Prepared for Housing Authority of Alameda

FINAL REPORT GEOTECHNICAL INVESTIGATION EVALUATION OF BUILDING DISTRESS 2815 SAN DIEGO ROAD ALAMEDA, CALIFORNIA

UNAUTHORIZED USE OR COPYING OF THIS DOCUMENT IS STRICTLY PROHIBITED BY ANYONE OTHER THAN THE CLIENT FOR THE SPECIFIC PROJECT

July 2, 2018 Project No. 17-1424

July 2, 2018 Project No. 17-1424

Mr. Michael Billington Housing Authority of the City of Alameda 701 Atlantic Avenue Alameda, CA 94501

Subject: Final Report Geotechnical Investigation to Evaluate Building Distress 2815 San Diego Avenue Alameda, California

Dear Mr. Billington,

The attached report presents the results of the geotechnical investigation we performed to evaluate the potential cause(s) of distress to the residential building located at 2815 San Diego Road in Alameda, California. Our investigation was performed in accordance with our Authorization to Provide Geotechnical Services dated April 23, 2018.

Built in the early 1940’s during the original construction of the Alameda Naval Air Station (NAS), the subject building is a one-story wood-framed single-family residence with plan dimensions of approximately 40 by 90 feet. It has a stucco exterior finish, a structurally supported wood floor above a crawl space, and is supported on a combination of continuous and isolated shallow footings. The building has experienced significant differential settlement since its construction, as evidenced by sloping floors, out-of- square door and window frames, and interior wall cracking.

We understand there are concerns whether the building can be occupied in its current condition and, if not, whether there are repairs can be made to make it occupiable.

Based on our investigation and research, we conclude the existing building was constructed sometime between about 1963 and 1968. It appears the current residence was constructed over a demolished pile-supported railroad structure constructed in the late 1800’s. We believe the pile foundations from this former structure were left in place. The existing building has experienced differential settlement of about 3-1/2 inches since its construction in the 1960’s. There are many areas within the building where the differential settlement exceeds one inch over a horizontal distance of 25 feet, which is the industry standard for “acceptable” differential settlement. The abrupt differential

Mr. Michael Billington Housing Authority of the City of Alameda July 2, 2018 Page 2 settlement that has occurred is likely the result of old timber piles forming “hard points’ beneath portions of the foundation. As the ground settles around these hard points, abrupt differential settlement occurs. The existing foundation has large cracks, as well as voids beneath portions of it. In our opinion, repair of the existing foundation is not feasible.

In the attached report, we have provided recommendations for two options: 1) a “short- term” consisting of demolishing the existing foundation and supporting the existing structure on a new mat foundation underlain by 12 inches of recompacted fill, and 2) a “long-term” option consisting of demolishing the existing structure and its foundation and constructing a new residence on a mat foundation underlain by four feet of recompacted fill and two layers of geogrid. For both options, we estimate the residence (new or existing) will settle between approximately 2 and 4 inches over the next 20 years.

The recommendations contained in our report are based on a limited subsurface exploration. Consequently, variations between expected and actual subsurface conditions may be found in localized areas during construction. Therefore, we should be engaged to observe grading and foundation installation during which time we may make changes in our recommendations, if deemed necessary.

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

Sincerely yours, ROCKRIDGE GEOTECHNICAL, INC.

Craig S. Shields, P.E., G.E. Principal Geotechnical Engineer

Enclosure

TABLE OF CONTENTS

1.0 INTRODUCTION ...... 1 2.0 SCOPE OF SERVICES ...... 2 3.0 SITE HISTORY ...... 2 4.0 FIELD INVESTIGATION ...... 3 4.1 Cone Penetration Tests ...... 3 4.2 Dynamic Penetrometer Tests and Hand-Auger Boring ...... 4 4.3 Floor-Level Survey ...... 4 5.0 SUBSURFACE CONDITIONS ...... 5 6.0 SEISMIC CONSIDERATIONS ...... 6 6.1 Regional Seismicity and Faulting ...... 6 6.2 Geologic Hazards ...... 9 6.2.1 Ground Shaking ...... 9 6.2.2 Liquefaction and Associated Hazards ...... 9 6.2.3 Lateral Spreading ...... 11 6.2.4 Cyclic Densification ...... 11 6.2.5 Ground Surface Rupture ...... 11 7.0 DISCUSSION AND CONCLUSIONS ...... 12 8.0 RECOMMENDATIONS ...... 13 8.1 Site Demolition and Grading ...... 14 8.2 Mat Foundation ...... 16 8.3 Seismic Design...... 17 9.0 LIMITATIONS ...... 18 FIGURES APPENDIX A – Logs of Cone Penetration Tests and Dynamic Penetrometer Tests APPENDIX B – Floor-Level Survey Results APPENDIX C – Historical Topographic Maps and Aerial Photographs

LIST OF FIGURES Figure 1 Site Location Map Figure 2 Site Plan Figure 3 Regional Geologic Map Figure 4 Regional Fault Map Figure 5 Seismic Hazard Zones Map Figure 6 1895 Topographic Survey

APPENDIX A Figures A-1 Cone Penetration Test Results, CPT-1 and CPT-2 and A-2 Figures A-3 Dynamic Penetrometer Test Results, DPT-1 through A-5 through DPT-3

APPENDIX B Floor-Level Survey Results

APPENDIX C Historical Topographic Maps – 1895, 1899, 1915, 1949 Historical Aerial Photographs – 1939, 1946, 1958, 1968

GEOTECHNICAL INVESTIGATION EVALUATION OF BUILDING DISTRESS 2815 SAN DIEGO ROAD Oakland, California

1.0 INTRODUCTION

This report presents the results the geotechnical investigation performed by Rockridge Geotechnical, Inc. to evaluate the potential cause(s) of distress to the residential building located at 2815 San Diego Road in Alameda, California. The subject building is located on the southwestern side of San Diego Road east of its intersection with Pan Am Way, as shown on the Site Location Map (Figure 1).

Built in the early 1940’s during the original construction of the Alameda Naval Air Station (NAS), the subject building is a one-story wood-framed single-family residence with plan dimensions of approximately 40 by 90 feet. It has a stucco exterior finish, a structurally supported wood floor above a crawl space, and is supported on a combination of continuous and isolated shallow footings. The building has experienced significant differential settlement since its construction, as evidenced by sloping floors, out-of-square door and window frames, and interior wall cracking.

We understand there are concerns whether or not the building can be occupied in its current condition and, if not, whether there are repairs can be made to make it occupiable.

The objectives of our services were to evaluate the potential cause(s) of settlement of the building settlement and to provide recommendations to mitigate the potential for future settlement, if feasible, and to provide recommendations for construction of a new building if it is not economically feasible to repair the existing structure.

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2.0 SCOPE OF SERVICES

The scope of our services was outlined in our proposal dated December 8, 2017. These services included investigating subsurface conditions at the site by performing cone penetration tests (CPTs) and dynamic penetrometer tests (DPTs), performing a floor-level survey in the building and examining its crawl space, and reviewing historical aerial photographs and topographic surveys of the site vicinity. We used the data acquired during our field investigation to perform engineering analyses to develop conclusions and recommendations regarding:

 potential cause(s) of the building settlement  estimated future settlement  measures to mitigate future settlement and repair of existing foundations (if feasible)  recommendations for new foundations if repair of existing foundations is not feasible.

3.0 SITE HISTORY

To evaluate the history of the site, we reviewed historical topographic maps from 1895, 1899, 1915, 1948, 1949, 1959, 1968, 1973, 1980, 1996 and 2012, and historical aerial photographs from 1939, 1946, 1958, 1963, 1968, 1972, 1982, 1993, 1998, 2005, 2009, 2010 and 2012 provided by EDR. Copies of the 1895, 1899, 1915, and 1948 historical topographic maps and the 1939, 1946, 1958, and 1968 historical aerial photographs are presented in Appendix C.

The topographic map from 1895, which is presented on Figure 6, shows the northwestern part of Alameda, including the land on which the NAS was constructed, consists of open water and marsh land with sinuous sloughs running throughout the marshes. The land is undeveloped except for a railroad line that runs in an east-west direction along the southern boundary of San Antonio Creek (currently the Oakland-Alameda Estuary) and connects to another railroad line running in a north-south direction along the future Main Street alignment. These two railroads are connected by a curved section of railroad that crosses over open water and, therefore, would have been supported on timber pile-supported trestles. Overlaying the 1895 and 2012

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topographic maps indicates the curved connecting section of railroad passes over the current location of 2815 San Diego Road.

The 1915 topographic map shows the curved connecting section of railroad being supported on filled land, indicating the fill directly benaeth 2815 San Diego Road was placed between 1899 and 1915. The 1939 aerial photograph indicates most of the land on which the NAS was constructed has been filled and the curved connecting section of railroad has been removed. In the 1946 aerial photograph, the entire NAS base has been constructed and a building occupies the 2815 San Diego Road site; however, in the 1958 aerial photograph, the 2815 San Diego Road site is once again vacant, indicating the original building constructed on the site was demolished about 15 years after its original construction. The current building occupying the 2815 San Diego Road site was then constructed between 1963 and 1968 in roughly the same footprint as the former building that occupied the site.

4.0 FIELD INVESTIGATION

Subsurface conditions around the perimeter of the building were investigated by performing two CPTs and three DPTs. Prior to our field investigation, we obtained a drilling permit from the Alameda County Public Works Agency (ACPWA) and a Marsh Crust Permit from the City of Alameda. We also contacted Underground Service Alert (USA) to notify them of our work and retained a private utility locator, Precision Locating, to check for buried utilities at the CPT locations. To provide additional data for evaluation of the building distress, we also performed a floor-level survey inside the building and examined the crawl space. Details of our field investigation and the floor-level survey are described further below.

4.1 Cone Penetration Tests

Middle Earth Geo Testing of Orange, California performed the CPTs, designated as CPT-1 and CPT-2, on December 21, 2017 at the approximate locations shown on the attached Site Plan (Figure 2). The CPTs were performed by hydraulically pushing a 1.4-inch-diameter cone-tipped probe with a projected area of 10 square centimeters into the ground. The cone-tipped probe measured tip resistance and the friction sleeve behind the cone tip measured frictional resistance.

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Electrical strain gauges within the cone continuously measured soil parameters for the entire depth advanced. Soil data, including tip resistance and frictional resistance, were recorded by a computer while the tests were conducted. Accumulated data were processed by computer to provide engineering information such as the types and approximate strength characteristics of the soil encountered. CPT-1 and CPT-2 were advanced to depths of approximately 90.6 and 60.5 feet below the existing ground surface (bgs), respectively. The CPT logs, showing tip resistance and friction ratio by depth, as well as pore pressure and soil behavior type, are presented on Figures A-1 and A-2 in Appendix A. Upon completion, the CPT holes were backfilled with cement grout.

4.2 Dynamic Penetrometer Tests and Hand-Auger Boring

To provide additional data on the properties of the upper fill soils immediately adjacent to the existing building and to check for possible voids, we performed three dynamic penetrometer tests, designated as DPT-1 through DPT-3, on December 15, 2017 at the approximate locations shown on Figure 2. The DPT is performed by manually driving a 1.4-inch-diameter cone-tipped probe with a 30-pound hammer falling 15 inches. The blow counts required to drive the probe are recorded at 10-centimeter intervals. The DPTs were each advanced to a depth of approximately 16-1/2 feet bgs. The DPT results are presented on Figures A-3 through A-5 in Appendix A.

We also hand-augered a five-foot-deep boring, designated as HA-1, adjacent to the DPT-1 location to obtain samples of the upper sand fill.

4.3 Floor-Level Survey

On December 15, 2017, a floor-level survey was performed in the building by Brad Hillebrandt with B. Hillebrandt Soil Testing, Inc. using a manometer. Because a floor plan of the existing residence was not available, an approximate floor plan was developed using taped measurements. The results of the floor-level survey are presented in Appendix B. The measurements shown are relative elevations (in inches) below a high point located (labelled as 0.0) in the room immediately to the right of the entryway.

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5.0 SUBSURFACE CONDITIONS

A regional geologic map prepared by Graymer, et al. (2000), a portion of which is presented on Figure 3, indicates the site is underlain by artificial fill (af). The results of our CPTs and DPTs indicate the site is blanketed by fill that extends to a depth of approximately 16 to 16-1/2 feet bgs. The upper approximately seven feet of fill consists of loose to dense, relatively clean sand. Between depths of about 7 and 14 feet bgs, the fill is significantly weaker and consists of soft to medium stiff clay. The lower 2 to 3 feet of the fill consists of loose to medium dense silty sand. We believe the portion of the fill below a depth of about five feet bgs was likely placed in water using a suction dredge, whereas, the upper fill was placed on dry land.

The fill is underlain by a weak and highly compressible marine clay deposit known locally as Bay Mud that extends to more than 90 and 60 feet at the CPT-1 and CPT-2 locations, respectively (i.e., the bottom of the Bay Mud layer was not reached with the CPTs). The upper portion of the Bay Mud deposit is very soft to soft. The strength of the Bay Mud slowly increases with depth due to the overburden pressure.

The depth to groundwater at the CPT-1 location was estimated to be at a depth of about 4.7 feet bgs by performing a pore pressure dissipation test. Available historic groundwater information presented in the California Geologic Survey (CGS) Seismic Hazard Zone Report for the Oakland West Quadrangle indicates the historic high groundwater at the site is approximately 5 feet bgs. The groundwater level is expected to fluctuate 2 to 3 feet due to tides and seasonally, depending on the seasonal rainfall.

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6.0 SEISMIC CONSIDERATIONS

The San Francisco Bay Area is considered to be one of the more seismically active regions in the world. The results of our evaluation regarding seismic considerations for the project site are presented in the following sections.

6.1 Regional Seismicity and Faulting

The site is located in the Coast Ranges geomorphic province of California that 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 plate and North American plate and subsequent strike-slip faulting along the San Andreas Fault system. The San Andreas Fault is more than 600 miles long from Point Arena in the north to the Gulf of California in the south. The Coast Ranges 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 Hayward, San Andreas and Calaveras faults. These and other faults in the region are shown on Figure 4. The fault systems in the Bay Area consist of several major right-lateral strike-slip faults that define the boundary zone between the Pacific and the North American tectonic plates. Numerous damaging earthquakes have occurred along these fault systems in recorded time. For these and other active faults within a 50-kilometer radius of the site, the distance from the site and estimated mean characteristic moment magnitude1 [Working Group on California Earthquake Probabilities (USGS 2008) and Cao et al. (2003)] are summarized in Table 1.

1 Moment magnitude 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.

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TABLE 1 Regional Faults and Seismicity

Approximate Direction from Maximum Fault Segment Distance from Site Site Magnitude (km) Total Hayward 8.6 Northeast 7.00 Total Hayward-Rodgers Creek 8.6 Northeast 7.33 N. San Andreas - Peninsula 21 West 7.23 N. San Andreas (1906 event) 21 West 8.05 N. San Andreas - North Coast 24 West 7.51 Mount Diablo Thrust 25 East 6.70 Total Calaveras 25 East 7.03 San Gregorio Connected 27 West 7.50 Green Valley Connected 30 East 6.80 Rodgers Creek 35 North 7.07 Monte Vista-Shannon 39 South 6.50 Greenville Connected 42 East 7.00 West Napa 42 North 6.70 Great Valley 5, Pittsburg Kirby Hills 47 East 6.70

In the past 200 years, four major earthquakes (i.e., Magnitude > 6) have been recorded on the San Andreas Fault. In 1836, an earthquake with an estimated maximum intensity of VII on the Modified Mercalli (MM) Intensity Scale occurred east of Monterey Bay on the San Andreas

Fault (Toppozada and Borchardt, 1998). The estimated moment magnitude, Mw, for this earthquake is about 6.25. In 1838, an earthquake occurred on the Peninsula segment of the San

Andreas Fault. Severe shaking occurred with an MM of about VIII-IX, corresponding to an Mw of about 7.5. The San Francisco Earthquake of 1906 caused the most significant damage in the history of the Bay Area in terms of loss of lives and property damage. This earthquake created a surface rupture along the San Andreas Fault from Shelter Cove to San Juan Bautista

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approximately 470 kilometers in length. It had a maximum intensity of XI (MM), an Mw of about 7.9, and was felt 560 kilometers away in Oregon, Nevada, and Los Angeles. The most recent earthquake to affect the Bay Area was the Loma Prieta Earthquake of October 17, 1989 with an Mw of 6.9. This earthquake occurred in the Santa Cruz Mountains about 90 kilometers southwest of the site.

In 1868, an earthquake with an estimated maximum intensity of X on the MM scale occurred on the southern segment (between San Leandro and Fremont) of the Hayward Fault. The estimated

Mw for the earthquake is 7.0. In 1861, an earthquake of unknown magnitude (probably an Mw of about 6.5) was reported on the Calaveras Fault. The most recent significant earthquake on this fault was the 1984 Morgan Hill earthquake (Mw = 6.2).

The U.S. Geological Survey's 2014 Working Group on California Earthquake Probabilities has compiled the earthquake fault research for the San Francisco Bay area in order to estimate the probability of fault segment rupture. They have determined that the overall probability of moment magnitude 6.7 or greater earthquake occurring in the San Francisco Region during the next 30 years (starting from 2014) is 72 percent. The highest probabilities are assigned to the Hayward Fault, Calaveras Fault, and the northern segment of the San Andreas Fault. These probabilities are 14.3, 7.4, and 6.4 percent, respectively.

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6.2 Geologic Hazards

Because the project site is in a seismically active region, we evaluated the potential for earthquake-induced geologic hazards including ground shaking, ground surface rupture, liquefaction,2 lateral spreading,3 and cyclic densification4. We used the results of the CPTs to evaluate the potential of these phenomena occurring at the project site.

6.2.1 Ground Shaking

The seismicity of the site is governed by the activity of the Hayward Fault, although ground shaking from future earthquakes on other faults will also be felt at the site. The intensity of earthquake ground motion at the site will depend upon the characteristics of the generating fault, distance to the earthquake epicenter, and magnitude and duration of the earthquake. We judge that strong to very strong ground shaking could occur at the site during a large earthquake on one of the nearby faults.

6.2.2 Liquefaction and Associated Hazards

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.

As shown on Figure 5, the site has been mapped within a zone of liquefaction potential on the map titled State of California Seismic Hazard Zones, Oakland West Quadrangle, Official Map,

2 Liquefaction is a phenomenon where loose, saturated, cohesionless soil experiences temporary reduction in strength during cyclic loading such as that produced by earthquakes. 3 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. 4 Cyclic densification is a phenomenon in which non-saturated, cohesionless soil is compacted by earthquake vibrations, causing ground-surface settlement.

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prepared by the California Geological Survey (CGS), dated February 14, 2003. ). The CGS has provided recommendations for the content of site investigation reports within seismic hazard zones in Special Publication 117 (SP 117) titled Guidelines for Evaluating and Mitigating Seismic Hazard Zones in California, dated September 11, 2008. SP117 recommends that at least one exploration point extend to a depth of at least 50 feet to evaluate liquefaction potential.

Liquefaction susceptibility was assessed using the software CLiq v2.0 (GeoLogismiki, 2016). CLiq uses measured field CPT data and assesses liquefaction potential given a user-defined earthquake magnitude and peak ground acceleration (PGA). We performed a liquefaction triggering analysis using the CPT data within the site vicinity in accordance with the methodology by Boulanger and Idriss (2014). Our analyses were performed assuming a high groundwater depth of 4 feet bgs. In accordance with the 2016 CBC, we used peak ground accelerations of 0.49 times gravity (g) in our liquefaction evaluation; this peak ground

acceleration is consistent with the Maximum Considered Earthquake Geometric Mean (MCEG) peak ground acceleration adjusted for site effects (PGAM). We also used a moment magnitude 7.33 earthquake, which is consistent with the mean characteristic moment magnitude for the Hayward Fault, as presented in Table 1.

Our liquefaction analyses indicate there are thin layers of potentially liquefiable fill between depths of about 4 and 6 feet bgs and between 14 and 16 feet bgs. We estimate settlement due to post-liquefaction reconsolidation of these layers following a Maximum Considered Earthquake

(MCE) event with PGAM of 0.49g would be up to approximately one inch and differential settlement could be up to 1/2 inch over a horizontal distance of 30 feet. These settlement estimates are for a “free-field” condition. Settlement of the existing building during a seismic event is expected to be larger than these estimates due to ratcheting displacement due to soil- structure interaction. In addition, because the uppermost liquefiable soil layer is relatively shallow, we conclude there is a relatively high potential for surface manifestations from liquefaction, such as sand boils. Where sand boils form, abrupt differential settlement of several inches could occur.

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6.2.3 Lateral Spreading

Lateral spreading occurs when a continuous layer of soil liquefies at depth and the soil layers move toward an unsupported face, such as an open slope cut, or in the direction of a regional slope or gradient by earthquake and gravitational forces. There is an unsupported face along the southern edge of the Oakland-Alameda Estuary, approximately 800 feet north of the site; however, considering the two potentially liquefiable layers encountered during our investigation are thin (two feet thick or less) and separated by a layer of cohesive soil that is not susceptible to lateral spreading, we conclude the potential for lateral spreading to occur at the site is low.

6.2.4 Cyclic Densification

Seismically induced compaction (also referred to as cyclic densification) of non-saturated granular soil (granular soil above groundwater table) can occur during an earthquake, resulting in settlement of the ground surface and overlying improvements. The sand fill encountered above the groundwater table at the site is sufficiently dense to resist cyclic densification. Therefore, we conclude the potential for settlement of the subject building due to cyclic densification is low.

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

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7.0 DISCUSSION AND CONCLUSIONS

Based on our field investigation and research of historical map and aerial photographs, we conclude the following:

1. The site which the 2815 San Diego Road building occupies was originally part of San Francisco Bay and is just west of the large marsh that underlies the northwestern part of Alameda (see Figure 6). The land in the immediate site vicinity was reclaimed by fill placement to support a former railroad line in the early 1900’s. Large-scale fill placement to construct the NAS occurred in the late 1930’s and early 1940’s.

2. The original residence constructed on the 2815 San Diego Road site in the early 1940’s likely experienced significant distress due to differential settlement, which made it uninhabitable. It appears it was demolished some time before 1958, although it is possible the building may have burnt down.

3. The current residence was built on the site between 1963 and 1968 on approximately the same footprint as the original residence that was demolished prior to 1958.

4. It appears the current residence was constructed over a demolished pile-supported railroad structure constructed in the late 1800’s. We believe the pile foundations from this former structure were left in place.

5. The Bay Mud layer beneath the site, which is more than 75 feet thick, is likely normally consolidated under the weight of the fill placed before the late 1930’s; however, settlement due to secondary compression (creep) is still occurring. We estimate the creep settlement over the next 20 years will be on the order of 2 to 4 inches.

6. The existing building has experienced differential settlement of about 3-1/2 inches since its construction in the 1960’s. There are many areas within the building where the differential settlement exceeds one inch over a horizontal distance of 25 feet, which is the industry standard for “acceptable” differential settlement. The abrupt differential

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settlement that has occurred is likely the result of old timber piles forming “hard points’ beneath portions of the foundation. As the ground settles around these hard points, abrupt differential settlement occurs. The existing foundation has large cracks, as well as voids beneath portions of it. In our opinion, repair of the existing foundation is not feasible.

7. Considering the ongoing settlement due to secondary compression of the Bay Mud and the “hard points” beneath the residence, ongoing maintenance consisting of shaving and/or reframing doors, adjusting and/or replacing windows, and patching of interior and exterior cracks will be required to address future differential settlement of the building. Because the doors and windows have been adjusted/shaved over the years to address the differential settlement, relevelling of the building would necessitate replacing most of the existing doors and windows. Relevelling would also result in severe cracking of the exterior stucco.

8. The building may settle up to several inches during and following a major earthquake due to liquefaction and, possibly, also due to collapse of voids beneath the existing foundations. This settlement will cause significant cracking of the interior and exterior walls and may result in jamming of doors and windows, making exiting the building immediately after the earthquake difficult. Therefore, if the building will be occupied in its current condition, it should be recognized it may be necessary to break windows and/or break down doors to exit after an earthquake.

8.0 RECOMMENDATIONS

In our opinion, the existing building may be occupied in the short-term provided the risk of jammed points of exits following an earthquake (#8 above) is adequately addressed. In the long- term, we believe it would be prudent to replace the building with a new structure to avoid long- term maintenance costs. To minimize the potential for total and differential settlement of a new structure under static and seismic conditions, it would be necessary to support the building on deep foundations; however, because of the significant thickness of the Bay Mud layer beneath

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the site, deep foundations are not economically feasible. One alternative to a deep foundation system would be to support the building on a mat foundation underlain by geogrid-reinforced engineered fill. Prior to placing the engineered fill, the existing piles beneath the building footprint should be extracted. Removal of these hard points will significantly reduce the differential settlement the new building will experience. We estimate total settlement of the new building would be between 2 and 4 inches over a period of 20 years and differential settlement would be on the order of one inch over a horizontal distance of 30 feet. After a major earthquake, we estimate an additional one inch of total settlement and 1/2 inch of differential settlement over a horizontal distance of 30 feet may occur.

Our recommendations for site preparation and grading, foundation design, and seismic design for a new residence are presented in the following sections.

8.1 Site Demolition and Grading

Site demolition should include the removal of all existing pavements, underground utilities and buried foundations, if any. In general, abandoned underground utilities should be removed to the property line or service connections and properly capped or plugged with concrete. Voids resulting from demolition activities should be properly backfilled with compacted fill under the observation of our field engineer following the recommendations provided later in this section. After demolition, existing piles should be located and extracted. The holes resulting from pile extraction should be filled immediately with granular fill. The on-site sand fill is acceptable to use for filling the voids resulting from pile removal.

After removal of the existing piles, the soil beneath the new building footprint should be excavated to a depth of four feet below existing grade. The base of the excavation should extend at least five feet beyond the perimeter of the proposed buildings except where constrained by property lines. The excavation subgrade should be track-walked to provide a firm surface for placement of the geogrid layer. Rubber-tired and vibratory compaction equipment should not be used. After placement of the geogrid layer, the on-site sandy fill should be placed in lifts not exceeding eight inches in loose thickness, moisture-conditioned to above optimum moisture

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content, and compacted to at least 95 percent relative compaction5. The second geogrid layer should be placed at a depth of 2-1/2 feet below pad grade, followed by placement and compaction of the on-site sandy fill to pad grade.

Two layers of Tensar TX140 geogrid should be placed in the fill beneath the new building. The first layer should be placed on the track-walked subgrade at a depth of four feet below the bottom of the mat foundation. The second layer should be placed at a depth of 2-1/2 feet below mat subgrade; however, it should be confirmed the geogrid will be below any proposed utility lines. The geogrid should be placed in accordance with the manufacturer’s specifications with a minimum overlap of two feet. The geogrid layers should extend at least five feet outside the proposed building footprint. Utility lines running parallel to the mat edge should be located at least three feet from the edge of the mat and should not penetrate the edge reinforcement. Where penetrations in the geogrid reinforcement are necessary to install utilities extending into the building, the geogrid should be repaired with a minimum overlap of one foot with nylon zip ties at 18 inches on center along the overlap.

Backfill for utility trenches is also considered fill, and it should be compacted according to the recommendations presented in this section. Special care should be taken when backfilling utility trenches within the building footprint and beneath pavements. Poor compaction may result in excessive settlement and damage to the building and/or pavements. If on-site sand or imported clean sand or gravel is used for trench backfill, it should be compacted to at least 95 percent relative compaction. Jetting of trench backfill should not be permitted.

Fill should consist of on-site soil or imported soil (select fill) that is free of organic matter, contains no rocks or lumps larger than three inches in greatest dimension, has a liquid limit of less than 40 and a plasticity index lower than 12, and is approved by the Geotechnical Engineer. Samples of proposed imported fill material should be submitted to the Geotechnical Engineer at least three business days prior to use at the site. The grading contractor should provide analytical

5 Relative compaction refers to the in-place dry density of soil expressed as a percentage of the maximum dry density of the same material, as determined by the ASTM D1557 laboratory compaction procedure.

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test results or other suitable environmental documentation indicating the imported fill is free of hazardous materials at least three days before use at the site. If this data is not available, up to two weeks should be allowed to perform analytical testing on the proposed imported material.

8.2 Mat Foundation

We recommend the mat foundation supported on recompacted fill be designed using allowable bearing capacities of 3,000 pounds per square foot (psf) for dead-plus-live loads and 4,000 psf for total loads (including seismic and wind loads). To evaluate the pressure distribution beneath the mat foundations under static conditions, we recommend using a modulus of vertical subgrade reaction of 40 pounds per cubic inch (pci); this value has been reduced to account for the size of the mat. To check whether the mat is sufficiently stiff to adequately limit differential settlement during an earthquake, the deflection of the mat should be checked by assuming a five-foot-wide strip along the building perimeter with a reduced modulus of 10 pci and a modulus of 40 pci for the rest of the mat. The perimeter of the mat should be thickened to provide at least nine inches of embedment below the lowest outside adjacent finished grade.

Lateral loads may be resisted by a combination of friction along the base of the mat and passive resistance against the vertical faces of the mat foundation. To compute lateral resistance, we recommend using an equivalent fluid weight of 300 pounds per cubic foot (pcf); the upper foot of soil should be ignored unless confined by a slab or pavement. Frictional resistance should be computed using a base friction coefficient of 0.35 where the mat is in direct contact with soil. Where a vapor retarder is placed beneath the mat, a base friction coefficient of 0.20 should be used. The passive pressure and frictional resistance values include a factor of safety of at least 1.5.

The mat foundation should take support below a zone-of-influence line extending up at an inclination of 1.5:1 (horizontal:vertical) from the bottom of utility trenches running parallel to the building perimeter. To avoid thickening the edge of the mat foundation adjacent to utility trenches, the portion of the trench that extends below the zone-of-influence line can be filled

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with controlled low-strength material (CLSM) with a 28-day unconfined compressive strength of at least 100 pounds per square inch (psi).

A vapor retarder meeting the requirements for Class A vapor retarders stated in ASTM E1745 should be placed directly on the subgrade soil below the mat in all living spaces. The vapor retarder should be placed in accordance with the requirements of ASTM E1643. These requirements include overlapping seams by six inches, taping seams, and sealing penetrations in the vapor retarder. If required by the structural engineer, the vapor retarder can be covered with two inches of sand to aid in curing the concrete.

Concrete mixes with high water/cement (w/c) ratios result in excess water in the concrete, which increases the cure time and results in excessive vapor transmission through the mat foundation. Therefore, concrete for the mat should have a low w/c ratio - less than 0.45. Water should not be added to the concrete mix in the field. If necessary, workability should be increased by adding plasticizers. In addition, the mat should be properly cured. Before the floor covering is placed, the contractor should check that the concrete surface and the moisture emission levels (if emission testing is required) meet the manufacturer’s requirements.

8.3 Seismic Design

We have assumed the new building will be designed using the seismic provisions in the 2016 CBC. We can provide updated seismic parameters in case the new building is delayed until the next code cycle. The latitude and longitude of the site are 37.7776° and -122.2755°, respectively. Section 1613A of the 2016 California Building Code (CBC) and Section 20.3.1 of ASCE 7-10 indicate if liquefiable soil is present at a site, it is classified as Site Class F and a site- specific response study is required; however, if the period of the structure is less than 0.5 second, the site class can be determined from Section 20.3 of ASCE 7-10. Based on our experience the period of the new building will be less than 0.5 second. Therefore, we recommend Site Class E be used for design. Hence, in accordance with the 2016 CBC, we recommend the following:

 SS = 1.50g, S1 = 0.60g

 SMS = 1.35g, SM1 = 1.44g

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 SDS = 0.9g, SD1 = 0.96g

 PGAM = 0.49g  Seismic Design Category D for Risk Categories I, II, and III.

9.0 LIMITATIONS

This 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 recommendations made in this report are based on the assumption that the subsurface conditions do not deviate appreciably from those disclosed in the CPTs or DPTs. If any variations or undesirable conditions are encountered during construction, we should be notified so that additional recommendations can be made.

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FIGURES SITE

Base map: The Thomas Guide Alameda County 01/4 1/2 Mile 2002 Approximate scale

2815 SAN DIEGO ROAD Alameda, California SITE LOCATION MAP ROCKRIDGE GEOTECHNICAL Date 12/26/17 Project No.17-1424 Figure 1 EXPLANATION CPT-1 Approximate location of cone penetration test by Rockridge Geotechnical Inc., December 20, 2017

DPT-1 Approximate location of dynamic penetrometer test by Rockridge Geotechnical Inc., December 15, 2017

HA-1 Approximate location of hand-auger boring by Rockridge Geotechnical Inc., ______, 2017 CPT-1 Project limits

DPT-2

DPT-1 HA-1

0 30 Feet CPT-2 Approximate scale

Base map: Google Earth, 2017.

41829 & 41875 OSGOOD ROAD San Francisco, California

SITE PLAN Date 03/05/15 Project No. 15-833 Figure 2 ROCKRIDGE GEOTECHNICAL QtQt SITE

afaf

QhyQhy QsQs

Base map: Google Earth with U.S. Geological Survey (USGS), Alameda County, 2016.

EXPLANATION

af Artificial Fill

Qhy Alluvium (late Holocene)

Qs Beach and dune sand (Quaternary) 02,000 4,000 Feet

Qt Marine terrace deposits (Pleistocene) Approximate scale Geologic contact: dashed where approximate and dotted where concealed, queried where uncertin

2815 SAN DIEGO ROAD Alameda, California REGIONAL GEOLOGIC MAP ROCKRIDGE GEOTECHNICAL Date12/26/17 Project No.17-1424 Figure 3 G W r e e a s t

t N V

Gr all a p e ee a y

n 4b

V H a

ayw ll e

y a r d-Ro

dge G

r rs Cr e a t eek Fault V a l l e y

05

SITE

Point Reye s Fault

Sa Gree n Andr M ou n nt v D ille e ia a b s Faul lo F T a h u ru lt t st

S a n

G

r e g o r i o

F a u l t H ayw

a rd -Rodgers Cr

C a l av e e e r k Fa a s

Fa ult ul

t

Mon te V is ta -S ha nno n Fa u lt Base Map: U.S. Geological Survey (USGS), National Seismic Hazards Maps - Fault Sources, 2008. EXPLANATION

Strike slip 0 5 10 Miles Thrust (Reverse)

Normal Approximate scale

2815 SAN DIEGO ROAD Alameda, California REGIONAL FAULT MAP ROCKRIDGE GEOTECHNICAL Date 12/26/17 Project No.17-1424 Figure 4 SITE

EXPLANATION 02,000 4,000 Feet Liquefaction; Areas where historic occurence of liquefaction, or local topographic, geological, geotechnical, and subsurface Approximate scale water conditions indicate a potential for permanent ground displacements.

Earthquake-Induced Landslides; Areas where previous occurence of Reference: landslide movement, or local topographic, geological, geotechnical, and State of California "Seismic Hazard Zones" Oakland West Quadrangle. subsurface water conditions indicate a potential for permanent ground Released on February 14, 2003 displacements.

2815 SAN DIEGO ROAD Alameda, California SEISMIC HAZARDS ZONE MAP ROCKRIDGE GEOTECHNICAL Date12/26/17 Project No.17-1424 Figure 5 SITE

00.5 1 MIle

Approximate scale

2815 SAN DIEGO ROAD Alameda, California 1895 TOPOGRAPHIC SURVEY ROCKRIDGE GEOTECHNICAL Date02/02/18 Project No.17-1424 Figure 6

APPENDIX A Logs of Cone Penetration Tests and Dynamic Penetrometer Tests

-3.5 -3.1 -2.0 -1.8 -2.1 -0.8 -0.6 -1.1 -3.2 -1.9 -1.8 -1.3 -3.0 -3.6 -2.7 -2.5 -1.6 -1.7 -1.7 -2.8 -1.4 -1.2 -1.8 -1.2 -2.4 -2.2 -3.1 -1.8 -1.1 -1.0 -0.9 -2.3 -0.2 -0.4 -3.1 -0.5 -0.3 -0.3 -2.2 -2.4 -0.5 -0.1 -3.6 -2.5 -1.2 -0.8 -0.2 -0.3 0.0 -0.4 -2.3 -2.6 -3.2 -2.6 -1.5 -1.7 -2.7 -2.7 -2.6 -1.7 -2.3 -2.4 -1.8 -1.9 -2.6 -3.5 -2.4 -2.0 -0.6 -1.7 -3.2 -3.1 -3.0 -2.1 -0.8

-3.4 -2.5 -2.1 -1.6 -1.8 -0.6 -0.6 -1.1 -1.2 -1.1 -1.1 -2.8 -3.1 -2.4 2815 San Diego Road, Alameda -2.7 12/15/2017 -2.1 -1.5 -1.2 -0.7 B.H. 0' 20'

APPENDIX B Floor-Level Survey Results

SBT legend 1. Sensitive fine grained 4. Clayey silt to silty clay 7. Gravely sand to sand Total depth: 90.55 ft, Date: 12/29/2017 2. Organic material 5. Silty sand to sandy silt 8. Very stiff sand to clayey sand Estimated Groundwater Depth: 4.9 feet 3. Clay to silty clay 6. Clean sand to silty sand 9. Very stiff fine grained Cone Operator: Middle Earth Geo Testing, Inc.

2815 SAN DIEGO ROAD CONE PENETRATION TEST RESULTS Alameda, California CPT-1 ROCKRIDGE GEOTECHNICAL Date 02/02/18Project No. 17-1424 Figure A-1 SBT legend 1. Sensitive fine grained 4. Clayey silt to silty clay 7. Gravely sand to sand Total depth: 60.53 ft, Date: 12/20/2017 2. Organic material 5. Silty sand to sandy silt 8. Very stiff sand to clayey sand Estimated Groundwater Depth: 5.0 feet 3. Clay to silty clay 6. Clean sand to silty sand 9. Very stiff fine grained Cone Operator: Middle Earth Geo Testing, Inc.

2815 SAN DIEGO ROAD CONE PENETRATION TEST RESULTS Alameda, California CPT-2 ROCKRIDGE GEOTECHNICAL Date 02/02/18Project No. 17-1424 Figure A-2 0

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2815 SAN DIEGO ROAD DYNAMIC PENETROMETER Alameda, California TEST RESULTS, DPT-1 ROCKRIDGE GEOTECHNICAL Date 12/26/17Project No. 17-1424 Figure A-3 0

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2815 SAN DIEGO ROAD Alameda, California DYNAMIC PENETROMETER TEST RESULTS, DPT-2 ROCKRIDGE GEOTECHNICAL Date 12/26/17Project No. 17-1424 Figure A-4 0

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2815 SAN DIEGO ROAD Alameda, California DYNAMIC PENETROMETER TEST RESULTS, DPT-3 ROCKRIDGE GEOTECHNICAL Date 12/26/17Project No. 17-1424 Figure A-5

APPENDIX C Historical Topographic Maps and Aerial Photographs

5134192.3

1939

= 500' 5134192.3

1946

= 500' 5134192.3

1958

= 500' 5134192.3

1968

= 500' Historical Topo Map 1949

This report includes information from the following map sheet(s). 0 Miles 0.25 0.5 1 1.5

NW N NE TP, Oakland West, 1949, 7.5-minute SITE NAME: 2815 San Diego Road ADDRESS: 2815 San Diego Road Alameda, CA 94501 W E CLIENT: Rockridge Geotechnical

SW S SE 5134192 - 2 page 12 Historical Topo Map 1915

This report includes information from the following map sheet(s). 0 Miles 0.25 0.5 1 1.5

NW N NE TP, San Francisco, 1915, 15-minute SITE NAME: 2815 San Diego Road ADDRESS: 2815 San Diego Road Alameda, CA 94501 W E CLIENT: Rockridge Geotechnical

SW S SE 5134192 - 2 page 14 Historical Topo Map 1899

This report includes information from the following map sheet(s). 0 Miles 0.25 0.5 1 1.5

NW N NE TP, San Francisco, 1899, 15-minute SITE NAME: 2815 San Diego Road ADDRESS: 2815 San Diego Road Alameda, CA 94501 W E CLIENT: Rockridge Geotechnical

SW S SE 5134192 - 2 page 15 Historical Topo Map 1895

This report includes information from the following map sheet(s). 0 Miles 0.25 0.5 1 1.5

NW N NE TP, San Francisco, 1895, 15-minute SITE NAME: 2815 San Diego Road ADDRESS: 2815 San Diego Road Alameda, CA 94501 W E CLIENT: Rockridge Geotechnical

SW S SE 5134192 - 2 page 16