OR DOT 20(3)_USG Final Geotech Report GR 22-20 Rev 2 signed.pdf

Upper Sunken Grade Slide Repair Project January 6, 2021

Table of Contents 1-i

Geotechnical Report No. 22-20-R2

Upper Sunken Grade Slide Repair Project

FINAL GEOTECHNCIAL REPORT

OR DOT 20(3)

Willamette National Forest, Linn County, Oregon

January 6, 2021

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Table of Contents i

TABLE OF CONTENTS

1 INTRODUCTION

1.1 Project Description

1.2 Purpose and Scope

1.3 Previous Geotechnical Reports

2 HIGHWAY CORRIDOR SETTING

2.1 Average Temperatures and Precipitation

2.2 Topography and Vegetation

2.3 Regional Geology

2.3.1 Site Seismicity

2.3.2 Liquefaction

3 SITE INVESTIGATION

3.1 Field Reconnaissance

3.2 Subsurface Investigation

3.3 Laboratory Testing

3.4 Groundwater

3.5 Slope Inclinometer Instrumentation

4 SITE CHARACTERIZATION

4.1 Site Reconnaissance Observations

4.2 Subsurface Profile

5 ANALYSIS AND DESIGN RECOMMENDATIONS

5.1 Soldier Pile Retaining Wall

5.1.1 Landslide Earth Loads

5.1.2 Active Earth Pressure

5.1.3 Passive Earth Pressure

5.1.4 Tieback Anchors

5.1.5 Soldier PIle and Lagging

5.1.6 Wall Drainage

ii Table of Contents

5.2 Soldier Pile Wall Fill Slope

5.3 Cut Slope

6 CONSTRUCTION CONSIDERATIONS

6.1 Excavation And Embankment

6.2 Anchored Soldier Pile Wall

6.3 Drainage

7 LIMITATIONS

8 REFERENCES

List of Figures iii

LIST OF FIGURES

Figure 1-1: Project Location Map

Figure 1-2: Vicinity Topographic Map

Figure 2-1: Geologic Map from DOGAMI

Figure 2-2: Five Percent Damped Design Response Spectrum

Figure 5-1: Cantilever Wall Lateral Pressure Distribution Diagram

Figure 5-2: Anchored Wall with Single Anchor Lateral Pressure Distribution Diagram

Figure 5-3: Anchored Wall with Multiple Anchors Lateral Pressure Distribution

LIST OF TABLES

Table 2-1: Climate Data Summary

Table 2-2: Summary of Seismic Parameters for Earthquake with 7% PE in 75 Years

Table 3-1: Borehole Location Summary

Table 3-2: Laboratory Test Descriptions

Table 3-3: Laboratory Test Results Summary

Table 3-4: Groundwater Summary

Table 3-5: Slope Inclinometer Details

Table 5-1: Soldier Pile Retaining Wall Design Summary

Table 5-2: Nominal Force Resultant per Height of Wall

APPENDICES

iv Appendices

APPENDICES

APPENDIX A

Project Photos A-1 to A-10

APPENDIX B

Figure B-1: Regional Faults and Folds Map Figure B-2: Seismic Design Ground Motion Parameters Figure B-3: Slide Site Plan Figure B-4: Slide Cross Section

APPENDIX C

Field Classifications for Soil and Rock Unified Soil Classification System Boring Log Legend Boring Logs USG-01 through USG-02 Boring Logs BH20-01 through BH20-04

APPENDIX D

Rock Core Photos: Figures D-1 to D-13

APPENDIX E

Laboratory Test Reports

APPENDIX F

Vibrating Wire Piezometer Plots: Figures F-1 to F-9

APPENDIX G

Slope Inclinometer Plots: Figures G-1 to G-7

APPENDIX H

Analyses Results: Figures H-1 to H-10

Introduction 1

1 INTRODUCTION

The Western Federal Lands Highway Division (WFLHD) of the Federal Highway Administration (FHWA), in cooperation with Oregon Department of Transportation ODOT (project partner), is proposing a landslide repair project on U.S. Highway 20 near MP 56.1. The progression of slide movement and roadway degradation results in ODOT Maintenance Department constantly patching the cracking in the roadway pavement.

This report provides geotechnical investigation results, analysis, design, and construction recommendations for the project. Accordingly, this report incorporates the findings of the preliminary work performed for the project and presents relevant final geotechnical recommendations for the preferred route.

1.1 PROJECT DESCRIPTION

Designated as the Over the River and Through the Woods Scenic Byway, the U.S. Highway 20 corridor provides exclusive access to large areas of National Forest Lands used for recreation and natural resource management, including management and monitoring of essential habitat.

U.S. Highway 20 is a two-lane paved road passing through the Willamette National Forest that functions as a key commerce and tourist transportation route from the mid-Willamette Valley to the Cascades and Central Oregon. Figure 1-1 and Figure 1-2 show the Project Location Map and Project Vicinity Map, respectively. Figure 1-2 shows the extent of the project indicated by the red line. The orange line shows a previously constructed wall by ODOT. The yellow and orange shaded areas are the extents of the earth flows in this area. The orange shaded area is the approximate extent of the landslide impacting the roadway in this project.

The project consists of an approximately 1000-foot stretch of highway between mainline stations 68+00 to 78+00 with about 400 feet of affected roadway length that includes constructing a both cantilevered and tieback solider pile wall to mitigate the headscarp portion of the landslide that is impacting the highway. Photos of the project are included in Appendix A.

1.2 PURPOSE AND SCOPE

The scope of work for the project included assessing relative stability along the existing and proposed alignments with the selection of the preferred option being discussed in this report.

The scope also included performing a subsurface investigation consisting of a total of four geotechnical borings. Other geotechnical scope items included site visits in to perform geotechnical landslide reconnaissance and instrumentation monitoring.

This report was prepared to compile the results of all the subsurface investigations, laboratory testing programs, instrumentation monitoring, geotechnical recommendations for materials, retaining wall design, and construction considerations and recommendations. The geotechnical borings and laboratory tests were completed by WFLHD and a drilling subcontractor under the direction of a WFLHD geotechnical engineer.

2 Introduction

1.3 PREVIOUS GEOTECHNICAL REPORTS

Previous geotechnical investigations were completed between 2007 and 2010 for six landslide sites along U.S. Highway 20 between MP 53.7 and MP 60.1 under WFLHD project number OR EMK 2005(1). The Upper Sunken Grade Project is one of the six landslides previously investigated. The results and recommendations from these previous works are contained in the following reports:

• South Santiam Highway, U.S. Route 20, Slope Stabilization, Phase 1, Geologic Reconnaissance and Landslide Interpretation, WFLHD Geotechnical Report No.11-07, dated October 2007.

• South Santiam Highway, U.S. Route 20, Slope Stabilization, Phase 1, Interim Report:

Landslide Data and Analysis, WFLHD Geotechnical Report No. 05-08, dated June 2008.

• South Santiam Highway, U.S. Route 20 Slope Stabilization Phase 1, Landslide Investigation and Analysis, Final Geotechnical Report, Volumes 1A, 1B, and 2, WFLHD Geotechnical Report No. 20-10, dated January 2011.

The reports dated 2007 and 2008 were prepared by the WFLHD Geotechnical Group. The 2011 report was prepared and submitted by Cornforth Consultants, Inc. to update geologic interpretations and provide conceptual risk reduction options for the six landslides along U.S.

Highway 20 based on a subsurface geotechnical investigation performed as part of their task order with WFLHD.

This report supersedes these previous reports for this project.

Figure 1-1: Project Location Map

Figure 1-2: Vicinity Topographic Map.

Project Location

Beginning of Project

Previously Constructed Wall End of Project

4 Highway Corridor Setting

2 HIGHWAY CORRIDOR SETTING

2.1 AVERAGE TEMPERATURES AND PRECIPITATION

Table 2-1 shows the average maximum and minimum temperatures, average monthly precipitation, and average monthly snowfall data from 1949 to 2016 for the Santiam Junction (357554) weather station (Western Regional Climate Center, 2019). Santiam Junction weather station is about 15 miles, directly east of the project. The elevation of the weather station is 3750 feet, and the project elevation is about 2225 feet. The data in Table 2-1 is expected to vary from the project location due to the 1525 feet of elevation difference.

Table 2-1: Climate Data Summary

2.2 TOPOGRAPHY AND VEGETATION

The general topography of the region consists of mountainous terrain between Sweet Home, Oregon, and Sisters, Oregon. The project is located in a relatively narrow V-shaped valley of Sheep Creek with a general relief of about 1400 feet from the top of the ridge to the valley bottom. Sheep Creek flows in a southwesterly direction until its confluence with the South Fork of the Santiam River.

Vegetation within the project is a Douglas Fir dominant forest with various other typical conifers such as mountain hemlock, western red cedar, Pacific yew, and deciduous, hardwoods such as big leaf maple and alder. The typical understory consists of Oregon grape, rhododendron, vine maple, ferns, and other shrubs.

Highway Corridor Setting 5

2.3 REGIONAL GEOLOGY

The following section is an excerpt from South Santiam Highway, U.S. Route 20, Geologic Reconnaissance and Landslide Interpretation (2007).

The reconnaissance area is within the Western Cascades of the Cascade Mountains physiographic province. Available geologic maps of the area (Geologic Map of the Salem 1–by-2 Quadrangle, Western, Oregon, by G.W. Walker and R.A. Duncan, 1989) indicate that the bedrock in the landslide area is called the Little Butte Volcanic Series. The age of this formation is between 17 and 32 million years. The regional bedrock includes multiple rock types, including basalt and basaltic andesite flows and breccias; and basalt to rhyodacite ash-flow tuffs, air-fall tuffs, lapilli tuffs, tuff breccias, mudflows (lahars), fine- to coarse-grained volcanic sediments, and conglomerates.

The volcanic stratigraphy has not been mapped in detail and is anticipated to be very complex. In addition, the bedrock exhibits low-grade metamorphism and alteration to clay and zeolite minerals.

Landforms in the area are primarily a result of uplift and erosion with transport of sediments from the various types and strengths of the geologic materials. Based on reconnaissance experience in the Cascade Mountains, gentler valley side slope topography can be typically associated with lower strength parent or weathered/altered material, while steeper slope topography can be associated with more resistant or less weathered material.

Broad landslide landforms impact U.S. Highway 20 corridor at numerous locations.

These landforms appear to occur on a westward dipping sequence of volcanic strata within the Little Butte Volcanic Series; however, the material type and stratigraphic sequence are not well understood. The primary type of landsliding in the South Santiam drainage consists of earthflows that coalesce and form broad aprons of slide debris. Much of the landslide terrain is ancient, comprising a geologic formation that has been present since the drainage was created over the past million years or so.

Also, a majority of the landslide terrain appears to be inactive. However, based on locally active slide areas, and distorted pavement where State Route 20 traverses nearby ancient slide terrain, the stability of most if not all of the ancient landslides are likely to be marginal.

Slides that are impacting the highway are mainly earthflows. Some are active along their entire length, while others are partial reactivations of larger earthflows. Some of the slides contain translating blocks of relatively intact rock. Common landforms that were observed and helpful in the interpretation of the landslides includes transverse and longitudinal pressure ridges, isolated blocks, nested slumps and earthflows, lateral shears, and toe bulges.

Figure 2-1 shows a geologic map excerpt from DOGAMI for the vicinity of the project.

6 Highway Corridor Setting

Figure 2-1: Geologic Map from DOGAMI

2.3.1 SITE SEISMICITY

Figure B-1 in Appendix B shows an excerpt from the USGS Quaternary Faults and Folds database web-based mapping program. This program shows an absence of Quaternary-aged (present to 1.6 million years old) faults within 6 miles of the project. Metolius and Sisters fault zones, considered to be of less than 130,000 years old, are located about 40 miles east-southeast of the project.

The American Association of State Highway and Transportation Officials (AASHTO) criteria for seismic design allows for some structural damage to occur when the structure is subjected to 1000-year strong ground motion. For Extreme Event I Limit State Design, an earthquake load and resistance factor of 1.00 should be used during the LRFD design of structures.

The procedures outlined in Section 3.10 of the 8th Edition of the American Association of State Highway and Transportation Officials LRFD Bridge Design Specifications Manual (2017) (AASHTO BDS) were followed to characterize the seismic hazard during the design activity.

These procedures use a design earthquake with a 7 percent probability of exceedance (PE) in 75 years, which equates to a return period of approximately 1000 years. Based on the

Project Location

Highway Corridor Setting 7

AASHTO BDS procedures, and the standard penetration resistance in the top 100 feet of the soil profile at the project site, the AASHTO Seismic Site Class for this project is Site Class C.

The USGS Seismic Design Map Web Services was used following 2011 AASHTO Guide Specifications 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 C using the site factors recommended by AASHTO (2017) to represent the seismic accelerations based on actual ground conditions. The results of this analysis are presented in Table 2-2. The values in Table 2-2 were used to develop the 5-percent-damped-design response spectrum shown in Figure 2-2 below for the project site.

The scaled peak ground acceleration, As, for an earthquake with a return period of approximately 1,000 years was determined to be 0.177g using the USGS Seismic Design Maps website. Seismic design for retaining walls is typically required by AASHTO (2017) where the As is over 0.4g; in addition, from Oregon Department of Transportation Geotechnical Design Guide, Subsection 6.2.3 states if the site falls under AASHTO “No Analysis” option, seismic design is not required. Embankment and cut slopes are generally not designed for earthquake loading for roadways except in close proximity to bridge abutments. No bridges are proposed for the project.

It should be noted that landslides and displacements of retaining structures could occur during strong ground shaking from an earthquake, and that movement of the existing large slide mass could also affect the stability of the roadway and existing areas not recently exhibiting active landslide movement.

Table 2-2: Summary of Seismic Parameters for Earthquake with 7% PE in 75 Years

8 Highway Corridor Setting

Figure 2-2: Five Percent Damped Design Response Spectrum

2.3.2 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). According to AASHTO BDS Article

11.5.4.2 seismic design is not mandatory for walls located in Seismic Zones 1 through 3 or where the site adjusted peak ground acceleration, As, is less than or equal to 0.4g.

Site Investigation 9

3 SITE INVESTIGATION

Geotechnical investigation for this project consisted of reviewing the previous geotechnical documents (literature review) listed in subsection Previous Geotechnical Information Subsection of this memorandum.

3.1 FIELD RECONNAISSANCE

In addition to the literature review, field reconnaissance site visits were performed by the Project Geotechnical Engineer on October 22, 2018 and June 15, 2019. Geomorphic mapping along the entire length of the active landslide was completed for the OR EMK 2005(1) project and that mapping was used during the recent 2018 and 2019, field reconnaissance. An interpreted plan map and cross section of the slide is provided on Figures B-1 and B-2, respectively. Grab samples from the cut slope and from near the toe of the slide were obtained during the 2007 field reconnaissance. During the recent 2018 and 2019 field reconnaissance the project geotechnical engineer evaluated the existing roadway conditions, determined boring locations, and identified drill access issues.

3.2 SUBSURFACE INVESTIGATION

One subsurface investigation boring was completed on November 14 and 15, 2007, for the OR EMK 2005(1) project. A second boring for the OR EMK 2005(1) project was completed on November 22 and 23, 2008. Information obtained from these two borings were used for previous conceptual design analyses and preliminary design work. Figure B-3 shows the approximate locations of the 2007 and 2008 explorations, Site Plan contained in Appendix B.

Boring USG-01 was completed in November 2007, and boring USG-02 was completed in November 2008. Cornforth Consultants Inc. logged these two borings. Boring logs and rock core photographs from these borings are contained in Appendix C and D, respectively. Table 3-1 shows a summary of the two previous borings drilled in 2007 and 2008 and four borings drilled in 2020. The collar elevation is the referenced ground surface in the boring logs.

Drilling operations for the 2007 and 2008 subsurface investigation were contracted with Haz- Tech Drilling, Inc. of Nampa, Idaho. The geotechnical test borings, USG-01 and USG-02, were completed using a CS-1000 track-mounted drill rig. The geotechnical borings were generally advanced with hollow-stem augers. Split barrel sampling of soils was performed by an automatic hammer. The split barrel samples were performed in the soil at 5-foot and 10-foot intervals, using 140-pound hammer dropped from 30 inches. A Modified California, 2.5-inch O.D., split barrel sampler was used to recover the disturbed soil samples. The results of penetration testing are shown on the Boring Logs. When relatively competent rock was encountered, the drillers switched to HQ3 (3.8-inch O.D.) core drilling methods. An inclinometer casing (2.75-inch O.D.) was installed in each boring to record ground movements over time. A vibrating-wire piezometer was also installed in boring USG-02. Ground water measurements were obtained in boring USG-01p by a 1-inch standpipe piezometer installed in a second boring located about 5 feet from boring USG-01i.

10 Site Investigation

Four additional subsurface investigation borings were drilled between July 16 and July 23, 2020 in support of the intermediate design phase and are shown in Figure B-3 located in Appendix B.

These borings were logged by a WFLHD Geotechnical Engineer. Drilling operations for the 2020 subsurface investigation were also contracted with Haz-Tech Drilling, Inc. and completed using a BK-75 truck-mounted drill rig. The borings were initially advanced with 8.25-inch O.D., hollow-stem auger until refusal after which the boring was advanced using coring methods.

Borings BH20-01 and BH20-02 were advanced through rock with PQ3 (4.8-inch O.D.) coring system and borings BH20-03 and BH20-04 were advanced through rock using HQ3 (3.8-inch O.D.) coring system.

Standard Penetration Tests (SPTs) were performed by an automatic hammer. The SPTs were performed in the soil at 2.5-foot and 5-foot intervals, using 140-pound hammer dropped from 30 inches. A standard, 2-inch O.D., split barrel sampler was used to recover the disturbed SPT soil samples. The results of penetration testing are shown on the Boring Logs.

The 3.34-inch O.D. inclinometer casing was installed in borings BH20-01 and BH20-02 to allow the inclinometer probe to pass with greater deformation in the casing. A 2.75-inch O.D.

inclinometer casing was installed in borings BH20-03 and BH20-04. A vibrating-wire piezometer was also installed in each boring.

Site Investigation 11

Table 3-1: Borehole Location Summary

Boring ID Approx.

Lat.

North

Approx.

Lon.

West

Collar Elev.6

Approx.

Station

Approx.

Offset 5

Boring Depth Purpose Instrumentation

USG-011,4 44.4066° 122.2232° 2234' 73+90 45' LT 55' &

25'

Landslide Characterization

Inclinometer, vibrating wire piezometer, and standpipe piezometer

USG-02 2 44.4065° 122.2233° 2224' 73+90 36' RT 91' Landslide Characterization

Inclinometer, vibrating-wire piezometer

BH20-01 3 44.4064° 122.2234° 2234' 74+43 10' RT 101'

Landslide Characterization, Fill Wall

Inclinometer, vibrating-wire piezometer

BH20-02 3 44.4065° 122.2231° 2226' 73+08 10' RT 100.5'

Landslide Characterization, Fill Wall

Inclinometer, vibrating-wire piezometer

BH20-03 3 44.4067° 122.2232° 2236' 74+38 42' LT 75'

Landslide Characterization, Wall Anchor

Inclinometer, vibrating-wire piezometer

BH20-04 3 44.4064° 122.2236° 2228' 73+08 42' LT 75'

Landslide Characterization, Wall Anchor

Inclinometer, vibrating-wire piezometer

Notes:

1 Boring drilled November 2007 2 Boring drilled November 2008 3 Borings drilled July 2020 4 USG-01 consists of two borings drilled about 5 feet of each other and will be discussed within this document as one boring. USG-01i contains slope inclinometer casing extending to a depth of 55 feet. USG-01p contains 1-inch nominal diameter standpipe piezometer installed to 25 feet bgs with a screen between 15 and 25 feet and sand to 14 feet with bentonite seal to the ground surface.

5 Offset is from proposed centerline 6 Elevation based on 2020 70% wall design plan plans

3.3 LABORATORY TESTING

Representative soil samples collected during the subsurface explorations were submitted to the WFLHD Materials Laboratory for soil index testing. Laboratory testing was conducted in accordance with appropriate AASHTO specifications to obtain engineering properties necessary for design. The results of the soil index tests were used to classify the samples according to the Unified Soil Classification System (USCS). The USCS classification of individual laboratory tested soil samples is reported on the boring logs. A summary of the laboratory tests performed, and a summary of the soil physical test results are shown in Table 3-2 and Table 3-3, respectively. Laboratory test reports are contained in Appendix E.

12 Site Investigation

Table 3-2: Laboratory Test Descriptions

Table 3-3: Laboratory Test Results Summary

Site Investigation 13

3.4 GROUNDWATER

Table 3-4 shows the depths groundwater was encountered during the two subsurface investigations and the piezometer pore water pressure depth. In addition, Table 3-4, provides the depths groundwater was encountered during the 2020 drilling and the depth the vibrating wire piezometer (VWP) was installed in each boring.

The boring logs from USG-01 and USG-02 do not indicate groundwater was encountered during drilling. Five manual groundwater readings were obtained from the boring USG-01p open standpipe piezometer between November 16, 2007 and April 9, 2008 and they are shown in Figure F-1 of Appendix F. A Durham Geo Slope Indicator (DGSI) vibrating wire piezometer, installed in boring USG-02, is assumed to have been connected to a DGSI VW MiniLogger data recorder at the time of installation. Data should have started being collected soon after USG-02 was completed on November 23, 2008; however, the only data found was a groundwater graph in Geotechnical Report No. 20-10 and it is presented shown in Figure F-2. Figures F-3, F-4, and F-5 shows the daily and cumulative precipitation data for USG-01p and USG-02 monitoring period. The origin of this precipitation data is unknown.

A DGSI vibrating wire piezometer and VW MiniLogger were installed in borings BH20-01 through BH20-04 using the grouted-in method with the porous tip up. Figures F-6 through F-9 show the date and the depth to groundwater from the ground surface for each boring relative to daily precipitation measurements for BH20-01 through BH20-04 starting on July 25, 2020 and continuing through December 12, 2020. In Figures F-6 through F-9, the red dashed line shows the depth of the porous stone of the piezometer while the blue line represents the groundwater level during the monitoring period. The figures also provide a graphical representation of the precipitation recorded at closest weather station to the project. The weather station used to collect precipitation data for the piezometer plots is managed by National Oceanic and Atmospheric Administration (NOAA), Jump Off Joe, OR, ID number USS0022E07S and it is located at 44.3895°N, -122.1706°W, about 3 miles straight-line east of the project at an elevation of 3520 feet. Data obtained from this weather station includes precipitation, temperatures, measured snow on the ground, and water equivalent of snow on the ground.

Precipitation data in these groundwater plots only accounts for non-frozen precipitation. Frozen precipitation is thawed and measured for the daily precipitation. Water equivalent of snow on ground (WESG) data is not shown or included in the precipitation data.

Groundwater levels in boring BH20-01 appear to have remained generally constant at about

30.5 feet bgs between July 25 to September 22, 2020. Between September 23rd and 25th, the weather station recorded 3 inches of precipitation at which the piezometer recorded a porewater pressure increase beginning on September 23 with the groundwater surface at a depth of about

29.9 feet bgs on September 27th. Thereafter, the porewater pressure decreased over a period of no precipitation resulting in the groundwater surface to drop to 30.5 feet on October 10th when 3.4 inches of precipitation was measured at the weather station. Between October 10th and 13th, the weather station measured 6.3 inches of precipitation which the groundwater surface elevation raised to 29.6 feet. Throughout the monitoring period precipitation and groundwater appears to correlate well. The highest point the groundwater reached was 29.2 feet on November 26, 2020.

14 Site Investigation

The piezometer in boring BH20-02 shows groundwater at 30 feet at the beginning of the data collection period and increased to 29.6 feet and then a steady decline until September 13, 2020 when it reached a low of 33.2 feet. The increase and decrease in the data between July 25 and September 13th may have be the result of drill water from BH20-03 influencing this piezometer;

however, it is unclear reason for the groundwater increase between September 14 and September 20 when the first precipitation was recorded. After September 20th, there appears to be a correlation between precipitation and groundwater depth.

It appears the piezometer at boring BH20-03 was installed very close to the groundwater depth at the time of drilling. The porewater pressure decreased from the end of July to September 28, 2020, to a low of 0.4 feet below the piezometer tip elevation of 19.5 feet. After 3 inches of precipitation that occurred between September 23rd through 25th there was a slight increase of the groundwater elevation. Another groundwater elevation increase occurred after the October 10th through 13th precipitation event that begins to exhibit a correlation between precipitation and groundwater response.

The poor correlation between porewater pressure and precipitation in boring BH20-04 may indicate the piezometer was not installed deep enough at this location.

Table 3-4: Groundwater Summary

3.5 SLOPE INCLINOMETER INSTRUMENTATION

Data reading sets from the instrumentation from the preliminary geotechnical explorations, USG-01 and USG-02, were downloaded to create the displacement plots contained in Appendix G. The inclinometer in boring USG-01 did not detect subsurface ground displacements between the initial baseline reading set in November 2007 and the final data collection set taken

Site Investigation 15 in January 2009. An additional reading set was obtained in USG-01 installation in June 2019 and then compared to the initial data set. The additional reading set indicated that movement within the USG-01 installation had not occurred and is shown in Figure G-1.

The inclinometer casing in the USG-02 installation became sheared before the third reading could be taken on March 26, 2009, indicating that movement of sufficient magnitude to cause the instrument to shear occurred at some time after the previous data set reading taken on 12/11/2008. The depth to shearing was measured at 38.1 feet bgs was determined after the inclinometer sheared with a fixed borehole extensometer. As can be seen in the displacement plot on Figure G-2, no discernable movement was observed in the inclinometer surveys prior to shearing. According to WFLHD Geotechnical Report No. 20-10, the displacement rate was estimated to be approximately 12 inches per year at a shearing depth of 38.1 feet bgs.

Inclinometer readings from borings BH20-01 and BH20-02 did not show any discernable movement until the November 16, and December 3, 2020 readings. This is thought to be due to no precipitation between July through November 4, 2020. Both inclinometer plot indicates about 0.1-inch of movement, primarily in the Ao direction, starting at about 38 feet below the ground surface. This time period correlates well with the increase precipitation that occurred.

Inclinometer plots for BH20-01 and BH20-02 are provided in Figures G-3 and G-4, respectively.

The inclinometer plots for borings BH20-03 and BH20-04 do not exhibit any displacement at this time and are provided in Figure G-5 and G-6. Table 3-5 provides details of each slope inclinometer.

Table 3-5: Slope Inclinometer Details

4 SITE CHARACTERIZATION

4.1 SITE RECONNAISSANCE OBSERVATIONS

The highway traverses across an active earthflow that merges with another earthflow farther down the slope to the east of the project. The roadway pavement is characterized by multiple asphalt patching with general subsidence between the lateral limits of the slide, and pavement distress caused by tension cracks within the slide limits. The earthflow is impacting approximately 315 feet along Highway 20.

According to WFLHD Geotech Report 20-10, the head scarp of the slide is approximately 20 feet upslope of the highway. An oblique-oriented backscarp on the downslope side of the highway delineates a down-dropped graben feature within the right side of the top of the slide.

The “oblique” graben could be the result of the right side of the slide moving faster or the slide rotating counterclockwise (plan view) as it moves downslope. Maintenance paving of the down-dropped road surface has built up a thick embankment of asphaltic concrete (14 feet at USG- 02). Further upslope of the highway, the ancient landslide material appears to thin and pinch out and appears to be left in-place upslope and behind the active slide. Below the highway, the condition of the ground surface and vegetation indicate that the slide is translating downslope.

On the east side of the upper portion of the Upper Sunken Grade Slide a bedrock ridge or promontory exists. About 450 feet from the east lateral scarp, a tieback soldier-pile wall supports the highway. This wall is about 260 feet long and was constructed in the late 1990s as mitigation of the Mile Point 56.2 Slide. Downslope of the ridge, the Upper Sunken Grade Slide merges with the adjacent larger slide area that, based on having more disturbed ground and leaning trees, appears to be more active than Upper Sunken Grade Slide.

The lower portion of Upper Sunken Grade Slide is not well defined as a separate slide feature.

However, the toe of the slide area appears to be actively moving downslope, and an over-steep slope between elevation approximately 2,050 and 1,990 feet is raveling and sloughing onto a gentler slope that toes at an alluvial bench alongside the river.

Surface water features include creeks along both sides of the upper portion of the landslide and the right margin of the lower portion of the slide. Above the slide at the left margin, a small creek flows over a waterfall and then through a culvert at the road that outlets and flows down the left side of the slide. Above the slide at the right margin, another creek flows through a culvert at the road that outlets and flows down the right side of the upper slide before it diverges away from the active slide. We did not observe the creek downslope and into the adjacent eastern slide area. A spring was also observed emitting from the left margin of the slide near the toe. Based on on-site observations, it is possible that the depth of the slide becomes shallower near the lateral limits.

Existing cut slope ratios are approximately 1V:1H near the beginning of the project and range between 1V:1H to 2V:1H near the end of the project. The cut slopes near the beginning of the

SITE CHARACTERIZATION 17

project are poorly vegetated while the slopes near the end have little to no vegetation due to the steepness and the presence of rocky outcroppings.

4.2 SUBSURFACE PROFILE

The following discussion is from the text of WFLHD Geotechnical Report No. 20-10 pertaining to the material encountered in borings USG-01 and USG-02; however, the boring logs descriptions contained in the 20-10 report vary from the text descriptions.

The boring log shows boring USG-01 encountered cobbley road fill in the first 2 feet of the boring and medium dense silty gravel between the depths of 2 and 6 feet.

Underlying the silty gravel, the material transitions with depth from medium-stiff, clayey sand with gravel to extremely soft rock consisting of decomposed volcanic tuff displaying highly jointed and slickenside characteristics between the depths of 6 and 40 feet. Between the depths of 40 feet and 55 feet, the material consists of soft to medium-hard rock, gray to green-gray, moderately weathered volcanic tuff to tuff breccia characterized by highly jointed and occasional slickensided joints.

Boring USG-02 encountered asphaltic concrete fill to a depth of 14 feet. Underlying the asphaltic concrete fill, the material transitions with depth from medium stiff, slightly clayey, silty sand (possible landslide debris) to extremely soft rock (R0) at approximately 32 feet bgs consisting of decomposed volcanic tuff displaying highly jointed and slickenside characteristics. At 50 feet bgs to the bottom of the boring at 91 feet bgs, the material consists of soft to medium hard rock, gray to green-gray, moderately weathered volcanic tuff characterized by highly jointed with occasional slickensided joints.

Boring BH20-01 was advanced to 101 feet bgs. The boring encountered loose to very dense silty sand with gravel to a depth of 29.5 feet, which is interpreted to be fill. Cobbles and chunks of asphalt are suspected to have overstated some of the SPT blow counts. Between the depths of 29.5 to 43 feet, a zone of medium dense to very dense silty sand and sandy silt was encountered that is interpreted to be completely weathered tuff. Highly weathered to slightly weathered, very weak (R1) to weak (R2) volcanic tuff was encountered from 43 to 101 feet.

Boring BH20-02 was advanced to 100.5 feet bgs. The boring encountered loose to very dense silty sand with gravel to a depth of 28 feet, which is interpreted to be fill. Cobbles and chunks of asphalt are suspected to have overstated some of the SPT blow counts. Between the depths of 28 to 55.5 feet, a zone of medium dense to very dense silty sand and clayey sand was encountered that is interpreted to be completely weathered tuff. Highly weathered to slightly weathered, very weak (R1) to weak (R2) volcanic tuff was encountered from 55.5 to 100.5 feet.

Boring BH20-03 was advanced to 75 feet bgs. Based on drill actions and driller’s comments the top 8 feet was gravelly then smoothed out and loose clayey sand was encountered to 17.5 feet.

Between the depths of 17.5 to 30 feet, a zone of very dense clayey sand interpreted as completely weathered tuff was encountered. Slightly weathered, very weak (R1) to weak (R2) volcanic tuff was encountered from 30 to approximately 52 feet. Between 52 and 75 feet the

18 SITE CHARACTERIZATION

strength of the rock decreased to extremely weak rock (R0) and the discontinuities are in poor condition.

Boring BH20-04 was advanced to 75 feet bgs. Based on drill actions and driller’s comments the top 6 feet was gravelly then smoothed out and loose clayey sand interpreted as completely weathered tuff was encountered to 40 feet. Slightly weathered, very weak (R1) to weak (R2) volcanic tuff was encountered between 40 and 75 feet.

Possible groundwater observations were noted in the boring logs, but it is not clear if the water is true ground water or residual drilling fluids after coring or casing advancer drilling methods began.

Analysis and Design Recommendations 19

5 ANALYSIS AND DESIGN RECOMMENDATIONS

5.1 SOLDIER PILE RETAINING WALL

Based on the site conditions and the geotechnical investigation, we recommend that the slide movements be mitigated with a solder pile retaining wall as listed in Table 5-1. Lateral earth pressures and forces for design of the wall are described in the sections below.

Table 5-1: Soldier Pile Retaining Wall Design Summary

5.1.1 LANDSLIDE EARTH LOADS

Due to the sliding mass of soil, the earth pressure driving loads on the wall will be higher than typical active earth pressures. Rather than model the earth forces using conventional lateral earth pressure diagrams, we modeled the landslide earth forces using limit equilibrium methods with the software SLIDE2 by Rocscience. Based on Ground Anchors and Anchor Systems, FHWA (1999), the force applied at the face should be located between 0.3H and 0.5H. During the analysis the force was checked at a range of locations between 0.3H and 0.5H for larger required loads. 0.4H was used in the analyses to maintain constant height. During field reconnaissance, tension cracks varied in location a maximum 27 feet from the proposed wall face. The resultant failure plane, in the analyses results, were within the observed tension crack locations. The force resultant, Pa is applied considering the interface friction between the soil and the wall resulting in the force applied to the vertical surface at a 9-degree angle above horizontal which is the interface friction angle of one half of the slide material friction angle. The analyses also include a 300 psf distributed load to account for the pavement section. The magnitude of the nominal force resultant per unit width of wall, Pa are provided in Table 5-2 for various wall heights. The force resultants models are the forces that are required to stabilize the landslide to a factor of safety of 1.0; therefore, the actual driving force should be considered the opposite vector for structural analysis as shown in Figure 5-1 to Figure 5-3. The force resultant analyses are provided in Appendix H. The soil parameters used in the analyses are shown in Table H-1. The force resultants on a downward angle of 9 degrees should be assumed to act on the pile spacing above the excavation base and broken into horizontal and vertical driving force vectors and distributed based on the teback spacing.

A load factor of 1.5 for cantilever walls and 1.35 for anchored walls is recommended for these earth forces per Table 3.4.1-2 from AASHTO BDS.

20 Analysis and Design Recommendations

5.1.2 ACTIVE EARTH PRESSURE

Below the slide plane, active earth pressures should be used to model driving forces. The active earth pressures should be assumed to act on the grouted pile diameter below the base of the excavation. For the case of cantilevered wall, the applied lateral force would be represented by a triangle pressure distribution. Figure 5-1 illustrates the recommended triangular pressure distributions. For the case of walls with one or more levels of anchors, the applied lateral pressure would be represented by a trapezoidal pressure distribution. Figure 5-2 and Figure 5-3 illustrates the recommended trapezoidal pressure distributions for a single anchor and multiple levels of anchors in the walls. These pressure diagrams assume the walls will be constructed from the top down. A load factor of 1.5 for cantilever walls and 1.35 for anchored walls is recommended for these earth forces per Table 3.4.1-2 from AASHTO BDS.

For the case of a cantilevered wall system, the applied active earth pressure would be represented by a triangular pressure distribution termed an equivalent fluid density. We recommend the nominal active pressure equivalent fluid density of 58H for the grouted pile diameter below the base of the excavation.

Table 5-2: Nominal Force Resultant per Height of Wall

Note: The force resultant is a vector acting at a downward angle of 9 degrees from horizontal.

Analysis and Design Recommendations 21

5.1.3 PASSIVE EARTH PRESSURE

We recommend a nominal passive pressure, in units of pounds per square foot (psf), equivalent to 288 times the embedded depth of the soldier pile. The passive pressure may be applied over

2.5 times the diameter of the concreted soldier pile section or the pile spacing, whichever is less, neglecting the upper 2 feet below the excavation base. Per AASHTO Bridge Design Specifications (BDS) (AASHTO, 2017) 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.

Figure 5-1: Cantilever Wall Lateral Pressure Distribution Diagram

22 Analysis and Design Recommendations

Figure 5-2: Anchored Wall with Single Anchor Lateral Pressure Distribution Diagram

Figure 5-3: Anchored Wall with Multiple Anchors Lateral Pressure Distribution

Analysis and Design Recommendations 23

5.1.4 TIEBACK ANCHORS

The anchor portion of the tieback must be fully located a sufficient distance behind the retained excavation face to develop resistance within the stable soil mass. As shown on Figure 5-2 and Figure 5-3, we recommend the anchorage be attained behind an assumed failure plane that is formed by a 60-degree angle from the base of the excavation and set back from the retained excavation face for a horizontal distance of one-fourth the height of the soldier pile above the bottom of the excavation. The zone above this plane is called the “no-load zone”. In addition to the typical no-load zone for conventional tieback walls, the bond zone of the anchors also needs to be located below the slide plane in stable materials approximately 15 feet below the roadway.

The no-load portion of the tieback should also be a minimum length of 40 feet measured along the tieback and protected with bond-breakers, such as plastic sheathing to avoid very soft rock found in borings BH20-03 and BH20-04. All tieback holes within the no-load zone should be immediately backfilled with neat cement grout to prevent loss of ground. The tendons should be specified for Class I Corrosion Protection for tieback anchors as recommended by the Post- Tensioning Institute (PTI) for permanent anchors (PTI, 2014).

With a tieback wall system, we estimate a nominal cement grout-soil adhesion of 100 pounds per square inch (psi) could be used based on AASHTO BDS Table C11.9.4.2-3 published values for sandstone. However, the actual adhesion value will depend on the contractor’s means and methods, such as using pressure grouting, secondary grouting, etc. Therefore, the design loads are typically provided on the plans, and it is up to the contractor to construct on anchor bond zone to resist the specified load. A minimum bond zone length of 10 feet is recommended. Per AASHTO BDS Table11.5.7-1 Resistance Factors for Permanent Walls, a reduction factor of 1.0 may be applied for design resistance where the pullout resistance is proof-tested during construction. Therefore, all tiebacks should be proof-loaded to at least 133 percent of design resistance. The tieback testing program should be approved by WFLHD and observed by a qualified geotechnical engineer.

For lock-off load check calculations on the upper row of tiebacks, a total unit weight of 120 pcf and friction angle of 30 degrees may be used for a passive coefficient, kp, equal to 5.11.

5.1.5 SOLDIER PILE AND LAGGING

Soldier piles for walls are typically set in pre-augered holes and backfilled with lean and structural concrete. Vertical loads on such piles could be resisted by a combination of friction and end-bearing below the base of the excavation. We recommend a nominal unit side friction value of 2,500 psf and a nominal unit end-bearing value of 15 ksf (kips per square foot) for design. Side friction should be neglected within the upper 2 feet below the base of excavation.

The end-bearing value is predicted on embedment of at least 20 feet below the base of the excavation.

An appropriate reduction factor should be applied for design resistance. Per AASHTO BDS Table 11.5.7-1 Resistance Factors for Permanent Walls, Article 10.5 applies for axial compressive resistance of vertical elements. Per AASHTO BDS Table 10.5.5.2.4-1 Resistance

24 Analysis and Design Recommendations

Factors for Geotechnical Resistance of Drilled Shafts, a reduction factor of 0.55 for side resistance in cohesive IGMs and 0.55 for tip resistance in cohesive IGMs.

We recommend lagging, or some other form of shoring, be installed in all areas. Due to soil arching effects, lagging, and concrete fascia may be designed for 50 percent of the lateral earth pressure used for walls design.

5.1.6 WALL DRAINAGE

Since the front of the wall will be backfilled, the wall weep holes will be covered. We recommend installing geocomposite sheet drain behind the wall in addition to the wall weep holes to reduce hydrostatic pressure against the back of the wall. The geocomposite drain should allow for flow on both side of the drainage core. This will allow for flow out of the weep holes as well. The wall drainage should be installed between all of the piles and attached to a 6-inch drainpipe at the toe of the wall.

5.2 SOLDIER PILE WALL FILL SLOPE

This wall is proposed to be pushed out to the outboard side of the existing roadway. Imported fill material behind the wall will be required to widen the existing roadway embankment prior to wall construction to create a solid soil block behind the wall. Select borrow, FP-14, 704.07, is recommended for the embankment material and FP-14 Section 204 for the construction.

5.3 CUT SLOPE

Cutting into the existing slopes along the inboard side of the road is not recommended. Cutting into the slopes may result in an unstable slope and/or slope that will be difficult for vegetation to grow.

Construction Considerations 25

6 CONSTRUCTION CONSIDERATIONS

6.1 EXCAVATION AND EMBANKMENT

Boulders and chunks of asphalt are anticipated to be encountered in the excavation of the wall.

In general, weathered rock within the front of the wall should be able to be excavated with heavy ripping equipment, although hydraulic breakers or other rock splitting methods may be needed for deeper cuts and harder rock areas.

The slope in front of the wall will be reconstructed to the designed grade. Material from the excavation is anticipated to be used. However, this material contains fine-grain soils that are anticipated to be moisture sensitive and may be difficult to work when the moisture content is outside of the optimum range.

Seepages may be encountered any time of the year.

6.2 ANCHORED SOLDIER PILE WALL

HP14x117 section steel beams will be mounted into 30-inch diameter drilled shafts with up to two rows of tieback anchors across the exposed portion of the wall. The wall will be temporarily timber lagged and completed with a concrete fascia.

Stability of the drilled shaft holes should be the responsibility of the contractor, which may need to include temporary casing or a slurry drilling method. After the drilling has reached the bottom, a cleanout run should be performed to reduce loose or disturbed material to less than 2 inches for soldier piles with permanent ground anchors and less than 12 inches of loose or disturbed materials for soldier piles without permanent ground anchors. Tremies should be used to place structural concrete and lean concrete backfill when more than 6 inches of water are in the bottom of the excavation or wet drilling methods are used. Excavation for shafts should not be started until a minimum of 12 hours after the backfill for adjacent solider piles have been placed.

Other requirements will be provided in FP-14 and the special contract requirements. Prompt and careful installation of lagging will reduce potential loss of ground. The requirements for lagging should be made the responsibility of the walls subcontractor to prevent soil failure, sloughing and loss of ground and to provide safe working conditions. We recommend any voids between the lagging and soil be backfilled. However, the backfill should not allow potential hydrostatic pressure to build-up behind the wall. Drainage behind the wall must be maintained. A lean concrete backfill may be considered for lagging backfill or a drainage mat can be placed against the cut soils.

Anchor holes should be drilled in a manner that will minimize loss of ground and not disturb previously installed anchors. During the drilling, caving could occur if loose, wet or saturated zones are encountered. Drilling with a casing would reduce the potential for loss of ground. The selected tieback anchor installation should be subject to performance testing and proof-loading.

Use of the design values presented herein is dependent on a well-constructed anchor. We recommend that structural grout be placed in the drilled tieback anchor hole by tremie methods such as pumping through a hose placed in the bottom of the hole. In this way, the grout is forced up through the anchor zone under pressure, with the resulting anchor more likely to be continuous. The grout should not be placed into the anchor zone by simple gravity methods such as flowing down a chute. We recommend that a qualified geotechnical engineer or their representative monitor all tieback installations.

6.3 DRAINAGE

Embankment in front of the wall may continue to slide down the slope with the rest of the landslide. There is a risk if the landslide continues down the slope it will create a bathtub effect between the wall and the head of the sliding fill. It is important to outlet subsurface drainage pipes either off of the slide or into the existing drainages that are also the lateral scarps.

Culverts that extend from the wall face should not be incorporated within an…