GR 14-21 AK BLM GFO 2019(1) Sourdough Campground Bridge Final Geotechnical and Foundation Report.pdf

Sourdough Campground Bridge Replacement February 2022

Table of Contents 1-i

Geotechnical Report No. 14-21

Sourdough Campground Bridge Replacement

FINAL GEOTECHNICAL AND FOUNDATION REPORT

AK BLM GFO 2019(1)

Valdez-Cordova Census Area, Alaska February 2022

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Table of Contents i

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Project Description

1.2 Purpose and Scope

2 GEOLOGY

2.1 Geologic Hazards

2.1.1 Site Seismicity

2.1.2 Seismic Source Selection and Design Earthquake

2.1.3 Liquefaction

3 SITE CONDITIONS

3.1 Average Temperatures and Precipitation

3.2 Scour

4 SUBSURFACE CONDITIONS

4.1 Subsurface Borings

4.2 Laboratory Testing

4.3 Groundwater

5 FOUNDATION ANALYSIS AND RECOMMENDATIONS

5.1 Bridge Layout and Foundation Considerations

5.2 Design Methodology

5.3 Axial and Lateral Resistance

5.4 AXIAL Group Considerations

5.5 Lateral Group Considerations

5.6 Lateral Spread

5.7 Service Limit State Analysis

5.8 Drag Load

5.9 Frost Heave

5.10 Pile Driving Considerations

5.11 Pile Corrosion

5.12 Abutment and Wing Wall Recommendations

6 EARTHWORK ANALYSIS AND RECOMMENDATIONS

6.1 Anticipated Foundation Excavation Uses

6.1.1 Waste 25

6.1.2 Foundation Excavation

ii Table of Contents

7 CONSTRUCTION

7.1 Wet Weather Earthwork

7.2 Construction Dewatering

7.3 Pile Driving

8 LIMITATIONS

9 REFERENCES

List of Figures iii

LIST OF FIGURES

Figure 1-1 Project Location Map

Figure 1-2 Site Vicinity Map

Figure 2-1 Excerpt from the Geologic Map of Alaska – Eastern part, and central Aleutians

Figure 2-2 Quaternary Faults Map for the Site Vicinity

Figure 2-3 Five Percent Damped Design Response Spectrum

Figure 2-4 Deaggregation Plot (USGS, 2021b)

Figure 2-5 Tabular Deaggregation Summary (USGS, 2021b)

Figure 5-1 Effects of Temperature on the Soil Stiffness (Shelman et al., 2014)

Figure 5-2 Definition of Loading Direction and Spacing for Group Effects (Figure

10.7.2.4.1 AASHTO LRFD Bridge Design Specifications, 9th Edition, 2020) iv List of Tables

LIST OF TABLES

Table 2-1 Summary of Seismic Parameters

Table 2-2 Deaggregated Major Earthquake Sources

Table 3-1 Climate Data Summary (1971 to 1996)

Table 3-2 Climate Data Summary (1981 to 2010)

Table 3-3 Scour Summary

Table 5-1 Final Factored Axial Abutment Loads

Table 5-2 Pile Tip Elevations

Table 5-3 East Abutment (BH20-01) Recommended Soil Parameters for Lateral Analysis

Table 5-4 West Abutment (BH20-02/B) Recommended Soil Parameters for Lateral Analysis

Table 5-5 AASHTO Recommended P-Multipliers for Lateral Analysis of Pile Groups

Table 5-6 Corrosivity Index Test Results

Table 5-7 Recommended Static Soil Parameters for Abutment and Wing Wall Design

List of Photos v

LIST OF PHOTOS

Photo A-1. View of Sourdough Campground Bridge Close-Up, Facing West

Photo A-2. View of Approach and Existing Bridge, Facing West

Photo A-3. View of Sourdough Creek From Bridge, Facing Downstream

Photo A-4. Test Boring Location BH20-01, Facing East

Photo A-5. Test Boring Location BH20-02, Facing East

Photo A-6. Test Boring Location BH20-02B, Facing West vi Appendices

APPENDICES

APPENDIX A – SITE PHOTOGRAPHS

Photo A-1. View of Sourdough Campground Bridge Close-Up, Facing West

Photo A-2. View of Approach and Existing Bridge, Facing West

Photo A-3. View of Sourdough Creek From Bridge, Facing Downstream

Photo A-4. Test Boring Location BH20-01, Facing East

Photo A-5. Test Boring Location BH20-02, Facing East

Photo A-6. Test Boring Location BH20-02B, Facing West

APPENDIX B – SITE MAPS

Figure B-1. Site Vicinity Map

Figure B-2. Site Aerial Photograph

Figure B-3. Site Exploration Map

APPENDIX C – SUBSURFACE EXPLORATION LOGGING

Descriptive Terminology for Boring Logs USCS Soil Group Classification Boring Legend Boring Log BH20-01 Boring Log BH20-02 Boring Log BH20-02B

APPENDIX D – LABORATORY TEST RESULTS

Soil Index Testing

Corrosivity Index Testing

One-Dimensional Consolidation Testing

APPENDIX E – SEISMIC ANALYSIS

USGS Unified Hazard Tool

Liquefaction Analysis

Lateral Spread Slope Stability Analysis

APPENDIX F – CALCULATIONS

Axial Capacity Analysis

Beam Moment Analysis

Neutral Plane Analysis

Soil Static Settlement

Introduction 1

1 INTRODUCTION

This report provides the geotechnical investigation results, analysis, and final foundation and geotechnical recommendations for the Sourdough Campground Bridge Replacement project. The project is located approximately 21 miles north of Gakona, Alaska and 250 feet west of Richardson Highway at MP 147.5 in the Valdez-Cordova Census Area, Alaska as shown in Figure 1-1 Project Location Map and Figure 1-2 Site Vicinity Map. Orthometric elevations referenced in this report are based on the NAVD88 vertical datum unless stated otherwise. The bridge provides access to the Sourdough Creek Campground managed by the U.S. Bureau of Land Management

(BLM).

1.1 PROJECT DESCRIPTION

The existing 50-foot long bridge over Sourdough Creek is a timber structure built in 1990 with glulam girders salvaged from an old state highway bridge. The proposed project involves removing and replacing the existing bridge with a new bridge spanning approximately 90 feet.

The proposed bridge will be located on the same alignment. A temporary crossing will be installed during construction for public access. Photos of the bridge are included in Appendix A.

1.2 PURPOSE AND SCOPE

The purpose of this report is to provide geotechnical recommendations for bridge foundation design, grading and site preparation, and construction considerations. The geotechnical scope of work included a visual reconnaissance along the proposed alignment, a subsurface exploration program consisting of two geotechnical test borings (BH20-01 and BH20-02/BH20-02B), ground temperature measurements, laboratory soil index testing, preparation of interim memorandums, participation in bridge foundation selection meetings, and preparation of this report. Geotechnical borings and ground temperature measurements were completed by a Robert Peccia Associates / Golder Associates representative and drilling subcontractor under the direction of a Western Federal Lands Highway Division (WFLHD) geotechnical engineer. Laboratory tests were completed by WFLHD materials testing staff and Kuo Testing Labs.

The following reports were previously prepared by WFLHD and are available by request:

Preliminary Geotechnical Fill Recommendations Memorandum, GM 29-20, dated June 19, 2020

Preliminary Foundation Recommendations Memorandum, GM 10-21, dated March 15, This report supersedes the previous memorandums for this project.

Figure 1-1 Project Location Map

Figure 1-2 Site Vicinity Map

4 Geology

2 GEOLOGY

The bridge site is located in a broad basin between mountainous terrain including the Alaska Range to the north, Talkeetna Mountains to the west, Chugach Mountains to the south, and Wrangell Mountains to the east. The Geologic Map of the Alaska – Eastern part, and central Aleutians (Wilson, et. al., 2015) maps the bridge location in a poorly consolidated surficial deposit, QTs, as shown in an excerpt in Figure 2-1 below. This deposit is described as silt to coarse gravel and semi-consolidated sandstone to conglomerate as an erosional remnant deposit. The unit includes deposits of fluvial, glaciofluvial, colluvial, eolian, shallow-marine, and local tuffaceous deposits. Deposit ages are Quaternary, uppermost Tertiary, and Pleistocene-aged.

Figure 2-1 Excerpt from the Geologic Map of Alaska – Eastern part, and central Aleutians

Sourdough Campground

Bridge

Geology 5

2.1 GEOLOGIC HAZARDS

Geologic hazards can include active faults, liquefaction and related phenomenon, soft or subsiding ground, strong ground shaking, and landsliding, among other natural hazards. Our search did not discover a published geologic hazard map for the vicinity. Based on the geologic mapping and the subsurface conditions encountered in our borings (discussed later in this report), it is our opinion that the relevant geologic hazards in the site vicinity include: active faults, liquefaction/lateral spreading, and strong ground shaking. These hazards are discussed further in the Site Seismicity section below.

2.1.1 SITE SEISMICITY

Information from the USGS Earthquake Hazards Quaternary Fault and Fold Database (USGS, 2022) of the United States was used to locate nearby active faults. According to the website, the Denali fault, west Muldrow-Alsek section, runs northwest to southeast near the site vicinity, as shown in Figure 2-2 below. The right-lateral strike-slip fault is described as 4254 kilometers (2643 miles) in length with south 84 degrees strike and a vertical dip direction. The slip rate is reported to be in the category of greater than 5 millimeters (0.2 inches) per year, and the age is categorized as historic (<150 years). The solid line of the mapping website indicates that the fault is well constrained. In 2002, the Denali Fault ruptured in a magnitude 7.9 earthquake at a depth of 5.0 km that triggered seiches in lakes as far away as Lake Union in Seattle, Washington and Lake Pontchartrain in Louisiana (Fuis & Wald, 2003).

Also of significance in the region, is the Aleutian Subduction Zone, which runs in a generally east-west arc off the southern coast of Alaska. The thrust fault is described as 6350 kilometers (3946 miles) in length with south 76 degrees west strike and a north dip direction. The slip rate is reported to be in the category of greater than 5 millimeters (0.2 inches) per year, and the age is categorized as historic (<150 years). The solid line of the mapping website indicates that the fault is well constrained. Subduction zone earthquakes are capable of generating earthquakes with moment magnitudes of 8 or higher.

AASHTO LRFD Bridge Design Specifications (9th Edition, 2020) use earthquake design ground motion with a 7 percent probability of exceedance in 75 years, approximately equivalent to a return period of 1,000 years. The seismic hazard at the site and the importance of the proposed bridge dictates the level of seismic analysis that is required for the design. The seismic hazard principles within the current AASHTO Specifications are: 1) small to moderate earthquakes should be resisted without significant damage; 2) realistic seismic ground motion intensities and forces should be used in the design; and 3) exposure to shaking from large earthquakes may cause damage to the bridge, but the probability of collapse will be low and the damage will be limited to areas of the structure that are accessible for inspection and repair.

The AASHTO LRFD Bridge Design Specifications state that a site-specific probabilistic analysis to develop design response spectra shall be used if the site is located within 6 miles of an active fault and studies shall be considered to quantify near-fault effects on ground motions to determine if these could significantly influence the bridge response. The Denali fault section is 52 miles north of the project, therefore, a site-specific probabilistic analysis is not needed.

6 Geology

Figure 2-2 Quaternary Faults Map for the Site Vicinity

The procedures outlined in Section 3.10 of the AASHTO LRFD Bridge Design Specifications were followed to characterize the seismic hazard. Based on Table C3.10.3.1-1—Steps for Site Classification of the AASHTO LRFD Bridge Design Specifications, the seismic Site Class for the Sourdough Campground Bridge site is Site Class D.

The U.S. Seismic Design Maps website (USGS, 2021a) was used to determine the peak ground acceleration and spectral accelerations at the project site based on Site Class B rock. The spectral accelerations were then scaled to Site Class D using the site factors recommended by AASHTO to represent the seismic accelerations based on actual ground conditions. The results of this analysis are presented below in Table 2-1.

Aleutian Subduction Zone

Denali Fault

Geology 7

Table 2-1 Summary of Seismic Parameters

Site Latitude 62.527388°

Site Longitude -145.516781°

Site Class at Project Site

(by Soil Type and Penetration Resistance in Top 100 feet of Soil Profile) D

Horizontal Peak Ground Acceleration on Site Class B Rock (PGA) 0.215

Horizontal Response Spectral Acceleration at a

Period of 0.2 sec on Site Class B Rock (Ss) 0.474

Horizontal Response Spectral Acceleration at a

Period of 1.0 sec on Site Class B Rock (S1) 0.217

Mapped Peak Ground Acceleration Coefficient, FPGA 1.37

Mapped Short-Period Spectral Acceleration Coefficient, Fa 1.421

Mapped 1-sec Period Spectral Acceleration Coefficient, Fv 1.967

Scaled Peak Ground Acceleration for Project Site, Site Class D (As) 0.295

Scaled Horizontal Response Spectral Acceleration at a

Period of 0.2 sec for Project Site, Site Class D (SDS) 0.673

Scaled Horizontal Response Spectral Acceleration at a

Period of 1.0 sec for Project Site, Site Class D (SD1) 0.426

AASHTO Seismic Zone 3

The values in Table 2-1 were used to develop the 5-percent-damped-design response spectrum shown in Figure 2-3 below for the project site.

Figure 2-3 Five Percent Damped Design Response Spectrum

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

E la s ti c S ei s m ic

C o ef fi c ie n t, C sm

Period, Tm (seconds)

8 Geology

2.1.2 SEISMIC SOURCE SELECTION AND DESIGN EARTHQUAKE

The USGS seismic parameters discussed in the previous section are more complicated in the Pacific Northwest and Alaska regions due to the presence of coastal subduction zone sources capable of large magnitude earthquakes that can cause strong ground motions hundreds of kilometers inland from the source. These subduction zone sources are aggregated with other seismic sources into the conservative code-based design seismic parameters presented above.

For the simplified liquefaction procedure discussed in the next section, however, subduction zone sources are commonly deaggregated from the other sources for more realistic liquefaction analyses. The deaggregation analyses performed for this study addresses earthquakes from the following sources:

1. Shallow crustal earthquakes in close proximity to the site. The source may be a identified fault or gridded seismic data from the USGS and should generally have a moment magnitude greater or equal to 6.

2. A subduction zone intraslab earthquake with a moment magnitude greater than 7 at depths ranging from 20 to 50 km and a maximum distance of 500 km.

3. A subduction zone interface earthquake with a minimum magnitude of 8.5 with a maximum distance of 500 km.

4. Other seismic sources of interest in the study.

The Unified Hazard Tool website (USGS, 2021b) was utilized to generate deaggregation data of seismic sources used in the USGS ground motions maps. Deaggregation results are shown in Figure 2-4 and Figure 2-5. The plot graphically shows the sources used to aggregate the code-based seismic parameters. The tabular data provides fault data of the largest contributors. This table was used to identify three major seismic sources: 1) local crustal faults with a high magnitude 6.11; 2) the interface subduction zone with a magnitude 9.2; and 3) the Denali fault with a magnitude 7.92. The data shows a relatively high contribution from intraslab sources at depths of 50 to 80 km, but no source data was provided. Furthermore, depths beyond 50 km are outside the range of the subduction zone ground motion models used for this study. The Denali fault was also included due to its long length, prominent presence in the region, and recent activity.

Geology 9

Figure 2-4 Deaggregation Plot (USGS, 2021b)

Figure 2-5 Tabular Deaggregation Summary (USGS, 2021b)

10 Geology

Table 2-2 Deaggregated Major Earthquake Sources

Source Type rrup (km) Mw 0 PGAGMM

Eastern Aleutian Region – 1964 Zone, Eastern Aleutian Segment 1

Interface 118.35 9.20 0.71 0.16

Shallow Crustal Seismicity

Grid 12.85 6.11 0.32 0.19

Denali – Totschunda System, Denali Center

System 83.18 7.92 1.40 0.09

The above source and fault information were input into four of the Next Generation Attenuation (NGA) West 2 (W2) models published in 2013 by the Pacific Earthquake Engineering Research Center (PEER). The four NGAW2 models used were: the Abrahamson, Silva, Kamai model (Abrahamson, et. al., 2013); the Boore, Stewart, Seyhan & Atkinson model (Boore, et. al., 2013);

the Campbell & Borzognia model (Campbell & Borzognia, 2013); and the Chiou & Youngs model (Chiou & Youngs, 2013). All four models were given equal weight of 25 percent. The Idriss NGAW2 model (Idriss, 2013), was not used since the model’s Vs,30 range was not applicable for the subject site. The NGA West-2 GMPE spreadsheet developed by Seyhan (2015) was used to perform the calculations, and a weighted average of the models were used to determine the ground motion model predicted peak ground acceleration (PGAGMM), which was taken to be the median spectral acceleration at a period of 0.01 s.

For the Aleutian subduction source, its fault data was input into an NGA-Sub model and spreadsheet developed by Abrahamson & Gülerce (2020) to determine the PGAGMM. Although this subduction zone model can be used globally, only the PGAGMM for the Alaska Adjusted model was considered for this study.

All models used an estimated average 240 m/s shear wave velocity, Vs,30, for the upper 100 feet of the site. This value was based on a combination of SPT-to-shear-wave-velocity correlations and engineering judgement. Resulting peak ground accelerations from the ground motion models are shown in Table 2-2.

2.1.3 LIQUEFACTION

Soil liquefaction is a phenomenon in which cohesionless soil deposits below the groundwater table lose a substantial amount of strength due to pore pressure generation resulting from earthquake strong ground shaking (Kavazanjian, 2011). A simplified liquefaction analysis was performed using the guidelines provided in the AASHTO LRFD Bridge Design Specifications and Soil Liquefaction During Earthquakes (Idriss & Boulanger, 2008).

Based on the analysis, liquefaction potential was limited to an approximately 10-foot thick zone of silt, silty sand, well-graded sand, and various gravel layers between depths of 10 to 20 feet, which correspond to approximate elevations of 1868 and 1878 feet. A deeper potentially liquefiable low plasticity silt layer was also encountered between elevations 1813 and 1817 feet

Geology 11 in boring BH20-02. Settlements up to approximately one-half foot and lateral spreading displacements in the order of one foot are estimated from liquefaction. Since the bridge superstructure will bear on deep foundations, vertical settlement from liquefaction may be mitigated by extending deep foundation elements beneath liquefiable layers. However, large lateral loads generated by lateral spreading of stream bank soils after a design earthquake event may damage the foundation to the point needing repair. The potential displacements will be discussed further in the Foundation section of this report. Detailed analysis results are included in Appendix E.

A slope stability analysis was performed for bridge assuming the 5-foot thick liquefaction susceptible layer on east and west abutments. The slopes were analyzed using RocScience’s Slide2 software (v9.0). Liquefied soil strengths were used to model the effects of liquefaction during and after a seismic event. The geometry was developed from the TSL plan sheet dated January 2021. Results of the stability analyses are presented in Appendix E.

12 Site Conditions

3 SITE CONDITIONS

The existing bridge is a 50-foot long two-lane structure with a 21-ton capacity, installed in 1990 with timber glulam girders and a timber deck. The bridge currently provides access to Sourdough Campground; however, it has been showing signs of foundation movement and needs replacement. The bridge is owned and maintained by the Bureau of Land Management (BLM).

Photos of the bridges and streams are included in Appendix A.

According to the Sourdough Campground Bridge Hydraulics Memo, the existing bridge is approximately one mile upstream of the Sourdough Creek confluence with the Gulkana River (Dillin, 2021). Site surface elevations range from approximately 1895 feet at the eastern approach near the beginning of project, to 1874 feet at the bottom of the channel, back to 1888 feet at the project terminus near the end of the western approach. Stream banks were observed to be well-vegetated both upstream and downstream of the existing bridge. Vegetation consists of grass, shrubs, trees, and other upland plants.

3.1 AVERAGE TEMPERATURES AND PRECIPITATION

Table 3-1 and Table 3-2 show the average maximum and minimum temperatures, average monthly precipitation, and average monthly snowfall data for the period from 1971 to 1996, and the average maximum and minimum temperatures and average monthly precipitation for the period from 1981 to 2010 for the Sourdough 1N, Alaska (508625) weather station, respectively (Western Region Climate Center, 2021).

Table 3-1 Climate Data Summary (1971 to 1996)

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

Avg. Max.

Temp. (F)

4.8 14.9 28.8 41.5 54.1 62.5 67.0 63.0 51.6 34.5 14.3 6.1 36.9

Avg. Min.

Temp. (F)

-13.7 -10.7 -1.7 14.9 28.2 36.9 41.5 36.8 28.1 14.8 -5.8 -11.4 13.2

Avg. Total Precip. (in.)

0.84 0.73 0.71 0.50 0.83 1.92 2.86 2.08 1.68 1.46 1.10 1.18 15.91

Avg. Total Snowfall

(in.)

6.6 8.5 6.0 3.5 1.0 0.3 0.0 0.0 1.7 11.0 10.3 10.1 58.9

Avg. Snow Depth (in.)

19 22 21 14 2 0 0 0 0 3 8 15 9

Notes:

1 Period of Record : 09/28/1971 to 09/30/1996.

2 Percent of possible observations for period of record: Max. Temp.: 91.1% Min. Temp.: 91.4% Precipitation: 91.8% Snowfall: 92% Snow Depth: 91%.

Site Conditions 13

Table 3-2 Climate Data Summary (1981 to 2010)

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

Avg. Max.

Temp. (F)

5.8 14.2 29.0 41.6 54.1 62.5 66.0 61.7 50.7 32.9 12.8 9.2 36.8

Avg. Min.

Temp. (F)

-12.5 -10.2 -1.5 13.9 27.9 36.5 40.6 36.5 27.8 13.4 -6.1 -8.1 13.2

Avg. Total Precip.

(in.)

0.87 0.75 0.79 0.54 0.80 1.90 2.68 2.25 1.76 1.46 1.22 1.21 16.24

Note:

1 Period of Record : 1981 to 2010.

3.2 SCOUR

The streams were evaluated by WFLHD hydraulics engineers. Scour elevations were provided by Matt Dillin of the WFLHD Hydraulics Group in a Scour Hydraulics Memo dated June 24, 2021.

The combined long term degradation and abutment scour elevation was estimated to be 1862.5 feet for the 500-year scour check flood event.

The hydraulics memo stated that a spill-through abutment is recommended at this location and that the construction area should be dewatered as needed to install the bridge structure, riprap, and conserved streambed material. Scour countermeasures consisting of 5-foot thick Class 5 riprap with class 1, type C geotextile filter fabric were recommended for each abutment.

Conserved gravel-cobble streambed material should be placed atop of installed riprap. The recommendations from the memo are summarized in Table 3-3.

Table 3-3 Scour Summary

Item 100 Year

Flood 500 Year

Flood

Scour Base Elevation (feet) (Channel Thalweg) 1875.3 1875.3

Contraction Scour Depth (feet) 1.5 4.0

Stream Degradation Scour Depth (feet) 1.5 1.5

Abutment (1&2) Scour Depth (feet) 9.6 11.3

Total Abutment (1 & 2) Scour Elevation (feet) 1864.2 1862.5

14 Subsurface Conditions

4 SUBSURFACE CONDITIONS

Three subsurface borings were completed with a CME-75 drill rig by a Denali Drilling representative and logged by a Golder Associates representative at the site on September 9, 10, and 11, 2020. Drilling consisted of one boring at the east abutment and two at the west abutment, in the approximate locations shown in Figure 3 of Appendix B. Borehole BH20-01 was advanced on the east side of the bridge in the westbound lane. Boreholes BH20-02 and BH20-02B were advanced on the west side of the bridge in the eastbound lane. Borehole BH20-02 was drilled in two separate advancements due to an irretrievable sampler terminating the hole at 32½ feet below ground surface (bgs). The boring was moved approximately 7 feet east and redrilled.

Sampling resumed at 30 feet bgs and is denoted as Borehole BH20-02B in subsurface exploration logs. Subsurface boring logs and material testing results may be found in Appendices C and D, respectively.

4.1 SUBSURFACE BORINGS

Samples were obtained using a large penetration test (LPT) sampling method consisting of a 3-inch outside diameter split spoon sampler driven with a 340-pound hammer dropped from 30 inches. The automatic hammer was rated with an average energy transfer efficiency of 88 percent as of January 2017. The field blowcounts (raw data) were recorded for each 6-inch interval and are presented on the boring logs. For analysis purposes, the blowcounts were corrected for sampler type/size, hammer size/drop, energy transfer and overburden to standard penetration test (SPT) (N1)60 values. The raw data ranged from 1 to 51 blows per six-inch interval. In both borings, a dense to very dense gravel layer is encountered between 70 and 80 feet bgs.

Exploration logs indicate approximately 7½ to 10 feet of embankment fill material consisting of poorly-graded sand with silt and gravel. Underlying fill soils, native silt, silty sand, and well-graded sand with silt were observed to approximately 12½ feet bgs. Gravels and sands were observed to approximate depths between 23 and 25 feet bgs where they were underlain by fat clay. Fat clay soils extended to approximately 70 feet bgs in both explorations and were underlain by approximately 5 to 10 feet of elastic silt. Elastic silts were underlain by gravelly soils extending to respective termination depths of 90½ and 81½ feet bgs in Boreholes BH20-01 and BH20-02.

Sealed PVC standpipes were installed in each borehole to their termination depths to allow access for temperature readings with depth at later dates. Based on temperature readings and conditions logged during the site investigation, frozen subsurface conditions were encountered at depths between 69 and 85 feet bgs in explored areas. According to the Permafrost Map of Alaska (Ferrains, O.J., Jr., 1965), the site is underlain by an area of discontinuous permafrost.

4.2 LABORATORY TESTING

Selected soil samples were submitted to the WFL Materials Laboratory for routine soil index testing and one-dimensional consolidation. Additional samples were sent to Kuo Testing Labs for subsequent corrosivity index testing. Laboratory test reports are contained in Appendix D for select samples from Test Borings BH20-01, BH20-02, and BH20-02B.

Subsurface Conditions 15

The following is a list of tests that were completed on selected soil samples, and the standards that were followed:

● Particle Size Analysis - AASHTO T 88

● Liquid Limit, Plastic Limit, and Plasticity Index – AASHTO T 89/90

● Natural Moisture – AASHTO T 265

● Apparent Specific Gravity – AASHTO T100

● pH Value – AASHTO T289

● Soil Classification – AASHTO DL145

● Minimum Resistivity – AASHTO T288

● One-Dimensional Consolidation – ASTM D2435 – Method B

The results of the soil index tests were used to classify the samples according to the Unified Soil Classification System (USCS). The USCS is reported on the boring logs where samples were selected for testing.

4.3 GROUNDWATER

Groundwater was observed within both boreholes at 10½ feet bgs an hour after completion of exploration activity, which is expected to correlate with the observed elevation of water in Sourdough Creek. According to the Sourdough Campground Bridge Scour Hydraulics Memo, the anticipated 100-year water surface elevation at the proposed bridge location is 1885 feet, correlating to approximately 1 to 2 feet bgs from existing grade (Dillin, 2021). Water surface elevation of the creek is discussed in greater detail in the above-referenced memo.

These groundwater measurements were taken during the summer; groundwater and soil moisture conditions should be expected to fluctuate with adjacent stream levels, precipitation, nearby irrigation, and other seasonal factors.

16 Foundation Analysis and Recommendations

5 FOUNDATION ANALYSIS AND RECOMMENDATIONS

5.1 BRIDGE LAYOUT AND FOUNDATION CONSIDERATIONS

The proposed bridge will be approximately 92 feet long by 27 feet wide with a precast prestressed girder (integral deck) superstructure. Both deep and shallow foundation concepts were considered for support of the superstructure. Site conditions, soil profile, scour, design loading, constructability, and cost were the major factors considered when evaluating foundation alternatives.

Shallow foundations were considered but dismissed due to the long-term settlement potential of the compressible clays present at the site.

Deep foundation types considered included drilled shafts and driven piles. Drilled shafts were considered but are not recommended due to relatively high cost and environmental impacts associated with removal and disposal of large quantities of spoils from shaft excavations. Non-displacement driven piles are the recommended foundation type. The recommended pile for abutment locations is a HP14x117 H-Pile. Both abutment foundations are expected to include a single row of four piles. Factored loads listed below in Table 5-1 are per abutment.

Table 5-1 Final Factored Axial Abutment Loads

Service Limit

State Strength Limit

State

Dead Load (kips) 436 548 Total Load (kips) 624 872

5.2 DESIGN METHODOLOGY

Pile foundations were designed in general accordance with the AASHTO LRFD Bridge Design Specifications. Piles were designed using a combination of side resistance (skin friction) and tip resistance (end bearing). Pile lengths and tip elevations were initially estimated to satisfy the maximum factored Strength Limit State axial loads. Then settlement for the maximum Service Limit State axial loads was calculated based on those pile lengths. The neutral plane method recommended in the Federal Highway Administration Driven Pile Design Manual (Hannigan et al, 2016a) was used to calculate the maximum loads in the piles. Settlement was evaluated using the equivalent footing method for the pile group.

5.3 AXIAL AND LATERAL RESISTANCE

Axial geotechnical resistances of the piles were calculated using the program RSPile (v3.0).

Geotechnical capacities of single piles are expected to exceed the structural capacity of the pile section where driven through stiff clay to very dense gravelly soils. Piles must be tipped into very dense gravels at the minimum tip elevation provided in Table 5-2 below. The top of gravelly soil was estimated based on the materials encountered in the borings. Based on anticipated abutment

Foundation Analysis and Recommendations 17 loads, the piles are expected to achieve the required capacity at pile elevations of approximately 1803 feet on the East Abutment and 1806 feet on the West Abutment. A geotechnical resistance factor of 0.65 was applied based on the criteria described in Table 10.5.5.2.3-1—Resistance Factors for Driven Piles of the AASHTO LRFD Bridge Design Specifications. The attached axial capacity graphs also show the unfactored and factored geotechnical resistance for a HP14x117 H-Pile. The information is provided for a HP14x117 H-Pile in the case that the final bridge loads increase and an increase in capacity is needed.

Table 5-2 Pile Tip Elevations

Location Estimated Tip

Elevation (feet)

Minimum Tip Elevation

(feet) East Abutment 1803 1806 West Abutment 1806 1812

Recommendations for soil parameters for lateral analysis of the piles for static and seismic conditions are provided in Table 5-3 and Table 5-4. The k values in Table 5-3 and Table 5-4 are based on the representative values for sands and clays in the LPile Technical Manual (Eisenhower and Wang, 2015). The values in Table 5-3 and Table 5-4 do not include resistance factors. A “frozen condition” P-multiplier has been provided where appropriate. Soil stiffness is dependent on temperature. P-multipliers of 1,000 for embankment fill and 250 for silt are reasonable upper bounds and are recommended based on research performed by Shelman et

al. (2014) summarized below in Figure 5-1.

18 Foundation Analysis and Recommendations

Figure 5-1 Effects of Temperature on the Soil Stiffness (Shelman et al., 2014)

Table 5-3 East Abutment (BH20-01) Recommended Soil Parameters for Lateral Analysis

Elevation (feet)

Soil Unit 8

LPILE

Material

(lb/ft3) c

(lb/ft2)

(deg) kpy

(lb/in3) ε50

Seismic Condition

P-Multiplier

Frozen Condition

P-multiplier

1884 - 1888 Existing

Embankment Fill Sand

(Reese) 125 0 36 125 - 1 1000

1881 - 1884 Submerged

Existing Embankment Fill

Sand (Reese)

63 0 36 125 - 1 1000

1878 - 1881 Submerged, Soft, Low Plasticity Silt

Soft Clay 48 500 0 - 0.01 0.15 250

1876 - 1878 Submerged, Med. Dense Silty Sand

Sand (Reese)

58 0 32 20 0.5 -

1863 - 1876

Submerged, Med. Dense

Gravel with Sand and Silt

Sand (Reese)

63 0 38 125 - 1 -

1807 - 1863 Submerged, Stiff

Clay and Silt

Stiff Clay w/free water

53 1000 0 1,500 0.005 1 -

Below 1807 Submerged Very

Dense Gravel with Sand

Sand (Reese)

63 0 38 125 - 1 -

Notes:

1Assumed groundwater elevation is 1884 feet.

= moist unit weight.

3 ' = effective unit weight.

4 ' = effective internal angle of friction.

5 ε50 = soil strain at maximum 50% deviatoric stress

6 k = reaction modulus (p-y modulus).

7 c = cohesion.

8 Relative density and consistency based on N60 values and may differ from the boring logs field descriptions.

Foundation Analysis and Recommendations 19

Table 5-4 West Abutment (BH20-02/B) Recommended Soil Parameters for Lateral Analysis

Elevation (feet)

Soil Unit

LPILE

Material

(lb/ft3) c

(lb/ft2)

(deg) kpy

(lb/in3) ε50

Seismic Condition

P-Multiplier

Frozen Condition

P-multiplier

1884 - 1887 Existing

Embankment Fill Sand

(Reese) 125 0 36 125 - 1 1000

1877 - 1884 Submerged

Existing Embankment Fill

Sand (Reese)

63 0 34 90 - 1 1000

1874 - 1877 Submerged, Loose Silty Sand Sand

(Reese) 58 0 32 20 0.15 -

1864 - 1874 Submerged, Med.

Dense Gravel with

Sand and Silt

Sand (Reese)

63 0 38 125 - 0.5 -

1813 - 1864 Submerged Stiff

Clay and Silt Stiff Clay w/free water 53 1000 0 1,500 0.005 1 -

Below 1813 Submerged Very

Dense Gravel with Sand

Sand (Reese)

63 0 38 125 - 1 -

Notes:

1Assumed groundwater elevation is 1884 feet.

= moist unit weight.

3 ' = effective unit weight.

4 ' = effective internal angle of friction.

5 ε50 = soil strain at maximum 50% deviatoric stress

6 k = reaction modulus (p-y modulus).

7 c = cohesion.

8 Relative density and consistency based on N60 values and may differ from the boring logs field descriptions.

5.4 AXIAL GROUP CONSIDERATIONS

For pile groups end-bearing in cohesionless soils such as the deep gravels, the axial pile group resistance is the sum of the resistance of all the piles in the group as long as the pile center-to-center spacing is at least 2.5 pile diameters. Anticipated pile spacing is 90 inches on-center, equating to approximately 6 pile diameters at center-to-center spacing. A group reduction factor was not applied. The axial capacity charts provided in Appendix E are intended to be used to estimate pile capacity and determine pile lengths for contract documents.

5.5 LATERAL GROUP CONSIDERATIONS

For pile groups, Section 10.7.2.4—Horizontal Pile Foundation Movement of the AASHTO LRFD Bridge Design Specifications recommends application of P-Multipliers (Pm) when pile center-to-center spacing is less than 5 pile diameters (B) to account for group effects in the direction of loading. The AASHTO recommended P-Multipliers are summarized in Table 5-5. As previously mentioned, anticipated pile spacing is 90 inches on-center, equating to approximately 6 pile diameters at center-to-center spacing. A resistance factor of 1.0 is specified in AASHTO LRFD Bridge Design Specifications Table 10.5.5.2.3-1—Resistance Factors for Driven Piles for lateral geotechnical resistance of a single pile or pile group. These multipliers apply only to Service Limit State analysis of horizontal pile foundation movement. The definition of the loading direction and spacing is illustrated in Figure 5-2.

20 Foundation Analysis and Recommendations

Table 5-5 AASHTO Recommended P-Multipliers for Lateral Analysis of Pile Groups

Pile Center-to-Center Spacing in the Direction of Loading

P-Multipliers, Pm

Row 1 Row 2 Row 3 and higher

3B 0.8 0.4 0.3

5B 1.0 0.85 0.7

Figure 5-2 Definition of Loading Direction and Spacing for Group Effects (Figure

10.7.2.4.1 AASHTO LRFD Bridge Design Specifications, 9th Edition, 2020).

5.6 LATERAL SPREAD

In addition to typical soil loading, lateral spreading of the bank soils during the earthquake is estimated on the order of 3 feet towards the creek. This soil movement has been modeled as a lateral load in the RSPile software, and the estimated displacement and moment demand on the piles results are shown in Appendix E. These graphs do not include lateral driving/resisting loads or driving/resisting moments at the top of the piles. LPile files with the soil model have been provided to the WFLHD Structural Group for additional structural analysis of the foundations, including possible inclusion of the soil movement during the Extreme I Event. It is likely that the H-piles would be damaged beyond repair after a design earthquake, and that the bridge may require replacement of the foundations. Ground improvement methods and other options for mitigating lateral spreading are feasible, but may be outside of the project budget. WFLHD can discuss the risk acceptance with the BLM and determine the appropriate design and acceptable level of risk for this low volume structure.

Limit equilibrium analysis using RocScience’s Slide2 v9.0 indicates that lateral flow after an earthquake is not likely to occur with factors of safety of 1.1 or greater based on an estimated liquefied shear strength of approximately 170 psf in the silt and silty sand layers from approximate elevations 1876 to 1881 feet in boring BH20-01.

Foundation Analysis and Recommendations 21

5.7 SERVICE LIMIT STATE ANALYSIS

Pile settlement was checked based on the Service Limit State axial pile loads. To estimate settlement, the pile group was modeled as an equivalent pier embedded at the estimated pile depth following the procedures recommended in the FHWA Driven Pile Manual (Hannigan et al, 2016). Structure settlement from axial loads is not anticipated if piles end-bear in very dense gravels as recommended. Settlement from elastic compression of piles was calculated based on permanent axial dead loads referenced in Table 5-1. The estimated total settlement at the top of the pile for the Service I Limit State is expected to be approximately 0.1 inches at the east and west abutments. Static settlement of surrounding soil was analyzed using RocScience’s Settle3 software with consolidation parameters obtained from laboratory testing of Shelby tube samples.

Embankment composition and dimensions were modeled based on the 95% Plan Set. Static settlement analysis of surrounding soil at east and west abutments due to placement of additional fill is estimated to be less than half an inch. Static settlement and neutral plane analyses may be found in Appendix F.

5.8 DRAG LOAD

Drag loads can develop in pile foundations from static settlement of surrounding soil relative to pile settlement, and if liquefaction occurs during a seismic event. Nominal drag loads have been calculated at locations where liquefaction is predicted to occur. The drag loads presented below should be combined with the factored dead load to check for the structural adequacy of the pile.

East Abutment – Nominal seismic drag load = 121 kips per pile West Abutment – Nominal seismic drag load = 98 kips per pile

Because these drag loads occur above the neutral plane elevation, the liquefaction event will have a limited effect on the pile.

Drag loads from static settlement of surrounding soil relative to pile settlement are unlikely to occur due to their relative difference. As mentioned previously, pile cap settlements at east and west abutments are estimated at approximately 0.1 inches. Static settlement of surrounding soil due to placement of additional embankment fill are estimated at less than half an inch. Analysis procedures outlined in Section 3.11.8—Downdrag of the AASHTO LRFD Bridge Design Specifications indicate that downdrag cannot be assumed to fully developed for relative settlement differences of less than 0.4 inches. Therefore, drag loads from static settlement of surrounding soil are not anticipated to develop.

5.9 FROST HEAVE

Frost heave and frost jacking can occur in cold regions when soil freezes. Frost heave occurs in permafrost and non-permafrost soils. Frost heave is made up of two components; the first consists of moisture migrating to the freezing front and the associated formation of ice lenses, and the second is the volumetric expansion of water as it freezes. The migration of water to the freezing front results in heaves that are much larger than the volumetric expansion due to water changing to ice (ASCE, 1996).

22 Foundation Analysis and Recommendations

Three requirements must be met for frost heave to occur:

Freezing temperatures in the soil, A source of water, such as the groundwater table, and Frost susceptible soil.

These requirements are potentially present at the east abutment location of the proposed bridge given the temperature data provided in Table 3-1 and Table 3-2, the drainage flowing below the bridge, and 5-foot thick layer of silt and silty sand present in the boring beneath the bottom of pile cap elevation at the east abutment location. As previously mentioned, the site is underlain by an area of discontinuous permafrost. Freezing temperatures were documented between 69 and 85 feet bgs during exploration and subsequent temperature readings from within PVC pipes set in boreholes. However, no frost heave susceptible soils were identified beneath the 5-foot silt and silty sand layers within explored areas.

Frost jacking of pile foundations occurs when, during the freezing season, the ground heaves and the adfreeze forces between the ground and the pile are large enough to lift the pile. When the ground thaws and settles back to its original level in the spring, the pile may not return to its original position because soil or water may have filled the void below it or there may be insufficient downward force to overcome the pile frictional resistance (ASCE, 1996).

Potential frost jacking forces were developed based on an average adfreeze adhesion force of 40 psi. 40 psi was chosen based on research published in Arctic and Subarctic Construction Foundation for Structures (Department of the Army and the Air Force, 1983). For an HP14x117 H-pile and a frost heave susceptible layer of 5 feet, the frost jacking force was calculated to 204 kips per pile. Jacking forces were estimated based on the true H-pile perimeter of 85 inches and not the boxed perimeter of 58.2 inches. This jacking force does not exceed the combined uplift resistance of the design dead load, pile weight, and skin friction from soils beneath frost heave susceptible layers.

5.10 PILE DRIVING CONSIDERATIONS

Hard driving conditions may be encountered at the target end-bearing gravel layer. The contractor should be prepared to remove or break up any obstructions encountered above the minimum pile tip elevations specified on the plans.

Piling installed using non-impact hammers such as vibratory hammers may result in loss of axial resistance ranging from 15 to 55 percent (Doherty, 2015). Therefore, non-impact hammers are only recommended for installing piling down to 20 feet above the estimated tip elevation. Impact hammers must be used to advance the piling when the tip is within 20 feet of the estimated tip elevation.

A resistance factor of φ = 0.65 is recommended to determine the factored capacity of the piles, assuming the dynamic load tests are performed to establish driving criteria for production piles.

Foundation Analysis and Recommendations 23

5.11 PILE CORROSION

The procedure recommended in AASHTO R 27 (2019), Assessment of Corrosion of Steel Piling for Non-Marine Applications, was generally followed to assess corrosion potential. Laboratory testing of the pH and resistivity of the fine-grained soils was performed. The results are summarized in Table 5-6. The pH and soil resistivity tests were performed at the WFLHD laboratory, and due to the low resistivity results (less than 2,000 ohm-cm), the samples were sent out to a chemical laboratory for sulfate and chloride testing. Resistivity values below 2,000 ohm-cm are generally indicative of soils in a “very-to-severely corrosive” category for buried metal objects. Corrosivity index results were communicated to WFLHD Structural Group for consideration.

Table 5-6 Corrosivity Index Test Results

Sample pH Resistivity (ohm-cm)

Sulfur (S) (%1)

Chloride (Cl) (mg/kg)

BH20-01 S-9 at 25’ 6.9 1100 0.04 211 BH20-02 S-5 at 25’ 7.1 610 0.04 657 BH20-02B S-1 at 30’ 7.1 590 0.04 553 Notes:

11% = 10,000 mg/kg

5.12 ABUTMENT AND WING WALL RECOMMENDATIONS

Semi-integral abutments are proposed for the Sourdough Campground Bridge. Bridge abutments will include the abutment end wall and wingwalls that will be approximately 10 feet tall. The wingwalls will extend approximately 20 feet back from the end wall.

Structural Backfill (FP-14 Subsection 704.04) should be used as backfill behind abutment end walls and wing walls. A drainage system should be installed behind all walls to prevent buildup of pore water pressure. The soil parameters listed in Table 5-7 are recommended to determine the lateral earth pressures exerted on retaining walls. The parameters listed in Table 5-7 assume a vertical wall with level ground at the top of the wall.

The values in Table 5-7 do not include load or resistance factors. Appropriate load factors should be used with these parameters when calculating lateral earth pressures. It is generally assumed that compacted backfill creates at-rest lateral earth pressure behind unyielding walls and that a slight movement at the top of the wall (0.002 x wall height) creates active lateral earth pressures behind walls. When passive lateral earth pressures are applicable, wall movements of 0.02 x wall height at the top of the wall are necessary to reach the earth pressures listed above.

The procedures outlined in Appendix A11 of AASHTO LRFD Bridge Design Specifications should be used to determine the seismic forces exerted on the walls.

24 Foundation Analysis and Recommendations

Table 5-7 Recommended Static Soil Parameters for Abutment and Wing Wall Design

Parameter Design Value1

Backfill Moist Unit Weight 130 pcf

Backfill Angle of Internal Friction 34 degrees

Backfill Interface Friction Angle, δ 26 degrees

Active Earth Pressure Coefficient, Ka 0.28

Active Equivalent Fluid Pressure 36 psf/ft

At-Rest Earth Pressure Coefficient, K0 0.44

At-Rest Equivalent Fluid Pressure 57 psf/ft

Passive Earth Pressure Coefficient, Kp 3.54

Passive Equivalent Fluid Pressure 460 psf/ft

Base Soil Moist Unit Weight 125 pcf

Base Soil Angle of Internal Friction 35 degrees

Coefficient of Sliding Along the Base2, tan δ 0.65

Equivalent Height of Soil for Vehicle Loading of Abutment End Walls3, heq 3.5 ft

Equivalent Height of Soil for Vehicle Loading of Abutment Wing Walls4, heq 2.0 ft

Notes:

1 Design values based on a drained condition behind the wall.

2 A resistance factor of 1.0 is recommended for sliding resistance of semigravity walls at the strength limit state (AASHTO Table 11.5.7-1)

3 Interpolated from Table 3.11.6.4-1 in AASHTO LRFD Bridge Design Specifications.

4 From Table 3.11.6.4-2 in AASHTO LRFD Bridge Design Specifications.

Earthwork Analysis and Recommendations 25

6 EARTHWORK ANALYSIS AND RECOMMENDATIONS

6.1 ANTICIPATED FOUNDATION EXCAVATION USES

Proposed grading operations within the Sourdough Campground Bridge Replacement project limits will require minor cuts and fills. Excavations will be in existing granular fill material and surficial silts and silty sands. Based on conversations with the WFL Design Team, it is understood that soil and rock excavated within the foundation excavation will be used for proposed fill locations. Based on this assumption, and the current understanding of materials in the proposed Sourdough Campground Bridge project limits, excavated materials have been divided into the following categories:

1) Waste

2) Foundation Excavation

6.1.1 WASTE

Waste is defined as materials that, based on the geotechnical investigation, laboratory testing, and experience, cannot be placed as embankment fill in a manner that results in adequate compaction, strength, and settlement characteristics using reasonable and accepted construction practices. Foundation excavation materials that are appreciably above optimum moisture content and soils containing organic material and wood debris, such as those generated during clearing and grubbing operations for the project should also be wasted. Waste materials should be transported offsite to a suitable disposal site or stockpiled locally for later use in landscaping and/or non-structural fill applications.

6.1.2 FOUNDATION EXCAVATION

The foundation excavation materials typically consist of granular sands and gravels with minor amounts of silt. Based on materials observed during the time of exploration, foundation excavation material may be used to construct the proposed embankment sections within the project limits.

Additional material meeting the Select Borrow specification per the FP 14 Section 704.07 may be imported to reach proposed roadway grades.

26 Construction

7 CONSTRUCTION

7.1 WET WEATHER EARTHWORK

Soils at the site will be susceptible to disturbance from construction equipment during wet weather. Construction vehicle operation may be difficult during periods of wet weather, especially when standing moisture is present at the surface, or when the moisture content of the material is several percentage points above optimum. On-site fine-grained soils will be particularly susceptible to disturbance and will generally provide inadequate support of construction equipment during wet weather conditions.

7.2 CONSTRUCTION DEWATERING

The rip rap and guide bank will be located at and below the elevation of the Sourdough Creek. If the contractor chooses to place these work elements in the dry, dewatering will be required. The contractor should be prepared for challenging conditions when dewatering including but not limited to dense coarse-grained soils with cobbles and boulders, potential obstructions from boulders or other material, and heaving soils.

7.3 PILE DRIVING

The reader is referred to Pile Driving Considerations for construction…