04_-_Geotechnical_Report_02132015.pdf

R E P O R T

GEOTECHNICAL INVESTIGATION

SOUTH BAY INTERNATIONAL

WASTEWATER TREATMENT PLANT

IMPROVEMENTS

SAN YSIDRO, CALIFORNIA

Prepared for

U.S. International Boundary and Water Commission (USIBWC) 4171 North Mesa, Suite C100 El Paso, Texas 79902-1441 Task Order No. IBM14T0013

URS Project No. 41011854.01

February 13, 2015

4225 Executive Square, Suite 1600 La Jolla, California 92037

858.812.9292 Fax: 858.812.9293

TABLE OF CONTENTS

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

1.1 Project Description ................................................................................................ 1-1

1.2 Background ............................................................................................................ 1-1

1.3 Purpose and Scope of Investigation ....................................................................... 1-2

Section 2 Geotechnical Investigation ............................................................................2-1

2.1 Previous Studies..................................................................................................... 2-1

2.2 Subsurface Exploration .......................................................................................... 2-1

2.3 Laboratory Testing ................................................................................................. 2-1

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

3.1 Geologic Setting .................................................................................................... 3-1

3.2 Tectonic Setting ..................................................................................................... 3-1

3.2.1 Faulting ..................................................................................................... 3-2

3.2.2 Historical Seismicity ................................................................................. 3-2

3.3 Surface Conditions ................................................................................................. 3-3

3.4 Geologic Units ....................................................................................................... 3-3

3.4.1 Fill ............................................................................................................. 3-3

3.4.2 Surficial Deposits ...................................................................................... 3-3

3.5 Groundwater .......................................................................................................... 3-4

Section 4 Discussions, Conclusions and Recommendations ......................................4-1

4.1 Seismic Hazards..................................................................................................... 4-1

4.1.1 Fault Rupture ............................................................................................ 4-1

4.1.2 Strong Ground Motions ............................................................................ 4-1

4.1.3 Soil Liquefaction and Secondary Effects .................................................. 4-2

4.1.4 Secondary Effects of Seismic Activity ..................................................... 4-2

4.2 Geologic Hazards ................................................................................................... 4-3

4.3 Design Groundwater Level .................................................................................... 4-3

4.4 Below Grade Tank Walls ....................................................................................... 4-3

4.4.1 Subsurface Drainage ................................................................................. 4-4

4.4.2 Backfill ...................................................................................................... 4-4

4.5 Shallow Foundations ............................................................................................. 4-4

4.6 Concrete Slabs-on-Grade ....................................................................................... 4-5

4.7 Pavements .............................................................................................................. 4-5

4.8 Corrosion Potential ................................................................................................ 4-5

4.9 Earthwork .............................................................................................................. 4-6

4.9.1 Subgrade Preparation ................................................................................ 4-6

4.9.2 Fill Materials ............................................................................................. 4-6

4.9.3 Fill Placement and Compaction ................................................................ 4-7

4.10 Construction Considerations .................................................................................. 4-7

4.10.1 Excavation Characteristics ........................................................................ 4-7

4.10.2 Temporary Slopes ..................................................................................... 4-7

4.10.3 Protection of Existing Structures .............................................................. 4-8

4.10.4 Groundwater Control ................................................................................ 4-8

4.10.5 Monitoring of Deep Excavations .............................................................. 4-8

TABLE OF CONTENTS

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4.10.6 Construction Observation and Testing ...................................................... 4-8

Section 5 Limitations .......................................................................................................5-1

Section 6 References ......................................................................................................6-1

List of Figures and Appendices

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Figures

Figure 1 Vicinity Map Figure 2 Site Plan Figure 3 Regional Geologic Map Figure 4 Regional Fault and Epicenter Map Figure 5 Typical Lateral Pressures for Below Grade SST Walls Figure 6 Typical Lateral Pressures for Below Grade EQ Tank Walls

Appendices

Appendix A Previous Geotechnical Explorations Appendix B Subsurface Exploration Appendix C Geotechnical Laboratory Testing

List of Acronyms and Abbreviations

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AC Asphalt Concrete ACI American Concrete Institute ASCE American Society for Civil Engineers ASTM ASTM International (formerly American Society for Testing and Materials) Cal/OSHA California OSHA CBC California Building Code CRR Cyclic Resistance Ratio CSR Cyclic Stress Ratio EI Expansion Index FHWA Federal Highway Administration ft feet H:V horizontal:vertical MSL Mean Sea Level NAVD88 North American Vertical Datum of 1988 PCA Portland Cement Association’s PCC Portland Cement Concrete pcf pounds per cubic foot PGA Peak Ground Acceleration psf pounds per square foot sf square foot SPT Standard Penetration Test tcf tons per cubic foot TI Traffic Index URS URS Corporation Americas USGS U.S. Geological Survey WCC Woodward Clyde Consultants

SECTIONONE Introduction

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SECTION 1 INTRODUCTION

This report presents the findings from the geotechnical investigation completed by URS Group, Inc.

(URS) for the proposed improvements at the South Bay International Wastewater Treatment Plant (SBIWTP). The site is located in San Ysidro, California immediately north of the International Border between the United States and Mexico. Figure 1, Vicinity Map shows the location of the site.

1.1 PROJECT DESCRIPTION

Our understanding of the project is based on the “in-progress” 60 percent design plans being prepared by URS. The purpose of the proposed project is to provide infrastructure improvements at the SBIWTP. To accomplish this, it is proposed to design and construct an offline Equalization (EQ) Tanks with a capacity of 3.5 million gallons, and three additional Secondary Sedimentation Tanks (SSTs), along with associated appurtenances, and connections. The proposed features are shown on Figure 2, Site Plan.

URS was provided with the as-built drawings of the existing treatment plant dated June 18, 2008, prepared by S&B Infrastructure, Ltd. Based on a review of the as-built drawings, the existing SSTs are each 20 feet wide. The bottom of the SSTs is 18 to 20 feet deep (elevation 39 to 40 feet North American Vertical Datum [NAVD] 88). The invert level of the influent channel is 18 feet deep (elevation 40 feet NAVD88) and that of the effluent gallery is approximately 30 feet deep (approximate elevation 27 feet

NAVD88).

The proposed SSTs, associated influent and effluent channels, and the effluent gallery will be designed similar to the existing features and will be located 13 feet west of the existing structures. Portions of the existing slope and access roads will be relocated to accommodate the new SSTs.

The proposed EQ Tanks are 145 by 220 feet in plan dimensions. The bottom of the tanks ranges between 18 and 20 feet deep (elevation ranging between 36 to 38 feet NAVD 88); the effluent channel at the northern end of the tanks is about 27 feet deep (elevation 30 feet NAVD88).

1.2 BACKGROUND

URS (as its predecessor Woodward Clyde Consultants (WCC) performed the geotechnical investigation for the design of the original SBIWTP in 1994. The results were summarized in a report titled “Geotechnical Investigation for the Proposed South Bay International Wastewater Treatment Plant, San Diego County, California,” dated March 18, 1994 (WCC, 1994). This investigation identified a liquefaction potential hazard at the site and provided ground modification alternatives, of which Deep Dynamic Compaction (DDC) was recommended as the preferred alternative.

We understand USIBWC adopted DDC for mitigation of liquefaction potential. Phase 1 of the DDC was performed prior to construction of the original treatment plant (period of DDC unknown), which has been identified as Phase 1 in available references (discussed below). Records of completion of DDC or the limits of the mitigation area were not available for our review at the time this report was prepared.

Phase 2 of the DDC was performed in the previous expansion area of the treatment plant in 2009 (area of existing SSTs). Records of completion of the Phase 2 DDC were not available for our review either;

SECTIONONE Introduction

J:\41011854 USIBWC - South Bay\014 WORK IN PROGRESS\Geotechnical Final Report\2015-02-13 SBIWTP Geotechnical Report (URS).docx 1-2 however, the SPT-Based Compliance Evaluation of Deep Dynamic Compaction study performed by Construction Testing & Engineering, Inc. (CTE, 2009) was provided for our review. A major portion of the currently proposed SSTs are located within the area of the Phase 2 DDC based on the CTE reports.

While no confirmation of depth of treatment is available in construction documents provided for our review, WCC (1994) indicated that the subsurface soils between elevations of about 50 to 15 feet NAVD88 have a moderate to high potential for liquefaction and recommended the DDC for this interval.

Based on this information, we have interpreted that the bottom of the proposed tanks are located in this previously improved zone.

We understand that after the completion of both phases of DDC, the craters created by the DDC were backfilled with engineered fill. Partial daily field records of observation and testing of fill placement and compaction for Phase 2 DDC, prepared by CTE, were provided for our review. We also understand that a major portion of the SBIWTP has been graded level by placing and compacting imported fill. No records of fill placement and compaction were available for our review.

1.3 PURPOSE AND SCOPE OF INVESTIGATION

The purpose of this geotechnical investigation is to assess the site subsurface conditions and provide geotechnical recommendations for the design and construction of the above referenced improvements.

The scope of our services included:

• Supervising the drilling and logging of four test borings.

• Completing laboratory testing on samples obtained from the borings.

• Preparing this report with findings, conclusions and geotechnical recommendations.

SECTIONTWO Geotechnical Investigation

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SECTION 2 GEOTECHNICAL INVESTIGATION

The geotechnical investigation included reviewing and interpreting data from previous subsurface investigations and completing additional subsurface explorations and geotechnical laboratory testing.

2.1 PREVIOUS STUDIES

As mentioned in the previous section, WCC completed a geotechnical investigation for the original SBIWTP and associated transboundary fugitive surface water flow collection and conveyance systems.

As part of this investigation, WCC performed several types of subsurface explorations including but not limited to cone penetrometer tests (CPTs), air rotary borings, hollow stem auger borings, and test pits.

Several of these explorations are at or close to the currently proposed improvement locations, as shown on Figure 2, Site Plan. Appendix A provides the logs of the relevant previous subsurface explorations.

2.2 SUBSURFACE EXPLORATION

Current subsurface exploration consisted of logging and obtaining soil samples from four test borings (designated B1 through B4) that were advanced to depths ranging from 22 to 46 feet below ground surface (bgs). Figure 2 shows the locations of the subsurface explorations. Appendix B discusses the methods of subsurface exploration and provides logs of the test borings.

2.3 LABORATORY TESTING

Geotechnical laboratory testing was completed on representative soil samples to review the field classifications and to evaluate the physical and engineering characteristics of the subsurface materials.

Representative soil samples were tested for moisture content, dry unit weight, plasticity index, grain size distribution, shear strength (direct shear), stiffness (consolidation), subgrade strength (R-Value), and corrosion potential. Testing was performed in general accordance with ASTM International standards.

The test results are shown at the corresponding sample locations on the logs of test borings in Appendix B. Appendix C provides the laboratory test result data sheets.

SECTIONTHREE Site Conditions

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SECTION 3 SITE CONDITIONS

Knowledge of the site conditions was developed by reviewing local geologic information, and interpreting previous and current subsurface explorations.

3.1 GEOLOGIC SETTING

The site is located in the coastal plain sub-province of the Peninsular Ranges Physiographic Province.

The Peninsular Ranges consist of a series of elongated northwest-trending mountain ranges with intervening valleys and structural basins. The coastal plain runs parallel to the coastline and flanks the western edge of the Peninsular Ranges in San Diego County.

The coastal plain sub-province is characterized by a broad wedge of Tertiary age sedimentary deposits that increase in thickness to the west and which are capped by Quaternary age terrace deposits. The terrace deposits define broad, relatively flat mesa surfaces. Rivers and secondary drainages have subsequently incised these Quaternary and Tertiary deposits to create the mesa and valley physiography that is characteristic of the San Diego area.

The project site is located within the coastal plains sub-province and lies adjacent to and south of a major valley cut by the Tijuana River. This area also marks the southern margins of a broad lowland area referred to as the South Bay embayment. The notable highlands to the south of this prehistoric marine embayment are referred to as the Border Highlands area. This highland area is underlain by marine and non-marine sandstones and conglomerates of the Tertiary to Quaternary age San Diego Formation and capped by terrace conglomerates mapped as very old paralic deposits (Kennedy and Tan, 2008).

The area of the proposed improvements is underlain by Holocene and late Pleistocene age young alluvial flood-plain deposits, within the Tijuana River valley. Toward the southwest, the ground surface rises slightly and is underlain by Pleistocene age sediments derived from fluvial and alluvial fan type processes. These deposits are mapped as late to middle Pleistocene age old paralic deposits (Kennedy and Tan, 2008), which have been described as terrace deposits and interpreted to by roughly 120,000 years old. Figure 3 presents a regional geologic map.

3.2 TECTONIC SETTING

The tectonic setting of the San Diego/Tijuana area is influenced by crustal plate boundary interaction between the Pacific and North American lithospheric plates. As part of the San Andreas fault system, this crustal interaction occurs along a broad zone of northwest striking, predominantly right slip faults. This zone spans the width of the Peninsular Ranges and extends offshore into the California Continental Borderland Province. At the latitude of San Diego, this zone extends from the San Clemente fault zone, located approximately 44 miles offshore of the San Diego coastline, to the San Andreas fault, located about 96 miles northeast of San Diego (Figure 4).

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3.2.1 Faulting

Geologic, geodetic, and seismic data indicate that the faults along the eastern margin of the transform plate boundary, including the San Andreas fault zone, San Jacinto fault zone, and Imperial faults, and their associated branches, are currently the most active and appear dominant in accommodating the majority of the motion between the two adjacent plates. Farther west, a smaller portion of the relative strike-slip plate motion is being accommodated by predominantly northwest-striking right lateral strike-slip faults, including the Elsinore, Rose Canyon, San Miguel, and Agua Blanca fault zones; and offshore faults including the Coronado Bank, San Diego Trough, and San Clemente fault zones.

The closest mapped active faults to the project site are the Rose Canyon and the Coronado Bank faults, approximately 4 and 12 miles to the west, respectively. The Rose Canyon fault is capable of producing a maximum earthquake magnitude (Mw) of 7.2. The Coronado Bank fault is capable of producing an Mw of

7.6 (CGS, 2002). The faults farther west offshore include the San Diego trough, and the San Clemente faults, which are approximately 22 miles and 44 miles southwest of the site, respectively. The major active fault zones east of the project site include the Elsinore, San Jacinto, and San Andreas, which are approximately 47, 68, and 96 miles to the northeast, respectively. Many of these faults have experienced historic seismic activity.

Other faults that appear to be secondary faults related to the complex stepping and splaying character of the Rose canyon fault in the San Diego Bay area include the La Nacion, San Ysidro, Point Loma and possibly some of the faults observed in the Border Highlands area. Based on our current understanding of these faults they are not considered active by State of California guidelines (CGS, 2007).

3.2.2 Historical Seismicity

San Diego has experienced strong seismic shaking and minor damage from local and distant earthquakes, but none have been very destructive (Agnew and others, 1979). A large earthquake in 1862 may have been centered locally (Anderson and others, 1989), and some researchers have suggested the 1862 event could have been in or near San Diego Bay. Paleoseismic studies suggest that the last large event on the Rose Canyon fault may have occurred on the order of 300 years ago.

More than 75 miles to the east of San Diego is the Salton Trough, a structural depression with a very high rate of seismicity. High slip rate faults in the Salton Trough include the southern San Andreas, Imperial and San Jacinto faults. The Imperial fault has ruptured twice with magnitudes (Mw) of 6.9 and 6.4 in the last 70 years. The San Jacinto has displayed the highest activity level of any fault in the State of California. The Coyote Creek fault segment of the San Jacinto fault zone ruptured in April 1968; this event is known as the Borrego Mountain Earthquake, and produced an Mw of 6.5.

The April 4, 2010 El Major-Cucapah earthquake, an Mw 7.2 event, occurred in northern Baja, approximately 30 miles south-southeast of Calexico, California. The San Diego area experienced light to moderate shaking during this event.

Our previous report (WCC, 1994) identified a series of potentially active northeasterly trending faults (referred to as Border Highlands faults), as well as a set of northwesterly trending faults, located in the

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3.3 SURFACE CONDITIONS

The site of the proposed EQ Basin is relatively level and is covered with sparse dry vegetation. Based on the recent topographic survey, the elevation in this area ranges between 55 and 57 feet (NAVD88).

The site of the proposed SSTs gently slopes to the west and south, with elevations ranging from 56 to 58 feet (NAVD88). A paved access road is located along the western boundary of the site. A slope, with an approximate inclination of 2:1 Horizontal:Vertical (H:V), exists at the southwest corner of the site. An existing dirt road (Border Patrol Road) traverses beyond this slope in a northwest-southeastwardly direction.

3.4 GEOLOGIC UNITS

Based on available geologic information, the site is underlain by various episodes of fill placement (associated with the original site grading and following two phases of ground improvement) and surficial deposits including alluvium, alluvial fan, and old alluvial fan deposits. The following paragraphs briefly describe these units.

3.4.1 Fill

We understand fill was placed during construction of the original SBIWTP treatment plant. While there are no as-built records for compaction of the fill, given the time of placement (post 1994), it is likely the fill was properly placed and compacted. All of the current borings encountered this fill from the surface to depths ranging from about 10 to 25 feet. The soils obtained from drive sampling were observed to primarily consist of silty sand to sandy silt with varying amounts of gravel and cobbles. Based on sampler drive resistance, the relative density was generally medium dense to very dense.

3.4.2 Surficial Deposits

Unconsolidated surficial deposits are present in the site area and include alluvium, alluvial fan deposits, and old fan deposits.

The alluvial deposits are associated with the Tijuana River channel and are present beneath the proposed EQ Basin area. These alluvial soils were observed to consist primarily of sands and sandy gravels with cobbles and boulders. Based on sampler drive resistance, the relative density was generally dense to very dense sands. Our borings encountered refusal in the coarse-grained alluvial cobbles and boulders.

Although the relative density of the alluvial materials in their natural state was expected to be loose to medium dense, the denser alluvium encountered in our borings is interpreted to be the result of DDC performed previously at the site.

Fan deposits were mapped in the area underlying the proposed SSTs during the previous geotechnical investigation. These deposits are generally derived from the conglomeratic San Diego Formation located upslope and are comprised of generally granular materials. Clayey fine sand, silty fine sand and silty fine

J:\41011854 USIBWC - South Bay\014 WORK IN PROGRESS\Geotechnical Final Report\2015-02-13 SBIWTP Geotechnical Report (URS).docx 3-4 sand with clay, with localized occurrences of sandy lean clay, was observed in subsurface investigations.

Older fan deposits underlie the fan deposits and were observed to be, poorly graded gravel with cobbles, silty sand and layers of sandy lean clay. The relative density was very dense based drive sampler resistance.

3.5 GROUNDWATER

Groundwater was encountered during drilling in Boring B-2 at a depth of 28.5 feet, which corresponds to an elevation of approximately 26 feet NAVD88. Previous explorations at the SBIWTP site encountered groundwater at elevations ranging between 29 to 37 feet NAVD88. Groundwater conditions at this site are highly subject to flow in the Tijuana River and the contributions from its tributaries. Other localized perched groundwater conditions may also exist within the site.

WCC (1994) expected maximum seasonal groundwater elevations to be on the order of 37 to 42 feet NAVD88, based on monitoring well data and historic groundwater data.

SECTIONFOUR Discussions, Conclusions and Recommendations

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SECTION 4 DISCUSSIONS, CONCLUSIONS AND

RECOMMENDATIONS

The discussions, conclusions, and recommendations presented in this report are based on the information provided to us, results of previous and current subsurface explorations and laboratory testing, engineering evaluations and analyses, literature research, empirical correlations, and professional judgment.

The following sections of this report provide recommendations for design and a discussion of construction considerations.

4.1 SEISMIC HAZARDS

4.1.1 Fault Rupture

In our opinion, the site is not underlain by an active fault and the fault rupture hazard is considered low.

Faults present in the Border Highlands adjacent to the site are potentially active but are not considered significant fault rupture hazards based on fault trenching and geomorphic evaluations performed for the previous investigation.

4.1.2 Strong Ground Motions

The site could be subject to moderate to strong ground shaking in response to a local or more distant, large-magnitude earthquake occurring during the expected life span of the development. Seismic design parameters in accordance with the 2013 California Building Code (CBC) are presented in the table below.

These parameters are developed based on Risk-Targeted Maximum Considered Earthquake (MCER) ground motion accelerations.

2013 CBC Seismic Coefficients South Bay International Wastewater Treatment Plant Improvements

Parameter Value Reference

Site Class D ASCE 7-10, Table 20.3-1

Mapped Spectral Acceleration - Short Period, Ss (g) 0.957 2013 CBC Figure 1613.3.1(1)1

Mapped Spectral Acceleration - 1 Sec. Period, S1 (g) 0.359 2013 CBC Figure 1613.3.1(2)1

Site Coefficient - Short Period, Fa 1.117 2013 CBC Table 1613.3.3(1)1

Site Coefficient - 1 Sec. Period, Fv 1.681 2013 CBC Table 1613.3.3(2)1

MCE2 Spectral Response Acceleration - Short Period, SMS (g) 1.069 2013 CBC Equation 16-37, SMS=FaSS

MCE2 Spectral Response Acceleration - 1 Sec. Period, SM1 (g) 0.604 2013 CBC Equation 16-38, SM1=FvS1

Design Spectral Response Acceleration - Short Period, SDS (g) 0.713 2013 CBC Equation 16-39, SDS=2/3*SMS

Design Spectral Response Acceleration - 1 Sec. Period, SD1 (g) 0.403 2013 CBC Equation 16-40, SD1=2/3*SM1

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Parameter Value Reference Notes:

1. Calculated using U.S. Seismic Design Maps web application developed by USGS.

2. MCE – Maximum Considered Earthquake.

3. Site coordinates estimated from ‘Google Earth’ computer program used to evaluate coefficients: 32.5426; -117.0633.

4.1.3 Soil Liquefaction and Secondary Effects

Liquefaction is a phenomenon where loose, saturated coarse-grained soils (with less than 50% passing the No. 200 sieve) lose their strength and acquire some mobility from strong ground motion induced by earthquakes. The secondary effects of liquefaction include sand boils, settlement, reduced soil shear strength, lateral spreading and global instability (flow slides) in areas with sloping ground.

Calculations to evaluate the potential for liquefaction were completed in general accordance with the guidelines provided by Idriss and Boulanger (2008). The results of the analyses indicate a very low potential for liquefaction at the location of EQ Tanks, and that up to 2 inches of liquefaction-induced settlement may occur in the area of the new SSTs, at a depth of 15 to 20 feet bgs. This estimate was based on an anomalous low blowcount encountered in a current boring (B4), which was located in the area previously improved by DDC based on CTE report (CTE, 2009). In our opinion, such an occurrence of loose soil is likely to be localized to an area that the DDC did not fully impact. Further, the bottom of the tanks and the associated channels and galleries will be located deeper than this depth. Hence, a potential for liquefaction to impact the proposed improvements should be low.

The potential for liquefaction was assessed at each current exploration in terms of a factor of safety. This factor of safety is defined as the Cyclic Resistance Ratio (CRR) required to resist liquefaction divided by the Cyclic Stress Ratio (CSR) generated by the design ground motion. A potential for liquefaction is interpreted to exist at each subsurface layer where the factor of safety against liquefaction is less than 1.1.

The empirical method proposed by Tokimatsu and Seed (1987) was used to calculate liquefaction-induced settlement. This method correlates the volumetric strain in liquefied soils to Standard Penetration Test (SPT) blowcount data and CSR.

The following input parameters were used:

• Earthquake Magnitude, Mw: 7.2

• Peak Ground Acceleration (PGA): 0.44g (Based on ASCE 7-10)

• Groundwater Level: +42 feet NAVD88

Considering the relative density of the fill above groundwater, the amount of seismic compaction should be low. Seismic compaction is the settlement that can occur in loose, relatively clean granular deposits above the water table during strong ground motion.

4.1.4 Secondary Effects of Seismic Activity

The secondary effects of seismic activity include tsunamis and seiches. Tsunamis are seismically induced waves generated by sudden movements of the ocean bottom during submarine earthquakes, landslides, or

J:\41011854 USIBWC - South Bay\014 WORK IN PROGRESS\Geotechnical Final Report\2015-02-13 SBIWTP Geotechnical Report (URS).docx 4-3 volcanic activity. Seiches are similarly generated but are waves in lakes or reservoirs. Based on the inland location, site elevation, and the location of the nearest large lake, the potential for damage due to a tsunami or seiche is non-existent.

The potential for slope instability from strong ground shaking is low since the relocated cut/fill slope near the proposed SSTs is relatively small and will be formed at an inclination of 2:1 H:V or flatter.

4.2 GEOLOGIC HAZARDS

Based on the relatively flat topography of the site, landslides should not be considered a potential hazard at the site. Collapsible soils are not present within the site and do not constitute a significant hazard to development. Due to the presence of predominately granular materials on site, expansion potential is not considered a hazard.

4.3 DESIGN GROUNDWATER LEVEL

Since fluctuations of the groundwater level are likely to occur due to seasonal effects, flow in the Tijuana River, and other factors, the maximum long-term groundwater level should be assumed to be at +42 feet

NAVD88.

4.4 BELOW GRADE TANK WALLS

The SSTs, EQ Tanks and their associated channels and galleries will be embedded to depths ranging from 18 to 33 feet below grade. We understand the walls of the SSTs will be designed for restrained conditions and those of the EQ Tanks will be designed to allow movement at the top of the walls.

These below grade walls should be designed for earth, seismic, hydrostatic and surcharge pressures.

Figures 5 and 6 present depictions of these pressures to design the walls at SSTs and EQ Tanks, respectively.

The earth pressures on these figures assume the walls will have a vertical back and will mainly retain properly processed, placed and compacted coarse grained soils (as recommended in Section 4.4.2). They also assume compaction within four feet of the wall will be completed with light hand-held or equivalent equipment; the lateral pressures would be higher if heavy equipment is used for soil compaction next to the walls.

The seismic pressure increment can be estimated as an equivalent fluid weighing 8 and 18 pounds per cubic foot (pcf) for SST and EQ Tanks, respectively. To estimate seismic pressure, the seismic pressure increment should be added to the active earth pressure.

a) SST Walls - The seismic earth increment for this restrained wall was developed by considering recommendations by Lew and others (2010), which provided a comprehensive review of the methods used to assess seismic earth pressures on deep subterranean retaining walls along with recommendations for design. The seismic pressure increment was developed with the use of a conservative estimate of the pseudostatic acceleration coefficient (Kh) of 0.25 times the PGA as recommended by Lew and others (2010).

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b) EQ Tank Walls – The seismic earth increment for this free-to-deflect wall was developed using Mononobe-Okabe method (Mononobe and Matsuo, 1929 and Okabe, 1926), with Kh equal to 0.5 times the PGA. This pressure should be added as an inverted triangular distribution to the active pressure, as shown on Figure 6.

4.4.1 Subsurface Drainage

The design of subterranean walls above elevation +42 feet NAVD88 should provide an adequate drainage system behind the wall to collect water from possible transient sources, such as surface runoff or leaking underground utilities, to reduce the potential for hydrostatic pressure buildup behind the wall. Drainage may be provided using free draining material (a minimum of 12 inches wide, measured horizontally immediately behind the wall, and consisting of ¾-inch crushed rock or gravel wrapped in filter fabric) along the back of the wall. Panel drainage systems, such as Miradrain 6000 or equivalent, may be used as an alternative. The collected groundwater will then drain into the natural soils.

4.4.2 Backfill

Backfill placed behind the below grade walls should meet the criteria listed below:

• No oversize materials greater than 6 inches in maximum dimension.

• An Expansion Index (EI) less than 30 and a Plasticity Index less than 25.

• A relatively well-graded particle size distribution with at least 60 percent (by weight) passing a 1-inch sieve and fines content (percent, by weight, passing the No. 200 sieve) not exceeding 30 percent.

All backfill should be placed and compacted as recommended in the Earthwork section of this report.

4.5 SHALLOW FOUNDATIONS

We understand the influent and effluent channels, and other lightly loaded structures will be supported on conventional continuous strip and spread footings founded in engineered fill or alluvium.

All spread or continuous footings should bear directly on engineered fill or alluvial soils. Footings should be embedded at least 24 inches below lowest adjacent grade and should be at least 18 inches wide.

Footings dimensioned as recommended may be designed using an allowable soil bearing pressure of 3,500 pounds per square foot (psf). The allowable bearing pressure may be increased by one-third for loads that include wind or seismic forces.

For foundations designed as recommended above, the total settlement due to foundation loads only is estimated to be about one inch, with differential settlements across a structure estimated to be approximately half that amount.

Resistance to lateral loads on the shallow foundations can be provided by passive resistance along the edge of the foundation and by frictional resistance along the bottom of the foundation. An allowable equivalent fluid weight of 250 pounds per cubic foot (pcf) may be used for passive resistance for footings

J:\41011854 USIBWC - South Bay\014 WORK IN PROGRESS\Geotechnical Final Report\2015-02-13 SBIWTP Geotechnical Report (URS).docx 4-5 poured neat against properly placed and compacted engineered fill, or native alluvial materials. The upper 12 inches of material in areas not protected by hardscapes should not be included in the estimate of passive resistance. If friction is to be used to resist lateral loads, an allowable coefficient of friction of 0.4 may be used between foundation concrete and engineered fill or native alluvial materials. If it is desired to combine frictional and passive resistance, a friction coefficient of 0.3 may be used. Passive resistance may be increased by one-third for loads that include wind or seismic forces.

4.6 CONCRETE SLABS-ON-GRADE

Concrete slabs-on-grade such as those supporting the tanks may be designed using a modulus of subgrade reaction of 200 tons per cubic foot (tcf). The Structural Engineer should design the thickness and reinforcement of concrete slabs-on-grade to accommodate concentrated loads and heavy distributed loads.

Expansion joints and crack control sawcuts should be included at regular intervals. URS should be consulted to provide geotechnical design parameters for special load bearing slabs-on-grade as needed.

4.7 PAVEMENTS

The relocated access road will be surfaced with Asphalt Concrete (AC) pavement. The structural design of AC pavement depends primarily on anticipated traffic conditions, subgrade soils, and construction materials. The laboratory R-Value test performed on a representative sample of the near-surface soils resulted in an R-Value of 12.

The table below provides recommended flexible pavement structural sections for assumed Traffic Indices of 5.0, 6.0 and 7.0, and a design R-Value of 12 for the subgrade.

Flexible Pavement Recommendations South Bay International Wastewater Treatment Plant Improvements

Traffic Index Asphalt Thickness (in)

Base Thickness (in)

5.0 3 9

6.0 4 10

7.0 4 14

The design assumes a pavement life of 20 years with normal maintenance. The sections recommended above assume properly prepared subgrade consisting of at least 12 inches of soil compacted to a minimum of 95% relative compaction, as determined by the latest version of ASTM D1557. The aggregate base should be placed at a minimum relative compaction of 95%. Aggregate base should conform to Section 26 of the Caltrans Standard Specifications or Section 200-2 of the “Standard Specification for Public Works Construction”.

4.8 CORROSION POTENTIAL

Laboratory corrosion suite tests performed on representative samples resulted in a resistivity between 900 and 1,200 ohm-centimeter (ohm-cm). It has been our experience with local Corrosion Engineers that

J:\41011854 USIBWC - South Bay\014 WORK IN PROGRESS\Geotechnical Final Report\2015-02-13 SBIWTP Geotechnical Report (URS).docx 4-6 resistivity results between 500 and 1,000 ohm-cm may be considered corrosive to metallic utility piping and conduits, and that resistivity between 1,000 and 2,000 ohm-cm may be considered moderately corrosive. A Corrosion Engineer should be consulted for additional design recommendations.

The results of the tests indicate the soil has a low to moderate potential for chloride attack, and a negligible potential for sulfate attack to concrete. Table 7-2 the Portland Cement Association’s (PCA) publication “Design and Control of Concrete Mixtures”, states that sulfate exposure from concentrations less than 0.10 percent is considered negligible (PCA, 1988).

4.9 EARTHWORK

Earthwork is expected to primarily consist of: a) mass excavation for the proposed below grade tanks and channels, b) forming the permanent cut/fill slope south of the new SSTs, c) backfilling the temporary excavations and underground utilities, and d) preparing the subgrade of the relocated access road.

Earthwork should be completed according to applicable County of San Diego Grading requirements, the California Building Code, and the Standard Specifications for Public Works Construction (i.e., Greenbook), most recent editions.

The following sections provide further recommendations for the general earthwork at the site.

4.9.1 Subgrade Preparation

Weeds, grass, trees, shrubs and construction debris within areas that are to be graded should be cleared and properly disposed of off-site. Roots and other vegetative matter should be removed and disposed either offsite or stockpiled for reuse in landscape areas. Existing infrastructure should be properly demolished and disposed at an appropriate facility offsite.

The bottom of the excavation should be observed by a Geotechnical Engineer or their field designate.

Localized loose or soft soils may be encountered at the bottom of the excavations, and such materials should be removed to a depth of 2 feet and recompacted to the satisfaction of the Geotechnical Engineer or their field designate. For planning purposes, we have estimated that loose/soft areas do not exceed 2 feet below the bottom of the excavation. Fill should be compacted in accordance with the recommendations presented in Section 4.9.3.

Following the clearing of weeds, grass and debris, and upon completion of removals, the surface within areas to receive fill should be scarified, moisture conditioned as necessary, and compacted prior to fill placement.

Areas temporarily vacated during earthwork should be similarly scarified, moisture conditioned and reworked to the satisfaction of a Geotechnical Engineer before placing additional fill to avoid drying out and lamination along the fill interface.

4.9.2 Fill Materials

Except for surficial organic materials, the onsite materials should be suitable for use as engineered fill.

On-site materials should be screened for particles greater than six inches in maximum dimension.

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Import materials are not expected to be needed, except for retaining wall backfill. A Geotechnical Engineer should review and test all import sources should they become necessary.

4.9.3 Fill Placement and Compaction

Fill soils should be placed in loose horizontal lifts that do not exceed 8 inches. Each layer should be spread evenly and thoroughly mixed during spreading to obtain a uniform moisture content and soil composition.

After each layer has been mixed and spread, it should be compacted to a relative compaction indicated by tests to be not less than 90 percent. Relative compaction is defined as the ratio (expressed in percent) of the in-place dry unit weight of the compacted fill divided by the maximum dry unit weight evaluated in accordance with ASTM D1557. Fill materials should be compacted at a moisture content maintained above the optimum moisture content determined in accordance with the above test method.

4.10 CONSTRUCTION CONSIDERATIONS

4.10.1 Excavation Characteristics

Excavations will be primarily in the engineered fill and dense to very dense alluvium. These materials contain varying amounts of gravel and cobbles, and interbeds of gravel. These materials should require moderate to heavy effort to excavate. Since the on-site materials are generally granular, they should be considered prone to caving and/or sloughing.

4.10.2 Temporary Slopes

The design and construction of temporary slopes, as well their maintenance and monitoring during construction, is the responsibility of the Contractor. The Contractor should have a geotechnical or geological professional evaluate the soil/rock conditions encountered during excavation to determine permissible temporary slope inclinations and other measures as required by California OSHA (Cal/OSHA). The Contractor's geotechnical or geological professional may use the factual information provided in this report, as well as any additional data they may need to acquire, to assess the stability of temporary slopes and prepare a specific temporary slope analysis and/or develop parameters to design temporary support systems.

Based on the existing data interpreted from site reconnaissance and subsurface explorations, the design of temporary slopes and benches for planning purposes may assume Cal/OSHA Soil Type ‘C’.

Existing infrastructure that is within a 2:1 H:V line projected up from the bottom edge (toe) of temporary slopes should be monitored during construction. Surface drainage should be directed away from the temporary slopes. Surface drainage should not run over the face of the temporary slopes or pond at the crest of the slopes. All sources of surcharge loads should be kept away from the crest of the temporary slope a horizontal distance that is greater than the maximum depth of excavation.

The Contractor should note the materials encountered in construction excavations could vary significantly across the site. The above assessment of Cal/OSHA soil type for temporary excavations is based on preliminary engineering classifications of material encountered in widely spaced excavations. The Contractor's geotechnical or geological professional should observe and map mass excavations and

J:\41011854 USIBWC - South Bay\014 WORK IN PROGRESS\Geotechnical Final Report\2015-02-13 SBIWTP Geotechnical Report (URS).docx 4-8 temporary slopes at regular intervals during excavation and assess the stability of temporary slopes, as necessary.

4.10.3 Protection of Existing Structures

Existing structures in close proximity to the proposed excavations, such as the buildings located at the northwest corner of the EQ Tanks and the northern end of the SSTs, should be properly protected in place during excavations for the proposed tanks. Shoring should be considered for supporting the effluent box located north of the proposed SSTs.

4.10.4 Groundwater Control

When construction extends below groundwater, the area ahead of the excavation must be dewatered to reduce the possibility of soil instability at the bottom of the excavation and to allow for dry, firm working conditions. The groundwater surface should remain at least 5 feet below the bottom of the excavation until the slabs and foundations are poured and adequate structure loads are in place to resist hydrostatic uplift. Based on current groundwater levels and proposed excavations, dewatering will need to achieve a groundwater drawdown of about 20 to 25 feet, with reference to a design groundwater elevation of 42 feet

NAVD88.

The Dewatering Contractor is responsible for designing, installing, and operating a dewatering system capable of lowering and maintaining the groundwater to the desired depths during construction. The Contractor should make their own interpretations and/or develop more specific design information using the factual information provided in this report.

Significant subsidence due to dewatering is not expected within the granular soils underlying the sites of the proposed improvements. Settlements should be estimated once the excavation and construction methods, and dewatering schemes are finalized.

4.10.5 Monitoring of Deep Excavations

The Contractor typically implements a monitoring program that should mitigate the risk of distress to existing infrastructure from potential horizontal or vertical movement (subsidence) of the ground surrounding the excavation to an acceptable level. The temporary slopes should b e inspected monthly, or more frequently during periods of inclement weather for evidence of movement and other forms of distress. Movement or other forms of distress should be reported to the Geotechnical Engineer.

4.10.6 Construction Observation and Testing

Geotechnical services during construction are anticipated to consist of the following:

• Continuous onsite observation and compaction testing by a Geotechnical Field Technician during earthwork.

• Observation by a Geotechnical Field Technician to verify that foundation excavations extend to proper depth and bearing strata.

• Preparation of an “As-Graded Soils and Geological Report.”

SECTIONFIVE Limitations

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SECTION 5 LIMITATIONS

We observed only a very small portion of the pertinent subsurface conditions. The recommendations made herein are based on the assumption that soil conditions do not deviate appreciably from those found during our field investigation.

Geotechnical engineering and geologic sciences are characterized by uncertainty. Professional judgments presented herein are based partly on our understanding of the proposed construction and partly on our general experience. Our engineering work and judgments rendered meet current professional standards;

we do not guarantee the performance of the project in any respect.

SECTIONSIX References

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SECTION 6 REFERENCES

Agnew, D. and others, 1979. “Earthquake History of San Diego in Earthquakes and Other Perils, San Diego Region,” Abbott, P.L. and Elliott, W.J. ed, p. 11-21.

Anderson, J. G., T. Rockwell, and D. C. Agnew, 1989. “A Study of the Seismic Hazards in San Diego,” Earthquake Spectra, Vol. 5, No. 2.

ASCE/SEI…