DRAFT_Preliminary GeoTechnical Report (CRCBF-14).pdf

Geotechnical | Environmental | Construction Inspection & Testing | Forensic Engineering & Expert Witnes s

Geophysics | Engineering Geology | Laboratory Testing | Industrial Hygiene | Occupational Safety | Air Quality | GIS

Geotechnical Evaluation

Boulder Flats 230kV Solar Interconnect

Eldorado Valley

Boulder City, Nevada

Burns & McDonnell

9400 Ward Parkway | Kansas City, Missouri 64114

February 28, 2024 | Project No. 305086001

Geotechnical | Environmental | Construction Inspection & Testing | Forensic Engineering & Expert Witness

Geophysics | Engineering Geology | Laboratory Testing | Industrial Hygiene | Occupational Safety | Air Quality | GIS

DRAFT

6700 Paradise Road, Suite E | Las Vegas, Nevada 89119 | p. 702.433.0330 | www.ninyoandmoore.com

Geotechnical Evaluation

Boulder Flats 230kV Solar Interconnect Eldorado Valley Boulder City, Nevada

Mr. Shae Pelkowski

Burns & McDonnell

9400 Ward Parkway | Kansas City, Missouri 64114

February 28, 2024 | Project No. 305086001

Alex Arndt , PE Senior Project Engineer

Brad Olsen, PE Principal Engineer

AMA/BLO/cas DRAFT http://www.ninyoandmoore.com/

Ninyo & Moore | Boulder Flats 230kV Solar Interconnect, Boulder City, Nevada | 305086001 R | February 28, 2024 i

CONTENTS

1 INTRODUCTION 1

2 SCOPE OF SERVICES 1

3 PROJECT DESCRIPTION 2

4 GENERAL SITE AND ALIGNMENT CONDITIONS 2

5 GEOLOGY 3

5.1 Geologic Setting 3

5.2 Potential Geologic Hazards 4

5.3 Ground Motions 5

5.4 Liquefaction Potential 5

6 FIELD EXPLORATION AND LABORATORY TESTING 6

7 SUBSURFACE CONDITIONS ENCOUNTERED 6

7.1 Laboratory Tests 7

7.2 Groundwater 8

8 FINDINGS AND CONCLUSIONS 8

9 RECOMMENDATIONS 10

9.1 Earthwork 10

9.1.1 Site Grading 10

9.1.2 Structural Fill and Backfill 11

9.1.3 Import Soil 12

9.1.4 Temporary Excavations 12

9.2 Structure Foundations 12

9.2.1 Conventional Spread Foundations 12

9.2.2 Mat Foundations 13

9.2.3 Drilled Shaft Foundations 13

9.2.3.1 Drilled Shaft Construction Considerations 14

9.3 Lateral Earth Pressures 15

9.4 Concrete Slab-On-Grade Floors 16

9.5 Pavement Sections 17

9.5.1 Flexible Pavement 17

9.5.2 Rigid Pavement 18

9.5.3 Pavement Considerations 18

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9.6 Exterior Concrete Flatwork 18

9.7 Concrete and Corrosion Considerations 19

9.7.1 Concrete 19

9.7.2 Buried Metal 20

9.8 Surface Drainage and Moisture Infiltration Reduction 20

9.9 Observation and Testing 20

9.10 Plan Review 21

9.11 Pre-Construction Meeting 21

10 LIMITATIONS 21

11 REFERENCES 23

TABLES

1 – Preliminary Substation Foundation Loading

2 – Preliminary Transmission Line Foundation Loading

3 – Nearest Geologic Hazard

4 – Seismic Design Criteria

5 – Summary of Laboratory Test Results

6 – Recommended Flexible Pavement Section Thickness

FIGURES

1 – Project Location

2 – Boring Locations

3 – Project Geology

4 – Lateral Earth Pressures for Yielding Retaining Wall

5 – Lateral Earth Pressures for Restrained Retaining Wall

6 – Retaining Wall Drainage Detail

APPENDICES

Appendix A – Exploratory Boring Logs

Appendix B – Laboratory Testing

Appendix C – Chemical Test Results

Appendix D – Field Resistivity Test Results

Appendix E – ReMi Survey Results

Appendix F – MFAD/LPILE Soils Data Summary Appendix G – General Drilled Shaft Recommendations

Ninyo & Moore | Boulder Flats 230kV Solar Interconnect, Boulder City, Nevada | 305086001 R | February 28, 2024 1

1 INTRODUCTION

In accordance with your request, Ninyo & Moore has performed a geotechnical evaluation for the proposed Boulder Flats 230 kV Solar Interconnect project located in the Eldorado Valley in

Boulder City, Nevada. The approximate location of the project site and alignment are shown on

Figure 1. The purpose of this study was to perform a geotechnical evaluation and to provide recommendations regarding the geotechnical aspects of the project.

2 SCOPE OF SERVICES

The scope of our evaluation for project included the following:

• Review of pertinent background data, including aerial photographs, previous geotechnical studies performed near the project area, in-house geotechnical and soils data, and published geologic maps and literature.

• Performance of safety meetings and completion of safety forms, as requested.

• Coordination and mobilization for subsurface exploration with markout of existing utilities performed by Underground Service Alert (USA).

• Oversight by a biological monitor for the geotechnical surveys conducted within the Bureau of Land Management (BLM) utilities corridors, Western Area Power Administration’s (WAPA) Mead Withdraw Areas, and switch station site.

• Performance of one refraction microtremor (ReMi) survey to evaluate seismic site class in general accordance with the 2018 International Building Code at the switch station site. The survey was performed to characterize the average subsurface shear wave velocity to a depth of approximately 100 feet below existing ground surface.

• Performance of a field resistivity test (Wenner four-pin method) measured at approximate “A” spacings of 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 75 and 100 feet at the switch station site. Soil resistivity measurements were taken along two roughly perpendicular traverses (N-S and E-W).

• Drilling, logging, and sampling of 22 exploratory borings to depths ranging from approximately 40 to 51.5 feet. Five borings were drilled at the switch station site and 17 borings were drilled along the proposed alignment.

• Performance of laboratory tests to evaluate the physical, chemical, and engineering properties of the subsurface soils. Laboratory tests performed include gradation, plasticity, maximum dry density and optimum moisture content, direct shear, chloride content, pH, reduction-oxidation potential, resistivity, sodium content, sulfate content, solubility potential (total salts), and sodium sulfate content.

• Compilation and analysis of the accumulated data.

• Preparation of this report presenting the findings, conclusions, and geotechnical recommendations for the project.

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3 PROJECT DESCRIPTION

The project includes design and construction of an approximately 3-acre switch station site and an approximately 9-mile long 230 kV transmission line. The proposed improvements are shown on Figure 2. Switch station structures are anticipated to consist of dead-end H-frames, transformers, breakers, a control building and other miscellaneous structures supported on either shallow concrete mat foundations or drilled shafts. Additional on-site improvements may include relatively short retaining walls and paved parking and access areas. Preliminary foundation loads and types for the proposed switch station structures are provided in the table below. We have assumed that cuts and fills for the switch station site will be approximately 10 feet or less.

Table 1 – Preliminary Substation Foundation Loading

Structure Foundation Type Axial Shear Moment

H-Frame Deadend 5.5ft Diam D.S. 30 kips 20 kips 1,000 kip-ft

Control Building 2.5ft Diam D.S. 30 kips 3 kips 20 kip-ft

Misc. Supports 3ft Diam D.S. 6 kips 3 kips 30 kip-ft

Transformer Slab 400 kips - -

Breaker/Misc. Slab 15 kips - -

Static Mast 4ft Diam D.S. 15 kips 15 kips 600 kip-ft

The proposed transmission line will be approximately 9 miles long and will extend generally in a southwest direction between the proposed switch station site and the future Boulder Flats solar site (by others). Transmission line structures will consist of tubular steel monopoles. Tangent structures are anticipated to be direct embedded poles with crushed rock or unreinforced concrete backfill. Angle and deadend structures are anticipated to be supported by drilled shafts.

Preliminary loads for the proposed transmission line structures are provided below.

Table 2 – Preliminary Transmission Line Foundation Loading

Structure Axial Shear Moment

Tangents 79 kips 27 kips 3100 kip-ft

Deadends 158 kips 110 kips 15,969 kip-ft

4 GENERAL SITE AND ALIGNMENT CONDITIONS

The proposed switch station site is located on undeveloped land and is bordered by undeveloped land to the east, west, and south, and an existing overhead transmission line to the north. Surficial soils at the switch station site and transmission line alignment are comprised primarily of coarse-grained (sandy and gravelly) soils, with some cobbles. The transmission line alignment also generally extends through undeveloped land, but crosses several developed roadways (I-11, US-95, Old US-95, Unnamed Gravel Pit Roadway, and Black Hills Drive), the alignment is

Ninyo & Moore | Boulder Flats 230kV Solar Interconnect, Boulder City, Nevada | 305086001 R | February 28, 2024 3 generally bordered by commercial properties, solar facilities, existing overhead transmission line infrastructure, or undeveloped land.

The topography at the switch station site is slightly to moderately undulatory and generally slopes downward from the north to the south. Several small ephemeral (typically dry) washes, generally up to a few feet wide and a few feet deep, are located in the area of the proposed improvements.

Washes near the switch station site and the northern portions of the alignment generally extend in a northeast-southwest direction, while washes along the southern half of the alignment extend in a northwest-southeast direction.

At the time of our field activities, the project site and alignment were covered with sparse to moderate native desert vegetation. This vegetation consisted primarily of small desert shrubs.

Indications of underground utilities were not noted at the switch station site but were observed along the transmission line alignment at several locations, generally near developed roadways.

Additionally, several existing overhead power lines were observed extending across portions of the proposed transmission line alignment.

5 GEOLOGY

Based on our field observations, subsurface exploration, and review of referenced geologic and soils data, the areas of the proposed improvements are underlain primarily by Quaternary and

Tertiary-age alluvium (native soil). Ninyo & Moore's findings regarding the geologic setting, potential geologic hazards, ground motions, and liquefaction potential are provided in the following sections.

5.1 Geologic Setting

The project is located in the northwest portion of the Eldorado Valley, within the Basin and Range physiographic province. The Eldorado Valley is a naturally formed structural basin as a result of block faulting, a fundamental characteristic of the Basin and Range physiographic province.

The Eldorado Valley is a closed drainage basin and extends in a northeast-southwest direction.

Surface water drains toward the dry lake in the central portion of the valley through ephemeral washes that extend from adjacent highlands and alluvial fan areas. Bordering the alluvium- and playa-filled valley, are relatively steep mountain ranges, including the Eldorado Mountains to the east, the River Mountains to the north and west, and the McCullough Range to the west.

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Based on our review of referenced geologic maps, the site/alignment are primarily underlain by

Quaternary-age alluvial deposits (native soil), composed primarily of coarse-grained soils (Qa, Qfy, Qfy1, Qfy2, Qfi, Qfie). The referenced geologic map (Hinz, N.H., Dee, S., and Johnsen, R., 2017) also indicates an older Tertiary sediment (Ts) soil/rock unit was previously mapped near boring B-9. The Tertiary sediments unit (Ts) is likely composed of decomposed soil-like units that may be well-indurated and could contain cobbles and small boulders.

5.2 Potential Geologic Hazards

Ninyo & Moore’s geotechnical study included an evaluation of the possible presence of geologic hazards, such as faults and ground fissures, in the project area. This evaluation included review of published geologic and soils maps and literature. Faults in the vicinity of the project include seismic (Class A) and subsidence-related (Class B) faults. Class A fault refers to a fault where geologic evidence indicates a Quaternary-active fault of tectonic origin (USGS, 2024). Class B fault refers to a fault where available geologic evidence is not strong enough to consider the fault to be of tectonic origin or to extend deep enough to be a source of significant earthquakes

(USGS, 2024).

Ground fissures, generally believed to be caused by erosion and differential stress resulting from regional subsidence due to withdrawal of groundwater, are known to occur near subsidence-related faults in southern Nevada (Bell J.W., 2001). Review of referenced geologic data does not indicate the presence of ground fissures at the switch station site or along the transmission line alignment, and no ground fissures were observed in the vicinity of the project site and alignments during our field activities. However, recent surficial ground cracks have been observed in the low-lying drainage area southwest of the dry lakebed (playa) (approximately 3 miles south of the transmission line south terminus). There are no geologic hazards, including faults and fissures, anticipated to impact development. The nearest mapped faults are summarized in the following table.

Table 3 – Nearest Geologic Hazard

Hazard Description

Nearest Seismic Fault (Class A) Black Hills fault approximately 1.5 miles west of the site.

Nearest Subsidence-Related Fault (Class B) Approximately 10 miles north of the site.

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5.3 Ground Motions

Ninyo & Moore performed a refraction microtremor (ReMi) survey to obtain the shear wave velocity profile to a nominal depth of approximately 100 feet at the subject site to evaluate Seismic

Site Class in general accordance with the 2018 International Building Code (ICC, 2018). The approximate location of the ReMi survey is shown on Figure 2, and additional information regarding the ReMi survey is provided in Appendix E. The results of the ReMi survey indicate a calculated average shear wave velocity of approximately 1,436 feet per second. Based on this information, the findings of our subsurface exploration program, and the 2018 International

Building Code, a Seismic Site Class C is appropriate for design of this project.

Using the American Society of Civil Engineers (ASCE) 7 Hazard Tool (http://asce7hazardtool.online/), estimated maximum considered earthquake spectral response accelerations for short

(0.2 second) and long (1.0 second) periods were obtained for the subject site, which is located at approximately 35.95072 degrees north latitude and -114.88963 degrees west longitude. Based on the referenced International Building Code (ICC, 2018), the parameters in the following table are characteristic of the subject site, for design purposes.

Table 4 – Seismic Design Criteria

Site Coefficients and Spectral Response Acceleration Parameters Values

Site Class C

Risk Category II

Site Coefficient, Fa 1.265

Site Coefficient, Fv 1.5

Mapped Spectral Response Acceleration at 0.2-second Period, Ss 0.587

Mapped Spectral Response Acceleration at 1.0-second Period, S1 0.188

Spectral Response Acceleration at 0.2-second Period Adjusted for Site Class, SMS 0.743

Spectral Response Acceleration at 1.0-second Period Adjusted for Site Class, SM1 0.283

Design Spectral Response Acceleration at 0.2-second Period, SDS 0.495

Design Spectral Response Acceleration at 1.0-second Period, SD1 0.188

5.4 Liquefaction Potential

Liquefaction is a phenomenon in which loose, saturated soils lose shear strength under short-term (dynamic) loading conditions. Ground shaking of sufficient duration results in the loss of grain-to-grain contact in potentially liquefiable soils due to a rapid increase in pore water pressure, causing the soil to behave as a fluid for a short period of time. To be potentially liquefiable, a soil is typically cohesionless with a grain-size distribution generally consisting of sand and non-plastic silt. It is generally loose to medium dense, saturated, and subjected to sufficient magnitude and duration of ground shaking.

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Liquefaction generally occurs within the upper 50 feet of ground surface. Based on review of publicly available groundwater wells (well log 124747), groundwater is anticipated to be deeper than 300 feet below ground surface (NDWR, 2024). Based on a screening analysis considering no saturated soil anticipated within 50 feet of ground surface, liquefaction hazard at the site may be considered low.

6 FIELD EXPLORATION AND LABORATORY TESTING

Ninyo & Moore’s subsurface exploration of the project site and transmission line alignment was performed in January 2024. The exploration included drilling, logging, and sampling of 22 small-diameter exploratory borings. The exploratory borings were advanced to depths ranging from approximately 40 to 51.5 feet utilizing a CME-850 limited access drill rig with 6-inch diameter hollow-stem augers. The locations of the exploratory borings are shown on Figure 2.

The elevations of the ground surface at each boring location, based on Mean Sea Level (MSL), were estimated from Google Earth (Google, 2024) data. Accordingly, the elevations recorded on the boring logs in Appendix A should be considered approximate.

The purpose of the borings was to evaluate subsurface conditions at the switch station site and along the transmission line alignment, and to collect bulk and relatively undisturbed soil samples for laboratory testing. Borings were logged by an experienced geotechnical engineer or geologist.

Standard Penetration Tests (SPT) and split-barrel sampler drives were obtained in the borings at varying depth intervals.

A refraction microtremor survey was performed at the switch station site (see Figure 2) to evaluate the shear wave velocity profile at the site. Additional information regarding the ReMi survey is provided in Appendix E.

7 SUBSURFACE CONDITIONS ENCOUNTERED

Native soil was encountered at the ground surface and extended to the total depths of our exploratory borings (up to approximately 51.5 feet). The encountered native soil consisted primarily of medium dense to very dense, poorly/well graded sand with silt and varying amounts of gravel; loose to very dense, silty or clayey sand with varying amounts of gravel and silty gravel with sand. A few layers of very stiff, sandy silt and sandy silty clay were also encountered in some of our borings near the existing dry lake bed. Significant amounts of cobbles (rock particles between 4 inches and 12 inches in nominal diameter) and boulders up to approximately 24 inches in nominal diameter were encountered in some of our exploratory borings.

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Caliche, which is a naturally occurring, highly cemented soil with rock-like characteristics, was not encountered in our subsurface explorations. However, due to its highly variable nature, caliche may be encountered in areas between and beyond our boring locations during earthwork and excavation operations. The following describes typical properties of caliche encountered in southern Nevada.

• Generally occurs in layers a few inches to several feet thick.

• Layers typically vary significantly in thickness, degree of cementation, and hardness over relatively short distances.

• Varies in composition from primarily fine-grained material to primarily coarse-grained material.

• Moderately hard, moderately cemented caliche can generally be gouged with a knife with difficulty and broken with a few hammer blows.

• Hard and very hard, strongly cemented caliche is difficult to scratch with a knife and breaks with difficulty with repeated hammer blows.

• Impedes earthwork operations, including grading and utility line trenching. Rock excavation methods are generally needed.

7.1 Laboratory Tests

Laboratory tests were performed on selected samples of native soil obtained from the borings.

Results of in-place moisture content and dry density tests are also presented on the boring logs in Appendix A. Additional information regarding the laboratory test procedures and results are provided in Appendix B and Appendix C. Laboratory test results are summarized in the following table.

Table 5 – Summary of Laboratory Test Results

Test Type Test Results Remarks

Atterberg Limits

Liquid Limit

Plastic Limit

Plasticity Index

Non Plastic and 40

Non Plastic and 28

Non Plastic and 12

Low plasticity

Direct Shear

Friction Angle

Cohesion

30 to 42 degrees

0 to 90 psf

Maximum Dry Density (pounds per cubic foot, pcf)/Optimum Moisture Content

119.3 pcf/10.6 to

126.8 pcf/9.6 percent

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Table 5 – Summary of Laboratory Test Results

Test Type Test Results Remarks

Resistivity (Saturated Condition) 860 to 6,700 Ohm-cm Moderately to severely corrosive to steel

Chloride Content 0.00033 to 0.015 percent Low corrosive potential to buried metal pH 7.57 to 8.27 --

Redox Potential 157 to 189 mV --

Sodium Sulfate Content 0.00079 to 0.065 percent Low chemical heave (salt heave) potential

Sodium Content 0.00081 to 0.021 percent --

Sulfate Content 0.00053 to 0.46 percent Low to severe sulfate exposure class (S2)

Total Salts (Solubility) 0.019 to 1.4 percent Low solubility potential

7.2 Groundwater

Groundwater was not encountered in our exploratory borings, which were advanced to depths of up to approximately 51.5 feet. Referenced well log data (State of Nevada, Division of Water

Resources (NDWR), 2024) indicate that the groundwater level is approximately 300 feet or more below the existing ground surface in the vicinity of the switch station site (Well Log 138902).

However, seasonal fluctuations in groundwater levels may occur. These fluctuations may be due to variations in ground surface topography, subsurface geologic conditions, rainfall, irrigation, and other factors. Evaluation of factors associated with groundwater fluctuations and surface water flow was beyond the scope of this study.

8 FINDINGS AND CONCLUSIONS

Based on findings of this evaluation, it is Ninyo & Moore’s opinion that there are no known geotechnical or geologic conditions that would preclude development of the proposed project at the subject site. Preliminary findings and conclusions regarding geotechnical aspects of the proposed project include the following:

• Significant amounts of cobbles and small boulders were encountered in our exploratory borings. Oversized materials (particles greater than approximately 4 inches nominal diameter) will be generated during excavation of soils at the subject site. These materials will need to be crushed prior to being used as structural fill or backfill, or removed from the site and disposed of in a suitable manner.

• Based on the relatively low moisture content and granular nature of some native soils encountered in our exploratory excavations, caving of soils should be anticipated during earthwork and excavation operations for this project.

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• Some on-site soils are salt-laden and concrete in contact with soil should be proportioned for durability to resist severe sulfate-exposure: Type V sulfate-resistant cement, maximum

0.45 water-to-cement ratio, and minimum 4,500-psi 28-day strength.

• Proposed structure foundations should be founded on medium dense to very dense native granular soils, stiff native fine-grained soils, or on adequately placed and compacted structural fill (reworked native or import soils).

• Caliche was not encountered in our exploratory borings. However, due to the highly variable nature of this rock-like cemented soil, caliche may be encountered during earthwork operations. Utilization of rock/heavy-duty excavation techniques should be anticipated where caliche is encountered.

• The switch station site and transmission line alignment are subject to occasional heavy rainfall and flash flooding. Swales and washes that extend through and near the project site may convey significant flows during and after rainfall events. Potential surface flooding and associated erosion may be significant design considerations for this project.

• Based on the findings of our subsurface evaluation and laboratory test results, native soils encountered in our explorations should generally be suitable for use as structural fill and backfill material for the project. However, it should be anticipated that some cobbles and possible boulders will be generated during grading and excavation operations for this project.

These oversize materials will need to be screened out or broken down/crushed prior to utilization as structural fill and backfill, or exported from the project site.

• Laboratory tests of representative soil samples collected from our subsurface explorations performed at the project site indicate the presence of soil that may be considered to be corrosive to steel. Appropriate measures should be taken to protect steel that will be in contact with on-site soils.

• In accordance with the referenced International Building Code, the seismic parameters provided in Table 4 in Section 5.3 are characteristic of the site and transmission line/ alignment, and may be used in design of the proposed structures.

• Review of published geologic data and our field observations do not indicate the presence of on-site geologic faults. It is Ninyo & Moore’s opinion that the potential for adverse effects of this fault on the project is low.

• Review of referenced geologic data does not indicate the presence of ground fissures at the switch station site or along the transmission line alignments, and no ground fissures were observed in the vicinity of the project during our field activities. However, ground cracking has been observed in areas approximately 3 miles south of the south terminus of the proposed transmission alignment. It is Ninyo & Moore’s opinion that the potential for adverse effects due to ground cracking is low.

• Review of published groundwater data indicates that the groundwater level at the project site is greater than 100 feet deep. Based on the anticipated depth of the static groundwater table, it is our opinion that there is a low potential for liquefaction of subsurface soils at the project site

• Groundwater was not encountered in our explorations to the total depths explored and is not anticipated to be a design concern.

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9 RECOMMENDATIONS

The following recommendations are intended for incorporation into the design and construction of the subject project.

9.1 Earthwork

The following subsections provide recommendations for earthwork, including site grading, structural fill and backfill, import soil, and temporary excavations.

9.1.1 Site Grading

Prior to grading, proposed structure and improvement areas should be cleared of any surface obstructions (including cobbles and boulders), debris, organics (including vegetation), and other deleterious material. Materials generated from clearing operations should be removed from the project site for disposal (e.g. at a legal landfill site).

We recommend that the full depth of any surficial loose and/or disturbed native soils be removed from proposed structure and improvement areas, including equipment slab/pad foundation, block screen/retaining wall, pavement, and concrete flatwork areas. These excavated soils may be processed and stockpiled for later use as structural fill or backfill, provided they comply with the recommendations presented in this report.

After the removals described above have been made, the exposed native soils should be scarified to approximately 6 inches, moisture-conditioned to approximately optimum moisture content, and compacted to 95 percent relative compaction, as evaluated by American Society for Testing and Materials (ASTM) Standard D 1557. The project’s geotechnical consultant should observe excavation bottoms and areas to receive fill at the time of grading to assess the suitability of the exposed material and to evaluate if removals down to more competent soils are needed.

Surface preparations should extend 5 feet or more beyond the exterior edges of planned equipment slab pad/foundations and 2 feet or more beyond planned pavement, exterior concrete flatwork, and retaining/screen walls, or to a lateral distance that is equivalent to the depth of compacted structural fill, whichever is greater.

Based on the relatively low moisture content and loose, granular nature of some native soils encountered in our exploratory excavations, caving of soils should be anticipated during earthwork and excavation operations for this project.

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Significant amounts of cobbles and boulders were encountered in our exploratory borings.

Oversized materials (particles greater than approximately 4 inches nominal diameter) will be generated during excavation of soils at the switch station site and along the transmission line/access road alignments. These materials will need to be crushed prior to being used as structural fill or backfill, or removed from the site and disposed of in a suitable manner. Bulking of this material should be anticipated when it is excavated, processed/crushed, and compacted. For planning purposes, approximately 10 percent bulking should be anticipated.

Soils excavated in areas of proposed project improvements may be re-used as structural fill and backfill provided they conform to recommendations provided in Section 9.1.2.

Caliche was not encountered in our exploratory borings. Due to its variable nature, layers of caliche may be present in subsurface soils between and beyond our explorations at the project site. Grading and excavation activities may be impeded by caliche and cemented soil layers, if encountered. Rock excavation techniques such as use of heavy-duty ripping equipment, heavy duty backhoe, headache ball, hoe-ram, and/or rock saw may be needed.

9.1.2 Structural Fill and Backfill

Based on the findings of our subsurface evaluation and laboratory test results, native soils encountered in our exploratory borings should generally be suitable for use as structural fill and backfill material for the project. However, oversize materials will likely be generated during excavation of the native soils at the switch station site. These materials will need to be screened out or crushed prior to use as structural fill and backfill, or removed from the site and disposed of in a suitable manner.

Structural fill and backfill soils should not contain significant amounts of organic matter, debris, other deleterious matter, or rocks or hard chunks larger than approximately 4 inches nominal diameter (over-size material). These soils should have a low solubility potential of

2.0 percent or less, and a very low to low expansion potential (Expansion Index, EI, less than

50, as evaluated by ASTM D 4829).

Soils used as structural fill and backfill should be moisture-conditioned to approximately optimum moisture content and placed and compacted in uniform horizontal lifts to a relative compaction of 95 percent or more, as evaluated by the ASTM D 1557. The optimal lift thickness of fill will depend on the type of soil and compaction equipment used, but should generally not exceed approximately 8 inches in loose thickness. Placement and compaction of structural fill should be performed in accordance with applicable building codes.

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Earthwork operations should be observed and compaction of structural fill and backfill materials should be tested by the project’s geotechnical consultant. Typically, one field test should be performed per lift for each approximately 500 cubic yards of fill placement in structural areas. Additional field tests may also be performed in structural and non-structural areas at the discretion of the geotechnical consultant.

9.1.3 Import Soil

Import soil should be similar to on-site soils consisting of coarse-grained material (50 percent or more retained on the No. 200 sieve). Import soils should have a low solubility potential of

2.0 percent or less, a low sulfate content (less than 1.0 percent), and a very low to low expansion potential (Expansion Index, EI, less than 50, as evaluated by ASTM D 4829).

Import soil should not contain significant amounts of organic matter, debris, other deleterious matter, or rocks or hard chunks larger than approximately 4 inches nominal diameter. We further recommend that proposed import material be evaluated by the project’s geotechnical consultant at the borrow source for its suitability prior to importation to the project site. Import soil should be moisture-conditioned and placed and compacted in accordance with the recommendations set forth in the previous section.

9.1.4 Temporary Excavations

Temporary slope surfaces should be kept moist to retard raveling and sloughing. Water should not be allowed to flow over the top of excavations in an uncontrolled manner.

Stockpiled material and/or equipment should be kept back from the top of excavations a distance equivalent to the depth of the excavation or more. Workers should be protected from falling debris, sloughing, and raveling in accordance with Occupational Safety and Health

Administration (OSHA) regulations (OSHA, 2016). Temporary excavations should be observed by the project’s geotechnical consultant so that appropriate additional recommendations may be provided based on the actual field conditions. Temporary excavations are time sensitive and failures are possible.

9.2 Structure Foundations

The following sections provide recommendations for conventional spread footings, mat foundations, and deep foundations planned for support of the proposed structures.

9.2.1 Conventional Spread Foundations

Structure foundations consisting of spread footings should extend 18 inches or more below the lowest adjacent finished grade and bear on medium dense to very dense native granular soils, or on adequately placed and compacted structural fill (reworked native or import soils).

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Structure footings should have a width of 12 inches or more. Footings should be reinforced in accordance with the project structural engineer’s recommendations.

An allowable bearing capacity of 2,000 pounds per square foot (psf) may be used for conventional spread footings with an embedment depth of 18 inches below adjacent finished grade and a width of 12 inches. The allowable bearing capacity may be increased by 500 psf for each additional 1 foot of width and 1,000 psf for each additional 1 foot of embedment up to 4,000 psf. The allowable bearing capacity, which was developed considering a factor of safety of 2.5, may be increased by one-third for short duration loads, such as wind or seismic.

Lateral resistance for footings is presented in Section 9.3. Seismic parameters for design of structures at the site are provided in Table 4 in Section 5.3. Foundations should be designed and constructed in accordance with the recommendations of a qualified structural engineer.

Based on our evaluation of spread footing bearing capacity, we anticipate that settlement of foundation material will be on the order of 1 inch or less. We estimate footing differential settlement of about ½-inch over a horizontal span of about 40 feet for spread footings.

9.2.2 Mat Foundations

Mat foundations should extend 12 inches or more below the lowest adjacent finished grade and bear on 6 inches of Type II Aggregate Base over undisturbed medium dense to very dense native granular soils, or on adequately placed and compacted structural fill (reworked native or import soils).

The aggregate base should be compacted to 95 percent relative compaction, as evaluated by ASTM D 1557. Mat foundations may be designed using an allowable net bearing capacity of 2,500 pounds per square foot (psf). The allowable bearing capacity may be increased by one-third when considering loads of a short duration such as wind or seismic forces. Mat foundations may be designed using an average modulus of subgrade reaction, k, of

200 pounds per square inch (psi) per inch.

9.2.3 Drilled Shaft Foundations

We understand that the drilled shaft foundations for the transmission line and other structures will be designed using the Electric Power Research Institute (EPRI) computer program

Moment Foundation Analysis and Design (MFAD) and/or LPILE (Ensoft, 2016). The design soil parameters for each boring along the transmission line alignment and for the switch station site for use with the MFAD/LPILE programs are presented in Appendix F. The soil parameters provided in Appendix F are based on encountered subsurface conditions, standard penetration test correlations, and laboratory test results.

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Drilled shafts at the switchyard site may be designed using the soil parameters presented in the table for Boring B-3 presented in Appendix F. The end bearing values provided in

Appendix F are based on the assumption that drilled shaft foundations will be approximately the dimensions provided in Table 1, and that the bottom of the shaft excavations will be cleaned of loose or other unsuitable material. The allowable uplift capacity of drilled shaft foundations may be computed using 70 percent of the allowable skin friction. We recommend that the upper 12 inches of soil be neglected when calculating uplift resistance. Allowable drilled shaft capacities should include a factor of safety of 2.5 and may be increased by one-third for short duration loads, such as wind or seismic. Potential scour should also be taken into consideration when evaluating lateral capacity of drilled shafts.

For axial loading, drilled shafts should be spaced at least 3B (where B is the diameter of the shaft) center-to-center. Group-effects of relatively closely spaced drilled shafts (spaced less than approximately 3B center-to-center) were not considered in our analysis. If closely spaced drilled shafts are used, group-effects should be evaluated, as appropriate.

For lateral loading, drilled shafts in a group may be considered to act individually when the center-to-center spacing is greater than 3B (where B is the diameter of the shaft) in the direction normal to loading and greater than 6B in the direction parallel to loading. If closely spaced drilled shafts/driven piles are used, group-effects should be evaluated, as appropriate.

9.2.3.1 Drilled Shaft Construction Considerations

As discussed previously, significant amounts of cobbles and boulders were encountered in some of our explorations. Accordingly, difficult drilled shaft excavation conditions should be anticipated.

Caving of the upper soils should be anticipated during drilled shaft excavation due to the relatively low moisture content and loose, granular nature of some of the surficial native soils encountered in our exploratory excavations. We recommend that the contractor be prepared to take appropriate measures during construction/excavation to reduce the potential for caving of the drilled holes, including the use of casing and/or drilling mud. In addition, we recommend that measures, such as placement of concrete by tremie method, are implemented so that aggregate and cement do not segregate during concrete placement. When possible, reinforcing steel and shaft concrete should be placed the same day the shaft excavation is drilled. To aid in reducing the potential for “blowout” between adjacent shaft excavations, it may be necessary to drill and fill the shafts alternately, allowing the concrete to cure 8 hours or more before drilling the adjacent shaft. Concrete compressive strength and steel reinforcement should be specified in accordance with recommendations of a qualified structural engineer.

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The bottom and sidewalls of each drilled excavation should be evaluated in the field during construction by the geotechnical consultant. If the encountered geotechnical conditions are significantly different than those used in design of the drilled shaft, our office should be notified and additional foundation recommendations, if warranted, will be provided upon request. The contractor should make provisions to provide for the integrity of the excavation and to make sure that the excavations are cleaned and straight, and that sloughed, loose, or soft soil is removed from the bottom of excavations prior to placement of concrete. Drilled shaft excavation bottoms need to be relatively clean.

Concrete should be placed in the drilled excavation as soon as practicable after drilling and evaluation by the geotechnical consultant. Concrete should have an ultimate strength not less than that specified, and, if concrete is not precast, should be workable and plastic so that it may be placed without segregation. Concrete should be cast-in-place against undisturbed earth in the hole in such a manner to provide for the exclusion of appreciable amounts of foreign matter in the concrete. The shafts should be adequately reinforced for lateral and uplift loads, as recommended by the project structural engineer. Additional general recommendations regarding drilled shaft construction are provided in Appendix G.

9.3 Lateral Earth Pressures

Retaining walls that are not restrained from movement at the top and having level, granular backfill behind the wall may be designed using an “active” lateral earth pressure as indicated on Figure 4.

Retaining walls that are restrained from movement at the top and having level, granular backfill behind the wall may be designed using an “at-rest” lateral earth pressure as indicated on Figure 5.

The locations of the resultant forces due to these lateral earth pressures are also provided on

Figure 4 and Figure 5. These lateral earth pressure values assume compaction within about 5 feet of the wall will be accomplished with relatively light compaction equipment and that very low to low expansive backfill will be placed behind the wall. These values also assume that retaining walls will have a height of approximately 10 feet or less.

The Seismic Design Category is anticipated to be C, and due to the relatively low seismic demand and risk category of the proposed structures, seismic earth pressure is not recommended to be included in the design of retaining walls.

Retaining walls should also be designed to resist an “active” and “at-rest” surcharge pressure as shown on Figure 4 and Figure 5. The value for “q” represents the pressure induced by adjacent light loads, uniform slab, or traffic loads plus any adjacent footing loads.

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For passive resistance to lateral loads, we recommend that a passive lateral earth pressure as shown on Figure 4 be used up to a value of 3,000 psf. This value assumes that the ground surface is horizontal for a distance of 10 feet, or three times the height generating the passive pressure, whichever is more. We recommend that the upper 12 inches of soil not protected by pavement or a concrete slab be neglected when calculating passive resistance. For frictional resistance to lateral loads, we recommend that a coefficient of friction of 0.41 may be used between soil and formed concrete contacts. Passive and frictional resistances may be used in combination, without restriction.

Measures should be taken so that hydrostatic pressure does not build up behind retaining walls.

Drainage measures, as indicated on Figure 6, should include open-graded gravel and perforated drain pipe or weep holes lined with polyvinyl chloride (PVC) pipe. Drain pipes should outlet away from structures and retaining walls. Retaining walls should be damp-proofed in accordance with the recommendations of a qualified civil engineer or architect.

9.4 Concrete Slab-On-Grade Floors

Concrete slab-on-grade floors should be designed by the project’s structural engineer based on anticipated loading conditions. Ninyo & Moore recommends that conventional concrete slab-on-grade floors for this project be founded on 6 inches of Type II Aggregate Base overlying a 6 inch thick zone of adequately placed and compacted structural fill. The structural fill thickness may include 6 inches of scarified and recompacted soils. Aggregate base underlying concrete slab-on-grade floors should be compacted to 95 percent of the laboratory maximum dry density

(ASTM D 1557).

Floor slabs should be 4 inches or more in thickness and reinforced with No. 4 steel reinforcing bars placed at 24 inches on-center both ways. Reinforcement of the slab should be placed at mid-height. We recommend that “chairs” be utilized to aid in the placement of the reinforcement.

Increased slab thickness and reinforcement may be recommended by the structural engineer. As a means to reduce shrinkage cracks, we recommend that conventional slab-on-grade floors be provided with control joints in accordance with the recommendations of a qualified structural engineer. Recommendations regarding concrete utilized in construction of floor slabs are provided in a subsequent section of this report.

As an alternative to slab reinforcement with steel reinforcing bars, post-tensioned slabs designed by a qualified structural engineer may be considered. Geotechnical recommendations for design of post-tensioned slabs-on-grade will be provided by Ninyo & Moore upon request.

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Ninyo & Moore recommends that a moisture barrier be provided by a membrane placed beneath concrete slab-on-grade floors, particularly in areas where moisture-sensitive flooring is to be used.

The membrane should consist of visqueen 10 mils in thickness, or an appropriate equivalent. The visqueen should overlie the previously described compacted base material.

9.5 Pavement Sections

Recommendations for flexible and rigid pavements are provided in this section.

9.5.1 Flexible Pavement

To form a basis for design of flexible pavement for on-site paved parking and access areas at the switch station site, we have assumed the following:

• A design Equivalent Single Axial Load (ESAL) value of 3,000, based on Traffic Index (TI) = 4.5 for automobile traffic; an ESAL value of 16,000, based on TI = 5.5 for delivery truck traffic; and an ESAL value of 65,000, based on TI = 6.5 for heavy duty truck and bus traffic areas are applicable.

• A reliability of 80 percent.

• A standard deviation of 0.45.

• An initial serviceability index of 4.2.

• A terminal serviceability index of 2.5.

• A subgrade resilient modulus (MR) of 13,400 pounds per square inch (psi) for an R-value of 50 (based on soil classification).

Using these values, structural numbers associated with the proposed parking and access areas were calculated using design procedures in accordance with the American Association of State Highway and Transportation Officials method of designing flexible pavement

(AASHTO, 1993) requirements. The following table presents recommended structural pavement sections placed over structural fill for on-site parking and access areas.

Table 6 – Recommended Flexible Pavement Section Thickness

Traffic Type Design

ESAL

Pavement (aasphalt = 0.35)

Base (abase = 0.12)

Subgrade

Structural Number Provided

Structural Number Needed

Asphalt Concrete

Thickness (Inches)

Type II Base Thickness (Inches)

Structural Fill Thickness (Inches)*

Automobile 3,000 2.5 4.0 6.0 1.36 0.88

Delivery Truck 16,000 2.5 4.0 6.0 1.77 1.25

Heavy Duty Truck 65,000 3.0 5.0 6.0 1.65 1.62

Note: *Structural fill below pavement sections may include 6 inches of scarified and recompacted native soil.

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If the assumed traffic or design ESAL values are not considered appropriate, this office should be notified. In providing these recommendations for pavement sections, we have assumed that asphalt concrete will be mixed and placed in accordance with Section 401 of the referenced Uniform Standard Specifications (RTC, 2024). We have also assumed that Type II

Aggregate Base will conform to Section 704.03.04 of the referenced Uniform Standard

Specifications. Type II Aggregate Base materials should be placed and compacted to at least

95 percent relative compaction, as evaluated by ASTM D 1557 and in accordance with

Section 302 of the Uniform Standard Specifications.

9.5.2 Rigid Pavement

Ninyo & Moore recommends that Portland cement concrete pavement be utilized in trash dumpster and other heavy traffic areas. Our experience indicates that refuse truck traffic and other heavy traffic can significantly shorten the useful life of asphalt concrete pavement. We recommend that, in dumpster approach and other heavy traffic areas, 650 pounds per square inch (psi) flexural strength Portland cement concrete, 7 inches thick, be placed over 6 inches of compacted Type II Aggregate Base over 6 inches of adequately placed and compacted structural fill. We also recommend that a qualified structural engineer be consulted for appropriate reinforcement of concrete pavement.

9.5.3 Pavement Considerations

We recommend that mix designs be made for the asphalt concrete and Portland cement concrete by an engineering company specializing in this type of work.