Geotechnical Investigation DRAFT V2.0

State Route 28 Central

Corridor Trail Phase 1 Sand Harbor to Thunderbird Cove

Washoe County, Nevada

January 23, 2026

Prepared for

AtkinsRéalis

Prepared By

Corestone Engineering, Inc.

Mr. Brian Janes, P.E.

10509 Professional Circle, Suite 103

Reno, NV 89521

January 23, 2026

Project No.: 5015-03-1

Corestone Engineering, Inc. 10751 Grayslake Dr, Reno, NV 89521

775-636-5916 Email: vimal@corestoneengineering.com

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DRAFT

RE: Geotechnical Investigation DRAFT V2.0

State Route 28 Central Corridor Trail Phase 1

Sand Harbor to Thunderbird Cove, Washoe County, Nevada

Dear Mr. Janes:

Corestone Engineering, Inc. is pleased to present the results of our geotechnical investigation for the above-referenced project. Our investigation consisted of research, field exploration, laboratory testing, and engineering analysis to allow formulation of geotechnical conclusions and recommendations for design and construction of the proposed shared use trail south of Sand Harbor along State Route (SR) 28 in Washoe

County, Nevada. Only preliminary design information was available at the time of this draft geotechnical report. Therefore, only preliminary geotechnical design recommendations and construction considerations are included in this report. Specific geotechnical design recommendations and associated construction considerations will be included in the final geotechnical report.

The proposed approximately 2-mile-long shared use trail for utilization by pedestrians and bicyclists will traverse on the west side of south side SR 28 between Sand Harbor beach and Thunderbird Cove in

Washoe County, Nevada. Due to existing steep grades along the trail, grading will require several hundred feet of retaining walls to support both cuts and fills.

The proposed trail alignment site generally includes a near surface sandy soil layer underlain by granitic bedrock that exhibit variable degrees of weathering and consistency. Onsite materials will provide adequate support trail improvements in cuts and will also be suitable to reuse as fills. The construction of retaining structures to establish design grades for the shared use trail will be difficult due to existing steep topography and subsurface materials consisting of bedrock and large granitic boulders.

We appreciate having the opportunity to work with you on this project. If you have any questions regarding the content of the attached report, please do not hesitate to contact us.

Sincerely, Corestone Engineering, Inc

Vimal P. Vimalaraj, P.E., G.E.

President/Principal Geotechnical Engineer PV:YK:PV:pv/lkv

Copies to: Addressee (PDF)

Table of Contents

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1.0 Introduction

2.0 Project Description

2.1 Project Location

2.2 Proposed Project Details

2.3 Design Standards

3.0 Site Conditions

4.0 Field Investigation

4.1 Geotechnical Reconnaissance

4.2 Boring Exploration

4.3 Materials Classification

4.4 Geophysical Testing

5.0 Laboratory Testing

5.1 Index Tests

5.2 Direct Shear Tests

5.3 R-Value Tests

5.4 Unconfined Compressive Strength Tests

5.5 Chemical Tests

6.0 Site Geology and Subsurface Conditions

6.1 Regional Geology and Seismicity

6.2 Site Geology

6.3 Subsurface Material Conditions

6.4 Groundwater

7.0 Geologic Hazards

7.1 Seismicity and Faults

7.2 Fault Rupture

7.3 Ground Motion

7.4 Liquefaction

7.5 Flood Plain

7.6 Other Geologic Hazards

8.0 Preliminary Design Recommendations

8.1 General Information

8.2 Seismic Design Criteria

8.3 Soil Nail Walls

8.4 MSE Retaining Walls

8.5 Underpass Ancillary Structures

Table of Contents

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8.6 Micropile Foundations

8.7 Structural Section for Shared Use Trail

8.8 Stability of Cut and Fill Slopes

8.9 Corrosion Evaluation

9.0 Construction Considerations

9.1 Site Preparation and Stabilization

9.2 Excavation Characteristics and Temporary Excavations

9.3 Fill Placement and Compaction

9.4 Shrinkage and Bulking

9.5 Erosion Control

10.0 Quality Control

11.0 Standard Limitations

12.0 References

Tables

1 - Laboratory Test Results Summary

2 - Rock Core Compressive Strength Test Results

3 - Seismic Design Criteria Site Parameters

4 - Design Ground Motion Parameters

5 - Soil Nail Walls Preliminary Design Parameters

6 - MSE Retaining Walls Preliminary Design Parameters

7 - CIP Retaining Walls Preliminary Design Parameters

8 - Corrosion Test Results Summary

Appendices

A-1 - Exploration Location Map

A-2 - Boring Logs

A-3 - USCS Soil Classification Chart

A-4 - Rock Core Photo Log

A-5 - Geophysical Test Modeling Results

B-1 - Index Test Results

B-2 - Direct Shear Test Results

B-3 - R-Value Test Results

B-4 - Direct Shear Test Results

B-3 - Chemical Test Results

Introduction

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1.0 Introduction

Presented herein are the results of Corestone Engineering, Inc.’s (CEI’s) geotechnical investigation, laboratory testing, and associated geotechnical design recommendations for the proposed State Route

(SR) 28 Central Corridor Trail Phase 1 project along the northeastern boundary of Lake Tahoe near Incline

Village community of Washoe County, Nevada. These recommendations are based on surface and subsurface conditions encountered in our explorations and on details of the proposed project as described in this report. The objectives of this study were to:

1. Determine general soil, bedrock, and groundwater conditions pertaining to design and construction of the proposed shared use trail project.

2. Provide geotechnical recommendations for design and construction of the project as related to the geotechnical conditions encountered in our study.

The project area covered by this report generally includes the limits of the proposed path alignment on the west side of SR 28 between Sand Harbor beach and Thunderbird Cove shown on Appendix A-1

(Exploration Location Map). Our investigation included field exploration, laboratory testing, and geotechnical engineering analysis to determine the physical and mechanical properties of the various on-site materials.

Results of our field exploration and testing programs are included in this report and form the basis for all geotechnical data, conclusions, and recommendations.

The services described above were conducted in accordance with the AtkinsRéalis USA Inc. Professional

Services Subconsultant Agreement Number 1015116 between AtkinsRéalis USA Inc. and CEI dated June

16, 2025. The geotechnical scope of work was coordinated as part of statement of qualifications submittal by the project team with Tahoe Transportation District.

Project Description

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2.0 Project Description

2.1 Project Location

The SR 28 Central Corridor Trail Phase 1 project is located on the east side of Lake Tahoe and spans between Sand Harbor State Park and Thunderbird Cove near the Incline Village community of Washoe

County in northern Nevada. The trail is approximately 2 miles in length and follows on the west (lake) side of SR 28. The trail will generally be located on steep slopes that drop from SR 28 to Lake Tahoe except for a small northern segment in the Sand Harbor area where the trail will be located in gentle to moderately sloping areas. The proposed trail segment is contained in Township 15 North, Range 18 East, Mount Diablo

Meridian in Placer, California. The trail will mostly lie within Nevada Department of Transportation (NDOT) right-of-way associated with SR 28. The latitude and longitude of the approximate midpoint of trail alignment are 39.1862 degrees and -119.9274 degrees, respectively. Access to the trail alignment is via SR 28.

2.2 Proposed Project Details

The project will include the design and construction of a shared use trail segment for utilization by pedestrians and bicyclists. Based on the 30 percent preliminary plans for the project prepared by Wood

Rodgers and dated March 2019, the overall SR 28 central corridor trail segment extends from Sand Harbor to the intersection of SR 28 and US Highway 50 (Spooner Junction) and is approximately 9 miles long. As noted above, Phase 1 covered in this geotechnical study is the northern, approximately 2-mile-long

Google EarthTM Isometric View of Proposed Trail – View to the North

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(775) 636-5916 Email: vimal@corestoneengineering.com segment of the trail and will extend the existing trail system south of Sand Harbor up to the unimproved trail access to Thunderbird Cove from SR 28. The proposed trail will include 10-foot-wide asphalt concrete pavement plus gravel shoulders on either side of the path. Gravel shoulders will generally be 2 feet wide but can vary in width based on the final design. The trail will also include view/rest areas at various locations where additional grading width will be necessary. The proposed trail will extend through public land that is owned by Nevada Division of State Lands (NDSL) in the northern portion and United States Forest Services

(USFS) in the southern portion; however, the NDOT right-of-way associated with SR 28 is present within the project site with the easement/special use from the landowners.

The design to refine the trail alignment and establish design grades is in progress at the time of this draft geotechnical report. Due to existing topography consisting of moderate to steep hillside slopes, grading for the trail will require several retaining walls to support cuts made adjacent to SR 28 and fills placed on the downhill side. In flatter areas of the trail alignment (near Sand Harbor and possibly in the extreme southern segment near Thunderbird Cove access), grading will be accomplished by cut and fill slopes to establish design grades. It is our understanding that nearly all the walls to retain cuts (or a single, long wall) on the side of SR 28 will include soil nail walls that are up to 15 feet in exposed height. The walls to retain fills are expected to include mechanically stabilized earth (MSE) type retaining structures with geogrid tiebacks and segmental block facing. Based on the retaining structures utilized in the trail north of Sand Harbor, “Redi-

Rock” block MSE retaining walls are expected to be utilized to retain fills on the project. Similar to existing trail north of Sand Harbor, the proposed trail segment may include elevated bridge segments where pre-cast bridge/boardwalk structures will be supported piers and abutments founded on micropile foundations.

The cast-in-place Portland cement concrete (PCC) pile cap for the micropiles at each substructure location will likely be placed into native grades and the construction of foundations for the bridge/boardwalk structures will likely require temporary excavations supported by soil nail walls. It is our understanding that the short gravity Redi-Rock block or rockery retaining walls located away from and without impacting the stability of SR 28 may be acceptable by NDOT and these walls may only be analyzed for static loading conditions.

It is our understanding that the overall trail project within the limits of Sand Harbor will add a new public entrance to Sand Harbor south of existing station approximately between trail Station 56+00 and 57+00.

The proposed trail alignment will cross the proposed entrance drive from SR 28 via a new PCC underpass structure consisting of precast 14 feet by 10 feet box culvert and associated ancillary structures (e.g. cast-in-place PCC wingwalls extending from the culvert on both sides to provide the needed grade separation in the adjacent grades. The trail finished grades within the underpass is expected to be about 10 to 15 feet below existing grades. Additional soil nail walls on the side of SR 28 and cut slopes will be necessary at the approaches to the underpass structure to establish design grades for the proposed trail.

Within the flatter areas, cuts and fills associated with the grading for trail will generally be on the order of 5 feet. Fill slopes will be graded at 2H:1V (horizontal to vertical) ratio. Cuts may be made at somewhat

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(775) 636-5916 Email: vimal@corestoneengineering.com steeper ratios (up to 1.5H:1V slope ratio) depending on the subsurface material conditions and vertical heights of the cut slopes.

2.3 Design Standards

The proposed retaining walls and other structures on the project will generally be designed and constructed per the NDOT standards generally utilizing Load resistance Factor Design (LRFD) method. In particular, applicable design standards/guidelines of American Association of State Highway and Transportation

Officials (AASHTO) LRFD Bridge Design Specifications, 9th Edition (AASHTO, 2020), 2024 NDOT

Geotechnical Manual (NDOT, 2024), and 2008 NDOT Structures Manual (NDOT, 2008) will be utilized on the project. The asphalt concrete pavement structural section for the trail may utilize local or NDOT standards, as applicable. The asphalt concrete structural section for the trail will likely be similar to the structural section of the existing trail north of Sand Harbor. As noted above, based on the initial conversations with NDOT, relatively short gravity block or rockery retaining walls located away from and without impacting the stability of SR 28 may be designed using alternate methodologies only considering static loading conditions.

Site Conditions

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3.0 Site Conditions

The project site is located along the northeastern perimeter of Lake Tahoe and extends for about 2 miles from Sand Harbor State Park area to Thunderbird Cove. The trail will mostly follow the narrow moderately to steep sloping land area between SR 28 and Lake Tahoe with the exception of the northern and southern segments where SR 28 is located somewhat away from Lake Tahoe. SR 28 within the project limits is an asphalt concrete paved undivided two-way, two-lane roadway with narrow shoulders and asphalt concrete paved parking strips at some locations. The grading for SR 28 within the project limits involved cuts into native granitic rocks on the eastern side and fill slopes on the western side. Underground water and sewer lines exist within the roadway limits of SR 28. Overhead power and communication lines exist on the east side of SR 28, generally within native areas behind the cut slopes associated with SR 28. Storm drain inlets with underground pipe crossings beneath SR 28 towards Lake Tahoe as well as PCC curbs and gutters also exist in some segments of SR 28 for management of runoff.

Most of the proposed trail will traverse through steep sloping land on the west side of SR 28 that includes an upper section of fill slope and then native slopes. In general, existing slopes vary between 1H:1V to

2H:1V but flatter grades also exist within the site. The northern segment of the trail in the Sand Harbor area and some limited distance to the south will lie in relatively flatter areas. Areas of granitic rock outcrops and/or partially buried massive granitic boulders also exist along the proposed trail alignment. Vegetation within the project site consists of a moderate to heavy population of mature pine trees and brush.

Typical Site Conditions – View to the North

Field Investigation

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4.0 Field Investigation

4.1 Geotechnical Reconnaissance

A CEI geotechnical engineer completed a site reconnaissance of project alignment to evaluate the geotechnical and geological conditions with respect to the proposed trail alignment. The geotechnical reconnaissance was utilized in the planning of field exploration. The exploration planning (locations, depths, etc.) generally followed NDOT and AASHTO guidelines with some adjustments that were necessary due to access conditions as well as the identified relatively uniform material conditions in our geotechnical reconnaissance.

4.2 Boring Exploration

Prior to boring exploration, CEI coordinated and obtained a temporary occupancy permit from NDOT to complete the field exploration within NDOT right-of-way associated with SR 28. Further, the field exploration activities were coordinated with the underlying landowners/agency (USFS and NDSL). For borings, CEI obtained utility clearance for public utilities via notifying Underground Service Alert. All field investigation tasks were completed during night shifts.

The SR 28 trail project alignment was explored between September 7th and 16th of 2025 by drilling 19 borings (B-01 through B-19) within the pavement limits of SR 28 as shown on Appendix A-1. It should be noted that the trail alignment runs west of SR 28 pavement within the existing steep native slopes with currently infeasible access for a drill rig. The borings were advanced using solid-flight auger and HQ coring techniques using a track mounted CME 55 drill rig.

Existing asphalt concrete pavement was drilled via auger and the auger drilling continued until practical refusal occurred on competent bedrock. Advancement of most of the borings to target depth was possible using auger drilling but a few borings required HQ coring techniques. In most cases, the borings were initially advanced using 4-inch or 6-inch outside diameter solid-flight augers and, where refusal occurred with auger drilling, the borings were advanced using HQ coring with 96-millimeter (mm) Drilling Exploration

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(775) 636-5916 Email: vimal@corestoneengineering.com outside diameter and 63.5-mm-inside-diameter core barrels. The maximum depth of exploration was approximately 41 feet below existing ground surface.

During conventional auger geotechnical drilling exploration, the subsurface soils and weathered rock were sampled in-place every 2 to 5 feet by use of a standard, 2-inch-O.D., split-spoon sampler driven by a 140-pound automatic drive hammer with a 30-inch stroke. The number of blows to drive the sampler the final 12 inches of an 18-inch penetration (Standard Penetration Test [SPT] - American Society for Testing and

Materials [ASTM] D 1586) into undisturbed soil and soil-like bedrock material is an indication of the density and consistency of the material.

A 3-inch-O.D., split-spoon sampler (modified California [MC] sampler) (ASTM D 3550) was also used to sample soils containing fine-grained soils, gravels, or where approximate in-place samples densities of subsurface materials were required. Sampling methods used were similar to the SPT but also included the use of 2-½-inch-diameter, 6-inch-long, brass sampling tubes placed inside the split-spoon sampler.

Because of the larger diameter of the sampler, blow counts are typically higher than those obtained with the

SPT and should not be directly equated to SPT blow counts. The logs indicate the type of sampler used for each sample. Per NDOT Geotechnical Manual, the blow counts using the larger MC sampler can be converted to equivalent SPT blow counts via multiplying a factor of 0.62 (NDOT, 2024).

Bedrock was continuously cored during HQ core drilling. Rock cores were extracted from the HQ core barrels and placed in plastic core boxes. Rock cores were sampled in accordance with ASTM D 2113 to identify various indicators regarding the geological, physical, and engineering nature of the bedrock.

Based on the calibration records, the auto hammers used in 2023 exploration and 2024 exploration have an efficiency of 82 percent. The field SPT blow counts (N values) reported in the boring logs may be multiplied by 1.37 to determine standard SPT blow counts using a hammer with the standard 60 percent efficiency

(N60 values).

Groundwater was measured at the time of exploration. Groundwater was only encountered in a northern boring near Sand Harbor.

Due to the relatively small diameter of the samplers, the maximum particle size that could be obtained was approximately 1 inch. The final logs may not, therefore, adequately represent the actual quantity or presence of coarse gravels, cobbles, or boulders. The subsurface soils at the site includes granitic cobbles and boulders based on the drilling conditions and the surficial rocks observed along the project alignment.

Where presence of oversize rocks is suggested by drilling response, they are noted in the boring logs.

4.3 Materials Classification

A geotechnical engineer examined and identified all soils and weathered rock materials sampled with the characteristics of soils in the field in accordance with American Society for Testing and Materials (ASTM) D

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2488. During exploration, representative bulk samples were placed in sealed plastic bags and returned to

Reno, Nevada for additional evaluation and laboratory testing assignments. Recovered rock cores were handled in accordance with ASTM D 5079. During HQ coring, the sampled core in each core run was logged, describing weathering, fracturing, strength, and quality of the rock as measured by Rock Quality

Designation. Rock Quality Designation is a scale describing the proportion of intact, durable rock within the formation. The scale, from 0 to 100 percent, is broken into the categories of Very Poor (0 to 25 percent), Poor (25 to 50 percent), Fair (50 to 75 percent), Good (75 to 90 percent), and Excellent (90 to 100 percent)

Rock Quality. Rock cores were placed in plastic core boxes and returned to our Reno, Nevada office for additional evaluation and laboratory strength testing assignments.

Additional soil classification was subsequently performed in accordance with ASTM 2487 (Unified Soil

Classification System [USCS]) upon completion of laboratory testing, as described in the Laboratory

Testing section. Logs of the test borings and test pits are presented as Appendix A-2 (Boring Logs), and a

USCS chart has been included as Appendix A-3 (USCS Soil Classification Chart). Photographs of rock cores are included as Appendix A-4 (Rock Core Photo Log).

4.4 Geophysical Testing

Geophysical testing/survey was completed along the trail alignment at selected locations using refraction microtremor (ReMi) method. The geophysical testing involved both shear wave velocity measurements as well as 2-dimensional (2-D) refraction geophysical tests/model. The geophysical field work was performed by Construction Materials Engineers, Inc.

(CME) of Reno, Nevada geotechnical personnel in accordance with the specific instructions provided by

CEI.

The ReMi shear wave velocity survey was completed along 2 survey lines (S-1 and S-2) to evaluate the average shear wave velocity within the upper 100 feet (30 meters) of subsurface materials (Vs30) within the project site to develop seismic design parameter for the seismic design of the project.

Methodology and results of this analysis are included in Appendix A-5 (Geophysical Test Modeling

Results.) The approximate locations of the geophysical survey lines are shown on Appendix A-

1. The measured Vs30 values vary from Shear Wave Velocity Profile for Line S-1

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(775) 636-5916 Email: vimal@corestoneengineering.com approximately 1.500 to 1,800 feet per second (fps). The appropriate seismic site class and design response spectrum for the project are presented later under Section 8.2 Seismic Design Criteria.

A 2-dimensional (2-D) ReMi refraction geophysical survey was also completed along 7 lines (R-1 through

R-7) for direct measurement of compressional or seismic wave velocities (P-wave) velocity and to allow for evaluation of excavation characteristics of site bedrock. Data was acquired using a Seismic Source DAQlink utilizing a 12-geophone array with predetermined geophone spacing. Data was processed and analyzed using the Seisopt @2-D software package. The resulting 2-D seismic velocity profiles modeled along the survey lines from the field measurements are shown on Appendix A-5 and the approximate locations of the refraction survey lines are shown on Appendix A-1.

Laboratory Testing

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5.0 Laboratory Testing

Corestone Engineering, Inc. subcontracted CME of Reno, Nevada to complete laboratory testing to analyze soil and rock characteristics (detailed below) in general accordance with the standards and methodologies described in Volume 4.08 of the ASTM Standards. Chemical (corrosion) testing detailed below were performed per AASHTO methods by SGS North America, Inc. of Reno, Nevada.

5.1 Index Tests

Samples of significant soil type were analyzed to determine their in-situ moisture content (ASTM D 2216), grain size distribution (ASTM D 422), and plasticity index (ASTM D 4318). In addition, selected MC tube samples from the borings were tested in the laboratory to determine unit weights and moisture contents

(ASTM D2937) of subsurface soils. The results of these tests are shown on Appendix B-1 (Index Test

Results). Test results were used to classify the soils according to ASTM D 2487 and to verify field logs, which were then updated as appropriate. Classification in this manner provides an indication of the soil's mechanical properties and can be correlated with published charts (NDOT, 2024; Bowles, 1996; Naval

Facilities Engineering Command [NAVFAC], 1986a and b) to use in geotechnical analysis.

5.2 Direct Shear Tests

Ten (10) direct shear tests (ASTM D 3080) were performed on representative samples of subsurface soils along the proposed trail alignment. The tests were run on remolded, inundated samples under various normal loads in order to develop a Mohr's strength envelope. The samples were screened to remove particles larger than the number 4 sieve prior to testing. The samples were remolded to approximate in-situ unit weights and moisture contents determined for MC tube samples obtained in the same soil strata.

Results of these tests are shown on Appendix B-2 (Direct Shear Test Results) and were used in various geotechnical analysis. Table 1 (Laboratory Test Results Summary) provides direct shear test results as well as USCS soil classifications, in-situ unit weights, and In-situ moisture contents for the subsurface soil samples.

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TABLE 1 - LABORATORY TEST RESULTS SUMMARY

Boring (B) No.

and Sample ID

Sample Depth (Feet)1

USCS Soil Type

Dry Unit Weight (pcf2)

Moisture Content (%)

Angle of Internal Friction (Degrees)

Cohesion (psf3)

B-01 F 15 SM 90.2 6.4 32.5 221

B-02 D 7.5 SW-SM 107.9 4.1 40.4 158

B-02 G 20 SM 99.9 6.1 34.1 177

B-03 E 10 SM 98.0 2.7 33.5 338

B-04 C 5 SW-SM 102.0 3.6 36.7 0

B-05 B 2.5 SW-SM 112.3 5.3 36.6 0

B-09 D 7.5 SW-SM 101.1 4.4 36.5 86

B-11 D 7.5 SW-SM 92.8 6.3 37.8 196

B-16 E 10 SW-SM 98.3 4.8 33.3 293

B-18 D 7.5 SP-SM 97.1 6.7 34.5 258

1 Depth below existing ground surface or pavement grade at the boring location.

2 Pounds per cubic foot.

3 Pounds per square foot.

5.3 R-Value Tests

Six (6) resistance value (R-value) tests (ASTM D 2844) were performed on representative samples of subsurface soils obtained from drill cuttings in selected borings to evaluate subgrade strength characteristics of site materials. Resistance value testing is a measure of subgrade strength and expansion potential and is used in the design of flexible pavements. Results of R-value tests are shown on Appendix

B-3 (R-Value Test Results). The measured R-values for granular sandy fills and native soils vary from 67 to

83 and indicate excellent subgrade soil strength characteristics.

5.4 Unconfined Compressive Strength Tests

Representative intact rock cores were tested to determine their unconfined compressive strength. The cores were trimmed to exhibit a height to diameter ratio of approximately 2 to 1. The unconfined compressive strength is a representation of the rock hardness. Unconfined compressive strength testing was performed in general accordance with ASTM D 7012. Results of unconfined compressive strength testing is summarized below in Table 2 (Rock Core Compressive Strength Test Results) and detailed results are shown on Appendix B-4 (Unconfined Compressive Strength of Rock).

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TABLE 2 - ROCK CORE COMPRESSIVE STRENGTH TEST RESULTS

Boring (B) No. Rock Core Identification Core Depth (ft)1 Compressive Strength (psi)2

B-05 RUN 1A 16.0 12,061

B-05 RUN 1B 17.5 10,793

B-05 RUN 1C 19.0 9.572

B-09 RUN 1 20.0 4,558

1 Depth below existing ground surface or pavement grade at the boring location.

2 Pounds per square inch.

5.5 Chemical Tests

Chemical testing was performed on representative samples of site soils along the proposed trail alignment to evaluate the site materials’ potential to corrode steel and PCC in contact with the ground. The sample was tested for pH, resistivity, chlorides, and soluble sulfates. The results of the chemical tests are shown on

Appendix B-5 (Chemical Test Results).

Site Geology and Subsurface Conditions

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6.0 Site Geology and Subsurface Conditions

6.1 Regional Geology and Seismicity

The project site is in the Tahoe Basin of the Sierra Nevada mountains. The Tahoe Basin is within the Sierra

Nevada Batholith consisting of mainly massive, Cretaceous age granitic rock subsequently overlain by

Tertiary age volcanic and volcaniclastic rock. Within late Tertiary and Quaternary time, Basin and Range style extensional regional faulting has extended into the Sierra Nevada, and the Tahoe Basin is a fault-bounded basin at the western edge of Basin and Range faulting. The current landscape has been shaped by an extensive Pleistocene age glacial history and continues to be shaped by fluvial and lacustrine processes and active faulting. Because of its geological settings, the project area has a high potential for strong seismic shaking.

6.2 Site Geology

Mapping by California Geological Survey (CGS) in Geological Map of Lake Tahoe, California and Nevada

(1:100,000 scale) shows most of trail alignment to lie in granitic rocks (Saucedo, 2005). The Craterous age granitic rock unit (Ksgr) is specifically noted by CGS as Monzogranite of Spooner Summit of Grose

(Saucedo, 2025). The northern segment within Sand Harbor area is mapped to lie in Quaternary age Talus deposits and Lacustrine terrace deposits Our field exploration reveals that native materials confirm to CGS geological mapping and generally includes granitic rocks weathered into sandy soils (decomposed granitic, hard granitic rocks and lake deposits consisting of sands.

6.3 Subsurface Material

Conditions Most of the borings were advanced (B-05 through B-19) were advanced within existing pavement of SR 28 and encountered approximately 4.5 to 11 inches of asphalt concrete pavement underlain by 5 to 10 inches of aggregate base. Northern borings near Sand

Harbor State Park were advanced within the roadway shoulders or native grades.

Subsurface soils generally include variable thickness of granular sand fill soils followed by native granular sandy soils with frequent granitic Portion of Geologic Map (Saucedo, 2005)

Site Geology and Subsurface Conditions

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(775) 636-5916 Email: vimal@corestoneengineering.com cobbles and boulders and then granite bedrock. Northern borings B-01 through B-03 encountered native medium dense to dense sand soils through 25 feet depth or more and then very dense sand soils that are likely weathered granitic rock sampled in the borings as soils to maximum depth of exploration, approximately 41 feet below existing ground surface. Granitic rock was encountered at variable depths of approximately 8 to 25 feet below existing pavement grade along the trail alignment south of Sand Harbor

State Park area. Strength and weathering of bedrock is variable, and rock encountered in most borings were drillable using conventional augers to result in materials that exhibit very dense (often refusal blowcounts) sand or gravel soil characteristics. The overburden soils encountered in the borings include colluvium as well as decomposed granitic sands resulting from severe/intense weathering of granitic rocks.

Drilling within upper native soils also indicated the presence of cobbles and/or boulders.

Most subsurface soils (including fills and weathered granitic rock sample as soil-like materials) include silty sand, well graded sand with silt, and poorly graded sand with silt soils (with variable but relatively low percentage of fine to medium gravels). These predominant sand soils within the project alignment contain less than 25 percent of non-plastic fines. There are some silty gravelly soil layers at variable depths across the project alignment.

6.4 Groundwater

Groundwater was encountered approximately 35 feet depth below existing ground surface in boring B-03 that was advanced near Sand Harbor State Park. Groundwater was not encountered in the remaining borings that were extended to a maximum depth of 41 feet below existing ground surface. In general, groundwater should not have adverse effect on the design and construction of the proposed trail project.

However, seasonal perched water may be present at or near the native ground surface during the spring thaw, percolating downslope over soil/bedrock interface during years with normal to heavy snowfall. The duration of saturated materials will depend on snow accumulation over the winter months, recent precipitation, and runoff conditions.

Geologic Hazards

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7.0 Geologic Hazards

7.1 Seismicity and Faults

The Lake Tahoe basin lies within an area with a high potential for earthquake shaking. It is generally accepted that a maximum credible earthquake in this area would be in the range of magnitude 7 to 7.5 along the Genoa fault system of the eastern Sierra Nevada. The most active segment of this fault system in the Tahoe area is located at the base of the eastern side of the mountains about 15 miles southeast of the project site.

Nevada Bureau of Mines and Geology (NBMG) MyHazards web-mapping tool (NBMG, 2025) shows multiple old Quaternary age (active within the past 750,000 years) of East Tahoe Fault Zone near the northern and southern ends of the trail alignment. These faults are shown on the figure to the right. The nearest Holocene age (active within the past 15,000 years) is the Incline Village Fault mapped approximately 2 miles west of the trail alignment. No fault of any age is mapped to cross the trail alignment, and the mapped nearby faults are sufficiently old to be considered as inactive.

7.2 Fault Rupture

There are no known ground rupture locations within or in the general area of the project site. The absence of ground faults passing through the site suggests the potential for ground rupture within the project site is negligible.

7.3 Ground Motion

Based on United States Geological

Survey (USGS) Uniform Hazard

Tool, the site modified peak ground acceleration is 0.50 g considering 7 percent probability of exceedance in

75 years (USGS, 2025). Additional Fault Activity Map for the Project Area (NBMG, 2025)

Geologic Hazards

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(775) 636-5916 Email: vimal@corestoneengineering.com information with respect to ground motion for the site is presented later in this report under Seismic Design

Criteria (Section 8.2).

7.4 Liquefaction

The submerged soils below groundwater table in northern segment of the trail alignment near Sand Harbor

State Park are very dense (likely weathered granitic rock). Granitic bedrock is present at relatively shallow depths along the remainder of trail alignment. Due to subsurface material type and consistency, the potential for soil liquefaction at the site is negligible.

7.5 Flood Plain

The Federal Emergency Management Agency (FEMA) has identified the site as lying-in unshaded Zone X, or outside of a 500-year flood zone (FEMA, 2009).

7.6 Other Geologic Hazards

A high potential for dust generation is present if grading is performed in dry weather.

With the presence of very dense soils and granitic rock materials from relatively shallow depth at the site, the potential for landslide is low.

No other geologic hazards were identified.

Preliminary Design Recommendations

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8.0 Preliminary Design

Recommendations

8.1 General Information

The site subsurface materials encountered in our exploration include granular sands underlain by granitic rock that are of variable quality and at variable weathering conditions. These materials will provide adequate support for the improvements.

At the time of this draft geotechnical report, only preliminary trail alignment, grading concepts, and structure details available. Therefore, preliminary geotechnical recommendations are provided in this section for anticipated retaining wall structures. Once the trail alignment is refined and design grades are established with specific structure details to support cuts and fills and other miscellaneous structures, CEI will develop final geotechnical design recommendations for each proposed structure on the project. We will also coordinate with structural engineers for the project to complete subsequent geotechnical analysis and then provide subsequent interim geotechnical design recommendations for the proposed walls and other structures via geotechnical design memorandums, and all these recommendations will be incorporated in the final geotechnical report for the project.

Any evaluation of the site for the presence of surface or subsurface hazardous substances is beyond the scope of this investigation. When suspected hazardous substances are encountered during routine geotechnical investigations, they are noted in the exploration logs and immediately reported to the client. No such substances were revealed during our exploration.

8.2 Seismic Design Criteria

Uniform seismic criteria for the seismic design of the project were developed utilizing the maximum of

AASHTO LRFD for 2 locations where shear wave velocity measurements were completed and regional acceleration values provided for Washoe County in 2008 Structures Manual (NDOT, 2008). The site input parameters utilized in determining the seismic criteria are presented in Table 3 (Seismic Design Criteria Site

Parameters). In accordance with guidelines from NDOT Geotechnical Manual (NDOT, 2024), the USGS

Uniform Hazard Tool (USGS, 2025) was utilized to determine seismic design parameters based on

AASHTO LRFD for 2 locations along trail alignment (AASHTO, 2020). The design response spectrum per

AASHTO LRFD considered a 20 percent increase in acceleration values at more than 1 second period to account for near fault effect with the presence of one or more Holocene age active fault within 6 miles of the project site.

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TABLE 3 – SEISMIC DESIGN CRITERIA SITE PARAMETERS

Parameters/Location Northern Trail Segment Southern Trail Segment

Latitude 39.19769 39.18104

Longitude -119.92784 -119.92561

Shear Wave Velocity (Vs30)1 1,801 fps 1,510 fps

Site Class2 C C

1 Based on average shear-wave velocity ReMi shear wave velocity survey (refer to Appendix A-5 for detailed results).

2 Per Table 7.4-1 of NDOT Geotechnical Manual (NDOT, 2024)

The seismic design criteria for the site utilizing the above site parameters and NDOT minimum values are also summarized in Table 4 (Design Ground Motion Parameters).

TABLE 4 – DESIGN GROUND MOTION PARAMETERS1

Parameters Values

Site Adjusted Horizontal Peak Ground Acceleration (As 2), g 0.50

Design Spectral Response at 0.2 Second (SDS), g 1.27

Design Spectral Response at 1.0 Second (SD1), g 0.65

AASHTO LRFD Seismic Zone 4

Mean Earthquake, M, Moment Magnitude3 6.65

Mean Site-to-Fault Source Distance, R, kilometers3 8.12

1 NDOT Minimum Controls (NDOT, 2008) except for SDS

Value is same as PGA since the site adjustment factor is 1.0.

3 Based on deaggregation analysis using USGS Uniform Hazard Tool (mean value for total component)

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The design response spectrum for the project is shown in the figure below.

8.3 Soil Nail Walls

Depending on the final design, the project will include multiple soil nail walls to retain cuts made on the side of SR 28 to establish design grades for the shared use trail. The preliminary plans prepared by Wood Rodgers show a long soil nail wall (about 3,000-foot-long) in the middle portion of the Phase 1 trail alignment where the trail will transverse through steep slopes. Height of the soil nail walls will vary, and it is our understanding that a maximum exposed wall height of 15 feet is anticipated. The approach segments of the trail alignment on either side of the new underpass structure in Sand Harbor area is also expected to require soil nail walls to retain cuts made on the sides of SR 28. Depending on the horizontal distance of trail alignment from SR

28 and the trail finished grade elevations, the soil nail wall will retain a variable height, broken slope and then

SR 28 roadway. Therefore, once design plans become available, several cases of soil nail walls should be analyzed depending on the heights, retaining conditions, and subsurface materials encountered in the exploration.

Design Response Spectrum

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To develop preliminary geotechnical recommendations for soil nail walls, we considered a few general wall design cases with maximum exposed wall heights of 10 and 15 feet.

8.3.1 Wall Geometry and Retaining Conditions

For the preliminary analysis, we considered a soil nail wall with the exposed height of 15 and 10 feet. In addition, we considered a 2H:1V and 1.5H:1V ratio retained slopes (existing west side slope of SR 28) with a maximum vertical height of 10 feet in the wall design geometry (slope surcharge). In the design cases considered, the shared use trail will be located 20 to 25 feet vertically below the adjacent SR 28.

8.3.2 Subsurface Material Conditions

The materials to be retained by soil nail walls will include an upper section of onsite soils (both roadway fills and native overburden soils) underlain by granitic bedrock with variable degree of weathering. The depth of soils will vary and our exploration also reveal variable consistency of native soils between borings. Based on the laboratory direct shear tests, we assigned an average angle of internal friction of 35 degrees and a minimal cohesion of 50 psf. The weathered granitic bedrock was assigned an angle of internal friction of 37 degrees and a conservative cohesion of 300 psf to represent the in-place condition. In the preliminary analysis, we utilized ultimate/nominal bond strengths of 15 and 30 psi, respectively, for overburden soils and weathered bedrock materials.

It should be noted that the construction of soil nail walls at the project site will be difficult with potential risks including the stability of overburden soils (fills and native) in temporary excavations, drilled holes stability in soils, and drilling in native bedrock materials as well as possible granitic boulders within upper soils.

8.3.3 Design Methodology

The preliminary analysis for soil nail walls was completed utilizing soil nail wall design methodologies in general accordance with the design guidelines provided in the Federal Highway Administration (FHWA) geotechnical engineering circular number 7 (FHWA, 2015). The computer program SNAIL Version 2.2.2 developed by the California Department of Transportation (Caltrans) was utilized in the design to determine the nail lengths at various levels to satisfy the factor of safety (or the applicable resistance factor) requirements against global stability and pullout failure (Caltrans, 2020). The design also accounts the requirements of NDOT Geotechnical Manual for soil nail walls such as minimal soil nail length of 15 feet

(NDOT, 2024). For seismic loading conditions, a horizontal design acceleration coefficient (kh) equal to half of site adjusted peak ground acceleration (As) or 0.25 was utilized in the analysis (NDOT, 2024). In the overall stability analysis, a minimum factor of safety of 1.5 (resistance factor of 0.65) was considered for Strength

Limit State and a minimum factor of safety of 1.1 was considered for Extreme Event Limit State.

8.3.3 Geotechnical Design Recommendations

The preliminary geotechnical recommendations for soil nail walls to retain cuts along the side of SR 28 are provided in Table 5 (Soil Nail Walls Preliminary Design Parameters).

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TABLE 5 – SOIL NAIL WALLS PRELIMINARY DESIGN PARAMETERS1

Maximum Exposed Wall Height 15 feet 15 feet 10 feet 10 feet

Height of Slope Above Soil Nail Wall 2H:1V Slope 10 feet 0 10 feet 0

1.5H:1V Slope 0 10 feet 0 10 feet

Minimum Length of Soil Nails2 15 feet 20 feet 15 feet 15 feet

Minimum Number of Soil Nail Courses/Levels 3 3 2 2

Maximum Vertical Soil Nail Spacing 5 feet

Maximum Horizontal Soil Nail Spacing 5 feet

Soil Nail Inclination from Horizontal 15 degrees

Maximum Distance from Top of Wall to First Level of Nails 3 feet

Maximum Distance Above Toe of Wall to Bottom Level of Nails 3 feet

Minimum Extension of Soil Nail Walls Below Trail Finished Grade 12 inches

Minimum Diameter of Soil Nails 6 inches

Minimum Diameter of Deformed Steel Soil Nail Bars (Grade 75) 1 inch

Minimum Base Plate Dimensions1 8 inches by 8 inches; 1-inch-thick

Minimum Thickness of Shotcrete (Initial and Final)1 4 inches / 8 inches

Minimum Shotcrete Reinforcement (Initial and Final)1 Per Structural Design

Minimum 28-Day Compressive Strength of Grout 3,000 psi

Minimum 28-Day Compressive Strength of Shotcrete (Initial and Final Facing)

2,500 psi / 4,000 psi

Soil Nail Verification Testing Minimum 2 or 5 percent of the total nails, whichever greater, for each wall

Soil Nail Wall Drainage 12-inch-wide Geocomposite drainage strips at Maximum 10 feet on Centers with Weep Holes

1 To be finalized and/or determined based on the internal stability analysis for the soil nail walls for the soil nail design loads. Soil nails were modeled in the SNAIL program with a selected default wall face (with internal analysis) to force soil nail pullout failures under static and seismic loading conditions. In addition to base plates, the nail heads will also need to include studs and any needed cross bar reinforcement 2 Regardless of wall height and retaining conditions, required minimum soil nail length is 15 feet per NDOT Geotechnical Manual.

8.4 MSE Retaining Walls

The project will include multiple MSE retaining walls with segmental block facing to support fills placed on the downhill side of the proposed trail alignment. It is expected Redi-Rock block MSE retaining walls will be utilized on the project. The maximum exposed height of the MSE retaining walls is anticipated to be 9 feet and the walls will retain relatively level cross grades associated with the proposed trail alignment. The

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(775) 636-5916 Email: vimal@corestoneengineering.com foundation soils associated with the MSE walls will be native overburden soils and/or granitic bedrock materials depending on the design grades. The walls will also retain an upper layer of native overburden soils and the bedrock materials. The reinforced zones of MSE walls should include MSE wall backfill materials that meet the requirements of NDOT Standard Specifications (NDOT, 2024). The preliminary geotechnical design recommendations for MSE retaining walls are provided in Table 6 (MSE Retaining Walls Preliminary Design

Parameters). The analyses were completed in general accordance with AASHTO LRFD design guidelines

(AASHTO, 2020) with specific requirements of NDOT Geotechnical Manual (NDOT, 2024). Additional discussion on the design methodology and associated analysis calculations for various wall heights will all be incorporated in the final geotechnical investigation report.

TABLE 6 – MSE RETAINING WALLS PRELIMINARY DESIGN PARAMETERS

Design Parameters MSE Wall Backfill1

Retained or Foundation Material

Native Soils Granitic Rock

Unit Weight (pcf) 125 120 150

Angele of Internal Friction (Degrees) 34 35 37

Cohesion (psf) 0 50 300

Maximum Total Wall Height, H (feet) 12

Minimum Embedment Depth (feet)2 Minimum of 2 or H/5

Minimum Geogrid Length of Reinforcement, L (feet)3 Minimum of 8 or 0.8*H

Maximum Vertical Spacing of Reinforcement (feet) 1.5

Seismic Horizontal Acceleration Coefficient, kh 0.25

Active Earth Pressure Coefficient for Level Grade5

Static, KA 0.29 0.27 0.19

Seismic, KAE 0.45 0.44 0.31

Nominal Bearing Resistance, qn (ksf)6 - 12

1 MSE wall backfill should meet NDOT Standard Specification.

2 Per NDOT Geotechnical Manual (NDOT, 2024). MSE walls on the project will be partially founded on native soils. Provide a minimum 4 feet level bench with a maximum of 1.5H:1V slope at the face.

3 Minimum 8-foot-long reinforcement is required per NDOT Geotechnical Manual. Geogrid reinforcement for upper 2 layers should be increased by 5 feet unless coordinated with NDOT. Geogrid type is to be based on internal analysis but Miragrid® 8XT for preliminary design and budget.

4 kh = 0.5As for walls…