C04_Attachment5_USFS_Lodgepole-Ninemile_GeoRpt.pdf

SUBMITTED TO:

DJ&A, P.C.

2000 Maple Street Missoula, MT 59808

BY:

Shannon & Wilson, Inc.

5900 W 38th Avene Wheat Ridge, CO 80212

(303) 825-3800 www.shannonwilson.com

GEOTECHNICAL REPORT

USFS Lodgepole and Lower Ninemile Bridge Designs

CHELAN COUNTY, WASHINGTON

December 2025 Shannon & Wilson No: 113461-001

USFS Lodgepole and Lower Ninemile Bridge Designs Geotechnical Report

113461-001 December 2025

PAGE INTENTIONALLY LEFT BLANK FOR DOUBLE-SIDED PRINTING

113461-001 December 2025 12/4/2025-113461-001_USFS Lodgepole and Ninemile_Geo Rpt i

Submitted To: DJ&A, P.C.

2000 Maple Street Missoula, MT 59808 Attn: Ammon Groesbeck, PE

Subject: GEOTECHNICAL REPORT, USFS LODGEPOLE AND LOWER NINEMILE

BRIDGE DESIGNS, CHELAN COUNTY, WASHINGTON

We are pleased to submit our Geotechnical Report for the above-referenced project. The enclosed report summarizes the subsurface conditions encountered in our exploration program, laboratory test results, and geotechnical engineering recommendations for the proposed project.

Shannon & Wilson participated in this project as a subconsultant to DJ&A, P.C (DJ&A). Our scope of services for the Project was specified in our Professional Services Agreement with DJ&A, dated November 5, 2024.

We appreciate the opportunity to be of service to you on this project. If you have questions concerning this report, or we may be of further service, please contact us.

Sincerely, SHANNON & WILSON

Walter J. Zitz Gregory R. Fischer, P.E., PhD Senior Geotechnical Staff President

BWV:WJZ/jma/azp

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CONTENTS

1 Introduction

2 Site and Project Description

3 Field Explorations and Laboratory Testing

4 Regional Geology and Subsurface Conditions

4.1 Regional Geology

4.2 Subsurface Conditions

4.2.1 Asphalt/Base Course

4.2.2 Overburden Soils

4.2.3 Bedrock

4.2.4 Groundwater

4.3 Subsurface Variation

5 Geologic Hazards

5.1 Corrosion Potential

5.2 Seismic Hazards

6 Geotechnical Recommendations

6.1 Seismic Ground Motion Design Parameters

6.2 Foundation Recommendations

6.2.1 Micropiles

6.2.1.1 Axial Resistance

6.2.1.2 Lateral Resistance

6.2.2 Shallow Spread Footings

6.2.2.1 Bearing Resistance

6.2.2.2 Lateral Resistance

6.3 Lateral Earth Pressures

6.4 Drainage

6.5 Permanent Slopes

7 Construction Considerations

7.1 Micropile Installation

7.2 Earthwork

7.2.1 Excavation Potential

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7.2.2 Foundation and Subgrade Preparation for Shallow

Foundations

7.2.3 Fill Placement and Compaction

7.3 Temporary Slopes

7.4 Dewatering

8 Document Review and Construction Observation

9 Limitations

10 References

Exhibits Exhibit 2-1: Site conditions at the Lodgepole Creek crossing in June 2025 Exhibit 2-2: Site conditions at the Lower Ninemile Creek crossing in May 2025 Exhibit 4-1: Summary of Pavement Section Thicknesses Exhibit 4-2: Summary of Groundwater Depths During Drilling Exhibit 5-1: Corrosion Testing Results Exhibit 5-2: Corrosivity Ratings Based on Water Soluble Sulfate Exposure Exhibit 6-1: Seismic Design Ground Motion Parameters Exhibit 6-2: Recommended Nominal Lateral Earth Pressure Parameters

Tables Table 1: Deep Foundation Design Parameters

Figures Figure 1: Vicinity Map Figure 2: Site and Exploration Plan Figure 3: Recommended Surcharge Loading for Temporary and Permanent Walls Figure 4: Recommended P-Multipliers for Horizontally Loaded Pile Groups

Appendices Appendix A: Subsurface Explorations Appendix B: Laboratory Test Results Important Information

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1 INTRODUCTION

This report summarizes subsurface conditions and provides geotechnical engineering recommendations for two new bridges along Forest Service Road (FSR) 6500 over Lodgepole Creek and Ninemile Creek (the Project) at the approximate locations shown on Figures 1 and 2. Our services were conducted in general accordance with our subconsultant agreement with DJ&A dated November 5, 2024.

Our scope of services included:

Coordinating a subsurface exploration program (which included observing and logging four geotechnical borings), clearing underground utilities, coordinating a drilling subcontractor, and measuring groundwater levels.

Performing geotechnical laboratory testing on selected samples retrieved from the borings.

Developing geotechnical recommendations for bridge foundations and associated wing walls.

Providing construction considerations, including earthwork, site grading, and foundation installation.

Preparing this Geotechnical Report.

The scope of our services did not include evaluating the presence of cultural resources or potentially contaminated soils at or around the site. If a service is not specifically indicated in this report, do not assume it was performed.

2 SITE AND PROJECT DESCRIPTION

The Project is located in the Wenatchee River Ranger District of the Okanogan-Wenatchee National Forest in Chelan County, Washington and includes the removal of two existing drainage culverts and replacement with single span bridges along FSR 6500 at the Lodgepole Creek crossing and the Lower Ninemile Creek crossing (see Figures 1 and 2).

The Upper Ninemile Creek crossing, along the FSR 6503-200 spur, is not included in the Project. FSR 6500 is a narrow, asphalt paved roadway which carries two-way traffic.

Exhibit 2-1 shows the site conditions at the existing Lodgepole Creek crossing at the time of our subsurface exploration and Exhibit 2-2 shows site conditions at the existing Lower Ninemile Creek crossing at the time of our site reconnaissance.

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Exhibit 2-1: Site conditions at the Lodgepole Creek crossing in June 2025 (view approximately east).

Exhibit 2-2: Site conditions at the Lower Ninemile Creek crossing in May 2025 (view approximately northeast).

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Based on information provided by the U.S. Forest Service (USFS), the existing culverts at the Lower Ninemile Creek crossing and the Lodgepole Creek crossing have recently been filling with sediment during spring runoff and major rainfall events, originating from burn scars uphill of the Project, attributed to 2022 forest fires. A debris flow in May 2023 completely buried the inlet of the corrugated metal pipe (CMP) culvert at the Lodgepole Creek crossing, causing drainage issues resulting in the roadway being overtopped and contributing to significant erosion of the roadway embankment (Exhibit 2-1), resulting in the closure of this section of FSR 6500.

The Lower Ninemile Creek crossing also experienced a debris flow in May 2023, depositing soil with cobbles and boulders material over the roadway and blocking the inlet and outlet of the existing bottomless arch culvert. We understand that the USFS was able to clear debris from the roadway and reestablish some surface water drainage via the roadside ditch and existing culvert, but the debris flow has continued with subsequent storms. We understand that the USFS maintains this portion of FSR 6500, clearing debris from the roadway after debris flow incidents. During our field exploration program, surface water was observed to flow from the Ninemile Creek, southwest through the roadside ditch to the next culvert below the roadway, and very little to no water was observed flowing from the downstream outlet of the culvert at the Ninemile Creek crossing. A bedrock outcrop was observed on the north side of the roadway near the proposed west abutment location. At the time of our field work, it was difficult to discern the original channel depths below the roadways due to surface disturbance (erosion and/or debris deposition); however, we estimate that the channel at the Ninemile Creek crossing was approximately 3 feet below the roadway, and the channel at the Lodgepole Creek crossing was approximately 8 feet below the roadway.

Based on bridge general layout sheets provided by DJ&A (DJ&A 2025a and 2025b), the Lodgepole Creek Bridge is expected to have a span length of approximately 112 feet and the Lower Ninemile Creek Bridge is expected to have a span length of approximately 80 feet, and these span lengths are intended to allow for debris flows to pass below the roadway.

The proposed maximum channel depths are indicated to be approximately 9.3 feet below the roadway grade at the Lower Ninemile Bridge, and approximately 23 feet below the roadway grade at the Lodgepole Bridge. Based on discussions with DJ&A, the bridges will be supported by micropiles or shallow foundations consisting of a precast concrete grade beam on a 6-inch thick geocell layer. Low height wingwalls may be included as part of the Project.

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3 FIELD EXPLORATIONS AND LABORATORY TESTING

Shannon & Wilson conducted a field exploration program to explore subsurface conditions at the Project site. The field exploration program consisted of drilling two borings at each structure; borings SW-B1-01 and SW-B1-02 were drilled at the Lodgepole Creek crossing and SW-B2-01 and SW-B2-02 were drilled at the Lower Ninemile Creek crossing. Boring locations are shown on Figure 2.

Appendix A describes the procedures used to complete the drilling and sampling of the geotechnical borings and presents the individual exploration logs and an explanation of the symbols and terminology used on the logs.

Shannon & Wilson completed geotechnical laboratory testing to determine properties of selected samples from the borings. The laboratory program included tests for natural water content, Atterberg limits, grain size distribution, percent fines, point load, and corrosion.

The laboratory test results are presented in Appendix B along with a brief discussion of the laboratory testing procedures. The natural water contents, Atterberg limits, percent fines and approximate unconfined compressive strength as determined by point load testing are also indicated on the individual boring logs in Appendix A.

4 REGIONAL GEOLOGY AND SUBSURFACE

CONDITIONS

4.1 Regional Geology

Based on a review of the geologic map of the Chelan quadrangle (Tabor and others, 1987) and the geologic map of the Skykomish River quadrangle (Tabor and others, 1993), surficial geology in the Project area consists of quaternary landslide deposits and alpine-glacier deposits overlying Late Cretaceous-aged (approximately 100.5 to 66 million years old) banded gneiss, schist, and amphibolite bedrock. Tabor and others describe the banded gneiss, schist, and amphibolite as interlayered heterogeneous light-colored tonalite to granodiorite gneiss, mica shist, and amphibolite.

4.2 Subsurface Conditions

The below sections present an overview of the subsurface conditions encountered in our borings at each site. Refer to the boring logs for additional detail.

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4.2.1 Asphalt/Base Course

The four borings were drilled through the existing roadway surface and encountered a composite pavement section consisting of asphalt pavement overlying a layer of aggregate base course material. The thickness of the pavement sections encountered is summarized in Exhibit 4-1.

Exhibit 4-1: Summary of Pavement Section Thicknesses

Boring No.

Asphalt Thickness (inches)

Base Course Thickness (inches)

SW-B1-01 2 6

SW-B1-02 2 6

SW-B2-01 3.5 6

SW-B2-02 2 6

NOTES:

Thicknesses were measured from hole annulus.

4.2.2 Overburden Soils

A layer of artificial fill consisting of silty sand with gravel (SM) with trace charcoal was encountered below the pavement section in boring SW-B2-01 and continued to a depth of approximately 4 feet below the ground surface (bgs). Below the pavement section and fill, all borings (excluding boring SW-B2-02) encountered colluvium consisting of silty sand with gravel and cobbles (SM); silty gravel with sand and cobbles (GM); and poorly graded gravel with silt sand and cobbles (GP-GM). Colluvium extended to depths ranging from 18.5 to

46.5 feet. A boulder was encountered in boring SW-B1-01 from a depth of approximately 2 to 5 feet. Glacial deposits consisting of outwash and till were encountered in the borings underlying the colluvium in all of the borings excluding boring SW-B2-02. Outwash was differentiated from the till based on grain size. Outwash consisted of silty sand (SM) with trace gravel and extended to depths ranging from 19.5 to 48.0 feet. Till was found underlying the outwash and extended to depths ranging from 32.0 to the termination of the boring at the Lodgepole Creek Bridge and to the top of rock at the east abutment of the Lower Ninemile Creek Bridge. Till consisted of silty sand with varying amounts of gravel and cobbles (SM); clayey sand with gravel and trace cobbles (SC); or poorly graded sand with silt and gravel (SP-SM). AASHTO soil classifications for the soil samples tested ranged from A-1-b(0) to A-4(0). Point load tests were conducted on gravel and cobble sized material recovered from sonic core samples of the overburden soils at the Lodgepole Creek Bridge, which indicated approximate compressive strength values ranging from 2,900 to 37,200 pounds per square inch (psi).

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4.2.3 Bedrock

Bedrock was encountered in borings SW-B2-01 and SW-B2-02 at depths of 32 and 0.7 feet bgs, respectively, and consisted of extremely weak to very weak micaceous schist banded with 6-inch to 2-foot-thick, strong to very strong marble layers. The upper 11 feet of the bedrock in boring SW-B2-01 and the upper 23 feet of bedrock in boring SW-B2-B2 was completely weathered. As noted on the boring logs, the degree of bedrock weathering was variable, but generally decreased with depth. Photographs of the bedrock sonic core samples from boring SW-B2-01 are included in Appendix A.

Point load tests were conducted on selected samples that were too small for unconfined compression strength (UCS) testing. The point load test results are shown in Table B-2 in Appendix B, and average values are indicated on the boring logs in Appendix A. The point load test results indicate approximate compressive strength values from bedrock ranging from 400 to 35,500 psi.

4.2.4 Groundwater

As indicated in Exhibit 4-2, where encountered, groundwater was measured during drilling at depths between 9 and 25 feet. Groundwater encountered in boring SW-B1-02 was sealed off during drilling, resulting in a dry borehole at the termination of the boring at 71.5 feet.

Groundwater measurements were recorded using a drop tape and are also noted on the boring logs included in Appendix A.

Exhibit 4-2: Summary of Groundwater Depths During Drilling

Boring No.

Groundwater Depth (feet)

Groundwater Elevation1

(feet) Measurement

Date

SW-B1-01 NE NA 6/11/2025

SW-B1-02 25 2,258 6/10/2025

SW-B2-01 9 2,288 6/13/2025

SW-B2-02 NE NA 6/12/2025

NOTES:

Based on ground surface elevations provided by DJ&A.

NE = Not Encountered, NA = Not Applicable

Groundwater fluctuations are possible and will depend on many factors such as seasonal variations, local precipitation, water levels in Ninemile Creek and Lodgepole creek, and runoff.

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4.3 Subsurface Variation

Our observations are specific to the locations, depths, and times noted on the logs and may not be applicable to all areas of the site. No amount of explorations or testing can precisely predict the characteristics, quality, or distribution of subsurface and site conditions.

Potential variation includes, but is not limited to:

The conditions between explorations may be different.

The passage of time or intervening causes (natural and human-induced) may result in changes to site and subsurface conditions.

Penetration test results in gravelly soils may be unrealistic. Actual soil density may be lower than estimated if the test was performed on a gravel or cobble.

If conditions that are different from those described herein are encountered during construction, we should be retained to review our description of the subsurface conditions and reconsider our conclusions and recommendations.

5 GEOLOGIC HAZARDS

5.1 Corrosion Potential

To assist in estimating the corrosion potential at the site, selected soil samples were tested for pH, resistivity, water soluble sulfates, and chlorides. The results are presented in Table B-1 in Appendix B and in Exhibit 5-1.

Exhibit 5-1: Corrosion Testing Results

Boring Sample USCS pH Resistivity (ohm-cm)

Water-Soluble Sulfates

Water-Soluble Chlorides

SW-B1-01 S-3 SM 8.0 6,800 0.11 0.079

SW-B2-02 S-4 SC 7.0 >100,000 <0.01 0.052

NOTES:

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

ohm-cm = ohm-centimeter, USCS = Unified Soil Classification System (see Log Key in Appendix A for definitions).

Soils with resistivity values less than 2,000 ohm-cm are considered corrosive for pile foundations per AASHTO (2020). The resistivity measured in the samples was 6,800 and >100,000 ohm-centimeters (ohm-cm).

The concentration of water-soluble sulfates measured in the samples was less than 0.01% and 0.11% by weight. Based on classifications as defined by the American Concrete Institute

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(ACI) 318-21 (ACI, 2021), as shown in Exhibit 5-2, these test results suggest a negligible to moderate exposure of sulfate attack for concrete exposed to the onsite soils (exposure class S0 to S1).

Exhibit 5-2: Corrosivity Ratings Based on Water Soluble Sulfate Exposure

Water Soluble Sulfate in Soil (Percent by Weight)

Sulfate Severity (Class)

<0.10 Negligible (S0)

0.10 - 0.20 Moderate (S1)

0.20 - 2.00 Severe (S2)

>2.00 Very Severe (S3)

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.

5.2 Seismic Hazards

Based on a geologic map by the United States Geological Survey (USGS), the nearest middle to late Quaternary Age fault to the Project area is the Straight Creek Fault, located approximately 13 miles west of the Project (Lidke, 2009). Based on the location and lack of recent movement of the Straight Creek Fault, it is our opinion that the potential for ground surface fault rupture at the Project site is low.

Liquefaction can occur in loose, saturated, cohesionless soils when subjected to earthquake ground shaking. The typical groundwater level at the proposed bridge locations is expected to coincide with the levels of Lodgepole and Ninemile Creeks. Based on the subsurface conditions encountered in the borings, which were drilled in close proximity to the proposed bridge abutment locations, groundwater was generally within medium dense to very dense silty sand with gravel and cobbles; therefore, it is our opinion that the risk of liquefaction is low.

6 GEOTECHNICAL RECOMMENDATIONS

6.1 Seismic Ground Motion Design Parameters

Using the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (AASHTO, 2020) criteria, and based on subsurface conditions encountered in our borings, the Project site classifies as Site Class C or D, depending on the boring

113461-001 December 2025 considered. We recommend conservatively assuming Site Class D conditions. If the seismic design is found to significantly affect the design, geophysical testing could be completed at the site to better characterize the seismic site class.

Ground motion parameters were determined for the Project site using the USGS U.S.

Seismic Design Map Web Application (USGS, 2025) and procedures recommended by AASHTO (2020). Recommended seismic design ground motion parameters are summarized in Exhibit 6-1.

Exhibit 6-1: Seismic Design Ground Motion Parameters

Value

Design Parameter Lodgepole Creek

Bridge Lower Ninemile

Creek Bridge

Peak Ground Acceleration (PGAB)1 0.224 g 0.220 g

Site Class D D

Short-period Spectral Acceleration, SS 0.514 g 0.504 g

Long-period Spectral Acceleration, S1 0.170 g 0.167 g

Site Factor, FPGA 1.352 1.361

Site Factor, Fa 1.389 1.397

Site Factor, FV 2.121 2.132

Peak Design Spectral Acceleration, AS 0.303 g 0.299 g

Short-period Design Spectral Acceleration, SDS 0.714 g 0.704 g

Long-period Design Spectral Acceleration, SD1 0.360 g 0.356 g

TO 0.101 sec. 0.101 sec.

TS 0.505 sec. 0.506 sec.

Seismic Zone2 3 3

NOTES:

PGAB = peak ground acceleration for a site underlain by Site Class B soil (very dense soil and soft rock).

Seismic Zone from AASHTO (2020) Table 3.10.6-1.

g = gravitational acceleration; PGA = peak ground acceleration, sec = second

6.2 Foundation Recommendations

The following sections provide geotechnical recommendations for the Project. As discussed in Section 2.0, we understand the proposed bridges will consist of single-span bridges with span lengths of approximately 105 feet at the Lodgepole Creek crossing and 80 feet at the Lower Ninemile Creek crossing. The following recommendations are based on preliminary information provided by DJ&A. Our recommendations may need to be updated as the design progresses. We have considered deep foundations consisting of micropiles and shallow foundations consisting of a pre-cast concrete grade beam placed on a 6-inch-thick

113461-001 December 2025 geocell layer, below the design scour depth, with a minimum horizontal distance from the front face of the grade beam to the channel slope of approximately 8 feet at both the Lodgepole Creek Bridge and Lower Ninemile Creek Bridge.

Driven H-piles were also considered as a deep foundation alternative; however, H-piles may have difficulty being driven to the target depth due to the presence of cobbles and boulders in the overlying colluvium at the Lodgepole Creek Bridge. This may require predrilling or coring an oversized hole to install H-piles to the target depth. Due to the added cost and uncertainty associated with predrilling for H-pile installation, H-piles were not considered the preferred deep foundation alternative.

6.2.1 Micropiles

6.2.1.1 Axial Resistance

The following summarizes our recommendations for the geotechnical design of micropiles to support the abutments. Structural design of the micropiles will be completed by others.

Micropiles should be designed in accordance with the FHWA Micropile Design and Construction Manual (Sabatini and others, 2005) and AASHTO (2020). Additional design recommendations for micropiles at the Lodgepole Creek and Lower Ninemile Creek Bridges are provided below:

Permanent casing should be included within the upper 10 feet of piles below the pile cap. The length of the permanent casing may need to be increased for structural design considerations.

Micropiles should be designed to develop axial resistance in the medium dense to very dense granular soils at the Lodgepole Creek Bridge. Micropiles at the Lower Ninemile Creek Bridge should be designed to develop axial resistance in the weathered bedrock and schist.

Axial resistance should be ignored along the cased section of the micropile.

The axial capacity of the micropiles can be designed based on the nominal grout bond strengths provided in Table 1. The Contractor should be responsible for determining the appropriate construction method to achieve these bond strengths.

The minimum bond length (below the bottom of casing) should be 10 feet.

AASHTO (2020) specifies a minimum center-to-center spacing of 30 inches or 3 pile diameters, whichever is greater.

AASHTO (2020) recommends a strength limit resistance factor of 0.55, an extreme event resistance factor of 1.0 for compression and 0.80 for uplift, and a service limit resistance factor of 1.0 on the bond strength. A strength limit resistance factor of 0.7 for nominal bond strength in compression and uplift may be used if sacrificial (verification) load testing is completed at each bridge, as discussed below.

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Corrosion protection is required for the reinforcing center bar according to the FHWA Micropile Design and Construction Manual (Sabatini and others, 2005). The corrosion protection should include grout cover. In addition, because of the permanent nature of the micropiles, we recommend one of the following: epoxy coating, galvanization, or encapsulation.

Corrosion protection for permanent casing should include a sacrificial steel section loss of 0.063 inches, in accordance with the FHWA Micropile Design and Construction Manual (Sabatini and others, 2005).

Total settlement of individual micropiles is anticipated to be less than ½ inch.

Differential settlement between adjacent micropiles would be expected to be about 50 to 75% of this value. This assumes the micropiles are constructed using good installation techniques and equipment based on criteria described in the FHWA Micropile Design and Construction Manual (Sabatini and others, 2005).

To verify the assumed bond strength used for design, one sacrificial (verification) load test should be completed at each bridge in accordance with the FHWA Micropile Design and Construction Manual (Sabatini and others, 2005). The verification load test can be conducted in either tension or compression. The micropiles used for the verification tests should be loaded to at least 2.0 times the design load.

At least one production proof tests should be completed at each substructure unit. The micropiles used for proof testing should be loaded to 1.6 times the design load. The micropiles tested may be incorporated as production micropiles, provided the proof tests satisfy the testing requirements.

6.2.1.2 Lateral Resistance

To determine the lateral load capacity of an individual micropile, and the deflection, shear, and moment along the pile, we have provided input parameters for the commercial software LPILE by Ensoft, Inc. (2022). LRFD design methods for lateral loading of piles utilize a resistance factor of 1.0 and are controlled by allowable deflection criteria.

Geotechnical parameters required for input into LPILE for an individual pile are presented in Table 1. If groups of piles are used, the effects of group interaction should be considered when evaluating the horizontal load-deflection behavior. When using the P-Y method of analysis, the values of P (load) should be factored appropriately to account for the group effects. Recommended values of P-multipliers for loading perpendicular and parallel to a line of piles are provided on Figure 4.

Because of the relatively small diameter of the micropiles, the lateral resistance is relatively minor and battered piles may be required to provide lateral resistance. If battered piles are utilized, the structural designer should confirm that the stiffer lateral response generated by battered piles is acceptable.

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6.2.2 Shallow Spread Footings

6.2.2.1 Bearing Resistance

For shallow foundations supported by a 6-inch-thick geocell mat, overlying medium dense to very dense sand and gravel or completely weathered bedrock, we recommend a nominal bearing resistance of 9,300 pounds per square foot (psf) for the Lodgepole Creek Bridge, and a nominal bearing resistance of 9,800 psf for the Lower Ninemile Creek Bridge. For these nominal values, we recommend the upper 2 feet of the foundation subgrade below the geocell mat be removed and replaced with Granular Fill, meeting the requirements in Section 7.2.3. The recommended nominal bearing resistance values have been reduced to consider the proximity to the adjacent channel slopes. A resistance factor of 0.45 should be applied to the nominal bearing resistance values. Based on the recommended bearing pressures above, we estimate foundation settlements would be on the order of 1 inch with differential settlement of about 1 inch between the bridge abutments. We recommend a minimum footing width of 3 feet and a minimum embedment distance of 1.5 feet from the closest graded surface.

6.2.2.2 Lateral Resistance

Lateral loads can be resisted by friction along the base of a shallow foundation. Precast footings may be designed using a nominal coefficient of sliding resistance of 0.8 and a resistance factor of 0.9 should be applied to the nominal value.

Passive earth pressure can also be provided. A resistance factor of 0.5 should be applied to the nominal passive resistance value provided in Exhibit 6-2. Because of the potential for scour, frost, or other disturbance of near-surface soils, passive resistance should be ignored in the upper 30 inches; however, the equivalent fluid pressure line may be project down from the ground surface. Because of the proximity of the foundation to the creek side slopes, we do not recommend using passive earth pressure in the direction toward the creek.

6.3 Lateral Earth Pressures

The lateral earth pressures acting on structure walls depend on many factors, including surcharge loads, the soil type/consistency, and whether the top of the wall is free to 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), the soil pressures will generally be less (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 structures. Our static earth pressure recommendations

113461-001 December 2025 are presented in Exhibit 6-2. The static lateral earth pressures assume the structure walls are backfilled with properly compacted Granular Fill material in a 1H:1V zone extending upward from the base of the structure from a point 1.5 feet behind the edge of the foundation. These pressures also assume a vertical wall face and a flat backslope. Clay, cobbles, and boulders are not suitable for reuse as backfill. We anticipate that the onsite sand and gravel would be suitable for reuse as backfill for structures if cobbles and boulders encountered are screened out.

Seismic active lateral earth pressure coefficients were determined based on the Mononabe- Okabe method (AASHTO, 2020), and seismic at-rest earth pressure coefficients were determined based on methods developed by Zhang and others (1998). Per AASHTO (2020), the pressure distribution shape for dynamic active earth pressures is triangular, with the resultant force (PAE) acting at the lower third of the wall height. The parameters are provided in Exhibit 6-2.

Exhibit 6-2: Recommended Nominal Lateral Earth Pressure Parameters

Nominal Equivalent Fluid Pressure1 (pcf)

Design Parameter Coefficient Above

Groundwater Below

Groundwater

Static At-Rest Earth Pressure Ko = 0.44 60 942

Static Active Earth Pressure Ka = 0.28 38 832

Static Passive Earth Pressure Kp = 3.7 505 331

Seismic At-Rest Earth Pressure Koe = 0.66 89 1102

Seismic Active Earth Pressure Kae = 0.50 67 982

NOTE:

Based on granular soil with an assumed moist unit weight of 135 pcf and a friction angle of 34 degrees for the retained soil.

Includes the combined hydrostatic pressure and equivalent fluid pressure.

pcf = pounds per cubic foot

Surcharge loads, such as traffic and construction equipment, will induce lateral loads on walls. The pressures provided above do not account for surcharge loads. Lateral loads due to various types of surcharges may be calculated by using the loading diagrams provided in Figure 3 and the earth pressure coefficient indicated above in Exhibit 6-2.

6.4 Drainage

Fluctuations of water levels in Lodgepole and Ninemile Creeks could potentially result in differential hydrostatic water pressures in wall backfill (i.e., water levels in the backfill may rise with the water level in the creek and then remain elevated after the creek level decreases, depending on wall drainage measures and the duration of elevated water levels

113461-001 December 2025 in the creek). The earth pressure parameters provided in Exhibit 6-2 assume that unbalanced hydrostatic pressures do not develop in the retained zone behind walls.

In general, materials with greater than about 3% fines content are not considered free draining. Granular Fill (Section 7.2.3) may not be free draining; therefore, a rapid drawdown condition could occur following flood events, i.e., water could be trapped in the backfill after flood waters recede. This condition could be addressed by designing the wall for appropriate water pressures or by including internal drainage features to reduce the potential for water to accumulate in the backfill. Appropriate drainage features could include:

Placement of a 12-inch-thick drainage layer behind the walls with a suitable discharge system (e.g., weep holes or a perforated collector pipe, daylighting to a suitable discharge point). Gravel backfill within the drainage layer should meet the requirements of Division 9-03.12(4) Gravel Backfill for Drains of the Washington State Department of Transportation (WSDOT) Standard Specifications for Road, Bridge, and Municipal Construction M41-10, 2025 (WSDOT, 2025).

Limiting the maximum fines content of the Granular Fill to 3%.

6.5 Permanent Slopes

Based on the general layout sheet for the Lodgepole Creek Bridge (DJ&A, 2025b), we anticipate a maximum cut depth of approximately 23 feet for the Lodgepole Creek channel at the bridge crossing, with permanent channel slopes of 1.5H:1V (horizontal to vertical) with riprap slope protection along the lower 10 feet of the channel slopes, and 2H:1V for the upper portion of the channel slope, with no riprap slope protection.

We recommend that all proposed slopes be protected from erosion; therefore, above the riprap, the soil slope should be seeded and/or planted with relatively fast-growing vegetation to reduce the potential for surface sloughing and erosion. Natural or geosynthetic erosion control blankets may be necessary to assist in erosion protection during vegetation growth.

7 CONSTRUCTION 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 and may result in reduced performance. Based on discussions with DJ&A, we understand the Federal Standard Specifications FP-24 (FHWA, 2024) will be used on this project. The following sections present additional construction and material considerations for this project.

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7.1 Micropile Installation

Micropiles should be constructed in accordance with our recommendations, the guidelines in Section 567 of FP-24 (FHWA, 2024), and the FHWA Reference Manual, “Micropile Design and Construction” (Sabatini and others, 2005).

The drilling and grouting process should be relatively continuous with minimal stoppage of work between the completion of drilling and cleaning of hole and placement of grout after setting the reinforcing bar, and under no circumstances after the day of drilling. We anticipate that the micropiles will be constructed below the groundwater table and that the grout should be placed using a tremie tube. Due to the presence of granular soils and the presence of water in the subsurface, the Contractor should be prepared to encounter flowing conditions in the overburden.

We recommend that micropile contractors who participate on this project be required to demonstrate that they have suitable equipment for this project and adequate experience in construction with similar subsurface conditions.

7.2 Earthwork

Earthwork, including site preparation, embankment construction, placement of fill, and subgrade preparation, should conform to the requirements provided in Division 200 of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-24 (2024).

7.2.1 Excavation Potential

We anticipate that excavation for the proposed construction will occur in the existing fill and overburden soils. We anticipate that excavation of the overburden soil and weathered bedrock can be accomplished using conventional excavating equipment, such as a dozers, rubber-tired backhoes or tracked hydraulic excavators; however, progress will be slower if bands of higher strength bedrock are encountered in the excavation. Boulders and cobbles may also be encountered within the overburden soils, and may be difficult to remove with conventional equipment depending on boulder size.

7.2.2 Foundation and Subgrade Preparation for Shallow Foundations

Proper subgrade preparation is required for adequate performance of shallow foundations.

If shallow foundations are used, subgrade excavations should be cleaned of all artificial fill, debris, or loose/soft material; approved by visual observation by a qualified geotechnical engineer prior to fill placement; and kept free of water at all times. Boring SW-B2-01 at the west abutment of the Lower Ninemile Creek bridge, encountered a layer of artificial fill

113461-001 December 2025 extending to 4 feet bgs, so we recommend over-excavating down to the medium dense to very dense native sands layer and backfilling with compacted Granular Fill to the base of the geocell layer. To provide more uniform foundation support, we recommend that this overexcavation and replacement be conducted at the east abutment as well, where completely weathered bedrock was encountered below the pavement section.

Prior to placing new fill materials, exposed subgrades should be stripped, scarified to a depth of 12 inches, moisture conditioned, and compacted in place to a dense and unyielding condition per the requirements described in Section 204 and 208 of FP-24 (2024). The compacted surface should be proof-rolled with a fully loaded, tandem-axle, 10-yard dump truck or equivalent. If it is not possible to perform a proof-roll, the subgrade should be probed and accepted by an experienced geotechnical engineer or materials testing technician. Areas that are identified as being loose, soft, or yielding during proof-rolling and/or probing should be removed, reconditioned and recompacted. Care should be taken during proof-rolling and subgrade preparation to avoid disturbing subgrade soils and supporting soils that will remain in place, as they can rut and pump under repeated construction traffic.

7.2.3 Fill Placement and Compaction

Granular Fill should consist of native or imported well-graded sand and gravel with less than 20% fines (material passing the U.S. Standard No. 200 sieve),a maximum particle size of 3 inches, and a maximum plasticity index (PI) of 10. Based on our laboratory testing, the onsite soils test results provided in Appendix B, suitable granular fill may be found onsite.

Fill materials should be placed in horizontal lifts, with the loose lift thickness not to exceed 12 inches for heavy equipment compactors and 6 inches for hand-operated compactors.

Compaction of backfill adjacent to walls can result in higher lateral earth pressures against the wall. As such, small equipment that is hand-operated should only be used within a line extending upward from the base of the structure at 0.5H:1V, or at least 3 feet from the wall, whichever is greater. Unless the structural engineer indicates differently, we recommend that backfill around structures be brought up in uniform horizontal layers and not be unbalanced around the structure.

All fill materials should be compacted to a dense/firm and unyielding condition. Granular Fill should be compacted to a minimum of 95% of its Modified Proctor maximum dry density (AASHTO T180). The moisture content of Granular Fill should be within ± 3% of the optimum moisture content.

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7.3 Temporary Slopes

We anticipate that the roadway will be closed to through traffic during construction and temporary excavations will be made to construct the Project (i.e., shoring will not be required). Temporary excavations should be sloped, as needed, to provide a safe, stable slope. Consistent with conventional construction practice, the Contractor should be responsible for temporary excavation slopes.

We recommend the temporary slopes be consistent with the Occupational Safety & Health Administration (OSHA) guidelines contained in 29 CFR 1926, Subpart P (1989). For cost estimating and planning purposes only, we recommend assuming temporary excavations above groundwater level are sloped at 1.5H:1V in granular soils, consistent with OSHA Type C soils. If groundwater is actively seeping into the excavation, flatter slopes will be required. The Contractor should continually classify the soils that are encountered as excavation progresses with respect to the OSHA system.

Consistent with conventional practice, the contract documents should require the Contractor to be responsible for the actual temporary excavation slopes, including methods, sequence, and schedule of construction. The Contractor is able to observe the nature and conditions of the subsurface materials encountered and should evaluate the factors discussed above. If instability is observed, slopes should be flattened or shored. All excavations should be accomplished in accordance with local, state, and federal safety regulations.

7.4 Dewatering

Based on the groundwater levels encountered in our explorations, excavations for foundations, abutments, and wing walls are not expected to extend below groundwater.

However, for excavations that extend near or below the Lodgepole and Ninemile Creek channel levels, the presence of groundwater may cause instability of open-cut excavations and subgrades. Groundwater levels will likely be influenced by the stages and seasonal changes of Lodgepole and Ninemile Creeks. The Contractor should consider these seasonal conditions in planning the work and the groundwater control measures.

We recommend that the groundwater level be kept a minimum of 3 feet below the bottom of the excavation elevation to reduce the potential for disturbance to the subgrade. The groundwater level should be drawn down prior to excavating and should be maintained in a dewatered state until the abutments and wing walls are constructed and backfilled.

Consistent with typical practice, the Contractor will be responsible for control of surface water and groundwater during construction, including the design of dewatering and

113461-001 December 2025 diversion features. In this regard, slope protection, ditching, sumps, dewatering wells, diversions, and other measures should be employed as necessary, to direct water away from excavations, to prevent ponding of water next to the work zone, and to permit completion of the work.

8 DOCUMENT REVIEW AND CONSTRUCTION

OBSERVATION

We recommend that we be retained to review the geotechnical portions of the plans and specifications to determine if they are consistent with our recommendations. In addition, because geotechnical design recommendations are developed from a limited number of explorations and tests, recommendations may need to be adjusted in the field. To this extent, we recommend that Shannon & Wilson be retained to monitor the geotechnical aspects of construction, particularly foundation installation, subgrade preparation, fill placement, and compaction. This monitoring would allow us to confirm that conditions encountered are consistent with those indicated by the explorations and provide expedient recommendations should conditions be revealed during construction that are different from those anticipated.

9 LIMITATIONS

This report was prepared for the exclusive use of DJ&A and the USFS for use in design of the Lodgepole Creek and Lower Ninemile Creek Bridge Designs project. It should be made available to prospective contractors and/or the Contractor for information on factual data only, and not as a warranty of subsurface conditions.

This report should not be used without our approval if any of the following occurs:

Conditions change due to natural forces or human activity under, at, or adjacent to the site.

Assumptions stated in this report have changed.

Project details change or new information becomes available such that our analyses, conclusions, and recommendations may be affected.

If the site ownership or land use has changed.

More than 5 years has passed since the date of this report.

If any of these occur, we should be retained to review the applicability of our analyses, conclusions, and recommendations.

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Unanticipated soil conditions are commonly encountered and cannot be fully determined by a limited subsurface exploration and laboratory testing program. Such unexpected conditions frequently require that additional expenditures be made to attain a properly constructed project. Therefore, some contingency fund is recommended to accommodate such potential extra costs.

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

Shannon & Wilson has prepared the attached document, "Important Information about Your Geotechnical Report," to assist you and others in understanding the use and limitations of our reports.

113461-001 December 2025

10 REFERENCES

American Association of State Highway and Transportation Officials. (2020). AASHTO

LRFD bridge design specifications, customary U.S. units (9th ed.). American Association of State Highway and Transportation Officials.

American Concrete Institute. (2021). Building code requirements for structural concrete and commentary (ACI 318-21) Farmington Hills, Michigan. American Concrete Institute.

DJ&A. (2025a, March). DJ&A. (2025b, August). Bridge general layout, Lodgepole Creek Bridge design, RD 6500000 MP 8.63, Structure No. 061707000037203, Okanogan-Wenatchee National Forest, Wenatchee River Ranger District, drawing sheet No. 6 (Sheet 6 of 21).

Ensoft, Inc. (2022). User’s manual for LPILE 2022: A program to analyze deep foundations under lateral loading. Ensoft, Inc.

Lidke, D. J. (Compiler). (2009). Fault No. 553, Straight Creek Fault. In Quaternary fault and fold database of the United States. U.S. Geological Survey.

https://earthquake.usgs.gov/hazards/qfaults/

Occupational Safety and Health Administration. (1989). 29 CFR Part 1926, Subpart P:

Excavations. U.S. Department of Labor.

Sabatini, P. J., et al. (2005). Micropile design and construction (FHWA NHI-05-039, Course No.

1320789). Federal Highway Administration; National Highway Institute.

Tabor, R. W., Frizzell, V. A., Whetten, J. T., Waitt, R. B., Swanson, D. A., Byerly, G. R., Booth, D. B., Hetherington, M. J., & Zartman, R. E. (1987). Geologic map of the Chelan 30-minute by 60-minute quadrangle, Washington (Miscellaneous Investigations Series Map I- 1661, scale 1:100,000). U.S. Geological Survey.

Tabor, R. W., Frizzell, V. A., Booth, D. B., Waitt, R. B., Whetten, J. T., & Zartman, R. E. (1993).

Geologic map of the Skykomish River 30 × 60 minute quadrangle, Washington (Miscellaneous Investigations Series Map I-1963, scale 1:100,000). U.S. Geological Survey.

Federal Highway Administration. (2024). Federal standard specifications for construction of roads and bridges on Federal Highway projects (FP-24 English Units). U.S. Department of Transportation and Federal Highway Administration.

http://flh.fhwa.dot.gov/resources/pse/specs/.

United States Geological Survey. (2025). U.S. seismic design maps web application. U.S.

Geological Survey. https://earthquake.usgs.gov/ws/designmaps/ https://earthquake.usgs.gov/hazards/qfaults/ http://flh.fhwa.dot.gov/resources/pse/specs/ https://earthquake.usgs.gov/ws/designmaps/

113461-001 December 2025

Washington State Department of Transportation. (2025). Standard specifications for road, bridge, and municipal construction (M 41-10). Washington State Department of Transportation. https://wsdot.wa.gov/publications/manuals/fulltext/M41- 10/SS2025.pdf

Zhang, J.-M., Shamato, Y., & Tokimatsu, K. (1998). Seismic earth pressure theory for retaining walls under any lateral displacement. Soils and Foundations, 38(2), 143–

163. https://doi.org/10.3208/sandf.38.2_143 https://wsdot.wa.gov/publications/manuals/fulltext/M41-10/SS2025.pdf https://wsdot.wa.gov/publications/manuals/fulltext/M41-10/SS2025.pdf

USFS Lodgepole and Ninemile Bridge Designs

Table 1 - Deep Foundation Design Parameters p-y Curve Effective Unit

Weight γ

Peak Friction Angle ϕ'

Undrained Cohesion su

Top Bottom (ksf) (pcf) (deg) (psf) 2,283 2,266 125 37 -

2,266 2,212 63 37

2,297 2,294 125 35 -

2,294 2,277 63 35 -

2,277 2,265 Dense to very dense, silty SAND with gravel - Sand (Reese) 63 37 -

2,265 2,254 Medium dense to dense, clayey SAND (Residuum) 2.5 Sand (Reese) 73 35 -

2,254 2,227 SCHIST: extremely weak to very weak, slightly weathered to fresh 8.0 Stiff Clay w/o free water 78 - 4,000

2,296 2,294 130 35 -

2,294 2,273 68 35 -

2,273 2,258 SCHIST: extremely weak to very weak, slightly weathered to fresh 8.0 Stiff Clay w/o free water 78 - 6,000

NOTES:

6 Adjustments to the design parameters may be required to account for elevation changes due to final grading, for the inclusion of additional materials at the abutments, or scour.

Structure Elevation

(feet)

3 Interpreted groundwater elevation was rounded up to the nearest foot corresponding to the Lodgepole Creek and Ninemile Creeks 100-year water surface elevations.

Location and Representative

Borings Micropile1,2

Nominal Side Resistance

LPILE Parameters4,5,6

Generalized Soil/Rock Description

1 The factored micropile side resistance should be determined by multiplying the nominal side resistance by the bearing material side resistance surface area (below the casing), and the specified resistance factor. Axial resistance in the overburden has been ignored at the east abutment of the Lower Ninemile Bridge due to strain compatibility between the overburden soils and bedrock.

East Abutment (based on SW-B2-01)

West Abutment (based on SW-B2-02) deg = degrees; ksf = kips per square foot; pcf = pounds per cubic foot; psf = pounds per square foot

2 The nominal side resistance values are based on a single pile and do not consider group action of closely spaced piles (closer than 30 inches or 3 daimeters, center-to-center).

4 The LPILE parameters are for a horizontal ground surface on the side of the foundation resisting lateral loading. Sloping ground surface modifications should be included as per Ensoft, Inc.'s recommendations for the LPILE program as necessary.

5 We recommend utilizing…