GR 04-22 AK FS CNF 1015(2) - Russian River Campground Phase II Geotechnical Report (REVISED 5-3-22).pdf

Russian River Campground Phase II May 2022

Table of Contents 1-i

Geotechnical Report No. 04-22

Russian River Campground Phase II

FINAL GEOTECHNICAL REPORT

AK FS CNF 1015(2)

Chugach National Forest, Kenai Peninsula Borough, Alaska May 2022

Table of Contents 1-ii

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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

3 SITE CONDITIONS

3.1 Climate Data Summary

3.2 Scour and Erosion Considerations

4 SUBSURFACE CONDITIONS

4.1 Subsurface Borings

4.2 Groundwater

4.3 Instrumentation

4.4 Laboratory Testing

5 ANALYSIS AND DESIGN RECOMMENDATIONS

5.1 Mechanically Stabilized Earth (MSE) Walls

5.2 Soldier Pile Retaining Walls

5.2.1 Lateral Earth Pressures

5.2.2 Traffic Surcharge

5.2.3 Seismic Considerations

5.2.4 Soldier Piles and Lagging

5.2.5 Tieback Anchors

5.3 Cut Slopes and Cut-Side Structures

5.4 Embankment Slopes and Fill-Side Structures

6 CONSTRUCTION CONSIDERATIONS

6.1 Excavation

6.1.1 Waste 35

Table of Contents ii

6.2 Frost Heave Susceptibility

6.3 Special Rock Embankments

6.4 MSE Walls

6.5 Soldier Pile Walls

6.6 Material and Aggregate Sources

6.6.1 MP33 Pit 38

6.6.2 MP35 Pit 38

7 LIMITATIONS

8 REFERENCES

List of Figures iii

LIST OF FIGURES

Figure 1-1 Project Location Map

Figure 1-2 Project Vicinity Map

Figure 2-1 Excerpt from Geology of the Prince William Sound and Kenai Peninsula Region, Alaska: Including the Kenai, Seldovia, Blying Sound, Cordova, and Middleton Island 1:250,000-scale Quadrangles

Figure 2-2 Quaternary Faults Map for the Site Vicinity

Figure 2-3 Five Percent Damped Design Response Spectrum

Figure 5-1 Fill-Side Single Row of Tiebacks Section

Figure 5-2 Cut-Side Cantilever Non-Gravity Section

Figure 5-3 Fill-Side Cantilever Non-Gravity Section

Figure 5-4 Static Active Pressure Coefficient Equations from AASHTO LRFD Bridge Design Specifications

Figure 5-5 Cut-Side Soldier Pile Axial Capacity

Figure 5-6 Fill-Side Soldier Pile Axial Capacity

List of Tables iv

LIST OF TABLES

Table 2-1 Summary of Seismic Parameters

Table 3-1 Climate Data Summary – Cooper Landing 5W, Alaska (1975 to 2014)

Table 3-2 Climate Data Summary – Cooper Landing Project, Alaska (1958 to 2004)

Table 4-1 Groundwater Summary

Table 5-1 MSE Wall Design Summary

Table 5-2 MSEW+ Soil Parameters

Table 5-3 Soldier Pile Retaining Wall Design Summary

Table 5-4 Lateral Earth Pressure Summary

Table 5-5 Static Lateral Earth Pressure Parameters for Cut-Side Cantilever Walls

Table 5-6 Static Lateral Earth Pressure Parameters for Fill-Side Cantilever Walls

Table 5-7 Lateral Earth Pressure Parameters for Fill-Side Anchored Walls

Table 5-8 Passive Lateral Earth Pressure Parameters for Cut-Side Soldier Pile Walls

Table 5-9 Passive Lateral Earth Pressure Parameters for Fill-Side Soldier Pile Walls

Table 5-10 Pseudostatic Lateral Displacement

Table 5-11 Special Rock Embankment Design Summary

List of Photos v

LIST OF PHOTOS

Photo A-1. Kenai River at Toe of Schooner Bend Slope Area Facing Southeast to Southwest, 2021

Photo A-2. Schooner Bend Landslide Area, Facing West, September 2009

Photo A-3. Schooner Bend Landslide Area, Cut-Side Failure, May 2013

Photo A-4. Schooner Bend Landslide Area, Cut-Side Failure Close-Up, May 2013

Photo A-5. Schooner Bend Landslide Area, Facing West from Middle of Slope, October

Photo A-6. Existing Bin Wall at Toe of Schooner Bend Landslide Area, Facing East, October 2014

Photo A-7. Jersey Barriers and Exposed Scarp at Cut-Side Portion of Schooner Bend Landslide Area, Facing East, September 2017

Photo A-8. Schooner Bend Landslide Area, Facing West, September 2017

Photo A-9. Jersey Barriers and Draped Geotextile at Cut-Side Portion of Schooner Bend Landslide Area, Facing West, June 2019

Photo A-10. Jersey Barriers and Scarp at Cut-Side Portion of Schooner Bend Landslide Area, June 2019

Photo A-11. Existing Gabion Basket Wall at Approximate Stationing STA 608+20, Facing West, June 2019

Photo A-12. Existing Bin Wall and Rip Rap Armoring at Toe of Schooner Bend Landslide Area, Facing West, June 2019

Photo A-13. Rip Rap Armoring at Toe of Schooner Bend Landslide Area, Facing West, June 2019

Photo A-14. Pavement Distress Adjacent to Monument for Boring RR16-01 Instrumentation, Facing West, June 2019

Photo A-15. Jersey Barriers and Draped Geotextile at Cut-Side Portion of Schooner Bend Landslide Area, Facing West, June 2019

Photo A-16. MP33 Pit Material Source, Approximately 120-Foot High Stockpile, July

Photo A-17. MP33 Pit Material Source, Approximately 120-Foot High Stockpile, Close- Up, July 2021

Photo A-18. MP33 Pit Material Source, Large Modular Concrete Blocks Observed Directly Left of Entrance, July 2021

Photo A-19. MP33 Pit Material Source, Facing West from East Side of Pit

Photo A-20. MP35 Pit Material Source, Vegetated Shale Waste Area, July 2021

List of Photos vi

Photo A-21. MP35 Pit Material Source, Ponding Water Observed in Areas of Fine- Textured Soils, July 2021

Photo A-22. MP35 Pit Material Source, Stockpiles of Processed Oversized Cobbles, July

Photo A-23. MP35 Pit Material Source, Stockpiles of Unprocessed Material to the Right of Entrance, Close-Up, July 2021

Photo A-24. MP35 Pit Material Source, Stockpiles of Unprocessed Material to the Right of Entrance, Close-Up, July 2021

Photo A-25. MP35 Pit Material Source Overview, Facing West from East Side of Pit

Appendices vii

APPENDICES

APPENDIX A – SITE PHOTOGRAPHS

See List of Photos

APPENDIX B – SITE MAPS

Figure B-1 through Figure B-6: Subsurface Exploration Location Maps

APPENDIX C – SUBSURFACE EXPLORATION LOGGING

Descriptive Terminology for Boring Logs

Boring Log RR16-01 through RR16-11

Test Pit Log RRTP16-12

APPENDIX D – LABORATORY TEST RESULTS

Soil Index Testing

Torsional Shear Strength Testing

APPENDIX E – INSTRUMENTATION DATA

RR16-01 Slope Inclinometer Chart

RR16-02 Slope Inclinometer Chart

RR16-03 Slope Inclinometer Chart

Bin-Wall Retaining Wall Survey Data Summary and Total Displacement Chart

Vibrating Wire Piezometer Data Summary Chart with Precipitation and Kenai River Discharge

Piezometer Calibration Records

APPENDIX F – CALCULATIONS

Walls 1 and 2 MSEW+ Analysis

Introduction 8

1 INTRODUCTION

This report provides the geotechnical investigation results, analysis, and design and construction recommendations for the Russian River Campground Phase II project. Russian River Campground Road is an access road located at MP 52.6 of the Sterling Highway near Cooper Landing, Alaska as shown in Figure 1-1 and Figure 1-2. Orthometric elevations referenced in this report are based on the NAVD88 vertical datum unless stated otherwise. The road provides access to Russian River Campground managed by the U.S. Forest Service (USFS). As shown in the 100% Plan Set, the Phase II project limits of the access road extend from roadway stationing STA 600+60 to STA 641+17.07. The project area includes an approximately 400-foot long section that crosses an active landslide.

1.1 PROJECT DESCRIPTION

Proposed roadway improvements include widening the existing road by five feet to accommodate a four-foot wide pedestrian walkway with a one-foot wide shoulder, and raising the roadway prism grade by one foot vertically. The USFS has requested that roadway improvements include design considerations for frost heave susceptible soils. Proposed improvements also include design and construction of mechanically stabilized earth (MSE) and soldier pile walls to satisfy roadway widening efforts and account for a section of roadway traversing an active landslide. Mitigative measures for the slide area have been implemented as far back as 1966 when the road was originally constructed. These measures included approximately 260 linear feet of a metal bin-wall retaining wall, and 1-foot to 3-foot diameter riprap placed at the toe of roadway embankment slopes. More recently, jersey barriers have been placed on the cut side of the active slide area to reduce the incidence of sloughing material or debris entering the roadway. Slope and bin-wall retaining wall movement were monitored as part of this project. Photos of the site and material sources are included in Appendix A.

1.2 PURPOSE AND SCOPE

The purpose of this report is to provide geotechnical recommendations for soldier pile and MSE wall 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 11 geotechnical test borings (RR16-01 through RR16-11), one test pit exploration (RRTP16-12), instrumentation installation and monitoring of slope movement and groundwater elevation, laboratory soil index testing, preparation of interim memorandums, and preparation of this report. Geotechnical borings and downhole instrumentation installation were completed and logged by a drilling subcontractor under the direction of a Western Federal Lands Highway Division (WFLHD) geotechnical engineer. Bin-wall retaining wall monitoring targets were affixed and surveyed by USFS staff. Vibrating-wire piezometer and slope inclinometer data were measured by AKDOT and WFLHD geotechnical staff. Laboratory tests were completed by WFLHD materials testing staff and Cooper Testing Laboratory.

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

Introduction 9

Preliminary Geotechnical Recommendations Memorandum, GM 25-19, dated August 19, Lateral Earth Pressures for Soldier Pile Retaining Walls Memorandum, GM 36-20, dated March 15, 2021

The following reports and documents were previously prepared by the USFS and their consultants were reviewed and summarized for this project:

Plans for Proposed Forest Development Road, Russian River No. 965 Russian River Campground No. 979, dated April 1966, by USDA Forest Service, Region 10, Chugach National Forest (USFS, 1966). These plans show the original 3.560 miles of road grading and 0.840 miles parking lots and spurs. The existing slide area is indicated on the Construction Area Map, but just along the river bank at what appears to be Schooner’s Bend. A quarry rock 1’-3’ triangular section buttress at the river’s edge is shown as the Typical Sections Thru Slide Area, including the along bin walls. A 1¼:1 (presumed H:V) embankment slope is shown above the quarry rock buttress.

Final Construction Report, File In Russian River Contract File #01-87, presumed date 1966, by Kenneth Saner, USDA Forest Service (Saner, 1966). Portions of this report discuss encountering a considerable amount of unstable blue clay in the excavation by a dozer from that project’s station 12+00 to 15+00. It appears that probing and digging were done to evaluate the extent of the unstable blue clay and an analysis was performed, but details of the analysis and resulting work were not included except to imply that the blue clay was wasted from the area.

Russian River Construction Reports, Russian River Road #0965 and Russian River Campground Road #0979, memo dated April 24, 1970, by William Odendahl, Acting Forest Engineer, USDA Forest Service (Odendahl, 1970). The seam of blue clay was revised to that project’s station 12+10 to 14+10. It appears to have been removed from the workzone for the bin wall and wasted elsewhere.

Materials Engineering Report, Stability Problem, Russian River Campground Access Road-Chugach NF, dated 1974, by W. Vischer (Vischer, 1974). This report discusses a field review of the stability problem on the access road to the campground. The road cuts are described as very unstable as they are glacial till deposits ranging from clayey silt to gravel, which can liquefy and flow when exposed to moisture. The natural, undisturbed stable slope angle is a maximum 1.5(H):1(V), but the existing cuts are unstable at slopes ranging from 2(H):1(V) to 1.1(H):1(V) with no vegetative cover. Retaining structures are discussed as the most feasible solution to the stability problem, including a sheet pile wall with tiebacks.

Stability Evaluation, Russian River Campground Access Road, Milepost 52.6 Sterling Highway, Cooper Landing, Alaska, dated March 2001, by Shannon & Wilson, Inc. This report presents exploration and lab testing results, and provides geotechnical recommendations that focus on the distressed roadway in the steep area where the river is cutting at the toe. The existing approximately 50-foot long by 4-foot high gabion wall is

Introduction 10 first mentioned in this report, as well as distress by overturning of the metal bin wall at the toe of the slope. Bedrock outcrops are reported at the bottom of the culvert on the uphill side of the road approximately 500 feet west of boring B4 and on the river edge approximately 120 feet west of boring B4. The bedrock is described as a laminar siltstone that strikes perpendicular to the road and dips almost vertically. A graywacke rock outcrop is also reported approximately 0.3 miles further west on the uphill side of the road.

Longitudinal cracking of the road is noted in the to the east and west of boring B1 (located in the roadway near the eastern end of the gabion wall). Ponding of water in the uphill ditch was noted. The report recommended drainage improvements using subdrains on the uphill side of the road and potentially placing a cut side wall to widen the road.

Additional Stability Studies, Russian River Campground Access Road, Milepost 52.6 Sterling Highway, Cooper Landing, Alaska, dated March 2002, by Shannon & Wilson, Inc.

This report presents three additional borings, four percussion probes, and refined geotechnical recommendations that focused on the water and soil conditions upslope of the distressed road area. Erratic rates of penetration of the percussion probe borings on gravel and cobbles were reported. The report recommended upslope cutting at an angle of 1H:1V up to 12 feet in height and realign the roadway and provide stability into the relatively stable uphill soils. The subdrain recommendation was placed on hold until additional groundwater measurements could justify the effectiveness of the subdrain system.

Russian River Campground Access Road, memo dated April 14, 2004, by Shannon & Wilson. This memo summarizes construction observations of the upslope cuts recommended in their 2002 Additional Stability Studies report. The slope was cut at 1.5H:1V but the final surface covering was not installed.

Russian River Campground Access Road Contract Modification, memo dated June 1, 2004, by Shannon & Wilson. This memo discusses a couple of local shallow sloughs that occurred on the uncovered cut slope during spring “breakup”. The memo recommends “springhead” drains be constructed to stabilize the slough areas and to provide for slope drainage to reduce similar conditions in the future.

Geotechnical Explorations, Russian River Campground Access Road, Milepost 52.6 Sterling Highway, Cooper Landing, Alaska, report dated April 2005, by Shannon & Wilson.

This report presents exploration data and geotechnical recommendations for other distressed portions of the roadway further west of the Schooner Bend slide. Large boulders are reported, as well as a peat layer below roadway fill materials. The peat layer was estimated to be approximately 0.5 to 4.5 feet thick and was buried approximately 2.5 to 7 feet below the ground surface. Loose fill materials and consolidation of the underlying peat layer were reported to be the likely conditions that resulted in a relatively shallow failure plane. A second possible deeper failure zone (~23 feet deep) was also discussed and thought to be from a lower consistency silt layer found within the very stiff to hard sandy silt matrix. However, the report ultimately stated that the more likely cause was lateral spreading of the loose fill materials through poor drainage and consolidation of the underlying peat. Recommendations included overexcavation of 3 feet below the pavement surface and replacement with new or recompacted fill, but only after subgrade

Introduction 11 observations could be confirmed by a qualified geotechnical engineer. An additional 2 feet of overexcavation (for a total of 5 feet) was recommended if it was not possible to have continuous direction from a qualified geotechnical engineer. Below the proposed 8-inch thick pavement section, 28 inches of Selected Material Type A per ADOT standards were recommended.

Field Visit Memo: Russian River Campground Access Road FSR 1015, Geotechnical Site Assessment and Prior Work Summary, dated February 27, 2014, USDA Forest Service, Region 6 & 10 Engineering, Geotechnical & Dams Services (Renteria, 2014). This memo discusses the author’s site observations and subject matter expertise in the project region regarding the currently planned roadway widening for pedestrian and bike access. Mr.

Renteria conducts a brief literature review, reporting on the documents listed above and others not listed. Notable information not discussed previously include the following: the gabion wall is discussed as being 6 feet tall by 70 feet long that was likely built in the 1970s or 1980s to help mitigate settlement at the road surface from then overturning metal bin wall. In 2003 and 2004, an underdrain is installed in the ditch at a depth of 3 to 5 feet at the base of the 1:1 cut recommended by Shannon & Wilson upslope of the roadway at Schooner’s Bend. The spring drains recommended by Shannon & Wilson were also installed on the cut slope in 2004. In the spring of 2005, the cut slope further sloughed and the USFS removed the slough, installed jersey barriers along the toe of the cut, and backfilled behind the barriers with fabric, rock, and a drain. At the time of the memo in 2014, the barriers had bowed out, the inside edge of the asphalt concrete shoved upwards, and slough material was reported to have flowed over the top of the barriers.

Most notably, this memo includes a discussion from a Alaska Department of Transportation and Public Facilities (AKDOT&PF) report for the adjoining Sterling Highway MP 45-60 project that states that the large cuts in glacial materials similar to that in the Cooper Landing area are rarely attempted due to the silty materials, precipitation/runoff, the effects of freeze/thaw, and the difficulty of revegetation that results in mudflows, slumps/shear failures, and surface failures. Based on this information and site observations, the memo states that conditions along the Russian River Campground access road are very similar to that discussed by AKDOT&PF. The memo lists the following examples which were presented in the AKDOT&PF report for mitigation of cuts in glacial soils, which include fill slopes at 1V:1.5H using riprap, French drains in ditches, fin drain in cutslopes, rock blanket for cutslopes, and cut-side buttress for wet till areas.

The memo also suggests that the current project elements might consist of: drainage, a cutslope buttress and/or retaining wall at MP 0.3 (Schooner Bend); drainage, deep patch and/or retaining walls further west (MP 0.7 and beyond); and associated new pavement in repair area and for additional width.

In general, the USFS documents warn against cutting along the project at angles steeper than 1V:1.5H due to the local issue of freeze-thaw slope creep and surficial movement downslope onto the roadway. Cut slopes at 1V:1.5H would result in very large and high cut slopes that would be difficult to revegetate and would likely cause additional maintenance problems. Based on our literature review and discussions with the USFS, the project cross-functional team has focused the project design to widen on the fill side of the roadway using retaining structures suitable for each road segment.

Introduction 12

This report supersedes these previous memorandums for this project. The above documents are not appended to this report, but can be made available upon request.

Figure 1-1 Project Location Map

Introduction 13

Figure 1-2 Project Vicinity Map

Geology 14

2 GEOLOGY

The project area is located on the Russian River near the western edge of the Chugach Mountains between Kenai Lake and Skilak Lake on the Kenai Peninsula. The Kenai Fjords are located to the south, lowlands are located to the west, and portions of the Chugach Mountains are located to the east and north. As shown in Figure 2-1 below, Geology of the Prince William Sound and Kenai Peninsula Region, Alaska: Including the Kenai, Seldovia, Blying Sound, Cordova, and Middleton Island 1:250,000-scale Quadrangles (Wilson, et. al., 2012), the project area is located within the vicinity of Holocene- to Pleistocene-aged alluvial and terrace deposits (Qat) and Pleistocene-aged glacial deposits of the Naptowne and Brooks Lake glaciations (Qgn). Alluvial and terrace deposits are described as unconsolidated, poorly-graded to well-graded silts, sands, gravels, cobbles, and boulders. Glacial deposits are described as unconsolidated, moderately weathered morainal deposits.

Figure 2-1 Excerpt from Geology of the Prince William Sound and Kenai Peninsula Region, Alaska: Including the Kenai, Seldovia, Blying Sound, Cordova, and Middleton Island 1:250,000-scale Quadrangles

Russian River Campground Phase II Project Area

Qat

Geology 15

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 of the United States (USGS, 2022) was used to locate nearby active faults. According to the database, the West Fork anticline fault (Fault ID: 5411) of the Cook Inlet folds runs southwest to northeast in the vicinity of the site as shown in Figure 2-2 below. The approximate 6.6-mile-long thrust fault is categorized as undifferentiated Quaternary (<1.6 million years). The dotted line shown indicates the location is inferred.

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 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 category 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 West Fork anticline fault section is 33 miles west of the project; therefore, a site-specific probabilistic analysis is not needed.

Geology 16

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 Russian River Campground Phase II project area is Site Class D.

The U.S. Seismic Design Maps website (USGS, 2022) 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 estimate the seismic accelerations based on ground conditions observed at the project site. The results of this analysis are presented below in Table 2-1.

West Fork Fault

Aleutian Subduction Zone

Geology 17

Table 2-1 Summary of Seismic Parameters

Site Latitude 60.479888°

Site Longitude -149.945535°

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) 0.525 g

Horizontal Response Spectral Acceleration at a

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

1.187 g

Horizontal Response Spectral Acceleration at a

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

0.486 g

Mapped Peak Ground Acceleration Coefficient, FPGA 1.000

Mapped Short-Period Spectral Acceleration Coefficient, Fa 1.025

Mapped 1-sec Period Spectral Acceleration Coefficient, Fv 1.514

Scaled Peak Ground Acceleration for Project Site, Site Class D (As) 0.525 g

Scaled Horizontal Response Spectral Acceleration at a

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

1.217 g

Scaled Horizontal Response Spectral Acceleration at a

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

0.736 g

AASHTO Seismic Zone 4

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

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

S p ec tr al A cc el er at io n

R es p o n se

, g

Period, Tm (seconds)

Site Conditions 18

3 SITE CONDITIONS

Russian River Road is in direct proximity to Russian River and Kenai River in a forest consisting generally of hardwood and spruce trees. Stumps and young vegetation in the area suggest prior logging activity. Site elevations range from approximately 390 feet at the start of the project to approximately 460 feet at the project terminus. In the Schooner Bend area, Russian River Road traverses a slope undercut at its toe by the Kenai River. Based on embankment sideslopes and generally topographic observations, the road appears to be sidecast in the hillside with minor cuts and fills to grade the roadway. In general, the stability of Russian River Road is related to the natural slope conditions.

The existing roadway is as narrow as approximately 22 feet in the Schooner Bend slide area with guardrail on the fill-side and concrete jersey barriers on the cut-side. We understand that the jersey barriers help retain soil debris that flows down the cut slope. A gabion wall approximately 4 feet high exposed is located several feet down slope from the roadway. Further down the roughly 30-foot high slope is a galvanized steel bin wall that is severely overturned and leaning over-vertical. The bin wall has had survey monitoring points placed on it, and the Forest Service collected survey data from April 14, 2016 to August 7, 2018. Based on this data, the higher magnitude movements were on the east end of the wall, which moved nearly 6 inches during this period. The west end of the wall moved approximately 1 inch or less during the monitoring period.

Below the bin wall is an area of riprap on the bank of the Kenai River. This reach of the Kenai River bends sharply, and the flow is directed towards the outer bend where the riprap and bin wall are located. We understand that above the roadway, the slope has many tension cracks and fails surficially resulting in debris on the roadway. A creek is located on the western end of the slide area, where the water reportedly goes underground several hundred feet above the roadway.

This subsurface water is not believed to be directly related to the slide, since it is relatively far from the most active eastern end of the slide area. Photos are included in Appendix A.

3.1 CLIMATE DATA SUMMARY

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 1975 to 2014 for the Cooper Landing 5W, Alaska (502149) and 1958 to 2004 Cooper Landing Project, Alaska (502144) weather stations (Western Region Climate Center, 2022).

Site Conditions 19

Table 3-1 Climate Data Summary – Cooper Landing 5W, Alaska (1975 to 2014)

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

Avg. Max.

Temp. (F)

25.3 30.1 36.9 47.1 57.9 65.6 68.1 66.2 56.8 42.8 29.8 26.5 46.1

Avg. Min.

Temp. (F)

9.1 11.9 14.7 25.5 32.9 40.5 45.8 43.4 36.3 25.9 13.6 11.3 25.9

Avg. Total Precip. (in.)

2.10 2.29 0.68 0.62 0.65 0.84 1.66 2.35 2.83 2.70 2.17 2.16 21.07

Avg. Total Snowfall (in.)

7.0 7.3 8.0 3.0 0.1 0.0 0.0 0.0 0.1 4.2 7.2 10.1 47.0

Avg. Snow Depth (in.)

9 11 10 3 0 0 0 0 0 1 3 7 4

Notes:

1 Period of Record : 02/01/1975 to 02/28/2014.

2 Percent of possible observations for period of record: Max. Temp.: 89.1% Min. Temp.: 89.0% Precipitation: 76.3% Snowfall: 88.1% Snow Depth: 80%.

Table 3-2 Climate Data Summary – Cooper Landing Project, Alaska (1958 to 2004)

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

Avg. Max.

Temp. (F)

26.3 30.0 35.5 44.2 53.7 61.8 65.4 63.1 54.3 42.0 33.6 29.0 44.9

Avg. Min.

Temp. (F)

13.5 15.1 18.3 27.8 35.3 42.1 47.9 46.9 40.8 30.4 22.8 17.6 29.9

Avg. Total Precip. (in.)

2.70 2.38 1.31 1.02 0.87 1.07 1.40 2.86 5.10 4.77 3.26 3.72 30.46

Avg. Total Snowfall (in.)

13.2 15.4 10.8 3.4 0.1 0.0 0.0 0.0 0.0 4.6 12.3 19.4 79.3

Avg. Snow Depth (in.)

9 12 13 6 0 0 0 0 0 1 2 7 4

Notes:

1 Period of Record : 12/02/1958 to 03/31/2004.

2 Percent of possible observations for period of record: Max. Temp.: 91.6% Min. Temp.: 91.5% Precipitation: 94.4% Snowfall: 94.9% Snow Depth: 95.1%.

3.2 SCOUR AND EROSION CONSIDERATIONS

The Kenai River impacts were evaluated by the WFLHD Hydraulic Group. General scour commentary was provided by Hydraulic Engineer Matt Dillin in a Hydraulics Wall Waterway Memo dated December 10, 2019. According to the memo, the channel appears generally stable through the project reach. However, proposed retaining walls at the active landslide location in Schooner Bend may be subject to erosive forces at 100-year water surface elevations. Mechanically stabilized earth (MSE) wall foreslopes should be inspected annually or after extreme flood events to check the setback distance from the wall toe to slope face is at least four feet.

Subsurface Conditions 20

4 SUBSURFACE CONDITIONS

Eleven subsurface borings were completed with a CME-75 truck-mounted drill rig and CS1000 portable drill rig by a HazTech Drilling representative. One test pit excavation was completed with an excavator by a HazTech Drilling representative. All subsurface explorations were logged by Collin M, a HazTech drilling inspector subcontractor. Approximate exploration locations are shown in Figures B-1 through B-6 of Appendix B. Exploration locations are approximate and were refined based on a site visit in July 2021. Borings RR16-01, RR16-02, and RR16-03 contain vibrating wire piezometer and slope inclinometer instrumentation installed in pipes within the borings.

Subsurface exploration logs may be found in Appendix C. Laboratory test results may be found in Appendix D. Instrumentation information may be found in Appendix E.

4.1 SUBSURFACE BORINGS

Samples were obtained using a SPT sampling method consisting of a 2-inch outside diameter split spoon sampler driven with a 140-pound hammer dropped from 30 inches. The automatic hammer was rated with an average energy transfer efficiency of 86.4 percent as of June 2016.

The field blowcounts (raw data) were recorded for each 6-inch interval and are presented on the boring logs. Borings RR16-02 (fill-side shoulder) and RR16-03 (above roadway) were performed in the active slide area and instrumented with inclinometers and piezometers.

Subsurface conditions in these two slide borings generally consist of medium dense, silty gravel with sand in the upper 7.5 to 12 feet below the ground surface (bgs). At 12 feet bgs in boring RR16-02 and 7.5 feet in boring RR16-03, a very stiff to hard, gray, non-plastic silt was encountered. From approximately 17.5 to 22 feet in boring RR16-02, the boring encountered a silty clayey sand with gravel becoming loose at approximately 21 feet bgs. We interpret this zone to be a shear zone with in the slide complex. From 22 feet bgs onward, the boring consistently encountered very dense silty sand with gravel interpreted to be undisturbed glacial till to the bottom of the boring at 100 feet bgs.

In the uphill boring RR16-03, the very stiff to hard silt extended down to approximately 21.3 feet bgs, where very dense silty gravel with sand (glacial till) was encountered to the bottom of the boring at 100 feet bgs. We interpret the upper approximately 13 feet of this boring to be within slide materials.

It should be noted that soil color described in the logs can vary depending on who is logging the hole, and light and other conditions at the time of logging. The blue clay described in the USFS documents referenced earlier in this report is likely the same material as the gray silt and gray silty clayey sand identified in the WFL logs discuss in this section.

4.2 GROUNDWATER

Groundwater was encountered in several of the explorations, and vibrating wire piezometers were installed in borings RR16-01, RR16-02, and RR16-03 along with a DGSI VW MiniLogger data recorder. Table 4-1 summarizes the groundwater depths encountered at the time of drilling, at

Subsurface Conditions 21 completion, and/or measured after the boring was complete in some cases. Piezometer data from the data loggers was downloaded on an approximately quarterly basis.

Table 4-1 Groundwater Summary

Exploration

Location1 GW Depth Logged in

Borehole at Time of Drilling (feet)

GW Depth Logged in

Borehole At Completion (feet)

GW Depth Logged in

Borehole After Drilling (feet)

GW Depth Range Logged in Piezometer

(feet) Latitude (°) Longitude (°)

RR16-01 60.4803 -149.9467 23.8 14.6 23.8 20.6 – 27.8

RR16-02 60.4800 -149.9456 32.5 32.5 32.8 29.2 – 35.6

RR16-03 60.4799 -149.9454 44.6 44.6 44.6 39.0 – 49.63

RR16-04 60.4798 -149.9461 27.4 21.5 27.4 -

RR16-05 60.4797 -149.9477 15.0 15.0 15.4 -

RR16-06 60.4796 -149.9498 34.5 34.5 34.5 -

RR16-07 60.4800 -149.9510 NE to 31.5 feet bgs - - -

RR16-08 60.4800 -149.9521 NE to 15.8 feet bgs - - -

RR16-09 60.4799 -149.9546 4.7 4.7 4.7 -

RR16-10 60.4797 -149.9567 NE to 12 feet bgs - - -

RR16-11 60.4794 -149.9595 NE to 8 feet bgs - - -

RR16-12TP 60.4817 -149.9423 NE to 6 feet bgs - - -

Notes:

1 Location not surveyed, approximate location based on site visit in July 2021.

2 NE = Not Encountered 3 Data between February 17, 2017 and June 13, 2017 was not available due to MiniLogger functionality issues.

Groundwater levels or seepage zones were estimated from the vibrating wire piezometer data, which is correlated to a water head or water level above the piezometer depth. All data is plotted on the attached charts, but water levels shown below the piezometer depth should be considered unreliable.

It should be noted that based on information from reports by others, including anecdotal information from the USFS staff, that the slopes uphill and downhill from the roadway are likely to contain numerous seepage zones. The glacial till and silt or clayey layers within can impede vertical groundwater infiltration and migrate it horizontally along the contacts between the various soil layers. These perched layers can be encountered just below the ground surface at random elevations and horizontal extents due to the chaotic nature of glacial till deposition and landslide movements.

Subsurface Conditions 22

4.3 INSTRUMENTATION

The relevant instrumentation installed along the preferred project alignment include three slope inclinometer, three vibrating wire piezometers, and 12 target plates to monitor existing bin wall movement. Bin wall monitoring targets were installed and surveyed by USFS with survey data sent to WFL for analysis. Inclinometer casings were installed in borings RR16-01, RR16-02, and RR16-03. VW piezometers were installed in borings RR16-01, RR16-02, and RR16-03.

The data indicates that the ground in this area is shearing at depths of approximately 14 feet in RR16-03 and 42.5 feet in RR16-02 . The total movement between February and September 2017 in boring RR16-03 is approximately 1.25 inches in the A-axis direction and 0.5 inch in the B-axis direction. The total movement between June and September 2017 in boring RR16-02 is less than

0.1 inches in the A-axis direction and approximately 0.2 inches in the B-axis direction. Several sets of instrumentation readings were not included due to being read in the wrong direction and containing bad checksums. Slope inclinometer and vibrating wire piezometers were monitored by both AKDOT and WFLHD staff. This shear zone(s) is being considered in the design of the cuts in this area.

Instrumentation documentation and analysis may be found in Appendix E.

4.4 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 Cooper Testing Laboratory for drained residual torsional shear strength testing. Laboratory test reports are presented in Appendix D for select samples from borings RR16-02 and RR16-03.

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

● pH Value – AASHTO T289

● Apparent Specific Gravity – AASHTO T100

● Soil Classification – AASHTO DL145

● Drained Residual Torsional Shear Strength – ASTM D6467

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

Analysis and Design Recommendations 23

5 ANALYSIS AND DESIGN RECOMMENDATIONS

The above information was analyzed to determine design recommendations for mechanically stabilized earth (MSE), soldier pile walls, and cut and fill slopes. The following sections provide our final recommendations for roadway design.

5.1 MECHANICALLY STABILIZED EARTH (MSE) WALLS

The results of the geotechnical investigations were to provide geotechnical data for design of retaining walls. Table 5-1 summarizes our recommendations for retaining walls.

Table 5-1 MSE Wall Design Summary

100% Plan Set

Design Stations

Approx.

Height

Range (ft) Backslope Conditions Comments

604+63 to 605+84 RT

(MSE Wall No. 1)

4 to 7 ½ 1V:3H shoulder, approximately 1 foot high

Designed as conventional MSE wall – global stability acceptable at approximate grid lengths L = H.

See plan and profile sheets for grid lengths.

610+90 to 620+26 RT

(MSE Wall No. 2)

4 to 18 1V:3H shoulder, approximately 1 foot high

Designed as conventional MSE wall – global stability acceptable at grid lengths L = H. Wall batter transitions from vertical at soldier pile wall terminus to 1H:18V in a 16-foot wall transition zone. Wall contains riprap armoring for foreslope for scour mitigation at Schooner Bend and a protective apron beneath culvert inlet where it passes through wall section.

The MSE walls will utilize 8-foot long by 18-inch high welded wire baskets stacked vertically for facing elements. Reinforcement geogrids are used for reinforcement within the soil mass. Wall stability was analyzed with ADAMA Engineering’s MSEW+. Parameters are included below in Table 5-2.

Analysis and Design Recommendations 24

Table 5-2 MSEW+ Soil Parameters

Material Friction Angle (°) Cohesion (pcf) Unit Weight (pcf)

Reinforced Fill 34 0 130

Retained Soils 32 0 125

Foundation Soils 28 50 120

Glacial Till 38 200 135

Notes:

1 MSEW+ calculations for the walls are included in Appendix F.

Static and seismic calculations for internal, external, and global stability for MSE walls may be found in Appendix F. The reinforced soil is a Select Granular Backfill as described in 704.08 of the FP-14, and a type III reinforcement geogrid as described in 714.04 is the reinforcing element.

The reinforcement layers have a vertical spacing of 18 inches with a reinforcement length to wall height ratio of approximately 1:1. Seismic analyses were based on the peak ground acceleration of 0.53 g, reduced for flexible walls per AASHTO assuming the walls can tolerate 8 inches of displacement during the design earthquake.

Water was modeled at the base of the wall to account for 100-year water surface elevations of the Kenai River. Less than an inch of immediate settlement is anticipated for MSE wall sections, therefore staged construction for this wall is not required.

5.2 SOLDIER PILE RETAINING WALLS

Based on the site conditions and the geotechnical investigation we make the following recommendation for the proposed solder pile retaining wall listed in Table 5-3. Soldier pile wall and tieback typical and detail sheets may be found in Appendix H.

Table 5-3 Soldier Pile Retaining Wall Design Summary

100% Plan Set

Design Stations

Approx. Height Range (ft)

Backslope Conditions

Comments

605+84 to 610+90 RT 10 to 32 Max 1V:5H

Fill-side soldier pile wall to facilitate roadway widening and pedestrian path development in addition to mitigating the active landslide in Schooner Bend

607+01 to 608+37 LT 4 to 9 Max 1V:2H Cut-side soldier pile wall to mitigate active landslide in Schooner Bend

Analysis and Design Recommendations 25

5.2.1 LATERAL EARTH PRESSURES

Lateral earth pressures summarized in Table 5-4 are based on residual shear strength friction angles of 27 and 31 degrees with the recommended pressure distributions illustrated in Figure 5-1, Figure 5-2, and Figure 5-3. For the case of a cantilevered wall system, the applied lateral pressure would be represented by a triangular pressure distribution termed an equivalent fluid density. We recommend the nominal active pressure equivalent fluid densities as shown in Table 5-4 for level (i.e., ≤1V:5H) and 1V:2H backslope conditions. Active pressures should be assumed to act on the pile spacing above the excavation base and the 30-inch diameter concrete drilled shaft below the base of the excavation.

Table 5-4 Lateral Earth Pressure Summary

Anchorage Type

Backslope Angle

Active 1 Pressure

Active 2 Pressure

Passive 1 Resistance

Passive 2 Resistance

Passive Lock-Off

Resistance

Seismic Pressure

Traffic Pressure

Fill-Side Wall with Tiebacks

Level (≤1V:5H)

See Figure

5-1 275 D psf - 900 D psf

12.6 H

psf

75 psf

Fill-Side, Cantilever

Wall

Level (≤1V:5H)

59 H psf 33 (H+D) psf 275 D psf - -

12.6 H

psf

75 psf

Cut-Side, Cantilever

Wall 1V:2H 68 H psf

(H+D) psf 595 D psf 309 D psf -

12.6 H

psf

75 psf

We recommend a nominal active pressure, in units of pounds per square foot (psf), equivalent to 59 times the excavation height. The active pressures are assumed to act over the pile spacing above the excavation base, and over the grouted diameter below the excavation base.

Analysis and Design Recommendations 26

Figure 5-1 Fill-Side with Tiebacks Section

Figure 5-2 Cut-Side Cantilever Non-Gravity Section

Analysis and Design Recommendations 27

Figure 5-3 Fill-Side Cantilever Non-Gravity Section

5.2.1.1 Active Earth Pressures

Design of these soldier pile walls should be based on “active” lateral earth pressures, which assume that the walls are free to deform on the order of 0.001 to 0.002 times the exposed height.

This lateral deformation is likely to be accomplished by vertical settlement of up to roughly 0.005 times the height of the wall, which may extend back from the side of the cut a distance equal to roughly the height of the cut. Lesser degrees of settlement may also occur within a setback extending twice as far back. A greater amount of lateral deformation could allow greater vertical settlements. If no structural elements are located within this zone, or if structural elements within the zone can tolerate this degree of settlement, then it would be appropriate to design with active earth pressures. Active earth pressure coefficient equations from the AASHTO Bridge Design Specifications are shown in Figure 5-4 were used for Strength I, Service I, and Extreme I limit states.

Analysis and Design Recommendations 28

Figure 5-4 Static Active Pressure Coefficient Equations from AASHTO LRFD Bridge Design Specifications

For the case of a cantilevered wall system, or walls with a single row of tiebacks, the applied lateral pressure would be represented by a triangular pressure distribution termed an equivalent fluid density (EFD). We recommend a nominal active pressure equivalent fluid densities for cut-side cantilever walls as shown in Table 5-5 and Figure 5-2, and for fill-side cantilever walls as shown in Table 5-6 and Figure 5-3. Though fill-side cantilever walls are not a final structure, the fill-side wall with a single row of tiebacks will function as a cantilever wall prior to tieback installation, and as such, should be evaluated. Figure 5-1 illustrates the recommended pressure distribution for fill-side walls with tiebacks. The values presented in Table 5-7 include active slide pressure. These pressure diagrams assume the walls will be constructed from the top down.

Active pressures should be assumed to act on the pile spacing above the design excavation base and the grouted pile diameter below the design base of the excavation. Please note that the design base may correspond with the estimated depth of the slide plane and not the final grade at the front of the wall.

Table 5-5 Static Lateral Earth Pressure Parameters for Cut-Side Cantilever Walls

Zone1

Total Unit

Weight (pcf)

Effective Friction Angle ' (deg)

Cohesion c (psf)

Wall or Pile

Friction (deg)

Active Earth

Coefficient ka

Equivalent Fluid

Density (pcf)

Ground Surface Angle from Horizontal (deg)

Active 1 2 105 28 0 14 0.64 68 27

Active 2 3 115 18 50 14 0.93 107 27

Notes:

1 See Figure 5-2 for zone depths. Zone depths vary and may consist of several soil layers.

2 Using a wall/lagging to soil friction coefficient of 0.50.

3 Using a grouted pile to soil friction coefficient of 0.75

Analysis and Design Recommendations 29

Table 5-6 Static Lateral Earth Pressure Parameters for Fill-Side Cantilever Walls

Zone1

Total Unit

Weight

(pcf)

Effective Friction Angle

' (deg)

Cohesion c (psf)

Wall or Pile

Friction

(deg)

Active Earth

Coefficient ka

Equivalent Fluid

Density (pcf)

Ground Surface

Angle from Horizontal (deg)

Active 1 2, 4 120 17.5 0 9 0.49 59 0

Active 2 3 135 35 0 27 0.24 33 0

Notes:

1 See Figure 5-3 for zone depths. Zone depths vary and may consist of several soil layers.

2 Using a wall/lagging to soil friction coefficient of 0.50.

3 Using a grouted pile to soil friction coefficient of 0.75 4 Composite values includes active slide pressure.

Table 5-7 Lateral Earth Pressure Parameters for Fill-Side Anchored Walls

Zone1

Total Unit

Weight (pcf)

Effective Friction Angle ' (deg)

Cohesion c (psf)

Wall or Pile

Friction (deg)

Active Earth

Coefficient ka

Equivalent Fluid

Density (pcf)

Ground Surface

Angle from Horizontal (deg)

Active 2,3 120 17.5 0 9 0.49 59 0

Notes:

1 See Figure 5-1 for zone depths. Zone depths vary and may consist of several soil layers.

2 Using a wall/lagging to soil friction coefficient of 0.50.

3 Composite values includes active slide pressure.

5.2.1.2 Passive Earth Pressures

Lateral resistance for the walls could be provided by passive pressure on the buried portion of the solider piles and a bracing system of tieback anchors, where needed. Figure A-1 illustrates the recommended truncated triangular pressure distribution for passive resistance. The recommended nominal passive pressures are shown in Table 5-8 and Figure 5-2 for the cut-side walls, and Table 5-9, Figure 5-1, and Figure 5-3 for the fill-side walls. Per AASHTO Bridge Design Specifications (2020, 9th Edition) Table 11.5.7-1 Resistance Factors for Permanent Walls, a reduction factor of 0.75 should be applied for passive resistance of vertical elements.

Analysis and Design Recommendations 30

Table 5-8 Passive Lateral Earth Pressure Parameters for Cut-Side Soldier Pile Walls

Zone1

Total Unit

Weight

(pcf)

Effective Friction Angle

' (deg)

Cohesion c (psf)

Wall or Pile

Friction

(deg)

Active Earth

Coefficient ka

Equivalent Fluid

Density (pcf)

Ground Surface

Angle from Horizontal (deg)

Passive 1 1 105 28 0 21 5.66 595 0

Passive 2 1 115 18 50 14 2.69 309 0

Notes:

1 Using a grouted pile to soil friction coefficient of 0.75.

Table 5-9 Passive Lateral Earth Pressure Parameters for Fill-Side Soldier Pile Walls

Zone1

Total Unit

Weight (pcf)

Effective Friction Angle ' (deg)

Cohesion c (psf)

Wall or Pile Friction

(deg)

Passive Earth

Coefficient kp

Equivalent Fluid

Density (pcf)

Ground Surface Angle from Horizontal (deg)

Passive 1 1,2,3 135 35 0 27 2.05 275 -27

Passive Lock-Off 1 130 30 0 23 6.99 900 0

Notes:

1 Using a grouted pile to soil friction coefficient of 0.75.

2 Assuming a 1V:2H effective slope in front of the wall.

3 Represents the shear zone falling away from the wall.

5.2.2 TRAFFIC SURCHARGE

An additional uniform lateral pressure 75 psf should be added to the above values to model traffic surcharges. This surcharge is approximately equivalent to a uniform vertical traffic live loads of 250 psf on the roadway and wall backfill and should be adequate to model typical wheeled and track-mounted construction vehicle loading, except for crane outrigger point loads and stockpile loads.