waa-skaman92135-1_WindRiverRoad_SupplementalGeotechReport.pdf

WA FLAP SKAMAN 92135(1)

Wind River Road Slide Reconstruction Supplemental Geotechnical Exploration and Recommendations Report

DISCLAIMER:

The following report was prepared for a landslide mitigation solution (Micropile and Cap Beam) that was not implemented in the final plans for the project. This report is provided for informational purposes, as it contains information pertinent to the design of the Mechanically Stabilized Earth wall, as well as additional geotechnical exploration data in the vicinity of the soldier pile retaining wall.

REPORT

Supplemental Geotechnical Exploration and Recommendations Report Wind River Road Slide Reconstruction Project - Skamania County, Washington

WA FLAP SKAMAN 92135(1)

Submitted to:

Robert Peccia and Associates, Inc.

3147 Saddle Drive Helena, MT 59601

Submitted by:

WSP USA Inc.

18300 Redmond Way, Suite 200, Redmond, Washington, USA 98052

+1 425 883 0777

US0033714.0878

May 23, 2025

May 23, 2025 US0033714.0878 i

Distribution List Western Federal Lands Highway Division ii

Table of Contents

1 INTRODUCTION

1.1 Project Description

1.2 Existing Site Conditions

1.3 Objective and Scope

1.4 Use of this Report

2 GEOTECHNICAL FIELD EXPLORATION

2.1 Borings

2.2 Laboratory Testing

3 GEOLOGIC SETTING

4 SUBSURFACE CONDITIONS

5 ENGINEERING ANALYSIS

5.1 Seismic Design

5.2 MSE Wall Recommendations

5.2.1 Overexcavation

5.2.2 Soil Parameters for the MSE Wall Design

5.2.3 Global Stability Analysis and Minimum Reinforcement Length

5.2.4 Factored Bearing Resistance

5.2.5 Sliding Resistance

5.2.6 External Stability

5.3 Micropile and Cap Beam Recommendations

5.3.1 Micropile Stabilization Analysis

5.3.1.1 Seismic Stability

5.3.2 Recommended Micropile and Cap Beam Design Criteria

6 CONSTRUCTION CONSIDERATIONS

7 ADDITIONAL SERVICES

8 LIMITATIONS

9 REFERENCES

iii

TABLES

Table 1: Summary of WSP Borings

Table 2: Summary of Soil Parameters for MSE Wall Design

Table 3: Global Stability Results for the MSE Wall

Table 4: Nominal and Factored Bearing Resistance for the MSE Wall

Table 5: Soil Units and Strength Parameters for Micropile Cap Beam Stability Analysis

Table 6: Global Stability Results for Initial Micropile Placement

Table 7: Global Stability Results of Cross-Section A-A' Post-Micropile Analysis

Table 8: Deformation-Based Seismic Stability Results

FIGURES

Figure 1: Site Location Map

Figure 2: Site Plan and Boring Location Map

Figure 3: Cross-Section A-A’

Figure 4: Cross-Section B-B’

APPENDICES

APPENDIX A

Boring Logs

APPENDIX B

Geotechnical Laboratory Testing

APPENDIX C

Global Stability Results (SLIDE Outputs)

APPENDIX D

Use of This Report

1 INTRODUCTION

WSP USA Inc. (WSP) presents this supplemental geotechnical report summarizing the results and findings of our geotechnical exploration and geotechnical recommendations for the Wind River Road Slide Reconstruction Project (herein referred to as “the Project”), located along Wind River Road in Skamania County, Washington (Figure 1). Our work was performed under the executed Task Order number 69056722F00122N and is based on several Microsoft Teams meetings with Robert Peccia and Associates, Inc. (RPA) and Western Federal Lands Highway Division (WFLHD) in 2024 and our review of the following:

Preliminary geotechnical report prepared by WFLHD’s Geotechnical Group (WFLHD 2024a)

Preliminary plan set for the project prepared by WFLHD (WFLHD 2024b)

Preliminary Mechanically Stabilized Earth (MSE) Wall plan, profile and detail sheets prepared by WFLHD (WHLHD 2024c)

1.1 Project Description

WSP understands that WFLHD is supporting Skamania County to mitigate ongoing slope movements along Wind River Road between Mile Post (MP) 0.2 and 0.4 caused by a suspected localized landslide. The slope movements are damaging the road and, as a result, Skamania County is constantly repairing pavement cracking and embankment sloughing. The Project also includes road shoulder widening approximately 100 feet to the southeast of the suspected landsliding. WFLHD will provide a full set of PS&E drawings for the proposed Project improvements, which include the following primary design elements:

Approximately 200 linear feet of micropiles with a cap beam to stabilize the ongoing slope movements, Approximately 170 linear feet a new MSE wall to accommodate road shoulder widening, Drainage improvements consisting of ditch regrading, replacing an existing 36-inch diameter cross culvert, and replacing two minor culverts.

Proposed ancillary improvements include replacing guardrails, construction of additional guardrails, and addition of a 4-foot-wide bicycle lane.

1.2 Existing Site Conditions

The existing site conditions observed by WSP were consistent with those described in WFLHD’s Geotechnical Report (WFLHD 2024). For completeness, WSP is including the “Surface Conditions” excerpt from WFLHD (2024a) with some modifications to incorporate the MSE wall and road widening section of the Project:

The project is along a 280-foot-long segment of Wind River Road, a hot mix asphalt (HMA) paved roadway in southern Skamania County, between the City of Carson, 0.5 miles to the east, and State Route (SR) 14, 0.25 miles to the west. The road traverses southwest-northeast across hilly terrain that slopes toward the Columbia River approximately 0.3 miles to the south and is within the Columbia River Gorge Scenic Area. The damaged road segment is between two parallel drainage channels and constructed atop embankment fill, which primarily spans the eastern drainage and its 36-inch diameter corrugated metal pipe (CMP) culvert. The road surface includes an approximately 140-foot-long asphalt patch with sealed and unsealed longitudinal cracks. The road segment to be widened is approximately 100 feet to the east of the damaged road segment and, in general, does not show signs of distress. A shallow (less than 10 feet below grade) 36-inch diameter CMP culvert crosses below Wind River Road within the limits of the proposed MSE wall.

The entire road segment has three lanes, one westbound and two eastbound, and gently curves through the project area, with a gradual slope down to the west that steepens towards the SR 14 roundabout. An unlined drainage ditch parallels the northside of the road and a guardrail spans the embankment where the 36-inch diameter CMP culvert crosses beneath it. A short driveway over the ditch leads to a flat, graded area and to the forestry access roads that traverse upslope for 200 feet to the north at 1V:4H (vertical:horizontal) slope to an upper bench. South of the road, the embankment slopes at 1V:1.3H for 25 feet, then flattens to 1V: 5H down to the drainage channel eastern drainage channel. The slopes south of the road within the proposed MSE wall section are generally 1V:1.3H to 1V:1.4H.

The slopes south of the road are heavily vegetated with large deciduous and coniferous trees with abundant underbrush. The property north of the road, based on aerial photographs, was timber harvested between July 2017 and February 2018, and smaller and younger trees are re-establishing, along with shrubs and grasses.

1.3 Objective and Scope

The objective of this supplemental geotechnical report is to provide the necessary geotechnical recommendations to support the design of the slide stabilization using micropiles and cap beam and the MSE wall. A summary of the of the work undertaken is presented below.

Geotechnical Field Exploration WSP performed a geotechnical field exploration between October 2, 2024, and October 3, 2024. The field exploration consisted of advancing three borings to depths of 36.5 to 57.0 feet below ground surface (bgs) using the mud rotary (MR) drilling method. The details of the geotechnical field exploration are presented in Section 2, and the exploration logs are presented in Appendix A.

Laboratory Testing Representative soil samples were collected during the field exploration and transported to a geotechnical laboratory for the index testing. The geotechnical laboratory testing included in-situ moisture content, grain size analysis, and Atterberg Limits. Appendix B contains the results of the laboratory testing.

Supplement Geotechnical Report WSP prepared this geotechnical report to provide a summary of the existing geotechnical data collected by WFLHD, subsurface and groundwater conditions encountered during our field exploration, and geotechnical recommendations to support the design of the proposed improvements.

1.4 Use of this Report

This report pertains to the proposed Wind River Road Slide Reconstruction Project and the improvements described in Section 1.1. The geotechnical recommendations provided herein are based on WSP’s understanding of the Project. If the design of the proposed corridor improvements is modified, WSP should be provided with an updated set of plans for review. Depending on the nature of the design modifications, the recommendations of this report may need to be updated to reflect the modified design. Appendix D has further information regarding the proper use and interpretation of this geotechnical report.

2 GEOTECHNICAL FIELD EXPLORATION

A summary of WSP’s geotechnical field exploration is provided in the following subsections. Details of the previous geotechnical field exploration performed by WFLHD are not included herein and can be found in their preliminary geotechnical report (WFLHD 2024a).

2.1 Borings

WSP conducted a subsurface exploration on October 2, and 3, 2024, that consisted of drilling three boreholes.

The boreholes were drilled to approximate depths of 36.5 to 57.0 feet below ground surface using a track-mounted B-57 drill rig with mud rotary drilling techniques. The mud rotary boreholes were completed by Holt Services, Inc. (Holt) under subcontract to WSP.

Prior to drilling, the borehole locations were cleared of underground utilities by Washington OneCall in accordance with local requirements. After the completion of drilling, boreholes were backfilled with drill cuttings mixed with bentonite chips and the surface grade was restored to previous conditions. The approximate coordinates, surface elevations, depths drilled, and date drilled for each exploration are summarized in Table 1. The locations of the subsurface explorations are shown in Figure 2.

Table 1: Summary of WSP Borings

Boring ID Date Drilled

Latitude Longitude Approximate

Surface Elevation (feet)

Depth Drilled (feet bgs)

BH-01-24 10/03/2024 45.717326 -121.825646 364.5 57.0

BH-02-24 10/02/2024 45.716661 -121.826599 342.5 51.5

BH-03-24 10/02/2024 45.716395 -121.826872 337.5 36.5

Drilling and sampling were performed in general accordance with ASTM D1586 and ASTM D2488 and under the full-time observation of a WSP geologist. From ground surface to 40 feet bgs, soil samples were obtained every 5 feet. The soil sampling interval was increased to every 2.5 feet from 40 feet to total depth.

Soil samples were primarily obtained using a standard penetration test (SPT) split spoon sampler consisting of a 2-inch outside diameter (O.D.), and 1.4-inch inside diameter (I.D.) split barrel. Soils collected inside the SPT samplers were visually classified in the field, placed in sealed plastic jars, and stored for future reference and laboratory testing. The SPT samplers were driven a total of 18 to 24 inches into the soil at the bottom of the boring (or until refusal, where refusal was taken as 50 blows for 6 inches or less). The SPT sampler was driven into the soil using an automatic 140-pound hammer free falling from a vertical distance of 30 inches. The total number of hammer blows required to drive the sampler the final 12 inches is termed the standard penetration resistance (N) value. The procedures employed in the field were generally consistent with those described in ASTM D1586. One thin-walled (“Shelby Tube”) sample was collected in BH-02-24. The Shelby Tube sampler consisted of a 3-inch O.D., 36-inch-long steel tube with a 0.065-inch wall thickness. The Shelby Tube sample was collected in general accordance with ASTM D1587.

Appendix A contains the logs (Record of Borehole). The logs show the earth materials encountered, uncorrected N-values, the locations of samples, and laboratory test results. The logs also show the borehole number, drilling date, and the name of the WSP geologist who logged the boring. The soils are characterized in accordance with ASTM D2487 (i.e., the Unified Soil Classification System). The boundaries between different soil types shown on the logs are approximate because the actual transition between layers may be gradual or between samples. The subsurface conditions depicted on the boring logs are for the specific date and location reported and therefore, are not necessarily representative of at other locations and times.

2.2 Laboratory Testing

Laboratory testing was performed on selected soil samples from our explorations. Testing was conducted in general accordance with ASTM International (ASTM) standards unless noted otherwise. The laboratory testing included natural moisture content (ASTM D2216), Atterberg limits (ASTM D4318), percent fines content (ASTM D2216), and particle (grain) size distribution (ASTM D6913). The laboratory test results are presented in Appendix B. Laboratory results for natural moisture content, grain size analyses and Atterberg limits are also presented in the boring records in Appendix A.

3 GEOLOGIC SETTING

A detailed discussion regarding the regional and site geologic setting and seismicity is provided in the preliminary geotechnical report (WFLHD 2024a). In general, WSP agrees with the information presented in the WFLHD (2024a) report.

4 SUBSURFACE CONDITIONS

The stratigraphic soil and rock units were interpreted from WFLHD’s previous borings (WFLHD 2024a) and WSP’s field explorations in 2024. The units are generally consistent with the mapped geologic units in the area.

The subsurface conditions along the Project generally consisted of a top-down sequence of road embankment fill, landslide debris, pyroclastic rocks, and the volcanic rocks of Stevenson Ridge. The conditions of these units as encountered in our explorations are as follows:

Road Embankment Fill—Approximately ten feet of road embankment fill was encountered. The fill encountered generally consisted of crushed rock, gravels and clean sands. Material was typically loose to medium dense, with poor recovery.

Reworked Old Landslide Deposits—Thick layers of reworked native material (old landslide deposits) were encountered directly below road embankment fill in BH-01-24 and BH-02-24. The deposits generally consisted of soft to firm clays and elastic silts with silty sands. The fine-grained portions of the material were typically low plasticity. Trace organics were observed in some of the samples. The coarse-grained portions of the consisted of fine to coarse sands and trace gravels. It is possible that the old landslide deposits were used for initial road fill and grading.

Native Pyroclastic Rocks (Upper)—Thick unit of stiff to very stiff clays, highly weathered pyroclastic rocks and sands.

Native Pyroclastic Rocks (Lower)—Thick unit of pyroclastic gravels, cobbles, and boulders in matrix of elastic silt and clay. Encountered in BH-01-24 and WFLHD prior Borings BH23-02, BH23-03, and BH23-04.

Very dense clays and gravels from weathered andesite. Coarse grained materials are medium dense to very dense. Fine grained deposits consisted of fat clays and elastic silts with medium to high plasticity.

Volcanic rocks of Stevenson Ridge—Native in-situ volcanic rock was encountered in the previous Western Federal Lands Highway Division borings from 2024. This unit was not encountered by WSP. WFLHD describes it as slightly weathered andesite breccia at an elevation of approximately 270 feet.

5 ENGINEERING ANALYSIS

5.1 Seismic Design

The seismic design parameters provided in the preliminary geotechnical report (WLFHD 2024a) should be used for the proposed Project improvements.

5.2 MSE Wall Recommendations

The following sections provide recommendations for the proposed MSE wall. Our recommendations are based on global stability and external stability (bearing, sliding, overturning) analyses of the proposed MSE wall configuration. The internal stability of the MSE wall will be checked by WSP once the wall type is selected by WFLHD and/or Skamania County. The recommendations provided herein will be re-checked after the wall type is selected and reinforcement strength and spacing are designed.

5.2.1 Overexcavation

A soft clay layer was encountered in our Boring BH-02-24 at the anticipated bottom of wall elevation. The soft clay layer is approximately 7.5 feet thick below the proposed bottom of wall elevation. WSP recommends overexcavation of the upper 2.5 feet of the clay layer below the bottom of the MSE wall. Overexcavation should extend laterally 2.5 feet beyond the MSE wall reinforcement and where practical, 2.5 feet beyond the front of the wall. Although the soft clay layer was not encountered in Boring BH-03-24, WSP recommends the 2.5-foot overexcavation along the entire MSE wall length to provide a uniform bearing surface.

5.2.2 Soil Parameters for the MSE Wall Design

Table 2 lists recommended soil parameters for the MSE wall design. The reinforced fill zone should consist of material meeting the gradation for “Select Granular Backfill” as specified in Section 704.03 of the 2024 Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-24; FHWA 2024).

Table 2: Summary of Soil Parameters for MSE Wall Design

Material Unit Weight (pcf)

Friction Angle (degree)

Cohesion (psf)

Active Earth Pressure Coefficient

Retained Soil 125 34 0 Ka = 0.25 Foundation Soil (Layer 1 = 2.5 feet thick) 135 38 0 N/A Foundation Soil (Layer 2) 105 26 100 N/A

Notes: pcf = pounds per cubic foot; psf = pounds per square foot; N/A = not applicable

5.2.3 Global Stability Analysis and Minimum Reinforcement Length WSP analyzed the global stability of the MSE wall using Spencer’s Method (Spencer 1967) in Slide2 slope stability software (Rocscience 2023). The stability analysis was performed at the critical section of the wall, which coincides with the maximum wall height of approximately 14 feet and is near Boring BH-02-24. Uniform vertical traffic surcharges of 250 and 125 psf were included for static and pseudo-static cases, respectively.

In the analyses, WSP assumed that the reinforced zone of MSE wall has infinite strength and the critical failure surfaces do not pass through the reinforced zones. This assumption must be validated by the wall designer while modeling the spacing, lengths, and the strengths of the reinforcing elements and evaluating the global stability.

WSP also assumed minimum 2.5 feet of embedment below the adjacent finished grade and a 4-foot horizontal bench in front of the wall as depicted on the MSE wall cross sections (WFLHD 2024c). For pseudostatic analysis, horizontal seismic load coefficient (kh) of 0.125 (i.e., 1/2 x As) was applied.

The FHWA Project Development and Design Manual (FHWA 2018) suggests an acceptable static factor of safety (FS) for a soil slope/retaining wall is 1.3, and the Washington State Department of Transportation (WSDOT) Geotechnical Design Manual suggests a seismic FS of 1.05 to 1.1 is acceptable for slopes and walls. The results of our stability analyses are provided in Table 3 and Slide model outputs are provided in Appendix C. The minimum reinforcement length to achieve the required FS is 1H feet, where H is the height of the MSE wall (including embedment depth). Therefore, WSP recommends that the reinforcement should extend at least 1H feet from the connection point at the wall facing or 8 feet, whichever is greater.

Table 3: Global Stability Results for the MSE Wall

Scenario Required FS Calculated FS

Static 1.3 1.3 Seismic (Pseudostatic) 1.05 1.05

5.2.4 Factored Bearing Resistance

The nominal bearing resistance calculations for the MSE wall were performed in accordance with Section

10.6.3.1.2 of AASHTO LRFD Bridge Design Specifications (AASHTO 2020). According to Table 11.5.7-1 in AASHTO (2020) resistance factors of 0.65 and 0.9 should be used to calculate the geotechnical bearing resistance of MSE walls in Strength Limit, Extreme Limit conditions, respectively. Nominal bearing resistance calculations also account for a two-layer soil system (i.e., dense compacted gravel overlying soft to medium stiff clay). The nominal and factored bearing resistance values are presented in Table 4.

The proposed MSE wall construction will replace existing road fill and native soils and require a small volume of new wall backfill to accommodate the road shoulder widening. This will result in a small net load increase on the underlying native soils (~450 psf). Based on our settlement analysis performed using Settle3 (Rocscience 2024) we estimate long-term total settlements of the proposed MSE wall to be on the order of 1 to 1.5 inches.

Differential settlement parallel to the wall face is anticipated to be less than 1 inch. Our settlement analysis assumes that the MSE wall will have the proper drainage at the back of the reinforced zone and below the MSE wall. We understand that a backdrain will be connected to an underdrain system that will outlet on the slope face.

We recommend the drain system be maintained to allow for proper drainage over the life of the structure.

Table 4: Nominal and Factored Bearing Resistance for the MSE Wall

Effective Footing Width (B’; feet)1

Nominal Bearing Resistance (ksf)2

Factored Bearing Resistance (ksf)

Strength Limit3 Extreme Limit4

8 10.7 3.1 9.6 10 11.0 3.2 9.9 12 11.5 3.4 10.3 14 12.0 3.5 10.8

Notes:

1. Minimum reinforcement length is 8 feet per FHWA (2023).

2. ksf = kips per square foot

3. Strength Limit includes a resistance factor of 0.65 (based on Table 11.5.7-1 in AASHTO 2020) and a slope reduction factor of 0.45

(based on Section 10.6.3.1.2c in AASHTO 2020) to account for the sloping ground conditions near the face of the MSE wall.

4. Extreme Limit includes a resistance factor of 0.9 based on Section 11.5.8 in AASHTO (2020).

5.2.5 Sliding Resistance

A nominal coefficient of friction value of 0.78 (corresponding to a friction angle of 38 degrees) can be used for sliding resistance between the bottom of the MSE wall and the subgrade soils. This assumes that the bottom of the MSE wall will be founded on compacted gravel fill as described in Section 6.0. A resistance factor of 1.0 can be applied based on Table 11.5.7-1 “Resistance Factors for Permanent Retaining Walls” in AASHTO (2020).

5.2.6 External Stability

WSP evaluated the external stability of the proposed MSE wall using MSEW+ software (ADAMA 2025). Using the soil parameters in Section 5.2.2 and an assumed reinforcement length of 1H feet from the global stability analyses (Section 5.2.3), we confirmed that the capacity demand ratios exceeded 1.0 for bearing, overturning, and sliding.

WSP will revisit the external stability analyses after the wall and reinforcement types are selected.

5.3 Micropile and Cap Beam Recommendations

5.3.1 Micropile Stabilization Analysis

WSP analyzed the global stability of the existing conditions along Cross Section A-A’ (Figure 3) using Spencer’s Method (Spencer 1967) in Slide2 slope stability software (Rocscience 2023). Since the inclinometer data from WFLHD (2024) clearly shows landslide movement and a localized shear zone, WSP assumed the existing slope has a static factor of safety (FS) below 1.0 (i.e., unstable condition). As such, we calibrated the soil properties of the upper clay layer and landslide shear zone until a FS of 0.85 was achieved (Figure C-3 in Appendix C). In our opinion, an FS of about 0.9 is representative of the active landslide conditions and corresponding rate of movement. The Slide model outputs are provided in Appendix C.

Table 5: Soil Units and Strength Parameters for Micropile Cap Beam Stability Analysis Soil Unit Unit Weight (pcf) Cohesion (psf) Friction Angle (degrees)

Roadway Embankment Fill 130 0 40 Soft to Firm Fat Clay 100 100 26

Fat Clay/Ash (landslide shear zone) 80 0 9 Stiff to Very Stiff Fat Clay 100 500 25

Very Dense Clay and Gravel 115 100 29 Andesite (bedrock) 145 5000 50

After analyzing the “existing conditions” scenario, WSP performed a micropile stabilization analyses in general accordance with Section 6.6 (“Design Concepts for Micropiles Used for Slope Stabilization”) in Sabatini et al.

(2005). Using the slope stability model for the existing conditions scenario, WSP incorporated a single vertical micropile and adjusted the shear capacity of the micropile element until an adequate FS was achieved. The Federal Highway Administration (FHWA) Project Development and Design Manual (FHWA 2018) and Geotechnical Technical Guidance Manual (FHWA 2007) suggest that an acceptable factory of safety for landslide remediation is between 1.2 and 1.5, with 1.25 being the most common. For this evaluation, WSP modeled a battered micropile pair system as a single vertical micropile element as suggested in Sabatini et al. (2005). This resulted in a static FS of approximately 1.25 (Figure C-4 in Appendix C).

The location of the vertical micropile element was selected so that an adequate FS was achieved on the downslope and upslope sides of the micropile element as presented in Appendix C (Figures C-5 and C-6) and summarized in Table 6. WSP considers a FS of less than 1.25 for the downslope portion of the landslide acceptable given the potential consequences of additional slope movement within this zone. As indicated on Figure C-5, the critical slip surfaces are south of the interpreted roadway embankment toe and micropile element.

Therefore, potential movement within the downslope zone is not expected to significantly impact the roadway.

Table 6: Global Stability Results for Initial Micropile Placement

Scenario Static FS

Entire Cross-Section A-A’ 1.25 Upslope zone (north of micropile location) 1.64

Downslope zone (south of micropile location) 1.11

After the micropile location was selected, WSP performed the remaining steps of the micropile analysis which included selection of a micropile cross-section, estimation of micropile length, evaluation of shear and bending moments acting on the micropile using LPILE software by Ensoft, Inc (Ensoft 2022), and calculation of micropile spacing based on an assumed batter of 30 degrees from vertical (i.e., 1.8V:1H). Using these micropile properties, WSP re-analyzed the global stability of Cross Section A-A’ for static conditions. The results are summarized in Table 7 and the Slide output ais presented in Figure C-7. Note that pseudostatic (seismic) stability analyses were not performed for the micropile stabilization because the primary purpose of the micropile cap beam is to mitigate the ongoing slope movement. Based on the results, the micropile configuration satisfies the static FS requirements for a landslide stabilization project.

Table 7: Global Stability Results of Cross-Section A-A' Post-Micropile Analysis Scenario Factor of Safety

Static 1.57 Pseudo-static 0.76

5.3.1.1 Seismic Stability

The micropile cap beam is not being designed for a seismic event, however pseudo-static and deformation-based seismic stability analyses were performed under the design-level earthquake (i.e., 1033-year return period earthquake) at the request of WFLHD. The purpose of the pseudo-static and deformation-based seismic stability analysis is to provide the ultimate owner, Skamania County, with an understanding of potential post-earthquake impacts.

The pseudo-static stability results in Table 7 show that the minimum required factor of safety of 1.05 is not achieved and therefore, a deformation-based analysis approach is warranted. The seismic slope displacements were analyzed using the procedure of Bray and Macedo (2023) which requires input parameters of yield acceleration (ky), fundamental period (Ts) of the critical slip plane, and the moment magnitude (Mw) of the design earthquake.

The yield acceleration (ky) is the horizontal acceleration that, when applied to the potential sliding mass in a pseudo-static slope stability analysis, results in a FS equal to one. The ky calculation was performed in Slide2 and results are presented in Appendix C (Figure C-9). The results of the displacement-based seismic stability analyses are summarized in Table 8.

Table 8: Deformation-Based Seismic Stability Results

Yield Acceleration (g) Fundamental Period, Ts (seconds)

Moment Magnitude Earthquake

50th Percentile Displacement (inches)

0.07 0.2 9.1 16

The estimated seismic slope displacements presented in Table 8 are considered reasonable and within the acceptable range of deformations for slopes during a design-level earthquake event (i.e., 12 to 18 inches).

5.3.2 Recommended Micropile and Cap Beam Design Criteria

Based on the micropile stabilization analysis, WSP recommends the following design criteria for the micropile and cap beam:

Two micropiles with a 30-degree batter and a total depth of 60 feet (measured as vertical depth).

Micropile section:

9.875-inch diameter drilled hole.

9.875-inch diameter steel casing with 0.875-inch wall thickness

No. 14 Grade 75 center bar (full length)

Annulus between casing and center backfilled with 4,000 psi grout.

Approximate total cap beam length of 205 feet with micropile spacing of 3.0 feet on center.

Required shear capacity of each micropile is 80 kips.

The structural engineer should design the cap beam assuming an active earth pressure with an equivalent fluid pressure of 43 pcf on the upslope side of the cap beam.

The micropile and cap beam should be located along the slope at the approximate location presented in Figure 2.

6 CONSTRUCTION CONSIDERATIONS

General

Excavations deeper than 4 feet should be sloped back at a maximum inclination of 1.5H:1V (horizontal:vertical). Heavy construction loads, such as those resulting from material stockpiles or heavy machinery, should be set back from the top of the excavation a minimum horizontal distance equal to the depth of the excavation. If there is insufficient room to slope the sides of the excavation, then the sides of the excavation should be braced and shored in accordance with OSHA standards, Washington Administration Code (WAC) 296-155 Part N, Appendices A and B, and all other applicable safety ordinances and codes. The contractor will be responsible for the structural design and safety of the temporary shoring system, and it is recommended that this design be submitted to WSP for review. The design of the temporary shoring system should account for all surcharge loads. All surface water should be diverted away from the temporary excavations. The contractor is ultimately responsible for site safety and temporary excavations. All excavations should be evaluated for stability prior to occupation by construction personnel.

Existing debris and obstructions (e.g., abandoned culverts) should be removed from within the footprint the MSE wall and micropile cap beam. Exposed deleterious, vegetative, inert, and oversized materials (materials greater than 6 inches in maximum dimension) partially exposed at the subgrade elevation should be stripped and isolated prior to removal of reusable soils. The soil exposed in excavation subgrades should be observed by WSP to confirm that the soil has the desired engineering properties. Additional removals (overexcavation) may be required during observation and testing of the exposed subgrade soil.

If contaminated soils are encountered, these soils should be stockpiled separately. The stockpiled soils should be placed on plastic and covered with an impermeable tarp/liner. These soils will have to be sampled and tested to identify the proper disposal method.

MSE Wall

Subgrade preparation and overexcavation below the MSE wall should meet the requirements of Section 209 of FP-24. As discussed in Section 5.2.1, the depth of overexcavation below the MSE wall should extend at least 2.5 feet below the bottom of the wall and should extend laterally beyond the MSE wall in the transverse and longitudinal directions, a distance of 2.5 feet or depth of overexcavation whichever is deeper.

Material placed below the MSE wall should be granular backfill and meet the gradation requirements of “Foundation Fill” (Table 704-1 in FP-24) or an approved equivalent. The fill should be compacted to a minimum relative compaction of 95% as measured by AASHTO T 180.

MSE Wall construction and materials should be in accordance with Section 255 – Mechanically Stabilized Earth Walls of FP-24.

Micropile and Cap Beam

Contractor should be prepared to handle difficult drilling conditions such as presence of boulders or large cobbles and artesian groundwater. As noted in the preliminary geotechnical report by WFLHD (2024a), artesian groundwater conditions were encountered Boring BH23-01.

Contractor should submit a work plan for review and approval by WSP, prior to commencing the micropile cap beam construction.

7 ADDITIONAL SERVICES

WSP should review the project’s construction documents before they are finalized. This review is necessary to verify that the geotechnical recommendations contained in this report have been properly interpreted and implemented into the project’s design. If WSP does not perform this review, then WSP can assume no responsibility for misinterpretation of the geotechnical recommendations provided herein.

The construction process is an integral design component with respect to the geotechnical aspects of a project.

Geotechnical engineering is not an exact science because of the variability of natural processes. Only a very small portion of the subsurface materials that will affect the performance of the proposed project have been observed, sampled, and tested. Unanticipated or changed conditions can occur during grading, excavating, wall construction, and micropile installation (see Appendix D). Proper geotechnical observation and testing during construction is necessary to allow the geotechnical engineer the opportunity to verify design assumptions.

Therefore, WSP should be retained during construction to observe compliance with the design concepts and geotechnical recommendations contained herein. WSP can recommend design changes if subsurface conditions or methods of construction differ from those assumed in this report.

8 LIMITATIONS

WSP prepared this supplemental geotechnical report for the exclusive use by Robert Peccia Associates and Western Federal Lands Highway Division for the specific application to the Wind River Road Slide Reconstruction Project (WA FLAP SKAMAN 92135(1)) in Skamania County, Washington. Use of this report by others or for another Project is at the user's sole risk. The findings, conclusions, and recommendations presented in this report were prepared in a manner consistent with the level of care and skill ordinarily exercised by other members of the geotechnical engineering profession currently practicing under similar conditions subject to the time limits and financial, physical, and other constraints applicable to the scope of work. No warranty, express or implied, is made.

Appendix D contains further information regarding the proper use and interpretation of this geotechnical report.

This report contains information that may be useful in the preparation of contract specifications and contractor cost estimates. However, this report is not written as a specification document and may not contain sufficient information for this use without proper modification.

WSP appreciates the opportunity to be of service on this important project. If you have any questions, please contact the undersigned.

Signature Page

WSP USA Inc.

Jason Cox, PE John Thornley, PE (AK) Vice President, Geotechnical Engineer Vice President, Geotechnical Engineer

MB/JC/JT/tp https://wsponline.sharepoint.com/sites/us-rpa-wflhd-wind-ri/shared documents/6. deliverables/us0033714.0878-rev0-supplemental geotechnical rpt-wind river rd slide reconstruction.docx

05/23/2025

9 REFERENCES

ADAMA Engineering (ADAMA). 2025. MSEW+. Version 2025.10. February 1.

American Association of State Highway and Transportation Officials (AASHTO). 2020. AASHTO LRFD Bridge Design Specifications, U.S. Customary Units. 9th Ed.

Bray, J.D. and Macedo, J. 2023. Seismic Slope Displacement Procedure for Interface and Intraslab Subduction Zone Earthquakes. Journal of Geotechnical and Geoenvironmental engineering, 149(11), p.04023104.

Ensoft. 2022. LPILE Software Version 12.07. November 15.

Federal Highway Administration (FHWA). 2007. Geotechnical Technical Guidance Manual. May

FHWA. 2018. Project Development and Design Manual (PDDM), U.S. Department of Transportation. FP-14. July.

Available on the web at: https://flh.fhwa.dot.gov/resources/design/pddm/.

FHWA. 2023. Design and Construction of Mechanically Stabilized Earth (MSE) Walls, FHWA GEC 011, Report No. FHWA-HIF-24-002. August.

FHWA. 2024. Standard Specifications for the Construction of Roads and Bridges on Federal Highway Projects, FP-24. Available on the web at:

Loehr, E.J., Lutenegger, A., Rosenblad, B., and Boeckmann, A. 2016. FHWA NHI-16-072, Geotechnical Site Characterization, Geotechnical Engineering Circular No. 5. November.

Rocscience Inc. 2023. Slide2. Version 9.031. Toronto, Canada.

Rocscience Inc. 2024. Settle3. Version 5.023. Toronto, Canada.

Sabatini, P. J., Tanyu, B., Armour, T., Groneck, P., and Keeley, J. 2005. FHWA NHI-05-039, Micropile Design and Construction. December.

Western Federal Lands Highway Division (WFLHD). 2024a. Geotechnical Memo 16-24, Preliminary Geotechnical Recommendations, Wind River Slide Reconstruction, WA FLAP SKAMAN 92135(1), Wind River Road, Skamania County, Washington. April 15.

WFLHD. 2024b. Plans for Proposed Project, WA FLAP Skaman 92135(1), Wind River Road Slide Reconstruction, Skamania County, Washington. May 23.

WFLHD. 2024c. Preliminary Wall Plan and Profile, Details, and Sections, WA FLAP Skaman 92135(1), Wind River Road Slide Reconstruction, Skamania County, Washington. September 16.

Figures

PROJECT SITE

in

US0033714.0878

TASK

FIGURE

2025-02-14

THR

AA

JTC

JTC 1

GEOTECHNICAL EVALUATION

WIND RIVER ROAD STABILIZATION

ROBERT PECCIA AND ASSOCIATES (RPA)

SITE LOCATION MAP

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REFERENCE(S)

© 2024 Microsoft Corporation © 2024 Maxar ©CNES (2024) Distribution Airbus DS © 2024 TomTom

A

A'

APPROXIMATE MICROPILE

AND CAP BEAM LOCATION

APPROXIMATE LIMITS OF

LANDSLIDE MAPPED BY WFLHD

B

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APPROXIMATE LOCATION OF

MECHANICALLY STABILIZED

EARTH WALL

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TASK

FIGURE

2025-02-14

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GROUND HYBRID", DELIVERED 2024-05-21.

REFERENCE(S)

HISTORICAL BORINGS BY OTHERS

WSP GEOTECHNICAL BORINGS

LEGEND

EL

EV

AT

IO

N (F

T)

EL

EV

AT

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N (F

T)

A'

HORIZONTAL DISTANCE (FT)

A

0+00 0+20 0+40 0+60 0+80 1+00 1+20 1+40 1+60 1+80 2+00 2+20 2+40 2+60 2+80 3+00 3+20 3+40 3+60 3+80 4+00 4+20 4+40 4+60 4+80 5+00 5+20 5+40 5+60 5+80 6+00 6+20

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: 3

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: 4

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MH

GP

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BEDROCK

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TD 80.1

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TD 91.5

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TD 123.5

in

US0033714.0878

TASK

FIGURE

0 3

2024-12-20

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and GRAVEL

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HORIZONTAL DISTANCE (FT)

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US0033714.0878

TASK

FIGURE

0 4

2024-12-14

MA

AA

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JC

CROSS-SECTION B-B'

PROJECT NO. REV.

PROJECTCLIENT

CONSULTANT

PREPARED

DESIGNED

REVIEWED

APPROVED

YYYY-MM-DD

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1'' = 30'

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STIFF to VERY STIFF CLAY

VERY DENSE CLAY and GRAVEL

BH

-2

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: 3

.0 F

T

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BH

-2

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.5 F

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-0

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: 3

.5 F

T

BH

-0

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EV

: 3

.0 F

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CULVERT OUTLET

ELEV: 337.0 FT

TD 57.0 FT

ELEV. 306.0 FT

TD 61.5 FT

ELEV. 303.5 FT

TD 91.5 FT

ELEV. 267.5 FT

TD 51.5 FT

ELEV. 291.0 FT

TD 37.0 FT

ELEV. 306.0 FT

50/6"

50/1"

50/4”

BEDROCK

APPENDIX A

Boring Logs

EP

TH

(f t)

MATERIAL PROFILE

DESCRIPTION

(GP-GM) Poorly graded GRAVEL with silt and sand, fine to coarse subangular to angular, little fine to coarse subangular to angular sand, few nonplastic fines; light brown to gray; loose to medium dense, wet.

(SM) Silty SAND, fine to medium, little nonplastic fines, few fine subangular gravel; grayish brown;

moist, dense.

(CH) Fat CLAY, high plasticity, some fine to coarse subrounded to subangular sand, trace fine subrounded gravel; light brown mottled orange and green, iron oxide staining, heterogeneous;

moist, medium stiff to stiff.

Continued on Next Page

U

SC

S G

P- G

M

SM

C H

ST

R

AT

A

PL

O

T

ELEV.

DEPTH

(ft)

0.0

351.0 12.0

348.0 15.0

338.0 25.0

SAMPLES

Hammer: Automatic, 140lb, 30" drop

ASTM D1586, Blows per 6 in 140-lb hammer, 30-in drop

N U

M

BE

R

S-01

S-02

S-03

S-04

S-05

S-06

S-07

TY

PE

SS

SS

SS

SS

SS

SS

SS

R

EC

BL

O

W S

10-10-10-12

2-2-4-4

4-5-5

9-13-18-18

15-24-14-10

1-3-4-8

5-5-10-11

N -V

AL

U

E

WATER CONTENT GRADATION %

G R

AV

EL

SA

N

D

FI

N

ES

O R

G

AN

IC

C

O N

TE

N

T

ADDITIONAL

OBSERVATIONS

2.50 ft: difficultly sampling loose gravels

7.50 - 10.00 ft: caving

10.00 ft: difficult drilling

24.80 ft: possible boulder - advanced to 27.5

G R

O U

N D

W

AT

ER

O

BS

ER

VA

TI

O

N S

RECORD OF BOREHOLE: BH-01 Sheet 1 of 3

CLIENT: WFLHD DATE: October 03, 2024 ELEVATION: 363.0 ft (Ground) PROJECT: WFLHD Wind River Road PROJECT NO: US0033714.0878 INCLINATION: 90.0° COORD SYS: Geographical Coordinates LOCATION: Skamania County, WA CONTRACTOR: Holt Services Inc. HORZ DATUM: NAD83

HAMMER TYPE: Automatic, 140lb, 30" drop

DRILL RIG: Mobile B57 DRILL METHOD: Mud Rotary

REV:

LOGGED: Madelaine Anderson DATE: Oct 03, 2024

Golder Log Imperial / Soil-Gradation 2_MP / Golder - 3 Imperial US / Golder US Auto (common in US) / 2025-02-17 CHECKED: J. Cox and B. Lockard DATE: Dec 11, 2024

Plastic & Liquid Limits (%) Water Content (%)

NP Nonplastic

0 25 50 75 10

TH

(f t)

MATERIAL PROFILE

DESCRIPTION

Possible boulder.

(SM) Silty SAND with gravel, fine to medium subangular sand, some nonplastic fines, little fine to coarse gravel; yellow to reddish brown, heterogeneous, stratified; wet, medium dense.

(CH) Fat CLAY, high plasticity, few fine to coarse subrounded to subangular sand; yellowish gray to brown, iron oxide staining; moist, very stiff.

(GP-GM) Poorly graded GRAVEL with silt and sand, fine to coarse subangular to angular, some coarse subangular to angular sand, trace nonplastic fines; dark gray, broken clasts; moist to wet, medium dense.

(CH) Fat CLAY with sand, high plasticity, few fine to coarse subrounded to subangular sand, trace fine subrounded gravel; dark gray, heterogeneous; moist, very stiff.

(SM) Silty SAND with gravel, fine to coarse, some fine to coarse subangular gravel, little low plasticity fines; orangish brown, heterogeneous; wet, medium dense.

Continued on Next Page

U

SC

S

SM

C H

G P-

G M

C H

SM

ST

R

AT

A

PL

O

T

ELEV.

DEPTH

(ft)

335.5 27.5

328.4 34.6

321.6 41.4

319.5 43.5

315.0 48.0

SAMPLES

Hammer: Automatic, 140lb, 30" drop

ASTM D1586, Blows per 6 in 140-lb hammer, 30-in drop

N U

M

BE

R

S-08

S-09

S-10

S-11

S-12

S-13

S-14

TY

PE

SS

SS

SS

SS

SS

SS

SS

R

EC

BL

O

W S

4-8-8-8

12-13-11-11

5-5-7-8

19-21-23-24

6-5-7-23

30-8-14-10

50/4"

N -V

AL

U

E

WATER CONTENT GRADATION %

G R

AV

EL

SA

N

D

FI

N

ES

O R

G

AN

IC

C

O N

TE

N

T

ADDITIONAL

OBSERVATIONS

G R

O U

N D

W

ER

VA

TI

O

N S

RECORD OF BOREHOLE: BH-01 Sheet 2 of 3

CLIENT: WFLHD DATE: October 03, 2024 ELEVATION: 363.0 ft (Ground) PROJECT: WFLHD Wind River Road PROJECT NO: US0033714.0878 INCLINATION: 90.0° COORD SYS: Geographical Coordinates LOCATION: Skamania County, WA CONTRACTOR: Holt Services Inc. HORZ DATUM: NAD83

HAMMER TYPE: Automatic, 140lb, 30" drop

DRILL RIG: Mobile B57 DRILL METHOD: Mud Rotary

REV:

LOGGED: Madelaine Anderson DATE: Oct 03, 2024

Golder Log Imperial / Soil-Gradation 2_MP / Golder - 3 Imperial US / Golder US Auto (common in US) / 2025-02-17 CHECKED: J. Cox and B. Lockard DATE: Dec 11, 2024

Plastic & Liquid Limits (%) Water Content (%)

NP Nonplastic

0 25 50 75 10

TH

(f t)

MATERIAL PROFILE

DESCRIPTION

(SM) Silty SAND with gravel, fine to coarse, some fine to coarse subangular gravel, little low plasticity fines; orangish brown, heterogeneous; wet, medium dense.

(GM) Silty GRAVEL with sand, coarse subangular, some fine to coarse sand, little low plasticity fines; orange red to grayish brown, heterogeneous, stratified; moist to wet, dense.

End of hole at 57.00 ft.

Boring backfilled with bentonite grout upon completion of drilling. Surface patched with asphalt. Groundwater not encountered during drilling.

U

SC

S

SM

G M

ST

R

AT

A

PL

O

T

ELEV.

DEPTH

(ft)

309.0 54.0

306.0

SAMPLES

Hammer: Automatic, 140lb, 30" drop

ASTM D1586, Blows per 6 in 140-lb hammer, 30-in drop

N U

M

BE

R

S-15

S-16

TY

PE

SS

SS

R

EC

BL

O

W S

8-10-11-26

46-28-19-25 N

-V

AL

U E

WATER CONTENT GRADATION %

G R

AV

EL

SA

N

D

FI

N

ES

O R

G

AN

IC

C

O N

TE

N

T

ADDITIONAL

OBSERVATIONS

52.00 ft: possible boulder

G R

O U

N D

W

ER

VA

TI

O

N S

RECORD OF BOREHOLE: BH-01 Sheet 3 of 3

CLIENT: WFLHD DATE: October 03, 2024 ELEVATION: 363.0 ft (Ground) PROJECT: WFLHD Wind River Road PROJECT NO: US0033714.0878 INCLINATION: 90.0° COORD SYS: Geographical Coordinates LOCATION: Skamania County, WA CONTRACTOR: Holt Services Inc. HORZ DATUM: NAD83

HAMMER TYPE: Automatic, 140lb, 30" drop

DRILL RIG: Mobile B57 DRILL METHOD: Mud Rotary

REV:

LOGGED: Madelaine Anderson DATE: Oct 03, 2024

Golder Log Imperial / Soil-Gradation 2_MP / Golder - 3 Imperial US / Golder US Auto (common in US) / 2025-02-17 CHECKED: J. Cox and B. Lockard DATE: Dec 11, 2024

Plastic & Liquid Limits (%) Water Content (%)

NP Nonplastic

0 25 50 75 10

TH

(f t)

MATERIAL PROFILE

DESCRIPTION

(SP) Poorly graded SAND, fine to coarse subrounded to subangular, little nonplastic fines, trace fine to coarse angular gravel; grayish brown; wet, medium dense.

(GP-GM) Poorly graded GRAVEL, fine to coarse subangular to angular, some medium plasticity fines, little fine to coarse sand;

grayish brown mottled orange, iron oxide staining; moist, loose.

(MH) Elastic SILT, high plasticity, little fine to coarse subrounded to subangular sand, trace fine subrounded to subangular gravel;

light grayish brown mottled orange and tan, iron oxide staining, heterogeneous, artificial debris;

moist, very soft to soft.

(SM) Silty SAND with gravel, fine to medium subrounded to subangular, some nonplastic fines, little fine subangular gravel;

grayish green mottled purplish gray, iron oxide staining, organic debris; moist, dense

Continued on Next Page

U

SC

S

SP

G P-

G M

M H

SM

ST

R

AT

A

PL

O

T

ELEV.

DEPTH

(ft)

0.0

334.5 8.0

329.5 13.0

318.5 24.0

SAMPLES

Hammer: Automatic, 140lb, 30" drop

ASTM D1586, Blows per 6 in 140-lb hammer, 30-in drop

N U

M

BE

R

S-01

S-02

S-03

S-04

S-05

S-06

TY

PE

SS

SS

SS

SS

TO

SS

R

EC

BL

O

W S

6-8-9

8-5-3

0-3-3

1-2-1

2-1-1

N -V

AL

U

E

WATER CONTENT GRADATION %

G R

AV

EL

SA

N

D

FI

N

ES

O R

G

AN

IC

C

O N

TE

N

T

ADDITIONAL

OBSERVATIONS

G R

O U

N D

W

ER

VA

TI

O

N S

RECORD OF BOREHOLE: BH-02 Sheet 1 of 3

CLIENT: WFLHD DATE: October 02, 2024 ELEVATION: 342.5 ft (Ground) PROJECT: WFLHD Wind River Road PROJECT NO: US0033714.0878 INCLINATION: 90.0° COORD SYS: Geographical Coordinates LOCATION: Skamania County, WA CONTRACTOR: Holt Services Inc. HORZ DATUM: NAD83

HAMMER TYPE: Automatic, 140lb, 30" drop

DRILL RIG: Mobile B57 DRILL METHOD: Mud Rotary

REV:

LOGGED: Madelaine Anderson DATE: Oct 02, 2024

Golder Log Imperial / Soil-Gradation 2_MP / Golder - 3 Imperial US / Golder US Auto (common in US) / 2025-02-17 CHECKED: J. Cox and B. Lockard DATE: Dec 11, 2024

Plastic & Liquid Limits (%) Water Content (%)

NP Nonplastic

0 25 50 75 10

TH

(f t)

MATERIAL PROFILE

DESCRIPTION

(SM) Silty SAND with gravel, fine to medium subrounded to subangular, some nonplastic fines, little fine subangular gravel;

grayish green mottled purplish gray, iron oxide staining, organic debris; moist, dense

(MH) Elastic SILT, high plasticity, little fine to coarse subrounded to subangular sand, trace fine to subrounded gravel; light brown mottled tan, heterogeneous; moist, medium stiff to stiff.

(SM) Silty SAND, fine to medium, some medium plasticity fines, little fine subangular gravel; grayish green mottled light green, chlorite;

moist to dry, dense.

(MH) Elastic SILT, high plasticity, little fine to medium sand; black mottled light brown and green, heterogeneous; moist, very stiff.

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