GR 04-24_Hawk Creek.pdf

SUBMITTED TO:

Jacobs Engineering Group, Inc.

707 17th Street, Suite 360 Denver, CO 80202

BY:

Shannon & Wilson 5900 W 38th Avenue Wheat Ridge, CO 80212

(303) 825-3800 www.shannonwilson.com

GEOTECHNICAL REPORT, GR 04-24

WA LINCOL 57860(1), Hawk Creek Road Improvements

LINCOLN COUNTY, WASHINGTON

April 30, 2024

Shannon & Wilson No: 111954

WA LINCOL 57860(1), Hawk Creek Road Improvements Geotechnical Report, GR 04-24

111954 April 30, 2024

PAGE INTENTIONALLY LEFT BLANK FOR DOUBLE-SIDED PRINTING

5/2/2024-111954_R1_ i

Submitted To: Jacobs Engineering Group, Inc.

707 17th Street, Suite 360 Denver, CO 80202 Attn: Ms. Sara Folk, PE

Subject: GEOTECHNICAL REPORT, GR 04-24, WA LINCOL 57860(1), HAWK

CREEK ROAD IMPROVEMENTS, LINCOLN COUNTY, WASHINGTON

Shannon & Wilson prepared this geotechnical report as a subconsultant to Jacobs Engineering Group, Inc. (Jacobs). The report provides our geotechnical engineering recommendations for the Hawk Creek Road Improvements Project in Lincoln County, Washington. Our work was completed in accordance with Task Order Number 148049541 under our Professional Services Master Agreement Number 148029282 with Jacobs.

We appreciate the opportunity to be of service to you on this project. If you have any questions or require further information, please contact me at 303-825-3800.

Sincerely, SHANNON & WILSON

Gregory R. Fischer, PE, PhD President

CRC:GRF/jma/ajg ii

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CONTENTS

1 Introduction

2 Project Description

3 Field explorations and laboratory testing

3.1 Subsurface Exploration Program

3.2 Geotechnical Laboratory Testing Program

4 Geologic Setting and General Subsurface Conditions

4.1 General

4.2 Regional Geology

4.3 United States Department of Agriculture (USDA) Soil Survey Map

4.4 Subsurface Conditions

4.5 Previous Subsurface Exploration Programs

4.6 Reconnaissance

4.6.1 Surface

4.6.2 Subsurface

5 Geologic Hazard Evaluation

5.1 Seismic Hazard Evaluation

5.1.1 Faults and Fault Rupture

5.1.2 Seismic Accelerations and Earthquake Magnitude

5.1.3 Liquefaction

5.2 Corrosive Soil

6 Geotechnical Recommendations

6.1 Soil Nail Walls (Cut Walls)

6.2 Cut Slopes

6.3 Fill Slopes

6.4 Hydraulic Structures

6.4.1 Foundations

6.4.2 Lateral Earth Pressure

6.5 Pavement

7 Constructon Considerations

7.1 Earthwork and Grading

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7.1.1 Clearing and Stripping

7.1.2 Excavation

7.1.3 Fill Materials and Compaction

7.1.4 Material Source Reconnaissance

7.1.5 Temporary Stability of Slopes and Interim Cuts for Nail Wall Construction

7.2 Soil Nail Walls

7.2.1 Drilling

7.2.2 Excavation Face Stability

7.2.3 Testing Requirements

8 Limitations

9 References iv

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Exhibits Exhibit 4-1: Surficial Geology of the Project Site Exhibit 4-2: Test Pit Log by Lincoln County Exhibit 5-1: Estimated Liquefaction-Induced Settlement Exhibit 5-2: Corrosion Testing Results Exhibit 5-3: Summary of Corrosion Tests and Associated Limits Exhibit 5-4: Corrosivity Ratings Based on Soil Resistivity Exhibit 6-1: Cut Wall Global Stability Factor of Safety (FS) - Existing Conditions Exhibit 6-2: Cut Wall Global Stability FS - Proposed Conditions2 Exhibit 6-3: Fill Slope Global Stability FS - Existing Conditions Exhibit 6-4: Fill Slope Global Stability FS - Proposed Conditions Exhibit 6-5: Recommended Factored Bearing Resistance Values Exhibit 6-6: Recommended Lateral Earth Parameters - Structural Backfill

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Figure 3: Recommended Surcharge Loading for Temporary and Permanent Walls

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Appendix C: Previous Geotechnical Data Important Information

1 INTRODUCTION

This report presents the results of our subsurface exploration program and geotechnical engineering recommendations for the proposed Hawk Creek Road Improvements Project (the Project) in Lincoln County, Washington. Our services were completed in accordance with a signed task order as part of our for professional services master agreement with Jacobs Engineering Group, Inc. (Jacobs) based on a scope of services developed by Western Federal Lands Highway Division (WFLHD) of the Federal Highway Administration (FHWA). Under this agreement we:

Performed a subsurface investigation program, which included coordinating subcontractors, coordinating utility locates, assisting in providing permitting for drilling borings, and observing and logging four supplemental geotechnical borings;

Completed geotechnical laboratory tests on select soil and bedrock samples from the supplemental borings;

Reviewed geotechnical explorations and laboratory tests completed for previous phases of the Project;

Evaluated geologic hazards;

Evaluated site conditions and provided recommendations for geotechnical aspects of the Project associated with the proposed cuts and fills; and

Prepared this geotechnical report, which incorporates data and pertinent opinions, conclusions, and recommendations from our previous studies for the Project.

The scope of our services did not include an evaluation regarding the presence or absence of wetlands. Further, we did not complete an environmental evaluation regarding the presence or absence of hazardous or toxic materials in the soil, surface water or air. If a service is not specifically indicated in this report, do no assume it was completed.

2 PROJECT DESCRIPTION

The Project is located about 13 miles northwest of Davenport, Washington (Figure 1). The Project will begin at the intersection of Miles Creston Road and Hawk Creek Road and extend approximately 0.65 miles west to a point approximately 500 feet west of the Hawk Creek Campground entrance and the National Park Service (NPS) boundary of the Lake Roosevelt National Recreation area.

Hawk Creek Road is the only access to the Hawk Creek Facilities of the Lake Roosevelt National Recreation Area. The Hawk Creek Facility, which includes a boat launch and campground, receives over 30,000 visitors each year and is both a long-term and day-use destination (Jacobs, 2021).

The existing topography of the roadway is mountainous with steep grade changes on either side of the roadway. The existing roadway currently varies in width from 15 to 20 feet. The narrow width and existing sharp, horizontal curves do not meet safety standards for the posted speed limit and type of traffic. We understand that the scope of the Project includes realigning the roadway to meet horizontal curve standards, adding hard surfacing to improve durability and reduce dust and erosion, updating drainage and improving roadside safety. Specifically, the road is designed with a 22- to 24-foot-wide paved roadway and a 1-foot-wide gravel shoulder on the fill slope side where guardrail is required.

Roadside ditches are a minimum of 6-inches below subgrade on the cut side. In areas with steep profile grades, and in areas with constraints, a 2-foot wide riprap-lined ditch is provided. Slight changes in roadway elevation are also proposed. To accommodate the road widening and limit impacts on private property, walls be required on the uphill (north) side of the alignment, including a proposed soil nail wall from Stations 20+00 to 26+00 (Jacobs, 2024). In addition, other cut slopes will be required on the uphill side and fills will be necessary along the downhill (south) side of the alignment.

The horizontal and vertical alignment of the proposed road and the extents and locations of cuts and fills have varied during the design process. Earlier packages did not include walls on the north side of the road but consisted of 1V:2H (vertical to horizontal) cut slopes. The recommendation sections of this report are based on the 95% design plans from Jacobs (2024). We understand that the roadway can be closed for construction.

In addition to the earthwork, culverts are planned as part of the Project, based on the 95% plans (Jacobs, 2024) and conversations with Jacobs, we understand that culverts are anticipated near Stations 19+50 and 41+50. The culvert near Station 19+50 will consist of a 24-inch diameter corrugated metal pipe (CMP). The culvert near Station 41+50 will be a 60-inch by 46-inch elliptical culvert with wingwalls at either end.

3 FIELD EXPLORATIONS AND LABORATORY TESTING

3.1 Subsurface Exploration Program

Shannon & Wilson completed two rounds of subsurface explorations at the approximate locations shown on Figure 2. In March 2022, we completed 14 borings, designated SW-01 to SW-14, along the roadway alignment and on the bluff above the roadway and at the toe of the slope below the roadway. These borings ranged in depths from 4 to 65 feet below existing grades. In February 2024, we completed an additional two borings, SW-103 and SW-105 at the top of bluff and an additional two borings, SW-108 and SW-109, at the toe of the slope in the NPS campground. The top of bluff borings extended 66.5 feet below grade, and the toe of slope borings extended 14 and 26.5 feet below grade.

Appendix A presents a discussion of the drilling, sampling, and testing procedures used to complete the current borings. Appendix A also presents the individual exploration logs and an explanation of the symbols and terminology used for the 2024 borings. Appendix C contains the boring logs from 2022.

3.2 Geotechnical Laboratory Testing Program

Geotechnical laboratory tests were completed on selected samples retrieved from the current borings to estimate index and engineering properties. Tests included natural water content, grain size distribution, Atterberg limits, and corrosion. Laboratory test methods and results are provided in Appendix B. The natural water content, fines content, and Atterberg limits are also shown on the individual boring logs included in Appendix A. The laboratory results from the 2022 borings are provided in Appendix C.

4 GEOLOGIC SETTING AND GENERAL SUBSURFACE

CONDITIONS

4.1 General

We based our understanding of the geology and subsurface conditions at the site on a review of regional geologic maps and the results of our subsurface exploration program.

4.2 Regional Geology

We reviewed 1:24,000-scale geologic mapping by Derkey and Hamilton (2009) covering the Project alignment. The following description of regional geology is based on this map:

Bedrock underlying the Project alignment is mapped as middle-Miocene-age (about 16-to 11-million-year-old) basaltic rocks of the Columbia River Basalt Group, specifically the Grande Ronde Basalt. This bedrock reportedly consists of dark gray to green-gray, fine-grained, locally vesicular basalt. An exposure of Grande Ronde Basalt is mapped at the surface along the western part of the alignment.

Surficial deposits consisting of Pleistocene-age (about 2.6-million- to 11,700-year-old) gravelly glacial flood deposits are mapped along much of the alignment. These deposits reportedly consist of gravel, boulders, cobbles, and sand with occasional silt and sand lenses.

Alluvium consisting of silt, sand, and gravel are mapped in the Hawk Creek floodplain south of the roadway and where the roadway crosses an ephemeral drainage at the east end of the Project alignment.

The easternmost part of the Project alignment is mapped as Pleistocene-age glaciolacustrine deposits of Glacial Lake Columbia. These deposits reportedly consist of interbedded silt, fine sand, and clay, with varying amounts of cobbles and boulders.

An annotated excerpt from the Derkey and Hamilton (2009) map is included as Exhibit 4-1.

Exhibit 4-1: Surficial Geology of the Project Site Qa = alluvium; Qgl = glaciolacustrine deposits; Qfg = glacial flood deposits, dominantly gravel; Mgrr2 = Grande Ronde Basalt Approximate Project Alignment Ephemeral Drainages Perennial (Flowing) Drainages Concealed Fault Locations, Uncertain

It should be noted that we did not observe bedrock at the ground surface to the extent indicated by the map. We did note bedrock at the western end of the project above the roadway and at the hairpin curve below the roadway (see Figure 2). Based on a recent geological map by the United States Geological Survey (USGS, 2021), the nearest fault to the Project site is the Frenchman Hills Structures, about 70 miles to the southwest. The fault is noted as Quaternary in age (within the past 1.6 million years) with less than 0.2 millimeters per year of movement.

4.3 United States Department of Agriculture (USDA) Soil Survey Map

We reviewed the United States Department of Agriculture (USDA, 2021) Soil Survey Map from the Natural Resources Conservation Service web soil survey tool. Several soil units were identified along the Project alignment, but most of the Project area is underlain by "Badge-Bakeoven-rock outcrop complex," a soil unit described as a sandy silt with gravel and cobbles. The soil survey map indicates "Riverwash" deposits (with no soil description) where the road crosses ephemeral stream courses at the east end of the alignment at approximate Stations 11+00 and 14+00 and in the floodplain of Hawk Creek south of the alignment; see Figure 2 for approximate stationing. Riverwash deposits are also shown underlying the hairpin turn near Station 30+00. The west end of the project and the Hawk Creek Campground are mapped as Nespelen silt loam, a soil unit described as a sandy silt or silty fine sand derived from glaciolacustrine and loess deposits. Nespelen silt loam is also mapped on the upland surface above the hairpin turn near Station 30+00 (USDA, 2021).

4.4 Subsurface Conditions

Subsurface conditions encountered in the explorations generally consisted of interbedded layers of loose to dense, silty sand to sand with silt and soft to medium stiff, lean to fat clay above the groundwater table. Differences from the above noted generalized conditions are as follows:

Very dense sand and gravel (with cobbles and/or boulders) was encountered at the bottom of borings SW-05, SW-06 and SW-13.

- Auger refusal was noted in borings SW-06 and SW-13 at a depth of 9.6 and 4.5 feet below grade, respectively.

Very loose sand was encountered below the water table at the toe of the slope in borings SW-12, SW-108 and SW-109.

The explorations were completed to evaluate geotechnical conditions at the site. Our observations are specific to the locations, depths, and times noted on the boring logs and may not be applicable to all areas of the Project. No amount of explorations or testing can precisely predict the characteristics, quality, or distribution of subsurface and site conditions. Section 8 describes the limitations of our geotechnical report as it relates to the Project and subsurface conditions.

4.5 Previous Subsurface Exploration Programs

We received a field log for a 20-foot-deep test pit excavated by Lincoln County Road and Bridge personnel on January 22, 2021. The test pit was excavated on the upland above approximate Station 16+00, and indicated soils consisting of silt, silty sand, and clay. No groundwater was noted. We did not receive reporting or lab testing associated with this log. The field log is shown below in Exhibit 4-2.

Exhibit 4-2: Test Pit Log by Lincoln County

4.6 Reconnaissance

A Shannon & Wilson site representative participated in the site kick-off meeting on August 31, 2021, and a subsequent site meeting on November 16, 2023. The Shannon & Wilson representatives observed geologic and geotechnical features along the Project alignment and evaluated drilling rig access to proposed boring locations. The information collected during the site reconnaissance were evaluated as part of the Project. Photographs of site features observed during reconnaissance are included in Appendix A.

4.6.1 Surface

Throughout much of the alignment, we observed soil cut slopes on the uphill side of the road. Soil cut slopes were up to about 30 feet high with slope angles ranging from about 36 to 43 degrees (approximately 1V:1.25H to 1V:1.1H). We noted that soil cut faces were typically vegetated with grass and scattered trees and exhibited evidence of minor raveling and rilling between vegetated areas. We did not observe this surficial raveling and rilling causing deposition on the roadway or other obvious maintenance concerns.

We observed fill slopes on the south (downhill) side of the roadway along most of the Project alignment. Fill slopes below the roadway appeared to consist of sidecast fill material similar to the cut slope material, except for the area of basalt boulders near the base of the fill at approximate Station 32+00. From about Stations 24+50 to 36+00, fill slopes ranged up to about 20 feet high and exhibited slope angles on the order of 32 to 42 degrees (1V:1.6H to 1V:1.1H). From about Stations 18+00 to 24+50, near the west end of the alignment, fill slopes were generally on the order of 30 degrees (1V:1.75H) and were covered with heavy vegetation that obscured the toe of the fill and underlying soils. From about Stations 21+00 to 24+00, we observed a narrow roadway (about 18 feet wide in some areas). At two areas in this station range, we observed indications of raveling or shallow slumping with an abrupt drop off of the downhill shoulder.

From approximate Stations 38+00 to 41+00, we observed that slopes were covered with deciduous trees unlike the low scrub observed elsewhere along the alignment. On aerial photos and topographic maps, there is an arcuate deviation in the bluff face that could be consistent with a relict or inactive slide area. However, we did not observe indications of sliding in the abandoned road grade parallel to and about 25 feet in elevation above the Project alignment in this area. We also did not observe indications that trees have displaced downhill in this area (e.g., tilting or "pistol butting" of trunks). We did observe indications of seepage in the uphill cut of the road grade near approximate Station 34+00, including wet soils and a profusion of poison ivy. Except for the area of seepage noted above the roadway at Station 34+00, we did not observe seepage elsewhere along the alignment.

4.6.2 Subsurface

We observed varying geologic conditions in outcrops and soil exposures along the alignment. We generally observed conditions to be consistent with mapping by Derkey and Hamilton (2009). The following bullets summarize the soils and bedrock conditions we observed along the alignment.

Stations 15+00 to 16+50 - We observed an outcrop of basalt in the approximately 40-foot-high cut north of the road in this area. The outcrop consisted of strong to very strong (unconfined compressive strength exceeding 7,200 pounds per square inch [psi]) basalt that was variably vesicular, gray, and exhibited whitish and iron-oxide staining. The basalt outcrop was cut by nearly vertical, undulating joints forming a columnar structure with individual columns on the order of 1 to 3 feet wide. We also observed areas of blocky rock mass with randomly oriented, undulating joints spaced at about 2 to 8 inches. The basalt was slightly to moderately weathered, with degree of weathering and fracturing appearing to increase toward the top of the approximately 40-foot-high outcrop.

Stations 16+50 to 28+00 - In this reach, we observed soils in cut slopes to consist of clayey sand to sandy clay soils with trace to few gravel and trace cobbles and boulders like those described above. Beginning at about Station 26+00, we observed a likely exposure of basalt in the dry, ephemeral stream channel south of the Project alignment at the base of the sidecast road fill. Reconnaissance observations and aerial photos also indicate basalt beds outcropping about 10 to 15 feet below roadway grade from about Stations 26+00 to 29+00. The top of the basalt outcrops in the ephemeral channel and below the roadway continue southward and form the escarpment at Hawk Creek Falls (about 150 feet south). These basalt beds may also extend to the west of Station 26+00 as mapped by Derkey and Hamilton (2009) but were not visible beneath heavy vegetation and apparent sidecast fill soils in this area.

Stations 28+00 to 42+00 - In this area we observed soils exposed in cuts to consist of relatively dry, clayey sand, to sandy clay with trace to few gravel, and trace cobbles and boulders. Shallow pits dug into cut slopes with a shovel exhibited soils containing sand grains that consisted of angular clay fragments. Hand samples behaved as a cemented sand when dry but softened when wetted. These soils likely represent flood gravels or colluvium developed from flood gravels. At approximate Station 22+00, we observed basalt boulders at the base of the fill slope below the road. It is unclear if these are fill deposits or if this may be detritus from near-surface basalt bedrock.

Stations 42+00 to 46+00 - Along this easternmost segment of the alignment, we observed soils along the road consisting of dry, lean clay to sandy lean clay with angular gravel and trace cobbles. This soil may be an alluvium reworked from glacial flood gravels.

Aerial photos indicate that the ephemeral stream at Station 41+50 was channelized into the culvert undercrossing.

5 GEOLOGIC HAZARD EVALUATION

Based on our subsurface explorations and our knowledge of the geology of the area, it is our opinion that there are no geologic hazards that preclude development at the site. However, there are a few potential geologic hazards that could pose a risk to the Project site if not adequately addressed during design, as described below.

5.1 Seismic Hazard Evaluation

5.1.1 Faults and Fault Rupture

The closest known fault hazard to the Project area is the Saddle Mountain structure, located approximately 90 miles southwest of the site. For this reason, it is our opinion that the potential for ground surface fault rupture at the site is low.

5.1.2 Seismic Accelerations and Earthquake Magnitude

Seismic accelerations and earthquake magnitude was determined based on procedures outlined in the current Washington Department of Transportation (WSDOT) Geotechnical Design Manual (GDM) (2022). We determined the subsurface conditions to be Site Class D based on blow count information and soil shear strength. Using the online Unified Hazard Tool (USGS, 2024) the peak ground acceleration (pga) and mean earthquake magnitude were 0.088g and 6.28, respectively. The site modified ground acceleration (As) was calculated to be 0.14 for Site Class D.

5.1.3 Liquefaction

Soil liquefaction is a phenomenon in which pore pressure in loose, saturated, granular soils increases during ground shaking to a level near the initial effective stress, thus resulting in a reduction of shear strength of the soil (a quicksand-like condition). As a result of this reduction in shear strength during liquefaction, vertical and lateral foundation restraint may also be reduced, and permanent ground deformations (i.e., settlement and lateral displacement) may occur. Liquefaction was evaluated according to procedures contained in

WSDOT GDM (2022).

We evaluated the potential liquefaction induced settlement for the design ground motion in each exploration using the procedures outlined in WSDOT GDM (2022). The groundwater depth used in our analysis corresponds to groundwater depth encountered in each individual boring. Our estimated liquefaction-induced settlement is tabulated in Exhibit 5- 1.

Exhibit 5-1: Estimated Liquefaction-Induced Settlement

Exploration Exploration

Type Total Depth

(feet)1

Design As (g)

Liquefiable Layer Depths (ft) Settlement (in)

SW-12 Boring 26.5 0.14 12 - 16 1 – 1½

SW-108 Boring 26.5 0.14 13 - 15 < ½

SW-109 Boring 14.0 0.14 7 – 10 ¼ – ¾

NOTES:

These borings are less than 80 feet deep and therefore additional settlement my occur.

5.2 Corrosive Soil

To assist in estimating the corrosion potential at the site, samples of soil were tested for pH, resistivity, water soluble sulfates, and chlorides. The results are presented below in Exhibit 5-2.

Exhibit 5-2: Corrosion Testing Results

Boring Depth (feet) pH

Resistivity (ohm-cm)

Sulfate Content (ppm)

Chloride Content (ppm)

SW-01 8 -- 2,670 -- --

SW-10 6 -- 6,450 -- --

SW-103 60 8.7 2,220 < 10 < 10

SW-105 65 8.3 1,720 93 < 13

NOTES:

Water-soluble sulfate and chloride content values are shown as percent weight in dry soil.

ppm = parts per million, ohm-cm = ohm-centimeter

The Geotechnical Engineering Circular (GEC) No. 7, Soil Nail Walls – Reference Manual (USDOT, 2015) provides recommendations on classifying a soil as “aggressive” and “non-aggressive” when assessing ground corrosion potential. Exhibit 5-3 provides a summary of the recommended tests and the threshold limit for those tests. The ground is classified as aggressive if just one test meets the criteria.

Exhibit 5-3: Summary of Corrosion Tests and Associated Limits

Test Units Non-Aggressive Aggressive pH -- 5.5 < pH < 10 5.5 > pH > 10

Resistivity ohm-cm > 3,000 ≤ 3,000

Sulfates ppm < 200 ≥ 200

Chlorides ppm < 100 ≥ 200

NOTES:

ppm = parts per million, ohm-cm = ohm-centimeter

Based on the SW-103 and SW-105 test results (in soils similar to those that the soil nails are anticipated to be located), the soils should be considered “aggressive” based on the measured resistivity values being less than 3,000 ohm-cm.

In addition to the GEC-7 Soil Nail Walls criteria, we considered the results of two resistivity tests near the culvert locations. The resistivity measured in the samples were 2,670 and 6,450 ohm-centimeters. Based on correlations developed by Roberge (2012) as shown in Exhibit 5-4, these values suggest moderately to highly corrosivity for metallic structures in contact with subsurface materials at the two culvert locations.

Exhibit 5-4: Corrosivity Ratings Based on Soil Resistivity

Soil Resistivity (ohm-cm)

Corrosivity Rating

>20,000 Essentially Noncorrosive

10,000-20,000 Mildly Corrosive

5,000-10,000 Moderately Corrosive

3,000-5,000 Corrosive

1,000-3,000 Highly Corrosive

<1,000 Extremely Corrosive

The test results and the above discussion are provided to assist the designer in the selection of project materials, concrete type, or other features with respect to corrosion. As appropriate, the designer should consider protective measures, such as coatings, upsizing for section loss, or using alternative materials to reduce the corrosion potential.

6 GEOTECHNICAL RECOMMENDATIONS

The following sections provide design recommendations for the Project. These recommendations are based on the 95% Plans set provided by Jacobs (2024). Our recommendations should be reviewed and revised if specific Project elements change.

6.1 Soil Nail Walls (Cut Walls)

Based on the provided 95% Plans (Jacobs, 2024), a soil nail wall is proposed from Station 20+00 to Station 26+00. Wall heights range from 3 to 17 feet with an average height of approximately 10 feet. The height of the slope behind the proposed wall ranges from approximately 110 feet at Station 20+00 to approximately 30 feet at Station 25+00. The backslope is generally 1V:1.3H with some isolated steeper slopes between Stations 20+00 and 24+00. From Stations 24+00 to 26+00 the 1V:1.3H slope generally ends within 50 feet horizontally from the wall toe and then the slope changes to a 1V:3H or flatter slope.

AASHTO LRFD (2020) does not specify factor of safety (FS) or resistance values for soil nail walls. Table 5.1 of GEC 7 (USDOT, 2015) and the WSDOT GDM (2022) require minimum FS values of 1.3 for walls that do not directly support other structures, such as is the case for Hawk Creek Road. Consistent with these guidelines, we recommend a minimum seismic FS of 1.1. We used a kh value of 0.07 in our seismic global stability analysis.

We understand that a contractor-selected vendor will design the soil nail walls per AASHTO, LRFD Bridge Design Specifications (AASHTO, 2020). Typical for vendor-designed soil nail walls, we evaluated global stability. For our global stability analysis, we used the Slide2 (Rocscience, 2023) computer program using the Bishop Simplified method to calculate the FS.

Due to the relative height of the slope compared to height of the wall (the slope being up to 10 times taller than the wall) our stability analysis looked at four conditions or limits to the slip surface boundaries.

Auto search – search limits were placed at 180 feet from the roadway center line and the outside edge of the roadway.

Full Slope – search limits were set so that the slip surface was forced to start at the top of slope and end within the width of the roadway.

Half Slope - search limits were set so that the slip surface was forced to start in the upper half of the roadway, generally between 75 and 90 feet from the center of the roadway, and end within the width of the roadway.

4 times H (4H) Slope search – search limits were set to 4 times the height of the wall horizontally for the proposed wall toe.

The 4H slope condition was based on recommendations from FHWA Manual for Design and Construction Monitoring of Soil Nail Wall (USDOT, 1998), which suggested that for infinite slope conditions, the global stability should be limited to a zone horizontally from the wall toe equal to 2 or 3 times the height of the wall. We increased the slope search distance to 4H to be more conservative. The other FS values were computed to confirm that if the FS was lower than the minimum criteria that it did not decrease from the soil nail wall construction; however, we did not attempt to achieve the minimum FS criteria if it was not satisfied.

Soil properties used in our analysis were based on field and laboratory test correlations and modified to achieve a minimum FS of 1.1 for the static conditions of the existing slope using the reasoning that the existing slope appears to be stable, and no failures were observed within the reach where soil nail walls may be placed. Therefore, using a FS of 1.1 for the existing conditions should be conservative.

The intent of our analysis was to determine a minimum nail configuration to satisfy requirements for global stability. We did not consider internal or external stability or temporary construction conditions. These should be completed by the designer of the nail wall.

Exhibits 6-1 and 6-2 contain a summary of our global stability results under existing conditions at the chosen cross-sections. Bolded values in Exhibits 6-1 and 6-2 indicate FS values that meet recommended minimum values. Non-bolded values do not meet recommended minimum values.

Exhibit 6-1: Cut Wall Global Stability Factor of Safety (FS) - Existing Conditions

Station Slope

Height (ft) Average Slope

Dimensions Condition Auto

Seach Full

Slope Half

Slope 4*H

Slope

21+00 102 1V:1.3H Long Term Static 1.09 1.13 1.13 1.07

Pseudo Static (Seismic) 0.97 0.982 0.98 0.96

22+50 88 1V:1.5H Long Term Static 1.06 1.19 1.19 1.30

Pseudo Static (Seismic) 0.91 1.04 1.04 1.15

25+00 30 1V:2.6H Long Term Static 1.16 1.62 -- 1.16

Pseudo Static (Seismic) 1.03 1.33 -- 1.03

Exhibit 6-2: Cut Wall Global Stability FS - Proposed Conditions2

Station Wall Height

(ft) Condition Auto

Seach Full

Slope Half

Slope 4*H

Slope

21+00 15 Long Term Static 1.141 1.27 1.29 1.35

Pseudo Static (Seismic) 1.001 1.12 1.13 1.20

22+50 8 Long Term Static 1.041 1.23 1.23 1.41

Pseudo Static (Seismic) 0.901 1.07 1.08 1.25

25+00 16.5 Long Term Static 1.35 1.51 -- 1.35

Pseudo Static (Seismic) 1.22 1.32 -- 1.22 Notes:

Minimum FS represents a slip surface above the proposed wall and therefore not indicative of wall stability.

See notes below for minimum nail dimensions to achieve global stability. Longer nails, additional rows, and tighter spacing may be required to meet internal and external stability requirements.

We recognize the minor decreases in FS between the existing and proposed conditions at Station 22+50 (Auto Search) and Staton 25+00 (Full Slope). At Station 22+50, the decrease is likely due to software programing. At Station 25+00, the soil nail wall with the properties described below meets the minimum recommended FS value, therefore, the potential decrease in FS is considered acceptable for that condition.

The below criteria regarding minimum soil nail length and maximum horizontal/vertical spacing of the soil nails are required to meet global stability and should be included in the Project criteria.

Stations 20+00 to 24+50,

- Wall heights greater than 8 feet: 5-foot horizontal and 5-foot vertical spacing with

20-foot nail length and a No. 9, Grade 75 reinforcing bar.

- Wall heights less than or equal to 8 feet: 6-foot horizontal and 6-foot vertical spacing with 8-foot nail length and a No. 9, Grade 75 reinforcing bar.

Stations 24+50 to 26+00, 5-foot horizontal and 5-foot vertical spacing with 10-foot nail length and a No. 9, Grade 75 reinforcing bar.

Our analysis only included global stability. Other external stability requirements (such as sliding and basal heave), internal stability analysis (pullout, nail size and facing connections) and temporary stability during construction, will need to be evaluated by the wall designer. Soil nail lengths may need to be increased to meet those design standards.

Similarly, the horizontal and vertical spacing may need to be decreased to satisfy internal and external stability requirements. For our analyses, we assumed the following:

A 6-inch diameter drill hole at 15 degrees below the horizontal.

A nominal grout to soil adhesion value of 5 pounds per square inch (psi). The actual grout-to-ground bond strength developed in the field will depend on the contractor’s installation methods, particularly the drilling and grouting procedures. The contractor will ultimately be responsible for confirming the design bond strength value in the field by load testing.

6.2 Cut Slopes

Cut slopes should be made at 1V:2H or flatter. Between Stations 16+00 and 20+00, walls can be avoided by steepening cut slopes to similar conditions (i.e., as steep as 1V:1.7H). While these cuts will not show adequate FS values in analyses, these cut slopes will be short (less than 5 feet high). In our opinion, there is low risk with these small cuts, particularly if they can be vegetated to reduce the potential for raveling that could progress further upslope.

6.3 Fill Slopes

The 95% plans (2024) and cross-sections provided by Jacobs indicate fill slopes of 1V:2H and 1V:3H. Where possible, for planning purposes, we suggest considering new fill slopes at 1V:3H, for revegetation and maintenance purposes. In areas where 1V:2H slopes are necessary, the use of more select fill material will be required. Such material would come from a borrow source. New fill slopes should be benched into the existing slopes. See Section 7.1.3 of this report for further discussion on material properties and placement.

The FHWA Project Development and Design Manual (USDOT, 2018) recommends a minimum FS value of 1.3 for new embankments; WSDOT GDM (2022) recommends a minimum FS value of 1.25 for new slopes that are not supporting structures or impacting adjacent structures. WSDOT GDM recommends a seismic FS of 1.1 for portions of a new embankment that could impact an adjacent structure (bridges, walls or nearby buildings).

We used a kh value of 0.07 in our seismic global stability analysis. The WSDOT GDM recommends a minimum FS of 1.05 for post-liquefied conditions. For post-liquefied conditions, the potentially liquefiable layer was modeled with a shear strength to effective strength ratio of 0.1 based on procedures outline by Idriss and Boulanger (2008). Global stability analysis was performed using Slide2 (Rocscience, 2023) computer program using the Bishop Simplified method to calculate the FS.

Similar to the soil nail wall analysis, the fill slope stability analysis looked at different conditions or limits to the slip surface boundaries.

Auto search – search limits are from edge of roadway to a point 180 feet from the roadway centerline or approximately 80 past the toe of the existing slope.

Full Slope – search limits set so that the slip surface is forced to start within the roadway width and end at or beyond the toe of the existing or new slope.

Based on the observations of the existing conditions, a minimum FS of 1.1 was assumed for the existing slopes. Soil properties used in our analysis were based on field and laboratory test correlations and modified to achieve a minimum FS of 1.1 for the static conditions of the existing slope.

Exhibits 6-3 and 6-4 contain a summary of our global stability results at the chosen cross-section. Bolded values in Exhibits 6-3 and 6-4 indicate FS values that meet recommended minimum values. Non-bolded values do not meet recommended minimum values.

Exhibit 6-3: Fill Slope Global Stability FS - Existing Conditions

Station Slope Height

(ft) Average Slope

Dimensions Condition Auto

Seach

Full Slope

21+50 28 1V:1.4H

Long Term Static 1.13 1.13

Pseudo Static (Seismic) 1.00 1.00

Post Seismic 1.13 1.13

Exhibit 6-4: Fill Slope Global Stability FS - Proposed Conditions

Station Wall Height

(ft) New Slope

Dimensions Condition Auto

Seach

Full Slope

21+50 34 1V:2H

Long Term Static 1.61 1.61

Pseudo Static (Seismic) 1.38 1.38

Post Seismic 1.12 1.12

As noted in Exhibit 6-4, the proposed embankment slopes will have a higher static and pseudo static (seismic) FS than existing conditions and meet the minimum criteria. The post-seismic FS is slightly lower in the proposed condition but still meets recommended minimum value for that condition. Exiting slopes that are taller than 28 feet may have post-seismic FS values lower than minimum requirements, and therefore, may experience post-seismic instability or movement. We assumed that WFLHD would determine that liquefaction mitigation for a roadway embankment in a rural area is not cost effective, which would be consistent with WSDOT design guidelines.

6.4 Hydraulic Structures

6.4.1 Foundations

Recommended factored bearing resistance for footings for the culvert at Station 41+50 is provided in Exhibit 6-5 based on resistance factor of 0.45. The factored bearing resistance may be increased by one-third for wind or seismic short-term loading.

Exhibit 6-5: Recommended Factored Bearing Resistance Values

Station Footing Width

(feet)

Factored Bearing Resistance

(psf)

41+50 1½ - 3 2,000 psf = pounds per square foot

We estimate approximately one inch of settlement under the provided factored bearing values.

6.4.2 Lateral Earth Pressure

The lateral earth pressures on structure walls depend on many factors, including surcharge loads, the soil type, and whether the top of the wall can yield or deflect laterally during and after excavation. If the top of the wall is free to yield a minimum of 0.1% of the wall height (i.e., 0.001 times the height [H]), the soil pressures will generally be lower (i.e., active condition), than if the wall is not able to yield this amount due to stiffness or resistance of the wall (i.e., at-rest condition). We anticipate that at-rest earth pressures will be appropriate for the structure. See Exhibit 6-6 for recommended lateral earth pressures values.

The lateral earth pressures assume the structure walls are backfilled with properly compacted Structural Backfill as described in Section 7.1.4.

Structural Backfill (or equivalent materials) should be placed in a 1V:1H zone extending upward from the base of the structure from a point 1.5 feet behind the wall. These parameters also assume a vertical wall face and a flat backslope.

Exhibit 6-6: Recommended Lateral Earth Parameters - Structural Backfill

Parameter Coefficient Equivalent Fluid Pressure

(pcf)1

Static Active Earth Pressure ka = 0.31 39

Static At-rest Earth Pressure ko = 0.47 59

Dynamic Active Earth Pressure kae = 0.38 48

Dynamic At-rest earth pressure kpe = 0.64 80

NOTES:

Based on granular soil, above the water table, with an assumed unit weight of 125 pcf and a friction angle of 32 degrees for the retained soil.

pcf = pounds per cubic foot

Surcharge loads from motor vehicles/traffic and construction equipment will induce additional lateral loads on structure walls. The pressures provided above do not account for surcharge loads or hydrostatic pressure. Although observed groundwater is below the bottom of the proposed foundation, drainage should be included to collect any water that infiltrates near any below grade structures. Lateral loads due to various types of surcharges may be calculated by multiplying the surcharge by the at-rest earth pressure coefficient (k0).

Surcharge loads should be applied as recommended in Figure 3.

6.5 Pavement

The existing Hawk Creek Road surface consists of aggregate. We understand that the proposed roadway will consist of 6 inches of aggregate base, overlain by 3 inches of asphalt concrete. Pavement design was not part of our scope of work; however, in our opinion, this section should be adequate for a local, recreational access road on a prepared subgrade.

7 CONSTRUCTON CONSIDERATIONS

The applicability of the design recommendations provided in this report is contingent on good construction practice. Poor construction techniques may alter conditions from those on which our recommendations are based, therefore, resulting in reduced foundation capacity and increased settlement. The following sections present additional construction and material considerations for this project.

7.1 Earthwork and Grading

Earthwork, including embankment construction, placement of fill, and soil nail cuts, should conform to the requirements provided in the latest edition of the FP-14 Standard Specifications for Construction of Roads and Bridges on Federal Highway Project (FHWA, 2014).

7.1.1 Clearing and Stripping

We recommend that brush and other vegetation be cleared, and roots and stumps be removed from all areas to be graded. Any existing surficial topsoil and any soil containing organics should be stripped and removed. The depth of this removal is anticipated to vary over alignment. Topsoil and organic-rich soils are not considered suitable for reuse as fill and should be removed from the site, or placed over the top of finished slopes to promote revegetation. Care should be taken to avoid disturbing subgrade soils and supporting soils that will remain in place, as they can rut and pump under repeated construction traffic. The final subgrade surface should be sloped to promote positive drainage.

7.1.2 Excavation

The majority of the native soils can likely be excavated using conventional excavating equipment, such as a rubber-tired backhoe or tracked hydraulic excavator. Cobbles and/or boulders may be encountered during excavation, thus requiring additional fill or shotcrete to fill voids. Shallow rock may be encountered at isolated areas, including but not limited to cut slopes (northside of roadway) between Stations 14+00 and 17+00 and the fill slopes (southside of roadway) between Stations 26+00 and 28+00.

The contractor should be prepared to excavate rock with equipment such as hoe rams and/or blasting. If blasting techniques are used, the contractor is responsible for any damage resulting from blasting-related ground vibrations and/or air-blast over- pressures.

The contractor will be responsible for developing a blasting plan with will include vibration and air-blast overpressure limits along with a monitoring plan. The vibration and air-blast overpressure limits should be based on atmospheric conditions, soil and rock conditions, blasting parameters, and proximity of buildings, structures, utilities, and sensitive natural features that may be subject to damage from ground vibrations or air-blasts. The contactor is also required to perform test blasting before beginning full-scale drilling and blasting.

7.1.3 Fill Materials and Compaction

General grading fill material used for construction should meet requirements for Unclassified Borrow as per Section 704.06 (USDOT, 2014), except the maximum particle size should be 4 inches. Fill should be placed in horizontal lifts, compacted as required in Section 204 (UDOT, 2014), and be deemed to be in a dense and unyielding condition. The thickness of loose lifts should not exceed 12 inches for heavy equipment compactors, and 6 inches for hand-operated compactors. All compacted surfaces should be sloped to promote drainage and mitigate ponding.

For fill placed against existing slopes, the slope should be benched or terraced in accordance with Section 204.09(d) (USDOT, 2014). New slopes should be vegetated via seeding and mulching to reduce the potential for erosion and surficial stability as detailed in Section 625 (USDOT, 2014). We recommend that a landscape architect be consulted to determine the best means to quickly revegetate the slopes. Either temporary or permanent erosion control blankets may be required to assist in establishing vegetation.

7.1.4 Material Source Reconnaissance

During our field reconnaissance, we visited two separate pit locations owned by Copenhaver Construction, Inc. (Copenhaver) of Creston, Washington. A representative of Copenhaver was not available at the time of our visit.

The nearest of the two Copenhaver pits is located near Creston, or about 13 miles southwest of the Project area near the intersection of Miles-Creston Road and US Highway 2. This pit appeared to mine a basalt exposure and had a wash plant at the site. We observed a large stockpile of approximately ½-inch crushed gravel at this location that was clean, angular, and composed of sound basalt. We also observed a large pile of a "pit run" material consisting of approximately 3/8-inch minus angular crushed rock with sand and non-plastic fines. The second Copenhaver pit near Wilbur, Washington, or about 23 miles west-southwest of the Project area along Miles Creston Road and US Highway 2, appeared to be developed in a flood gravel deposit and had many stockpiles of various materials ranging from clean gravels to mixtures of clay, sand, and fine gravel.

Based on our observations, we believe the Copenhaver pits can likely provide import materials that may be necessary for this project.

7.1.5 Temporary Stability of Slopes and Interim Cuts for Nail Wall Construction

The contractor will be responsible for slope stability during construction. As such, the soil nail designer should evaluate the stability of temporary slope and excavation configurations that may be required during construction. Additionally, for soil nail walls, we recommend that the contractor implement a monitoring program to monitor the performance of the nail wall and slope during construction.

7.2 Soil Nail Walls

7.2.1 Drilling

Based on the subsurface information, the contractor should anticipate encountering granular and fine-grained soils during drilling and select drill tooling and methods appropriate for these conditions. We anticipate that cased drill holes will be required to reduce the potential for hole collapse during soil nail and grout placement. Additionally, grout loss and quantity overruns may occur during grouting of soil nails installed at the site, particularly if the very dense granular soils are encountered during drilling. Finally, cobbles and boulders may be encountered that result in slow and difficult drilling.

7.2.2 Excavation Face Stability

Sloughing of bench cuts during or after excavation is possible in the granular soils at the proposed wall locations, and temporary stabilization may be required. Mitigation measures to improve stand-up time may include applying a flash coat of shotcrete as the ground is excavated and installing nails through the existing material before excavating.

7.2.3 Testing Requirements

Soil nail testing typically consists of verification testing of sacrificial nails at the outset and throughout construction to confirm the assumed bond strength, with additional proof testing of production nails. We recommend verification testing 1% of the total number of production nails and proof testing 4% of production nails.

Because of the quantity of soil nails and native soil layering required for the Project, the Project team may consider implementing preconstruction verification testing to better characterize the bond strength assumed for design. Such testing may allow for optimization of the design and reduce the risk of changes to the design occurring during construction.

8 LIMITATIONS

This geotechnical report was prepared for the exclusive use of Jacobs Engineering Group, Inc. and Western Federal Lands Highway Division and their selected representatives. The report should not be made available for use by others or for purposes other than those described in this report, except for contractors bidding the work or constructing the project.

In this regard, the report only should be relied on for information based on factual data, such as those indicated by the exploration logs and laboratory test results included herein.

Within the limitations of the scope, schedule and budget, the analyses, conclusions and recommendations presented in this geotechnical report were prepared in accordance with generally accepted professional geotechnical engineering principles and practices in this area at the time the report was prepared. We make no other warranty, either express or implied.

The analyses, conclusions, and recommendations contained herein are based on our understanding of the Project, as described in this report, and subsurface conditions as presented in the explorations. We assumed that the subsurface conditions at structure locations area are not significantly different from those disclosed by the explorations performed by us for this project. Unanticipated soil conditions are commonly encountered and cannot be fully determined from explorations. Such unexpected conditions frequently require that additional expenditures be made to attain properly constructed projects.

The scope of our services did not include an assessment or evaluation regarding the presence or absence of hazardous or toxic materials in the soil, surface water, groundwater, or air, on or below the site, or for the evaluation or disposal of contaminated soils or groundwater, should any be encountered. We would be pleased to provide you with a scope for these services at your request, should these services become necessary.

As an integral part of this report, we have prepared the attached “Important Information About Your Geotechnical Report,” to assist you and others who might read this report in understanding its use and limitations.

9 REFERENCES

American Association of State Highway and Transportation Officials (AASHTO), 2020, AASHTO LRFD bridge design…