Attachment_E_Earth_Systems_Geotechnical_Report_Jan_2022.pdf

Valley Sanitary District 45500 Van Buren Street Indio, California 92201

File No.: 304996‐001 Doc. No.: 21‐12‐711

Geotechnical Engineering Report Proposed Valley Sanitary District Training Offices & Lab Buildings

45‐500 Van Buren Street Indio, Riverside County, California

December 20, 2021

© 2021 Earth Systems Pacific Unauthorized use or copying of this document is strictly prohibited without the express written consent of Earth Systems Pacific.

Earth Systems________________________________________________ 79‐811 Country Club Drive, Suite B | Bermuda Dunes, CA 92203 | Ph: 760.345.1588 | www.earthsystems.com

December 20, 2021 File No.: 304996‐001 Doc No.: 21‐12‐711

Valley Sanitary District 45500 Van Buren Street Indio, California 92201

Attention: Mr. Ronald Buchwald

Subject: Geotechnical Engineering Report

Project: Proposed Valley Sanitary District Training Offices & Lab Buildings 45‐500 Van Buren Street Indio, Riverside County, California

Earth Systems Pacific (Earth Systems) is pleased to submit this geotechnical engineering report for the referenced project located at 45‐550 Van Buren Street in Indio, Riverside County, California. This report presents our findings and recommendations for site grading and foundation design, incorporating the information provided to our office. The site is suitable for the proposed development, provided the recommendations in this report are followed in design and construction. This report should stand as a whole, and no part of the report should be excerpted or used to the exclusion of any other part.

This report completes our scope of services in accordance with our proposal (BER‐21‐8‐003) with an authorization date of October 28, 2021. Other geotechnical related services that may be required, such as plan reviews, responses to agency inquiries, and grading observation and testing are additional services and will be billed according to the Fee Schedule in effect at the time services are provided. Unless requested in writing, the Client is responsible to distribute the report to the appropriate governing agency and other members of the design team. Please review the Limitations (Section 6) of this report as they are vital to the understanding of this report.

We appreciate the opportunity to provide our professional services. Please contact our office if there are any questions or comments concerning this report or its recommendations.

Respectfully submitted, EARTH SYSTEMS PACIFIC

Anthony Colarossi, PE Mark S. Spykerman, EG Project Engineer Principal Engineering Geologist

RCE 60302 EG 1174

GER/ac/mss/klp/cgj

Distribution: 1/Valley Sanitary District 1/Ron Buchwald (rbuchwald@valley‐sanitary.org)

1/BER

i

EARTH SYSTEMS PACIFIC

TABLE OF CONTENTS

Page

Section 1 INTRODUCTION

1.1 Background and Proposed Project Description

1.2 Site Description

1.3 Purpose and Scope of Services

Section 2 METHODS OF EXPLORATION AND TESTING

2.1 Field Exploration

2.2 Laboratory Testing

Section 3 DISCUSSION

3.1 Soil Conditions

3.2 Groundwater

3.3 Collapse Potential/Consolidation Potential/Swell Potential

3.4 Expansive Soils

3.5 Corrosion Potential

3.6 Geologic Setting

3.7 Geologic Hazards

3.7.1 Seismic Hazards

3.7.2 Other Hazards

Soil and Rock Engineering Properties Discussion and Loading Assumptions for Screening Level Evaluation:

Soil Slope Factor of Safety and Drainage Condition Assumptions:

Section 4 CONCLUSIONS

Section 5 RECOMMENDATIONS

5.1 Site Development – Grading

5.1.1 Cut and Fill Slopes

5.2 Excavations

5.3 Utility Trenches

STRUCTURES

5.4 Foundations

5.5 Slabs‐on‐Grade

5.6 Retaining Walls and Lateral Earth Pressures (granular backfill)

5.7 Seismic Design Criteria

5.8 Driveways and Parking Areas

5.9 Surface and Subsurface Site Drainage

Section 6 LIMITATIONS AND ADDITIONAL SERVICES

6.1 Uniformity of Conditions and Limitations

6.2 Additional Services

REFERENCES

APPENDIX A

Plate 1 – Site Location Map Plate 2 – Exploration Location Map Table A‐1 Fault Parameters Table A‐2 Historic Earthquakes 2019 CBC Seismic Design Parameters

Table of Contents, continued ii

Terms and Symbols Used on Boring Logs Soil Classification System

Log of Borings (3 pages) Site Class B‐1 (1 page) Liquefaction and Dry Seismic Settlement (2 pages) Continuous Footing Settlement (1 page) Spread Footing Settlement (1 page)

APPENDIX B

Laboratory Test Results

December 20, 2021 1 File No.: 304996‐001 Doc. No.: 21‐12‐711

Geotechnical Engineering Report Proposed Valley Sanitary District Training Offices & Lab Buildings

45500 Van Buren Street Indio, Riverside County, California

Section 1

INTRODUCTION

1.1 Background and Proposed Project Description

Earth Systems experience at the Valley Sanitary District site is noted from past reports prepared in 2001, 2003, 2010, and 2019 (see Reference Section under “Earth Systems”) along with additional reports (field density testing, District Wetlands, nutrient analysis, pond slope protection, and temporary slope stability evaluation during construction (File No.: 07641‐08)—if desired please contact Earth Systems for those reports.

The Valley Sanitary District’s proposed project is located at the existing treatment plant at 45‐ 500 Van Buren Street, Indio, California, see Plate 1 (Site Location Map). This geotechnical engineering report has been prepared for two proposed buildings at the site: 1) Office Training and 2) Laboratory. Based upon the architectural plans provided, we understand the proposed Office/Training and Laboratory buildings will have an approximate footprint area as shown in Figure 1. Appurtenant site work is anticipated to include underground utilities, hardscape, flatwork improvements, and landscaping. Preliminary grading and foundation plans, and appurtenant site work plans were not available at the time that this report was prepared. We have assumed site grades will be similar in elevation to the surrounding grades (+‐3 feet). The proposed site layout along with our exploration locations is presented in Plate 2 in Appendix A.

We have assumed the buildings will be one to two story in height made of wood, metal, light steel frame, or reinforced masonry construction and founded on shallow permanent foundations. No below grade basement levels are anticipated. Column loads are anticipated not to exceed approximately 50 kips for spread footings and 3 kip/lf for continuous footing loads.

Preliminary design loading was not provided by the structural engineer. If actual structural loading exceeds these assumed values, we will need to re‐evaluate the given recommendations.

1.2 Site Description

The project site is located at 45‐500 Van Buren Street in the city of Indio, Riverside County, California. Coordinates of the farthest north of the proposed lab building is approximately 33.7157°N/116.1981°W. Topographically, the site is generally flat and level, however, there is a sludge pond located south of the proposed Office/Training building—additional measurements of the slope are found in Section 3.7.2.1 (Slope Stability). Site elevations range from approximately ‐31 feet to ‐27 feet (msl) based on Google Earth. Drainage is by sheet flow to the east where the Whitewater Channel is located. No evidence of significant erosion was observed during the site visit. Access to the site is via Van Buren Street, a paved public roadway.

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Figure 1 Site Plan of Proposed Buildings

1.3 Purpose and Scope of Services

The purpose for our services was to evaluate the site soil and geologic conditions at our exploration locations and to provide professional opinions and recommendations, from a geologic and geotechnical point of view, regarding the proposed development of the site. We understand that the proposed site improvements will be developed under the regulation of the current California Building Code (CBC 2019). The scope of services included:

Task 1 ‐ Literature and Photograph Reviews

We reviewed select geologic and geotechnical literature pertaining to the project. This included a review of various hazard, fault, and geologic maps prepared by the California Geological Survey (CGS), the U.S. Geological Survey (USGS), Riverside County and other governmental agencies as they relate to the project area. Select historical aerial photographs were reviewed using Google

Earth Pro and Historical Aerials website.

Task 2 – Utility Clearance, USA Dig Alert

Each of our proposed field explorations were located and marked in the field and cleared with known utility lines as identified by Underground Service Alert (USA), “Dig Alert”. Also, a client provided as‐built drawing of utilities was used to mark the exploration locations. Our exploration

December 20, 2021 3 File No.: 304996‐001 locations were located in the field by pacing with sighting from landmarks identified on the project site plan.

Task 3 – Field Exploration

We evaluated the general subsurface conditions at the site by drilling three small‐diameter (8‐ inch) hollow‐stem auger borings, to depths from approximately 21½ feet to 51½ feet below the existing ground surface (bgs). A representative from Earth Systems maintained a log of the subsurface conditions encountered and obtained samples for visual observation, classification and laboratory testing. Plate 2 shows the approximate locations of the borings.

Task 4 – Laboratory Testing

Laboratory tests were performed on selected samples to evaluate the physical characteristics of the materials encountered during our field exploration. Laboratory testing included moisture content, dry unit weight, maximum dry density/optimum moisture content, consolidation/collapse potential, Expansion Index, and R‐Value. The testing was performed in general accordance with American Society for Testing and Materials (ASTM) or appropriate test procedures. Selected samples were also tested for a preliminary screening level of corrosion potential (pH, electrical resistivity, water‐soluble sulfates and water‐soluble chlorides).

Task 5 – Analysis and Report

Earth Systems analyzed the field data obtained, performed engineering analyses, and provided recommended design parameters for earthwork and foundations for the structure as described within. Our report includes:

A description of the proposed project including a site plan showing the approximate boring locations;

A description of the surface and subsurface site conditions including groundwater conditions, as encountered in our field exploration;

A description of the site geologic setting and possible associated geology‐related hazards, including liquefaction, subsidence, and seismic settlement analysis;

A discussion of regional geology and site seismicity;

A description of local and regional active faults, their distances from the site, their potential for future earthquakes;

A discussion of other geologic hazards such as ground shaking, landslides, flooding, and tsunamis;

A discussion of site conditions, including the geotechnical suitability of the site for the general type of construction proposed;

A “General Procedure” seismic analysis including geotechnical seismic design coefficients in accordance with the 2019 CBC.

Recommendations for imported fill for use in compacted fills;

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Recommendations for foundation design including parameters for shallow foundations and subgrade preparation;

Anticipated total and differential settlements for the recommended foundation system;

Recommendations for site preparation, earthwork, and fill compaction specifications;

Screening‐level slope analysis of nearby sludge pond slope near proposed office building;

Discussion of anticipated excavation conditions; , including shrinkage and/or bulking.

Recommendations for underground utility trench backfill;

Recommendations for stability of temporary trench excavations;

Recommendations for slabs‐on‐grade, including recommendations for reducing the potential for moisture transmission through interior slabs;

Recommendations for collapsible or expansive soils (if applicable);

Asphalt concrete pavement and Portland cement driveway recommendations for auto, driveways, and fire lanes (Traffic Index values of 5.0 for parking areas and fire lanes, and

7.0 for driveways);

A discussion of the corrosion potential of the near‐surface soils encountered during our field exploration;

An appendix, which includes a summary of the field exploration (computer generated boring logs) and laboratory testing program (computer generated plots).

Not Contained in This Report: Although available through Earth Systems, the current geotechnical scope of our services does not include:

An environmental Phase 1 assessment.

A study for the presence or absence of wetlands, hazardous or toxic materials in the soil, surface water, groundwater, or air on, below, or adjacent to the subject property, nor exploration or delineation of past development or remaining structures/debris.

The client did not direct Earth Systems to provide any service to investigate or detect the presence of moisture, mold, or other biological contaminates in or around any structure, or any service that was designed or intended to prevent or lower the risk or the occurrence of the amplification of the same. Client is hereby informed that mold is ubiquitous to the environment, with mold amplification occurring when building materials are impacted by moisture. Site conditions are outside of Earth Systems’ control, and mold amplification will likely occur or continue to occur in the presence of moisture. As such, Earth Systems cannot and shall not be held responsible for the occurrence or recurrence of mold.

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Section 2

METHODS OF EXPLORATION AND TESTING

2.1 Field Exploration

The subsurface exploration program consisted of advancing three exploratory borings. The borings were drilled to depths ranging from approximately 21½ to 51½ feet bgs using a Mobile B‐61 truck‐mounted drill rig equipped with 8‐inch hollow‐stem augers provided by CalPac Drilling of Calimesa, California. The borings were advanced to observe soil profiles and obtain samples for laboratory testing. The approximate boring locations are shown on Plate 2, in Appendix A.

The locations shown are approximate, established by consumer grade Global Positioning System (GPS) accurate to ± 15 feet in conjunction with pacing based upon the control provided.

An engineer from Earth Systems maintained a log of the subsurface conditions encountered and obtained samples for visual observation, classification and laboratory testing. Subsurface conditions encountered in the borings were categorized and logged in general accordance with the Unified Soil Classification System (USCS) and ASTM D 2487 and 2488 (current edition). Our typical sampling interval within the borings was approximately every 2½ or 5 feet to the full depth explored; however, sampling intervals were adjusted depending on the materials encountered onsite. Samples were obtained within the test borings using a Modified California (MC) ring sampler (ASTM D 3550 with those similar to ASTM D 1586). The MC sampler has an approximate 3‐inch outside diameter and a 2.4‐inch inside diameter. The ring sampler was mounted on a drill rod and driven using a rig‐mounted 140‐pound automatic hammer falling for a height of 30 inches. The number of blows necessary to drive the MC sampler within the borings was recorded.

Bulk samples of the soil materials were obtained from the drill auger cuttings, representing a mixture of soils encountered at the depths noted. The depth to groundwater, if any, was measured in the boreholes. Following drilling, sampling, and logging, the borings were backfilled with the cuttings and tamped upon completion. Our field exploration was provided under the direction of a State of California Registered Engineer from our firm.

Design parameters provided by Earth Systems in this report have considered an estimated 72% hammer efficiency based on test data provided by the drilling subcontractor and accepted SP 117A criteria. Since the MC sampler was used in our field exploration to collect ring samples, the N‐values using the California sampler can be roughly correlated to SPT N‐values using a conversion factor that may vary from about 0.5 to 0.7. In general, a conversion factor of approximately 0.63 from a study at the Port of Los Angeles (Zueger and McNeilan, 1998 per SP 117A) is considered satisfactory. A value of 0.63 was applied in our calculations for this project.

The final logs of the borings represent our interpretation of the contents of the field logs and the results of laboratory testing performed on the samples obtained during the subsurface exploration. The final logs are included in Appendix A of this report. The stratification lines represent the approximate boundaries between soil types, although the transitions may be gradual. In reviewing the logs and legend, the reader should recognize the legend is intended as a guideline only, and there are a number of conditions that may influence the soil characteristics observed during drilling. These include, but are not limited to cementation, oversize rocks, variations in soil moisture, presence of groundwater, and other factors.

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The boring logs present field blowcounts per 6 inches of driven embedment (or portion thereof) for a total driven depth attempted of 18 inches. The blow counts on the logs are uncorrected (i.e. not corrected for overburden, sampling, etc.). Consequently, the user must correct the blow counts per standard methodology if they are to be used for design and exercise judgment in interpreting soil characteristics, possibly resulting in soil descriptions that vary somewhat from the legend.

2.2 Laboratory Testing

Samples were reviewed along with field logs to select those that would be analyzed further.

Those selected for laboratory testing include, but were not limited to, soils that would be exposed and those deemed to be within the influence of the proposed structures. Test results are presented in graphic and tabular form in Appendix B of this report. Testing was performed in general accordance with American Society for Testing and Materials (ASTM) or other appropriate test procedure. A selected sample was also tested for a screening level of corrosion potential (pH, electrical resistivity, water‐soluble sulfates, and water‐soluble chlorides). Earth Systems does not practice corrosion engineering; however, these test results may be used by a qualified corrosion engineer in designing an appropriate corrosion control plan for the project.

Our testing program consisted of the following:

Particle Size Analysis to classify and evaluate soil composition. The gradation characteristics of selected samples were made by percent passing the #200 sieve and sieve analysis procedures (ASTM D 6913).

Density and Moisture Content of select samples of the site soils (ASTM D 2937 & 2216).

Maximum Dry Density/Optimum Moisture Content tests to evaluate the moisture‐density relationship of typical soils encountered (ASTM D 1557).

Consolidation and Collapse (or swell) Potential to evaluate the compressibility and hydroconsolidation (collapse), or swell potential of the soil upon wetting (ASTM D 5333) under various loads.

Expansion Index tests to evaluate the expansive nature of the soil. The samples were surcharged under 144 pounds per square foot at moisture contents of near 50% saturation. Samples were then submerged in water for 24 hours and the amount of expansion was recorded with a dial indicator (ASTM D 4829).

Chemical Analyses (Soluble Sulfates and Chlorides (ASTM D 4327), pH (ASTM G 187), and Electrical Resistivity/Conductivity (APHA 2320‐B) to evaluate the potential for adverse effects of the soil on concrete and steel.

R‐Value for pavement section analysis (CTM 301).

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Section 3

DISCUSSION

3.1 Soil Conditions

The field exploration indicates that site soils consist generally of silty sand, poorly graded sand with silt, silty clay, and lean clay (Unified Soils Classification System symbols of SM, SP‐SM, CL‐ ML, and CL) to the maximum depth of exploration of 51½ feet below the ground surface. Boring B‐1 and B‐2 was similar to CPT‐1 for the Hypochlorite Storage Building (Earth Systems, 2007).

Boring B‐3 became coarser grained with lesser amounts of silt. The underlying coarse‐grained soils encountered were found to be loose to medium dense (the deepest defined as loose was at boring B3 at 10 feet). Fine grained soil layers were found at various depths in borings B‐1 and B‐ 2; however, they were not observed in boring B‐3. The fine grained soils were observed as lean clay and silt soils. The fine grained soil consistency varied from “Firm” to “Very Stiff”.

The upper 35‐foot soils were observed, in general, to vary from damp to moist. Below this depth soils were observed, in general, to have increased moisture. Laboratory moisture contents varied from 4.3 to 23.1 percent by weight in the upper 35‐foot soil profiles. Below 35 feet the moisture contents varied from 39.7 to 40.5 percent.

The surface soils are typical of alluvium and aeolian (windblown); unconsolidated and semi‐ consolidated. Site soils expected in the upper 7 feet during construction are generally cohesionless. The boring logs provided in Appendix A include more detailed descriptions of the soils encountered.

The site lies within a recognized blow sand hazard area. Fine particulate matter (PM10) can create an air quality hazard if dust is blowing. Watering the surface, planting grass or landscaping, or placing hardscape normally mitigates this hazard. Site soils are classified as Type C in accordance with Cal OSHA.

3.2 Groundwater

Earth Systems reviewed both current and historic groundwater levels near the project site. For this report, we used information dated back to 1968 for use as historic information. We also provide a brief discussion of the moisture contents of the soils found during the exploration and the ability of water features to produce a perched water table.

Exploration Information: Free groundwater was not encountered in the borings during our explorations conducted on November 12, 2021; however, very moist soils appeared at a depth of 35 feet below the ground surface (bgs) and continued with very moist conditions to a maximum depth of 50 feet bgs. We performed 14 moisture content tests of the soil samples recovered and obtained values varying between 4% to 40% at depths below surface ranging between 2½ feet and 51½ feet below the ground surface (bgs). Saturation levels at or above 85% were observed at 30 to 50 bgs indicating potential near groundwater levels; however, no free water was observed on any samples.

Nearby Well Information: We researched the California Department of Water Resources (DWR) groundwater database and found one well (Local Well 337345N1162245W001) located

December 20, 2021 8 File No.: 304996‐001 approximately 2 miles northwest of the project site. That well had readings taken on September 27, 2021, and indicates the groundwater elevation is approximately 79.92 ft below mean sea level (MSL). From Section 1.2, the project low elevation is 31 below MSL. Therefore, the groundwater depth at the site is approximately 48 feet below the ground surface.

Historic Groundwater Information: From observation of a 1961 Ground Water Basin Subdivisions and Contours of Ground Water Levels Map, published by the Resources Agency of California Department of Water Resources Southern District (Department of Water Resources Bulletin 108), the historic groundwater contour nearest the project is between contour ‐50 and ‐60.

Figure 2 Historic Groundwater Map

Groundwater Depth used for Liquefaction Analysis: Based on the historic groundwater map and past liquefaction analysis, Earth Systems assumes the depth to historic groundwater is approximately 19 feet below the ground surface. Since liquefaction requirements require depths of groundwater to be within 50 feet of the surface, we provide liquefaction settlement calculation, see Section 3.7.2.3. In addition, we provide dry seismic ground settlement calculations settlements located in layers above groundwater table.

3.3 Collapse Potential/Consolidation Potential/Swell Potential

Collapsible soil deposits generally exist in regions of moisture deficiency. Collapsible soils are generally defined as soils that have potential to suddenly decrease in volume upon increase in moisture content even without an increase in external loads. Soils susceptible to collapse include loess, weakly cemented sands and silts where the cementing agent is soluble (e.g. soluble gypsum, halite), valley alluvial deposits within semi‐arid to arid climate, and certain granite residual soils above the groundwater table. In arid climatic regions, granular soils may have a potential to collapse upon wetting. Collapse (hydro‐consolidation) may occur when the soils are lubricated or the soluble cements (carbonates) in the soil matrix dissolve, causing the soil to densify from its loose configuration from deposition.

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The degree of collapse of a soil can be defined by the Collapse Potential (CP) value, which is expressed as a percent of collapse of the sample using the Collapse Potential Test (ASTM Standard Test Method D 5333). Based on the Naval Facilities Engineering Command (NAVFAC) Design Manual 7.1, the severity of collapse potential is commonly evaluated by the following Table 1, Collapse Potential Values.

Table 1 Collapse Potential Values

Collapse Potential Value Severity of Problem

0‐1% No Problem

1‐5% Moderate Problem

5‐10% Trouble

10‐20% Severe Trouble

> 20% Very Severe Trouble

Table 1 can be combined with other factors such as the probability of ground wetting to occur on‐site and the extent or depth of potential collapsible soil zone to evaluate the potential hazard by collapsible soil at a specific site. A hazard ranking system associated with collapsible soil as developed by Hunt (1984) is presented in Table 2, Collapsible Soil Hazard Ranking System.

Table 2 Collapsible Soil Hazard Ranking System

Degree of Hazard Definition of Hazard

No Hazard No hazard exists where the potential collapse magnitudes are non‐ existent under any condition of ground wetting.

Low Hazard Low hazards exist where the potential collapse magnitudes are small and tolerable, or the probability of significant ground wetting is low.

Moderate Hazard

Moderate hazards exist where the potential collapse magnitudes are undesirable, or the probability of substantial ground wetting is low, or the occurrence of the collapsible unit is limited.

High Hazard High hazard exists where potential collapse magnitudes are undesirably high and the probability of occurrence is high.

The results of collapse potential tests performed on four selected samples from depths ranging from 5 to 15 feet below the ground surface indicated a collapse potential on the order of between

0.4 to 0.5 percent. Based on the above criteria, field, and laboratory findings, we estimate the potential for collapse is “No Problem”.

3.4 Expansive Soils

Expansive soils are characterized by their ability to undergo significant volume change (shrink or swell) due to variations in moisture content. Changes in soil moisture content can result from rainfall, landscape irrigation, utility leakage, roof drainage, perched groundwater, drought, or other factors, and may cause unacceptable settlement or heave of structures, concrete slabs

December 20, 2021 10 File No.: 304996‐001 supported‐on‐grade, or pavements supported over these materials. Depending on the extent and location below finished subgrade, expansive soils can have a detrimental effect on structures.

Based on our visual observations and testing, we found isolated layers of expansive soils at shallow depths.

One sample of near surface soils was tested for Expansion Index. The test result indicated the sample had an Expansive Index (EI) of the onsite soils of “very low”, as defined by ASTM D 4829.

Clay soils were also found at depths as shallow as 7.5 feet below the ground surface (see boring B‐2 and deeper layers at B‐1. Clay layers did not appear to be continuous. Field testing of clay layers indicated low strength plasticity threads near the plastic limit indicating low expansive pressures. Samples of building pad soils should be observed and/or tested during grading to confirm or modify these findings.

3.5 Corrosion Potential

Two samples of the near‐surface soils within the site was tested for potential corrosion of concrete and ferrous metals. Soils in the upper 0 to 5 feet were tested as a blended (composite) and individual samples. The tests were conducted in general accordance with the ASTM Standard Test Methods to evaluate pH, resistivity, and water‐soluble sulfate and chloride content. The test results are presented in Appendix B. These tests should be considered as only an indicator of corrosivity for the samples tested. Other earth materials found on site may be more, less, or of a similar corrosive nature.

Water‐soluble sulfates in soil can react adversely with concrete. ACI 318 provides the relationship between corrosivity to concrete and sulfate concentration, presented in the table below:

Table 3

Water‐Soluble Sulfate in Soil (ppm)

Corrosivity to Concrete

0‐1,000 Negligible

1,000 – 2,000 Moderate

2,000 – 20,000 Severe

Over 20,000 Very Severe

In general, the lower the pH (the more acidic the environment), the higher the soil corrosivity will be with respect to ferrous structures and utilities. As soil pH increases above 7 (the neutral value), the soil is increasingly more alkaline and less corrosive to buried steel structures, due to protective surface films, which form on steel in high pH environments. A pH between 5 and 8.5 is generally considered relatively passive from a corrosion standpoint. High chloride levels tend to reduce soil resistivity and break down otherwise protective surface deposits, which can result in corrosion of buried steel or reinforced concrete structures. Soil resistivity is a measure of how easily electrical current flows through soils and is the most influential factor. Based on the findings of studies presented in ASTM STP 1013 titled “Effects of Soil Characteristics on Corrosion” (ASTM, 1989), the approximate relationship between soil resistivity and soil corrosivity was developed as shown in Table 4.

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Table 4

Soil Resistivity (Ohm‐cm) Corrosivity to Ferrous Metals

0 to 900 Very Severely Corrosive

900 to 2,300 Severely Corrosive

2,300 to 5,000 Moderately Corrosive

5,000 to 10,000 Mildly Corrosive

10,000 to >100,000 Very Mildly Corrosive

Test results indicated pH values of 7.7 and 7.8, chloride contents of 43 and 82 ppm, sulfate contents of 169 and 48 ppm and minimum resistivities of 1,720 and 2,480 Ohm‐cm. Previous testing onsite indicated resistivities of 58 Ohm‐cm, Chloride contents of 5,056 ppm, and Sulfates of 4,500 ppm, as well as results similar to those tested currently (June and October, 2003, Earth Systems). Although Earth Systems does not practice corrosion engineering, the corrosion values from the soils tested are normally considered as being “very severely” corrosive to buried metals and as possessing a potential “severe” exposure to sulfate attack for concrete as defined in American Concrete Institute (ACI, 2011) 318, Section 4.3. The results of chemical testing have been provided in Appendix B, and previous reports. The above values can potentially change based on several factors, such as importing soil from another job site and the quality of construction water used during grading and subsequent landscape irrigation.

3.6 Geologic Setting

Regional Geology: The site lies near the northeast margin of the central Coachella Valley, a part of the Colorado Desert geomorphic province. To the northeast are the Little San Bernardino Mountains which are part of the Peninsular Ranges geomorphic province. A significant feature within the Colorado Desert geomorphic province is the Salton Trough, a large northwest‐trending structural depression that extends approximately 180 miles from the San Gorgonio Pass to the Gulf of California. Much of this depression in the area of the Salton Sea is below sea level.

The Coachella Valley forms the northerly part of the Salton Trough and contains a thick sequence of Miocene to Holocene sedimentary deposits. Mountains surrounding the Coachella Valley include the Little San Bernardino Mountains on the northeast, foothills of the San Bernardino Mountains on the northwest, and the San Jacinto and Santa Rosa Mountains on the southwest.

These mountains expose primarily Precambrian metamorphic and Mesozoic granitic rocks.

Within the immediate site area, native geologic lithologic units consist of a mix of younger (Holocene) alluvium and lake deposits of the ancient Lake Cahuilla.

Active faults in the immediate vicinity (within 30 miles) of the site include the San Andreas, San Jacinto, Blue Cut, Pinto Mountain, Burnt Mountain, and Eureka Peak faults (See Table A‐1). The closest active faults are multiple traces of the San Andreas fault zone that traverse along the northeast margin of the valley. The site does not lie within a currently designated Alquist‐Priolo Earthquake Fault zone or Riverside County designated fault zone.

Local Geology: The project site is located within the central portion of the Coachella Valley within the limits of mapped lakebed deposits associated with the ancient Lake Cahuilla (ancestral Salton Sea). The Little San Bernardino Mountains are located to the northeast. The site is cross gradient

December 20, 2021 12 File No.: 304996‐001 and approximately 0.2 miles southwest of the Whitewater River channel. Sediments within this area consist of fine‐ to medium‐grained sands with interbedded clays, silts of alluvial, aeolian (wind‐blown) and lacustrine (lake) origins. The site is not located in a frost zone.

The project site is located in a mapped area where surficially, a mix of alluvial fan deposits, dunes sands, and lake deposits are prevalent. Thin deposits of artificial fill resulting from site modification are also present across the site and are generally undifferentiated from the underlying native deposits.

No active faults are currently mapped in the immediate project vicinity. The closest mapped Holocene‐active faults are segments of the San Andreas fault located approximately 1.7 miles northeast of the project site.

3.7 Geologic Hazards

Geologic hazards that may affect the region include seismic hazards (ground shaking, surface fault rupture, soil liquefaction, and other secondary earthquake‐related hazards), slope instability, flooding, ground subsidence, and erosion. A discussion follows on the specific hazards to this site.

3.7.1 Seismic Hazards

Seismic Sources: Several active faults or seismic zones lie within 50 miles of the project site as shown on Table A‐1 in Appendix A. The primary seismic hazard to the site is strong ground shaking from earthquakes along regional faults including the San Andreas fault, San Jacinto fault, and faults associated with the Eastern California shear zone.

Surface Fault Rupture: The project site does not lie within a currently delineated State of California, Alquist‐Priolo Earthquake Fault Zone (CGS 2018). Well‐delineated fault lines cross through this region as shown on California Geological Survey (CGS) Fault Activity Map (2010);

however, no active faults are mapped in the immediate vicinity of the site. The closest active faults are traces/segments of the San Andreas fault zone, located approximately 1.7 miles northeast of the site. Therefore, active fault rupture is unlikely to occur at the project site. While fault rupture would most likely occur along previously established fault traces, future fault rupture could occur at other locations.

Review of select aerial photographs reveal that the site is located in an area predominated by alluvial fan, lineal dune (wind) patterns, and regressive shoreline features associated with ancient Lake Cahuilla. No significant evidence of fault related lineaments was noted.

Historic Seismicity: The site is located within an active seismic area in southern California where large numbers of earthquakes are recorded each year. Approximately 40 magnitude 5.5 or greater earthquakes have occurred within 38 miles of the project since 1856 (See Table A‐2).

Six notable historic seismic events (5.9 M or greater) have significantly affected the Coachella Valley in the last 100 years. They are as follows:

Desert Hot Springs Earthquake – On December 4, 1948, a magnitude 6.5 ML (6.0MW) earthquake occurred east of Desert Hot Springs. This event was strongly felt in the Indio area.

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Palm Springs Earthquake – A magnitude 5.9 ML (6.2MW) earthquake occurred on July 8, 1986 in the Painted Hills, causing minor surface creep of the Banning segment of the San Andreas fault. This event was strongly felt in the Indio area and caused structural damage, as well as injuries.

Joshua Tree Earthquake – On April 22, 1992, a magnitude 6.1 ML (6.1MW) earthquake occurred in the mountains 9 miles east of Desert Hot Springs. Structural damage and minor injuries occurred in the Coachella Valley as a result of this earthquake.

Landers and Big Bear Earthquakes – Early on June 28, 1992, a magnitude 7.5 MS (7.3MW) earthquake occurred near Landers, the largest seismic event in Southern California for 40 years. Surface rupture occurred just south of the town of Yucca Valley and extended some 43 miles toward Barstow. About three hours later, a magnitude 6.6 MS (6.4MW) earthquake occurred near Big Bear Lake. No significant structural damage from these earthquakes was reported in the Indio area.

Hector Mine Earthquake – On October 16, 1999, a magnitude 7.1 MW earthquake occurred on the Lavic Lake and Bullion Mountain faults north of Twentynine Palms. While this event was widely felt, no significant structural damage has been reported in the Coachella Valley.

Seismic Risk: While accurate earthquake predictions are not possible, various agencies have conducted statistical risk analyses. In 2013, the California Geological Survey (CGS) and the United States Geological Survey (USGS) presented new earthquake forecasts for California (USGS UCERF3). We have used these maps in our evaluation of the seismic risk at the site which estimate a 37% conditional probability that a magnitude 6.7 or greater earthquake may occur in 30 years (2014 as base year) along the nearby San Andreas fault. For a magnitude 8 earthquake locally on the San Andreas fault, the probability is approximately 8% for the same time period.

For the nearest segment (Anza) of the San Jacinto fault, the conditional probability for a similar magnitude earthquake is about 14%. Recent estimates suggest a nearly 98% probability of a nearby magnitude 5 earthquake in the next 50 years.

The primary seismic risk at the site is a potential earthquake along the San Andreas fault that is about 1.7 miles from the site and is considered as fault Type A per the CGS. Geologists believe that the San Andreas fault has characteristic earthquakes that result from rupture of each fault segment. The estimated mean characteristic earthquake is magnitude 7.7 for the Southern Segment of the fault (USGS, 2002). However, recent standard of practice suggests a maximum magnitude of 7.9 be used for analysis, assuming a multi‐segment rupture event.

The local segment has the longest elapsed time since rupture of any part of the San Andreas fault.

The last rupture occurred about 1680 AD, based on dating by the USGS near Indio (WGCEP, 2008). This segment has also ruptured on about 1020, 1300, and 1450 AD, with an average recurrence interval of about 220 years. The San Andreas fault may rupture in multiple segments, producing a higher magnitude earthquake. Recent paleoseismic studies suggest the San Bernardino Mountain Segment to the north and the Coachella Segment may have ruptured together in 1450 and 1690 AD (WGCEP, 1995).

Secondary seismic hazards related to ground shaking include soil liquefaction, ground subsidence, tsunamis, and seiches. Other hazards include flooding and slope instability. The site is far inland, so the hazard from tsunamis is non‐existent. At the present time, several water

December 20, 2021 14 File No.: 304996‐001 storage tanks and ponds are located in the immediate vicinity of the site. Therefore, localized flooding hazards from seiches may be possible.

Site Acceleration and Seismic Coefficients: In developing site‐specific seismic design criteria, the characteristics of the earth units underlying the site are an important input to evaluate the site response at a given site. Based on the results of our 2021 field exploration at the site, the project site is underlain by stiff and medium dense alluvium and lacustrine deposits with an estimated averaged shear wave velocity of about 229 m/s (average blowcount of 16) for the upper 100 feet of soil based on our deep boring at the site. Based on the above information, we classify the site soil profile for site response as Site Class F due to potential for liquefaction, according to ASCE 7‐ 16, and D based on blowcount. The D characterization is defined as a soil profile consisting of stiff soil with shear wave velocities between 180 and 360 m/s, and field blowcount of 15 to 50 blows/ft.

Earthquake horizontal peak ground motions of approximately 0.98 g is estimated based upon a 2% probability of exceedance in 50 years (CGS Ground Motion Interpolator, 2008). Acceleration values provided are estimates only. Actual acceleration values may be more or less than those provided and could exceed 1 g assuming a maximum considered earthquake event occurs on the nearby San Andreas fault. Vertical accelerations are typically 1/3 to 2/3 of the horizontal accelerations, but can equal or exceed the horizontal accelerations depending upon the local site effects and amplification.

2019 CBC Seismic Coefficients: The seismic and site coefficients given in Chapter 16 of the 2019 California Building Code are provided in Section 5.7 of this report.

3.7.2 Other Hazards

3.7.2.1 Slope Stability: An existing slope is located near the proposed Training Office building.

The slope is part of the existing sludge ponds. A 2019 google aerial found the pond to be inundated with probable effluent.

Near the south perimeter of the proposed office buildings there is a paved road and south of the road is an existing pond, see Figure 3 below. The existing sludge pond has a descending slope along the road.

There is also a taller slope along the existing Whitewater Channel. That slope is beyond the scope of work outlined in this and previous projects between Earth Systems and Valley Sanitary District.

If the District is interested in a specific analysis of the Whitewater Channel slope, please contact Earth Systems.

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Figure 3 Slope Cross Section A‐A' Location

Slope Stability Location and Conditions Cross Section A‐A’: One cross section was produced:

Section A‐A’ and was used to study static and pseudostatic (seismic) load conditions.

Measurements of depth and horizontal distance of the slope is 13 feet and 60 feet or a slope inclination of approximately 4.6(H):1(V). As‐builts plans (Southwest Consulting, 2010) show the road is 24 feet in width. We assumed a 3‐foot setback between the top of slope and back of curb.

Earth Systems performed a slope stability analysis for existing and proposed conditions for the static and pseudostatic (seismic) loading conditions. That foundation selected was equal to a 4 foot spread footing having a 2000 pound per square foot (psf) load applied to it.

We used boring B‐1 to represent the underlying soil conditions. Historic groundwater is 19 feet bgs; however, we used 26.5 feet, which is more likely for the seismic event scenario. We also used a water table of 19 feet below the existing surface, which is a conservative. We also used a water table set at the top of the sludge pond and accounted for some migration of water laterally above the assumed clay layers.

For the slope analysis, we used the Janbu and Bishop Simplified Methods provided in the software called Slide2 (Rocscience), which provided the results for static and seismic modeling.

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Soil and Rock Engineering Properties Discussion and Loading Assumptions for Screening Level Evaluation:

Soil property values varied depending on the results of boring B‐1 and its lab tests. Undrained shear values were used for static analysis and peak values were used for seismic. Engineered fill and native soil strength parameters are presented in Table 5 below. Soil used as compacted engineered fill should have at least these minimum properties such that at least the minimum factor of safety is achieved. Ultimate cohesion values shown in Table 5 are taken from field testing using pocket penetrometers and the value halved to represent undrained shear values.

Although drained conditions can exist during static loading, they typically have higher soil strength values than undrained condition. Earth Systems used the undrained cohesion values for static loading because it became clear that seismic conditions governed. Soil material used for engineered fill shall be, in general, free draining material and have an Expansive Index designation of “Very Low”.

Table 5 Slope Stability Soil Parameters Used for Slope Stability Analysis in Static Condition

Table 5 above shows undrained shear strengths taken from field testing. Strength for the seismic condition was reduced by 80 percent (%) per SP 117A. Historic groundwater is 19 feet bgs;

however, we used 26.5 feet, which is more likely for the seismic event scenario. Therefore, unit weights vary between inundated and non‐inundated soils.

Load Assumptions: Project plans show an approximate 24‐foot‐wide access road built between the proposed office building/s and sludge pond. Earth Systems assumes typical vehicles will be light vehicles and typical vac trucks. We used a value of 450 psf for the static loading conditions but only 100 psf for the seismic loading conditions because the likelihood of maximum roadway loading and the seismic event occurring simultaneously are low. Per the 2019 California Building Code, for pedestrian yards and terraces, a 100 psf surcharge minimum is required.

Seismic Loading: Seismic coefficients input in Slide2 were determined for a pseudostatic analysis.

Earth Systems determined seismic coefficients from procedures provided by DMG Special Publication 117, Guideline for Analyzing and Mitigating Landslide Hazards in California (pp 78).

Earth Systems estimated the maximum horizontal acceleration PGAM is 1.07g and the Earthquake Magnitude is 7.9. The maximum horizontal acceleration is determined using site coordinates and the use of an OSHPD and Structural Engineers Association of California web site https://seismicmaps.org/ having seismic design maps. A magnitude earthquake is obtained from

December 20, 2021 17 File No.: 304996‐001 a seismic hazards study that was performed for this project by Earth Systems. Using Figure 1.b from Special Publication 117 (pp 30), the factor (feq)related to the seismicity of the site is 0.48.

From the equation (Equation 11.1) to determine seismic coefficient (Keq) yields 0.49 (maximum used) for the horizontal direction and the vertical is ⅓ the horizontal direcƟon or 0.16.

Table 6 Loading Assumption

Load Condition Section Observed

Pressure Load (psf)

Seismic Coefficients

Roadways A‐A’ 450

Seismic A‐A’ 0.49H and 0.16V

Roadways A‐A’ 100

Soil Slope Factor of Safety and Drainage Condition Assumptions:

Static and Seismic FOS: From page 55 of DMG Special Publication 117 (Guidelines for Analyzing and Mitigating Landslide Hazards in California): “Historically, the most commonly required factors of safety in southern California have been 1.5 for static long‐term slope stability. A Factor of

Safety (FS) of 1.5 is required for Riverside County (2000) for static conditions. Earth Systems also used a screening process to check for the stability of the psuedostatic condition (seismic). This procedure used was Makdisi and Seed Method (SP 117, page 88) and it allows a FOS of 1.0 to check for estimated displacements. Earth Systems assumes a 5‐centimeter (cm) horizontal displacement or less is considered adequate (SP 117, page 91) “for slip surfaces intersecting stiff improvements (such as building, pools, etc) computed median displacements should be maintained at < 5 cm”.

Earth Systems used the Screening Analysis Procedure detailed in DMG SP117 on page 78 because pseudostatic slope analysis shows multiple slip failure planes with FOS of less than 1.1 (see Figure 5). The procedure is implemented by entering a horizontal seismic coefficient (k) into a conventional slope stability calculation to obtain a Factor of Safety of 1. The seismic coefficient represents the fraction of the weight of the sliding mass that is applied as an equivalent horizontal force acting through the centroid of the mass. This method estimates the displacement empirically.

Slope Analysis Results: For the slope analysis, we used the Janbu and Bishop Simplified Methods in software called Slide2 (Rocscience), which provided the results for static and seismic modeling.

Shear parameters used in the static and seismic case were estimated from project soils, see Table

5 and seismic reduction writeup below Table 5. Historic groundwater is 19 feet bgs; however, we used 26.5 feet, which is more likely for the seismic event scenario as a standard typical condition, rather than the low probability of a historic high groundwater and the design earthquakes occurring simultaneously.

As shown in Figures 4 to 7, our analysis indicates the proposed building has a low potential to cause slope stability failures. However, note the existing seismic event is predicted to cause slope failures. Earth Systems assumed sludge pond liquid could infiltrate into the roadway and some of

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