Exhibit_E_-_Preliminary_Geotechnical_Report.pdf

Prepared for County of San Mateo

1350 Ocean Avenue • Emeryville, CA 94608 • (510) 420-5738 www.rockridgegeo.com

PRELIMINARY GEOTECHNICAL STUDY

PROPOSED RESIDENTIAL BUILDING

710 HAMILTON STREET

REDWOOD CITY, CALIFORNIA

UNAUTHORIZED USE OR COPYING OF THIS DOCUMENT IS STRICTLY

PROHIBITED BY ANYONE OTHER THAN THE CLIENT FOR THE SPECIFIC

PROJECT

July 27 2026

Project No. 26-3071

1350 Ocean Avenue • Emeryville, CA 94608 • (510) 420-5738 www.rockridgegeo.com

July 27, 2026

Project No. 26-3071

Ms. Lidia Mata

Real Property Services, County Executive's Office

County of San Mateo

555 County Center, 4th Floor

Redwood City, California 94063

Subject: Preliminary Geotechnical Study

Proposed Residential Building

710 Hamilton Street

Redwood City, California

Dear Ms. Mata:

We are pleased to present the results of our preliminary geotechnical study for the due diligence evaluation of the property (APN 052-364-110) at 710 Hamilton Street in Redwood City, California. Our study was performed in accordance with our proposal dated June 22, 2026.

The subject property is located in downtown Redwood City and is bordered by Marshall Street to the north, Winslow Street to the west, Hamilton Street to the east, and an existing commercial property and parking lot to the south. The site has maximum plan dimensions of about 100 by

200 feet (approximately 0.5 acres). The site is relatively flat and is currently occupied by the San

Mateo County Law Library building, along with associated parking, driveways, and hardscape.

We understand the County of San Mateo is considering re-developing the property and is conducting its due diligence review. Plans for the property are still being developed, but may include construction of a 6- to 8-story residential building.

Based on the results of our preliminary geotechnical study, we conclude there are no geotechnical or geological conditions that would preclude development of the site as proposed.

Based on nearby subsurface information, the primary geotechnical considerations for the proposed development are: (1) the potential presence of loose to medium dense granular layers within the upper 30 feet that may be susceptible to liquefaction, (2) the potential presences of highly expansive near-surface soil, (3) relatively shallow groundwater, and (4) providing adequate foundation support while limiting total and differential settlement to tolerable levels.

We preliminarily conclude the proposed building may be supported on a mat foundation, either bearing on unimproved soil (if a site-specific investigation confirms the liquefaction hazard is small) or bearing on a ground improvement system; conventional spread footings underlain by ground improvement may also be considered.

Ms. Lidia Mata

County of San Mateo

July 27, 2026

Our preliminary geotechnical study was based on a review of subsurface information from the site vicinity; no site-specific field exploration or laboratory testing was performed. Prior to final design, a design-level geotechnical investigation should be performed to evaluate subsurface conditions within the site and provide final conclusions and recommendations regarding the geotechnical aspects of the project.

We appreciate the opportunity to provide our services to you on this project. If you have any questions, please call.

Sincerely, ROCKRIDGE GEOTECHNICAL, INC.

Clayton J. Proto, P.E., G.E. Linda H. J. Liang, P.E., G.E.

Associate Engineer 7/27/26 Principal Engineer 7/27/26

Enclosure

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SCOPE OF SERVICES

3.0 DATA REVIEW

4.0 SUBSURFACE CONDITIONS

4.1 Groundwater

5.0 SEISMIC CONSIDERATIONS

5.1 Regional Seismicity

5.2 Geologic Hazards

5.2.1 Ground Shaking

5.2.2 Ground Surface Rupture

5.2.3 Liquefaction and Liquefaction-Induced Settlement

5.2.4 Cyclic Densification

6.0 PRELIMINARY CONCLUSIONS AND RECOMMENDATIONS

6.1 Foundations and Settlement

6.1.1 Alternative A - Mat Foundation on Unimproved Soil

6.1.2 Alternative B - Mat or Spread Footings Supported on Ground

Improvement

6.1.3 Ground Improvement Methods

6.2 Expansive Soil

6.3 Below-Grade Construction

6.4 Seismic Design

7.0 ADDITIONAL GEOTECHNICAL SERVICES

8.0 LIMITATIONS

REFERENCES

FIGURES

APPENDIX A – Logs of Cone Penetration Tests and Borings in the Site Vicinity

LIST OF FIGURES

Figure 1 Site Location Map

Figure 2 Site Plan

Figure 3 Regional Geologic Map

Figure 4 Regional Fault and Historic Seismicity Map

Figure 5 Earthquake Zones of Required Investigation Map

26-3071 1 July 27, 2026

PRELIMINARY GEOTECHNICAL STUDY

PROPOSED RESIDENTIAL BUILDING

REDWOOD CITY, CALIFORNIA

1.0 INTRODUCTION

This report presents the results of the preliminary geotechnical study performed by Rockridge

Geotechnical, Inc. (Rockridge) for the due diligence evaluation of the property at 710 Hamilton

Street (APN 052-364-110) in Redwood City, California. The site is located in downtown

Redwood City and is bordered by Marshall Street to the north, Winslow Street to the west, Hamilton Street to the east, and an existing commercial property and parking lot to the south, as shown on the Site Location Map (Figure 1) and Site Plan (Figure 2). The site has maximum plan dimensions of about 100 by 200 feet (approximately 0.5 acres). The site is relatively flat and is currently occupied by the San Mateo County Law Library building, along with associated parking, driveways, and hardscape.

We understand the County of San Mateo is considering re-developing the property and is conducting its due diligence review. Plans for the property are still being developed, but may include construction of a 6- to 8-story residential building.

2.0 SCOPE OF SERVICES

Our preliminary geotechnical study was performed in accordance with our proposal dated June

22, 2026. The purpose of the study was to evaluate whether there are any adverse geotechnical or geological conditions that may significantly affect the feasibility and costs associated with development of the site. Our scope of services consisted of reviewing available geotechnical and geological data in the site vicinity and performing engineering analyses to develop preliminary conclusions and recommendations regarding:

• anticipated soil and groundwater conditions at the site

• site seismicity and seismic hazards, including the potential for liquefaction and liquefaction-induced ground failure

• the most appropriate foundation type(s) for the proposed new building

26-3071 2 July 27, 2026

• preliminary design criteria for the recommended foundation type(s), including vertical and lateral capacities

• estimates of foundation settlement under both static and seismic conditions

• 2025 California Building Code (CBC) site class and mapped design spectral response acceleration parameters

• construction considerations.

No site-specific subsurface exploration, such as drilling test borings or performing cone penetration tests (CPTs), was performed as part of this preliminary geotechnical study.

3.0 DATA REVIEW

For our preliminary geotechnical study, we reviewed available subsurface information in our files and publicly available databases for subsurface conditions in the site vicinity. Specifically, we reviewed the following reports (Reports I through IV):

I. Kleinfelder (2017), County Office Building, Project 20181527: 2 borings (COB-1 to 51.5 feet, COB-2 to 90 feet), approximately 200 feet northwest of site

II. Kleinfelder (1981), Job B11571, Winslow and Broadway: 1 boring to 31.5 feet (CGS borehole database), approximately 100 feet south of site

III. Essel Environmental Consulting (2016), 801 Brewster Avenue: 4 CPTs, 2 borings to 50 feet, approximately 800 feet northwest of site

IV. Rockridge Geotechnical, Inc. (2025), 910 Marshall Street: 8 CPTs, 2 borings to maximum depth of 134 feet, approximately 1,500 feet east of site (for deeper stratigraphy)

The locations of the previous borings and CPTs relative to the site are shown in Figure 2.

Selected logs of the borings and CPTs from these previous investigations are presented in

Appendix A.

4.0 SUBSURFACE CONDITIONS

The regional geologic map prepared by Pampeyan (1993), a portion of which is shown on Figure

3, indicates the site is underlain by Holocene-age fine-grained alluvium (Qaf). Explorations on the adjacent parcels and in the immediate site vicinity indicate the site is underlain by alluvial deposits consisting predominantly of stiff to hard sandy clay and clay, interbedded with

26-3071 3 July 27, 2026 occasional loose to medium dense granular layers (clayey sand, silty sand, poorly graded sand, and clayey gravel). Granular layers were generally encountered within the upper 30 feet; their thickness, density, and lateral extent vary considerably between explorations, and individual layers cannot be reliably correlated between exploration locations. The clay is stiff to very stiff to a depth of about 50 feet below the ground surface (bgs) and very stiff to hard below, to the maximum depth explored of about 130 feet bgs in the greater downtown area. The top of bedrock is estimated to be about 300 feet deep based on regional geologic data.

Atterberg limits tests on samples of the near-surface clay from the adjacent parcels indicate liquid limits of about 44 to 47 and plasticity indices of about 27 to 32, indicating that the near-surface clay is highly expansive1. Undocumented fill associated with the existing building and prior site development may also be present.

4.1 Groundwater

Groundwater was measured during the investigation for Report II (see Section 3.0) at a depth of

9 feet bgs. Quarterly to annual readings of monitoring wells at 455 County Center (formerly 590

Hamilton Street, approximately 400 feet to the north) from 2003 through 2009, obtained from the

State Water Resources Control Board GeoTracker website, indicated groundwater depths fluctuating between about 9 and 15 feet bgs. According to the California Geological Survey

(CGS) Seismic Hazard Zone Report for the Palo Alto 7.5-Minute Quadrangle, historic high groundwater levels in the site vicinity are less than 10 feet bgs.

Groundwater levels at the site are expected to fluctuate several feet seasonally, with potentially larger fluctuations annually, depending on rainfall. Considering the potential for higher levels during unusually wet years, we preliminarily recommend a design-high groundwater depth of 5 feet bgs be considered for the project. If a below-grade level is included in the project, the excavation will likely extend below the groundwater level, as discussed in Section 6.3.

1 Expansive clay undergoes volumetric changes with changes in moisture content (i.e., swells when wetted and shrinks when dried).

26-3071 4 July 27, 2026

5.0 SEISMIC CONSIDERATIONS

The results of our evaluation regarding seismic considerations for the site are presented in the following sections.

5.1 Regional Seismicity

The site is located within the Coast Ranges Geomorphic Province of California, which is characterized by northwest-trending valleys and ridges. These topographic features are controlled by folds and faults that resulted from the collision of the Farallon and North American plates and subsequent strike-slip faulting along the San Andreas Fault system. The San Andreas Fault is more than 600 miles long and extends from Point Arena in the north to the Gulf of California in the south. The Coast Ranges Geomorphic Province is bounded on the east by the Great Valley and on the west by the Pacific Ocean.

The major active faults in the area are the San Andreas, Hayward, and Monte Vista-Shannon

Faults. These and other faults in the region are shown in Figure 4. For these and other active faults within about 50 kilometers of the site, the distance from the site and maximum scenario moment magnitude2 [Petersen et al. (2014) & Thompson et al. (2016)] are summarized in

Table 1. These references are based on the Third Uniform California Earthquake Rupture

Forecast (UCERF3), prepared by Field et al. (2013).

2 Moment magnitude (Mw) is an energy-based scale and provides a physically meaningful measure of the size of a faulting event. Moment magnitude is directly related to average slip and fault rupture area.

26-3071 5 July 27, 2026

TABLE 1

Regional Faults and Seismicity

Fault Segment

Approximate

Distance from

Site (km)

Direction from Site

Scenario

Moment

Magnitude

Monte Vista - Shannon 5.3 Southwest 7.14

Total North San Andreas

(SAO+SAN+SAP+SAS)

6.8 Southwest 8.04

North San Andreas (Peninsula, SAP) 6.8 Southwest 7.38

San Gregorio (North) 20 West 7.44

Total Hayward + Rodgers Creek

(RC+HN+HS+HE)

23 Northeast 7.58

Hayward (South, HS) 23 Northeast 7.00

Butano 24 Southwest 6.93

Hayward (North, HN) 33 North 6.90

Total Calaveras (CN+CC+CS+CE) 33 East 7.43

Calaveras (North, CN) 33 East 6.86

Zayante-Vergeles (2011 CFM) 35 Southwest 7.48

Calaveras (Central, CC) 37 East 6.85

Hayward (Extension, HE) 39 East 6.18

Las Positas 39 East 6.50

North San Andreas (Santa Cruz Mts, SAS) 40 Southeast 7.15

Contra Costa - Lafayette Connected 42 Northeast 7.20

Franklin 43 Northeast 7.10

Mount Diablo Thrust 44 Northeast 6.67

Sargent 46 Southeast 6.71

Zayante-Vergeles 50 Southeast 7.00

Damaging earthquakes have occurred along many of these faults in recorded history, as depicted in Figure 4 (USGS, 2021). Notable historic earthquakes which have impacted the Bay Area in recorded history include:

• 1838 San Andreas Earthquake, Mw = 7.4 (estimated)

• 1865 San Andreas Earthquake, Mw = 6.5 (estimated)

• 1868 Hayward Earthquake, Mw = 7.0 (estimated)

• 1906 Great San Francisco Earthquake (San Andreas Fault), Mw = 7.9 (estimated)

• 1989 Loma Prieta Earthquake (San Andreas Fault), Mw = 6.9

• 2014 South Napa Earthquake, Mw = 6.0

26-3071 6 July 27, 2026

As a part of the UCERF3 project, researchers estimated that the probability of at least one

Mw ≥ 6.7 earthquake occurring in the greater San Francisco Bay Area during a 30-year period

(starting in 2014) is 72 percent. The highest probabilities are assigned to sections of the Hayward

(South), Calaveras (Central), and San Andreas (Santa Cruz Mountains) faults. The respective probabilities are approximately 25, 21, and 17 percent.

5.2 Geologic Hazards

Because the site is in a seismically active region, we preliminarily evaluated the potential for earthquake-induced geologic hazards including ground shaking, ground surface rupture, liquefaction,3 lateral spreading,4 and cyclic densification.5 We used the results of nearby geotechnical investigations to preliminarily evaluate the potential of these phenomena occurring at the site.

5.2.1 Ground Shaking

The seismicity of the site is governed by the activity of the San Andreas Fault, although ground shaking from future earthquakes on other faults, including the Hayward and Monte Vista-

Shannon Faults, will also be felt at the site. The intensity of earthquake ground motion at the site will depend upon many factors, including the characteristics of the generating fault, distance to the earthquake rupture surface, magnitude and duration of the earthquake, and local subsurface conditions. We judge that strong to violent ground shaking could occur at the site during a large earthquake on one of the nearby faults.

5.2.2 Ground Surface Rupture

Historically, ground surface displacements closely follow the trace of geologically young faults.

The site is not within an Earthquake Fault Zone, as defined by the Alquist-Priolo Earthquake

3 Liquefaction is a phenomenon where loose, saturated, cohesionless soil experiences temporary reduction in strength during cyclic loading such as that produced by earthquakes.

4 Lateral spreading is a phenomenon in which surficial soil displaces along a shear zone that has formed within an underlying liquefied layer. Upon reaching mobilization, the surficial blocks are transported downslope or in the direction of a free face by earthquake and gravitational forces.

5 Cyclic densification is a phenomenon in which non-saturated, cohesionless soil is compacted by earthquake vibrations, causing ground-surface settlement.

26-3071 7 July 27, 2026

Fault Zoning Act, and no known active or potentially active faults exist on the site. Therefore, we conclude the probability of fault offset at the site from a known active fault to be nil. In a seismically active area, the remote possibility exists for future faulting in areas where no faults previously existed; however, we conclude the probability of surface faulting, and consequently secondary ground failure from previously unknown faults, is very low.

5.2.3 Liquefaction and Liquefaction-Induced Settlement

When a saturated, cohesionless soil liquefies, it experiences a temporary loss of shear strength created by a transient rise in excess pore pressure generated by strong ground motion. Soil susceptible to liquefaction includes loose to medium dense sand and gravel, low-plasticity silt, and some low-plasticity clay deposits. Flow failure, lateral spreading, differential settlement, loss of bearing strength, ground fissures, and sand boils are evidence of excess pore pressure generation and liquefaction. The site is mapped within a liquefaction hazard zone, as shown on the map titled Earthquake Zones of Required Investigation, Palo Alto Quadrangle, prepared by the California Geological Survey (2006), a portion of which is shown on Figure 5.

Explorations in the site vicinity indicate the upper 30 feet of soil frequently include granular layers which are loose to medium dense; where these layers are below the groundwater table, they are potentially susceptible to liquefaction during a large earthquake. Depending on their thickness, density, and location relative to foundations, these layers may pose a hazard with respect to liquefaction-induced settlement and loss of bearing strength. Post-liquefaction volumetric settlement of liquefied layers is typically on the order of 3 percent of the liquefied layer thickness; based on the thicknesses of the granular layers encountered in nearby explorations, free-field liquefaction-induced settlement of about 2 to 3 inches appears plausible.

Settlement will be greater where a liquefiable layer is within the zone of influence of a shallow foundation, and liquefaction within this zone may also result in a loss of bearing capacity.

Because the thickness, density, and lateral extent of the granular layers vary considerably between nearby explorations, the liquefaction potential of the site cannot be definitively established from off-site explorations. Site-specific CPTs and/or borings should be performed as part of a design-level geotechnical investigation to evaluate the liquefaction hazard within the

26-3071 8 July 27, 2026 building footprint; the viability of a foundation system without ground improvement depends on the outcome of that investigation, as discussed in Section 6.1.

Given the relatively flat site grades and the absence of a nearby free face, we preliminarily conclude the potential for lateral spreading at the site is low.

5.2.4 Cyclic Densification

Cyclic densification, also known as differential compaction, can occur in non-saturated sand

(sand above the groundwater table) during an earthquake, leading to settlement of the ground surface and overlying structures. Given the shallow groundwater level and the predominance of clay in the near-surface soil, we preliminarily conclude the potential for cyclic densification of soil above the groundwater table at the site is low.

6.0 PRELIMINARY CONCLUSIONS AND RECOMMENDATIONS

From a geotechnical standpoint, we preliminarily conclude the proposed residential building may be constructed as planned, provided the recommendations of a design-level geotechnical investigation are incorporated into the project plans and specifications and implemented during construction. The primary geotechnical considerations for the proposed development are:

• the potential presence of loose to medium dense granular layers within the upper 30 feet that may be susceptible to liquefaction during a large earthquake,

• the potential presence of highly expansive near-surface soil,

• relatively shallow groundwater, which will likely be encountered if a below-grade level is constructed, and

• providing adequate foundation support and limiting total and differential settlement to tolerable levels.

Our preliminary conclusions and recommendations regarding these issues are presented in the following sections.

6.1 Foundations and Settlement

Building height and structural loads have not yet been determined for the proposed development.

For preliminary evaluation of a 6- to 8-story residential building, we estimated static settlement

26-3071 9 July 27, 2026 for a mat foundation supported at or near existing grade under a range of assumed loading conditions and summarized in Table 2.

TABLE 2

Preliminary Foundation Settlement Estimates

Assumed Average

Foundation Pressure

(D+L, psf)

Assumed

Maximum Column

Load (D+L, kips)

Estimated Total

Static Settlement

(inches)

500 400 3/4 to 1-1/4

1,000 800 1-1/2 to 2

1,500 1,200 2 to 2-1/2

*Notes: (1) D+L refers to dead plus live loads

(2) psf = pounds per square foot

The primary settlement mechanisms anticipated to govern foundation performance are: (1) static settlement of the clay deposits under the weight of the proposed building, and (2) post-earthquake settlement of potentially liquefiable granular layers within the upper 30 feet, as discussed in Section 5.2.3. The appropriate foundation alternative depends on whether the estimated static settlements are acceptable to the project structural engineer and architect and on the liquefaction hazard identified by the site-specific geotechnical investigation:

• Alternative A - A mat foundation bearing on firm native soil is appropriate if the static settlements are tolerable and the liquefaction hazard is found to be low (Section 6.1.1).

• Alternative B – Foundation system consisting of a mat or spread footings bearing on ground improvement foundation is appropriate if the static settlements exceed tolerable limits or the liquefaction hazard is found to be high (Section 6.1.2).

Final selection should be made after structural loads are confirmed, settlement tolerances are established by the structural engineer and architect, and a site-specific geotechnical investigation is performed.

6.1.1 Alternative A - Mat Foundation on Unimproved Soil

Under Alternative A, the building is supported on a reinforced concrete mat foundation bearing on firm native soil. This alternative is appropriate where the liquefaction hazard is relatively low, 26-3071 10 July 27, 2026 meaning the granular layers beneath the site are sufficiently dense, clayey, or thin that estimated liquefaction-induced settlements are within tolerable limits. The mat should be designed to accommodate the static settlements presented in Table 2 and any residual seismic settlements estimated from site-specific data. For preliminary planning, we anticipate an allowable bearing pressure of 3,000 psf for dead-plus-live loads may be used, with a one-third increase for total loads including wind or seismic.

6.1.2 Alternative B - Mat or Spread Footings Supported on Ground Improvement

Under Alternative B, the building is supported on a mat foundation or conventional spread footings bearing on a ground improvement system. This alternative is appropriate where the estimated static settlements exceed tolerable limits, or where the liquefaction hazard is relatively high, or both. The ground improvement is designed to reduce static settlement and, where required, to mitigate the liquefaction potential of the potentially liquefiable granular layers within the upper 30 feet. In addition to reducing settlement and mitigating liquefaction, ground improvement stiffens the soil mass and increases bearing capacity. Based on recent design-level work in the site vicinity, we anticipate a properly designed and constructed ground improvement system can increase the maximum allowable bearing pressure for shallow foundations to about

4,000 to 6,000 psf for dead-plus-live loads; these pressures may be increased by one third for total loads, including wind and seismic loads. The allowable bearing pressures include factors of safety of at least 2.0 and 1.5 for dead-plus-live and total loads, respectively. Denser ground improvement configurations can be used where applied pressures are higher.

Ground improvement systems are typically designed and installed under design-build contracts by specialty contractors, who determine the size, spacing, length, and strength of the elements based on required performance criteria. The final allowable bearing pressures, estimated settlements, and modulus of vertical subgrade reaction should be developed by the design-build contractor and reviewed by us and the project structural engineer. Suitable ground improvement methods for the anticipated conditions are discussed in Section 6.1.3.

26-3071 11 July 27, 2026

6.1.3 Ground Improvement Methods

Given the depths of the potentially liquefiable layers, candidate ground improvement methods include rigid inclusions (RIs), vibro-replacement stone columns (VSCs), and deep soil mixed

(DSM) columns or panels. These methods differ in what they can address (static settlement, liquefaction, or both), depending on how each element is installed and how it interacts with the surrounding soil. Method selection should be made during design based on the governing improvement requirement, contractor availability, constructability considerations associated with the shallow groundwater level, spoil management requirements, tolerable vibrations, and cost.

These systems are typically designed and installed by specialty contractors under design-build contracts, who finalize the size, spacing, length, and strength of the elements based on the targeted improvement levels.

Rigid Inclusions

RIs are unreinforced or lightly reinforced grout columns, typically 14 to 18 inches in diameter.

RIs are installed using non-displacement, partial-displacement, or full-displacement methods.

They reduce static settlement by transferring foundation loads to firmer soil below. However, due to relatively low area replacement ratio, installation of RIs does not meaningfully densify the surrounding granular soil. As a result, RIs do not mitigate liquefaction. Moreover, where RIs extend through a liquefiable layer, their small cross-sectional area, high stiffness contrast with the surrounding soil, and the high stresses developed under foundation loading make them less able to accommodate the kinematic loading and displacement demands imposed by the liquefiable layer, raising integrity concerns under seismic loading. RIs are therefore appropriate only where ground improvement is needed to reduce static settlement and the liquefaction hazard is low; extending them through a significant liquefiable interval generally precludes their use.

Vibro-Replacement Stone Columns

VSCs are constructed by advancing a vibrating probe into the ground and backfilling with crushed rock using a bottom-feed system. The installation densifies and reinforces the surrounding granular soil, mitigating liquefaction, and the resulting composite also reduces static settlement. VSCs thus address both static settlement and liquefaction. Because the probe densifies the soil through vibration, ground vibrations generated during installation should be

26-3071 12 July 27, 2026 evaluated with respect to adjacent structures and utilities, particularly in the downtown setting; a pre-construction survey and vibration monitoring are typically warranted. Their effectiveness for liquefaction mitigation depends on the soil being sufficiently granular to densify; the anticipated area replacement ratio is on the order of 10 to 15 percent. A pre-production test section with post-improvement CPTs at the midpoints between columns should be performed to confirm the design spacing mitigates liquefaction potential.

Deep Soil Mixing

DSM constructs columns or panels of soil-cement by mechanically mixing cement slurry with in-situ soil. Because the method does not rely on densification, it is effective across a wide range of soil types. DSM reduces static settlement. DSM, configured as isolated columns or a lattice, can provide partial mitigation of liquefaction effects. When configured in a closed-cell geometry, DSM mitigates liquefaction of the enclosed soil regardless of the soil type, at an anticipated area replacement ratio of about 40 to 50 percent. DSM installation generates spoil on the order of one-third of the treated volume; because of its cement content, the spoil develops strength upon curing and is generally suitable for use as structural fill, though its elevated pH typically requires characterization for off-site disposal.

6.2 Expansive Soil

Atterberg limits tests performed on samples of the near-surface clay in the site vicinity indicate the near-surface clay is highly expansive. Expansive soil is subject to volume changes with fluctuations in moisture content, leading to movement and cracking of foundations, pavements, slabs, and below-grade walls. Consequently, improvements should be designed and constructed to withstand the effects of expansive soil.

Where the building will have a slab-on-grade floor supported on footings, a minimum of 12 inches of non-expansive soil (imported non-expansive fill or lime-treated onsite soil) should be placed on the prepared subgrade to mitigate the long-term effects of expansive subgrade soil.

Non-expansive soil is not required beneath mat foundations. Positive surface drainage should be provided around the building, roof downspouts should discharge into controlled drainage

26-3071 13 July 27, 2026 facilities, and water-intensive landscaping adjacent to the building and flatwork should be avoided.

6.3 Below-Grade Construction

As discussed in Section 4.1, groundwater has been encountered at depths of about 9 to 15 feet bgs in the site vicinity. If a below-grade level is included in the project, the excavation

(anticipated to be on the order of 12 to 15 feet deep, including the foundation excavation) will likely encounter groundwater during construction.

Where there is insufficient space to slope excavations in accordance with Cal-OSHA requirements, temporary shoring will be required. A soldier-pile-and-timber-lagging system is anticipated to be suitable; cantilevered walls are generally suitable for excavation depths up to about 12 feet, with tiebacks or internal bracing required for deeper excavations. Installing soldier piles through granular soil below groundwater may require casing or drilling slurry, and concrete should be placed by tremie methods where water is present. The shoring designer should establish deflection tolerances and a monitoring program; typical tolerances are about 1/2 inch near buildings or sensitive utilities and up to 1 inch elsewhere.

Temporary dewatering will likely be required to provide a firm, relatively dry base for foundation construction. Due to the low permeability of the clayey soil, a passive system operating within the excavation footprint (sumps and/or a drainage blanket) may be appropriate.

However, an active dewatering system may be necessary if there is a continuous layer of highly permeable soil near the bottom of excavation. The dewatering system should maintain groundwater at least 3 feet below the excavation bottom until sufficient building weight is available to resist hydrostatic uplift, as determined by the structural engineer.

Where proposed improvements extend below the design groundwater level, foundations and below-grade walls should be waterproofed and designed to resist hydrostatic pressures.

6.4 Seismic Design

The latitude and longitude of the site are approximately 37.4872° and -122.2306°, respectively. If potentially liquefiable soil is present beneath the site, as discussed in Section 5.2.3, the site is

26-3071 14 July 27, 2026 designated Site Class F in accordance with the 2025 CBC and ASCE 7-22. This designation requires a site response analysis to be performed to evaluate the potential for amplification of seismic forces and also has additional detailing requirements for certain structural components.

However, a site response analysis is not required if the structure has a fundamental period of 0.5 seconds or less. Based on an estimated VS100 of about 750 feet per second, we preliminarily recommend using the more critical of the Site Class D and DE seismic design parameters if any of the following apply:

• a site-specific investigation demonstrates that liquefiable soil is not present beneath the site,

• the liquefaction potential is mitigated by ground improvement, or

• the structure has a fundamental period of 0.5 seconds or less (for seismic design parameter determination only).

For design in accordance with the 2025 CBC (ASCE 7-22), assuming the more critical of Site

Class D or DE designation, we preliminarily recommend the following mapped values:

TABLE 3

Mapped Two-Period Seismic Design Parameters

Preliminary, Critical of Site Class D and DE

Hazard Level Parameter Value

MCER

Ss 2.05 g

S1 0.79 g

SMS 2.10 g

SM1 3.07 g

Seismic Design

Category

(Risk Category I-III)

E

PGAM 0.73 g

DE

SDS 1.40 g

SD1 2.05 g

26-3071 15 July 27, 2026

7.0 ADDITIONAL GEOTECHNICAL SERVICES

The preliminary conclusions and recommendations presented herein are based on subsurface data from the site vicinity and are not intended for final design. Prior to final design, we should be retained to perform a design-level geotechnical investigation, including site-specific borings and/or CPTs, to evaluate subsurface conditions beneath the site, refine the liquefaction and settlement analyses, and provide final conclusions and recommendations.

8.0 LIMITATIONS

This preliminary geotechnical study has been conducted in accordance with the standard of care commonly used as state-of-practice in the profession. No other warranties are either expressed or implied. The conclusions and recommendations made in this report are based on the assumption that subsurface conditions at the site do not deviate appreciably from those disclosed in previous subsurface investigations in the site vicinity and are preliminary in nature. The preliminary recommendations presented in this report are developed exclusively for the proposed development described in this report and are not valid for other locations and construction in the site vicinity.

26-3071 16 July 27, 2026

REFERENCES

American Society of Civil Engineers (ASCE). (2022). ASCE/SEI 7-22: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. ASCE Publications.

Boulanger, R. W., and Idriss, I. M. (2014). CPT and SPT Based Liquefaction Triggering

Procedures. Report No. UCD/CGM-14/01, Center for Geotechnical Modeling, University of

California, Davis, CA.

California Geological Survey. (2006). Seismic Hazard Zone Report for the Palo Alto 7.5-Minute

Quadrangle, San Mateo and Santa Clara Counties, California. California Department of

Conservation.

California Department of General Services. (2025). California Building Code. State Government of California.

Federal Highway Administration (FHWA). (2013). Deep Mixing for Embankment and

Foundation Support. Publication No. FHWA-HRT-13-046.

Field, E.H., et al. (2013). Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3)

- The Time-Independent Model (Open-File Report 2013-1165). United States Geological Survey.

Pampeyan, E.H. (1993). Geologic Map of the Palo Alto and Part of the Redwood Point 7-1/2'

Quadrangles, San Mateo and Santa Clara Counties, California. United States Geological Survey.

Petersen, M.D., et al. (2014). Documentation for the 2014 Update of the United States National

Seismic Hazard Maps (Open-File Report 2014-1091). United States Geological Survey.

Thompson, E.M., et al. (2016). Shakemap earthquake scenario: Building Seismic Safety Council

2014 Event Set (BSSC2014). United States Geological Survey.

United States Geological Survey. (2021). Earthquake Hazards Program: ANSS Comprehensive

Catalog of Earthquake Events and Products. United States Department of the Interior.

FIGURES

ROCKRIDGE

GEOTECHNICAL

SITE LOCATION MAP

SITE

Project No. FigureDate 1

0 1,000 Feet

Approximate scale

Base map: USGS The National Map, 2026

26-307107/22/26

Redwood City, California

Approximate scale

100 Feet

Base map: Google Earth, 2026.

07/22/26 26-3071 2

SITE PLAN

Date Project No. Figure

710 HAMILTON STREET

Redwood City, California

ROCKRIDGE

GEOTECHNICALR

EXPLANATION

Project limits

C2Q-9B2

KLF COB-2

KLF COB-1

KLF COB-1

Approximate locations of borings by others

−− Depositional or intrusive contact, dashed where approximately located, dotted where concealed

−− Dashed where approximately located, small dashes where inferred, dotted whereconcealed, queried where location is uncertain.

−− Shows fold axis, dotted where concealed

Younger alluvium (Holocene)

Artifical Fill (Holocene)

Fine-grained alluvium (Holocene)

Medium-grained alluvium (Holocene)

Older alluvium (Pleistocene)

Sandstone

Qya

Qf

Qoa

Qam

Qaf fsBorehole to basement - showing elevation of top of basement (Franciscan Complex). Datum is mean sea level

Younger alluvium (Holocene)

REGIONAL GEOLOGIC MAP

0 1/2 Mile

Approximate scale

1/4

EXPLANATION

Base map: Geology of the Palo Alto and Part of the Redwood Point 7-1/2’ Quadrangles, San Mateo and Santa Clara Counties, California.

by Earl H. Pampeyan, 1993.

Borehole that did not reach basement (Franciscan Complex)-1147

-358

Qac

ROCKRIDGE

GEOTECHNICAL Project No. FigureDate 3

Younger landslide deposits (Holocene)Qyl

Bay mud (Holocene)Qm

Chert

Metamorphic rock

Greenstone fm fc fg

26-307107/22/26

710 HAMILTON STREET

SITE

1906, M7.9

1838, M7.4

1868, M7.0

1989, M6.9

2014, M6.0

San Andreas (SAN)

San Andreas (SAP) Hayward (HN)

Hayward (HS)

San Andreas (SAS)

Monte Vista-Shannon

Calaveras (CN)

Calaveras (CC)

Calaveras (CS)

San G regorio (North)

San G regorio (South)

Greenville

G reen Valley

Rodgers Creek

Maacama

W est Napa

Concord

G reat Valley (M idland)

Reliz

Great Valley (Orestimba)

1865, M6.5

Symbol Magnitude (Mw)

7.5+

7.0 - 7.4

6.5 - 6.9

6.0 - 6.4

5.0 - 5.9

Historic Earthquake Epicenters

Pre-1900 1900-1970 1971-2021

Project No. FigureDate

REGIONAL FAULT AND HISTORIC

SEISMICITY MAP

20 Miles

Approximate scale

0 10

32 Kilometers0 16

EXPLANATION

Faults (National Seismic Hazard Model)

ROCKRIDGE

GEOTECHNICAL 4

SITE

26-307107/22/26

0 4,000 Feet

Approximate scale

2,000

ROCKRIDGE

GEOTECHNICAL Project No. FigureDate 5

Earthquake-Induced Landslide Zones Areas where previous occurrence of landslide movement, or local topographic, geological, geotechnical and subsurface water conditions indicate a potential for permanent ground displacements such that mitigation as defined in Public Resources Code Section 2693(c) would be required.

Liquefaction Zones Areas where historical occurrence of liquefaction, or local geological, geotechnical and ground water conditions indicate a potential for permanent ground displacements such that mitigation as defined in Public Resources Code Section 2693(c) would be required.

EARTHQUAKE ZONES OF REQUIRED

INVESTIGATION MAP

Reference:

Earthquake Zones of Required Investigation Palo Alto Quadrangle California Geological Survey Released October 18, 2006

26-307107/22/26

710 HAMILTON STREET

SITE

APPENDIX A

Logs of Cone Penetration Tests and Borings in the Site Vicinity

LOG OF BORING C2Q_9B2 (CGS BOREHOLE DATABASE COMPILATION)

Compiled from CGS Seismic Hazard Zonation Program borehole database export tables (sample_location, excavation_compl, fluid_pen, lithology, sample_data, pen_test_results, sample_analysis)

BORING INFORMATION

Official name (CGS)

Well name

Data source

Site name

Operator (investigator)

Job number

Date completed

Excavation method

Total depth (ft)

Reference elevation (ft)

Latitude (NAD 27)

Longitude (NAD 27)

Groundwater depth (ft)

Groundwater note

STRATIGRAPHY (lithology table) Layer Top Depth (ft) Bottom Depth (ft) Top Elev.

(ft) Lith. Code Description

(verbatim from database)

1 0 1 10.0 AC

2 1 2 9.0 FILL

3 2 6 8.0 CL

4 6 10.5 4.0 SM

5 10.5 18.5 -0.5 CL

6 18.5 31.5 -8.5 CL

SAMPLES, PENETRATION RESISTANCE, AND LABORATORY TEST RESULTS

Sample Depth (ft) Stratum Sampled Blows/ft

(Cal) Blows, Last

6 in.

Dry Density

(pcf) Moisture

Content (%) Liquid

Limit (%) Plasticity Index (%)

% Pass #4

% Pass #200

1 2.0 - 2.5 CL (2-6 ft) 48 24 112 17 47 32

2 6.0 - 6.5 SM (6-10.5 ft) 40 20 101 14 85 14

3 11.0 - 11.5 CL (10.5-18.5 ft) 34 17 104 22

4 16.0 - 16.5 CL (10.5-18.5 ft) 72 36

5 21.0 - 21.5 CL (18.5-31.5 ft) 38 19 104 20

6 26.0 - 26.5 CL (18.5-31.5 ft) 40 20

7 31.0 - 31.5 CL (18.5-31.5 ft) 30 15

NOTES

6. Groundwater at 9 ft measured at time of drilling (12/29/1981); may not represent stabilized level.

7. Lithologic descriptions and lab values transcribed verbatim from the CGS database export; original Kleinfelder log not reviewed. Boring drilled 1981 - conditions may have changed.

brown silty clay, very moist, medium stiff, mottled; at 14 mottled orange gray brown silty clay, moist, stiff, some caliche dark brown silty clay, moist, stiff, trace pebbles; at 5 lighter brown, some sand

AC over AB brown silty clay with chunks of orange brick, fill brown silty sand, medium dense, some angular gravel more uniform color, gray brown silty clay, trace of sand

1. All samples obtained with a California sampler (2-inch ID) driven with a 140-lb hammer falling 30 inches; hammer type (rope-and-cathead vs. automatic) not recorded in database.

2. Blow counts are raw California sampler blows per foot as reported in pen_test_results (blow_count field); the database spt_equivalent and nm (ASTM D1586) fields are blank. Values are NOT SPT N-values and have not been corrected for sampler type, hammer energy, or overburden.

3. 'Blows, Last 6 in.' transcribed from the pen_test_results remarks field (e.g., '24 for 6 inches').

4. Top elevations computed as reference elevation (10 ft) minus depth; elevation datum not stated in database.

5. Coordinates are NAD 27 datum as published by CGS.

37.48693994

-122.2295166

Measured 12/29/1981 (at time of drilling); source remark:

'ground water measured atd'

B11571

12/29/1981

Solid stem auger, 6-inch diameter

31.5

000107_00023_37122D2

C2Q_9B2

City of Redwood City

Winslow and Broadway streets [source spelling: 'Wisnlow']

Kleinfelder and Associates

KLF COB-2

KLF COB-1

FORMER

CREDIT

UNION

BUILDING

EXISTING

LATHROP

HOUSE

PROPOSED

OFFICE

BUILDING

LEGEND

SOIL BORING

(By Kleinfelder, 2017)

NOTE: All locations are approximate.

The information included on this graphic representation has been compiled from a variety of sources and is subject to change without notice. Kleinfelder makes no representations or warranties, express or implied, as to accuracy, completeness, timeliness, or rights to the use of such information. This document is not intended for use as a land survey product nor is it designed or intended as a construction design document. The use or misuse of the information contained on this graphic representation is at the sole risk of the party using or misusing the information.

N

FIGURE

DRAWN BY:

PROJECT NO.

CHECKED BY:

REVISED:

DATE:

SITE PLAN

COUNTY OF SAN MATEO GOVERNMENT CENTER

COUNTY OFFICE BUILDING

REWOOD CITY, CALIFORNIA

20181527

JDS

OK

09/01/2017

REFERENCE:

IMAGE CREATED FROM DATA COMPILED BY GOOGLE

EARTH PRO., IMAGERY DATED 11-02-2016.

SCALE IN FEET

80 160

SCALE: 1" = 80'