aka-fnsb-tanana1_GeotechnicalReport.pdf

AK FNSB TANANA(1)

WFL Report 08-21 Tanana River Recreation Access Improvements

Final Geotechnical Report

September 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

Tanana River Recreation Access Improvements September 2021 i

TABLE OF CONTENTS

INTRODUCTION

1.1 Purpose and Scope

1.2 Project Description

2 GEOLOGY AND GEOLOGIC HAZARDS

2.1 Geologic Hazards

2.2 Site Seismicity

2.3 Liquefaction — Earthquake Induced

3 SITE CONDITIONS

3.1 Surface Conditions

3.1.1 South Lathrop Street Extension-Section 1 (Station 74+80 to

Station 88+60)

3.1.2 South Lathrop Street Extension-Section 2 (Station 58+20 to

Station 74+80)

3.1.3 Parking Lots, Vaulted Restrooms, ADA Pathway (Station 5+00 to

Station 20+80), and Northlake Lane Extension (Station 100+00 to

Station 111+20)

3.2 Subsurface Conditions

3.3 Climate

4 EXPLORATION METHODS AND BOREHOLE LOCATIONS

5 LAB SOILS CHARACTERIZATION

5.1 Laboratory Testing

5.2 Gradations

5.3 Moisture Content

5.4 Atterberg Limits

5.5 Density and Unit Weight

5.6 Consolidation Test

6 DETERMINED SUBSURFACE CONDITIONS

6.1 South Lathrop Street (Section 1) Soil Profile – Station 74+80 to Station

88+60

6.2 South Lathrop Street (Section 2) Soil Profile – Station 58+20 to Station

74+80

6.3 Northlake Lane Extension Soil Profile – Station 100+00 to Station 106+40

6.4 South Lathrop Street Non-motorized Boat Launch Improvements Soil

Profile

6.5 Swimming Beach Parking Lot and Vault Toilets Soil Profile

6.6 Motorized Boat Launch Parking Lot Soil Profile

6.7 ADA Pathway Soil Profile-Station 5+45 to Station 9+70

6.8 Groundwater

ii

7 ANALYSES AND RESULTS

7.1 Design Traffic Loads

7.2 Slope Stability Analysis

7.3 Settlement Analysis

8 RECOMMENDATIONS

8.1 Site Preparation

8.2 Excavation

8.3 Embankment

8.3.1 Pavement and Structural Section Recommendations

8.3.2 Suitability of Excavation Material

8.3.3 Embankment Slopes/Erosion Controls

8.4 Impact to Existing Levee

8.5 Construction Window

8.6 Settlement Mitigation

8.6.1 Permafrost

8.6.2 Foundation Soil Consolidation Settlement

9 LIMITATIONS

10 REFERENCES

APPENDIX A — BOREHOLE LOGS ...................................................................................... A-1

APPENDIX B — CROSS SECTIONS .......................................................................................B-1

APPENDIX C — SUMMARY OF LAB AND FIELD CHARACTERISTICS .........................C-1

APPENDIX D — GRAIN SIZE DISTRIBUTION .................................................................... D-1

APPENDIX E — MOISTURE CONTENTS .............................................................................. E-1

APPENDIX F — ATTERBERG LIMITS RESULTS ................................................................ F-1

APPENDIX G — CONSOLIDATION & OTHER LABORATORY TESTS ........................... G-1

APPENDIX H — CORRECTED SPT BLOW COUNTS BY DEPTH ..................................... H-1

APPENDIX I — LIQUEFACTION ANALYSIS ........................................................................ I-1

APPENDIX J — SETTLEMENT ANALYSIS (ROCSCIENCE SETTLE3) .............................. J-1

APPENDIX K — SLOPE STABILITY ANALYSIS (ROCSCIENCE SLIDE2) ..................... K-1

APPENDIX L — PROJECT PHOTOS ....................................................................................... L-1 iii

TABLE OF FIGURES

Figure 1-1 Project Location and Vicinity Map

Figure 2-1 Five Percent Damped-Design Response Spectrum

Figure 4-1. Boreholes Advanced in Each Project Area

Figure H-1. Corrected SPT blow counts (N1)60 versus depth below ground surface for

South Lathrop Street (Section 1)................................................................... H-2

Figure H-2. Corrected SPT blow counts (N1)60 versus depth below ground surface for

South Lathrop Street (Section 2)................................................................... H-3

Figure H-3. Corrected SPT blow counts (N1)60 versus depth below ground surface for the

Northlake Lane extension ............................................................................. H-4

Figure H-4. Corrected SPT blow counts (N1)60 versus depth below ground surface for the non-motorized boat launch improvements.................................................... H-5

Figure H-5. Corrected SPT blow counts (N1)60 versus depth below ground surface for the swimming area parking lot and vault toilets ................................................. H-6

Figure H-6. Corrected SPT blow counts (N1)60 versus depth below ground surface for the motorized boat parking lot ............................................................................ H-7

Figure H-7. Corrected SPT blow counts (N1)60 versus depth below ground surface for the

ADA pathway ............................................................................................... H-8

Figure I-1. Factor of Safety Against Liquefaction Versus Depth for South Lathrop Street

(Section 1) ....................................................................................................... I-2

Figure I-2. Factor of Safety Against Liquefaction Versus Depth for South Lathrop Street

(Section 2) and Northlake Lane extension ...................................................... I-3

Figure I-3. Factor of Safety Against Liquefaction Versus Depth for the non-motorized boat launch improvements .............................................................................. I-4

Figure I-4. Factor of Safety Against Liquefaction Versus Depth for the swimming area parking lot and vault restrooms....................................................................... I-5

Figure I-5. Factor of Safety Against Liquefaction Versus Depth for the motorized boat parking lot ....................................................................................................... I-6

Figure I-6. Factor of Safety Against Liquefaction Versus Depth for the ADA pathway ............. I-7 iv

LIST OF TABLES

Table 2-1 Summary of Seismic Parameters

Table 2-2 Liquefaction Settlement Analysis Results

Table 3-1 Climate Data Summary

Table 4-1 Boreholes Advanced in each Project Area

Table 5-1: Laboratory Tests Performed on Collected Samples

Table 5-2. Representative Gradation Properties

Table 5-3. Consolidation Strain

Table 6-1. Design Profile for South Lathrop Street (Section 1)

Table 6-2. Design Profile for South Lathrop Street (Section 2)

Table 6-3. Design Profile for Northlake Lane Extension

Table 6-4. Design Profile for South Lathrop Street Non-Motorized Boat Launch

Improvements

Table 6-5. Design Profile for Swimming Beach Parking Lot and Vault Toilets

Table 6-6. Design Profile for Motorized Boat Parking Lot

Table 6-7. Design Profile for ADA Pathway

Table 7-1 Soil Parameters Used for Geosynthetic Reinforced Embankment Design

Table 7-2. Geosynthetic Properties Used in Slope Stability Model

Table 7-3. Factor of Safety Summary for Slope Stability Analysis

Table 7-4. Summary of Settlement Analysis Results

Table 8-1. Excavation Recommendations by Station

Table E-1. Average Moisture Content by Depth for South Lathrop Street (Section 1) ............... E-2

Table E-2. Average Moisture Content by Depth for South Lathrop Street (Section 2) ............... E-2

Table E-3. Average Moisture Content by Depth for Northlake Lane Extension ......................... E-2

Table E-4. Average Moisture Content by Depth for South Lathrop Street Non-Motorized

Boat Launch Improvements ........................................................................... E-2

Table E-5. Average Moisture Content by Depth for Swimming Beach Parking Lot and

Vault Toilets................................................................................................... E-2

Table E-6. Average Moisture Content by Depth for Motorized Boat Parking Lot ..................... E-3

Table E-7. Average Moisture Content by Depth for ADA Pathway ........................................... E-3 v

Table G- 1. Summary of Consolidation Parameters ................................................................... G-2

Introduction 1

INTRODUCTION

This report provides the geotechnical investigation results, analysis, and recommendations for the Tanana River Recreation Access (TRRA) Improvements Project. The project is located near

Fairbanks, Alaska, within the Fairbanks North Star Borough (FNSB).

The Geotechnical Report is organized following guidelines in the Federal Highway

Administration (FHWA) Project Development and Design Manual (PDDM), the project-specific contract supplements to PDDM geotechnical requirements (SOW Appendix B), and the FHWA

Geotechnical Technical Guidance Manual (TGM). All data from the field investigation and laboratory soils testing are presented in the report appendices.

The geotechnical report contains 12 appendices:

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

✓ Appendix B – Cross Sections presents the general subsurface conditions that were encountered in the project area.

✓ Appendices C through G – 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; Atterberg Limits test results for fine-grained samples; and specific gravity, densities, and consolidation test results.

✓ Appendix H – Corrected SPT Blow Counts by Depth presents the (N1)60 Standard

Penetration Test (SPT) blow counts for each borehole.

✓ Appendix I – Liquefaction Analysis presents the factor of safety against liquefaction by depth for each project area.

✓ Appendix J – Settlement Analysis presents the results of a settlement analysis of the

South Lathrop Street (Section 2) embankment.

✓ Appendix K – Slope Stability Analysis presents the results of a slope stability analysis of the South Lathrop Street (Section 2) embankment.

✓ Appendix L – Project Photos presents photos taken during the geotechnical investigation and laboratory testing program.

1.1 PURPOSE AND SCOPE

This report provides geotechnical recommendations for the design of a new roadway and recreation area facilities, as well as for the expansion of existing parking lots and paving of existing road surfaces. The geotechnical scope of work included a visual reconnaissance along the proposed alignment, a subsurface exploration program consisting of 20 geotechnical test borings, laboratory soil index testing, preparation of interim memorandums, and preparation of this report. The geotechnical borings were completed by PND Engineers, Inc. (PND) and

Discovery Drilling LLC (Discovery). All laboratory testing of soil samples was performed by

PND.

Introduction 2

The following report was previously prepared by PND for Western Federal Lands Highway

Division (WFLHD) for the TRRA project and is available upon request: Geotechnical Memo

01-20, Preliminary Geotechnical Memo, dated September 2020

This report supersedes previous reports and memos for this project.

1.2 PROJECT DESCRIPTION

The project begins at the intersection of South Lathrop Street and Sanduri Avenue and extends south into the Tanana Lakes Recreation Area (TLRA). A vicinity map showing the extent of the project and the different project areas is provided in Figure 0-1. The project consists of establishing new sections of roadway for South Lathrop Street, an extension of Northlake Lane, and various other improvements. The first section of the South Lathrop Street addition is a

1,200-linear-foot section extending south from Sanduri Avenue. This section will begin at the existing railroad crossing and will create a direct route to TLRA. Section 2 consists of 1,700 linear feet of roadway extending south from the levee into TLRA. A culvert will be installed in the existing slough approximately 1,300 linear feet south of the Tanana Levee.

The Northlake Lane extension consists of a 500-linear-foot section of new roadway that will connect Northlake Lane to the extension of South Lathrop Street. Other improvements associated with this project include the expansion and resurfacing of the Swim Beach parking lot, addition of approximately 1,500 linear feet of ADA-compliant pathway, installation of 3 new vault restrooms, addition of an ADA-compliant parking area at the non-motorized boat launch, and improvements to the motorized boat launch parking lot.

Figure 0-1 Project Location and Vicinity Map

Geology and Geologic Hazards 3

2 GEOLOGY AND GEOLOGIC HAZARDS

Fairbanks is located within a large arcuate band of alluvial sediments extending from the Alaska

Range to the Yukon-Tanana Uplands that make up the Tanana Lowlands physiographic province. Lowlands consist of low benches cut by the Tanana River, as well as oxbow lakes and sloughs representing former channels of the Tanana River or its tributaries. Vegetated floodplains are also present. Former slough channels typically have surficial deposits of organic silt and peat. The soils within the lowlands typically consist of silty overburden deposits underlain by interbedded alluvial sands and gravels of varying thickness. Deposits within the

Tanana lowlands tend to be laterally discontinuous. The project area has not been glaciated. The depth to bedrock in the project area is unknown but has been estimated at depths of 500 feet below the ground surface (bgs).

The proposed alignment for the extension of South Lathrop Street is underlain by the Tanana-

Mosquito complex, consisting of poorly drained alluvial silty soils. Moving toward the Tanana

River, and an area of previous gravel extraction, the other project areas are made up of the moderately well-drained Riverwash and Eielson-Piledriver complexes consisting of sands and gravels.

Fairbanks is located within a subarctic zone underlain by discontinuous permafrost. Permafrost is classified as ground that has remained below 32°F for at least 2 years. Permafrost in Fairbanks has been measured at greater than 200 feet thick in some locations. The layer of soil that experiences annual freeze/thaw cycles is known as the active layer. The active layer in Fairbanks is highly dependent on surficial soils, vegetation, and snow depth. Areas with an organic layer or thick snow cover typically have an active layer between 3 and 5 feet thick; roads and parking lots kept free of snow can have an active layer greater than 10 feet.

2.1 GEOLOGIC HAZARDS

Relevant geologic hazards in the vicinity of the site include active seismic faults, strong ground shaking, liquefaction-caused lateral spreading and settlement potential, and flooding. Although permafrost in its frozen state is not considered a hazard, thawing permafrost can be a hazard and is sensitive to human disturbance. These hazards are discussed in more detail in the following sections.

2.2 SITE SEISMICITY

Information from the United States Geological Survey (USGS) Earthquake Hazards Program website (USGS, 2020) was used to locate nearby faults. The project area is located approximately 45 miles north of the Northern Foothills thrust fault system and 90 miles south of the Tintina strike-slip fault system. Earthquakes in the Fairbanks area are typically shallow (less than 25 miles deep). According to USGS information, the nearest active fault is the Chatanika thrust fault, located approximately 24 miles north of the project area.

The procedures outlined in Section 3.10 of the 8th Edition of the 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 soils encountered at the project site, the AASHTO seismic site class for the TRRA site is expected to be Site Class E. As such, a site-specific procedure is not required for this project.

Geology and Geologic Hazards 4

The USGS interactive deaggregation website (USGS, 2020) was used to determine the peak ground and spectral accelerations at the project site based on Site Class B and C soils. The spectral accelerations were then scaled to Site Class E using the site factors recommended by the

AASHTO LRFD Manual to represent the seismic accelerations based on assumed ground conditions. The results of this analysis are presented in Table 2-1 and Figure 2-1.

Table 2-1 Summary of Seismic Parameters

Site Latitude 64.80052187° N

Site Longitude 147.74431774° W

Site Class at Project Site

(By Soil Type and Penetration Resistance in Top 100 feet of Soil Profile) E (assumed)

Horizontal Peak Ground Acceleration on Site Class B Rock (PGA) 0.36

Horizontal Response Spectral Acceleration at a

Period of 0.2 sec on Site Class B Rock (Ss) 0.65

Horizontal Response Spectral Acceleration at a

Period of 1.0 sec on Site Class B Rock (S1) 0.11

Mapped Peak Ground Acceleration Coefficient, FPGA 1.02

Mapped Short-Period Spectral Acceleration Coefficient, Fa 1.4

Mapped 1-sec Period Spectral Acceleration Coefficient, Fv 3.47

Scaled Peak Ground Acceleration for Project Site, Site Class E (As) 0.37

Scaled Horizontal Response Spectral Acceleration at a

Period of 0.2 sec for Project Site, Site Class E (SDS) 0.91

Scaled Horizontal Response Spectral Acceleration at a

Period of 1.0 sec for Project Site, Site Class E (SD1) 0.38

AASHTO Seismic Zone 3

Geology and Geologic Hazards 5

The seismic parameters given in Table 2-1 were used to develop the 5% damped-design response spectrum shown in Figure 2-1 below for the project site.

Figure 2-1 Five Percent Damped-Design Response Spectrum

2.3 LIQUEFACTION — EARTHQUAKE INDUCED

Soil liquefaction is a condition where large deformation and a high loss of strength occur in the soil mass. Earthquake-induced liquefaction generally requires a combination of conditions to occur, including a high groundwater table (saturated soils), strong earthquake ground shaking, and loose, poorly graded sands or silts.

A liquefaction potential analysis was performed on the unfrozen soils sampled during this investigation based on SPT blow counts, laboratory tests results, and the analysis procedure, as described by Youd and Idriss (2001).

All borehole locations at the project site have a layer or layers of potentially liquefiable soils.

The predominate soil types that are potentially liquefiable are loose- to medium-dense gravelly sands and silts. The full range of liquefiable soils, plotted by depth, are shown in Appendix I.

Table 2-2 shows the potential settlements of native soils due to liquefaction in three different conditions of freeze and water table depth. Settlement values in Table 2-2 are calculated from the average values of the following three methods:

• Idriss and Boulanger, 2008

• Ishihara, 1992

• Tokimatsu and Seed, 1987, as modified by Lee, 2007

Table 2-2 demonstrates that the average potential settlement due to earthquake-induced liquefaction ranges from about 2 to 7 inches over a range of conditions. The majority of liquefaction settlement analyses were based on the liquefaction behavior of sand. The gravel, silt, and organic soils interbedded with the sand materials have likely undergone soil aging, a time-dependent increase in strength and stiffness, which will generally decrease the amount of liquefaction and liquefaction settlement (Mitchell and Soga, 2005).

Geology and Geologic Hazards 6

Table 2-2 Liquefaction Settlement Analysis Results

Winter Water Table (Exploration Data)

Water Table at Active layer Depth

Water Table at 1-ft bgs with Assumed

4-ft Sub-ex

Borehole ID

Average Liquefaction

Settlement (in)

Average Liquefaction

Settlement (in)

Average Liquefaction

Settlement (in)

So u th L at h ro p

S t.

Se ct io n

(S

TA

+8 t o

+6

BH-1 Frozen/Bonded — —

BH-2 3.8 3.4 4.7

BH-3 2.4 5.5 6.8

BH-4 4.8 4.9 6.2

BH-5 NA NA NA

Average 3.7 4.6 5.9

So u th L at h ro p

S t.

Se ct io

ST

A

+2 t o

+8 0)

BH-6 Frozen/Bonded — —

BH-7 Frozen/Bonded — —

BH-8 2.5 2.2 3.7

BH-9 5.4 6.0 6.7

BH-10 3.8 3.6 4.4

BH-15 9.1 8.7 9.4

Average 5.2 5.1 6.1

Motorized Boat Launch BH-11 2.2 2.0 2.7

N o n -m o to ri ze d

B o at L au ch

Im p ro ve m en t BH-12 0.8 7.6 8.7

BH-13 6.9 6.4 7.4

BH-14 5.6 5.2 6.3

Average 4.4 6.4 7.4

Swimming

Beach Parking Lot/Vault

Toilet

BH-16 0.0 7.9 8.9

BH-17 4.3 4.7 5.8

Average 2.2 6.3 7.3

ADA Pathway (STA 5+00 to

20+80)

BH-18 0.1 3.5 3.5

BH-19 4.1 3.7 3.7

Average 2.1 3.6 3.6

Project Site Average 3.3 4.7 5.5

Site Conditions 7

3 SITE CONDITIONS

3.1 SURFACE CONDITIONS

The project area consists of old channels that have been cut off from the Tanana River, flood-control structures (levees and groins), roads, and trails. Undeveloped areas south of the Tanana

Levee are covered by natural vegetation primarily consisting of low shrubs and alder and willow trees, with occasional stands of birch, spruce, and tundra. The developed areas south of the levee consist of tall gravel embankments with armor rock on the side slopes (groins) or unreinforced side slopes (roads and parking areas).

3.1.1 South Lathrop Street Extension-Section 1 (Station 74+80 to Station 88+60)

The South Lathrop Street (Section 1) alignment overlies cleared areas leading up to the Tanana

Levee, with an existing gravel trail present along part of the alignment between the levee and

South Lathrop Street, as well as a paved road section that extends to Sanduri Avenue. The existing paved road section is in fair condition but is not believed to have been constructed to any design standard. The gravel trail was not constructed on an embankment and has potholes and signs of rutting. The levee itself is approximately 12 feet tall with low vegetation on the side slopes. The top of the levee is used by passenger vehicles and ATVs and has evidence of potholes along the gravel surface. Generally, the levee is in good condition. No evidence of frost heaving was noted.

3.1.2 South Lathrop Street Extension-Section 2 (Station 58+20 to Station 74+80)

The South Lathrop Street (Section 2) alignment is located south of the Tanana Levee. During the site visit in June 2020, the area was observed to be saturated with standing water and had marsh/bog across much of the area. This saturated condition is expected along most of the alignment that is south of the Tanana Levee during spring, summer, and fall months. There is an overgrown remnant of an access road south of the levee and approximately halfway through the marsh that can be used to access a portion of the alignment but does not appear to be significantly built up. The intersection of this relic access road with the proposed alignment is small and will not affect recommendations for construction. The Tanana-Mosquito complex is poorly drained, resulting in stagnant and slow-moving water. These conditions lend themselves to seasonal ground movement and frost jacking.

3.1.3 Parking Lots, Vaulted Restrooms, ADA Pathway (Station 5+00 to Station 20+80), and Northlake Lane Extension (Station 100+00 to Station 111+20)

Previous geotechnical reports (2007 and 2005) were focused on gravel extraction studies.

Emphasis was not on road and pad construction methods or settlement properties of overburden materials. Local knowledge suggests that the existing parking lot for the swim beach has been unofficially expanded to the west by end-dumping gravel fill material off the edge of the pad without removing compressible organics. No evidence of frost heaving was noted in the existing road prism or construction along the project site; fencing at the Motorized Boat Launch parking area did not exhibit any signs of frost jacking.

Site Conditions 8

3.2 SUBSURFACE CONDITIONS

Sampled conditions of the project site concur with the global geological characterization of the general area. The project site can be characterized by five soil types: peat, organic silt, silt, fine-grained sands, and gravels. Soil strata are deposited alluvium or water-deposited soils. The surficial 5 to 12 feet generally consists of organic silts, silt, fine sands, and some peat. This layer could be considered a modern or active depositional area. The layers below consist of mostly gravelly soils and were likely deposited in an earlier geological era with higher river energy that allowed for the transport of larger particles. Drilling response, blow counts, and liner samples indicated void lenses and arching in looser soil layers – likely due to the thawing of older ice lenses or buried aufeis.

The in-situ soils consist of frost-susceptible organic silt and organics overlying silty sand and sand to depths of approximately 10 feet bgs. The underlying soils were sandy gravel with silt and silty sandy gravel. The active layer thickness is expected to be between 3 and 8 feet bgs, depending on the thickness of the organics layer. The permafrost table was variable, with permafrost beginning at the bottom of the active layer in some areas and below 40 feet bgs in others. Based on geotechnical data from 2007, we believe that the area is underlain by permafrost at various depths. Ground temperature data is not available at this time. The water table appears to be shallow and subject to seasonal standing water depending on the water level of the Tanana River, which is high during breakup and after rains.

Historical geotechnical data suggests the area is underlain by continuous permafrost, with an active layer thickness of approximately 5 feet in undisturbed soils. The results of this geotechnical investigation, however, suggest that continuous permafrost begins at depths between the bottom of the active layer to depths below 42 feet bgs. This was evidenced by the subsurface conditions encountered in BH-9, where the soils were thawed from 5 feet bgs to the boring termination depth of 42 feet bgs, while BH-6 and BH-7 were continuously frozen from the ground surface, indicating that permafrost begins at the bottom of the active layer in these locations. Table 4-1 presents the locations, termination depths, and the depth of permafrost in boreholes that were advanced in each project area.

Site Conditions 9

Figure 4-1 presents the borehole locations graphically. The estimated depth of permafrost for

BH-6 and BH-7 was based on a calculated active layer thickness and may not be representative of the actual depth. Removal of the in-situ organics and tree clearing will result in a deeper active layer at these locations. Failure to encounter permafrost in a borehole does not indicate an absence of permafrost. Rather, the borehole may not have extended deep enough to delineate the top of the permafrost.

Settlement on the order of 18 inches was observed at the Swim Beach parking lot in September

2020. The settlement appears to be subsidence of soils rather than frost heaving. Settlement may be caused by poor compaction of fill material during construction or failure to remove compressible organic materials prior to construction of the parking lot. A complete hydraulic analysis has been performed for this project, which included recommendations for revetment

Exploration Methods and Borehole Locations 10 rock. Surficial native soils found on the project site generally will be sensitive to erosional forces.

3.3 CLIMATE

Table 3-1 shows the average maximum and minimum temperatures, monthly precipitation, and monthly snowfall data for the period from 1929 to 2016 for the Fairbanks International Airport

(502968) weather station (Western Region Climate Center, 2017).

Fairbanks Freezing Degree Day Index: 6900

Fairbanks Thawing Degree Day Index: 4000

Design Surface Freezing Days Index: 6900

Design Surface Thawing Days Index: 6800

Table 3-1 Climate Data Summary

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

Avg. Max.

Temp. (F) -1.4 8.5 23.7 42.8 60.1 70.8 72.3 66.3 54.7 32.3 11.4 1.7 36.9

Avg. Min.

Temp. (F) -19.0 -13.9 -3.4 20.4 37.9 49.3 52.0 46.8 35.7 17.4 -5.0 -15.2 16.9

Avg. Total Precip. (in) 0.57 0.44 0.35 0.28 0.58 1.34 1.96 1.84 1.05 0.77 0.67 0.69 10.53

Avg. Total Snowfall (in) 10.4 8.6 6.0 3.0 0.7 0.0 0.0 0.0 1.3 10.3 12.6 12.2 65.2

Avg. Snow Depth (in) 17 21 19 9 0 0 0 0 0 2 7 13 7

Notes:

1) Period of Record: 12/01/1929 to 06/09/2016.

2) Percent of possible observations for period of record: Max. Temp.: 99.8%; Min. Temp.: 99.8%; Precipitation: 99.9%;

Snowfall: 99.9%; Snow Depth: 99.8%.

4 EXPLORATION METHODS AND BOREHOLE LOCATIONS

The geotechnical program was conducted between March 10 and 16, 2021, using a Geoprobe

7822DT tracked drill rig. The field sampling advanced an MC7 liner through the frozen upper soils using a direct push, then switched to a 3-inch, outside-diameter split spoon sampler that was advanced using a 340-pound automatic drop hammer falling 30 inches per stroke in the thawed soils. 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 20 boreholes were advanced along the proposed South Lathrop Street extension at the location of proposed vault toilets and within the sections proposed for paving and resurfacing.

The in-situ soils within the project area generally consist of between 1 and 5 feet of organic silt

Exploration Methods and Borehole Locations 11 and organics mat underlain by a deposit of silt and silty sand extending to approximately 9 feet bgs. This is underlain by sandy gravel and sandy silty gravel with occasional interbedded sand.

Permafrost was encountered at 36 feet bgs in BH-15, and continuous permafrost beneath the active layer was present in BH-6 and BH-7 at 24.5 and 27 feet bgs, respectively. Failure to identify permafrost during drilling does not indicate a lack of permafrost, but rather that it begins deeper than the borehole depth at that location.

Samples were typically collected in continuous 5-foot liners when drilling through the frozen surficial soils. After advancing through the frozen soils, samples were collected every 5 feet using a split spoon sampler to obtain representative samples of the subsurface soil conditions.

The depth to unfrozen soils varied from 3 feet bgs (BH-9) to greater than 27 feet bgs (BH-7).

The recovered soils were field-classified following the Unified Soils Classification System

(USCS), according to ASTM D2488. The borehole logs are attached to this report in

Appendix A.

Table 4-1 Boreholes Advanced in each Project Area

Area Borehole Latitude Longitude Elevation*

(ft) Depth

(ft)

Permafrost Depth**

(ft)

South Lathrop Street (Section 1)

BH-1 64.806001° N 147.744728° W 443 15 -

BH-2 64.805627° N 147.744797° W 443 27 -

BH-3 64.804602° N 147.744624° W 440 27 -

BH-4 64.803595° N 147.744636° W 439 27 -

BH-5 64.802479° N 147.744400° W 451 27 -

South Lathrop Street (Section 2)

Northlake Lane extension

BH-6 64.801398° N 147.744611° W 441 24.5 4.5ⴕ

BH-7 64.800560° N 147.744807° W 442.9 27 3ⴕ

BH-8 64.799896° N 147.744972° W 442 27 -

BH-9 64.799290° N 147.745094° W 438 42 -

BH-10 64.798083° N 147.744354° W 440 25.5 -

BH-15 64.801148° N 147.742650° W 441 41 36

Non-motorized boat launch improvements

BH-12 64.797262° N 147.742629° W 448 27 -

BH-13 64.796442° N 147.740951° W 443 27 -

BH-14 64.796146° N 147.739644° W 442 17 -

BH-20 64.794976° N 147.738621° W 444 4 -

Parking Lots & Vault Toilets

BH-11 64.797407° N 147.744768° W 441 17 -

BH-16 64.800388° N 147.736433° W 447 26 -

BH-17 64.799961° N 147.736551° W 446 27 -

ADA Pathway

BH-18 64.799655° N 147.737520° W 442 17 -

BH-19 64.799408° N 147.740228° W 441 17 -

* Elevations according to the North American Vertical Datum of 1988 (NAVD88). Horizontal Datum WGS 84.

**Permafrost was not encountered in boreholes noted with a dash.

ⴕ Calculated active layer depth of in-situ soil conditions. Active layer thickness will increase with removal of surface organics and clearing.

Exploration Methods and Borehole Locations 12

Figure 4-1. Boreholes Advanced in Each Project Area

Lab Soils Characterization 13

5 LAB SOILS CHARACTERIZATION

5.1 LABORATORY TESTING

Representative samples from the MC7 liners and SPT split spoon sampler were collected and sealed in freezer bags to maintain in-situ moisture content. The samples were then transported to the PND Soils-Material Laboratory in Anchorage for testing. All samples were kept frozen from the time they were collected until they were tested. Tests were performed to ASTM standards where applicable. A total of 214 lab tests were performed (Table 5-1).

Table 5-1: Laboratory Tests Performed on Collected Samples

Test Type Quantity

Moisture Content with Classification

(ASTM D2487 / D2488 / D2216) 142

Gradation of Soils (ASTM D6913) 17

Fines Wash (ASTM D1140) 6

Atterberg Limits (ASTM D4318) 4

Density (ASTM D7263) 39

1-Dimensional Consolidation Test (ASTM D2435) 3

Hydrometer 1

Soil Salinity 1

Specific Gravity (ASTM D854) 1

The results of the laboratory testing program are shown in appendices C, D, E, F, and G. The appendices include a comprehensive tabulated summary of test results and soil classifications.

The appendices also include grain-size distribution plots and moisture contents by depth for each of the project areas. Lab results are shown in the borehole logs at corresponding columns with given soil lithologies.

Gradations

Table 5-2 shows average or representative soil fractions of the soil types sampled based on gradations and fines washes. D50 defines the soil particle size that 50% of the soil in the sample is smaller than. P10 describes the percent of soil 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 1.5 inches across the project site when it was encountered.

Collected silts and organic silts had trace to moderately heavy amounts of mica. The organic silts contained between 8 to 20% fine amorphous organic material. An organic burn on the mineral silts indicated 1% organic content by mass.

Lab Soils Characterization 14

Table 5-2. Representative Gradation Properties

Borehole ID Material Type

Soil Type Fractions (%) D50 (mm)

P10 (%) Gravel Sand Fines

South Lathrop Street (Section 1)

BH-01 Silt (ML) 0 4.5 95.5 - 100

BH-03 Sandy Poorly-graded gravel (sGP) 53.2 42.3 4.5 5.3 37.0

BH-05

Sandy Poorly-graded Gravel with Silt

(s(GP-GM)) 48.4 45.4 6.3 3.8 45.6

South Lathrop Street (Section 2) and Northlake Lane extension

BH-06

Sandy Poorly-graded Gravel with Silt

(s(GP-GM)) 53.8 36.8 9.4 4.8 37.1

BH-07 Sandy Well-graded Gravel (sGW) 60.6 35.7 3.7 7.6 30.3

BH-15 Sandy Poorly-graded gravel (sGP) 63.0 33.7 3.3 9.0 30.2

Non-motorized boat launch improvements

BH-12 Silty, Gravelly Sand (gSM) 38.3 46.4 15.3 1.2 52.4

BH-13 Silty Sand (SM) 14.0 68.8 17.2 0.2 83.8

BH-20

Sandy Poorly-graded Gravel with Silt

(s(GP-GM)) 53.5 38.3 8.2 5.4 38.3

Parking lots and vault toilets

BH-16

Sandy Poorly-graded Gravel with Silt

(s(GP-GM)) 54.2 38.8 6.9 5.7 37.5

BH-17 Sandy Poorly-graded gravel (sGP) 62.1 33.9 4.0 7.4 25.7

Sand and gravel 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. The fines fraction of gravelly material samples ranged from 1 to 16%, with gravelly fill material in the existing roadway having 9% fines on average and in-situ gravelly material having 5% on average. The gradation curves for the samples are shown in Appendix C.

5.2 MOISTURE CONTENT

Moisture content by soil type and depth are shown in Appendix E in Table E-1 through Table E-

7. Graphical plots of moisture content versus depth are shown for individual boreholes. Moisture contents are also shown in the lab “Summary of Sample Characteristics” in Appendix B and

“Borehole Logs” in Appendix A.

5.3 ATTERBERG LIMITS

Four Atterberg Limits tests were completed to verify field and visual classifications of fine-grained samples. The determination identified the fines type in relation to the soil material above and below the sample location. All Atterberg Limits tests that were performed indicated a silt

(ML) soil type. The Atterberg Limits tables and plots are found in Appendix F.

5.4 DENSITY AND UNIT WEIGHT

Density and total-unit-weight calculations of in-situ soils were performed on soil samples collected in acrylic or brass liners. Density tests were performed on samples from all boreholes

Determined Subsurface Conditions 15 except for BH-17 and BH-18. Appendix G provides the density and wet- and dry-unit weights of all samples that were tested.

5.5 CONSOLIDATION TEST

Three consolidation tests were performed at the PND soils lab on samples collected from boreholes BH-8 and BH-15. The soil types identified for these two boreholes were organic silt

(ML) and silt with sand (MLs), respectively. Strains at final loadings are shown in Table 5-3.

Consolidation test samples were very disturbed, with some ice lensing in the frozen state and friable characteristics during sampling. The high initial strain at seating loads indicate there were likely internal voids or freeze/thaw bridging in the samples. However, based on visual inspection and soil sample softness, the strains occurring after the initial loadings would appear to be fairly representative.

Table 5-3. Consolidation Strain

Sample ID Total Strain

Final Load Before Unload

(tsf)

BH-8, Sample 1 34.7 31.4

BH-15, Sample 1 17.2 31.4

BH-15, Sample 2 17.6 31.4

The tests were performed in accordance with ASTM D2435. A summary of the test parameters is shown in Table G-1, followed by strain-logp curves in Appendix G.

6 DETERMINED SUBSURFACE CONDITIONS

The determined soil profiles and properties for each project area, based on the corrected SPT blow counts, lab results, and lithology, are provided in Table 6-1 through Table 6-7. Appendix H shows corrected SPT blow counts versus depth bgs for the boreholes located in each project area.

Blow counts were corrected to (N1)60 using standard correlations found in most geotechnical texts. The bonded (frozen) condition of the active layer did not allow for split spoon sampling or blow count characterization. Where the ground was frozen, continuous coring methods with acrylic liner were utilized. The liner samples were tested for density/unit weight. Densities were then correlated to blow count and strength characteristics. These engineering properties were utilized in settlement and stability analyses. The following sections provide idealized design profiles for each project area.

Graphical soil lithological cross sections are shown in Appendix B for the project site.

6.1 SOUTH LATHROP STREET (SECTION 1) SOIL PROFILE – STATION 74+80 TO STATION

88+60

Four feet of medium-dense, poorly graded Sandy Gravel (sGP) and Sandy Gravel with Silt

(s(GP-GM)) fill with a nominal maximum aggregate size of 2 inches. This is overlying 6 feet of firm Silt (ML) and Silt with Sand (MLs) above medium-dense, well- to poorly graded Sandy

Gravel (sGW and sGP) and poorly graded Sandy Gravel with Silt (s(GP-GM)) with a nominal maximum aggregate size of 2 inches. Discontinuous lenses of poorly graded Sand (SP) are present at depths greater than 10 feet bgs.

Determined Subsurface Conditions 16

Table 6-1. Design Profile for South Lathrop Street (Section 1)

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak (deg)

Cohesion (psf)

0 - 4 s(GP-GM) (fill) 15 130 34 NA

4 - 10 ML 18 100 31 NA

> 10 s(GP-GM) 15 120 33 NA

6.2 SOUTH LATHROP STREET (SECTION 2) SOIL PROFILE – STATION 58+20 TO STATION

74+80

Five feet of very soft Organic Silt (OL) overlying 5 feet of soft Silt (ML) to Sandy Silt (sML) and loose Silty Sand (SM). This is underlain by 10 feet of medium-dense, poorly to well-graded

Sandy Gravel (sGP and sGW) and poorly graded Sandy Gravel with Silt (s(GP-GM)), with occasional poorly graded Sand with Silt and Gravel ((SP-SM)g) that has a nominal max aggregate size of 1 inch. Beneath this layer is medium-dense, poorly to well-graded Gravel with

Sand (GPs and GWs) with a maximum aggregate size of 2.5 inches. Occasional layers of poorly graded Sand (SP) are present at depths greater than 10 feet bgs.

Near-surface permafrost was encountered in BH-6 and BH-7 and deep permafrost in BH-15.

Permafrost in BH-6 and BH-7 extended to full depth of the borehole (approximately 25 feet bgs).

It is unknown how far the near-surface permafrost extends towards BH-8, but it could be assumed that near-surface permafrost extends approximately 500 feet through South Lathrop

Street (Section 2). The full depth of the proposed embankment will provide substantial thermal resistance under the main mass of the embankment. However, considerable thaw will occur at the shoulder fore-slope and any denuded areas adjacent to the toe of the embankment. The thaw and associated thaw subsidence will likely contribute to considerable differential settlement that will be detrimental to the quality and serviceability of the asphalt wear course in this area.

Table 6-2. Design Profile for South Lathrop Street (Section 2)

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak

(deg) Cohesion

(psf)

0 - 5 OL 2 70 22 NA

5 - 10 sML 4 90 26 NA

10 - 20 s(GP-GM) 11 120 32 NA

> 20 GPs 15 130 34 NA

6.3 NORTHLAKE LANE EXTENSION SOIL PROFILE – STATION 100+00 TO STATION 106+40

Five feet of very soft Silt with Sand (MLs) to very loose Sandy Silt (SM) underlain by medium-dense, poorly graded Sandy Gravel with Silt to Sandy Gravel (s(GP-GM) to sGP) with occasional interbedded poorly graded Sand (SP). Gravelly materials have a nominal maximum aggregate size of 3 inches.

Determined Subsurface Conditions 17

Table 6-3. Design Profile for Northlake Lane Extension

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak (deg)

Cohesion (psf)

0 - 5 MLs 2 85 25 NA

> 5 s(GP-GM) 10 110 34 NA

6.4 SOUTH LATHROP STREET NON-MOTORIZED BOAT LAUNCH IMPROVEMENTS SOIL

PROFILE

Five feet of dense Sandy Gravel to Sandy Gravel with Silt (sGP to s(GP-GM)) with a nominal maximum aggregate size of 2.5 inches overlying 5 feet of Silt and Silty Sand (ML and SM). This is underlain by Sandy Gravel with Silt (s(GP-GM)) to Sandy Gravel (sGW) with interbedded

Sand (SP) and a nominal maximum aggregate size of 2 inches.

Table 6-4. Design Profile for South Lathrop Street Non-Motorized Boat Launch Improvements

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak (deg)

Cohesion (psf)

0 - 5 s(GP-GM) (fill) 15 115 34 NA

5 - 10 ML 4 85 26 NA

> 10 (GP-GM)s 15 115 34 NA

6.5 SWIMMING BEACH PARKING LOT AND VAULT TOILETS SOIL PROFILE

Six feet of medium-dense Gravelly Silty Sand (s(GP-GM)) with a nominal max aggregate size of

2.5 inches. This is overlying 3 feet of Silt (ML), 1 foot of fibrous Peat (PT), and Sandy Gravel

(sGP) with a nominal max aggregate size of 1.5 inches. Occasional interbedded Sand (SP) layers are presents at depths below 10 feet bgs.

Table 6-5. Design Profile for Swimming Beach Parking Lot and Vault Toilets

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak (deg)

Cohesion (psf)

0 - 6 s(GP-GM) (fill) 14 115 32 NA

6 - 9 ML 6 90 28 NA

9 - 10 PT 1 70 - NA

> 10 sGP 15 120 36 NA

6.6 MOTORIZED BOAT LAUNCH PARKING LOT SOIL PROFILE

Three feet of Gravelly Sand (gSP) fill with a nominal max aggregate size of 1 inch followed by 3 feet of Silt (ML) and 5 feet of Sandy Silt (sML). This is overlying 3 feet of Sandy Gravel (sGP) with a nominal max aggregate size of 1 inch, 2 feet of sand (SP), and Gravel with Sand (GWs) that has a nominal max aggregate size of 1 inch.

Determined Subsurface Conditions 18

Table 6-6. Design Profile for Motorized Boat Parking Lot

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak (deg)

Cohesion (psf)

0 - 3 gSP (fill) 10 115 34 NA

3 - 6 ML 4 85 26 NA

6 - 11 sML 15 110 32 NA

11 - 14 sGP 15 115 34 NA

14 - 16 SP 7 105 31 NA

>16 GWs 7 110 32 NA

6.7 ADA PATHWAY SOIL PROFILE-STATION 5+45 TO STATION 9+70

Five feet of loose, poorly graded Sand with Silt (SP-SM) to Sandy Silt (sML) and Silt (ML). This is underlain by a 1.5-foot-thick layer of soft Organic Silt (OL) and medium-dense, poorly graded

Sandy Gravel (sGP) to Gravel with Sand (GPs) with a nominal max aggregate size of 2 inches.

Occasional lenses of poorly graded Sand (SP) are present at depths greater than 6.5 feet bgs.

Table 6-7. Design Profile for ADA Pathway

Depth

(ft) Soil Type

Design (N1)60

(blows/ft)

Total Unit Weight

(pcf) φ'peak (deg)

Cohesion (psf)

0 - 5 sML 4 85 26 NA

5 - 6.5 OL 3 85 22 NA

> 6.5 sGP 15 120 34 NA

6.8 GROUNDWATER

Groundwater was encountered at approximately the bottom of the active layer across the project area, at a nominal depth of approximately 10 feet. Groundwater levels are based on observations during sample collection. Seasonal groundwater is expected to be shallow and at the same elevation as standing water in adjacent ponds and stagnant sloughs, between approximately 3 and

8 feet bgs. Water levels will fluctuate during seasonal high water. Standing water is present along much of the South Lathrop Street (Section 2) alignment south of the Tanana Levee during spring, summer, and fall months.

Analyses and Results 19

7 ANALYSES AND RESULTS

7.1 DESIGN TRAFFIC LOADS

The following design values were provided in the Scoping Report dated September 25, 2019, and conform to Federal Highway Administration (FHWA) and Alaska Department of Transportation

& Public Facilities (DOT&PF) design standards. These loads were used to develop the pavement and structural section recommendations shown in the drawings and discussed in the following section. This design vehicle was also incorporated into the slope stability analysis, discussed in the following section.

• Design AADT: 750

• 5% trucks

• 3% growth rate

• 15-year design life

• Design vehicle: Motor home and boat trailer (MH/B) and 250 psf live vehicle surcharge load

• ESAL: 216,387

7.2 SLOPE STABILITY ANALYSIS

A two-dimensional limit equilibrium slope stability analysis was performed on the cross-section of the proposed South Lathrop Street embankment located between station 63+00 and 70+50 using Rocscience’s Slide2 limit equilibrium software. The analysis considered the stability of the proposed road embankment under seismic and highway traffic loading conditions. Analyses were performed assuming different amounts of the uppermost organic silt layer would be removed and replaced with imported fill within a 1:1.5 (vertical to horizontal) projection from the toe of roadway aggregate. The imported fill was assumed to have properties consistent with Select

Borrow material, and the groundwater table was assumed to be located 1 foot below the existing ground surface for all analyses.

Pseudo-static analyses were performed to evaluate the stability of the embankment when subjected to the site-class-adjusted design earthquake accelerations given in Table 2-1. The external traffic load for stability was modeled as a 250 psf traffic surcharge that spanned the travel width of the road.

The analysis incorporated 1 layer of Miragrid 3XT geogrid (or equal) located at the elevation of the existing ground surface. The minimum factor-of-safety for static (long-term) conditions is between 1.3 and 1.5, as noted in section 4.7 of FHWA’s Geotechnical Technical Guidance

Manual. For seismic conditions, the minimum factor of safety is FS=1.1 at a given acceleration.

Table 7-3 presents a summary of the factors of safety according to Spencer’s Method that were calculated for the differing amounts of organic silt removed and the corresponding embankment thicknesses. Refer to Appendix K for slope stability analysis parameters, graphic outputs, and results.

Table 7-1 shows the soil parameters that were used in the slope stability model of the proposed geosynthetic reinforced embankment for South Lathrop Street (Section 2). Select Borrow

Analyses and Results 20 beneath the existing ground surface was given a lower friction angle than the Select Borrow above the existing ground surface in this analysis to account for the possibility of less material compaction during construction.

Table 7-1 Soil Parameters Used for Geosynthetic Reinforced Embankment Design

Material Unit Weight

(pcf) Friction Angle

(deg) Cohesion

(psf)

Select Borrow (Above Existing Ground) 135 38 0

Select Borrow (Below Existing Ground) 135 36 0

Subbase Grading B 135 40 0

Roadway Aggregate, Method 2 135 40 0

Asphalt 150 0 2500

The materials listed in Table 7-1 were assumed to be underlain by the idealized soil profile for

South Lathrop Street (Section 2), discussed in Section 4 of this report. One layer of Miragrid

3XT (or equal) geogrid was placed at the existing ground surface for all scenarios. The material properties assigned to the geogrid in the slope stability model are given in Table 7-2. The geogrid should be used in conjunction with a non-woven geotextile separator (placed between the native soils and the imported fill) to prevent fines migration into the borrow material.

Table 7-2. Geosynthetic Properties Used in Slope Stability Model

Geosynthetic Ultimate Tensile Strength (lbs/ft)

Creep Reduction Factor RFcr

Installation Reduction Factor

RFid

Deterioration Reduction Factor RFd

Miragrid 3XT (or equal) 3,500 1.44 1.25 1.1

Table 7-3. Factor of Safety Summary for Slope Stability Analysis

Excavation Depth (ft)

Embankment Thickness (ft)

FS

Seismic Load

100% PGA

FS

Seismic Load

50% PGA

FS

250 psf Traffic

Surcharge Load

0 6.75 0.70 1.01 1.70

1 7.75 0.71 1.01 1.58

2 8.75 0.72 1.02 1.60

3 9.75 0.73 1.03 1.65

4 10.75 0.74 1.04 1.66

5 11.75 0.75 1.06 1.70

A sensitivity analysis for groundwater depth, friction angle of Select Borrow, and surcharge load offset was performed. These analyses indicated that the proposed embankment satisfies the factor of safety requirements under the 250 psf surcharge load for all conditions that can be reasonably expected.

Analyses and Results 21

7.3 SETTLEMENT ANALYSIS

A settlement analysis was conducted for the proposed South Lathrop Street (Section 2) embankment using Rocscience’s Settle3 software. The analysis considered the placement of a

6.75-foot-thick road embankment on the design profile.

Material properties for the upper 10 feet of soil underlying the road embankment were determined from the…