorly Graded Gravel with Sand (GW, GP), Poorly Graded Sand with Gravel (SP), Poorly Graded Sand with Silt and Gravel (SP-SM), Silty Sand (SM), with instances of Poorly Graded Gravel with Silt and Sand (GP-GM) and Silty Gravel with Sand (GM). Cobble-sized particles greater than 3.0 inches in diameter should be expected throughout the foundation soils of the Moose Creek Dam. A photograph of poorly graded gravel with sand and cobbles encountered in AP-2988 is shown in Figure 3.

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Figure 3. Typical coarse-grained foundation soils encountered in AP-2988 at a depth of approximately 44 feet below the Moose Creek Dam crest.

In addition, fine-grained soil layers consisting of Silt with Sand, Sandy Silt (ML) and Organic Silt (OL) with sand, gravel, and organic inclusions were also encountered in the Moose Creek Dam foundation soils. A photograph of Sandy Organic Silt encountered in AP-2984 is shown in Figure 4.

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Figure 4. Typical fine-grained foundation soil encountered in AP-2984 at a depth of 44 to 45 feet below the Moose Creek Dam crest.

7.5 Bedrock

The northern end of the Moose Creek Dam abuts a natural rock outcrop of highly weathered schist bedrock. During a geophysical investigation performed by Golder Associates Inc. in 2020, seismic refraction methods were used to assess the depth and extent of bedrock near the north abutment below the Moose Creek Dam centerline. That report is presented in Annex D as the “Geophysical Survey Report Chena River Lakes Flood Control Project, Barrier Wall Modification”. Figure 5 provides typical surface conditions on the crest of the Moose Creek Dam near station 493+00 and it shows the general surface condition of highly weathered bedrock at the north abutment.

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Figure 5. Looking north near station 493+00 drilling AP-2993. Highly weathered schist bedrock was initially encountered in AP-2993 at a depth of 11 feet below the Moose Creek Dam crest, elevation 523.4 feet NAVD88.

During initial construction of the Dam, rock excavation was necessary at the north abutment.

Figure 6, taken from the 1983 Moose Creek Dam Foundation / Embankment Criteria and Performance Report, illustrates foundation excavation limits after bedrock was exposed and cleared of all soil and rubble at the north abutment. Elevations reported on Figure 6 use the older NGVD29 vertical datum. It is necessary to convert from NGVD29 to NAVD88 by adding 5.06 feet when comparing elevations reported on exploration logs and contract drawings.

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Figure 6: Excavation limits taken from the 1983 Moose Creek Dam Foundation / Embankment Criteria and Performance Report. Elevations reported in NGVD29 vertical datum.

Bedrock was encountered in two test borings drilled on the crest of the Moose Creek Dam and in four other test borings drilled outside of the Dam embankment area. Table 2 lists the test borings where bedrock was encountered. The bedrock encountered was identified as schist in varying condition from slightly weathered to highly weathered bedrock. Sonic drilling action was very destructive to the weathered schist bedrock samples. During sonic sampling the bedrock broke down along foliation planes to form plated angular particles and fines. Photographs of highly weathered and slightly weathered bedrock encountered in AP-2993 are shown in Figures 7 and 8.

Table 2. Summary of bedrock extents

AP- Number Top of Bedrock Elevation (NAVD) 88 (feet)

AP-2885 474.8

AP-2886 472.3

AP-2887 450.3

AP-2991 447.3

AP-2992 442.8

AP-2993 523.4

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Figure 7. Sonic drilling sample of highly weathered schist bedrock encountered in AP-2993 between elevations 514.4 and 504.4 feet NAVD88. A depth of 20 to 30 feet below the Moose Creek Dam crest.

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Figure 8. Sonic drilling sample of slightly weathered schist bedrock encountered in AP-2993 at a depth of approximately 36 feet below the Moose Creek Dam crest, elevation 498.4 feet NAVD88.

7.6 Groundwater

Groundwater depths and elevations recorded while drilling are presented on the exploration logs.

Groundwater was encountered in all the borings except for some shallow test pits and test borings drilled in frozen zones. In some test borings where a slurry mixture of bentonite and water was used, an approximate groundwater elevation was inferred from the transition of soil moisture content from moist to wet. Groundwater elevations can fluctuate seasonally with changes in precipitation, snowmelt, permafrost, runoff conditions, and with changes in the water level in the Chena River and Tanana River. During Moose Creek Dam flood control operations of the Chena River, impounding water within the floodway causes groundwater on the upstream and downstream side of the Moose Creek Dam to rise simultaneously with the impounded water. On the downstream side of the Dam, relief wells begin to flow, and artesian groundwater conditions are observed on the downstream side of the Moose Creek Dam.

Starting in 2018 the Cold Regions Research and Engineering Laboratory (CRREL) automated 39 piezometers along the Moose Creek Dam to measure fluctuations in groundwater elevations.

Results of this effort are provided in Annex H as “TR-19-26-2020 Update Web-Based Monitoring of Piezometers for USACE Moose Creek Dam, November 2020”.

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7.7 Ground Temperatures

In addition to measuring groundwater elevations with automated piezometers, CRREL collected groundwater temperatures in select monitoring wells. These results are also provided in Annex H as “TR-19-26-2020 Update Web-Based Monitoring of Piezometers for USACE Moose Creek Dam, November 2020”. During the summer months, ground temperatures of thawed soil and or groundwater at the Chena Flood Control Project range from 45° to 65° Fahrenheit (F) near the surface and quickly drop to around 39° F. Ground temperatures continue to drop with depth and can be between 39° and 33° F to a depth of 25 feet below the ground surface depending on site improvements at the surface, vegetation cover, and the proximity of discontinuous permafrost.

Ground temperatures 25 feet below the ground surface are generally at or below 35° F.

7.8 Seasonally Frozen Soils

Seasonally frozen soils were encountered from depths ranging from the ground surface to 19.0 feet below the ground surface. The annual frost zone or active layer is the top layer of ground subject to annual freezing and thawing. The deepest seasonally frozen soils were encountered in the Moose Creek Dam embankment on 12 April 2011 in AP-2907 at a depth of 19.0 feet. Seasonal frost at the Chena Flood Control project can penetrate to 20 feet or deeper during a cold winter in areas of little or no snow cover. Seasonally frozen soils can be encountered during excavations late into the summer months.

7.9 Permafrost Soils

Permafrost is defined as the thermal condition in which the temperatures in earth materials remain below 32°F/0°C for at least two consecutive winters and the intervening summer; moisture in the form of water and ground ice may or may not be present. The extent of permafrost at the Chena Flood Control project is considered discontinuous. Moose Creek Dam embankment soils are generally free of permafrost soils with sporadic discontinuous permafrost located in the foundation soils below the Dam. During the extent of these site investigations, permafrost soils were encountered in 13 test borings. Table 3 lists the test borings where permafrost was encountered and the vertical extent of the frozen soils. During the 2017 CRREL geophysical investigation, discontinuous permafrost was interpreted to exist between Moose Creek Dam stations 245+00 and 310+00 at elevations between 475 feet to 280 feet NAVD88.