Attachment_3_-_CRS_Geotechnical_Report.pdf

Cahuilla Ranger Station – Facility Replacement Geotechnical Report

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Jacobs Project No.: D3290106 Bureau of Land Management Contract No.: 140L4318D0002

July 23, 2021

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Cahuilla Ranger Station - Facility Replacement Geotechnical Report

Jacobs Project No.: D3290106

July 7, 2021

Bureau of Land Management

Contract No.: 140L4318D0002

Draft Basis of Design Bureau of Land Management ii

Cahuilla Ranger Station - Facility Replacement

Project No: D3290106

Document Title: Geotechnical Report

Document No.: NA

Revision: 1

Document Status: Final

Date: July 7, 2021

Client Name: Bureau of Land Management

Client No: 140L4318D0002

Project Manager: John Lory, PMP

File Name: BLM_Cahuilla Ranger Station_Geotechnical Report.docx www.jacobs.com

© Copyright 2021 Please select a legal entity from the Change Document Details option on the Jacobs ribbon. The concepts and information contained in this document are the property of Jacobs. Use or copying of this document in whole or in part without the written permission of Jacobs constitutes an infringement of copyright.

Limitation: This document has been prepared on behalf of, and for the exclusive use of Jacobs’ client, and is subject to, and issued in accordance with, the provisions of the contract between Jacobs and the client. Jacobs accepts no liability or responsibility whatsoever for, or in respect of, any use of, or reliance upon, this document by any third party.

Document history and status

Revision Date Description Author Checked Reviewed Approved

0 10/05/2020 Draft Geotechnical

Report M. Jafari J. Lee D. Jankly

1 07/07/2021 Final Geotechnical

Report

A. Tran (Edits from Draft to Final) NA V.R. Nadeswaran

Kirk Arradaza Snapshot iv

CONTENTS

1. Introduction .................................................................................................................................................................. 1-1

1.1 Purpose and Scope of Work ...................................................................................................................................... 1-1

1.2 Project Description ....................................................................................................................................................... 1-1

2. Field Investigation and Laboratory Testing .......................................................................................................... 2-1

2.1 Field Investigation ........................................................................................................................................................ 2-1

2.2 Laboratory Testing........................................................................................................................................................ 2-2

3. Site Conditions ............................................................................................................................................................. 3-1

3.1 Regional Geology .......................................................................................................................................................... 3-1

3.2 Site Geology .................................................................................................................................................................... 3-1

3.3 Topography ..................................................................................................................................................................... 3-1

3.4 Subsurface Soil Conditions ........................................................................................................................................ 3-1

3.5 Groundwater Condition .............................................................................................................................................. 3-2

4. Geologic and Seismic Hazards .................................................................................................................................. 4-1

4.1 Faulting and Surface Rupture .................................................................................................................................. 4-1

4.2 Seismic Evaluation ........................................................................................................................................................ 4-1

4.3 Liquefaction and Seismic Settlement Evaluation ............................................................................................. 4-2

4.4 Seismic Slope Stability ................................................................................................................................................ 4-2

4.5 Flooding ............................................................................................................................................................................ 4-2

5. Subsurface Soil Characteristics ................................................................................................................................ 5-1

5.1 Soil Compressibility Potential .................................................................................................................................. 5-1

5.2 Expansive Soil ................................................................................................................................................................. 5-1

5.3 Soil Corrosivity ............................................................................................................................................................... 5-1

5.4 Frost Penetration Depth ............................................................................................................................................. 5-1

5.5 California Bearing Ratio .............................................................................................................................................. 5-2

6. Geotechnical Recommendations ............................................................................................................................. 6-1

6.1 Foundation Recommendations ............................................................................................................................... 6-1

6.2 Concrete Slab-on-Grade ............................................................................................................................................ 6-1

6.3 Pavement Design Parameters .................................................................................................................................. 6-2

6.4 Windblown Sediment and Associated Maintenance ....................................................................................... 6-2

6.5 Earthwork ......................................................................................................................................................................... 6-2

6.5.1 Site Preparation ............................................................................................................................................ 6-2

6.5.2 Subgrade Preparation ................................................................................................................................. 6-2

6.5.3 Fill Placement ................................................................................................................................................ 6-3

6.5.4 Temporary Excavations .............................................................................................................................. 6-3

6.5.5 Geotechnical Observation ......................................................................................................................... 6-3 v

6.6 Review of Construction Plans and Specifications ............................................................................................. 6-3

7. Limitations .................................................................................................................................................................... 7-1

8. References ..................................................................................................................................................................... 8-1

LIST OF TABLES

2-1 Summary of Borings .................................................................................................................................................................... 2-1

4-1 Seismic Design Parameters....................................................................................................................................................... 4-1

5-1 Summary of Corrosion Testing ................................................................................................................................................ 5-1

5-2 Summary of California Bearing Ratio .................................................................................................................................... 5-2

6-1 Foundation Recommendations ............................................................................................................................................... 6-1

LIST OF FIGURES

1-1 Site Vicinity Map

2-1 Boring Location Map

3-1 Geologic Map

APPENDIXES

A Boring Logs

B Laboratory Test Results vi

EXECUTIVE SUMMARY

The following is an executive summary of the geotechnical design report for the Cahuilla Ranger Station – Facility Replacement. The regional and local geology, subsurface and groundwater conditions, geologic and seismic hazards, subsurface soil characteristics, and geotechnical recommendations are provided in this report.

The project site is located on the west side of Gecko Road, approximately 1,000 feet south of the Intersection of State Route 78 (Ben Hulse Highway) and Gecko Road in Imperial County, California. Replacement of Cahuilla Ranger Station Facility is proposed, which includes functionality consolidation for improved efficiency, operations and mission readiness.

The geotechnical field investigation was conducted between August 11 and 12, 2020. Jacobs Engineering Group, Inc. (Jacobs)’s field study consisted of advancing three hollow-stem auger borings to evaluate the subsurface conditions beneath the site. Available geologic maps and pertinent existing geotechnical reports were reviewed and referenced. Based on the geotechnical boring data obtained during the investigation, the stratigraphy within the upper 61.4 feet consists of sand dune deposits, generally composed of a medium dense to very dense, fine-grained sand with minor amount of silts. The site is situated on the western side of the Algodones Dunes, also known as the Sand Hills. At the highest points, the dunes are 200 feet or more in thickness (Morton, 1977), and are underlain by alluvial deposits (Guptil et al, 1986). Bedrock and groundwater were not encountered within the upper 61.4 feet of the subsurface.

A State of California Alquist-Priolo Earthquake Fault Zone map has not been prepared for the site (CGS, 2018).

Additionally, per the Quaternary Fault and Fold database (USGS and CGS, 2006), no active or potentially active (Quaternary age) faults are mapped adjacent to or crossing the site. The closest mapped (Holocene) active faults nearest the site include the Imperial Fault and Brawley Seismic Zone, located approximately 18 miles west-southwest of the site. The southern end of the San Andreas Fault is mapped approximately 40 miles northwest of the site, at the east side of the Salton Sea. The potential for ground surface rupture associated with known active faults is considered low for the site. However, the site is located in the seismically active Southern California area.

The structures should be designed to tolerate seismic shaking.

Based on the subsurface investigation and laboratory test results, compressibility and expansion potential are considered low. Results of corrosion testing indicate that site soils are generally non-corrosive to concrete and steel structural elements. Excavation of on-site soil should be achievable with conventional heavy equipment.

However, access on the windblown loose dune sand condition should be considered to select the equipment types.

Recommendations for earthwork, foundation, and pavement design are provided in Section 6. This Executive Summary should not be relied upon without consulting the associated report that details the geotechnical study conducted for the project. This report is subject to the limitations included in Section 7.

For Official Use Only 1-1

1. Introduction

1.1 Purpose and Scope of Work

The current Cahuilla Ranger Station site is partially developed and includes a bunkhouse facility, modular ranger building, parking areas for personal and government vehicles, storage containers, fuel pad, covered storage buildings for equipment and vehicles, asphalt road, and a perimeter security fence. The current Ranger Station consists of a modular building assembled in the mid- to late-1980s to provide space for law enforcement rangers, emergency medical services, and visitor services staff. This building is over 30 years old and not large enough to accommodate the current visitor and staff needs; to provide a safe and healthy environment and better manage recreational experiences, it is proposed to replace the existing facility with a new Cahuilla Ranger Station.

This report provides geotechnical recommendations for the proposed replacement of Cahuilla Ranger Station Facility, which includes functionality consolidation for improved efficiency, operations and mission readiness. This report was prepared in accordance with Scope of Architect-Engineer Services (SAES) for Cahuilla Ranger Station and Site Layout Design, by Bureau of Land Management (BLM), dated January 26, 2020. The geotechnical recommendations in this report are provided in accordance with the American Society of Civil Engineers (ASCE) 7-16 and 2018 International Building Code (IBC).

1.2 Project Description

The project includes services necessary for completing the design and construction documentation for a new, approximately 12,500-square-foot facility to replace the existing Cahuilla Ranger Station at the BLM’s Administrative Site, located at the Imperial Sand Dunes Recreation Area (ISDRA), east of Brawley in Imperial County, California. The new facility will provide administrative office and visitor contact space, law enforcement space, emergency medical services space, employee sleeping rooms, and storage space for various vehicles for government employees while working at the Cahuilla Ranger Station site at the ISDRA. The project includes demolition of the existing buildings and constructing a new facility in approximately the same area. The geotechnical recommendations are based on the conceptual design and program developed by the BLM.

The existing and proposed facility is located on the west side of Gecko Road, approximately 1,000 feet south of the Intersection of State Route 78 (Ben Hulse Highway) and Gecko Road in Imperial County, California. A site vicinity map is presented on Figure 1-1 (figures are located at the end of the report).

For Official Use Only 2-1

2. Field Investigation and Laboratory Testing

2.1 Field Investigation

A geotechnical field investigation was performed between August 11 and 12, 2020 for the proposed building and site re-development. The investigation consisted of drilling three hollow-stem auger borings to the maximum explored depth of 61.4 feet as summarized in Table 3-1. A final site plan was not available prior to the start of the field investigation; therefore, drilled boring locations were distributed across the site so as to characterize the general subsurface conditions. The boring locations are provided in Figure 2-1. Based on our review of the final project plans, the borings are outside but within 100 feet of the proposed structure footprint.

The borings were drilled using a track-mounted Central Mine Equipment (CME) 75 drill rig with 8-inch outer-diameter hollow-stem augers and sampled with a 140-pound automatic hammer, free falling at a height of 30 inches. Details of the drill rig and associated hammer energy are provided in the boring logs presented in Appendix A.

Table 2-1. Summary of Borings

Location Boring Number Latitude (°) Longitude (°) Elevation a

(feet) Exploration Depth

(feet)

Cahuilla Ranger Station

B-01 32.972739 -115.172389 227.5 31.5

B-02 32.973019 -115.172550 227.0 61.4

B-03 32.973458 -115.172800 226.0 31.5 a Elevations in this report are based on the North American Vertical Datum of 1988 (NAVD 88)

Drilling investigation was performed under the continuous technical supervision of Jacobs’ field engineer, who maintained detailed logs of the soil encountered, classified the materials according to the Unified Soil Classification System, and assisted in obtaining soil samples.

Prior to the field exploration, the boring locations were cleared for potential underground utility conflicts using a radiodetection transmitter, hand-held magnetometer, shallow focus metal detector, and ground penetrating radar unit. The goal of utility locating was to identify detectable subsurface interferences at the boring locations. In addition, the locations of the existing water line from the active well and septic system leach lines were identified to support the site utility plans.

Bulk soil samples were obtained from drill cuttings between the ground surface and approximately 5 feet below ground surface (bgs) and placed into polyethylene bags. Drive samples were obtained with two different types of split‐barrel samplers. Standard Penetration Tests (SPT) were conducted using standard 2‐inch outside-diameter, 1.375‐inch inside-diameter split‐spoon samplers in accordance with ASTM International (ASTM) D1586. Larger diameter (3‐inch outside-diameter, 2.4-inch inside-diameter) samples were collected using a “California” sampler in accordance with ASTM D3550. The SPT and California samplers were driven into the bottom of the borehole using a 140‐pound automatic hammer, free falling at a height of 30 inches. The penetration resistance (or “blow count”) in blows per 6 inches of driving was recorded on the logs. Where high resistance was encountered and the sampler did not advance the full sampling length, sampler refusal was indicated as the number of blows per number of inches penetrated. In accordance with Burmister’s correction, a factor of 0.65 was applied to all California ring sampler blow counts to convert to equivalent SPT blow counts. Penetration test samples were placed in sealable containers to protect the natural moisture.

For Official Use Only 2-2

Upon drilling completion, all borings were backfilled with non-shrink bentonite grout from the bottom of the borehole to near ground surface and subsequently with tamped soil cuttings at the ground surface.

2.2 Laboratory Testing

Laboratory testing on samples from the borings included the following tests:

• California Bearing Ratio (CBR) (ASTM D1883)

• Corrosion – pH (ASTM D4972), Minimum Resistivity (ASTM G187), Sulfate and Chloride (ASTM D4327)

• Direct Shear (ASTM D3080)

• Modified Proctor Compaction (ASTM D1557)

• Moisture Content Determination (ASTM D2216)

• Particle-Size Distribution, with or without hydrometer (ASTM D7928/D6913)

• Unit Weight Determination (ASTM D2937)

The laboratory test results are presented in Appendix B. Details of the soil characteristics based on the laboratory results are discussed in Section 5.

For Official Use Only 3-1

3. Site Conditions

3.1 Regional Geology

The project site is located within the Colorado Desert geomorphic province of California (CGS, 2002), also known as the Salton Trough. The Salton Trough is a rift valley, the northward extension of the Gulf of California, currently cutoff from the gulf by sediment deposited by the Colorado River. The Salton Trough area is characterized by relatively flat terrain composed of ancient lacustrine deposits and sand dunes. Although alluvial deposits are also present at the east margin of the Trough. The geology of the Salton Trough is complex. The Trough is a depressed structural block generated as the Pacific Plate (including Central and Southern and Baja California) is being pushed away (by spreading of the earth’s crust at rift zones) from the North American Plate. The northmost rift zone comprises the Salton Sea area, where the plate boundary transitions to a transform boundary (i.e. the San Andreas Fault system). At the Salton Sea, the right-lateral slip from the San Andreas Fault is transferred to the Imperial Fault, via the Brawley Seismic Zone. The Salton Trough is filled with sediment deposited by the Colorado River.

Over geologic time, the Colorado River has switched flow direction numerous times; the river has previously flowed towards the west, resulting in the ancient Lake Cahuilla, which extended as far north as Indio. The change in flow direction of the Colorado River is likely triggered by major earthquakes in the Trough area and their associated ground ruptures (Philibosian et al 2011). These westerly flows of the Colorado River resulted in the infilling of the Salton Trough with over 20,000 feet of sediment (Morton, 1977).

3.2 Site Geology

As shown on the geologic map (Figure 3-1), the Cahuilla Ranger Station site is underlain by sand dunes (Unit Qs).

Based on the borings drilled, the subsurface soils at the Station are composed of medium dense to very dense, fine-grained sand with minor amount of silts. The site is situated on the western side of the Algodones Dunes, also known as the Sand Hills. At the highest points, the dunes are 200 feet or more in thickness (Morton, 1977), and are underlain by alluvial deposits (Guptil et al, 1986). The project site is located approximately 7 miles east of the ancient Lake Cahuilla shoreline (Philibosian et al, 2011). The ancient shoreline is easily identifiable on aerial photographs, the shoreline generally coincides with the limits of the agricultural land.

3.3 Topography

The topography of the site is depicted on the boring location map (Figure 2-1). The site is relatively flat, and elevations across the site vary from approximately 220 feet to 230 feet. The site is bounded on the north, west, and south by dune slopes up to 20 feet tall, which are inclined at 2.5H:1V (horizontal to vertical) and flatter.

3.4 Subsurface Soil Conditions

Based on the current investigation, the subsurface condition at the site generally consists of fine-grained, poorly-graded sand with trace to few silts. The relative density varies from medium dense to very dense. Bedrock was not encountered up to the maximum explored depth of 61.4 feet. Windblown sediments of dune sand were observed at the surface, which presented loose condition. The sediment thickness may vary depending on seasons and winds.

For Official Use Only 3-2

3.5 Groundwater Condition

Groundwater was not encountered during the current field investigation up to the maximum explored depth of

61.4 feet.

Based on groundwater modeling prepared for the United States Department of Energy (LLNL, 2008), the 2003 groundwater table in the general site vicinity was at approximate elevation 20 feet, or roughly 200 feet bgs.

Groundwater levels may fluctuate locally due to rainfall, withdrawal, and recharge. However, based on the above information, the groundwater table is not anticipated to be encountered during the construction of the proposed structures and developments.

For Official Use Only 4-1

4. Geologic and Seismic Hazards

Geologic hazards are adverse geologic conditions that can cause damage or loss of property and life. The potential geologic hazards listed below were considered for the proposed structures.

4.1 Faulting and Surface Rupture

A State of California Alquist-Priolo Earthquake Fault Zone map has not been prepared for the site (CGS, 2018).

Additionally, per the Quaternary Fault and Fold database (USGS and CGS, 2006), no active or potentially active (Quaternary age) faults are mapped adjacent to or crossing the site. The closest mapped (Holocene) active faults nearest the site include the Imperial Fault and Brawley Seismic Zone, located approximately 18 miles west-southwest of the site. The southern end of the San Andreas Fault is mapped approximately 40 miles northwest of the site, at the east side of the Salton Sea. The potential for ground surface rupture associated with known active faults is considered low for the site.

There is a fault mapped on the 1962 Geology Map for the area (labeled as “San Andreas Fault Zone” on Figure 3-1), immediately southwest of the project site. This fault trace was mapped in the 1950s; however, it is not currently included in the 2006 USGS and CGS Fault and Fold database.

4.2 Seismic Evaluation

The soil profile type is classified based on the average shear wave velocity and/or average SPT blow count (N-value) within the upper 30 meters (100 feet) of the subsurface. Site-specific shear wave velocity measurements were not taken during this exploration. Field SPT blow counts with applied energy correction (N60) were extrapolated to the depth of 100 feet to estimate the average N-value for seismic site classification. An average N-value between 15 and 50 blows per foot for the upper 100 feet of the subsurface was estimated, which corresponds to Site Class D according to the ASCE 7-16 classification.

Seismic parameters for structure design are based on the ASCE 7-16 requirements. The seismic design parameters were estimated using the online web interface developed by Structural Engineers Association of California (SEAOC) and California’s Office of Statewide Health Planning and Development (OSHPD). The maximum considered earthquake (MCE) defined in ASCE 7-16 corresponds to an earthquake ground motion with a 2 percent probability of exceedance in 50 years or return period of 2,475 years. The risk-targeted maximum considered earthquake (MCER) ground motion parameters Ss and S1 are mapped with a target risk of 1 percent probability of structural collapse in 50 years. The acceleration response parameters and the correction factors for the design earthquake are presented in Table 4-1.

Table 4-1. Seismic Design Parameters Parameter Criteria

Project site geographic coordinates 32.973458oN

115.172800oW

Site class (based on ASCE 7-16) D

Risk category III

Ss – Mapped MCER spectral acceleration at short periods (g) 1.013

S1 – Mapped MCER spectral acceleration at 1-second period (g) 0.374

Fa – Site coefficient at short periods 1.095

For Official Use Only 4-2

Table 4-1. Seismic Design Parameters Parameter Criteria

Fv – Site coefficient at 1-second period 1.926

SMS – Site-adjusted MCER spectral acceleration at short periods (g) 1.109

SM1 – Site-adjusted MCER spectral acceleration at 1-second period (g) 0.720

SDS – Site-adjusted design spectral response acceleration at short periods (g) 0.740

SD1 – Site-adjusted design spectral response acceleration at 1-second period (g) 0.480

PGA – Mapped MCEG peak ground acceleration (g) 0.416

PGAM – Site-adjusted MCEG peak ground acceleration (g) 0.493

Notes:

°N = degrees North

°W = degrees West g = acceleration caused by gravity

MCEG = geometric mean maximum considered earthquake

4.3 Liquefaction and Seismic Settlement Evaluation

Liquefaction refers to the loss of strength or stiffness experienced by susceptible soils during earthquakes or other rapid loading. Three simultaneous conditions are required for liquefaction to occur: (1) cohesionless soils of loose to medium density; (2) saturated conditions; and (3) rapid, large strain, cyclic loading, normally provided by earthquake motions. Dry sand settlement or seismic compaction refers to earthquake-induced densification of loose to medium dense unsaturated cohesionless soils resulting in ground settlement. The total seismic settlement is the combined dry sand settlement (above the groundwater table) plus liquefaction settlement (below the groundwater table).

Potentials for liquefaction and seismic settlement at the site are expected to be low due to the relatively deep groundwater table and the underlying medium dense to very dense soil at the site.

4.4 Seismic Slope Stability

There are no steep slopes within the site; therefore, potential for seismic-induced landslides is considered low. New fill should be placed no steeper than 2H:1V ratio on competent soil. Based on the current grading plan, the proposed slopes are to be inclined at 3H:1V or flatter and therefore seismic slope stability is not a significant concern for this project.

4.5 Flooding

The site is not within a 100-year flood plain based on project hydraulics study. Therefore, potential for site flooding is low.

For Official Use Only 5-1

5. Subsurface Soil Characteristics

Select samples retrieved from exploratory borings were tested as discussed in Section 2.2. Based on the laboratory test results, details of the soil characteristics are provided in the sections below. Boring logs and laboratory test data are provided in Appendixes A and B, respectively.

5.1 Soil Compressibility Potential

Typically, fine-grained soils such as clays and silts will experience time-dependent settlements and are considered compressible under footing loads. Based on the field investigation, the soils at the site are predominantly medium dense to very dense granular soil. Therefore, the soil compressibility at the site is considered to be low.

5.2 Expansive Soil

Expansive soils are typically fine-grained soils with high plasticity that shrink and swell with changes in moisture.

Based on the granular soil on the near surface, very low expansion potential is anticipated.

5.3 Soil Corrosivity

Soil samples collected during the field investigation were tested for minimum resistivity, pH, chloride content, and sulfate content. The results are summarized in Table 5-1.

Table 5-1. Summary of Corrosion Testing

Boring Number Sample Depth

(feet) Soil Type

Minimum Resistivity (ohm-cm) pH Sulfate Content (ppm)

Chloride Content (ppm)

B-01 1 to 5 SP-SM 10,720 8.95 11.3 5.0

B-02 0 to 5 SP 20,100 8.93 16.7 7.0

B-03 1 to 4 SP 14,740 9.04 7.7 3.2

Notes:

ohm-cm = ohms-centimeter pm = parts per million

According to American Concrete Institute (ACI) 318-19 (Table 19.3.1.1) corrosion guidelines, test results indicate a non-corrosive environment to concrete. Therefore, there are no restrictions to cement type and concrete in contact with the soils in this area may be batched using Type II cement. The results also show no corrosion potential to steel structural elements. Adequate concrete cover over reinforcing steel should be provided in accordance with good construction practices and design standards. It is recommended that a qualified corrosion engineer be retained to evaluate the general corrosion potential with respect to construction materials at this site and to review the proposed design.

5.4 Frost Penetration Depth

The project site is situated within a non-frost area. Therefore, frost-protection is not considered.

For Official Use Only 5-2

5.5 California Bearing Ratio

CBR values were determined based on the information from the laboratory test data performed on the representative subgrade materials. The CBR test was performed in accordance with ASTM D1883. The CBR test results at 95 percent relative compaction are summarized in Table 5-2.

Table 5-2. Summary of California Bearing Ratio

Boring Number Depth (feet)

Soil Type CBR a

B-01 1 to 5 SP-SM 12

B-03 1 to 5 SP 8 a CBR results for 0.1 inch of penetration, interpolated at approximately 95 percent relative compaction of maximum dry density determined from ASTM D1557 Test Procedure.

For Official Use Only 6-1

6. Geotechnical Recommendations

The following recommendations are based on the collected data, site conditions, available information, and subsurface exploration.

6.1 Foundation Recommendations

Based on the subsurface conditions, the site is generally on granular soils suitable for a shallow foundation (square, mat, or continuous footing). At the time this report was prepared, the spread footing sizes were not available for the structure.

Minimum footing widths for square and continuous footings are provided with the allowable bearing capacities, which can be increased per additional foot in width up to the maximum static bearing pressures. The allowable bearing capacities were estimated as the ultimate bearing capacities divided by a factor of safety of 3, the bearing pressures at the tolerable settlement for the foundation (1 inch), or allowable bearing pressures typically recommended based on our experience with similar soils, whichever is lower. The allowable net bearing pressure was estimated considering dead and sustained live loads. The net allowable bearing pressure can be increased by 30 percent for transient load, such as wind and seismic loading.

Foundation recommendations are summarized in Table 6-1. If the dimensions of the design square or continuous footings are significantly different from those considered, the allowable bearing pressures will need to be re-evaluated.

Table 6-1. Foundation Recommendations

Foundation Recommendation Square Footing Continuous Footing

Min. Footing Embedment Below Subgrade (feet) 1.5 1.5

Min. Footing Width (feet) 2 1.5

Allowable Soil Bearing Pressure (psf) 3,000 2,500

Bearing Pressure Increase Per Additional Foot in Width (psf) 500 500

Maximum Static Bearing Pressure (psf) 3,500 3,000

Lateral Sliding Resistance – Coefficient of Friction a 0.25 0.25

Lateral Bearing Pressure (psf/ft) 150 150 a Coefficient to be multiplied by the dead load. Frictional coefficient is fully mobilized at 0.25-inch movement per IBC Table 1806.2. The total sliding resistance is determined as the lateral bearing pressure plus the lateral sliding resistance.

Note:

psf = pounds per square foot

6.2 Concrete Slab-on-Grade

The ground floor slab (slab-on-grade) should be underlain by compacted free-draining granular materials. A minimum of 6 inches of free-draining material is recommended with positive drainage to prevent a potential submersed foundation with perched water. The free-draining granular material should satisfy the requirements listed in ACI 360R-10 Section 4.6.4 and should not contain more than 5 percent fines (passing No. 200 sieve).

For Official Use Only 6-2

Design of structural slab requires the modulus of subgrade reaction to evaluate the differential settlement and the stress distribution across the foundation. The slab foundation will be supported by compacted granular backfill material (further discussed in Section 6.5.3), which will have a conservative unit modulus of subgrade reaction, Kv1, value of 150 pounds per cubic inch (pci). This unit value is applicable for a 1-foot wide square plate. For other foundation dimensions, the modulus of subgrade reaction, Kb, should be estimated using the following formula:

𝐾𝐾𝑏𝑏 = 𝐾𝐾𝑣𝑣1 × �𝐵𝐵+1

2𝐵𝐵

, where B = foundation width in feet

6.3 Pavement Design Parameters

Design CBR values for the subgrade were determined based on the information from the laboratory test data performed on the representative subgrade materials. The CBR test results are provided in Section 5.5 and Appendix B of this report. Following design parameter was recommended for flexible pavement design:

• California Bearing Ratio = 8

For the parking lot and driveway pavement section, a low-volume road design value of 100,000 18-kip equivalent single axial load (ESAL) was assumed employing an equivalent number of passes of HS20-44 vehicle, 3-axle truck, and passenger vehicles over the life span. Pavement sections were analyzed based on AASHTO Guide for Design of Pavement Structures (1993).

The recommended pavement section for the driveway and parking consists of minimum 4 inches of Hot Mix Asphalt (HMA) over 6 inches of Aggregate Base (AB) over the compacted subgrade (see Section 6.5). The finished grade should have a minimum 1 percent slope to allow for drainage. Permeable pavers at the parking stalls should be design separately to meet Low Impact Design (LID) requirements as needed.

6.4 Windblown Sediment and Associated Maintenance

As indicated in this report, the site and its surroundings are comprised of sand dunes. Windblown sediment (sand and silt) will be deposited on and against the proposed improvements, similar to occurrences at the current Station. Regular maintenance should be anticipated to remove windblown sediments as needed. In addition, heating, ventilation, and air conditioning (HVAC) and other systems should be designed to withstand these conditions; regular maintenance may also be required.

6.5 Earthwork

6.5.1 Site Preparation

Site preparation will include the demolition of existing structures, removal of surface vegetation, and any trash or debris in the areas where structure foundations will be located. Loose, soft, or wet material should also be removed and replaced with competent backfill. On-site granular soil is considered suitable for re-use as backfill if it is free of deleterious materials. Site clearing, grubbing, and earthwork should be performed in accordance with the project specifications. Surficial vegetation, organic topsoil, and deleterious material should be stripped and completely removed from the site.

6.5.2 Subgrade Preparation

To provide a uniform bearing layer below the proposed foundations and pavement section structures, it is recommended that the soils within 24 inches below the bottom of footing should be removed and recompacted

For Official Use Only 6-3 and 8 inches below the pavement section should be scarified and recompacted. The soils should be moisture conditioned to within ±2 percent of optimum moisture content, and recompacted to 95 percent relative density per ASTM D1557 Test Procedure.

6.5.3 Fill Placement

The backfill and fill materials should conform with the requirements of the project specifications. The fill materials should be sand or gravel materials with a liquid limit of less than 35, plasticity index of less than 12, and a maximum of 25 percent fines passing through a No. 200 sieve.

Fill materials should be placed in layers not exceeding 8 inches in loose thickness, moisture conditioned within ±2 percent of optimum moisture content, and compacted to a minimum of 95 percent of the maximum dry unit weight as determined in accordance with ASTM D1557 Test Procedure.

Permanent fill should be placed in a gradient no steeper than 2H:1V, and temporary fill, such as surcharge fill, should be placed in a gradient no steeper than a gradient of 1.5H:1V.

6.5.4 Temporary Excavations

Excavations may be either sloped back or shored. Excavation of the temporary cut slopes greater than 12 inches in height should be no steeper than a gradient of 1.5H:1V or as the field conditions dictate to provide safe and stable slopes. The excavations should not extend into the influence zone of adjacent structures or utility lines, unless the excavations are shored. In general, the influence zone may be obtained by projecting a line sloping outward and downward at 1H:1V from the edge of the existing structure.

All temporary excavations should be performed in accordance with the safety requirements of Occupational Safety and Health Administration (OSHA) excavation regulations along with local, and state requirements. It is the responsibility of the contractor to provide stable excavation for the temporary cuts at the project site.

The eventual evaluation of the safety of construction slopes is the responsibility of the contractor and should be based on the contractor’s approach to the field work. We recommend that the actual safe inclination of the construction slopes and other safety precautions for the proposed excavation be evaluated and determined by the contractor at the time of construction.

6.5.5 Geotechnical Observation

Subgrade preparation and fill placement should be observed by a geotechnical specialist. Variations in soil and geologic conditions may be encountered during construction. To permit correlation between the exploration data and the actual conditions encountered during construction, a qualified geotechnical specialist should perform on-site observation during construction.

6.6 Review of Construction Plans and Specifications

The geotechnical recommendations in this report are based on the geotechnical field investigation and available plans at the time of preparing this report. The geotechnical designer should review construction plans and specifications to verify that the geotechnical recommendations have been incorporated into the design. If the nature, design, or location of the proposed development at the site varies from those used to develop the recommendations provided in this report, the recommendations should be re-evaluated.

For Official Use Only 7-1

7. Limitations

The conclusions and recommendations contained in this report are professional opinions, intended for the use of Jacobs and its design consultants. This report has been prepared solely for the design of the improvements described and may not contain sufficient information for other uses. The recommendations should not be extrapolated to areas not covered by this report or used for other facilities without the review and approval of Jacobs.

Our investigation and evaluations were performed in accordance with generally accepted local standards using that degree of care and skill ordinarily exercised under similar circumstances by geotechnical consultants practicing in this or similar localities. No other warranty, express or implied, is made as to the professional opinions included in this report.

The recommendations for this project are dependent upon proper quality control of foundation installation and other construction activities. The owner and contractor should implement proper quality control procedures to ensure that the subgrade soils, quality of fill, placement and compaction of fill, foundation excavation and installation, etc. meet the assumptions in this report. If different field conditions are encountered during construction, Jacobs should be notified and remediation measures should be implemented as necessary. A geotechnical specialist should observe key stages of the foundation construction to verify that the subgrade conditions meet the requirements of the geotechnical report.

For Official Use Only 8-1

8. References

American Associate of State Highway (AASHTO). 1993. Guide for Design of Pavement Structures. ISBN 1-56081- 055-2

American Concrete Institute (ACI). 2010. Guide to Design of Slabs-on-Ground, ACI 360R-10.

American Concrete Institute (ACI). 2019. Building Code Requirements for Structural Concrete and Commentary, ACI 318-19.

American Society of Civil Engineers (ASCE). 2019. Minimum Design Loads and Associated Criteria for Buildings and Other Structures including Supplement 1, ASCE/SEI 7-16.

ASTM International (ASTM). 2017. Book of Standards. West Conshohocken, PA. www.astm.org.

Bureau of Land Management (BLM). 2020. Scope of Work, Architectural and Engineering Design Services for the Cahuilla Ranger Station and Site Layout Design, Title I and II Services. January 26.

California Geological Survey. 1962. Geologic Map of California, San Diego-El Centro Sheet. Scale 1:250,000.

California Geological Survey (CGS). 2002. California Geomorphic Provinces. CGS Note 36. December.

California Geological Survey (CGS). 2018. Earthquake Fault Zones, A Guide for Government Agencies, Property owners/Developers, and Geoscience Practitioners for Assessing Fault Rupture Hazards in California. Special Publication 42.

Guptil, P.D., R.W. Ruff, E.M. Gath. 1986. Geology of the Imperial Valley, California. South Coast Geological Society Field Trip Guidebook.

Lawrence Livermore National Laboratory (LLNL). 2008. Groundwater Availability Within the Salton Sea Basin, A Final Report. Report No. LLNL-TR-400426. Prepared for the U.S. Department of Energy. January 29.

Morton, P.K. 1977. Geology and Mineral Resources of Imperial County California. CGS County Report 7.

Palmer, T.D., J.H. Howard, and D.P. Lande. 1975. Geothermal Development of the Salton Trough, California and Mexico. Prepared for the U.S. Energy and Research Development Administration Contract No. W-7405-Eng-48.

April 1.

Philibosian, B., T. Fumal, and R. Weldon. 2011. San Andreas Fault Earthquake Chronology and Lake Cahuilla History at Coachella, California. Bulletin of the Seismological Society of America. Vol. 101, No. 1, pp13-38.

February.

United States Geological Survey (USGS) and California Geological Survey (CGS). 2006. Quaternary Fault and Fold Database of the United States. https://earthquake.usgs.gov/hazards/qfaults/. Accessed September 18, 2020.

http://www.astm.org/ https://earthquake.usgs.gov/hazards/qfaults/

For Official Use Only

Figures

Figure 1-1 Site Vicinity Map Cahuilla Ranger Station Imperial County, California

Figure 2-1 Boring Location Map Cahuilla Ranger Station Imperial County, California

© 2021 Microsoft Corporation, © 2021 Maxar, © CNES (2021) Distribution Airbus DS, © 2021 TomTom. Annotation by Jacobs, 2021.

100 50 100

Approximate scale in feet

Legend

Hollow-Stem Auger Boring

Cahuilla Ranger Station Building Limits

Construction Limits

Plan North

AutoCAD SHX Text B-01

AutoCAD SHX Text 31.5'

AutoCAD SHX Text B-01

AutoCAD SHX Text 31.5'

AutoCAD SHX Text B-02

AutoCAD SHX Text 61.4'

AutoCAD SHX Text B-02

AutoCAD SHX Text 61.4'

AutoCAD SHX Text B-03

AutoCAD SHX Text 31.5'

AutoCAD SHX Text B-03

AutoCAD SHX Text 31.5'

AutoCAD SHX Text

GECKO RD

AutoCAD SHX Text

GECKO RD

AutoCAD SHX Text

GECKO RD

AutoCAD SHX Text

GECKO RD

AutoCAD SHX Text

CAHUILLA RANGER STATION GECKO ROAD

AutoCAD SHX Text

CAHUILLA RANGER STATION GECKO ROAD

Reference: California Geological Survey. 1962. Geologic Map of California, San Diego-El Centro Sheet. Scale 1:250,000.

Figure 3-1 Geologic Map Cahuilla Ranger Station Imperial County, California

North

0 42

Approximate scale in miles

LEGEND

Geologic Units

Dune sand

Alluvium

Quaternary lake deposits

Pleistocene nonmarine

Ql

Qc

Qal

Qs

Approximate Location of Cahuilla Ranger Station

Note: This map was published in 1962 and the fault locations shown are no longer accurate. This includes the San Andreas Fault as shown on this figure, see text for discussion.

D3290106 Figure3-1_CahuillaRangerStation_GeoMap_rev1

Appendix A Boring Logs

2.5

1.5

1.5

1.5

1.5

1.5

Comments

Comments and observations regarding drilling or sampling made by the driller or field personnel

Tests

Field and laboratory tests include the following:

Wc Water Content (ASTM D2216)

DD Dry Density (ASTM D2937)

CBR California Bearing Ratio (ASTM D1883)

CR Corrosion Suite (ASTM D4972, G187, D4327)

DS Direct Shear (ASTM D3080)

MAX Maximum Dry Density and Optimum Moisture Content (ASTM D1557)

PA Particle-Size Analysis (ASTM D7928 / D6913), with or without hydrometer

PI Atterberg Limits (ASTM D4318)

0.0

2.5

5.0

7.5

10.0

15.0

4.0

6.5

9.0

11.5

16.5

1-2-3 (5)

4-5-6 (11)

30-40-50/1" (90/7")

40-50/2" (50/2")

50/3"

Samples bgs = below ground surface

Sample Type - Sample Number

[BULK] Bulk sample collected from drill cuttings

[SPT] Standard split-spoon driven sampler, 2.0-inch (51-mm) outside diameter, 1.4-inch (35-mm) inside diameter, without liners

[CAL] California split-spoon driven sampler, 3.0-inch (76-mm) outside diameter, 2.4-inch (64-mm) inside diameter, with liners

Penetration Test Results

Number of blows required to advance drive sampler (SPT or CAL) over three 6-inch (152-mm) increments. Number in parenthesis is the total number of blows required to advance the sampler 12-inch (304-mm) beyond the first 6-inch (152-mm) interval. Where sampler refusal is encountered and the sampler does not advance the full sampling interval, the number in the parenthesis is the total number of blows per number of inches advanced.

Driven sampler is advanced using a 140-lb (63.5-kg) hammer with the 30-inch (762-mm) drop. The blow counts given have not been modified to account for field and/or depth conditions.

General Notes

1) Soil classifications are based on the Unified Soil Classification System. Classifications and descriptions made in the field have been modified based on the results of laboratory testing.

2) Boring logs depict subsurface conditions only at the specific locations and times the boring was advanced. Logs do not necessarily reflect strata variations that may exist between boring locations.

3) The percentage of gravel, sand, and fines is reported using a proportional descriptor, estimated from field visual classification or through laboratory particle-size analysis. The proportional descriptor criteria are as follow:

trace - less than 5% few - between 5 and 10% little - between 15 and 25% some - between 30 and 45% mostly - between 50 and 100%

4) To convert from uncorrected CAL sampler blow count to equivalent uncorrected SPT sampler blow count, multiply by 0.65.

B-1

S-2

S-3

S-4

S-5

S-6

SOIL NAME, USCS GROUP SYMBOL, COLOR,

MOISTURE CONTENT, RELATIVE DENSITY OR

CONSISTENCY, SOIL STRUCTURE, MINERALOGY

STANDARD

PENETRATION

TEST RESULTS

ELEVATION :

LEGEND

SOIL DESCRIPTION

SOIL BORING LOG

PROJECT NUMBER

COMMENTS

D3290106

START: END:

BORING NUMBER:

INTERVAL (ft)

RECOVERY (ft)

SHEET 1 OF 1

DEPTH OF CASING, DRILLING RATE,

DRILLING FLUID LOSS, TESTS, AND

INSTRUMENTATION

LOGGER :

TYPE-#

DRILLING CONTRACTOR :

DRILLING EQUIPMENT AND METHOD :

LOCATION : LAT/LONG : PROJECT : Cahuilla Ranger Station - Facility Replacement

ELEVATION (ft)

GROUNDWATER EL : Not Encountered

6"-6"-6" (N)

S O

IL

B

O R

IN

G

L O

G

C A

H U

IL

LA

R A

N G

E R

S T

A T

IO

N

L

IB

R A

R Y

.G

LB

C

A H

U

IL

LA

R

A N

G E

R S

T A

T

IO

N B

O R

IN

G

L O

G S

.G P

J;

C

H 2M

G E

O T

E C

H .G

D T

7/

7/

GRAVEL

AND

GRAVELLY

SOILS

CLAYEY GRAVELS, GRAVEL - SAND -

CLAY MIXTURES

WELL-GRADED SANDS, GRAVELLY

SANDS, LITTLE OR NO FINES

POORLY-GRADED SANDS,

GRAVELLY SAND, LITTLE OR NO

FINES

SILTY SANDS, SAND - SILT

MIXTURES

CLAYEY SANDS, SAND - CLAY

MIXTURES

INORGANIC SILTS AND VERY FINE

SANDS, ROCK FLOUR, SILTY OR

CLAYEY FINE SANDS OR CLAYEY

SILTS WITH SLIGHT PLASTICITY

INORGANIC CLAYS OF LOW TO

MEDIUM PLASTICITY, GRAVELLY

CLAYS, SANDY CLAYS, SILTY CLAYS,

LEAN CLAYS

ORGANIC SILTS AND ORGANIC

SILTY CLAYS OF LOW PLASTICITY

INORGANIC SILTS, MICACEOUS OR

DIATOMACEOUS FINE SAND OR

SILTY SOILS

INORGANIC CLAYS OF HIGH

PLASTICITY

SILTS

AND

CLAYS

MORE THAN 50%

OF MATERIAL IS

LARGER THAN

NO. 200 SIEVE

SIZE

MORE THAN 50%

OF MATERIAL IS

SMALLER THAN

NO. 200 SIEVE

SIZE

MORE THAN 50%

O…