Attachment_H_Leighton_Geotechnical_Report_-_11-20-2020.pdf

UPDATED GEOTECHNICAL EXPLORATION

VALLEY SANITARY DISTRICT

WASTEWATER TREATMENT PLANT

PROPOSED EXPANSION

45500 VAN BUREN STREET

INDIO, RIVERSIDE COUNTY, CALIFORNIA

Prepared For:

STANTEC CONSULTING SERVICES, INC.

300 North Lake Avenue, Suite 400 Pasadena, California 91101-4109

November 20, 2020

Project No. 12926.001

Friday, November 20, 2020

Project No. 12926.001 Stantec Consulting Services, Inc.

300 North Lake Avenue, Suite 400 Pasadena, California 91101-4109

Attention: Mr. Venugopal Kolli, PE, PMP, ENV SP

Area Manager

Subject: Updated Geotechnical Exploration

Valley Sanitary District Wastewater Treatment Plant Proposed Expansion 45500 Van Buren Street Indio, Riverside County, California

In accordance with the October 1, 2020 Subconsultant Agreement, we have completed this Updated Geotechnical Exploration for proposed Valley Sanitary District plant improvements in Indio. This report summarizes our geotechnical findings, conclusions and recommendations regarding proposed plant improvements to be constructed north of remaining sewage ponds. This report augments and updates our previously conducted geotechnical exploration at this plant, which was documented in our May 8, 2006 “Phase 2 Geotechnical Exploration” report.

This site is not within a currently designated Alquist-Priolo Earthquake Fault Zone, and there is no currently available evidence of surface fault rupture through this site. However, as is the case for most of Southern California, strong ground shaking has and will occur at this site, primarily due to relatively close proximity of the active San Andreas Fault located to the north and east.

Riverside County and others (see References) have previously identified this plant as being within an area of potential liquefaction. Although groundwater was encountered in our previous and current ten borings at depths of 37 feet or more (46 feet or more currently) below existing grade; reportedly, groundwater has been as shallow as 19 to 25 feet below existing grade at the plant. Site soils were predominantly sand, silt and clay mixtures; encountered silts and clays are expected to be less susceptible to liquefaction.

Nevertheless, based on reported historical high groundwater levels, some liquefaction was calculated to occur at this site, due to a postulated large magnitude earthquake on the nearby San Andreas Fault (Leighton, 2005).

41945 Boardwalk, Suite V Palm Desert, CA 92211

760.776.4192 760.776.4080 Fax www.leightongroup.com

Valley Sanitary District Plant Expansion 12926.001

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Our recommendations to mitigate liquefaction and static differential settlement are based on all proposed new plant components being constructed atop newly placed properly-compacted backfill. Most of these proposed improvements will be constructed within the area of previously drained and partially-filled Pond 1 (sewage lagoon). However, separate from the other improvements, a new grit chamber will be constructed in the northwestern portion of the plant just south of the existing influent pump station, where 7½-feet of undocumented fill was encountered. Liquefaction and differential settlement mitigation recommendations are a function of new structure bearing elevations as follows:

Grit Chamber: The grit chamber may extend as much as 10-feet below existing grade, but if founded shallower, then all existing undocumented fill extending to 7½-feet (possibly more) below existing grade should be excavated from the grit chamber pad area. Bottom of this excavation should then be covered with an 18-inch thick ¾-inch crushed rock layer, wrapped-in/surrounded-with (top and bottom) Mirafi 140N non-woven filter fabric, or equivalent. Properly compacted fill should then be place up to design bottom elevation of the grit chamber.

Digester 2, Sludge Holding Tanks and Thickener Building: All of these improvements will be constructed within the previously drained and newly backfilled Pond 1 area. All remnants of Pond 1 lining and piping should be removed from these proposed new structure areas. These structures should bear on a zone of uniform thickness properly compacted fill extending below these structure footprints conceptually 12-feet, reinforced with a Tensar BX 1300 bi-axial geogrid at the bottom of the fill at or below reference elevation 454 feet (-28 feet MSL), possibly deeper to remove loose slough. To reduce differential settlement between thee structures with connecting conduits, this area should be mass graded as one compacted fill mass.

In all cases, properly compacted fill should be compacted to at least 95% relative compaction, relative to the ASTM D1557 modified Proctor laboratory maximum density.

More detailed recommendations are presented in this report. We appreciate the opportunity to be of service to Stantec. If you have questions or if we can be of further service, please contact us at your convenience at 866-LEIGHTON, directly at the phone extension and/or e-mail address listed below.

Respectfully submitted, LEIGHTON CONSULTING, INC.

Thomas C. Benson, Jr., GE 2091 President and CEO

TCB:tcb Extension 8771, tbenson@leightonconsulting.com

Distribution: (1) addressee (PDF via e-mail) mailto:tbenson@leightonconsulting.com

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T A B L E O F C O N T E N T S Section Page

1.0 INTRODUCTION

1.1 Plant Location and Description

1.2 Proposed Plant Improvements

1.3 Purpose and Scope of Exploration

2.0 FINDINGS

2.1 Site Description

2.2 Geotechnical Setting

2.3 Subsurface Conditions

2.3.1 Earth Materials:

2.3.2 Groundwater:

2.4 Seismic Considerations

2.5 Secondary Seismic Hazards

2.5.1 Liquefaction:

2.5.2 Seiches, Tsunamis, Inundation Due to Large Water Storage Facilities:

2.6 Differential Subsidence and Ground Fissuring

3.0 CONCLUSIONS AND RECOMMENDATIONS

3.1 Conclusions and Recommendations Summary

3.2 2019 California Building Code (CBC) Seismic Design Parameters

3.3 Earthwork

3.3.1 Site Preparation and Remedial Grading:

3.3.2 Suitability Of Site Soils for Fills:

3.3.3 Import Soils:

3.3.4 Pipeline Trenches:

3.3.5 Drainage:

3.4 Grit Chamber Mat Foundations

3.5 Spread Footings

3.5.1 Minimum Footing Dimensions and Embedment:

3.5.2 Allowable Vertical Bearing:

3.5.3 Lateral Loads:

3.5.4 Settlement Estimates:

3.6 Retaining Walls

3.6.1 Static Lateral Earth Pressures:

3.6.2 Retaining Wall Surcharges:

3.6.3 Incremental Earthquake Induced Earth Pressures:

3.6.4 Drainage:

3.7 Concrete Slabs-On-Grade

3.8 Sulfate Attack and Ferrous Corrosion Protection

3.8.1 Sulfate Exposure:

3.8.2 Ferrous Corrosivity:

3.8.3 Corrosivity Test Results:

3.9 Pavements

3.9.1 Hot Mix Asphalt (HMA) Pavement Sections:

3.9.2 Portland Cement Concrete (PCC) Pavements:

3.9.3 Paving Materials:

4.0 CONSTRUCTION CONSIDERATIONS

4.1 Temporary Construction Excavations

4.2 Temporary Shoring

4.3 Construction Review

5.0 LIMITATIONS

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REFERENCES

Tables Page Table 1. Fill Depth Table 2. Depth to Groundwater Table 3. Current (2020) Borings Liquefaction Characterization Table 4. 2019 CBC Site Categorization and Site Coefficients Table 5. Retaining Wall Design Earth Pressures Table 6. Sulfate Concentration and Exposure Table 7. Soil Resistivity and Soil Corrosivity Table 8. Results of Corrosivity Testing Table 9. Asphalt Pavement Section Thickness Table C-1. Historical Earthquakes .............................................................................. C-1

Figures Figure 1 - Site Location Map Figure 2 - Boring Location Map Figure 3 - Regional Fault Map Figure 4 - Alquist-Priolo Zone Figure 5 - Liquefaction Hazard Map

Appendices Appendix A – Subsurface Exploration Appendix B – Geotechnical Laboratory Testing Appendix C – Site-Specific Seismic Analyses Appendix D – Earthwork and Grading Guide Specifications Appendix E – GBA’s Important Information About This Geotechnical-Engineering Report

1 . 0 I N T R O D U C T I O N

1.1 Plant Location and Description

As shown on Figure 1, Site Location Map, this Valley Sanitary District (VSD) plant is bound by the Whitewater River Storm Channel to the northeast, Cabazon Indian Reservation land to the east, Vista Del Norte (not depicted on Figure 1) to the south, and Van Buren Street to the west. Access to the plant is via Van Buren Street. This is a relatively flat area draining southeast towards the Salton Sea; roughly 10- to 30-feet below mean sea level. We understand that reference elevations used by VSD and Stantec are 481.69 feet higher than USGS mean sea level (MSL).

As can be seen on Figure 2, Boring Location Map, proposed improvements discussed in this report will be constructed predominantly within drained and partially-backfilled Pond 1, located due east of the main plant facilities and in the northeastern portion of the plant parcel, north of Fourth Street within the plant. One exception is a proposed new grit chamber, which is to be constructed in the northwestern portion of the plant just south of the existing influent pump station.

1.2 Proposed Plant Improvements

New wastewater process improvements are proposed solely within the existing Valley Sanitary District’s plant. Although phases of plant expansion may be proposed in the future, currently proposed improvements are predominantly to be locate either within partially-backfilled Pond 1 or to the north, which is, for the most part, within areas we previously explored on March 21, 2005. However, since our 2005 borings were drilled, what was Pond 1 has been drained and partially-backfilled, and developed with Digester 1 and other facilities. Also, the proposed grit chamber is to be constructed well north of our previously explored area in and around backfilled Pond 1. Hence the need for updated subsurface exploration.

More specifically, we understand that Stantec will be designing the following five structures, sludge drying beds and appurtenant plant improvements; generally described from north to south:

Grit Chamber: A 22-foot-diameter, 10-foot-high hopper grit chamber is proposed to be installed in the northwest-central portion of the plant just south of the existing influent pump station. This will be located just west of Plant- Avenue B. A prefabricated system is being considered, and this is expected to

-2-be installed at-least partially below existing grade (≤10 feet below grade). We presume this could also be a conventionally-reinforced concrete hopper basin.

Digester 2: A new 85-foot-diameter digester identical to existing Digester 1 will be constructed just west of Digester 1. This area was Pond 1 back in 2006 at the time of our prior subsurface exploration. It has since been partially backfilled.

Digested Sludge Holding Tank: Located west-southwest of Digester 1 and south of proposed Digester 2, this proposed 188,000-gallon holding-tank footprint is also currently proposed over a portion of the backfilled Pond 1 area.

Thickener Building: This building will be located just west of Plant-Avenue D generally southeast of the digester sludge holding tank. We envision this as a single-story lightweight steel structure. This building footprint is also currently proposed over a portion of the backfilled Pond 1 area.

Digestion/Biogas Flare: Recently added to this project, a lightweight biogas flare will be constructed south of the existing digester, between Plant-Avenues D and E.

Sludge Drying Bed Extension: Sludge drying beds consisting of concrete swales and ditches will be extended south into existing Pond 2, without any structures. This will be limited to grading and concrete-on-grade improvements.

Various Pumps and Switchgear Pads: Miscellaneous equipment pads will be required, presumably in close proximity to the above-described improvements.

1.3 Purpose and Scope of Exploration

Purposes of this updated geotechnical exploration were to (1) explore subsurface conditions under Pond 1 that had been recently graded and explore the new grit chamber pad, and (2) based on these findings, develop geotechnical recommendations for design of these proposed plant improvements described in the prior section. In accordance with our October 1, 2020 Subconsultant Agreement, scope of our updated geotechnical exploration included the following:

Research: We reviewed available site-specific information, including review of various geotechnical reports (including our 2005 and 2006 subsurface exploration), geologic maps and aerial photographs listed in the references at the end of this report.

Subsurface Exploration: We drilled, sampled and logged five hollow-stem-auger borings (LB-6 through LB-10) to depths of 26¼- to 51½-feet on October 16, 2020. A description of our field exploration along with boring logs are presented in Appendix A, Subsurface Exploration. Our prior (March 21, 2005)

-3-boring logs for the plant (LB-1 through LB-5) are also included in Appendix A.

Approximate locations of all ten borings (from March 21, 2005 and October 16, 2020) are approximately mapped on Figure 2, Boring Location Map.

Geotechnical Laboratory Testing: Geotechnical laboratory testing of selected soil samples obtained from these exploratory borings was completed at our in-house geotechnical laboratory. Test procedures and results are presented in Appendix B, Geotechnical Laboratory Testing.

Geotechnical Analyses: Geotechnical engineering analyses were performed as directed by a California licensed Geotechnical Engineer (GE). Probabilistic and deterministic seismic hazard analyses were performed as presented in Appendix C, Site-Specific Seismic Analyses.

Geotechnical Update Report: This report was prepared to present our augmented/updated geotechnical conclusions and recommendations regarding currently proposed wastewater treatment plant improvements (primarily within backfilled Pond 1). This report includes, Appendix D, Earthwork and Grading Guide Specifications, as current standard specification for grading and earthwork to be used to complete backfilling of Pond 1.

This report does not address the potential for hazardous materials in soil and/or groundwater. Presented in Appendix E, GBA’s Important Information About This Geotechnical-Engineering Report, is information about limitations common for all geotechnical reports.

2 . 0 F I N D I N G S

2.1 Site Description

As shown on the aerial photograph reproduced on Figure 2, the eastern half of the site consists of the recently drained Pond 1, while the western half of the site consists of the operational plant. The site and region are relatively flat, draining gently to the southeast towards the Salton Sea. The Whitewater River (or storm channel) is the northeastern boundary of the site, so the site is in a relatively low-lying area. Based on the “Indio” 1972 USGS quadrangle (reproduced as Figure 1), the plant is approximately 12 feet or more below mean sea level (the Salton Sea is approximately 235 feet below mean sea level). We understand that reference elevations used by Stantec are 481.69 feet higher than USGS mean sea level

(MSL).

We were told in 2006 that the bottom of Pond 1 was at reference elevation 457 feet (roughly 25 feet below mean sea level). However, a recently provided topographic

-4-map of the plant shows remnants of Pond 1 that have not been backfilled, west of the recently constructed digester, at elevation 455 feet.

We understand that all of the existing facilities have been functioning adequately, and that there is no reported evidence of subsidence or ground movement at this plant.

2.2 Geotechnical Setting

This plant is located in Coachella Valley within the Colorado Desert geomorphic province. The Coachella Valley along with the Imperial Valley to the south comprise an elongated structural depression known as the Salton Trough. The Salton Trough is considered to be the landward extension of the Gulf of California, and is believed to be part of a complex rift resulting from tectonic spreading at the northern extension of the East Pacific Rise. The active San Andreas Fault defines the eastern margin of the Salton Trough. As shown on Figure 3, Regional Fault Map, this site is approximately 1.3 miles southwest from the active San Andreas Fault Zone. This Coachella segment of the San Andreas Fault Zone is a right-lateral strike slip fault with a slip rate of 25 millimeters per year (mm/year), ± 5mm/year (CGS, 2003). Total length of this fault segment is 96 kilometers (km) ± 10 km (CGS, 2003) and has an estimated recurrence interval ranging from 140 years to over 300 years (SCEC, 2004). Figure 4, Alquist-Priolo Zone, shows that this plant is not within the currently designated Alquist-Priolo Fault Zone.

The Coachella Valley is now filled with as much as 12,000 feet of sediments (USGS, 2002). This site is located in the alluvial valley with surficial loess and deeper lacustrine deposits. Pond 1 has been partially backfilled, presumably with new compacted fill, up to reference elevations 466.5 feet to 470 feet (roughly 12 feet below mean sea level) to support new plant components constructed and operating since 2006.

2.3 Subsurface Conditions

2.3.1 Earth Materials: Fill soils were encountered in all five of our recent borings to depths up to 12 feet, as tabulated below:

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Table 1. Fill Depth

Boring Date Drilled

Surface Elevation

(feet)*

Bottom of Fill (feet) Relative Compaction **(percent) Depth Elevation

LB-1

3/21/2005

-12 no t d et ec te d

-12

LB-2 -12 -12

LB-3 -12 -12

LB-4 -11 -11

LB-5 -12 -12

LB-6

10/16/2020

-17 7.5 -25 74% LB-7 -16 6.5 -23 91 to 93% LB-8 -15 12 -27 88 to 94% LB-9 -16 11.5 -28 88 to 95%

LB-10 -16 8 -24 86 to 90%

*Based on site topography provided by Stantec, adjusted to mean sea level (-481.69 feet).

NE=not encountered within depth drilled.

** Relative to the ASTM D1557 modified Proctor laboratory maximum density

Fill soils had not been previously reported to be encountered by Earth Systems Southwest in their three geotechnical reports for plant improvements (June 6, 2003, October 13, 2003 and June 11, 2004, all referenced at the end of this report). However, we expect that fill soils were placed to construct the perimeter berm around Pond 1. As tabulated above, fill soils in the Pond 1 area had ASTM D1557 modified Proctor relative compactions ranging from 86% to 95%. In contrast, at the proposed grit chamber site, existing fill was very loose, at 74% relative compaction. We are unaware of any fill placement documentation for this plant and site.

Native site soils consisted predominantly of lacustrine deposits of silt and silty sand that was primarily medium dense, often finely laminated, with layers of low to high plasticity clay. During our previous investigation, the average fines content for 32 soil samples tested was 71 percent, with a standard deviation of 27 percent. Fines content in our current borings ranged from 47% to 81%.

Atterberg Limits showed fairly consistent low-plasticity clays (CLs). Prior collapse tests showed ¼-percent collapse or less, which is essentially negligible. A detailed description of the soils encountered is presented on the logs in Appendix A.

2.3.2 Groundwater: As tabulated below, groundwater was encountered in eight deeper borings (deeper than 26¼ feet) immediately after drilling:

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Table 2. Depth to Groundwater

Boring Date Drilled

Surface Elevation

(feet)*

Depth To Groundwater (feet) Depth Of

Boring (feet) Depth Elevation

LB-1

3/21/2005

-12 60 -72 71½

LB-2 -12 40 -52 71½

LB-3 -12 55 -67 76½

LB-4 -11 50 -61 71½

LB-5 -12 37 -49 76½

LB-6

10/16/2020

-12 -NE- < -38 26¼

LB-7 -12 45.7 -58 51½

LB-8 -12 46.8 -59 51½

LB-9 -12 48.2 -60 51½

LB-10 -12 -NE- < -38 26¼

*Based on site topography provided by Stantec, adjusted to mean sea level.

NE=not encountered within depth drilled.

Locations of our borings are depicted on Figure 2. We note that LB-1 was drilled in the northwest corner of the site away from the ponds, while LB-5 was drilled on the levee between what was then Pond 1 and Pond 2. Groundwater elevations could possibly be a function of the proximity to these ponds when the ponds are full of effluent, due to percolation. Discounting LB-5, encountered groundwater was 40- to 60-feet deep.

Our first five borings were drilled to a maximum depth of 76½ feet in late March 2005, which was after a rainy winter for the area. Earth Systems Southwest reported encountering groundwater at a depth of 26 feet in their deeper boring drilled on April 11, 2003, at the administration building addition. Southland Geotechnical reported groundwater as shallow as 18½-feet deep in a boring drilled on January 10, 1997 for the headworks located in the northwestern portion of the plant. They also described (but did not reference) a 1981 Converse report, where groundwater was reportedly 13 to 15 feet deep, “suggesting groundwater levels have dropped since then.” Local groundwater conditions can fluctuate due to the type and integrity of pond linings (if any), precipitation, or other factors not initially observed at the time of exploration.

2.4 Seismic Considerations

As shown on Figure 4, Alquist-Priolo Zone, this site is not located within a currently designated Alquist-Priolo Special Studies Zone, or County of Riverside Fault Hazard Zone. The nearest known active fault is the southern segment of the San Andreas Fault Zone located approximately 1.3 miles (2.1 kilometers) northeast of the site.

Ground rupture commonly occurs along pre-existing faults, triggered during a seismic event.

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Since active faults are not mapped to cross the site, the possibility of plant damage due to ground-surface fault-rupture is considered low. For most structures in Southern California, seismically induced damage is predominantly caused by ground shaking or seismically induced ground settlement. Lurching due to shaking from distant seismic events is not considered a significant site-specific hazard, although it is a possibility throughout Southern California.

2.5 Secondary Seismic Hazards

2.5.1 Liquefaction: Liquefaction of saturated cohesionless soils can be caused by strong ground motion resulting from earthquakes. For liquefaction to occur, the site must have (1) shallow groundwater, (2) loose, clean saturated sands, and

(3) a potential for sustained strong ground shaking. If one of these three components does not exist at the site, then liquefaction will not occur.

As depicted on Figure 5, Liquefaction Hazards Map, from Riverside County, the site is within a mapped “study required” liquefaction hazard zone.

Previously, we performed a liquefaction evaluation for this plant site for our Phase 1 Geotechnical Report. Our analyses were based on the following liquefaction evaluation components:

Groundwater: Groundwater was encountered in our previous borings as shallow as 37 feet below existing grade, immediately after drilling.

However, it has been reported that groundwater has been as shallow as 19 feet at this site. For analyses, we assumed a depth to groundwater of 19 feet.

Soil Gradation: Most of the tested samples consisted of silts and clays with reduced or no liquefaction potential. During our previous investigation, all but three of 32 samples tested had greater-than (>) 35-percent fines (silts and clays). Nine of the 13 Atterberg Limits tests had Liquid Limit (LL) of 35 percent or greater. Liquefaction is unlikely in these nine samples with high Liquid Limits.

Sand Density: Standard Penetration Test (SPT) blow counts (N-values) ranged from 6 to 31 for samples obtained at depths greater than 19 feet.

However, many of these low blow counts were in silts and clays. Cone Penetrometer Test (CPT) tip resistances were measured below 100 tons-per-square-foot (tsf) at depths ranging from 35 feet to 55 feet, in the potential liquefaction zone at depths greater than 19 feet below the surface.

Ground Shaking: Due to the close proximity of the site to the active San Andreas Fault, which is capable of a large magnitude earthquake, strong peak horizontal ground accelerations approaching 1g were modeled. A summary of our site-specific seismic analysis is presented in Appendix C.

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To update/augment our liquefaction analyses, soils in our five current borings at the depth range of 19- to 50-feet are characterized as follows:

Table 3. Current (2020) Borings Liquefaction Characterization

Boring SPT N-values

Liquid Limits Liquefaction Potential Low High

LB-6* dense to very dense, low moisture NP low due to density LB-7 13 27 40 some in sand LB-8 17 23 ML-CL some in sand LB-9 17 27 46 some in sand

LB-10* dense, low moisture NP low due to density

*Only 26¼ feet deep.

These results are consistent with our prior borings and liquefaction evaluation results. Liquefaction evaluation should consider the relative probabilities of high groundwater levels existing simultaneously at the time of local strong ground shaking, which is a low probability of occurrence.

Although site soils are silty and clayey, liquefaction in the sand strata was still calculated to occur at this site with shallow groundwater and very high probabilistic ground motions. Liquefaction due to an Upper Bound Earthquake, with ten-percent probability of exceedance within a 100-year period, could result in on the order of 2¼-inches of liquefaction induced settlement at the plant, with differential settlement on the order of an inch over a distance of 100 to 300 feet.

2.5.2 Seiches, Tsunamis, Inundation Due to Large Water Storage Facilities:

Due to the distance between large bodies of water and this site (see Figures 1, 3 and 4), the possibility of seiches and tsunamis impacting the site is considered remote. The nearest large body of water is the Salton Sea, which is lower in elevation than the site. We are unaware of any large water reservoir up-gradient from this site. This report does not address conventional flood hazard risk.

2.6 Differential Subsidence and Ground Fissuring

Land subsidence associated with groundwater-level declines has been recognized as a potential problem in the Coachella Valley (USGS, 2002). In 1996, Coachella Valley Water District (CVWD) entered into a cooperative agreement with the USGS to monitor land subsidence in the lower Coachella Valley. A documented area of subsidence (USGS, 2002) is west of the site in La Quinta and Indian Wells.

However, we are unaware of any documented subsidence in the Indio area. To manage land subsidence, CVWD proposes to implement “Preferred Alternative No.

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4 - Groundwater Recharge.” Implementation of this plan alternative will decrease the potential for future land subsidence to “minimal” levels (CVWD, 2002). Based on CVWD’s stated basin management objectives, and distance to currently mapped land subsidence areas, we consider the potential for differential subsidence and ground fissuring to be low at this site. See:

https://www.usgs.gov/centers/ca-water-ls/science/land-subsidence-coachella-valley

3 . 0 C O N C L U S I O N S A N D R E C O M M E N D A T I O N S

3.1 Conclusions and Recommendations Summary

Based on our findings discussed in the prior section, the subject site is not within a currently designated Alquist-Priolo Earthquake Fault Zone or County Fault Zone.

This site lies within a County Liquefaction Hazard Zone, and is subject to liquefaction based on our site-specific evaluation presented in our previous 2005 report. Strong ground shaking at this site is expected due to the close proximity of the San Andreas Fault. This could result in as much as 2¼-inches of liquefaction induced settlement at this site.

There are numerous ways to mitigate this liquefaction potential, and we can explore different approaches to mitigating liquefaction-induced settlement, based on interaction with the design team. However, we recommend the following based on our current understanding of the proposed grading and improvements. Our recommendations to mitigate liquefaction and static differential settlement are based on all proposed new plant components being constructed atop newly placed properly-compacted backfill. Most of these proposed improvements will be constructed within the area of previously drained and partially-filled Pond 1 (sewage lagoon). However, separate from the other improvements, a new grit chamber will be constructed in the northwestern portion of the plant just south of the existing influent pump station, where 7½-feet of undocumented fill was encountered.

Liquefaction and differential settlement mitigation recommendations are a function of new structure bearing elevations as follows:

Grit Chamber: The grit chamber may extend as much as 10-feet below existing grade, but if founded shallower, then all existing undocumented fill extending to 7½-feet (possibly more) below existing grade should be excavated from the grit chamber pad area. Bottom of this excavation should then be covered with an 18-inch thick ¾-inch crushed rock layer, wrapped-in/surrounded-with (top and bottom) Mirafi 140N non-woven filter fabric, or https://www.usgs.gov/centers/ca-water-ls/science/land-subsidence-coachella-valley

-10-equivalent. Properly compacted fill should then be place up to design bottom elevation of the grit chamber.

Digester 2, Sludge Holding Tanks and Thickener Building: All of these improvements will be constructed within the previously drained and newly backfilled Pond 1 area. All remnants of Pond 1 lining and piping should be removed from these proposed new structure areas. These structures should bear on a zone of uniform thickness properly compacted fill extending below these structure footprints conceptually 12-feet, reinforced with a Tensar BX 1300 bi-axial geogrid at the bottom of the fill at or below reference elevation 454 feet (-28 feet MSL), possibly deeper to remove loose slough. To reduce differential settlement between thee structures with connecting conduits, this area should be mass graded as one compacted fill mass.

In all cases, properly compacted structural fill should be compacted to at least 95% relative compaction, relative to the ASTM D1557 modified Proctor laboratory maximum density, at or above optimum moisture content.

We should review the project grading (Civil) and foundation (Structural) plans and specifications prior to release for bidding and construction. Such review is important to determine whether the geotechnical recommendations have been effectively implemented. Additional geotechnical studies may be warranted based on these reviews. Review findings should be documented in a letter or by stamp on approved drawings.

Additional more detailed geotechnical recommendations are presented in the following subsections.

3.2 2019 California Building Code (CBC) Seismic Design Parameters Strong ground shaking can be expected at this site during moderate to severe earthquakes in the general region. This is common to virtually all of southern California. Sand alluvium over clayey lacustrine deposits are expected below this plant. Therefore, based on Chapter 16 of the 2019 California Building Code (CBC) and ASCE 7-16, the following site-specific seismic coefficients could be used for seismic design of structures, as listed below:

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Table 4. 2019 CBC Site Categorization and Site Coefficients

Categorization /Coefficient Design Value Site Longitude (decimal degrees) -116.1967° Site Latitude (decimal degrees) 33.7165°

Site Class Definition (ASCE 7-16, Chapter 20) D Mapped Spectral Response Acceleration at 0.2s Period, Ss (Figure 1613.3.1(1)) 2.271 Mapped Spectral Response Acceleration at 1s Period, S1 (Figure 1613.3.1(2)) 0.964

Short Period Site Coefficient at 0.2s Period, Fa (Table 1613.2.3(1)) 1

Long Period Site Coefficient at 1s Period, Fv (Table 1613.2.3(2)) 1.857

Adjusted Spectral Response Acceleration at 0.2s Period, SMS (Eq. 16-36) 2.27

Adjusted Spectral Response Acceleration at 1s Period, SM1 (Eq. 16-37) 2.37

Design Spectral Response Acceleration at 0.2s Period, SDS (Eq. 16-38) 1.51

Design Spectral Response Acceleration at 1s Period, SD1 (Eq. 16-39) 1.58

Site Modified Peak Ground Acceleration (PGAM, g) 1.075 Long-period transition period (TL, seconds) 8

3.3 Earthwork

Earthwork should be performed in accordance with the following recommendations and the Earthwork and Grading Guide Specifications included in Appendix D of this report. In case of conflict, the following site-specific recommendations should supersede guide specifications in Appendix D.

3.3.1 Site Preparation and Remedial Grading: Remedial grading for the Pond 1 area, grit chamber and pavements are provided as follows:

Pond 1 Area: Prior to grading, Pond 1 should be cleared of surface and subsurface obstructions. Sludge and debris should be disposed of offsite or through an approved waste stream for the plant. Abandoned plant piping, if any, should be completely removed from the pond area. Pond lining should also be removed. Voids created by removal of buried material should be backfilled with properly compacted soil in general accordance with the recommendations of this report.

To reduce the potential for excessive static and/or dynamic differential settlement under new plant improvements, we recommend that existing pond perimeter slope concrete surfaces be saw cut at least 8-feet horizontally beyond the proposed structure footprints. Demolished concrete can either (1) be disposed of, or (2) pulverized to particles less-than (<) 3-inches in largest dimension and mixed with soils for use as backfill. These areas should then also be overexcavated to at least 8-feet horizontally

-12-beyond the proposed structure footprints, down to reference elevation 454 feet (-28 feet MSL), possibly deeper to remove loose slough. A mass overexcavation and backfill compaction is recommended to reduce differential settlement between new facilities. Before any backfill is placed, the exposed native subgrade should be covered with a Tensar BX 1300 bi-axial geogrid.

Grit Chamber: The grit chamber may extend as much as 10-feet below existing grade, but if founded shallower, then all existing undocumented fill extending to 7½-feet (possibly more) below existing grade should be excavated from the grit chamber pad area. Bottom of this excavation should then be covered with an 18-inch thick ¾-inch crushed rock layer, wrapped-in/surrounded-with (top and bottom) Mirafi® 140N non-woven filter fabric, or equivalent. Properly compacted fill should then be place up to design bottom elevation of the grit chamber.

Pavement Areas and Sludge Drying Beds: In pavement and concrete flatwork areas, a minimum remedial removal and recompaction of 18-inches below existing grade or finish grade, whichever is deeper, should be performed. However, localized deeper overexcavation may be required in soft or wet subgrades.

Soft and rutting subgrades may have to be stabilized using crushed rock with the geogrid. In-place testing of removal bottoms in alluvial/lacustrine areas should be performed during grading to determine the competency of the alluvial/lacustrine materials being left in place. A minimum criterion of 95 pounds-per-cubic-foot (pcf) or 85 percent relative compaction (ASTM D1557- 02ε1) should be considered in general accordance with the County of Riverside requirements. Deeper overexcavation may be required if this criterion is not achieved. All structural fill within the limits of overexcavation described above and within structure footprints should be compacted throughout to 95 percent of the ASTM D1557 laboratory maximum density, at or slightly above optimum moisture.

3.3.2 Suitability Of Site Soils for Fills: Sludge is not considered suitable for use as fill. Deeper alluvial soils (sands) should generally be considered suitable for re-use as compacted fills provided the soil conforms to the following criteria.

However, site clays should not be used to backfill retaining walls. Fill soils should have an organic content (based on loss-on-ignition) of less-than (<) 2-percent. If cobbles and boulders larger than 3-inches in largest diameter are encountered or produced during grading, these oversized cobbles and boulders should be disposed of in non-structural and non-pavement areas or broken down to particles no-greater-than (≤) 3-inches in largest diameter.

3.3.3 Import Soils: Import soils and/or borrow sites should be evaluated by Leighton

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Consulting, Inc, prior to import. Import soils should be uncontaminated, granular in nature, free of organic material (loss on ignition less-than 2 percent), have very low expansion potential (with an Expansion Index less than 20) and have a low corrosion impact to the proposed improvements. It may be prudent to have Leighton Consulting, Inc. test proposed import soils for the presence of hazardous materials such as pesticides, metals, etc., based on the identified source and past use of the borrow site.

3.3.4 Pipeline Trenches: Pipeline and all utility conduit trenches should be backfilled with compacted fill or Controlled Low Strength Material (CLSM) in accordance with Sections 306-12 of the Standard Specifications for Public Works Construction, (“Greenbook”), 2018 Edition. Specifically, backfill for narrow trenches should be in accordance with Section 306.12.2 or backfill for wider mechanically-compacted trenches should be in accordance with Section 306-12.3. Prior to backfilling trenches, pipes/conduits should be bedded in and covered with either:

(1) Sand: A uniform, granular material that has a Sand Equivalent (SE) of

(≥) 30 or greater and a maximum particle size of ¾-inches (or as specified by the pipe manufacturer), water densified (flooded/jetted) in place, or

(2) CLSM: Controlled Low Strength Material (CLSM) conforming to Section 201-6 of the Standard Specifications for Public Works Construction, (“Greenbook”), 2018 Edition. CLSM is desirable to mitigate liquefaction and backfill areas that cannot be density-tested due to lack of safe access.

Pipe bedding should extend at least 4-inches below the pipeline invert and at least 12 inches over the top of the pipeline. Site soils are unsuitable for use in the pipe zone as defined above.

Above the pipe zone, trenches can be backfilled with excavated onsite soils free of oversized material greater-than (>) 3-inches (8 cm) in largest dimension, any debris and/or organic matter. Oversized rock (cobbles and/or boulders) should either be removed or pulverized for use in backfill. Gravel larger than ¾-inch (2 cm) in diameter should be mixed with at least 80-percent soil (by weight) passing the No. 4 sieve. Native soil backfill over the pipe bedding zone should be placed in thin lifts, moisture conditioned as necessary, and mechanically compacted using a minimum standard of 90 percent relative compaction, relative to the ASTM D1557 modified Proctor laboratory maximum dry density. Any imported base should be compacted to 95 percent relative compaction, relative to the ASTM D1557 laboratory maximum density, regardless of depth.

3.3.5 Drainage: Site clays will expand and silts and sands will consolidate when wetted. All drainage should be directed away from structures and pavements by means of approved permanent/temporary drainage devices. Adequate

-14-storm drainage of any proposed pad should be provided to avoid wetting of foundation soils. The proposed low-lying pad should be designed to rapidly discharge stormwater to prevent ponding on the pad. Irrigation adjacent to buildings should be avoided when possible. As an option, sealed-bottom planter boxes and/or drought resistant vegetation should be used within 5-feet of buildings.

3.4 Grit Chamber Mat Foundations

The bottom of the grit chamber bearing on newly-placed properly compacted fill over native soils can be designed as a mat foundation with an allowable overall static bearing pressure of 1,500 pounds-per-square-foot (psf). Locally, bearing pressures should not exceed 2,000 psf. These bearing values may be increase by one-third for short-term transient loading conditions, except for seismic loading.

Total static settlement of this 22-foot-diameter, 10-foot-high hopper grit chamber is estimated to be on the order of ½-inch or less, with differential static settlement anticipated to be on the order of ¼-inch or less for uniformly loaded structures bearing on the uniform thickness zone of properly compacted fill (95%). An additional total liquefaction induced settlement on the order of 2¼-inches is estimated for this site, resulting from a local large magnitude earthquake on the San Andreas Fault. This total settlement, however, will not be manifest as differential settlement, which is expected to be on the order of an inch.

3.5 Spread Footings

Shallow spread footings bearing on a zone of newly placed properly compacted/reinforced fill may be used to support structures in the Pond 1 area, as described in the following subsections.

3.5.1 Minimum Footing Dimensions and Embedment: For these single-story bay-high structures, footings should be embedded at least 18-inches below lowest adjacent finish grade. Footing embedment is measured from lowest adjacent finished grade, considered as the top of interior slabs-on-grade or the finished exterior grade, excluding landscape topsoil, whichever is lower. Footings located adjacent to utility trenches or vaults should be embedded below an imaginary 1:1 (horizontal:vertical) plane projected upward and outward from the bottom edge of the trench or vault, up towards the footing. Continuous/strip footings should have a minimum width of 12-inches, while column footings should have a minimum width of 24-inches. It is essential that we observe footing excavations before reinforcing steel is placed.

3.5.2 Allowable Vertical Bearing: For footings founded on newly placed properly compacted fill soil (as described in Section 3.5.1 of this report), an allowable

-15-net vertical bearing capacity of 2,500 pounds-per-square-foot (psf) should be used for design. This allowable net bearing pressure may be increased by 500 psf for each additional foot of width, to a maximum vertical bearing value of 3,500 psf. These bearing values may be increased by one-third when considering short-term seismic or wind loads. All continuous perimeter or interior footings should be reinforced with at least one No. 5 bar both top and bottom.

Conventional footings/slab may be enhanced by structurally tying the slabs-on-grade to the perimeter and interior footings as directed by the Structural Engineer (SE) for this project. The slab and footings may be placed (poured) monolithically to further integrate the structural system as a means of reducing the potential for structural damage due to dynamically induced settlement at this site. Exterior (not within the main foundation) isolated footings should be tied either together with a grade beam/ tie beam or back to the main foundation to increase lateral stability performance under the design seismic load(s). The need for tie beam/grade beam for these foundations should be determined by the structural consultant for the project.

3.5.3 Lateral Loads: Lateral loads may be resisted by friction between the footings and the supporting subgrade. A maximum allowable frictional resistance of

0.35 may be used for design of concrete structures poured on properly compacted fill. In addition, lateral resistance may be provided by passive pressures acting against foundations poured neat against properly compacted granular fill. We recommend that an allowable passive pressure based on an equivalent fluid pressure of 300 pounds-per-cubic-foot (pcf) be used in design.

These friction and passive values have already been reduced by a factor-of-safety of 1.5.

3.5.4 Settlement Estimates: For settlement estimates, we assumed that column loads will be no more than 75 kips, with bearing wall loads not exceeding 10 kips per foot of wall. If greater column or wall loads are required, we should re-evaluate our foundation recommendation, and re-calculate settlement estimates. Settlement of newly placed properly compacted fill materials is expected to predominantly occur during and within 60 days following fill placement. However, some settlement of the remaining loose native sands will occur slowly as these soils are wetted by excess irrigation water percolation.

Clarifiers and buildings located on compacted fill soils (as recommended in Section 3.3.1) should be designed in anticipation of ½ inch of total settlement and ¼ inch of differential settlement within a 40-foot horizontal run. The majority of this settlement is anticipated to occur during construction as the load is applied. These short-term settlements and angular distortions are from imposed building loads and do not include dynamic settlements. Dynamic liquefaction induced settlement on the order of 2¼-inches is estimated for this site, resulting from a local large magnitude earthquake on the San Andreas Fault. This total

-16-settlement, however, will not be manifest as differential settlement, which is expected to be on the order of an inch.

3.6 Retaining Walls

3.6.1 Static Lateral Earth Pressures: Earth retaining walls should be designed to resist lateral earth pressures from properly compacted backfill. These various walls are also supporting hydrostatic pressures from the interior, in addition to exterior earth pressures. We assume these walls are integral with the various structures and supported by mat foundations as recommended previously in this report. A sliding friction coefficient of 0.35 can be used for sustained loads when concrete is cast on properly compacted fill, unformed. When formed and cured concrete is backfilled with compacted fill, the friction coefficient should be reduced to 0.25. All permanent subterranean walls are assumed to have level backfill. The following table provided lateral earth pressures for retaining wall no more than 20 feet high:

Table 5. Retaining Wall Design Earth Pressures

Below Grade Wall Condition

Equivalent Fluid Pressure (pounds-per-cubic-foot)

Drained Submerged Active (cantilever) 35 80 At-Rest (braced) 55 90

Passive (resistance) 300* *145

*Not to exceed 1500 psf

Cantilever walls that are designed to yield at least 0.001H, where H is equal to the wall height, may be designed using the active condition. Rigid walls and walls braced at the top (such as wet wells) should be designed using the at-rest condition. Passive pressure is used to compute soil resistance to lateral structural movement. Site clays should not be used to backfill retaining walls.

All retaining walls should be backfilled with non-expansive sands (EI≤20).

Backfill should be placed in temporary cut slope excavations, behind retaining walls, and compacted to at least 90 percent relative compaction (ASTM D1557) using light compaction equipment. Concrete in the walls should attain the specified 28-day strength before backfilling is begun, or as allowed by the project Structural Engineer. If heavy compaction equipment is used, the walls should be temporarily braced.

3.6.2 Retaining Wall Surcharges: In addition to the above lateral pressures due to retained earth, surcharge due to loads on the wall backfill, such as an adjacent structure, should be considered in design of the retaining wall. Vertical surcharge loads behind the wall on or in the backfill within a 1:1 (horizontal:vertical) plane projection up and out from the wall toe, should be

-17-considered as lateral and vertical surcharge. Unrestrained (cantilever) walls should be designed to resist one-third of these surcharge loads applied as a uniform horizontal pressure on the wall. Braced walls should also be designed to resist an additional uniform horizontal-pressure equivalent to one-half of uniform vertical surcharge-loads.

3.6.3 Incremental Earthquake Induced Earth Pressures: Seismic incremental loads need not be added to retaining walls with stem heights on the order of (≤) 6-feet or less, with adjacent level backfill. However, for taller walls, incremental seismic earth pressures of 25 pounds-per-cubic-foot (pcf) can be applied as an inverted triangle for design, at the discretion of the Structural Engineer, in addition to static earth and surcharge pressures presented above.

This is based on traditional Mononobe-Okabe (1929) equations. Traditionally, this incremental seismic earth pressure has been applied as an inverted triangle (inverted equivalent fluid pressure), with largest dynamic earth pressure occurring at the top of the wall (upper ground surface). Resultant seismic earth pressure force has traditionally been applied at approximately 0.6H from the bottom of the wall, where H is the wall (stem) height (e.g. Seed and Whitman, 1970).

However, studies (Sitar, et. al., 2010, U.C. Berkeley) suggest a uniform pressure distribution is likely closer to actual lateral seismic loads, so a uniform pressure of 12.5H (psf) applied as a uniform/rectangular pressure distribution can also be considered (based on current research and observations), at the discretion of the Structural Engineer. It is important to consider that for level backfill and in areas without shallow groundwater, both case history reviews and centrifuge test results suggest all of these approaches above are conservative. Seismic incremental loads need not be added to at-rest earth pressures, but can be added to active earth pressures in combination for design consideration.

3.6.4 Drainage: Adequate drainage may be provided by a subdrain system positioned behind earth retaining walls. Typically, this system consists of a 4-inch minimum diameter perforated pipe placed near the base of the wall (perforations placed downward). The pipe should be bedded and backfilled with pervious backfill material described in Section 300-3.5.2 of the Standard Specifications for Public Works Construction (Green Book), 2018 Edition. This pervious backfill should extend at least 2 feet horizontally from the back face of the wall and to within 2 feet of backfilled finished grade. This pervious backfill and pipe should also be wrapped in filter fabric, such as Mirafi 140N or equivalent, placed as described in Section 300-8.1 of the Standard Specifications for Public Works Construction (Green Book), 2018 Edition. The subdrain outlet should be connected to a free-draining outlet or sump.

Miradrain, Geotech Drainage Panels, or Enkadrain drainage geocomposites, -18-or similar, may be used for wall drainage as an alternative to the Class 2 Permeable Material or drain rock backfill, particularly where horizontal space is limited…