00-31-32-01_1-3-B-15_2023_05_31_Geotech_SBIWTP_Ninyo__More.pdf

Geotechnical Findings Report South Bay International Wastewater

Treatment Plant Expansion 2995 Clearwater Way San Diego, California

Arcadis 9620 Chesapeake Drive, Suite 106 | San Diego, California 92123

May 31, 2023 | Project No. 109539001

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

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

5710 Ruffin Road | San Diego, California 92123 | p. 858.576.1000 | www.ninyoandmoore.com

Geotechnical Findings Report South Bay International Wastewater Treatment Plant Expansion 2995 Clearwater Way San Diego, California

Mr. Peter Tymkiw, PE Arcadis 9620 Chesapeake Drive, Suite 106 | San Diego, California 92123

May 31, 2023 | Project No. 109539001

Nissa M. Morton, PG, CEG Senior Project Geologist

William R. Morrison, PE, GE Principal Engineer

Gregory T. Farrand, PG, CEG Principal Geologist

NMM/WRM/GTF/mp http://www.ninyoandmoore.com/

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 i

CONTENTS

1 INTRODUCTION 1

2 SCOPE OF SERVICES 1

3 SITE AND PROJECT DESCRIPTION 1

4 SUBSURFACE EXPLORATION 3

4.1 Previous Subsurface Explorations 3

5 LABORATORY TESTING 4

6 GEOLOGY 4

6.1 Regional Geologic Setting 4

6.2 Site Geology 5

Fill 5

Alluvium 5

Old Paralic Deposits 6

San Diego Formation 6

6.3 Groundwater 6

7 GEOLOGIC HAZARDS 7

7.1 Faulting and Seismicity 7

Ground Rupture 8

Seismic Site Class 8

Strong Ground Motion 8

Liquefaction and Seismically Induced Settlement 10

Lateral Spread 11

Surface Manifestation of Liquefaction 12

7.2 Tsunamis and Seiches 12

7.3 City of San Diego Seismic Safety Study 12

7.4 Landslides and Slope Stability 13

7.5 Hydro-Consolidation or Soil Collapse 13

8 CONCLUSIONS 13

9 RECOMMENDATIONS 15

9.1 Earthwork 15

Pre-construction Conference Site Preparation 15

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Site Preparation 15

Remedial Earthwork 15

Excavation Characteristics 16

Temporary Excavations 17

Temporary Shoring 17

Construction Dewatering 18

Excavation Bottom Stability 19

Materials for Fill 19

Compacted Fill 20

Lateral Pressures for Thrust Blocks 21

Pipe Bedding and Modulus of Subgrade Reaction 21

Utility Pipe Zone Backfill 21

Utility Trench Zone Backfill 22

Drainage 22

9.2 Seismic Design Considerations 22

9.3 Shallow Foundations 23

9.4 Mat Foundations 24

Mat Foundations 24

Mat Foundations – Lateral Resistance 25

9.5 Interior Slabs-on-Grade 25

9.6 Underground Structures 25

9.7 Retaining Walls 26

9.8 Preliminary Flexible Pavement Design 26

9.9 Corrosivity 27

9.10 Concrete 27

9.11 Plan Review and Construction Observation 27

10 LIMITATIONS 28

11 REFERENCES 30

TABLES

1 – Encountered Depths to Groundwater during Subsurface Exploration 6

2 – Principal Active Faults 7

3 – 2022 California Building Code Seismic Design Criteria 23

4 – Recommended Preliminary Flexible Pavement Sections 26

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FIGURES

1 – Site Location 2 – Exploration Locations 3 – Fault Locations 4 – Geology 5 – Acceleration Response Spectra 6 – Seismic Safety Study 7 – Lateral Earth Pressures for Braced Excavation below Groundwater 8 – Lateral Earth Pressures for Temporary Cantilevered Shoring 9 – Thrust Block Lateral Earth Pressure Diagram 10 – Lateral Earth Pressures for Underground Structures 11 – Lateral Earth Pressures for Yielding Retaining Walls 12- Lateral Earth Pressures for Restrained Retaining Walls 13- Retaining Wall Drainage Detail

APPENDICES

A – Boring Logs and CPT Data B – Geotechnical Laboratory Testing C – Liquefaction Analysis

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 1

1 INTRODUCTION

In accordance with your request and authorization, Ninyo & Moore has performed a geotechnical evaluation for the proposed South Bay International Wastewater Treatment Plant expansion project located at 2995 Clearwater Way in San Diego, California (Figure 1). The purpose of our geotechnical evaluation was to assess the general geologic conditions at the site and to develop geotechnical recommendations regarding the design and construction of the proposed improvements. This report presents our findings, conclusions, and recommendations for the project based on our background review, site reconnaissance, subsurface evaluation, laboratory testing, and geotechnical analyses.

2 SCOPE OF SERVICES

Ninyo & Moore’s scope of services for this evaluation included the following:

• Reviewing readily available published and in-house geologic and geotechnical literature, topographic maps, geologic and geologic hazard maps, fault maps, flood zone maps, project plans, historical stereoscopic aerial photographs, groundwater data, and other background information.

• Performing a field reconnaissance of the site, which included marking the exploration locations for utility clearance by Underground Service Alert (USA).

• Acquiring a boring permit from the County of San Diego Department of Environmental Health and Quality (DEHQ).

• Performing a subsurface exploration consisting of the drilling, logging, and sampling of ten small diameter exploratory borings and the performance of nine cone penetration test soundings (CPTs) at locations selected in consultation with Arcadis. Logging of the borings was performed by a representative from Ninyo & Moore. Relatively undisturbed and bulk soil samples were obtained at selected intervals from the borings. The collected samples were transported to our in-house geotechnical laboratory for testing.

• Performing geotechnical laboratory testing on representative soil samples to evaluate design parameters and soil characteristics. Laboratory testing included an evaluation of in-place moisture content and dry density, sieve (gradation) analysis, consolidation, shear strength, expansion index, and soil corrosivity (pH, electrical resistivity, sulfates and chlorides).

• Compiling and performing a geotechnical engineering analysis of the data obtained from our background review, subsurface exploration and laboratory testing.

• Preparing this geotechnical report presenting our findings, conclusions, and geotechnical recommendations for the design and construction of the subject project.

3 SITE AND PROJECT DESCRIPTION

The South Bay International Wastewater Treatment Plant (SBIWTP) is located at

2995 Clearwater Way in the San Ysidro area of San Diego, California, California (Figures 1

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 2 and 2). The existing facility is situated on approximately 35 acres of land on the south side of the

Tijuana River Valley. The facility presently consists of various sedimentation tanks, sludge tanks, storage tanks, grit chambers, chemical facilities, pump stations, associated piping, and various operations and maintenance buildings. The facility was first constructed in the mid 1990’s and has undergone various improvement and upgrade projects since that time. We understand that deep dynamic compaction (DDC) was utilized during construction of the existing facility to mitigate liquefaction hazards at the site. The existing facility is relatively level with elevations ranging from approximately 50 feet above mean sea level (MSL) in the northwest portion of the site adjacent to Clearwater Way to approximately 60 feet above MSL along the southern perimeter. The project site also includes roughly 20 acres of land adjacent to and southwest of the existing facility that is currently unoccupied. This area has been used as a construction staging area in the past and a fill pad was previously built up in the southwestern portion along Monument Road. Topography in this area consists of gentle to moderately steep slopes that generally descend to the northeast towards the facility. Elevations in this area range from approximately 45 feet to 80 feet above

MSL. An approximately 5- to 12-foot high slope ascends up to the existing facility along the boundary of these two areas.

We understand that the current expansion project will increase the treatment capacity of SBIWTP to 60 million gallons per day (MGD). Based on information provided by Arcadis, proposed expansion improvements will include an expansion and construction of the following structures:

- Secondary sedimentation tanks;

- Primary clarifier;

- Activated sludge tanks;

- Dissolved air flotation tank;

- Headworks;

- Solids dewatering tanks; and

- Associated pipelines and pipe galleries.

In addition, construction that will expand to the southwest portion of the project site will include new digesters, boilers, and a control building. We understand that the proposed improvements will be constructed at variable depths, ranging from at-grade structures to structures founded at depths of up to 25 feet below the ground surface (bgs). A site plan showing the approximate locations and dimensions of the proposed improvements was not available at the time of this evaluation.

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4 SUBSURFACE EXPLORATION

Our subsurface exploration was performed between January 9, 2023 and January 13, 2023 and consisted of drilling ten small-diameter exploratory borings (B-1 through B-10) and the advancement of nine cone penetration test (CPT) soundings (CPT-1 through CPT-9A). The borings and CPTs were performed at locations selected in consultation with Arcadis. Due to shallow refusal in the subsurface materials, multiple attempts were made to advance CPT soundings CPT-5B, CPT-7B, and CPT-9A. Prior to commencing the subsurface exploration, Underground Service Alert (USA) was notified to mark out locations of the existing underground utilities in the vicinity of the borings and CPTs.

The borings were drilled to depths up to approximately 48 feet using a truck-mounted drill rig equipped with 8-inch, continuous-flight, hollow-stem augers. During the drilling operations, the borings were logged and sampled by personnel from Ninyo & Moore. Representative bulk and relatively undisturbed in-place soil samples were obtained from the borings. The samples were then transported to our in-house geotechnical laboratory for testing. The nine CPTs were advanced to depths ranging from approximately 10 to 49 feet using a truck-mounted continuous push system. Shear wave measurements were performed at the bottom of selected CPT soundings to provide shear wave velocity data to be used in our analysis.

The purpose of the explorations was to evaluate subsurface conditions and to collect samples for geotechnical laboratory testing. The approximate locations of the exploratory borings and CPTs are shown on Figure 2. Logs of the borings and a summary of the CPT data are included in

Appendix A.

4.1 Previous Subsurface Explorations

Several subsurface explorations have previously been performed at the site by various consultants. Woodward-Clyde performed a geotechnical evaluation for the SBIWTP and adjacent areas in 1994 that consisted of the logging and sampling of 74 exploratory borings, 44 exploratory test pits, seven fault trenches, and the performance of 42 CPTs (Woodward-Clyde, 1994a). The exploratory borings extended to depths up to approximately 98 feet. Based on the results of their subsurface exploration and analysis, Woodward-Clyde recommended ground modifications at the site, including DDC. As part of compliance evaluations for the DDC performed at the plant, Construction Testing & Engineering (CTE) conducted a subsurface evaluation in 2009 that consisted of 26 exploratory borings (CTE, 2009a; 2009b). While our records of the CTE borings are incomplete, we understand that the borings extended to depths up to approximately 31 feet.

In 2014, Stantec, performed a geotechnical study for the new administration building at the site

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 4

(Stantec, 2014). Subsurface exploration performed as part of Stantec’s study included five exploratory borings that extended to depths of approximately 46½ feet. URS performed a geotechnical investigation in 2015 for various improvements at the existing facility, including new equalization tanks and three secondary sedimentation tanks (URS, 2015). Subsurface exploration performed as part of URS’s study included four exploratory borings that extended to depths of approximately 46 feet. The subsurface exploration data from these previous evaluations have been incorporated in this report, where appropriate.

5 LABORATORY TESTING

Geotechnical laboratory testing of representative soil samples included tests to evaluate in-situ moisture and density, sieve (gradation) analysis, consolidation, shear strength, expansion index, and soil corrosivity. The results of the in-situ dry density and moisture content testing are presented on the boring logs in Appendix A. Descriptions of the geotechnical laboratory test methods used and the results of the other geotechnical laboratory tests performed are presented in Appendix B.

6 GEOLOGY

Our findings regarding regional and site geology and groundwater conditions are provided in the following sections.

6.1 Regional Geologic Setting

The project site is situated in the coastal plain section of the Peninsular Ranges Geomorphic

Province. This geomorphic province encompasses an area that extends approximately 900 miles from the Transverse Ranges and the Los Angeles Basin south to the southern tip of Baja California

(Norris and Webb, 1990; Harden, 2004). The province varies in width from approximately 30 to

100 miles. In general, the province consists of rugged mountains underlain by Jurassic metavolcanic and metasedimentary rocks, and Cretaceous igneous rocks of the southern

California batholith. The portion of the province in western San Diego County that includes the project area consists generally of alluvium underlain by uplifted and dissected Tertiary-, and

Quaternary-age sedimentary rocks.

The Peninsular Ranges Province is traversed by a group of sub-parallel faults and fault zones trending roughly northwest. Several of these faults are considered to be active. The Elsinore, San Jacinto, and San Andreas faults are active fault systems located east of the project area and the Rose Canyon, Newport-Inglewood, Coronado Bank, San Diego Trough, and San Clemente

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 5 faults are active faults located southwest of the project area (Figure 3). Major tectonic activity associated with these and other faults within this regional tectonic framework consists primarily of right-lateral, strike-slip movement. The Rose Canyon fault is the nearest active fault and has been mapped approximately 5 miles west of the site. An unnamed fault, which is not considered active by the State of California, is mapped as being concealed beneath the southwestern portion of the site. Further discussion of faulting relative to the site is provided in the Faulting and

Seismicity section of this report.

6.2 Site Geology

Geologic units encountered during our subsurface exploration included fill and alluvium. Other units mapped at the site and encountered in previous site evaluations include old paralic deposits and materials of the San Diego Formation (Kennedy and Tan, 2008). Generalized descriptions of the earth units encountered during our field reconnaissance and subsurface exploration and mapped in the vicinity of the project site are provided in the subsequent sections. Additional descriptions of the encountered subsurface units are provided on the boring logs in Appendix A.

The regional geologic map of the site is shown on Figure 4.

Fill Fill soils were encountered in each of our exploratory borings at the ground surface and extended to depths of up to approximately 24 feet. As observed in the borings, these fill soils generally consisted of various shades of brown and gray, moist to wet, loose to very dense, sandy silt, silty sand, and poorly-graded sand with silt. Scattered to numerous amounts of gravel and cobbles were observed within the fill. Refusal within the fill materials occurred at

CPT-1, CPT-4, CPT-5B, and CPT-7B. As noted previously, we understand that DDC was performed within the existing facility as part of the initial site development. Documentation regarding the placement of existing fills was not available for our review.

Alluvium Quaternary-age alluvium was encountered underlying the fill soils in each of our borings and extended to the total depths explored in the borings of approximately 48 feet. As encountered during our subsurface exploration, the alluvium generally consisted of various shades of brown and gray, moist to wet, medium dense to very dense, sandy silt, silty sand, clayey sand, well-graded sand, well-graded sand with silt, poorly-graded sand, poorly-graded sand with silt, and clayey and silty gravel. Gravel and cobbles were encountered within the

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 6 alluvium. Refusal was met on gravel/cobble in the alluvium at each boring location. Refusal within the alluvium was also encountered at CPT-2, CPT-3, CPT-6, CPT-8, and CPT-9A.

Old Paralic Deposits While not encountered during our subsurface evaluation, materials mapped as Quaternary-age old paralic deposits are mapped across much of the site (Figure 4). As encountered in previous explorations in the project area, these materials are anticipated to consist of brown, sandy clay, silty sand with gravel and cobbles, clayey sand, and silty gravel (Woodward-Clyde, 1994a;

1994b). Boulders may be present within the old paralic deposits.

San Diego Formation While not encountered during our subsurface evaluation, materials of the Tertiary-age San Diego

Formation have been encountered in previous subsurface explorations at the site and are anticipated to underlie the alluvium and old paralic deposits on the site at depth. As encountered in previous site explorations performed by Woodward-Clyde, the San Diego Formation in the project area is anticipated to consist of brown, clayey and silty sandstone with gravel and cobble conglomerate. Boulders were observed within materials of the San Diego Formation in a previous boring that was down-hole logged by Woodward-Clyde (1994a; 1994b).

6.3 Groundwater

Groundwater was encountered in our borings B-1 through B-7, B-9, and B-10 at depths ranging from approximately 19 feet to 27 feet below the ground surface. Fluctuations in the depth to groundwater will occur due to flood events, seasonal precipitation, variations in ground elevations, subsurface stratification, irrigation, groundwater pumping, storm water infiltration and other factors. The groundwater depths encountered during our subsurface exploration are summarized in Table 1.

Table 1 – Encountered Depths to Groundwater during Subsurface Exploration Boring Encountered Depth to Groundwater (feet)*

B-1 21½ B-2 21½ B-3 21 B-4 21½ B-5 21½ B-6 22½ B-7 19 B-8 -- B-9 27 B-10 26½

Notes:

* Encountered water depths at time of drilling exploratory borings.

-- Groundwater not encountered during drilling.

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 7

7 GEOLOGIC HAZARDS

In general, hazards associated with faulting and seismic activity include strong ground motion, ground rupture, and liquefaction. These considerations and other potential geologic hazards such as tsunamis, and landsliding are discussed in the following sections.

7.1 Faulting and Seismicity

The site is not located within a State of California Earthquake Fault Zone (EFZ) (formerly known as an Alquist-Priolo Special Studies Zone) (Hart and Bryant, 2007). However, like the majority of Southern California, the site is located in a seismically active area and the potential for strong ground motion is considered significant during the design life of the proposed improvements. The Rose Canyon Fault Zone, the nearest active fault system, has been mapped approximately 5 miles west of the site. We also note that a potentially active fault has been mapped by the City of San Diego (2008; Figure 6) in the southwestern portion of the site. Table 2 lists mapped active fault zones near the site.

Table 2 – Principal Active Faults

Fault Approximate

Fault-to-Site Distance miles (kilometers)

Maximum Moment Magnitude

(Mmax) Rose Canyon 5 (8) 6.9 Coronado Bank 12 (19) 7.4 Elsinore (Julian Segment) 47 (76) 7.4 Newport-Inglewood (Offshore Segment) 47 (76) 7.0 Earthquake Valley 51 (82) 6.8 Elsinore (Coyote Mountain Segment) 52 (83) 6.9 Elsinore (Temecula Segment) 55 (89) 7.1 San Jacinto (Coyote Creek Segment) 67 (108) 7.0 San Jacinto (Borrego Segment) 68 (109) 6.8 Laguna Salada 70 (113) 7.3

As noted previously and shown on the City of San Diego seismic hazard map (Figure 6), an unnamed fault has been mapped as being concealed in the southwest portion of the site. As mapped, this fault extends a total length of approximately 800 feet, trending generally north-south, and terminates within the SBIWTP site. This fault is one of similar trending faults in the area that are shown as potentially active on the City of San Diego seismic hazard map

(Figure 6) and mapped as displacing Tertiary-age deposits on the regional geologic map

(Figure 3). These faults are not considered active by the State of California (Hart and

Bryant, 2007). Woodward-Clyde performed a study of these faults in 1994 that included aerial photograph interpretations, geologic field mapping, and the performance of fault trenches.

The fault mapped in the southwestern portion of the site, described as the “Eastern Fault”, was delineated based on an interpretation of aerial photographs and did not possess

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 8 geomorphic expression within Pleistocene deposits north of Monument Road

(Woodward-Clyde, 1994). Woodward-Clyde indicated the fault was not visible in exposures of road cuts and opined that it defined the eastern edge of a graben feature. The “Eastern Fault” is also not included in the USGS database of Quaternary-active faults (USGS, 2020).

Based on the information and discussion provided above, we do not consider the unnamed fault mapped in the southwestern portion of the site to be active. Due to the relatively minor geomorphic expression, it is out opinion that the fault is a relatively old feature, whereas active faulting in the area generally occurs along the Rose Canyon Fault Zone to the west. In general, hazards associated with seismic activity include strong ground rupture, strong ground motion, liquefaction, and tsunamis. These hazards are discussed in the following sections.

Ground Rupture Based on our review of the referenced literature, maps, and our site reconnaissance, active faults are not known to cross the project vicinity. Therefore, the potential for ground surface rupture due to faulting at the site is considered low. However, lurching or cracking of the ground surface as a result of nearby seismic events is possible.

Seismic Site Class As previously discussed, we performed shear wave measurements at the bottom of selected

CPT soundings to provide shear wave velocity data to be used in our analysis. In order to select the appropriate Seismic Site Class, we reviewed the results of the shear wave velocity measurements from the CPTs, as well as mapped shear wave velocity data (Wald and Allan, 2008). Shear wave data resolution generally decreases with depth, due to the loss of sensitivity of the dispersion curve to changes in shear wave velocity as depth increases.

Based on comparison of the average shear wave velocities in our CPTs and the mapped shear wave velocity for the site area with ASCE 7-16 Table 20.3-1, we consider the site to be

Seismic Site Class D.

Strong Ground Motion The 2022 California Building Code (CBC) specifies that the Risk-Targeted, Maximum

Considered Earthquake (MCER) ground motion response accelerations be used to evaluate seismic loads for design of buildings and other structures. Per the 2022 CBC, a site-specific ground motion hazard analysis shall be performed for structures on Site Class D with a mapped MCER, 5 percent damped, spectral response acceleration parameter at a period of

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 9

1 second (S1) greater than or equal to 0.2g in accordance with Sections 21.2 and 21.3 of the

American Society of Civil Engineers (ASCE) Publication 7-16 for the Minimum Design Loads and Associated Criteria for Building and Other Structures as updated by ASCE 7-16

Supplements 1, 2, and 3 (ASCE, 2018; 2021a; 2021b). We calculated that the S1 for the site is equal to 0.348g using the 2023 Structural Engineers Association of California

[SEAOC]/Office of Statewide Health Planning and Development [OSHPD] seismic design tool (web-based); therefore, a site-specific ground motion hazard analysis was performed for the project site.

The site-specific ground motion hazard analysis consisted of the review of available seismologic information for nearby faults and performance of probabilistic seismic hazard analysis (PSHA) and deterministic seismic hazard analysis (DSHA) to develop acceleration response spectrum (ARS) curves corresponding to the MCER for 5 percent damping. Prior to the site-specific ground motion hazard analysis, we obtained the mapped seismic ground motion values and developed the general MCER response spectrum for 5 percent damping in accordance with Section 11.4 of ASCE 7-16. The average shear wave velocity (VS) for the upper 30 meters of soil (VS30) is mapped to be 364 meters per second (m/s) (Wald and

Allan, 2008) and the depths to VS = 1,000 m/s and VS = 2,500 m/s are assumed to be

40 meters and 220 meters, respectively (Southern California Earthquake Center [SCEC]

Community Velocity Model Version 11.9.0 Basin Depth). These values were evaluated using the Open Seismic Hazard Analysis software developed by USGS and SCEC (2023c).

The 2014 new generation attenuation (NGA) West-2 relationships were used to evaluate the site-specific ground motions. The NGA relationships that we used for developing the probabilistic and deterministic response spectra are by Chiou and Youngs (2014), Campbell and Bozorgnia (2014), Boore, Stewart, Seyhan, and Atkinson (2014), and Abrahamson, Silva, and Kamai (2014). The Open Seismic Hazard Analysis software developed by USGS (USGS, 2023c) was used for performing the PSHA. The Calculation of Weighted Average 2014 NGA

Models spreadsheet by the Pacific Earthquake Engineering Research Center (PEER) was used for performing the DSHA (Seyhan, 2014).

PSHA was performed for earthquake hazards having a 2 percent chance of being exceeded in 50 years multiplied by the risk coefficients per ASCE 7-16. The maximum rotated components of ground motions were considered in PSHA with 5 percent damping. For the

DSHA, we analyzed accelerations from earthquakes on active faults within the region using the Building Seismic Safety Council 2014 scenario catalog (USGS, 2023b) from a connected rupture of the Rose Canyon and Newport Inglewood faults was evaluated to be the controlling

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 10 earthquake. Hence, the deterministic seismic hazard analysis was performed for the site using this event and corrections were made to the spectral accelerations for the

84th percentile of the maximum rotated component of ground motion with 5 percent damping.

The site-specific MCER response spectrum was taken as the lesser of the spectral response acceleration at any period from the PSHA and DSHA, and the site-specific general response spectrum was determined by taking two-thirds of the MCER response spectrum with some conditions in accordance with Section 21.3 of ASCE 7-16 as updated by Supplements 1, 2, and 3. Figure 5 presents the site-specific MCER response spectrum and the site-specific design response spectrum. The general mapped design response spectrum calculated in accordance with Section 11.4 of ASCE 7-16 is also presented on Figure 5 for comparison.

The site-specific spectral response acceleration parameters, consistent with the 2022 CBC, are provided in Section 9.3 for the evaluation of seismic loads on buildings and other structures. The site-specific Maximum Considered Earthquake Geometric Mean (MCEG) peak ground acceleration (PGAM) was calculated as 0.541g.

Liquefaction and Seismically Induced Settlement Liquefaction is the phenomenon in which loosely deposited granular soils with silt and clay contents of less than approximately 35 percent and non-plastic silts located below the water table undergo rapid loss of shear strength when subjected to strong earthquake-induced ground shaking. Ground shaking of sufficient duration results in the loss of grain-to-grain contact due to a rapid rise in pore water pressure, and causes the soil to behave as a fluid for a short period of time. Liquefaction is known generally to occur in saturated or near-saturated cohesionless soils at depths shallower than 50 feet below the ground surface.

Factors known to influence liquefaction potential include composition and thickness of soil layers, grain size, relative density, groundwater level, degree of saturation, and both intensity and duration of ground shaking.

According to the City of San Diego Seismic Safety Study (2008), the project vicinity is partially mapped within Geologic Hazard Category 31, which indicates a high potential for liquefaction. As noted in the previous sections, the site is underlain by granular fill soils and alluvium, and groundwater was encountered as shallow as approximately 19 feet bgs in our borings.

Accordingly, we evaluated the liquefaction potential at the project site. Based on the results of our subsurface exploration, the liquefaction potential of the upper soils was evaluated using subsurface data from our exploratory CPT sounding CPT-3 (situated adjacent to boring B-3). For purposes of our liquefaction assessment, a historic high groundwater level was assumed to be

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 11 at a depth of 14 feet. Liquefaction evaluation was performed using the characteristic magnitude of 7.0 associated with the Rose Canyon fault and an MCEG peak ground acceleration with adjustment for site class effects (PGAM) of 0.541g as obtained from our site-specific ground motion hazard analysis (Section 7.1.2). The liquefaction analysis was performed using the computer program LiquefyPro (CivilTech Software, 2007). The analysis for CPT sounding CPT-3 was performed using the modified Robertson and Wride Method (1997).

As a result of liquefaction, the proposed structures may be subject to several hazards, including liquefaction-induced settlement. In order to estimate the amount of post-earthquake settlement, the method proposed by Tokimatsu and Seed (1987) was used for the evaluation of dynamic settlement using the computer program LiquefyPro (CivilTech Software, 2007).

The amount of soil settlement during a strong seismic event depends on the thickness of the liquefiable layers and the density and/or consistency of the soils. Liquefaction calculations were performed based on our CPT sounding CPT-3. Calculations performed using data obtained from CPT-3 indicate that portions of the alluvial materials are potentially liquefiable during a design seismic event, with estimated seismic settlements on the order of 1 inch.

Because the CPTs provide continuous subsurface data, while the standard penetration test

N-values are obtained at 5-foot intervals in each boring, we consider the estimated seismic settlement of 1 inch based on the CPT data to be a more reliable estimate of the settlement that could result from a design seismic event than settlements estimated from SPT N-values obtained from exploratory borings. Differential settlements of approximately one-half of the total settlement over a horizontal span of 40 feet are anticipated. Our liquefaction analysis results are presented in Appendix C.

Liquefaction and seismic settlement should be considered during the design of improvements in the vicinity of CPT sounding CPT-3. Based on our review of blow count data in borings

B-1, B-2, and B-4 through B-10, as well as the data from CPT-1, CPT-2, and CPT-4 through

CPT-9A, we conclude that liquefaction and seismic settlement are not design considerations in the vicinity of those explorations.

Lateral Spread Lateral spreading of ground surface during an earthquake usually takes place along weak shear zones that have formed within a liquefied soil layer. Lateral spread has generally been observed to take place in the direction of a free-face (i.e., retaining wall, slope, channel) but has also been observed to a lesser extent on ground surfaces with very gentle slopes. An empirical model developed by Bartlett and Youd (1995, revised 2002) is typically used to estimate the amount of

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 12 horizontal ground displacement within a site. For a site located in proximity to a free-face, the amount of lateral ground displacement is strongly correlated with the distance of the site from the free-face. Other factors such as earthquake magnitude, distance from the earthquake epicenter, thickness of the liquefiable layers, and the fines content and particle sizes of the liquefiable layers also affect the amount of lateral ground displacement.

Evaluation of the borings where there is a potential for liquefaction indicates that they are either situated within a flat area away from free faces, or the corrected SPT N-values of the potentially liquefiable soils are greater than 15, which Youd et al (2002) considers to be too dense and dilative for lateral spreading to occur. Accordingly, lateral displacement is not a design consideration for the SBIWTP facility.

Surface Manifestation of Liquefaction Based on the design curves developed by Ishihara (1985) the potential for surface manifestation of liquefaction (i.e., ground subsidence, sand boils, and/or seismically induced bearing failure) at the MBGPF and secondary sites is considered low.

7.2 Tsunamis and Seiches

Tsunamis are long wavelength seismic sea waves (long compared to the ocean depth) generated by sudden movements of the ocean bottom during submarine earthquakes, landslides, or volcanic activity. Seiches are similar oscillating waves on inland or enclosed bodies of water. The site is located outside of mapped tsunami inundation areas (Graehl and Wilson, 2022). In addition, seiches are not considered to be a hazard at the site, due to the absence of lakes or other large enclosed bodies of water.

7.3 City of San Diego Seismic Safety Study

Based on our review of the City of San Diego Geologic Hazards map (City of San Diego, 2008), the developed area of the site is located in Hazard Category 31 and the southwest portion of the site is located in Hazard Category 53 (Figure 6). Hazard Category 31 designates an area of high potential for liquefaction due to the presence of shallow groundwater, major drainages, and hydraulic fill. Hazard Category 53 delineates level or sloping terrain with unfavorable geologic structure, with low to moderate risk. In addition, an area in the southwestern portion of the site is mapped within Hazard Category 12, which is defined as an area underlain by a fault that is

“potentially active, inactive, presumed inactive, or activity unknown”. As previously discussed, based on our background review and site evaluation, the unnamed fault mapped in the

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 13 southwestern portion of the site is not considered to be active. The portion of the seismic safety element depicting the treatment plant site is included as Figure 6.

7.4 Landslides and Slope Stability

Based on our review of published geologic literature and aerial photographs, and our site reconnaissance, no landslides or related features are known to underlie or be adjacent to the project site. Therefore, the potential for landslides at the project site is considered low.

Global slope stability is not anticipated to be a design consideration at the project due to the relatively competent nature of the subsurface materials. However, surficial stability and erosion may be design considerations in hilly portions of the project area. Following development of grading plans, surficial stability and erosion of proposed slopes may be further evaluated.

7.5 Hydro-Consolidation or Soil Collapse

Hydro-consolidation or soil collapse typically occurs in recently deposited, Holocene (younger than

10,000 years old) soils that were deposited in an arid or semi-arid environment. Soils prone to collapse are commonly associated with man-made fill, wind-laid sands and silts, and alluvial fan and mudflow sediments deposited during flash floods. These soils typically contain minute pores and voids. The soil particles may be supported by clay or silt, or chemically cemented with carbonates. When saturated, collapsible soils undergo rearrangement of their grains, and the water removes the cohesive (or cementing) material, this results in rapid substantial settlement.

Based on our subsurface exploration and laboratory testing data, the hydro-consolidation potential of the soils in the vicinity of borings B-4 and B-8 is moderate to severe. However, the samples exhibiting hydro-consolidation during our laboratory testing program (Appendix B) consisted of existing fill soils.

The presence of the potentially compressible fill soils at these locations is not expected to affect the proposed improvements at these locations, as foundations supporting the improvements in the vicinity of boring B-4 will extend below the collapsible layer, and earthwork is expected to remove the collapsible soils in the vicinity of boring B-8. The presence of potentially collapsible soils should be further evaluated by the geotechnical consultant during earthwork construction and supplemental recommendations may be provided as warranted.

8 CONCLUSIONS

Based on our review of the referenced background data, subsurface exploration, and laboratory testing, it is our opinion that construction of the proposed improvements is feasible

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 14 from a geotechnical standpoint provided the recommendations presented in this report are incorporated into the design and construction of the project. In general, the following conclusions were made:

• The areas of the planned improvements are generally underlain by fill soils and alluvium.

Fill was encountered in each of our borings and CPTs to depths up to approximately 24 feet below the ground surface. Gravel, cobbles, and possible boulders are expected to be present in the fill and alluvium. Refusal was encountered in each of our exploratory borings at depths ranging from approximately 26 to 48 feet.

• Groundwater was encountered in nine of our borings at depths ranging from approximately 19 feet to 27 feet below the ground surface. Fluctuations in the groundwater level may occur due to variations in ground surface topography, subsurface geologic conditions and structure, rainfall, irrigation, and other factors. Consideration should be given to dewatering methods where groundwater conditions are encountered during construction.

• The onsite fill soils and alluvium should be generally excavatable using heavy duty earthmoving equipment in good working condition. Scattered gravel and cobbles were encountered within the existing fill soils and alluvium. Due to the potential for gravel, cobbles, and possible boulders within the subsurface materials, the contractor should anticipate encountering difficulty in performing excavations within these materials. The contractor should anticipate handling oversize material and difficult excavation should be anticipated in materials containing cobbles and oversize materials.

• Caving and/or sloughing conditions may be encountered when performing excavations in loose zones within existing fill and/or alluvium.

• Onsite soils, derived from the earthwork operations are generally considered suitable for reuse as compacted fill and trench backfill. However due to the presence of gravel and cobbles, processing of the onsite soils (including screening and/or rock crushing) should be anticipated.

• The site is not located within a State of California Earthquake Fault Zone (Alquist-Priolo Special Studies Zone). The closest known major active fault is the Rose Canyon Fault Zone, which is located approximately 5 miles west of the project site.

• Our evaluation indicates that some of the granular portions of the alluvium at the site are potentially liquefiable during a design seismic event. Seismic induced settlements could occur within the alignment as a result of liquefaction at the site. In light of this, consideration should be given to the use of flexible connections, appropriately spaced shut off valves, or other measures to mitigate the effects of liquefaction on the pipeline due to a design seismic event.

• Our laboratory testing indicate that two areas of the site are underlain by soils susceptible to hydro-collapse. However, these materials consist of existing fill soils. Foundations supporting the improvements in one of these areas will be constructed below the collapsible layer, while earthwork is expected to remove the collapsible soils in the second area. The presence of potentially collapsible soils should be further evaluated by the geotechnical consultant during earthwork construction

• Based on the results of our soil corrosivity tests completed to date, American Concrete Institute (ACI) 318, and Caltrans (2021) criteria, onsite soils the project site would be classified as corrosive.

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 15

9 RECOMMENDATIONS

Based on our understanding of the project, the following recommendations are provided for the design and construction of the project. The proposed site improvements should be constructed in accordance with the requirements of the applicable governing agencies.

9.1 Earthwork

In general, earthwork should be performed in accordance with the recommendations presented in this report. Ninyo & Moore should be contacted for questions regarding the recommendations or guidelines presented herein.

Pre-construction Conference Site Preparation We recommend that a pre-construction conference be held. The International Boundary and

Water Commission (IBWC) and/or its representatives, the civil engineer, Ninyo & Moore, and the contractor should be in attendance to discuss the work plan and project schedule and earthwork recommendations.

Site Preparation Site preparations should begin with the removal of existing site improvements, vegetation, utility lines, asphalt, concrete, and other deleterious debris from areas where excavation and/or grading is planned. Tree stumps and roots should be removed to such a depth that organic material is generally not present. Clearing and grubbing should extend to the outside of the proposed excavation and fill areas. The debris and unsuitable material generated during clearing and grubbing should be removed from areas to be graded and disposed of at a legal dumpsite away from the project area.

Remedial Earthwork Existing fill soils were encountered in each of our exploratory borings at the ground surface and extended to depths of up to approximately 24 feet. The undocumented fill was observed to be variable in density and is considered to be potentially compressible and unsuitable for the support of the planned structure loads in its current condition. In addition, laboratory testing indicates that some portions of the fill are prone to hydro consolidation. Therefore, in order to provide adequate support for proposed buildings and other settlement-sensitive structures, we recommend that the existing undocumented fill soils and upper portions of the alluvium, if encountered, within the building/structure pads be over-excavated and replaced

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 16 with compacted, engineered fill. For the purposes of this report, the building/structure pad is defined as the structural footprint of the building (including foundations for attached overhangs, canopies, and other building appurtenances) plus a horizontal distance of 5 feet.

In light of the above, we recommend that overexcavation be performed within the proposed structural footprints to a depth where the existing fill is removed, or 2 feet below the bottom of the foundations, whichever is deeper. This over excavation should extend to the horizontal limits of the building pad as previously defined, where feasible. The lateral extents of the overexcavation may be modified in the field based on site constraints such as existing structures and property lines. The extent and depths of removals and overexcavations should be evaluated by Ninyo & Moore’s representative in the field based on the materials exposed.

Subsequent to removal, the resulting surface should be scarified to a depth of approximately

6 inches, moisture conditioned, and recompacted to a relative compaction of 90 percent as evaluated by the ASTM International (ASTM) Test Method D 1557 prior to placing new fill.

Recompacted areas, structural fills, and backfill materials that are placed at a relative compaction of 90 percent as evaluated by ASTM Test Method D 1557 are considered to be in accordance with the standard of care in southern California. Once the resulting removal surface has been recompacted, the overexcavation should be backfilled with generally granular soils that possess a very low to low expansion potential (i.e., an expansion index

[EI] less than 50). These materials are anticipated to consist of import soils or the soils derived from onsite excavations that have been processed to meet the soil characteristics recommended in the “Materials for Fill” section of this report.

Excavation Characteristics The results of our field exploration program indicate that the project alignment is underlain by fill soils and alluvium, with materials comprising old paralic deposits and the San Diego Formation at depth. Excavation of the fill soils, alluvium, and much of the old paralic deposits should be generally feasible with heavy-duty excavation equipment in good working condition. Due to the potential for gravel, cobbles, and possible boulders within the subsurface soils, the contractor should anticipate encountering difficulty in performing excavations within these materials. The contractor should anticipate caving and/or sloughing conditions when performing excavations due to the presence of loose soil within the fill and alluvium.

Excavations that extend into the old paralic deposits and San Diego Formation may encounter highly cemented zones or concretions. Such zones can exhibit difficult ripping

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 17 conditions and the use of rock wheel/saw and/or rock breaking equipment should be anticipated. Disposal or crushing of the resulting oversized material should be anticipated.

Temporary Excavations For temporary excavations, we recommend that the following Occupational Safety and

Health Administration (OSHA) soil classifications be used:

Fill and Alluvium Type C Old Paralic Deposits Type B San Diego Formation Type A

Upon making excavations at the site, the soil classifications and excavation performance should be evaluated in the field by the geotechnical consultant in accordance with the OSHA regulations. Temporary excavations should be constructed in accordance with OSHA recommendations. For trenches or other excavations, OSHA requirements regarding personnel safety should be met using appropriate shoring (including trench boxes) or by laying back the slopes to no steeper than 1.5:1 (horizontal to vertical) in fill soils and alluvium.

Temporary slopes having an inclination of 1:1 (horizontal to vertical) or flatter may be construction within materials comprising old paralic deposits. A ¾:1 (horizontal to vertical) temporary slope may be constructed in the San Diego Formation. Temporary excavations that encounter seepage may be shored or stabilized by placing sandbags or gravel along the base of the seepage zone. Excavations encountering seepage should be evaluated on a case-by-case basis. Onsite safety of personnel is the responsibility of the contractor.

Temporary Shoring In areas with limited space for construction (where temporary excavations may not be laid back at the recommended temporary slope inclination), a shoring system may be utilized to stabilize the excavation sidewalls during construction. Shoring systems are anticipated to be constructed through fill and alluvium. The shoring system should be designed using the magnitude and distribution of lateral earth pressures presented on Figure 7 for braced shoring and Figure 8 for cantilever shoring. The recommended design earth pressures are based on the assumptions that: (a) the shoring system is constructed without raising the ground surface elevation behind the shoring; (b) that there are no surcharge loads, such as soil stockpiles, construction materials, or vehicular traffic; and (c) that no loads act above a 1:1 plane extending up and back from the base of the shoring system. For shoring subjected to the

Ninyo & Moore | 2995 Clearwater Way, San Diego, California | 109539001 | May 31, 2023 18 above-mentioned surcharge loads, the contractor should include the effect of these loads on lateral earth pressures acting on the shoring wall.

Settlement of the ground surface may occur behind the shoring wall during excavation.