Geocon_-_Geotechnical_Report_-_Final.pdf

GEOTECHNICAL INVESTIGATION

PROPOSED PARKING STRUCTURE

AND MEZZANINE

1382 BELL AVENUE

TUSTIN, CALIFORNIA

JANUARY 20, 2026

PROJECT NO. W1585-88-03

PREPARED FOR:

Orange County Public Works

Santa Ana, California

2807 McGaw Avenue ■ Irvine, CA 92618 ■ Telephone (949) 491-6570 ■ oc@geoconinc.com

Project No. W1585-88-03 January 20, 2026

Orange County Public Works c/o Casey Garrison 601 N. Ross Street, 4th Floor Santa Ana, CA 92701

Subject: GEOTECHNICAL INVESTIGATION

PROPOSED PARKING STRUCTURE AND MEZZANINE

1382 BELL AVENUE, TUSTIN, CALIFORNIA

Ladies and Gentlemen:

In accordance with your authorization of our proposal dated June 24, 2025, we have performed a geotechnical investigation for the proposed 3- to 4 story parking structure and mezzanine project located at 1382 Bell Avenue in Tustin, California. The accompanying report presents the findings of our study and our conclusions and recommendations pertaining to the geotechnical aspects of proposed design and construction. Based on the results of our investigation, it is our opinion that the improvements can proceed as proposed, provided the recommendations of this report are followed and implemented during design and construction.

If you have any questions regarding this report, or if we may be of further service, please contact the undersigned.

Very truly yours, GEOCON WEST, INC.

Andrew Sover, M.S.

PE 92879

Jelisa Adams

GE 3092

Gerald A. Kasman

CEG 2251

(email) Addressee

1382 Bell Avenue

Geotechnical Investigation

TABLE OF CONTENTS

1. PURPOSE AND SCOPE

2. SITE AND PROJECT DESCRIPTION

3. BACKGROUND REVIEW

4. GEOLOGIC SETTING

5. SOIL AND GEOLOGIC CONDITIONS

5.1 Artificial Fill

5.2 Young Alluvium

6. GROUNDWATER

7. GEOLOGIC HAZARDS

7.1 Surface Fault Rupture

7.2 Seismicity

7.3 Seismic Design Criteria

7.4 Liquefaction Potential

7.5 Slope Stability

7.6 Earthquake-Induced Flooding

7.7 Tsunamis, Seiches, and Flooding

7.8 Oil Fields & Methane Potential

7.9 Subsidence

8. CONCLUSIONS AND RECOMMENDATIONS

8.1 General

8.2 Soil and Excavation Characteristics

8.3 Minimum Resistivity, pH, and Water-Soluble Sulfate

8.4 Grading

8.5 Shrinkage

8.6 Mat Foundation Design - Parking Structure

8.7 Conventional Foundation Design - Interior Mezzanine

8.8 Miscellaneous Foundations

8.9 Lateral Design

8.10 Concrete Slabs-on-Grade

8.11 Preliminary Pavement Recommendations

8.12 Retaining Wall Design

8.13 Retaining Wall Drainage

8.14 Elevator Pit Design

8.15 Elevator Piston

8.16 Temporary Excavations

8.17 Surcharge from Adjacent Structures and Improvements

8.18 Stormwater Infiltration

8.19 Surface Drainage

8.20 Plan Review

LIMITATIONS AND UNIFORMITY OF CONDITIONS

LIST OF REFERENCES

TABLE OF CONTENTS (CONTINUED)

MAPS AND ILLUSTRATIONS

Figure 1, Vicinity Map Figure 2, Site Plan Figure 3, Regional Fault Map Figure 4, Regional Seismicity Map Figure 5, Overall MCE Vertical Settlements Figures 6 and 7, Retaining Wall Drainage Detail – Mat Foundation Figures 8 and 9, Retaining Wall Drainage Detail – Conventional Foundation Figure 10 and 11, Percolation Test Results

APPENDIX A

FIELD INVESTIGATION

Figures A1 through A8, Boring Logs Figures A9 through A12, CPT Logs

APPENDIX B

LABORATORY TESTING

Figures B1 through B4, Direct Shear Test Results Figures B5 through B14, Consolidation Test Results Figures B15 and B16, Expansion Index Test Results Figures B17 and B18, Modified Compaction Test Results Figures B19 and B20, Corrosivity Test Results

APPENDIX C

SITE RESPONSE ANALYSIS REPORT

APPENDIX D

CPT LIQUEFACTION ANALYSIS

APPENDIX E

DATA BY OTHERS

Project No. W1585-88-03 - 1 - January 20, 2026

GEOTECHNICAL INVESTIGATION

1. PURPOSE AND SCOPE

This report presents the results of a geotechnical investigation for the 3- to 4-story parking structure and mezzanine project located at 1382 Bell Avenue in Tustin, California (see Vicinity Map, Figure 1).

The purpose of the investigation was to evaluate subsurface soil and geologic conditions underlying the areas of proposed construction and, based on conditions encountered, to provide conclusions and recommendations pertaining to the geotechnical aspects of proposed design and construction.

The scope of this investigation included a site reconnaissance, field exploration, laboratory testing, engineering analysis, and the preparation of this report. The site was initially explored on October 15, 2025, by excavating five 3-inch-diameter borings using hand auger and manual digging equipment to maximum depths of approximately 16 feet beneath the existing ground surface. Additionally, four cone penetration tests (CPTs) were advanced using a 30-ton CPT rig to depths of approximately 100 feet beneath the ground surface. Percolation testing was performed in two borings to measure the infiltration rate of site soils.

The site was further explored on December 13, 2025, by coring the interior concrete slab of the existing structure and excavating two 3-inch diameter borings using hand auger and manual digging equipment to depths of approximately 10½ feet beneath the existing concrete slab. Two attempts were made to core at location B6, but it was found that additional concrete was present under the slab-on-grade.

An attempt was made to core through the additional concrete object but was unsuccessful as the concrete object and slab were in excess of a combined 15 inches of thickness.

The approximate locations of the exploratory borings and CPT soundings are depicted on the Site Plan (see Figure 2). A detailed discussion of the field investigation, including boring and CPT logs, is presented in Appendix A.

Laboratory tests were performed on selected soil samples obtained during the investigation to evaluate pertinent physical and chemical soil properties. Appendix B presents a summary of the laboratory test results.

The recommendations presented herein are based on analysis of the data obtained during the investigation and our experience with similar soil and geologic conditions. References reviewed to prepare this report are provided in the List of References section.

Project No. W1585-88-03 - 2 - January 20, 2026

If project details vary significantly from those described herein, Geocon should be contacted to determine the necessity for review and possible revision of this report.

2. SITE AND PROJECT DESCRIPTION

The subject site is located at 1382 Bell Avenue in Tustin, California. The site is currently occupied by a two-story office building with associated paved parking. The site is bounded by a single-story building to the southeast, by a single-story commercial business park to the northwest and southwest, and Bell Avenue to the northeast. The site is relatively level, and surface water drainage at the site appears to flow to the city streets.

Based on the information provided by the Client, it is our understanding that the proposed project will consist of constructing a 3- to 4-story on-grade parking structure within the southwest area of the site as well as an interior mezzanine addition within the existing structure. The approximate footprints of the proposed improvements are shown on a satellite image of the site on the Site Plan (see Figure 2).

The project structural engineer has indicated that the mezzanine will be structurally independent from the existing structure and foundation system.

Based on the preliminary nature of the design at this time, wall and column loads were not available.

It is anticipated that column loads for the proposed parking structure will be up to 600 kips, and wall loads will be up to 6 kips per linear foot. It is anticipated that column loads for the proposed mezzanine structure will be up to 50 kips, and wall loads will be up to 2 kips per linear foot.

Once the design phase and foundation loading configuration proceeds to a more finalized plan, the recommendations within this report should be reviewed and revised, if necessary. Any changes in the design, location, or elevation of any structure, as outlined in this report, should be reviewed by this office. Geocon should be contacted to determine the necessity for review and possible revision of this report.

Project No. W1585-88-03 - 3 - January 20, 2026

3. BACKGROUND REVIEW

A prior geotechnical investigation was performed at the subject site by R McCarthy Consulting, Inc. and presented in a report dated July 21, 2021. The prior report was prepared for a seismic retrofit of the existing structure to support its use as a critical facility. Field work consisted of four exploratory borings to depths between 11.5 and 51.5 feet, four infiltration borings to approximate depths of 4.5 to 5 feet, and the advancement of four cone penetration tests (CPT) to depths between 50 and 100 feet below the ground surface. The locations of the explorations are presented on the Site Plan (Figure 2).

Groundwater was encountered at a depth of approximately 25 feet in boring B1 and a pore pressure dissipation test at CPT-1 suggested groundwater may be as shallow as 20 feet below the existing ground surface. Laboratory tests were performed on selected soil samples obtained during the explorations.

Raw data files of the prior CPTs were not available for our use. Copies of the prior boring and CPT logs are provided in Appendix E.

The prior report is referenced for completeness of background review; relevant information contained within the prior report by R McCarthy Consulting Inc., such as depth to groundwater, was considered.

However, the findings, conclusions, and recommendations presented herein are based on analyses of data obtained through our own site exploration, laboratory testing, and analyses. Geocon West, Inc. is the Geotechnical Consultant of Record and will be providing all necessary geotechnical consultation, plan review, design recommendations, inspection, and testing services for this project.

4. GEOLOGIC SETTING

The subject site is situated in the southeastern portion of the Orange County Coastal Plain, a relatively flat-lying alluvial surface with an average slope of less than 20 feet per mile. The lowland surface is bounded by the Puente Hills and the Chino Hills to the north, the Peralta Hills and Santa Ana Mountains to the east, the San Joaquin Hills to the south, and the Pacific Ocean to the south and southwest.

Prominent structural features within the Orange County Coastal Plain include the central lowland plain and the northwest trending line of low hills and mesas underlain by the Newport-Inglewood Fault Zone that include Newport Mesa, Huntington Beach Mesa, Bolsa Chica Mesa, and Landing Hill.

5. SOIL AND GEOLOGIC CONDITIONS

Based on published geologic maps of the area, the site is underlain by artificial fill and Holocene-age young alluvial fan deposits that consist of clay, silt, sand with lenses of gravel and cobbles (United States Geological Survey [USGS], 2006). Detailed stratigraphic profiles of the materials encountered at the site are provided on the boring and CPT logs in Appendix A.

Project No. W1585-88-03 - 4 - January 20, 2026

5.1 Artificial Fill

Artificial fill was encountered in our explorations to a maximum depth of 2½ feet below the existing ground surface. Fill was also encountered to a maximum depth of 5 feet below the existing ground surface in the prior borings by R. McCarthy Consulting (2021). The artificial fill generally consists of brown to dark gray sandy clay that can be characterized as slightly moist to moist and soft to stiff.

The fill is likely the result of past grading or construction activities at the site. Deeper fill may exist between excavations and in other portions of the site that were not directly explored.

5.2 Young Alluvium

Holocene-age young alluvial fan deposits were encountered beneath the fill in each boring to the maximum depth explored of 16 feet below existing ground surface. The alluvium generally consists of light brown to brown interbedded silty sand, sand, and sandy clay. The alluvial soils are characterized as slightly moist to moist and soft to stiff or loose to medium dense.

6. GROUNDWATER

Based on a review of the Seismic Hazard Zone Report for the Tustin 7.5 Minute Quadrangle (California Division of Mines and Geology [CDMG], 1998), historic high groundwater is reported at approximately 10 feet below the ground surface. Groundwater information presented in this document is generated from data collected in the early 1900’s to the late 1990s. Based on current groundwater basin management practices, it is unlikely that groundwater levels will ever exceed the historic high levels.

Groundwater was not encountered in our borings, drilled to a maximum depth of approximately 16 feet below the existing ground surface; however, perched groundwater was encountered at 14 feet in boring B5. Prior site explorations performed in 2021 reported groundwater depths ranging from 20 to 25 feet below the ground surface.

Based on the lack of groundwater in our borings, the reported historic high groundwater levels in the immediate area (CDMG, 1998), and the depth of proposed construction, static groundwater is not anticipated to be encountered unless deep excavations are required during construction. Furthermore, groundwater is not expected to have a detrimental effect on the project. It is not uncommon for groundwater levels to vary seasonally or for groundwater seepage conditions to develop where none previously existed (especially in impermeable fine-grained soils which are heavily irrigated or after seasonal rainfall), groundwater seepage levels encountered during construction may be actually higher than those encountered during our investigation. In addition, recent requirements for stormwater infiltration could result in shallower seepage conditions in the region. Proper surface drainage of irrigation and precipitation will be critical for future performance of the project. Recommendations for drainage are provided in the Surface Drainage section of this report (see Section 8.19).

Project No. W1585-88-03 - 5 - January 20, 2026

7. GEOLOGIC HAZARDS

7.1 Surface Fault Rupture

The numerous faults in Southern California include Holocene-active, pre-Holocene, and inactive faults.

The criteria for these major groups are based on criteria developed by the California Geological Survey (CGS, formerly known as CDMG) for the Alquist-Priolo Earthquake Fault Zone Program (CGS, 2018).

By definition, a Holocene-active fault is one that has evidence of surface displacement within Holocene time (about the last 11,700 years). A pre-Holocene fault has demonstrated surface displacement during Quaternary time (approximately the last 1.6 million years) but has had no known Holocene movement.

Faults that have not moved in the last 1.6 million years are considered inactive.

The site is not within a state-designated Alquist-Priolo Earthquake Fault Zone (CGS, 2025a; 2025b;

2017) for surface fault rupture hazards. No Holocene-active or pre-Holocene faults with the potential for surface fault rupture are known to pass directly beneath the site. Therefore, the potential for surface rupture due to faulting occurring beneath the site during the design life of the proposed development is considered low. However, the site is located in the seismically active Southern California region, and could be subjected to moderate to strong ground shaking in the event of an earthquake on one of the many active Southern California faults. The faults in the vicinity of the site are shown in Figure 3, Regional Fault Map.

The closest surface trace of an active fault to the site is the San Joaquin Fault located approximately 2 miles to south. Other nearby active faults are the Elsinore Fault and Puente Hills Fault, located approximately 14½ miles north and 19½ miles northwest of the site, respectively (USGS, 2006; Ziony and Jones, 1989). The active San Andreas Fault Zone is located approximately 45 miles northeast of the site (USGS, 2006; Ziony and Jones, 1989).

Several buried thrust faults, commonly referred to as blind thrusts, underlie the Southern California area at depth. These faults are not exposed at the ground surface and are typically identified at depths greater than 3.0 kilometers. The October 1, 1987, Mw 5.9 Whittier Narrows earthquake and the January 17, 1994 Mw 6.7 Northridge earthquake were a result of movement on the Puente Hills Blind Thrust and the Northridge Thrust, respectively. These thrust faults and others in the Los Angeles area are not exposed at the surface and do not present a potential surface fault rupture hazard at the site; however, these deep thrust faults are considered active features capable of generating future earthquakes that could result in moderate to significant ground shaking at the site.

Project No. W1585-88-03 - 6 - January 20, 2026

7.2 Seismicity

As with all of Southern California, the site has experienced historic earthquakes from various regional faults. The seismicity of the region surrounding the site was formulated based on research of an electronic database of earthquake data. The epicenters of recorded earthquakes with magnitudes equal to or greater than 5.0 in the site vicinity are depicted on Figure 4, Regional Seismicity Map.

A partial list of moderate to major magnitude earthquakes that have occurred in the Southern California area within the last 100 years is included in the following table.

LIST OF HISTORIC EARTHQUAKES

Earthquake Date of Earthquake Magnitude Distance to

Epicenter (Miles) Direction to

Epicenter (Oldest to Youngest) Long Beach March 10, 1933 6.4 39 ENE Tehachapi July 21, 1952 7.5 10 SW

San Fernando February 9, 1971 6.6 111 NW Whittier Narrows October 1, 1987 5.9 58 NW

Sierra Madre June 28, 1991 5.8 28 NW Landers June 28, 1992 7.3 39 NNW Big Bear June 28, 1992 6.4 87 ENE

Northridge January 17, 1994 6.7 67 ENE Hector Mine October 16, 1999 7.1 53 NW

Ridgecrest China Lake Fault July 5, 2019 7.1 108 ENE

The site could be subjected to strong ground shaking in the event of an earthquake. However, this hazard is common in Southern California and the effects of ground shaking can be minimized if the proposed structures are designed and constructed in conformance with current building codes and engineering practices.

7.3 Seismic Design Criteria

A site response analysis has been performed for the project based on Chapter 21 of ASCE 7-22.

The analysis incorporates the seismic CPT measurements performed at the site. The site response analysis report is included as Appendix C. The seismic design parameters generated from the site-response analysis are summarized in the following table.

Project No. W1585-88-03 - 7 - January 20, 2026

SEISMIC DESIGN PARAMETERS

Parameter Value

SMS

MCER 5% Damped Spectral Response Acceleration at Short

Periods Adjusted for Site Class 1.389g

SM1 MCER 5% Damped Spectral Response Acceleration at a Period of 1 Second Adjusted for Site Class 1.737g

SDS

Design, 5% Damped Spectral Response Acceleration at Short

Periods 0.926g

SD1 Design, 5% Damped Spectral Response Acceleration at a Period of 1 Second 1.158g

PGAM Maximum Considered Earthquake Geometric Mean (MCEG) Peak Ground Acceleration 0.66g

7.4 Liquefaction Potential

Liquefaction is a phenomenon in which loose, saturated, relatively cohesionless soil deposits lose shear strength during strong ground motions. Primary factors controlling liquefaction include intensity and duration of ground motion, gradation characteristics of the subsurface soils, in-situ stress conditions, and the depth to groundwater. Liquefaction is typified by a loss of shear strength in the liquefied layers due to rapid increases in pore water pressure generated by earthquake accelerations.

The current standard of practice, as outlined in the “Recommended Procedures for Implementation of DMG Special Publication 117, Guidelines for Analyzing and Mitigating Liquefaction in California” and “Special Publication 117A, Guidelines for Evaluating and Mitigating Seismic Hazards in California” requires liquefaction analysis to a depth of 50 feet below the lowest portion of the proposed structure.

Liquefaction typically occurs in areas where the soils below the water table are composed of poorly consolidated, fine- to medium-grained, primarily sandy soil. In addition to the requisite soil conditions, the ground acceleration and duration of the earthquake must also be of a sufficient level to induce liquefaction.

The State of California Seismic Hazard Zone Map for the Tustin Quadrangle (CDMG, 1998) indicates that the project property is located within an area identified as having a potential for liquefaction.

Also, according to the Safety Element of the Orange County General Plan (2004), the project property is located within an area identified as having a potential for liquefaction. The reported historic high groundwater level in the area is approximately 10 feet below ground surface (CDMG, 1998). Based on these considerations, a historic high groundwater depth of 10 feet was used in the liquefaction analyses.

Project No. W1585-88-03 - 8 - January 20, 2026

Liquefaction analyses of the CPT soundings were performed using the program CLiq (Version 3.5.4.2).

The analysis uses the 2001 NCEER method of analysis. This semi-empirical method is based on correlations with the data collected from the CPT soundings and includes consideration of dry-dynamic settlement above the water table.

The liquefaction analysis was performed for the Maximum Considered Earthquake level by using a groundwater level of 10 feet below the ground surface, a magnitude 7.7 earthquake, and a peak horizontal acceleration of 0.66g (PGAM). The results of the enclosed liquefaction analyses included herein for CPTs 1 through 4 indicate that the alluvial soils could be susceptible to the liquefaction induced settlements summarized in the table below during Maximum Considered Earthquake ground motion. A summary of the anticipated liquefaction induced settlements is provided as Figure 5;

calculations and output from CLiq are provided as Appendix D.

Liquefaction Induced Settlements Maximum Considered Earthquake

CPT Number CPT-1 CPT-2 CPT-3 CPT-4

Liquefaction Settlement (in)

0.4 0.24 0.58 0.31

The design recommendations provided in this report are intended to reduce the impact of the liquefaction hazard on the proposed development. The project owner should understand that the implementation of the recommendations presented in this report are not intended to completely prevent all damage to proposed structures and improvements during the occurrence of strong ground shaking. The intent is to provide recommendations which mitigate the hazard for structural collapse or loss of life due to strong seismic events. Complete mitigation of the hazard is not economically feasible.

The anticipated differential settlement is taken as the difference in the seismic settlement predicted at each CPT for the Maximum Considered Earthquake hazard level over the distance between the CPTs.

The resulting differential settlement is less than the differential settlement threshold for all Risk Categories and structure types per Table 12.13-3 in Section 12.13.9 of ASCE 7-22.

Therefore, the anticipated seismically induced settlements do not preclude the use of shallow foundations for this project. The project structural engineer is solely responsible for confirming that the foundation and superstructure are capable of accommodating the anticipated combined static and seismic total and differential settlements without the loss of the ability to support gravity loads.

Project No. W1585-88-03 - 9 - January 20, 2026

7.5 Slope Stability

As previously indicated, the site is relatively flat with less than 2 feet of vertical relief across the site.

The site is not located within an area identified as having a potential for slope instability (Orange County, 2004). Additionally, the site is not within an area identified as having a potential for seismic slope instability (CDMG, 1998). Based on this, the potential for slope stability hazards to impact the site is considered low.

The site is not located within zone of required investigation for earthquake-induced landslides (CDMG, 1998; CGS, 2025a). There are no known landslides near the site, nor is the site in the path of any known or potential landslides. Therefore, the potential landslide hazards to adversely affect the proposed development is also considered low.

7.6 Earthquake-Induced Flooding

Earthquake-induced flooding is inundation caused by the failure of dams or other water-retaining structures due to earthquakes. The Orange County Safety Element (2004) indicates that the site is not located within a designated dam inundation area. Therefore, the potential for inundation at the site, as a result of an earthquake-induced dam failure, is considered low.

7.7 Tsunamis, Seiches, and Flooding

The site is not located within a coastal area. Therefore, tsunamis are not considered a significant hazard at the site.

Seiches are large waves generated in enclosed bodies of water in response to ground shaking. No major water-retaining structures are located immediately up gradient from the project site. Therefore, flooding resulting from a seismically induced seiche is considered unlikely.

The site is within an area of minimal flooding (Zone X) as defined by the Federal Emergency Management Agency (FEMA, 2025).

7.8 Oil Fields & Methane Potential

Based on a review of the California Geologic Energy Management Division (CalGEM) Well Finder Website, the site is not located within an oil field. A plugged dry oil or gas well was documented within ½-mile of the site, the well was abandoned in October 1956 due to no commercial showings of oil or gas (CalGEM, 2025). Due to the voluntary nature of record reporting by the oil well drilling companies, wells may be improperly located or not shown on the location map and undocumented wells could be encountered during construction. Any wells encountered during construction will need to be properly abandoned in accordance with the current requirements of the CalGEM.

Project No. W1585-88-03 - 10 - January 20, 2026

Since the site is not located within the boundaries of a known oil field, the potential for the presence of methane or other volatile gases at the site is considered low. However, should it be determined that a methane study is required for the proposed development, it is recommended that a qualified methane consultant be retained to perform the study and provide mitigation measures as necessary.

7.9 Subsidence

Subsidence occurs when a large portion of land is displaced vertically, usually due to the withdrawal of groundwater, oil, or natural gas. Soils that are particularly subject to subsidence include those with high silt or clay content. The site is located within an area of historic subsidence related to groundwater pumping (USGS, 2025b). Subsidence related to groundwater pumping commonly occurs in such small magnitudes and over such large areas that it is generally imperceptible at an individual locality.

Accordingly, it affects only regionally extensive structures sensitive to slight elevation changes, such as canals and pipelines. The rate of elevation change is usually uniform over a large enough area that it does not result in differential settlements that would cause damage to individual buildings. Therefore, the potential for subsidence to adversely impact the proposed project is considered low.

Project No. W1585-88-03 - 11 - January 20, 2026

8. CONCLUSIONS AND RECOMMENDATIONS

8.1 General

8.1.1 It is our opinion that neither soil nor geologic conditions were encountered during the investigation that would preclude the construction of the proposed improvements, provided the recommendations presented herein are followed and implemented during design and construction.

8.1.2 At the time of preparing this report, site development plans were not available for our review.

Once plans are available, they should be provided to our office for review and incorporation into an updated report.

8.1.3 Up to 2½ feet of existing artificial fill was encountered during our site investigation, and up to 5 feet of artificial fill was encountered in prior site exploration. The existing fill encountered is believed to be the result of past grading and construction activities at the site.

Deeper fill may exist in other areas of the site that were not directly explored. Future demolition of the existing improvements which occupy the site will likely disturb the upper few feet of soil below those existing improvements. It is our opinion that the existing fill, in its present condition, is not suitable for direct support of proposed foundations or slabs.

If needed, the existing fill and site soils are suitable for re-use as engineered fill provided the recommendations in the Grading section of this report are followed (see Section 8.4).

8.1.4 The enclosed seismically-induced settlement analyses indicate that the site soils could be susceptible to up to approximately 0.58 inches of seismic settlement as a result of the Maximum Considered Earthquake peak ground acceleration (PGAM). The anticipated seismic differential settlement is below the differential settlement threshold for conventional shallow foundations set forth in Chapter 12 of ASCE 7-22. The grading and foundation design recommendations presented herein are intended to reduce the effects of settlement on proposed improvements.

8.1.5 The upper alluvial soils are moderately compressible (see Figures B5 to B14) and may be subject to settlements upon application of foundation loading. Based on this consideration, it is recommended that the proposed parking structure be supported on a reinforced concrete mat foundation system deriving support in newly placed engineered fill.

Project No. W1585-88-03 - 12 - January 20, 2026

8.1.6 As a minimum, it is recommended that the upper 5 feet of existing earth materials within the proposed parking structure footprint area be excavated and properly compacted for foundation and slab support. The depth of excavation is intended to be measured from the grade at the time of our borings. At a minimum, proposed foundations should be underlain by at least 3 feet of newly placed engineered fill. Deeper excavations should be conducted as necessary to remove all encountered fill or soft soils as necessary at the direction of the Geotechnical Engineer (a representative of Geocon). The limits of existing fill and/or soft soil removal will be verified by the Geocon representative during site grading activities.

The excavation should extend laterally a minimum distance of three feet beyond the building footprint areas, including building appurtenances, or a distance equal to the depth of fill below the foundation, whichever is greater. Recommendations for earthwork are provided in the Grading section of this report (see Section 8.4).

8.1.7 Additional grading should be conducted as necessary to maintain the required 3-foot thick blanket of newly placed engineered fill below foundations. The grading contractor should verify all bottom of footing elevations prior to commencement of grading activities to ensure that grading is conducted deep enough to provide the required engineered fill blanket below foundations.

8.1.8 Prior to the placement of any fill, excavation bottoms should be densified by proof-rolling with heavy equipment in the presence of the Geotechnical Engineer (a representative of Geocon). During the compaction procedure, compression and shrinkage should be expected as the soils densify. All excavation bottoms must be approved in writing by the Geotechnical Engineer (a representative of Geocon) before placing fill or construction materials.

8.1.9 The grading contractor should be aware that the site soils are currently at or above optimum moisture content. Conditions could change seasonally and could worsen if the soil is subjected to heavy precipitation. If the soils are in excess of 3 percent above optimum moisture content at the time of construction and grading, the soils will likely require some spreading and drying activities in order to achieve proper compaction.

Project No. W1585-88-03 - 13 - January 20, 2026

8.1.10 The alluvial soils anticipated to be exposed at the excavation bottom may be very moist and could be subject to excessive pumping. Operation of rubber-tire equipment on the subgrade soils may cause excessive disturbance of the soils. Excavation activities to establish the finished subgrade elevation must be conducted carefully and methodically to avoid excessive disturbance to the subgrade. Stabilization of the excavation bottom will likely be required in order to provide a firm working surface upon which heavy equipment can operate.

Recommendations for bottom stabilization and earthwork are provided in the Grading section of this report (see Section 8.4).

8.1.11 It is anticipated that stable excavations for the recommended grading associated with the proposed parking structure can be achieved with sloping measures. However, if excavations in close proximity to an adjacent property line and/or structure are required, special excavation measures may be necessary in order to maintain lateral support of offsite improvements. Excavation recommendations are provided in the Temporary Excavations section of this report (Section 8.16).

8.1.12 It is recommended that the proposed mezzanine structure interior to the existing structure be supported by new foundations consisting of conventional shallow spread foundations deriving support in the undisturbed alluvium found at and below a depth of 2 feet.

8.1.13 Where a proposed foundation will be deeper than an existing adjacent foundation, the proposed foundation must be designed to resist the surcharge imposed by the existing foundation. The surcharge area may be defined by a 1:1 projection down and away from the bottom of the foundation.

8.1.14 All foundation excavations must be observed and approved in writing by the Geotechnical Engineer prior to placement of steel or concrete. Recommendations for a mat foundation system are provide in the Mat Foundation Design section (Section 8.6). Recommendations for a conventional foundation system in the Conventional Foundations Design section (Section 8.7).

Project No. W1585-88-03 - 14 - January 20, 2026

8.1.15 Foundations for small outlying structures, such as block walls up to 6 feet high, planter walls or trash enclosures, which will not be tied to existing or proposed structures, may be supported on conventional foundations bearing on a minimum of 12 inches of newly placed engineered fill, which extends laterally at least 12 inches beyond the foundation area.

Where excavation and proper compaction cannot be performed, foundations may derive support directly in the undisturbed alluvial soils at or below a depth of 2½ feet and should be deepened as necessary to maintain a minimum of 12 inch embedment into recommended bearing materials. If the soils exposed in the excavation bottom are soft or loose, compaction of the soft soils will be required prior to placing steel or concrete. Compaction of the foundation excavation bottom is typically accomplished with a compaction wheel or mechanical whacker and must be observed and approved in writing by a Geocon representative.

8.1.16 Where new paving is to be placed, it is recommended that existing fill soils and soft alluvial soils be excavated and properly compacted for paving support. The client should be aware that excavation and compaction of existing fill in the area of new paving is not required;

however, paving constructed over existing uncertified fill or unsuitable soils may experience increased settlement and/or cracking and may therefore have a shorter design life and increased maintenance costs. As a minimum, the upper 12 inches of soil should be scarified and properly compacted for paving support. Paving recommendations are provided in the Preliminary Pavement Recommendations section of this report (see Section 8.11).

8.1.17 Based on the results of the percolation testing, stormwater infiltration is considered feasible for this project. Discussion on the percolation test results are provided in Section 8.18.

8.1.18 Once the design and foundation loading configuration for the proposed structure proceeds to a more finalized plan, the recommendations within this report should be reviewed and revised, if necessary. Based on the final foundation loading configurations, the potential for settlement should be re-evaluated by this office.

8.1.19 Any changes in the design, location or elevation, as outlined in this report, should be reviewed by this office. Geocon should be contacted to determine the necessity for review and possible revision of this report.

8.1.20 The most recent ASTM standards apply to this project and must be utilized, even if older ASTM standards are indicated in this report.

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8.2 Soil and Excavation Characteristics

8.2.1 The in-situ soils can be excavated with moderate effort using conventional excavation equipment. Some caving should be anticipated in unshored excavations, especially where granular soils are encountered.

8.2.2 It is the responsibility of the contractor to ensure that all excavations and trenches are properly shored and maintained in accordance with applicable OSHA rules and regulations to maintain safety and maintain the stability of existing adjacent improvements.

8.2.3 All onsite excavations must be conducted in such a manner that potential surcharges from existing structures, construction equipment, and vehicle loads are resisted. The surcharge area may be defined by a 1:1 projection down and away from the bottom of an existing foundation or vehicle load. Penetrations below this 1:1 projection will require special excavation measures such as sloping or shoring. Excavation recommendations are provided in the Temporary Excavations section of this report (see Section 8.16).

8.2.4 The upper five feet of existing site soils encountered during the investigation are considered to have a “low” to “medium” expansive potential (EI = 37 and 56) and are classified as “expansive” in accordance with the 2025 California Building Code (CBC) Section 1803.5.3.

The recommendations presented herein assume that foundations and slabs will derive support in these materials.

8.3 Minimum Resistivity, pH, and Water-Soluble Sulfate

8.3.1 Potential of Hydrogen (pH) and resistivity testing as well as chloride content testing were performed on representative samples of soil to generally evaluate the corrosion potential to surface utilities. The tests were performed in accordance with California Test Method Nos. 643 and 422 and indicate that the soils are considered “ severely corrosive” with respect to corrosion of buried ferrous metals on site. The results are presented in Appendix B (Figure B19 and B20) and should be considered for design of underground structures. Due to the corrosive potential of the soils, it is recommended that PVC, ABS or other approved plastic piping be utilized in lieu of cast-iron when in direct contact with the site soils.

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8.3.2 Laboratory tests were performed on representative samples of the on-site soil to measure the percentage of water-soluble sulfate content. Results from the laboratory water-soluble sulfate tests are presented in Appendix B (Figure B19 and B20) and indicate that the on-site soil possess a sulfate exposure class of “S0” to “S2” to concrete structures as defined by 2025 CBC Section 1904 and ACI 318-19 Chapter 19. The table below presents a summary of concrete requirements set forth by 2025 CBC Section 1904 and ACI 318. The presence of water-soluble sulfates is not a visually discernible characteristic; therefore, other soil samples from the site could yield different concentrations. Additionally, over time, landscaping activities (i.e., addition of fertilizers and other soil nutrients) may affect the concentration.

REQUIREMENTS FOR CONCRETE EXPOSED TO

SULFATE-CONTAINING SOLUTIONS

Exposure Class

Water-Soluble Sulfate (SO4)

Percent by Weight

Cement Type (ASTM C150)

Maximum Water to

Cement Ratio by Weight1

Minimum Compressiv e Strength

(psi)

S0 SO4<0.10 No Type Restriction n/a 2,500

S1 0.10<SO4<0.20 II 0.50 4,000

S2 0.20<SO4<2.00 V 0.45 4,500

S3 SO4>2.00

Option 1 V+Pozzolan or Slag 0.45 4,500

Option 2 V 0.40 5,000

1 Maximum water to cement ratio limits do not apply to lightweight concrete

8.3.3 Subsequent to grading of the building pad, it is recommended that additional testing be performed to confirm the corrosivity characteristics of the soil which will be in direct contact with buried metal and concrete.

8.3.4 Geocon West, Inc. does not practice in the field of corrosion engineering and mitigation.

If corrosion sensitive improvements are planned, it is recommended that a corrosion engineer be retained to evaluate corrosion test results and incorporate the necessary precautions to avoid premature corrosion of buried metal pipes and concrete structures in direct contact with the soils.

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8.4 Grading

8.4.1 Earthwork is anticipated to include grading for preparation of the building pad, excavation of site soils for the foundations and utility trenches, as placement of backfill for trenches, and preparation of paving.

8.4.2 A preconstruction conference should be held at the site prior to the beginning of excavation operations with the owner, contractor, civil engineer, geotechnical engineer, and building official in attendance. Special soil handling requirements can be discussed at that time.

8.4.3 Earthwork should be observed, and compacted fill tested by representatives of Geocon West, Inc. The existing fill and alluvial soil encountered during exploration are suitable for re-use as an engineered fill, provided any encountered oversize material (greater than 6 inches) and any encountered deleterious debris are removed.

8.4.4 Grading should commence with the removal of existing vegetation and existing improvements from the area to be graded. Deleterious debris such as wood and root structures should be exported from the site and should not be mixed with the fill soils.

Asphalt and concrete should not be mixed with the fill soils unless approved by the Geotechnical Engineer. Existing underground improvements planned for removal should be excavated and the resulting depressions properly backfilled in accordance with the procedures described herein. Once a clean excavation bottom has been established it must be observed and approved in writing by the Geotechnical Engineer (a representative of Geocon West, Inc.).

8.4.5 As a minimum, it is recommended that the upper 5 feet of existing site soils within the proposed parking structure footprint area be excavated and properly compacted for foundation and slab support. The depth of excavation is intended to be measured from the grade at the time of our borings. Deeper excavation should be conducted as necessary to completely remove all existing artificial fill or soft soil at the direction of the Geotechnical Engineer (a representative of Geocon). The excavation should extend laterally a minimum distance of 3 feet beyond the building footprint area or a distance equal to the depth of fill below the foundation, whichever is greater. The limits of existing fill and/or soft fill soils removal will be verified by the Geocon representative during site grading activities.

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8.4.6 Additional grading should be conducted as necessary to maintain the required 3-foot thick blanket of newly placed engineered fill below foundations. The grading contractor should verify all bottom of footing elevations prior to commencement of grading activities to ensure that grading is conducted deep enough to provide the required engineered fill blanket below foundations.

8.4.7 Prior to the placement of any fill, excavation bottoms should be densified by proof-rolling with heavy equipment in the presence of the Geotechnical Engineer (a representative of Geocon). During the compaction procedure, compression and shrinkage should be expected as the soils densify. All excavation bottoms must be approved in writing by the Geotechnical Engineer (a representative of Geocon) before placing fill or construction materials.

8.4.8 The grading contractor should be aware that the site soils are currently at or above optimum moisture content. Conditions could change seasonally and could worsen if the soil is subjected to heavy precipitation. If the soils are in excess of 3 percent above optimum moisture content at the time of construction and grading, the soils will likely require some spreading and drying activities in order to achieve proper compaction.

8.4.9 If determined to be excessively soft , stabilization of excavation bottoms may be required in order to provide a firm working surface upon which engineered fill can be placed. Should this condition exist, rubber tire equipment should not be allowed in the excavation bottom until it is stabilized or extensive soil disturbance could result. Track mounted equipment should be considered to minimize disturbance to the soils. It is suggested that excavation and grading be performed during the summer season to promote moisture control of the soils.

8.4.10 One method of subgrade stabilization would consist of introducing a thin lift of 3- to 6-inch diameter crushed angular rock into the soft excavation bottom. The use of crushed concrete will also be acceptable. The crushed rock should be spread thinly across the excavation bottom and pressed into the soils by track rolling or wheel rolling with heavy equipment.

It is very important that voids between the rock fragments are not created so the rock must be thoroughly pressed or blended into the soils. All subgrade soils must be properly compacted and proof-rolled in the presence of the Geotechnical Engineer (a representative of Geocon West, Inc.).

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8.4.11 An additional method of subgrade stabilization would be to place a minimum 12-inch thick layer of aggregate base over Tensar InterAx NX850 geogrid or equivalent extruded (nonwoven) geotextile. The Tensar geogrids should be installed taught and should overlap in accordance with the manufacturer’s recommendations. Prior to placing the geogrid, excessively soft or wet materials should be removed and the resulting excavation bottom should be free of loose material. Non-vibratory compaction methods should be used for compaction of the base material. The aggregate base should be compacted to a dry density of at least 95 percent of the laboratory maximum density near the optimum moisture.

If pumping of the subgrade continues, a thicker layer of aggregate base may be placed. It is very important that subgrade stabilization be performed uniformly across the entire excavation bottom.

8.4.12 It is anticipated that stable excavations for the recommended grading associated with the parking structure can be achieved with sloping measures. However, if excavations in close proximity to an adjacent property line and/or structure are required, special excavation measures such as slot cutting may be necessary in order to maintain lateral support of the existing offsite improvements. Excavation recommendations are provided in the Temporary Excavations section of this report (Section 8.16).

8.4.13 All fill and backfill soils should be placed in horizontal loose layers approximately 6 to 8 inches thick, moisture conditioned to 2 to 3 percent above optimum moisture content and properly compacted to a minimum 90 percent of the maximum dry density in accordance with ASTM D 1557 (latest edition).

8.4.14 Although not anticipated for this project, all imported fill should be observed, tested, and approved by Geocon West, Inc. prior to bringing soil to the site. Import fill should consist of the characteristics presented in the table below.

SUMMARY OF IMPORT FILL RECOMMENDATIONS

Soil Characteristic Values Expansion Potential “Low” (Expansion Index of 50 or less)

Particle Size Maximum Dimension Less Than 6 Inches

Free of Debris Corrosivity Less Detrimental Than Existing Onsite Soils

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8.4.15. Where new paving is to be placed, it is recommended that existing fill and soft alluvium be excavated and properly compacted for paving support. As a minimum, the upper 12 inches of soil should be scarified, moisture conditioned to 2 to 3 percent above optimum moisture content, and compacted to at least 92 percent relative compaction, as determined by ASTM D 1557 (latest edition). Paving recommendations are provided in Preliminary Pavement Recommendations section of this report (see Section 8.11).

8.4.16 Foundations for small outlying structures, such as block walls up to 6 feet high, planter walls or trash enclosures, which will not be tied to existing or proposed structures, may be supported on conventional foundations deriving support on a minimum of 12 inches of newly placed engineered fill which extends laterally at least 12 inches beyond the foundation area.

Where excavation and proper compaction cannot be performed, foundations may derive support directly in the undisturbed alluvial soils at and below a depth of 2½ feet below the existing ground surface and should be deepened as necessary to maintain a minimum 12 inch embedment into the recommended bearing materials. If the soils exposed in the excavation bottom are soft or loose, compaction of the soils will be required prior to placing steel or concrete. Compaction of the foundation excavation bottom is typically accomplished with a compaction wheel or mechanical whacker and must be observed and approved by a Geocon representative.

8.4.17 Utility trenches should be properly backfilled in accordance with the following requirements.

The pipe should be bedded with clean sands (Sand Equivalent greater than 30) to a depth of at least 1 foot over the pipe, and the bedding material must be inspected and approved in writing by the Geotechnical Engineer (a representative of Geocon).