HWAD NEW BOMB Drainage Improvements Project- Geotechnical Investigation Report.pdf

PDF 4 MB Posted Sep 18, 2025

Attached to: Hawthorne Army Depot New Bomb Request for Information
Solicitation number: W519TC-25-I-HWAD
Issued by: Department of the Army Materiel Command Contracting Command Rock Island Arsenal

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Written by GovTribe AI · Sep 18, 2025

This document is a geotechnical investigation report for the HWAD New Bomb Drainage Improvements Project at the Hawthorne Army Depot in Nevada. The project involves designing and constructing 3 detention basins and 1 retention basin to control potential storm water runoff from a canyon used to store and dispose of unwanted ordinance. The basins will be located on piedmont alluvial fans downstream of a steep mountain canyon, with a total site area of approximately 13 acres.

The investigation included 22 test pits excavated to depths of 10-17 feet, with soil analysis revealing predominantly poorly graded sand with silt and gravel. The site soils are light brown, dry, medium dense, and contain angular cobbles up to 8 inches in diameter. Slope stability analyses were conducted for various embankment configurations, including 35.5-foot weirs with a 2.5H:1V slope and 25-foot cut slopes. The study determined the slopes will be stable under static and seismic conditions, with factors of safety exceeding recommended minimums. Recommendations include proper compaction, moisture conditioning, and rip rap lining to prevent erosion and ensure structural integrity.

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Black Eagle Consulting

Geotechnical Investigation

HWAD New Bomb Drainage Improvements

Project Hawthorne Army Depot

Hawthorne, Nevada Project Number:

TS-W519TC-24-R-0003

September 4, 2024

Prepared for Bighorn Consulting

Table of Contents

Introduction

Project Description

Development Information Grading Concepts

Site Conditions

Existing Conditions Topography Vegetation

Exploration

Test Pits Material Classification

Laboratory Testing

Index Tests Laboratory Moisture-Density Relation Tests Hydraulic Conductivity Tests Chemical Tests

Geologic and General Soil Conditions

Geologic Hazards

Seismicity Faults Ground Motion and Liquefaction Floodplains Other Geologic Hazards

Discussion and Recommendations

General Information Site Preparation Trenching, Excavation and Utility Backfill Mass Grading Subsidence and Shrinkage Seismic Design Parameters Slope Stability Erosion Control Site Drainage Concrete Spandrel Gravel Surfacing

Anticipated Construction Problems

Table of Contents

T Quality Control

Standard Limitations Clause

References

Tables

1 - Maximum Allowable Temporary Slopes 2 - Seismic Design Criteria Using 2018 International Building Code 3 - Design Soil Parameters 4 - Global Stability Analysis Results 5 - Minimum Required Properties for Separator Geotextile

Plates

1 - Plot Plan 2 - Test Pit Logs 3 - USCS Soil Classification Chart 4 - Index Test Results 5 - Compaction Test Reports 6 - Hydraulic Conductivity Test Results

Appendices

A - Slope Stability Analyses B - Seismic Design Coefficient

Introduction

Introduction Presented herein are the results of Black Eagle Consulting, LLC’s (BEC’s) geotechnical investigation, laboratory testing, and associated geotechnical design recommendations for the proposed drainage basins to be located within the Hawthorne Army Depot in Mineral 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, and groundwater conditions pertaining to design and construction of the proposed drainage project.

2. Provide recommendations for design and construction of the project as related to these geotechnical conditions.

The area covered by this report is shown on Plate 1 (Plot Plan). Our investigation included field exploration, laboratory testing, and engineering analysis to determine the physical an 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 conclusions and recommendations.

The services described above were conducted in accordance with the BEC Professional Geotechnical Agreement dated January 12, 2024, which was signed by Mr. Eric Anderson, President of Bighorn Consulting. This work was conducted for the Hawthorne Army Depot Project Number TS-W519TC-24-R- 0003.

Project Description

Project Description The proposed New Bomb Basin site consists of an approximate 13-acre area located west of State Highway 359 approximately 21 miles south of Hawthorne, Nevada. The site is located in Sections 33 and 34, Township 5 North, Range 30 East, Mount Diablo Meridian. The site is bordered to the west by steep mountainous terrain, to the north and south by moderately sloping alluvial fans and to the east by State Route 359. The site is presently undeveloped land adjacent the previous Bomb Basin. Access to the site is obtained by from State Highway 359

Development Information The proposed New Bomb Basin project will provide a series of new retention basins downstream of a high mountain valley with an ephemeral wash subject to flash flooding. The purpose of the retention basins are to reduce peak flows and retain sediment potentially laden with munitions, and preventing discharge from overflowing existing culverts beneath State Highway 359. The existing detention basins were filled in September 2023 during a flash flooding event, necessitating construction of a new facility.

The new facilities will include a rip rap lined inlet channel, a concrete spandrel crossing an unimproved access road, and a 3-cell detention basin with earthen embankment weirs. Intermediate flows from the access road will also be collected in a roadside channel and conveyed to a retention basin northeast of the proposed main detention basins. The basins will include embankments with rip rap lining the upstream and downstream faces to prevent scour or erosion of the embankment weirs.

Grading Concepts The project will involve cut and fill on the order of 15 and 35 feet to establish design grades. The site materials are exclusively granular sands that are suitable for support of the spandrel in cut and can be reused as structural/ embankment fill. Generally, the embankment weirs will require over-excavation to provide uniform densified embankment fills through their entire height. The perimeter longitudinal berms will lie in cut or fill. The site materials are well below optimum moisture content and considerable water will be necessary to moisture condition fill soils and provide dust control.

Site Conditions

Site Conditions – view to west

Site Conditions Existing Conditions The site is accessed from an unimproved road off of State Highway

359. Pole Line Road parallels State Highway 359 approximately 500 feet to the west. A power transmission line is located between the highway and Pole Line Road. Between the access road and new basin site are the old basins.

The old basins consist of 3 radial basins growing in size downslope (to the east) and were filled by debris during flash flooding in the fall of 2023. South of the old basins another unimproved road extends toward the mouth of the canyon, with an overhead power line on its southern shoulder. This power line will remain, and will be near the northern grading limits for the new basins. Unimproved parking areas are present north and south of the access road at the range front. The main access road continues up into the mountains with a drainage channel on its southern shoulder. A small building is located within the southern parking area.

Topography The project is located downstream of a steep, narrow mountain canyon. The proposed retention facilities will be located on piedmont alluvial fans downslope of the mouth of the canyon. The site is approximately 1800 feet long from the mouth of the canyon to the crest of the outlet structure, with approximately 245 feet of vertical relief, and an average slope of approximately 13 percent to the east-southeast. The alluvial fan surface includes a few smaller intermittent drainages that are steeply incised up to about 5 feet deep.

These drainages will be intercepted and routed around the main retention basins.

Vegetation The site is moderately vegetated by desert brush up to about 5 feet tall. The adjacent hills include pinion pines.

Exploration

Test Pit Exploration

Exploration Test Pits The site was explored on July 17 – 18, 2024, by excavating 22 test pits using a John Deere 200D trackhoe. Locations of the test pits are shown on Plate 1. The maximum depth of exploration was 17 feet below the existing ground surface.

Bulk samples for index testing were collected from the trench wall sides at specific depths in each soil horizon. The test pits were backfilled immediately after exploration. Backfill was loosely placed and the area regraded to the extent possible with equipment on hand.

Material Classification A geotechnical engineer examined and identified all soils in the field in accordance with American Society for Testing and Materials (ASTM) D2488. During test pit exploration, representative bulk samples were placed in sealed plastic bags and returned to our Reno, Nevada, laboratory for testing. Additional soil classification was subsequently performed in accordance with ASTM D2487 (Unified Soil Classification System [USCS]) upon completion of laboratory testing, as described in the Laboratory Testing section.

Logs of the test pits are presented as Plate 2 (Test Pit Logs), and a USCS chart has been included as Plate 3 (USCS Soil Classification Chart).

Elevations shown on Plate 2 are approximate and have been estimated from site topography from Bighorn Engineering. Elevations should be closely compared and converted to actual surveyed elevations on the project site once survey and grading plans are available.

Laboratory Testing

Grain Size Analysis

Laboratory Testing

All soils testing performed in the BEC soils laboratory is conducted in general accordance with the standards and methodologies described in Volumes 4.08 and 4.09 of the ASTM Standards.

Index Tests Samples of each significant soil type were analyzed to determine their in-situ moisture content (ASTM D2216), grain size distribution (ASTM D6913), and plasticity index (ASTM D4318). The results of these tests are shown on Plate 4 (Index Test Results). Test results were used to classify the soils according to ASTM D2487 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 to published charts (Bowles, 1996; Naval Facilities Engineering Command [NAVFAC], 1986a and b) to evaluate bearing capacity, lateral earth pressures, and settlement potential.

Laboratory Moisture-Density Relation Tests Moisture-density relation tests (ASTM D1557) were performed on representative samples of the proposed embankment fill materials. The maximum density shown by this test is compared with field densities to determine the percent of relative compaction. The moisture density curve is included as Plate 5 (Compaction Test Report).

Hydraulic Conductivity Tests Falling head hydraulic conductivity tests (ASTM D5084) were performed on remolded samples of embankment fill and foundation soils and results are included as Plate 6 (Hydraulic Conductivity Report).

The tests were performed at various overburden pressures to simulate foundation and embankment characteristics.

Chemical Tests Chemical testing is being conducted to determine the site soils potential to corrode buried Portland cement concrete (PCC) and metal. Results will be advanced in an Addendum to the report.

Geologic and General Soil Conditions

Geologic and General Soil Conditions The site lies in an area mapped by the United States Geological Survey (Stewart and Carlsen, 1978) as Quaternary Age Alluvium. The consists of piedmont alluvial fans with moderately incised drainages. The alluvial fans coarsen upslope and include cobbles and boulders near the mouth of the canyon and near its main channel. The sit soils are consistent with the geologic map.

The site soils are exclusively granular well-graded to silty sands. The site soils are described as light brown, dry to slightly moist, medium dense, and as containing 6 to 13 percent non-plastic fines, mostly fine to medium sand, and 13 to 24 percent subangular to angular gravel. Occasional interbedded layers include up to 10 percent cobbles.

Groundwater was not encountered during exploration and is expected to lie at a depth below that which would affect design or construction.

Geologic Hazards

Geologic Hazards Seismicity Much of the western United States is a region of moderate to intense seismicity related to movement of crustal masses (plate tectonics). By far, the most seismically active regions, outside of Alaska, are in the vicinity of the San Andreas Fault system of western California. Other seismically active areas include the Wasatch Front in Salt Lake City, Utah, which forms the eastern boundary of the Basin and Range physiographic province, and the eastern front of the Sierra Nevada mountains, which is the western margin of the province. The Hawthorne area lies near the eastern base of the Sierra Nevada, within the western extreme of the Basin and Range. It must be recognized that there are probably few regions in the United States not underlain at some depth by older bedrock faults. Even areas within the interior of North America have a history of strong seismic activity.

The Hawthorne Army Depot area lies within an area with a high potential for strong earthquake shaking.

Seismicity within the Hawthorne area is considered about average for the western Basin and Range Province (Ryall and Douglas, 1976). 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 frontal fault system of the eastern Sierra Nevada. The most active segment of this fault system in the Reno area is located at the base of the mountains near Hawthorne and the Walker Lake Valley, some 10 miles north of the project.

Faults An earthquake hazards map is not available for the project area. The Nevada Bureau of Mines and Geology’s (NBMG’s) MyHazards web-mapping tool (NBMG, 2024) show the Wassuk Range fault zone as passing through the piedmont, range front interface within the western portion of the site and extends north and south of the project. The fault is identified as Quaternary in age (less than 1.8 million years old).

Aerial image analysis and site reconnaissance indicate the main alluvial fan surface is Pleistocene in age and consistent with the geologic map. The alluvial surface has smoothed surfaces without bar and swale morphology, and local drainages are incised over 2 meters into the fan surface. Locally incision is greater than 4 meters, and indicative of a surface abandoned over 35,000 years ago. No obvious fault scarps are present within the mapped fault zone indicating the most recent age of movement for this fault segment is mid- to early-Pleistocene in age.

The Nevada Earthquake Safety Council (1998) has developed and adopted the criteria for evaluation of Quaternary age earthquake faults. Holocene Active Faults are defined as those with evidence of movement within the past 10,000 years (Holocene time). Those faults with evidence of displacement during the last 130,000 years are termed Late Quaternary Active Faults. A Quaternary Active Fault is one that has moved within the last 1.6 million years. An Inactive Fault is a fault without recognized activity within Quaternary time (last 1.6 million years). Holocene Active Faults normally require that occupied structures be set back a

Geologic Hazards

1345 Capital Boulevard, Suite A | Reno | Nevada 89502 | 775-359-6600 8 3839 N. Carson Street, Suite 101 | Carson City | Nevada 89706 | 775-222-0080 www.blackeagleconsulting.com minimum of 50 feet (100-foot-wide zone) from the ground surface fault trace. An Occupied Structure is considered a building, as defined by the International Building Code (IBC), which is expected to have a human occupancy rate of more than 2,000 hours per year (International Code Council [ICC], 2018).

The setback from Quaternary Active Faults is left to the judgment of the geologist/engineer; however, no Critical Facility is permitted to be placed over the trace of a Late Quaternary Active Fault. A Critical Facility is defined as a building or structure that is considered critical to the function of the community or the project under consideration. Examples include, but are not limited to, hospitals, fire stations, emergency management operations centers and schools.

Based on the geologic map, the fault in the vicinity of the project is a Quaternary Active Fault. The mapped fault is located within the vicinity of the Spandrel, upslope of the detention basins. Based on the age of the mapped fault and lack of surface fault expression, it is our opinion the mapped fault does not require detailed investigation and does not pose a significant risk to the project.

Recurrence intervals for Nevada earthquakes along faults that have been studied are estimated to be in the range of 6,000 to 18,000 years in western Nevada (Bell, 1984). The very active eastern boundary faults of the Sierra Nevada mountains may have a shorter recurrence interval of 1,000 to 2,000 years. Many of the smaller faults may be the result of one-time events in response to movement along a better developed and more active fault system a considerable distance away.

Ground Motion and Liquefaction The United States Geological Survey seismic design maps that have been incorporated with the American Society of Civil Engineers (ASCE) Online ASCE 7 Hazard Tool indicate that there is a 2 percent probability that a bedrock ground acceleration of 0.62 g will be exceeded in any 50-year interval (ASCE, 2024). Only localized amplification of ground motion would be expected during an earthquake. Some amplification of ground motion could result from the site’s location on the margin of the Great Basin physiographic province.

Because the site area is underlain by dense granular soils and groundwater is relatively deep, liquefaction potential is minimal. The duration of full pools within the detention basin limit the possibility of fully saturated soils, and the embankments will be controlled embankment fills with moderate slopes.

Floodplains The Federal Emergency Management Agency (FEMA) has not printed a Flood Insurance Rate Map (FIRM) panel for the project site (FEMA, 2012a & 2012b). The site is subject to flash flooding, and the detention facilities are being designed based on 100-year and 500-year flood events.

Other Geologic Hazards A high potential for dust generation is present if grading is performed in dry weather. No other geologic hazards were identified.

Discussion and Recommendations

Discussion and Recommendations General Information The project will consist of the design and construction of 3 detention basins and 1 retention basin to control potential storm water runoff from a canyon used to store and dispose of unwanted ordinance. Flows out of a mountain canyon will be collected and conveyed across PCC spandrel, then by a rip rap lined channel into the detention basins. The retention basin will collect runoff from the access road downstream of the canyon mouth separately. The detention basins will be separated by 2 embankment weirs approximately 35 feet in height with pool depths of approximately 12 to 17 feet. The basin side slopes will be in a combination of cut and fill, while the weirs will be constructed of embankment fills.

The site soils are exclusively granular well-graded to silty sands with moderate amounts of gravel and limited cobbles. The soils are suitable for reuse as embankment fill and support of the improvements in cut.

Over-excavation of the base of the weirs and side slopes will be necessary to ensure consistent material properties and bonding of the embankments to the original ground. The site soils have high hydraulic conductivity and are erodible such that proper rip rap lining of the slopes will be necessary. Global stability analyses of 3 critical slopes for the project indicate that embankment weirs up to 2.5H:1V (horizontal to vertical), composite side slopes up to 2.2H:1V, and native cut slopes up to 2H:1V will be stable.

The recommendations provided herein, and particularly under Site Preparation, Mass Grading, and Quality Control, are intended to minimize risks of structural distress related to consolidation or expansion of native soils and/or structural fills. These recommendations, along with proper design and construction of the embankments and associated improvements, work together as a system to improve overall performance. If any aspect of this system is ignored or is poorly implemented, the performance of the project will suffer. Sufficient quality control should be performed to verify that the recommendations presented in this report are followed.

Structural areas referred to in this report include all areas beneath embankment fills or basin slopes, concrete slabs, and exterior pavements as well as pads for any minor structures. The width of the structural area shall extend laterally from the outer edge of concrete slabs, exterior pavements and building pads at least one-half the vertical distance from native subgrade. The term engineer, as presented below, pertains to the civil or geological engineer that has prepared the geotechnical engineering report for the project or who serves as a qualified geotechnical professional on behalf of the owner.

All compaction requirements presented in this report are relative to ASTM D1557.

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.

Site Preparation This section describes the necessary operations to prepare the existing ground for site improvements;

grading and filling are described in later sections of this report.

All vegetation shall be stripped and grubbed from structural areas and removed from the site. A stripping depth of 0.3 to 0.4 feet is anticipated. Roots greater than ½ inch in diameter shall be removed, where necessary, to a minimum depth of 12 inches below finished grade. Large roots (greater than 6 inches in diameter) shall be removed to the maximum depth possible. Resulting excavations shall be backfilled with structural fill compacted to 90 percent relative compaction.

The test pits were excavated by excavator at the approximate locations shown on Plate 1. Locations were determined in the field by approximate means. All test pits were backfilled upon completion of the field portion of our study, and the backfill was compacted to the extent possible with equipment on hand.

However, the backfill was not compacted to the requirements presented herein under Mass Grading. If structures, concrete flatwork, pavement, utilities or other improvements are to be located in the vicinity of any of the test pits, the backfill should be removed and recompacted in accordance with the requirements contained in this report. Failure to properly compact backfill could result in excessive settlement of improvements located over test pits.

The site soils are exclusively granular and are suitable for reuse as structural/ embankment fill. The native soils have a medium dense consistency that could result in variable density materials in the embankment weirs. Therefore, the weirs shall be over-excavated a minimum of 12 inches beneath their base elevation.

The underlying grade shall be moisture conditioned to within 2 percent of optimum moisture, and recompacted to at least 90 percent relative compaction. The weirs can then be constructed of densified embankment fill in accordance with the mass grading section.

Any side/lateral embankments constructed for the basins shall be over-excavated a minimum of 12 inches beneath the original ground surface, prepared as above and can then be constructed of densified embankment fill.

All areas to receive structural fill or structural loading shall be scarified at least 12 inches deep, moisture conditioned to within 2 percent of optimum moisture and then be densified to at least 90 percent relative compaction. In all cases, the final surface shall be smooth, firm, and exhibit no signs of deflection.

If wet weather is experienced, surface soils may be well above optimum moisture and impossible to compact. In some situations, moisture conditioning may be possible by scarifying the top 12 inches of

1345 Capital Boulevard, Suite A | Reno | Nevada 89502 | 775-359-6600 11 3839 N. Carson Street, Suite 101 | Carson City | Nevada 89706 | 775-222-0080 www.blackeagleconsulting.com subgrade and allowing it to air-dry to near optimum moisture prior to compaction. Where this procedure is ineffective or where construction schedules preclude delays, mechanical stabilization will be necessary.

Mechanical stabilization may be achieved by over-excavation a minimum of 18 inches and replacement with near-optimum moisture embankment fill. If soft, saturated soils are deeper, the engineer shall be consulted to determine stabilization measures. Regardless of which alternate is selected, a test section is recommended to determine the required thickness of stabilization.

Trenching, Excavation and Utility Backfill The site soils will be excavatable using conventional heavy earthwork equipment. Occasional cobbles are present in the soils and are typically less than 12 inches in diameter. Sloughing and raveling of the site soils in excavated slopes and trenches should be expected. Densified structural fill with adequate moisture will be more stable than native soils, but will still ravel if allowed to dry out.

Temporary trenches with near-vertical sidewalls should be stable to a depth of approximately 4 feet.

Temporary trenches are defined as those that will be open for less than 24 hours. Excavations to greater depths will require shoring or laying back of sidewalls to maintain adequate stability. Regulations contained in Part 1926, Subpart P, of Title 29 of the Code of Federal Regulations (2010) require that temporary sidewall slopes be no greater than those presented in Table 1 (Maximum Allowable Temporary Slopes).

TABLE 1 - MAXIMUM ALLOWABLE TEMPORARY SLOPES

Soil or Rock Type Maximum Allowable Slopes1 for Deep Excavations less than 20 Feet Deep2

Stable Rock Vertical (90 degrees)

Type A3 3H:4V (53 degrees)

Type B 1H:1V (45 degrees)

Type C 3H:2V (34 degrees)

Notes:

1 Numbers shown in parentheses next to maximum allowable slopes are angles expressed in degrees from the horizontal. Angles have been rounded off.

2 Sloping or benching for excavations greater than 20 feet deep shall be designed by a registered professional engineer.

3 A short-term (open 24 hours or less) maximum allowable slope of 1H:2V ([horizontal to vertical] 63 degrees) is allowed in excavation in Type A soils that are 12 feet or less in depth. Short-term maximum allowable slopes for excavations greater than 12 feet in depth shall be 3H:4V (53 degrees).

The State of Nevada, Department of Industrial Relations, Division of Occupational Safety and Health Administration (OSHA) has adopted and strictly enforces these regulations, including the classification system and the maximum slopes. In general, Type A soils are cohesive, non-fissured soils with an unconfined compressive strength of 1.5 tons per square foot (tsf) or greater. Type B are cohesive soils with an unconfined compressive strength between 0.5 and 1.5 tsf. Type C soils have an unconfined compressive strength below 0.5 tsf. Numerous additional factors and exclusions are included in the formal

1345 Capital Boulevard, Suite A | Reno | Nevada 89502 | 775-359-6600 12 3839 N. Carson Street, Suite 101 | Carson City | Nevada 89706 | 775-222-0080 www.blackeagleconsulting.com definitions. The client, owner, design engineer, and contractor shall refer to Appendix A and B of Subpart P of the previously referenced Federal Register for complete definitions and requirements on sloping and benching of trench sidewalls. Appendices C through F of Subpart P apply to requirements and methodologies for shoring.

On the basis of our exploration, the native soils are Type C. All trenching shall be performed and stabilized in accordance with local, state, and OSHA standards.

Utility Trench Backfill The maximum particle size in trench backfill shall be 4 inches. Bedding and initial backfill 12 inches over the pipe will require import and shall conform to the requirements of either Class A backfill (Standard Specifications for Public Works Construction [SSPWC], 2016) or the utility having jurisdiction. Bedding and initial backfill shall be densified to at least 90 percent relative compaction. Native granular soil will provide adequate final backfill as long as oversized particles are excluded, and it shall be placed in maximum 8-inch-thick loose lifts which are compacted to a minimum of 90 percent relative compaction in all structural areas.

Mass Grading The site soils are exclusively granular sandy soils with moderate gravel content. The soils include about 5 to 10 percent cobbles up to 12 inches in diameter but are typically less than 8 inches in diameter.

Native granular soils will be suitable for structural fill provided particles larger than 6 inches are removed.

Oversized rock can be stockpiled for later use as erosion protection. Grading shall not be performed with or on frozen, organic or other deleterious materials. If imported structural fill is not expected on this project, and ample borrow materials are available adjacent the project if necessary. Any borrow sources shall be evaluated by the geotechnical engineer at least 2 weeks prior to commencing borrow activities.

All fill placed on hillsides steeper than 5H:1V (horizontal to vertical) shall include a base keyway into the native soils. The keyway shall be at least 10-feet wide and be at least 12-inches deep. Subsequently, the sloping surface shall be smoothed and prepared in accordance with the Site Preparation section. Vertical seams in the fill shall be avoided.

Any structural fill within the weir and berm embankments shall be placed in maximum 8-inch-thick loose lifts each densified to at least 95 percent relative compaction. All other structural fill shall be densified to a minimum 90 percent relative compaction. Nonstructural fill shall be densified to at least 85 percent relative compaction to minimize consolidation and erosion. All structural fill shall be moisture conditioned to within 2 percent of optimum moisture per ASTM D 1557.

The surface of each lift shall be rough to promote bonding between fill layers. If a sheepsfoot compactor is used for compaction and the surface is rough, fill may be placed over the dimpled surface. If smooth drum

1345 Capital Boulevard, Suite A | Reno | Nevada 89502 | 775-359-6600 13 3839 N. Carson Street, Suite 101 | Carson City | Nevada 89706 | 775-222-0080 www.blackeagleconsulting.com rollers are used, or the surface is smoothed by rubber tire traffic the surface shall be lightly scarified 2 inches parallel to the crest of the weirs.

Subsidence and Shrinkage Subsidence of about 0.1 feet should be anticipated from construction traffic. Granular alluvial soils excavated and recompacted in structural fills should experience quantity shrinkage of approximately 15 to 20 percent, including removal of oversized particles. In other words, 1 cubic yard of excavated granular alluvium will generate about 0.8 to 0.85 cubic yards of structural fill at 95 percent relative compaction.

Fill Settlement All deep fills settle through internal consolidation from the weight of overlying fill material. Granular soils, as will be involved in this project, experience this internal compression very rapidly upon receiving the surcharge weight of the overlying lifts of fill. Fills on the order of 20 feet are anticipated within the embankment weirs. No extensive time delays will be necessary before finish grading and rip rap placement commences over these fills.

Seismic Design Parameters The 2018 IBC (ICC, 2018), requires a detailed soils evaluation to a depth of 100 feet to develop appropriate soils criteria. Based on our experience with the subsurface soil conditions and geology at the New Bomb Basin site as well as our previous deep borings and geophysical surveys in the general area of the project site lying on the same or similar geological deposits, a Site Class D is appropriate. The Site Class D soil profile is for stiff soils with a shear velocity between 600 and 1,200 feet per second, or with an N (Standard Penetration Test) value between 15 and 50, or an undrained shear strength between 1,000 and 2,000 pounds per square foot (psf).

The 2018 IBC seismic design loads are based on the ASCE 7-16 Standards titled Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE, 2017) which utilize the 2,475-year return period earthquake to develop design spectra. The recommended seismic design criteria using the 2018 IBC are presented in Table 2 (Seismic Design Criteria Using 2018 International Building Code). It is noted that for Site Class D and the site location, the determination of site coefficient (Fv) as well as site-adjusted and design spectral response values at long periods (SM1 and SD1, respectively) are not provided for the site.

However, the seismic design of the embankments will be based on the short period spectral acceleration provided.

TABLE 2 - SEISMIC DESIGN CRITERIA USING 2018 INTERNATIONAL BUILDING CODE (ASCE, 2024)

Approximate Latitude 38.243

Approximate Longitude -118.626

Spectral Response at Short Periods, Ss, percent of gravity 142.5

Spectral Response at 1-Second Period, S1, percent of gravity 48.0

Site Class D

Risk Category III

Site Coefficient Fa, decimal 1.0

Site Adjusted Spectral Response at Short Periods, SMS, percent of gravity 142.5

Design Spectral Response at Short Periods, SDS, percent of gravity 95.0

Seismic Design Category D

*These values assume the use of seismic response coefficient (Cs) to calculate seismic base shear is determined by the structural engineer in accordance with Section 11.4.8 (Exception Note 2 for Site Class D) and Section 12.8.1 of ASCE 7-16 (ASCE, 2017).

For temporary conditions, such as a partially filled or draining detention basin, a 475-year return period earthquake event was used to develop spectral acceleration for the site. The 475-year event (or 10 percent chance in 50 years) was determined using the United States Geological Survey (USGS, 2024) Unified Hazard Tool and results in a bedrock peak ground acceleration of 0.37 g, and site-specific (Site Class D) design peak ground acceleration of 0.22 g.

Slope Stability The project will include embankment fills on the order of 35 feet tall and cut slopes on the order of 25 feet tall. The slopes will be rip rap lined. Additionally, composite slopes with fill overlying native soils will be constructed in the basin side slopes.

Stability of cut and filled surfaces involves 2 separate aspects. The first concerns true slope stability related to mass wasting, landslides, or the en masse downward movement of soil or rock. Stability of cut and fill slopes is dependent upon shear strength, unit weight, moisture content, and slope angle. Static and pseudo-static global stability analyses for basin drained and full conditions were analyzed.

Worst case fill and cut slope scenarios were analyzed for global stability, as described below. The main embankment weirs will be approximately 35 feet tall at a slope of 2.5H:1V (horizontal to vertical). The side slopes will be on the order of 22 feet tall at a slope of 2.2H:1V. At the inlet to basin No. 1 (upstream basin) is a cut slope 24.5 feet tall at a slope of 2H:1V. The global stability analyses evaluated the drained (or normal condition) of the slopes and a condition with a wetting soil front when the basins are filled (temporary condition). Based on the hydraulic conductivity of the soils an estimated 3 to 5 feet of moisture

1345 Capital Boulevard, Suite A | Reno | Nevada 89502 | 775-359-6600 15 3839 N. Carson Street, Suite 101 | Carson City | Nevada 89706 | 775-222-0080 www.blackeagleconsulting.com penetration into the slopes was used in the temporary condition. Additionally, we anticipate the basins will drain in less than 7 days.

Circular failure surfaces were analyzed under both static and pseudo-static (seismic) conditions using the Slide 2 computer modeling system (Rocscience, 2018). The simplified Bishop Method of slices was used in the analysis. The design soil parameters are summarized below in Table 3 (Design Soil Parameters).

TABLE 3 – DESIGN SOIL PARAMETERS

Design Case Wet Unit Weight Friction Angle Cohesion

Native Soils 115 pcf1 33 degrees 50 psf

Fill Soils 125 pcf 34 degrees 25 psf

Saturated Soils (temporary condition) 135 pcf 30 degrees 0

Rip Rap 140 pcf 48 degrees 0

1 Pounds per cubic foot.

In the pseudo-static analysis, a seismic coefficient (kh) equal to half the design ground acceleration for the site at zero period was used (Federal Highway Administration [FHWA], 1998). For the normal basin drained condition, ground acceleration is based on a 2,475-year return period earthquake (Bedrock PGA = 0.618 g;

Design PGA = 0.38 g; kh = 0.19 g). For the temporary wetting front condition, ground acceleration is based on the 475-year return period earthquake (Bedrock PGA = 0.369 g; Design PGA = 0.226 g; kh = 0.19 g).

The calculated minimum factor of safety values for global stability of the slopes are presented in Table 4 (Global Stability Analysis Results) The global stability analyses results are included in Appendix A (Slope Stability Analyses) and seismic coefficient calculations are included as Appendix B (Seismic Design Coefficient). The calculated minimum factor of safety values exceed the recommended minimum factor of safety values (U.S. Bureau of Reclamation, 2011) of 1.3 and 1.1 under static and pseudo-static loading conditions, respectively. Our analysis confirms the global stability of the proposed embankment slopes.

TABLE 4 – GLOBAL STABILITY ANALYSIS RESULTS

Design Case

Minimum Factor of Safety

Static (Min FS = 1.3) 1

Pseudo-Static (Min FS = 1.1) 1

35.5-foot Fill Slope (2.5H:1V) Drained

1.96 1.24

35.5-foot Fill Slope (2.5H:1V)

Undrained

1.66 1.23

22.5-foot Fill Slope (2.5H:1V) Drained 1.80 1.18

22.5-foot Fill Slope (2.2H:1V) Undrained 1.48 1.12

25-foot Cut Slope (2H:1V) Drained

1.66 1.11

25-foot Cut Slope (2H:1V)

Undrained

1.49 1.15

1 Minimum factor of safety values under seismic loading conditions are in parentheses.

Erosion Control The embankment slopes and channels will require rip rap lining for erosion control. The rip rap shall be sized by the civil engineer based on water volume and velocity. Rip rap shall include a minimum 6 inches of bedding material. The rip rap and bedding shall be in accordance with Section 200.07 of the SSPWC (2016). A separator geotextile is required between the native soil/ embankment fill and the bedding material. A minimum 12 inches overlap between roll edges is required. Care shall be taken so that the geotextile is not damaged during placement of bedding and rip rap. The separator geotextile shall meet or exceed the following minimum properties presented in Table 5 (Minimum Required Properties for Separator Geotextile).

TABLE 5 - MINIMUM REQUIRED PROPERTIES FOR SEPARATOR GEOTEXTILE

Trapezoid Strength (ASTM D4533) 80 x 80 lbs.

Puncture Strength (ASTM D4833) 500 lbs.

Grab Tensile Strength/Elongation (ASTM D4632) 200 x 200 @ 50 %

For larger rip rap (Class 900), there may be large voids in the surface of the rip rap that could be filled with smaller rip rap to improve aesthetics and access if needed.

Exterior slopes up to 3H:1V can be stabilized by revegetating the slopes while 2H:1V slopes should be rip rap lined. For slopes that will not receive concentrated flows, 4 to 6 inch minus rip rap is acceptable for erosion control.

Dust potential at this site will be high during dry periods. Temporary (during construction) and permanent (after construction) erosion control will be required for all disturbed areas. The contractor shall prevent dust from being generated during construction in compliance with all applicable city, county, state, and federal regulations. The contractor shall submit an acceptable dust control plan to the governing agency prior to starting site preparation or earthwork. Project specifications should include an indemnification by the contractor of the owner and engineer for any dust generation during the construction period. The owner will be responsible for mitigation of dust after accepting the project.

In order to minimize erosion and downstream impacts to sedimentation from this site, best management practices with respect to stormwater discharge shall be implemented.

Site Drainage Surface Drainage The ponding of water on finished grade or at the edge of pavements shall be prevented by grading the site in accordance with IBC (ICC, 2018) requirements.

Any roads along the crest of the embankment sides should be graded to drain into the basins.

Concrete Spandrel All concrete slabs shall be directly underlain by imported Type 2, Class B aggregate base (SSPWC, 2016).

The thickness of base material beneath PCC flatwork shall be 6 inches. Aggregate base courses shall be densified to at least 95 percent relative compaction.

Final design of the concrete pavement shall be performed by the project structural engineer. A coefficient of subgrade reaction (K-value) of 200 pounds per cubic inch shall be used for design of concrete slabs.

The structural section for exterior concrete pavements shall be a minimum of 6 inches of 4,000 pounds per square inch (psi) concrete overlying 6 inches of Type 2, Class B aggregate base (SSPWC, 2016). These exterior rigid pavements meet the criteria of the American Association of State Highway and Transportation Officials (1993) method for concrete with a 28-day flexural strength of 570 psi (approximately 4,000 psi compressive strength).

The Hawthorne area is a region with exceptionally low relative humidity. As a consequence, concrete flatwork is prone to excessive shrinking and curling. Concrete mix proportions and construction techniques, including the addition of water and improper curing, can adversely affect the finished quality of concrete and result in cracking, curling, and the spalling of slabs. We recommend that all placement and curing be performed in accordance with procedures outlined by the American Concrete Institute (2019) and this report. Special considerations shall be given to concrete placed and cured during hot or cold weather temperatures, low humidity conditions, and windy conditions such as are common in the Walker Lake area.

Proper control joints and reinforcement shall be provided to minimize any damage resulting from shrinkage, as discussed below. In particular, crack-control joints shall be installed on maximum 10-foot centers and shall be installed to a minimum depth of 25 percent of the slab thickness. Saw-cuts, zip strips, and/or trowel joints are acceptable; however, saw-cut joints must be installed as soon as initial set allows and prior to the development of internal stresses that will result in a random crack pattern.

Concrete shall not be placed on frozen in-place soils.

Gravel Surfacing The majority of the project will include unimproved dirt drives for access to the basins. Any roadways that will include significant traffic (e.g. loaded delivery trucks) can include gravel-surfacing consisting of 6 inches of Type 2, Class B aggregate base. Minor roads with minimal traffic can include 4 inches of aggregate base. As an alternate, any well-graded 1-1/2 inch minus material could be used for gravel-surfacing and could potentially be generated onsite. Alternate gravel-surfacing should have a minimum R-value of 60, and meet the intent of Select Natural Base (Section 200.01.07 of the SSPWC, 2016).

Drainage and Maintenance Unsurfaced and gravel-surfaced roads shall include sufficient transverse and longitudinal slope to prevent the ponding of water on subgrade soils. V-ditches or drainage swales should be included along the shoulders of high traffic roadways. The swale should be at least 12 inches beneath the road subgrade. All dirt and gravel roads will require periodic maintenance including grading, watering and compaction in order to maintain drainage and ride quality. If drainage is inadequate, maintenance requirements will be higher.

The application of a dust palliative will aid in dust control and may increase maintenance intervals.

Anticipated Construction Problems

Anticipated Construction Problems Depending on the season of construction, soft, wet surface soils may make it difficult for construction equipment to travel and operate. Sloughing of clean soils in slope excavations and trench sidewalls should be expected, particularly if soils are allowed to dry. At the time of exploration, the site soils were well below optimum moisture and significant quantities of construction water could be necessary to achieve the required moisture content of embankment materials.

Quality Control

Quality Control All plans and specifications should be reviewed for conformance with this geotechnical report and approved by the engineer prior to submitting them to the building department for review.

The recommendations presented in this report are based on the assumption that sufficient field testing and construction review will be provided during all phases of construction. We should review the final plans and specifications to check for conformance with the intent of our recommendations. Prior to construction, a pre-job conference should be scheduled to include, but not be limited to, the owner, architect, civil engineer, general contractor, earthwork and materials subcontractors, building official, and engineer. The conference will allow parties to review the project plans, specifications, and recommendations presented in this report and discuss applicable material quality and mix design requirements. All quality control reports should be submitted to and reviewed by the engineer.

During construction, we should have the opportunity to provide sufficient on-site observation of preparation and grading, over-excavation, fill placement, foundation installation, and paving. These observations would allow us to verify that the geotechnical conditions are as anticipated and that the contractor's work is in conformance with the approved plans and specifications.

Standard Limitations Clause

Standard Limitations Clause This report has been prepared in accordance with generally accepted geotechnical practices. The analyses and recommendations submitted are based on field exploration performed at the locations shown on Plate

1. This report does not reflect soils variations that may become evident during the construction period, at which time re-evaluation of the recommendations may be necessary. We recommend our firm be retained to perform construction observation in all phases of the project related to geotechnical factors to ensure compliance with our recommendations.

It is anticipated that the site will be graded cut to fill. As such, minor deviations from the recommendations and assessments presented in this report are anticipated. Fills are to be generated on site using cut-to-fill methods and will not be entirely purchased from a commercial borrow source. Therefore, the potential exists for soils within the building pads to fall outside the material limits recommended in this report. Unless these deviations can be proven to be fundamental to any observed distress or performance issue, such deviations should not be considered a failure to adhere to the recommendations presented in this report or a design flaw but should be considered an acceptable variation in mass grading when on-site materials are used as the fill source.

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