ak-a2020161_GeotechnicalReport.pdf

AK ANCHORAGE T 2016(1)

WFL Report GR15-20 Connecting Anchorage Trail Improvements

Final Geotechnical Report

January 2021

Prepared for:

U.S. Department of Transportation Federal Highway Administration Western Federal Lands Highway Division 610 East Fifth Street Vancouver, WA 98661

Prepared by:

PND Engineers, Inc.

1506 West 36th Avenue Anchorage, AK 99503

AK ANCHORAGE T 2016(1) January 2021 Connecting Anchorage Trail Improvements Final Geotechnical Report

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TABLE OF CONTENTS

1 INTRODUCTION

2 PROJECT DESCRIPTION

3 GEOLOGY

3.1 Geology & Climate

3.2 Seismicity

3.3 Liquefaction

4 SITE CONDITIONS

5 SUBSURFACE CONDITIONS

5.1 Subsurface Conditions

5.2 North Fork Campbell Creek TOA Trail Bridge

5.3 South Fork Campbell Creek Main Channel TOA Trail Bridge

5.4 South Fork Campbell Creek Side Channel TOA Trail Bridge

5.5 North Gasline Trail Bridge

5.6 Laboratory Testing

5.7 Soil Type and Description Based on Lab Testing

5.7.1 Soil Type and Gradations

5.7.2 Moisture Content

5.7.3 Atterberg Limits

5.7.4 Density and Unit Weight

5.8 Determined Subsurface Conditions

5.8.1 North Fork Campbell Creek TOA Trail Bridge

5.8.2 South Fork Campbell Creek Main Channel TOA Trail Bridge

5.8.3 South Fork Campbell Creek Side Channel TOA Trail Bridge

5.8.4 North Gasline Trail Bridge

5.9 Groundwater Depth

6 ANALYSIS AND DESIGN RECOMMENDATIONS

6.1 Foundation Loads

6.2 Foundation Discussion

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6.2.1 Shallow Foundation Recommendations

6.2.2 Pile Foundation Recommendation

7 CONSTRUCTION CONSIDERATIONS

8 LIMITATIONS

9 REFERENCES

APPENDIX A — Borehole Logs................................................................................................................... A-1

APPENDIX B — Summary of Lab and Field Characteristics ....................................................................... B-1

APPENDIX C — Grain Size Distribution ..................................................................................................... C-1

APPENDIX D — Moisture Content by Depth ............................................................................................ D-1

APPENDIX E — Atterberg Limits Results ................................................................................................... E-1

APPENDIX F — Corrected SPT Blow Counts .............................................................................................. F-1

APPENDIX G— Factored Soil Bearing Resistance of Continuous Footing ................................................ G-1

APPENDIX H — Pile Capacity .................................................................................................................... H-1

APPENDIX I — Site Conditions Photo Log .................................................................................................. I-1

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TABLE OF FIGURES

Figure 1. Vicinity Map

Figure 2. As-built Borehole Locations for BH-1 and -2 for the North Fork TOA Bridge 2 Location

Figure 3. As-built Borehole Locations for BH-3, -4, -5, and -6 for the South Fork TOA Bridge 1 and Side Channel TOA Bridge 3 Locations

Figure 4. As-built Borehole Location for BH-7 for the North Gasline Bridge Location

Figure 5. Design Reponses Spectrum – 7% Probability of Exceedance in 75 Years

Figure F-1. Corrected SPT Blow Counts (N1)60 vs. Depth at BH-1 and BH-2) ............................................. F-2

Figure F-2. Corrected SPT Blow Counts (N1)60 vs. Depth at BH-3 .............................................................. F-3

Figure F-3. Corrected SPT Blow Counts (N1)60 vs. Depth at BH-6 .............................................................. F-4

Figure F-4. Corrected SPT Blow Counts (N1)60 vs. Depth at BH-7 .............................................................. F-5

Figure G-1. Factored Soil Bearing Resistance of Continuous Footing for A) In-situ Soils and B) on

Compacted Structural Fill Material at the North Fork TOA Bridge 2 Location ......................... G-2

Figure G-2. Factored Soil Bearing Resistance of Continuous Footing for A) In-situ Soils and B) on Compacted Structural Fill Material at the South Fork TOA Bridge 1 Location ......................... G-3

Figure G-3. Factored Soil Bearing Resistance of Continuous Footing for A) In-situ Soils and B) on Compacted Structural Fill Material at the Side Channel TOA Bridge 3 Location. .................... G-4

Figure G-4. Factored Soil Bearing Resistance of Continuous Footing for A) In-situ Soils and B) on Compacted Structural Fill Material at the North Gasline Bridge Location............................... G-5

Figure H-1. A) Compressive Pile Capacity under the Service 1 Limit State and B) Tensile Pile Capacity

Indicating Heave Force at North Fork TOA Bridge 2 ................................................................ H-2

Figure H-2. A) Compressive Pile Capacity under the Service 1 Limit State and B) Tensile Pile Capacity Indicating Heave Force at South Fork TOA Bridge 1. ............................................................... H-3

Figure H-3. A) Compressive Pile Capacity under the Service 1 Limit State and B) Tensile Pile Capacity Indicating Heave Force at Side Channel TOA Bridge 3 ............................................................. H-4

Figure H-4. A) Compressive Pile Capacity under the Service 1 Limit State and B) Tensile Pile Capacity Indicating Heave Force at North Gasline Bridge. ..................................................................... H-5

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LIST OF TABLES

Table 1. Summary of Seismic Parameters (7% PE in 75 Years)

Table 2. Trail Conditions by Station

Table 3. As-built Borehole Coordinates and Elevation Organized by Bridge Location

Table 4. Summary of Laboratory Tests

Table 5. Representative Gradation Properties

Table 6. Average Moisture Content by Depth (North Fork TOA Bridge 2)

Table 7. Average Moisture Content by Depth (South Fork TOA Bridge 1)

Table 8. Average Moisture Content by Depth (South Fork Side Channel TOA Bridge 3)

Table 9. Moisture Content by Depth (North Gasline Bridge)

Table 10. Atterberg Limit Results

Table 11. Density and Unit Weight of Soil from the Floodplain Between South Fork TOA Bridge 1 and Side Channel TOA Bridge 3

Table 12. North Fork TOA Bridge 2 Idealized Soil Profile

Table 13. South Fork TOA Bridge 1 Idealized Soil Profile

Table 14. Side Channel TOA Bridge 3 Idealized Soil Profile

Table 15. North Gasline Trail Bridge Idealized Soil Profile

Table 16. Groundwater depth at borehole locations

Table 17. Summary of Preliminary Unfactored and Factored Axial Loads per Abutment

Table 18. Service 1 Limit State Bearing Capacity of In-situ Soils and Structural Fill Material for a 2-, 4-, and 6-foot Footing Width at Each Bridge Location

Table 19. Approximate Settlement of a 2-, 4-, and 6-foot Effective Footing with at the Center and Corner of the Footing Placed on In-situ soils and Structural Fill Material

Table 20. Recommended Soil Parameters for Abutment Wall Design

Table 21. Minimum Embedment Depth and Uplift Resistance Depth for Piles at Each Bridge Location under a 111.2 Kip Unfactored Axial Load

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Geotechnical Report

1 INTRODUCTION

This report presents geotechnical investigation results, analysis, and recommendations for the Connecting Anchorage Trail Improvements project. The objective of the geotechnical investigation was to characterize surface and subsurface trail conditions and soil conditions at proposed bridge sites along an approximately 1.6-mile-long stretch of the Tour of Anchorage trail in Far North Bicentennial Park (FNBP) in Anchorage, Alaska, and a bridge crossing on the easternmost edge of FNBP on North Gasline Trail. A vicinity map showing the extents of the project is provided as Figure 1. The geotechnical scope of work for this project was performed by PND Engineers, Inc. (PND), under contract to the Federal Highway Administration (FHWA) Western Federal Lands Division (WFLHD), and included visual assessment along the trail length, with explorations at 200- to 400-foot intervals, a drilling program during August 2020 consisting of seven boreholes, laboratory testing of samples, and engineering analysis and recommendations.

The Geotechnical Report is organized following guidelines in the FHWA Project Development and Design Manual (PDDM), project-specific contract supplements to the PDDM geotechnical requirements (SOW Appendix B), and the FHWA Geotechnical Technical Guidance Manual (TGM). No prior geotechnical reports or investigations have been found for the project area, aside from general reports about regional geology. All data from the field investigation and laboratory soils testing is presented in the report appendices.

The Geotechnical Report contains nine appendices:

Appendix A – Borehole Logs presents the complete borehole log set from the exploratory borings of this investigation.

Appendices B-E – Present laboratory soils test results, including a summary of lab results and field characteristics for all the samples, grain size distribution of representative gravelly material encountered across the project area, moisture content of samples in each borehole, and Atterberg test results for fine grained samples.

Appendix F – Corrected SPT Blow counts presents the (N1)60 SPT blow counts for each borehole where a Standard Penetration Test was performed.

Appendix G – Bearing Capacity of Continuous Footings presents the bearing capacity of a continuous footing placed on in-situ soil and on structural fill material at each bridge location, which is the recommended foundation design.

Appendix H – Pile Capacity presents the compressive and tensile capacity of piles for a deep foundation alternative at each bridge location.

Appendix I – Site Conditions Photo Log presents trail alignment photos corresponding to visual geotechnical assessment of the subject TOA trail and North Gasline trail segments.

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Figure 1. Vicinity Map

2 PROJECT DESCRIPTION

Far North Bicentennial Park is the largest park within the Municipality of Anchorage, encompassing over 4,000 acres along the foothills of the Chugach Mountains. The park encompasses Hilltop Ski Area, the Alaska Botanical Garden, and more than 80 miles of multi-use trails. The Tour of Anchorage (TOA) Trail begins at Hilltop Ski Area, traverses FNBP, and terminates at Kincaid Park in west Anchorage. The Connecting Anchorage Trail Improvements project focus is to enhance accessibility to the park recreational resources for all user types while maintaining pristine habitat in the park, and to provide connectivity between FNBP, and the adjacent BLM Campbell Tract and Chugach State Park trail systems.

The project includes trail upgrades to an approximately 1.6-mile stretch of the TOA Trail, from the north side of FNBP to just north of the Campbell Creek Science Center and Campbell Airstrip, bridge replacements at both North Fork (NF) and South Fork (SF) Campbell Creek on the TOA Trail, and a new bridge on the North Gasline Trail. See Figure 1 for the project location. The approximate locations of the boreholes and the associated bridge locations are shown in Figures 2, 3 and 4.

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Figure 2. As-built BH-1 and BH-2 Locations, NF Campbell Creek TOA Bridge

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Figure 3. As-built BH-3, -4, -5 and -6 Locations, SF Campbell Creek Main and Side Channel TOA Bridges

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Figure 4. As-built BH-7 Location, North Gasline Trail Bridge

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3 GEOLOGY

3.1 GEOLOGY & CLIMATE

The Anchorage Bowl is bounded on the northwest by Knik Arm of Cook Inlet, on the south by Turnagain Arm, and on the east by the Chugach Mountains. Far North Bicentennial Park is located on the east side of Anchorage up against the Chugach Mountain foothills. The north and south forks of Campbell Creek traverse the park flowing west to Cook Inlet. Campbell Creek is the longest stream in the lowlands of Anchorage and originates in a glaciated mountain valley. The creek flows to the lowlands where it meanders through swamps. Soils in the Cook Inlet area around Anchorage are “podzol” soils, which are typified by an ash-gray color that results from leaching of the soluble salts and organic matter. Leaching removes iron compounds and causes the light color. To the east, poorly consolidated rocks of Tertiary age are thought to underlie most of the lowland at depths of several hundred feet. Unconsolidated deposits of Quaternary age effectively bury all older rocks in the lowland area. Subsurface soils in FNBP reflect soils expected in foothills locations, with primarily sandy gravel to gravelly sand, with fines deposits along river and creek channels typically consisting of silt, but with instances of clay.

The topography of the project area is generally undulating and hilly, with some areas of steep slopes and lowland wetland complexes. Soils generally consist of peat and silt loams (NRCS 2020b), and vegetation ranges from wetland vegetation and black spruce, to dense forest with cottonwoods, spruce, and birch.

The project area is generally undeveloped other than the trail system, which consists of gravel surfacing.

The North Gasline Trail also includes overhead power and buried gas lines.

Climate in Anchorage, Alaska, is subarctic, defined by short, cool summers. The average daytime summer temperature ranges from 55 to 78°F with average daytime winter temperatures ranging from 5 to 30°F.

Average annual precipitation as recorded at the Ted Stevens Anchorage International Airport is 16.63 inches.

3.2 SEISMICITY

Information from the U.S. Geological Survey (USGS) Earthquake Hazards Program website was used to locate nearby faults. No active faults are located near the project. According to USGS information, the closest active fault is approximately 30 miles from the project (Castle Mountain Fault).

The procedures outlined in Section 3.10 of the 8th Edition of the American Association of State Highway and Transportation Officials (AASHTO) LRFD Manual (2017) were followed to characterize the seismic hazard. These procedures use a design earthquake with a 7% probability of exceedance (PE) in 75 years, which equates to a return period of approximately 1,000 years. Based on the AASHTO procedures, and the standard penetration resistance in the top 100 feet of the soil profile at the project site, the AASHTO seismic Site Class for soils at each bridge location in the Far North Bicentennial Park is Site Class C.

The USGS Seismic Hazards Tool was used to determine the peak ground acceleration and spectral accelerations at the project site based on Site Class C Very Dense Soil and Soil Rock. The spectral accelerations were then scaled to Site Class C using the site factors recommended by AASHTO (2017) to represent the seismic accelerations based on actual ground conditions. The results of this analysis are presented in Table 1.

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Table 1. Summary of Seismic Parameters (7% PE in 75 Years) Site Class at Project Site (by Soil Type and Penetration Resistance in Top 100 feet of Soil Profile) C Horizontal Peak Ground Acceleration (PGA) 0.55 Horizontal Response Spectral Acceleration at a Period of 0.2 sec (Ss) 1.245 Horizontal Response Spectral Acceleration at a Period of 1.0 sec (S1) 0.497 Scaled Peak Ground Acceleration for Project Site, Site Class C (As) 0.550 Scaled Horizontal Response Spectral Acceleration at a Period of 0.2 sec for Project Site, Site Class C (SDS) 1.245 Scaled Horizontal Response Spectral Acceleration at a Period of 1.0 sec for Project Site, Site Class C (SD1) 0.646

AASHTO Seismic Zone 4

The values in Table 1 were used to develop the 5%-damped-design response spectrum shown in Figure 5 for the project site. An analysis of all project bridge crossings showed very small differences in seismic parameters. Only one response spectrum is provided to represent the project.

Figure 5. Design Reponses Spectrum – 7% Probability of Exceedance in 75 Years

3.3 LIQUEFACTION

Following the AASHTO procedures, this site was determined to be in Seismic Zone 4. A site in Seismic Zone 4 requires a liquefaction analysis if the ground water level is within 50 feet of the ground surface and low plasticity silts and sands within the upper 75 feet have corrected standard penetration test blow counts less than or equal to 25 blows per foot (bpf). Shallow ground water is present at the bridge sites, Final Geotechnical Report

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but dense soils present at the sites have corrected SPT blow counts exceeding 25 bpf, so liquefaction is not expected at the site during a design seismic event.

4 SITE CONDITIONS

A visual assessment with explorations at 200- to 400-foot intervals was performed by PND to determine the spatial distribution and characteristics of the soil and bedrock units that exist within the project corridor. The visual assessment was conducted during a site-walk. The site-walk evaluated the condition of the trail, including presence of rutting, low spots, presence and condition of surface course, and standing water. No proposed cuts exceeding 6 feet are anticipated for this project, so only visual methods were used and no test pits or borings were conducted for the explorations.

The following observations were made:

- No bedrock exposures or areas of suspected shallow bedrock presence were observed.

- No areas of existing or potential landslides, fill failures or slumps, or rockfall problems were observed.

- No existing structure foundations, retaining walls, pavement, material sources, significant cut slopes, or significant embankments were observed, other than existing bridges.

- No areas that might require significant cut slopes or retaining walls for the project were observed.

- Areas of wet or soft soils, surface rutting, and poor drainage were observed along the TOA Trail.

Photos in Appendix I show the existing site conditions. Table 2 provides a station-to-station description of observed surface soil conditions.

Table 2. Trail Conditions by Station

ALIGNMENT

STATION

TRAIL

CONDITION COMMENTS

TOUR OF ANCHORAGE TRAIL ALIGNMENT

10+00

GOOD Dry, light rutting, firm 12+00 14+00 16+00 FAIR Dry, geotextile fabric showing 18+00

GOOD Dry, light rutting, firm 20+00 22+00 24+00 26+00 28+00

POOR Wet, silty, no gravel, light rutting 30+00 32+00

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Table 2. Trail Conditions by Station (continued)

ALIGNMENT

STATION

TRAIL

CONDITION COMMENTS

TOUR OF ANCHORAGE TRAIL ALIGNMENT

34+00

VERY GOOD Dry, new gravel, firm 36+00 38+00 40+00 42+00

POOR Localized wet spots, significant rutting, moderate to poor compaction

44+00 46+00 48+00 50+00 52+00 POOR Wet, moderate to poor compaction 54+00 GOOD Dry, light rutting 56+00 58+00 FAIR Light rutting, moderate compaction 60+00 62+00 GOOD Light rutting, firm 64+00 FAIR Light rutting, moderate compaction 66+00 VERY GOOD Dry, new gravel, firm 68+00 GOOD Light rutting, moderate compaction 70+00 FAIR Moderate compaction 72+00 POOR Wet, loose compaction 74+00 FAIR Light rutting, moderate compaction 76+00 VERY GOOD Dry, new gravel, firm 78+00

GOOD

Dry

80+00 Dry, geotextile fabric showing 82+00 Dry, light rutting 84+00

POOR Wet, silty, no gravel 86+00 88+00 90+00 92+00 POOR Localized wet spots, significant rutting, moderate to poor compaction 94+39.48

NORTH GASLINE TRAIL ALIGNMENT

100+00

FAIR Light rutting, no surface course, moderate compaction 102+00 105+00

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5 SUBSURFACE CONDITIONS

The drilling at boreholes (BH) 1-3, 6, and 7 was performed using a Geoprobe 6712DT tracked drill rig. At BH-7 a CME-75 truck-mounted drill was used beginning at 20 feet below ground surface (bgs) to the bottom of hole. The field sampling advanced a 3-inch outside diameter split spoon sampler advanced using a 340-pound automatic drop-hammer falling 30 inches per stroke. The number of blows required to drive the sampler for each 6-inch interval, for a total distance of 24 inches, was recorded on the borehole logs. The blow counts shown on the borehole logs are field values that have not been corrected for overburden, rod length, sample size, or other factors.

A total of seven boreholes were advanced, one near each proposed bridge abutment located off the existing trail, except for the south abutment of the North Gasline Trail bridge. It is assumed that the replacement bridges will be along the same alignment as the existing structures. The approximate locations of the boreholes and the associated bridge locations are provided in Table 3.

Table 3. As-built Borehole Coordinates and Elevation Organized by Bridge Location

Bridge Borehole Latitude Longitude Elevation

(MOA72 ft) Depth

(ft)

NF Campbell Creek TOA Bridge

BH-1 61.17398°N 149.7947°W 194.4' 27

BH-2 61.17379°N 149.7946°W 195.3' 47

SF Campbell Creek TOA Bridge (Main)

BH-3 61.16829°N 149.7844°W 229.8' 46

BH-4 61.16816°N 149.7848°W 227.5' 16

SF Campbell Creek TOA Bridge (Side)

BH-5 61.16769°N 149.7849°W 227.9' 14

BH-6 61.16748°N 149.7850°W 228.7' 36

North Gasline Trail Bridge BH-7 61.17153°N 149.7190°W 434.1' 42

Samples were collected every 2.5 feet for the first 10 feet, then every 5 feet for the remainder of the borings to obtain representative samples of the subsurface soil conditions. The recovered soils were field-classified following the United Soils Classification System (USCS) according to ASTM D2488.

The drilling at BH-4 and BH-5 was performed using a Geoprobe 54LT tracked drill rig. The field sampling advanced a 48-inch-long, 2-inch O.D. rod with a dual tube configuration by direct push percussion.

Continuous core samples were collected every 4 feet. Recovered plastic liners were examined and a 12-inch intact sample was cut from the total length to be tested for density. The remaining liner was cut length-wise and smaller representative samples from each end of the retrieved 12-inch intact sample were also obtained. The recovered soils were field classified following the United Soils Classification System (USCS) according to ASTM D2488. The smaller/lighter Geoprobe 54LT was utilized at boreholes BH-4 and BH-5 to allow crossing of the existing small load capacity bridge. The smaller drill did not have the capacity to drive through the very dense soils encountered below 15 feet depth. Representative samples were transported to the PND Anchorage Soil-Material Lab for field verification and lab testing.

5.1 SUBSURFACE CONDITIONS

Boreholes were completed August 17-22, 2020, and were advanced to depths of 14 to 47 feet depending on subsurface conditions. The borehole logs are attached to this report as Appendix A. The following is a general description of subsurface soil, rock, and groundwater conditions at each bridge location during the field investigation:

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5.2 NORTH FORK CAMPBELL CREEK TOA TRAIL BRIDGE

BH-1 on the north side of the crossing reached a maximum 25 feet drilled depth and BH-2 on the south side reached a maximum drilled depth of 47 feet. Groundwater was encountered between 2 and 3 feet bgs. Both boreholes were similar with the upper 20 feet composed of a sandy well-graded to poorly-graded gravel with silt with layers of gravelly poorly-graded sand with silt. A very dense light gray glacial till layer composed of very fine sand was reached between 15 and 20 feet bgs. Drilling depth of BH-1 was curtailed because the soil profiles were in line with BH-2. Below 25 feet, BH-2 was composed of mostly gravelly poorly-graded sand with silt. At 40 feet bgs, the material was a very dense sandy silt with gravel and fragments of slate.

5.3 SOUTH FORK CAMPBELL CREEK MAIN CHANNEL TOA TRAIL BRIDGE

BH-3 on the north side of the crossing reached a maximum 46 feet drilled depth and BH-4 on the south side reached a maximum drilled depth of 16 feet. BH-3 was advanced using a hollow-stem auger and a 340 lb hammer to drive the split spoon sampler. BH-4 was advanced using a direct push drill that used percussion to drive the sampler into the ground. Groundwater was encountered between 2.5 and 3.5 feet bgs. Material in the top 15 feet was a sandy gravel to a gravelly sand with silt. Material below 15 feet was mostly poorly-graded sand with clay with increasing density with increasing depth. The glacial till began around 20 feet bgs with layers of clay with sand followed by clayey gravel and clayey sand. The direct push drill was unable to drill through the dense clayey sand which began at the 12.5 feet bgs.

5.4 SOUTH FORK CAMPBELL CREEK SIDE CHANNEL TOA TRAIL BRIDGE

BH-5 on the north side of the crossing reached a maximum 14 feet drilled depth and BH-6 on the south side reached a maximum drilled depth of 35 feet. BH-5 was advanced using a direct push drill that used percussion to drive the sampler into the ground. BH-6 was drilled using a hollow-stem auger and a 340 lb hammer to drive the split spoon sampler. Groundwater was encountered between 1 and 2 feet bgs.

Material on both sides were very similar. The lighter direct push drill was unable to push through the gravel material that began at 11 feet bgs. Material at the crossing was composed of mostly silty sand with gravel to sandy silty gravel from 5 feet bgs to about 25 feet bgs, followed by a 5-foot layer of silt that sits atop a silty gravel with sand at 30 feet bgs. The very dense material was reached at about 20 feet bgs.

5.5 NORTH GASLINE TRAIL BRIDGE

Only BH-7 on the north side was advanced. The proposed, optional BH-8 was inaccessible due to the creek crossing and steep terrain on the south side access route, and was not drilled. BH-7 advanced to 41.5 feet below ground surface (bgs). Groundwater was encountered at 17 feet bgs. The top 20 feet of the borehole was composed of sandy gravel to gravelly sand with silt. Cobbles were encountered beginning at approximately 5 feet bgs. Below 20 feet the soil was composed of coarse sandy well-graded gravel with silt with nominal gravel particle size ranging from 1.5 to 3 inches.

Very dense gravelly deposits make up the majority of material encountered. Corrected standard penetration test (SPT) values, also known (N1)60 values, ranged from 9 at the immediate ground surface (0 to 2 feet) to greater than 100 blows per foot (bpf) indicating the soils range from loose to very dense with majority very dense.

(N1)60 values, ranged from 9 at the immediate ground surface (0 to 2 feet) to greater than 100 blows per foot (bpf) indicating the soils range from loose to very dense with majority very dense. Corrected blow counts are provided in Appendix F for each bridge location.

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5.6 LABORATORY TESTING

Representative samples from the large split-spoon sampler and the direct push dual tube sampler were collected and sealed in freezer bags to maintain in-situ moisture content. The samples were then transported to the PND Soil Material Laboratory in Anchorage for testing. All tests were performed to ASTM standards where applicable. A total of 97 lab tests were performed (Table 4).

Table 4. Summary of Laboratory Tests Test Type Quantity

Moisture Content with Classification

(ASTM D2487 / D2488 / D2216) 73

Gradation of Soils (ASTM D6913) 8 Fines Wash (ASTM D1140) 4 Atterberg Limits (ASTM D4318) 3 Density (ASTM D7263) 9

5.7 SOIL TYPE AND DESCRIPTION BASED ON LAB TESTING

5.7.1 SOIL TYPE AND GRADATIONS

Table 5 shows representative gravel, sand, and fines fraction of the gradations performed on the soils including fines-washed samples only. D50 defines the particle size at which 50% of the soil is smaller than.

P10 describes the percent passing the #10 sieve (2-mm), a useful value in characterizing gravel behavior.

Although the P10 particle size is classified as coarse sand, when present in sufficient quantity in the gravel matrix it behaves synergistically with the gravel particles to increase strength and stiffness in compacted fill materials. Nominal average gravel particle size was approximately ½-inch.

Table 5. Representative Gradation Properties

Borehole ID Material Type

Soil Type Fractions (%) D50

(mm) P10 (%) Gravel Sand Fines

BH-1 Silty Gravelly Sand (gSM) 36 42 22 0.8 56 BH-1 Silty Gravel with Sand (GMs) - - 30 - - BH-2 Silty Sand with Gravel (SMg) 25 44 31 0.3 68 BH-2 Poorly-Graded Gravel w Silt and Sand ((GP-GM)s) - - 7 - - BH-3 Poorly-Graded Sandy Gravel with Silt (s(GP-GM)) 53 37 10 5.5 32 BH-3 Clayey Gravel with Sand (GCs) 29 28 42 0.2 65 BH-6 Silty Gravel with Sand (GMs) 31 29 39 0.2 64 BH-6 Sandy Silty Gravel (sGM) - - 33 - - BH-7 Well-Graded Sandy Gravel with Silt (s(GW-GM)) 61 33 6 9.8 27 BH-7 Well-Graded Sandy Gravel with Silt (s(GW-GM)) - - 7 - -

The majority of the particles were sub-rounded to sub-angular in texture. Angular aggregate encountered during sampling was likely fractured during drilling action and not representative of actual conditions.

Fines fraction of gravelly material samples ranged from 7% to 42%. The gradation curves for the samples are in Appendix C.

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5.7.2 MOISTURE CONTENT

Table 6 through Table 8 provide a summary of the average representative moisture contents at each bridge location. Table 9 provides a summary of the actual moisture content at the North Gasline Trail Bridge location as seen from BH-7. These characterizations are generalized to simply capture the general soil properties by depth. Moisture content (MC) by depth for individual boreholes are shown in Appendix D, the attached lab Summary of Sample Characteristics in Appendix B, and Borehole Logs in Appendix A.

Table 6. Average Moisture Content by Depth (NF Campbell Creek TOA Bridge) Depth

(ft) Soil Type MC Average

0 – 2 Peat (PT) to Organic Silt (OL) 74.5

2 – 15 Silty Gravelly Sand (gSM) to Poorly-Graded Silty Gravel with Sand ((GP-GM)s) 11.0 15 – 30 Silty Sandy Gravel (sGM) 8.0 30 – 47 Silty Sand with Gravel (SMg) 9.1

Table 7. Average Moisture Content by Depth (SF Campbell Creek Main Channel TOA Bridge) Depth

(ft) Soil Type MC Average

0 – 4 Peat (PT) to Organic Silt (OL) 33.9

4 – 14 Poorly-Graded Sandy Gravel with Silt (s(GP-GM)) 12.5 14 – 20 Lean Clay with Sand (CLs) 12.9 20 – 24 Clayey Gravel with Sand (GCs) 8.9 24 – 35 Clayey Sand with Gravel (SCg) 13.8 35 – 46 Lean Clay (CL) to Sandy Clayey Gravel (sGC) 11.3

Table 8. Average Moisture Content by Depth (SF Campbell Creek Side Channel TOA Bridge) Depth

(ft) Soil Type MC Average

0 – 2 Peat (PT) to Organic Silt (OL) 15.1

2 – 6 Poorly-Graded Gravelly Sand with Silt (g(SP-SM)) to

Poorly-Graded Sandy Gravel with Silt (s(GP-GM)) 18.4 6 – 20 Gravelly Silty Sand (gSM) to Sandy Silty Gravel (sGM) 7.4

20 – 36 Sandy Silty Gravel (sGM) 10.8

Table 9. Moisture Content by Depth (North Gasline Trail Bridge) Depth

(ft) Soil Type MC Average

0 – 3 Organic Silt (OL) 32.3

3 – 11 Poorly-Graded Sandy Gravel (sGP) 4.5 11 – 20 Poorly-Graded Gravelly Sand with Silt (g(SP-SM)) 3.8 20 – 42 Well-Graded Sandy Gravel with Silt (s(GW-GM)) 6.2

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5.7.3 ATTERBERG LIMITS

Three Atterberg Limit tests were completed to verify field and visual classifications. The determination identified the fines type in relation to the soil material above and below the sample location. The subsurface material was very similar at each bridge location with the exception of SF Campbell Creek Main Channel TOA Trail Bridge where lean clay (CL) was found. A noticeable change in color and consistency was observed during drilling once this layer was encountered. The Atterberg Limit charts are found in Appendix E. Table 10 provides the liquid limit, plastic limit, and plastic index for the three Atterberg Limit tests performed.

Table 10. Atterberg Limit Results Depth

Bridge Location Sample

No.

From (ft)

To (ft)

Liquid Limit (%)

Plastic Limit

Plasticity Index

(%) Soil Type NF Campbell Creek TOA 10 36 37 16.5 14.24 2.2 Silt (ML) SF Campbell Creek TOA (Main Channel) 12 35.5 37 28.2 20.25 8

Lean Clay

(CL)

SF Campbell Creek TOA (Side Channel) 5 10.5 11.5 13.9 13.11 0.8 Silt (ML)

5.7.4 DENSITY AND UNIT WEIGHT

Density and unit weight calculations of in-situ soils were performed on soil samples from BH-4 and -5 in the floodplain between the TOA Trail SF Campbell Creek Main and Side Channel bridges. This was possible because of the use of the percussion driven 54LT Geoprobe continuous core sampling. Table 11 provides the density and unit weight of samples from BH-4 and -5. These surficial soils were sampled to depths of 16 feet bgs and were composed of silts and sands to sands with gravel.

Table 11. Density and Unit Weight of Soil from the Floodplain Between TOA Trail SF Campbell Creek Main and Side Channel Bridges

Bridge Location Borehole

ID

Sample

No

Sample Depth

(ft)

Bulk Density (g/cm3)

Dry Density (g/cm3)

Bulk Unit

Weight (lb/ft3)

Dry Unit

Weight (lb/ft3) USCS

SF Campbell Creek Main Channel TOA

BH-4 1 3 2.25 2.02 140.2 126.2 s(GP-GM) BH-4 3 7 2.22 2.03 138.4 127.0 sGW

BH-4 4 11 2.44 2.25 152.4 140.2 GCs

BH-4 8 13 2.31 2.07 144.2 129.3 CL

BH-4 10 15 2.20 1.89 137.6 118.3 CL

SF Campbell Creek Side Channel TOA

BH-5 1 2.5 1.93 1.53 120.7 95.7 (SP-SM)g

BH-5 2 6.5 2.39 2.21 148.9 138.0 GPs

BH-5 3 10.5 2.49 2.30 155.4 143.7 SMg

BH-5 4 13 2.47 2.31 154.2 144.0 sGM

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5.8 DETERMINED SUBSURFACE CONDITIONS

Based on the corrected SPT blow counts and lab results, the determined soil profile and properties are provided in Table 12 through Table 15. Figures F-1 through Figure F-4 show corrected SPT blow counts versus depth for each bridge location. Blow counts were corrected using standard correlations found in geotechnical texts. The following sections address each bridge crossing subsurface condition:

5.8.1 NORTH FORK CAMPBELL CREEK TOA TRAIL BRIDGE

The soil profile can be generally described as:

2 feet of loose Peat (PT) and Organic Silt (OL) overlying 13 feet of very dense gravelly Silty Sand (gSM) and very dense poorly-graded Silty Gravel with Sand ((GP-GM)s). This is underlain by 15 feet of very dense Sandy Silty Gravel (sGM) on top of very dense Silty Sand with Gravel (SMg).

Table 12. North Fork TOA Bridge 2 Idealized Soil Profile

Thickness

(ft) Layer

Design (N1)60

(blows/ft)

Assumed Bulk Unit Weight

(pcf) φ’peak (deg)

Cohesion (psf)

0 – 2 PT/OL 7 40 26 —

2 – 15 gSM/(GP-GM)s 65 140 40 —

15 – 30 sGM 61 140 40 — 30 – 47 SMg 54 130 38 —

5.8.2 SOUTH FORK CAMPBELL CREEK MAIN CHANNEL TOA TRAIL BRIDGE

The soil profile can be generally described as:

4 feet of loose Peat (PT) and Organic Silt (OL) overlying 10 feet of medium-dense to dense poorly-graded Sandy Silty Gravel (s(GP-GM)). This is underlain by 6 feet of dense Lean Clay with Sand (CLs) on top of 4 feet of very dense Clayey Gravel with Sand (GCs). Below is 11 feet of dense Clayey Sand with Gravel (SCg) on top of 11 feet of Sandy Clayey Gravel (sGC).

Table 13. SF Campbell Creek TOA Bridge (Main) Idealized Soil Profile

Thickness

(ft) Layer

Design (N1)60

(blows/ft)

Assumed Bulk Unit Weight

(pcf) φ’peak (deg)

Cohesion (psf)

0 – 4 PT/OL 7 40 26 —

4 – 14 s(GP-GM) 36 135 38 —

14 – 20 CLs 43 110 15 1000 20 – 24 GCs 80 140 40 — 24 – 35 SCg 39 130 38 — 35 – 46 sGC 70 140 40 —

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5.8.3 SOUTH FORK CAMPBELL CREEK SIDE CHANNEL TOA TRAIL BRIDGE

The soil profile can be generally described as:

2 feet of loose Peat (PT) and Organic Silt (OL) followed by 4 feet of medium-dense poorly-graded Gravelly Silty Sand (g(SP-SM)) to poorly-graded Sandy Silty Gravel (s(GP-GM)) overlying 14 feet of very dense Gravelly Silty Sand (gSM) to Sandy Silty Gravel (sGM). This is underlain by 16 feet of very dense Sandy Silty Gravel (sGM).

Table 14. TOA Trail SF Campbell Creek Side Channel Bridge Idealized Soil Profile

Thickness

(ft) Layer

Design (N1)60

(blows/ft)

Assumed Bulk Unit Weight

(pcf) φ’peak (deg)

Cohesi on (psf)

0 – 2 PT/OL 7 40 26 —

2 – 6 g(SP-SM)/s(GP-GM) 20 135 36 —

6 – 20 gSM/sGM 61 140 40 — 20 – 36 sGM 96 140 40 —

5.8.4 NORTH GASLINE TRAIL BRIDGE

The soil profile can be generally described as:

3 feet of dense Organic Silt (OL) overlying 8 feet of very dense poorly-graded Sandy Gravel (sGP). This is underlain by 9 feet of very dense poorly-graded Gravelly Silty Sand (g(SP- SM)) on top of very dense well-graded Sandy Silty Gravel (s(GW-GM)).

Table 15. North Gasline Trail Bridge Idealized Soil Profile

Thickness

(ft) Layer

Design (N1)60

(blows/ft)

Assumed Bulk Unit Weight

(pcf) φ’peak (deg)

Cohesion (psf)

0 – 3 OL 45 110 26 —

3 – 11 sGP 65 140 40 —

11 – 20 g(SP-SM) 70 135 40 — 20 – 42 s(GW-GM) 66 140 40 —

5.9 GROUNDWATER DEPTH

Groundwater was identified at an average depth of 2.2 feet below-ground-surface (bgs) between six boreholes along the stretch of TOA trail traversing the existing and proposed new bridge locations.

Groundwater was encountered at 17 feet bgs at BH-7 at the North Gasline trail. Table 16 provides the encountered groundwater depth at each borehole during exploration.

Table 16. Groundwater depth at borehole locations

Borehole Groundwater Depth, (ft bgs) Borehole Groundwater Depth, (ft bgs)

BH-1 1.0 BH-5 1.0

BH-2 3.0 BH-6 2.0

BH-3 3.5 BH-7 17.0

BH-4 2.5

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6 ANALYSIS AND DESIGN RECOMMENDATIONS

6.1 FOUNDATION LOADS

Axial loads, for all bridges, were provided by the structural engineer and are summarized in Table 17.

Table 17. Summary of Unfactored and Factored Axial Loads per Abutment

Load Type

Unfactored Load (kips)

Strength I Limit State Service I Limit State Load

Factor (γ)

Factored Load (kip)

Load Factor

(γ)

Factored Load (kips)

Dead load of superstructure and substructure (DC) 59 1.25 74 1.00 59 Dead load of wearing surface (DW) 0 1.50 0 1.00 0 Vehicular live load (LL) 52 1.75 91 1.00 52 Total 111 - 165 - 111

Total unfactored axial load per abutment, for the Strength I Limit State, is 111 kips. For a 15-ft-long continuous footing at each abutment, resulting in a Strength I unfactored vertical load of about 7,400 lbs/ft.

6.2 FOUNDATION DISCUSSION

Both shallow and deep foundation concepts were considered for support of the structures at this location.

Site conditions, subsurface conditions, scour, design loading, constructability, and cost were the major factors considered when evaluating foundation alternatives. The AASHTO design vehicle is an H10 Maintenance Vehicle.

A shallow foundation such as a strip footing/sill or Geosynthetic Reinforced Soil (GRS) abutment wall would need to bear below the estimated scour depth. Scour would need to be controlled either through installation of rock revetments or retaining wall structures.

Driven piles are a feasible deep foundation alternative for support of the proposed structures. Pile foundations would also require scour protection for trail embankment at bridge abutments.

6.2.1 SHALLOW FOUNDATION RECOMMENDATIONS

Design of a shallow foundation must consider the bearing capacity of the underlying soils, as well as the potential for settlement and the effects of seasonal frost action. Frost line at the site is estimated to be at 4 feet below ground surface. AASHTO and FHWA recommend placing the footing below the frost line.

However, based on the lightweight bridge design, the flexibility of the structure, and intended use and cost there would be negligible negative effects placing the footing within the frost zone. Based on the subsurface material and water table elevation, the surficial 15 feet of soil can be considered to have negligible to moderate frost susceptibility (Frost group F1 and F2). Two separate analyses were performed to characterize bearing capacity; 1) underlying in-situ soils with excavated organic surface layer and 2) bearing capacity of gravel fill placed on in-situ gravelly soils underlying the removed organic surface layer.

Soils at the site will require excavation of at least 3 feet to remove surface organics; footings should be placed on a minimum of 12 inches of structural fill. Structural fill should conform to Structural Backfill as https://www.fhwa.dot.gov/publications/research/infrastructure/structures/11027/11027.pdf

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defined by FHWA FP-14 Section 704.04 modified for a maximum of 8% passing the No. 200 sieve. The analysis assumed the footings are located adjacent to a gradual slope at the creek banks.

6.2.1.1 Maximum Allowable Bearing Pressure

o Static Loads (Dead and Normal Live): See Figures in Appendix G o Transient Loads (Wind and Seismic): Increase Static Loads by 33%

The effective bearing area of eccentrically loaded footings will be less than the actual footing dimensions, and may vary depending on anticipated design loads and eccentricity. The allowable bearing capacity can be estimated from Figures G-1 through G-13 in Appendix G for different footing widths. Expected settlements will depend on footing dimensions and loads applied to the structure. An applied vertical load of 7400 pounds per foot was assumed for this analysis based on information provided by the structural engineer. The bearing capacity analysis may need to be updated after structural loads, footing types and dimensions have been determined by the structural engineer. An analysis of the Service 1 Limit State bearing capacity is provided in Table 18. These values for Service 1 Limit State bearing capacity are based on an effective footing width of 2, 4, and 6 feet. Values are presented for the bearing capacity of in-situ soils below the organic layers and a structural fill material placed where organic soils have been excavated over underlying in-situ soils. Settlements are limited to 1 inch. Expected settlements are described in the following section.

Table 18. Service 1 Limit State Bearing Capacity of In-situ Soils and Structural Fill Material for a 2-, 4-, and 6-foot Footing Width at Each Bridge Location

Bridge Location

Effective Footing Width

(ft)

In-situ Soil Structural Fill

Material Bearing

Capacity (ksf) Bearing

Capacity (ksf)

TOA Trail NF Campbell Creek Bridge

2 4.4 3.0 4 7.1 4.9 6 9.7 6.6

TOA Trail SF Campbell Creek Main Bridge

2 3.7 3.7 4 5.6 5.6 6 7.3 7.3

TOA Trail SF Campbell Creek Side Channel Bridge

2 1.9 2.6 4 3.3 4.6 6 4.6 6.3

North Gasline Trail Bridge 2 6.5 4.5 4 12.3 8.5 6 17.4 11.9

6.2.1.2 Settlement

Consolidation settlement of in-situ foundation soils will be negligible. Minor construction or elastic settlement of fill material or native soils can be expected. Table 19 shows the expected settlements for native soils and structural fill material at the center and corners of a proposed 2-, 4-, and 6-foot effective footing width. Estimated settlements are for the bearing capacities defined in Table 17 and Table 18, respectively.

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Table 19. Approximate Settlement of a 2-, 4-, and 6-foot Effective Footing with at the Center and Corner of the Footing Placed on In-situ soils and Structural Fill Material

Bridge Location

Effective Footing Width

(ft)

Center of Strip Footing Corner of Strip Footing

In-situ Soil Structural

Fill Material In-situ Soil Structural

Fill Settlement (in)

TOA Trail NF Campbell Creek Bridge

2 0.07 0.05 0.04 0.02 4 0.25 0.17 0.12 0.09 6 0.50 0.35 0.25 0.17

TOA Trail SF Campbell Crk Main Channel

2 0.06 0.06 0.03 0.03 4 0.20 0.20 0.10 0.10 6 0.38 0.38 0.19 0.19

TOA Trail SF Campbell Crk Side Channel

2 0.03 0.04 0.02 0.02 4 0.12 0.16 0.06 0.08 6 0.24 0.33 0.12 0.16

North Gasline Trail Bridge

2 0.11 0.07 0.05 0.04 4 0.43 0.29 0.21 0.15 6 0.90 0.62 0.45 0.31

6.2.1.3 Soil Parameters for Abutment/Retaining Wall Designs

Table 20. Recommended Soil Parameters for Abutment Wall Design Parameter Design Value* Moist Unit Weight, (lbs/ft3) 138 Angle of Internal Friction (°) 38 Active Earth Pressure Coefficient, Ka 0.2 Active Equivalent Fluid Pressure, (lbs/ft3) 33 At-Rest Earth Pressure Coefficient, K0 0.4 At-Rest Equivalent Fluid Pressure, (lbs/ft3) 53 Passive Earth Pressure Coefficient, Kp 4.2 Passive Equivalent Fluid Pressure, (lbs/ft3) 580 Wall-Backfill Interface Friction Angle, δ (°) 20

6.2.2 PILE FOUNDATION RECOMMENDATION

As an alternative to the recommended shallow foundations, pile-supported structural foundations would also provide suitable support at each bridge abutment. Table 21 shows analyses results for pile capacity.

The analysis assumed 12-inch-diameter open-end piles. The compressive pile capacity was calculated to provide a minimum embedment depth and the tensile pile capacity was calculated to provide the depth to resist uplift forces due to frost heave. An unfactored axial load of 56 kips was applied in each pile analysis. The design considers two piles per abutment. Down-drag loads from elastic settlement, consolidation, and possible future seismic loads should be accounted for in the final design. Compressive and tensile capacity charts are available in Appendix H.

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Table 21. Minimum Embedment Depth and Uplift Resistance Depth for Piles at Each Bridge Location under a 111.2 Kip Unfactored Axial Load

Bridge Location

Minimum Embedment Depth; bgs

(ft)

Uplift Resistance Depth; bgs

(ft) TOA Trail NF Campbell Creek 28 14 TOA Trail SF Campbell Creek Main Channel 34 18 TOA Trail SF Campbell Creek Side Channel 28 16 North Gasline Trail Bridge 20 8

7 CONSTRUCTION CONSIDERATIONS

In addition to bridge foundation recommendations, trail surface improvements are anticipated to remove areas of soft soils and rutting along the TOA trail. We recommend that these improvements may consist of excavation of up to 2 feet of surficial peat and organic silt soils, where present, use of separation geotextile, selected borrow material subgrade, and aggregate surface course.

At bridge abutments, where strip footings or sills are recommended, consolidation settlement of in-situ foundation soils will be negligible. Minor construction or elastic settlement of fill material or native soils can be expected. Driven piles are a feasible deep foundation alternative for support of the proposed structures. However, based on the load support requirements for the proposed structure, cost, and limited access pile foundations would only be recommended due to scour or river meander stresses.

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8 LIMITATIONS

The information submitted in this report is based on our interpretation of data from a field and lab geotechnical investigation conducted for this project and other sources discussed in this report. Effort was made to obtain information which is representative of the actual conditions at the site. However, actual subsurface conditions will vary and additional information may be discovered that could impact our recommendations. If conditions significantly different from those indicated in this report are encountered by subsequent investigations or during construction, the recommendations of this report should be reviewed by PND.

This report was prepared by PND Engineers, Inc. for use on this project only. PND is not responsible for conclusions, opinions or recommendations made by others based on data presented in this report. This report is submitted to Federal Highway Administration, Western Federal Lands Highway Division.

GEOTECHNICAL REPORT

Connecting Anchorage Trail Improvements Project

AK ANCHORAGE T 2016(1)

Anchorage, Alaska

January 2021

Prepared by:

Kannon Lee, Geotechnical Engineer

Reviewed by:

Torsten Mayrberger, Principal Geotechnical Engineer

Approved by:

Jim Campbell, Principal Geotechnical Engineer

1/22/21

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9 REFERENCES

AASHTO LRFD Bridge Design Specifications, Eighth Edition (2017) American Association of State Highway and Transportation Officials, Washington, D.C.

Alaska Department of Transportation & Public Facilities, Geotechnical Procedures Manual, May 2007.

Alaska Department of Transportation & Public Facilities, Alaska Flexible Pavement Design Manual, July 2020.

Alaska Department of Transportation & Public Facilities, Standard Specifications for Highway Construction (2020), Section 703 Aggregates.

ASTM D420-98, “Standard Guide to Site Characterization for Engineering Design and Construction Purposes”, American Society for Testing and Materials, ed. 2012.

ASTM D2216-10, “Laboratory Determination of Water Content of Soil and Rock by Mass”, American Society for Testing and Materials, ed. 2010.

ASTM D2487-11, “Classification of Soils for Engineering Purposes (Unified Soil Classification System)”, American Society for Testing and Materials, ed. 2010.

ASTM D2488-09a, “Description and Identification of Soils (Visual Manual Method)”, American Society for Testing and Materials, ed. 2010.

ASTM D4083 – 89, “Standard Practice for Description of Frozen Soils (Visual Manual Procedure)”, American Society for Testing and Materials, ed. 2010.

ASTM D4318-17, “Standard Test Methods for Liquid Limit, Plastic Limit, and Plasticity Index of Soils”, American Society for Testing and Materials, ed. 2010.

ASTM D6913-04, “Particle-Size Distribution (Gradation) of Soils Using Sieve Analysis”, American Society for Testing and Materials, ed. 2010.

Federal Highway Administration, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14 (2020), Section 200 Earthwork

Federal Highway Administration, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14 (2020), Section 300 Aggregate and Base Courses.

Federal Highway Administration, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects FP-14 (2020), Section 700 Material.

Idriss, I.M., and Boulanger, R.W. (2008). Soil Liquefaction During Earthquakes, Earthquake Engineering Research Institute Monograph No. MNO-12.

Lew, M., Sitar, N., Atik, L., Pourzanjani, M., and Hudson, M. (2010). “Seismic Earth Pressures on Deep Building Basements.” Structural Engineers Association of California, 2010 Convention Proceeds.

MMI Engineering, Inc. (2013). “Downtown Anchorage Seismic Risk Assessment & Land Use Regulations to Mitigate Seismic Risk.” Prepared for the Municipality of Anchorage Planning Department and the Geotechnical Advisory Commission.

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NRCS. (2020b). Web Soil Survey. <https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx> (May 14, 2020).

Ostadan, F. and White, W. (1998). “Lateral Seismic Soil Pressure: An Updated Approach.” US-Japan SSI Workshop, USGS, Menlo Park, CA, September 22-23, 1998.

U.S. Geological Survey, USGS Earthquake Hazards Program website, 2008 Interactive Deaggregations, http://geohazards.usgs.gov/deaggint/2008/, accessed July 2020.

Western Regional Climate Center (WRCC). Anchorage Park Strip, Alaska: Period of…