DRAFT GEOTECHNICAL ENGINEERING STUDY. Mixed-Use Development 4 th Street and University Avenue Berkeley, California

Transcription

1 GEOTECHNICAL ENGINEERING STUDY Mixed-Use Development 4 th Street and University Avenue Berkeley, California Prepared for: BHV CenterStreet Properties, LLC 500 La Gonda Way, Suite 295 Danville, California Prepared by: GEOSPHERE CONSULTANTS, INC Crow Canyon Road, Suite 210 San Ramon, California Geosphere Project No A

2 TABLE OF CONTENTS 1.0 INTRODUCTION Purpose and Scope Site Description Proposed Development Validity of Report PROCEDURES AND RESULTS Literature Review Field Exploration Laboratory Testing GEOLOGY AND SEISMICITY Site and Geologic Setting Seismic Setting FIELD AND LABORATORY FINDINGS Subsurface Soil Conditions Groundwater GEOLOGIC HAZARDS Seismic Induced Hazards Consolidation Settlement Expansive Soils CONCLUSIONS AND ENGINEERING RECOMMENDATIONS Seismic Coefficients Site Grading Utility Trench Construction Temporary Excavation Slopes and Shoring Building Foundations Concrete Slabs-on-Grade Below-Grade Walls Pavement Design Observation and Testing During Construction LIMITATIONS AND UNIFORMITY OF CONDITIONS REFERENCES... 26

3 TABLE OF CONTENTS (cont d.) FIGURES Figure 1 Site Vicinity Map Figure 2 Exploration Site Plan Figure 3 Historical Creek Channel Map Figure 4 Site Vicinity Geologic Map Figure 5 Regional Fault Map Figure 6A Schematic Geologic Cross Section A-A Figure 6B Schematic Geologic Cross Section B-B APPENDIX A FIELD EXPLORATION Boring Logs Cone Penetration Test Results Summary Table of Boring Logs by Others APPENDIX B LABORATORY TEST RESULTS Liquid and Plastic Limits Test Report Particle Size Distribution Report Consolidation Test Report Unconfined Compression Test R-Value Test Report Compaction Test Report APPENDIX C LIQUEFACTION EVALUATION

4 GEOTECHNICAL ENGINEERING STUDY Project: Client: Mixed-Use Development 4 th Street at University Avenue, Berkeley, California BHV CenterStreet Properties, LLC Danville, California 1.0 INTRODUCTION 1.1 Purpose and Scope The purposes of this study were to prepare a Geotechnical Engineering Study, evaluate the subsurface conditions at the site and prepare geotechnical recommendations for the proposed development. We have provided specific recommendations regarding suitability and settlement concerns relative to the proposed structural design. The scope of this study included the field exploration, laboratory testing, engineering analysis of the collected samples and test results, and preparation of this report. The conclusions and recommendations presented in this report are based on the limited samples collected and analyzed during this study, and on prudent engineering judgment and experience. This study did not include an in-depth assessment of potentially toxic or hazardous materials that may be present on or beneath the site. 1.2 Site Description The proposed project is located west of 4 th Street in Berkeley, and bound by 4 th Street on the east, University Avenue on the south, Hearst Avenue on the north, and the Union Pacific Railroad (UPRR) tracks on the west, as shown on Figure 1, Site Vicinity Map. The geographic coordinates of the proposed site improvements are approximately degrees north latitude and degrees west longitude. The proposed project site is about 96,260 square feet in area, and is mostly occupied by a paved parking lot. A small, single-story building, currently occupied by H2 Fitness Studio occupies the southwest corner of the property. The property measures approximately 238 feet by 405 feet, and is essentially level. Project site elevations range between approximately 9.0 at the northwest and southwest corners of the property, to about 11.1 near the northeast corner of the property, based on City of Berkeley (COB) Datum (El COB Datum = El NGVD 1929 or Mean Sea Level Datum). The local topography slopes west toward San Francisco Bay.

5 1.3 Proposed Development TBD 1.4 Validity of Report This report is valid for three years after publication. If construction begins after this time period, Geosphere should be contacted to confirm that the site conditions have not changed significantly. If the proposed development differs considerably from that described above, Geosphere should be notified to determine if additional recommendations are required. Additionally, if Geosphere is not involved during the geotechnical aspects of construction, this report may become wholly or in part invalid; since Geosphere s geotechnical personnel need to verify that the subsurface conditions anticipated preparing this report are similar to the subsurface conditions revealed during construction. Geosphere s involvement should include foundation and grading plan review; observation of foundation excavations; grading observation and testing; testing of utility trench backfill; observation and testing of subgrade preparation in hardscape and pavement areas, and flexible pavement sections. 2

6 2.0 PROCEDURES AND RESULTS 2.1 Literature Review Pertinent geologic and geotechnical literature pertaining to the site area, and previous geotechnical studies performed by others for projects in the site vicinity were reviewed. These included United States Geological Survey (USGS), California Geological Survey (CGS), and other online resources, and other applicable government and private publications and maps, as included in the References section. 2.2 Field Exploration Our field exploration program consisted of performing a combination of drilled test borings and Cone Penetration Tests. A total of four Cone Penetration Tests were performed at the site on April 3, 2014, and five test borings were drilled at the site on April 7 and 8, 2014 at the locations shown on Figure 2, Site Plan and Site Geology Map Test Borings The five test borings, designated B-1 through B-5, were drilled to depths between approximately 35 and 60 feet below ground surface using a truck mounted drill rig equipped with an eight-inch solid-stem auger and a rotary wash auger. In general, the solid-stem auger was used until groundwater was first encountered, at which point drilling was switched to the rotary-wash method. A Geosphere Registered Professional Engineer visually classified the materials encountered in the borings in general accordance with the Unified Soil Classification System as the borings were advanced. Relatively undisturbed soil samples were recovered at selected intervals using a three-inch outside diameter Modified California split spoon sampler containing six-inch long brass liners, or by hydraulically pushing a thin-walled, three-inch outside diameter, 36-inch long metal Shelby tube. A two-inch outside diameter Standard Penetration Test (SPT) sampler was used to obtain SPT blow counts and obtain disturbed soil samples. The samplers were driven by using a mechanical-trip, 140- pound hammer with an approximate 30-inch fall utilizing N-rods as necessary. Resistance to penetration was recorded in the field as the number of hammer blows required to drive the sampler the final foot of an 18-inch drive. Bulk samples were obtained in the upper few feet of the borings from the auger cuttings as needed. Following the completion of drilling, the boreholes were backfilled using a cement grout in accordance with City of Berkeley Environmental Health permit requirements. Excess drill cuttings as well as drilling fluid were collected and removed from the site. 3

7 For reporting purposes, all of the blow counts recorded using Modified California (MC) split spoon samplers in the field were subsequently converted to equivalent SPT blow counts using appropriate modification factors suggested by Burmister (1948); i.e., multiplied by a factor of 0.65 assuming a liner sample with an inner diameter of 2.5 inches. Therefore, the boring logs provided in this report all show equivalent SPT blow counts for the MC sampler in lieu of blow counts recorded in the field. The boring logs with descriptions of the various materials encountered in each boring, the penetration resistance values, and some of the laboratory test results are presented in Appendix A. The ground surface elevations indicated on the soil boring logs are approximate (rounded to the nearest ½ foot) and were estimated using a topographic site survey prepared by Moran Engineering, dated September 13, Elevations were based on City of Berkeley (COB) Datum (El COB Datum = El NGVD 1929 or Mean Sea Level Datum) Cone Penetration Tests As a part of the geotechnical exploration for the project, four Cone Penetrometer Tests (CPTs) designated CPT-1 to CPT-4, were conducted on April 3, 2014 to depths between 63 and 80 feet. Middle Earth Geo Testing, Inc., of Orange, California conducted the CPTs for this project, using a specially designed, truck-mounted, 25-ton cone apparatus. The instrumented cone assembly used for this project included a cone tip with a 60-degree apex, diameter of 35.6 millimeters (mm), and a projected cross sectional area of 10 square centimeters (cm 2 ), a sleeve segment with a surface area of 150 cm 2, and a pore pressure transducer near the base (shoulder) of the cone tip. Prior to the start of the test, the truck was jacked up and leveled on four pads to provide a stable reaction for the cone thrust. During the test, the instrumented cone was hydraulically pushed into the ground at a rate of about 20 millimeters per second (about 4 feet per minute), and continuous readings of cone tip resistance, sleeve friction, and pore pressure were digitally recorded. As the cone advanced, additional cone rods were added. PC-based data acquisition hardware in the CPT truck received electric signals from strain gauges mounted in the cone assembly, and generated graphical logs including cone resistance, friction resistance, friction ratio, and pore pressure ratio versus depth. CPT data was subsequently processed based on generally accepted soil behavior type correlations (Robertson et al., 1989) to interpret soil classification, and other properties such as SPT N-value and undrained shear strength were also estimated through correlations. CPT data plots and detailed tabulated logs are included in Appendix A. Seismic shear wave velocities were also measured at CPT-3 using a conventional cone assembly that also included a 4

8 three-component array of geophones. The seismic source consisted of a heavy wooden beam that was held firmly against the ground by stabilizing jacks at the side of the CPT truck. Seismic waves were generated at each test depth by striking one end of the beam with a sledge hammer attached to a rod string. An accelerometer mounted on the beam monitored "time zero", or the instant at which seismic waves were generated. The geophones mounted in the cone assembly monitored the waveform arrivals. Seismic data were acquired to at 5-foot intervals up to a depth of 30 feet, and measured at 10-foot intervals thereafter to the bottom depth of the hole (80 feet). Test results are included in Appendix A. 2.3 Laboratory Testing Laboratory tests were performed on selected samples to determine some of the physical and engineering properties of the subsurface soils. The results of the laboratory testing are presented on the boring logs, and included in Appendix B. The following soil tests were performed for this study: Dry Density and Moisture Content (ASTM D2216 and ASTM 2937) In-situ dry density and/or moisture tests were conducted on 23 samples to measure the in-place dry density and moisture content of the subsurface materials. These properties provide information to assist in evaluating the physical characteristics of the subsurface soils. Test results are shown on the boring logs. Atterberg Limits (ASTM D4318 and CT204) - Atterberg Limits tests were performed on three samples of cohesive soils encountered at the site. Liquid Limit, Plastic Limit, and Plasticity Index are useful in the classification and characterization of the engineering properties of soil, and help to evaluate the expansive characteristics of the soil and determine the USCS soil classification. Test results are presented in Appendix B, and on the boring logs. Particle Size Analysis (Wet and Dry Sieve - ASTM D422, D1140, and CT202) - Sieve analysis testing was conducted on two selected samples to measure the soil particle size distribution and the total percentage of fines (i.e., percent passing the USCS No. 200 sieve). This information is useful for characterizing the soil type according to USCS, and to assist in the evaluation of liquefaction susceptibility of granular soils or soils of relatively low cohesion. Test results are presented in Appendix B. Consolidation Test (ASTM D2435) - Consolidation tests were performed on three relatively undisturbed samples of the subsurface clay soils to assist in evaluating the compressibility characteristics of these materials. Consolidation test results are used in the analyses of site settlement occurring due to the squeezing of pore water out of saturated, 5

9 compressible materials in response to added loading from sources such as from new fill or structure (e.g., building foundation) loads. The results of the consolidation tests are included in Appendix B. Unconfined Compressive Strength Test (ASTM D2166) - Unconfined compression tests were performed on a total of two relatively undisturbed samples of the clayey subsurface soils to evaluate the undrained shear strengths of these materials. The unconfined tests were performed on samples having a diameter of 2.88 inches and heights of 4.5 and 5.0 inches. Failure was taken as the peak normal stress. The results of these tests are presented on the boring logs at the appropriate sample depths. R-Value Test (ASTM D2844 and CT301) One R-value test was conducted on a bulk composited sample of nearsurface materials collected from cuttings generated from Borings B-1, B-2 and B-4 to provide data on prospective pavement subgrade materials for use in new pavement section design. Test results are presented in Section 6.6 and in Appendix B. Compaction Test (ASTM D1557) - One laboratory compaction test was performed on a composited sample of nearsurface onsite soils to measure the maximum dry density and optimum moisture content of the material. Test results are used to establish a reference standard for measuring the degree of compaction of the material when placed and mechanically compacted during construction grading operations. The results of the test are presented in Appendix B. 6

10 3.0 GEOLOGY AND SEISMICITY 3.1 Site and Geologic Setting The site is located within the central portion of the Coast Ranges geomorphic province of California on the eastern side of San Francisco Bay. The Coast Ranges province consists of numerous small to moderate linear mountain ranges trending north to south and northwest to southeast. This province is characterized by northwest-trending faults and folds, erosion and deposition within the broad transform boundary between the North American and Pacific plates. Translational motion along the plate boundary occurs across a distributed zone of right-lateral shear expressed as a nearly 50-mile-wide zone of northwest-trending, near-vertical active strike-slip faults. This motion occurs primarily along the active San Andreas, Hayward, Calaveras and San Gregorio faults. Historically, the San Francisco Bay region is very seismically active and has been subjected to strong ground shaking from several large earthquakes. In particular, the southern Hayward Fault experienced a M6.8 earthquake in The site is located at the historical mouth of Strawberry Creek where the creek emptied into San Francisco Bay. The Creek and Watershed Map of Oakland and Berkeley (Sowers and Richard, 2009), as excerpted on Figure 3, Historical Creek Channel Map, shows the southern two-thirds of the site to have been occupied by a willow grove marshland circa 1850 that were representative of the creek channel and flood plain area of Strawberry Creek during the same time period. The same map shows the creek mouth emptying into a small tidal inlet of San Francisco Bay at the southwest corner of the site, and the northernmost portion of the site to be part of the land mass outside the marsh area, on the northeast shore of the tidal inlet. Various historical accounts indicate a portion of this mass to have been the site of Ohlone Indian shell mounds that were leveled during the initial development of this area in the latter part of the 19 th Century, when the tidal inlet and the willow grove marshland were filled in. The shell mounds as reported in published literature, and shown on a map by Radbruch (USGS, 1957) are estimated to have occupied the current adjacent Truitt and White property on the west side of the UPRR tracks, and an area east of 4 th Street, centered on Hearst Avenue. An 1895 USGS topographic map (San Francisco 15 Quadrangle) was reviewed that showed the mouth of Strawberry Creek, originally west of the Truitt and White property) to have been filled, and the creek channel to have been diverted to the south side of University Avenue, as well as showing development within proposed project site. Currently, Strawberry Creek is contained within an underground storm drain or culvert below University Avenue, emptying into San Francisco Bay west of the Eastshore Freeway (I-80) on the south side of University Avenue. 7

11 Geologic maps indicate native materials underlying the site and Strawberry Creek alluvial and marsh deposits to consist of Quaternary-age alluvial fan deposits of the Temescal Formation. A generalized geologic map of the site vicinity is presented as Figure 4, Site Vicinity Geologic Map. The Radbruch geologic map also shows deeper soft Bay Mud deposits located adjacent to the southwest corner of the proposed site within the old tidal inlet, and creek flood plain deposits extending across the site in a southwest to northeast direction, crossing to the east side of 4 th Street. 3.2 Seismic Setting Regional transpression has caused uplift and folding of the bedrock units within the Coast Ranges. This structural deformation occurred during periods of tectonic activity that began in the Pliocene and continues today. The site is located in a seismically active region that has experienced periodic, large magnitude earthquakes during historic times. This seismic activity appears to be largely controlled by displacement between the Pacific and North American crustal plates, separated by the San Andreas fault zone. This plate displacement produced regional strain that is concentrated along major faults of the San Andreas fault system including the San Andreas, Hayward, and Calaveras faults. These major fault zones extend through the Bay Area in a northwesterly direction and have produced approximately 12 earthquakes in the last two centuries that were strong enough to cause structural damage. The site is approximately 2.4 miles southwest of the Hayward fault. In addition, the Calaveras and Concord-Green Valley faults are mapped approximately 14 southeast and 16 miles northeast of the site, respectively, and the San Andreas fault is mapped as about 16 miles southwest of the site. In 2007, the Working Group on California Earthquake Probabilities (WGCEP 2007), in conjunction with the United States Geological Survey (USGS), published an updated report evaluating the probabilities of significant earthquakes occurring in the Bay Area over the next three decades. WGCEP 2007 estimated that there is a 93 percent probability that at least one magnitude 6.7 or greater earthquake will occur within the San Francisco Bay region over the next 30 years. This probability is an aggregate value that considered seven principal Bay Area fault systems and unknown faults (i.e., background values). 8

12 4.0 FIELD AND LABORATORY FINDINGS 4.1 Subsurface Soil Conditions Subsurface conditions below the project site were interpreted based on the results of our test borings and Cone Penetration Tests (CPTs) performed for this study, the results of our laboratory testing, and a review of previous as well as current subsurface explorations by others, including test borings conducted for an archaeological investigation (Archeo-Tec, 1999), a geotechnical investigation for the northeast corner of the site (T&R, 1999), and concurrent environmental test borings performed by Northgate Environmental in March The researched and newly generated data confirmed subsurface conditions consistent with the topographic and geologic history of the site; i.e., the site s location on filled marshland of Strawberry Creek within the approximately southern four-fifths of the site, and adjacent older, relatively competent alluvial deposits along the northern edge of the site bordering Hearst Avenue. Detailed descriptions of the various subsurface soil units encountered during subsurface explorations are described in the following paragraphs. Asphalt Concrete Paved Section Except for the single story fitness center building occupying the southwest corner of the site, the site was essentially covered by asphalt concrete (AC) pavement. Test borings indicate the thickness of asphalt paving varying between 2 and 6 inches in thickness, and typically between 3 and 4 inches in thickness. The AC was typically in worn condition, and underlain by 0 to 12 inches of baserock. Artificial Fill Below the pavement section, the portion of the site situated within the former Strawberry Creek floodplain/ marsh area was underlain by artificial fill believed to have been placed sometime in the latter half of the 19 th century during initial development of the area. Figure 2 shows an assessment of the approximate limit of marshland deposits based on available boring data, as well as the approximate original location of the Strawberry Creek channel, based on old survey maps as well as Sowers and Richard (2009). The thickness of marshland fill encountered in our borings varied between 4 and 8 feet in total thickness, and in our CPTs (CPT-2 through CPT-4) interpreted fill thicknesses were on the order of 4 feet. Logs of the 1999 archaeological borings performed throughout the site suggest similar fill thicknesses, with an average fill thickness of about 6 feet. The fills generally consisted of black to dark brown, loose to medium dense, clayey to silty sand to sandy silt, or black, stiff fat clay. These soils are weak and typically very compressible, and considered to be generally unsuitable for engineering support of major structures (i.e., not 9

13 including minor structures such as kiosks). Other fill encountered in borings outside the marshland area at the northwestern corner of the site, including Geosphere Boring B-4, consisted of about 3 feet of stiff, black fat clay in the Geosphere boring, which was relatively consistent with borings performed by others. Intermediate Clays and Gravels Over portions of the former marshland area, native clays and gravels situated in the soil profile between the surficial fills and underlying marshland deposits were encountered. Medium dense, dark brown clayey gravel deposits were encountered in Boring B-1 between approximate depths of 4 and 6 feet, which may be associated with deposition from Strawberry Creek. Additionally, similar deposits were encountered in two archaeological borings in close proximity to Boring B-1. Gray to dark gray, soft to medium-stiff sandy clay was encountered in Borings B-2 and B-3 between approximate depths of 5 and 8-1/2 feet. Similar materials were described in the Archeo-Tec (1999) report, where the material was described as an indeterminate layer below the fill, consisting of dark gray to black silty clay encountered beginning at depths of 5 to 7 feet, generally continuing to depths of 10 to 11 feet. These soils also appeared to be soft and compressible, and generally unsuitable for building support. Marsh and Tidal Deposits Artificial fill and intermediate clays and gravels were underlain by soft, highly compressible marshland and/or tidal deposits generally consisting of dark gray to black, soft, highly expansive fat clays, with minor amounts of fine sand and gravel. These deposits were encountered to approximate depths of 41 and 50 feet in Borings B-1 and B-5, respectively, and to the depth of exploration in Borings B-2 and B-3, drilled to respective depths of 40 and 45 feet. These clay deposits were found to be soft to very soft to depths of 10 to 15 feet, and medium stiff below these depths, becoming stiff at depths on the order of 30 feet to the bottom of the deposit where explored. CPT-2 and CPT-3, advanced near the central western and southwestern portions of the property, respectively, encountered what was interpreted as soft clay to respective depths of about 15 and 10 feet, becoming medium stiff to depths of about 28 feet, below which the clays were stiff in consistency to the bottom of the layer at respective depths of 39 and 37 feet. CPT-4, advanced along the central east edge of the property, encountered soft silty clay to a bottom depth of about 10 feet, underlain by denser alluvial sediments. 10

14 Consolidation tests performed on three samples of this soil stratum indicated the soils to be highly compressible. Atterberg limit tests performed on two samples recovered from depths of 10 feet (Boring B-3) and 19.5 feet (Boring B-5) resulted in measured Liquid Limits (LL) of 77 and 84, and corresponding respective Plasticity Indices (PI) of 52 and 60, indicative of very high plasticity and critically high expansion potential. Alluvial Soils Relatively competent alluvial soils likely consisting primarily of Temescal Formation deposits were encountered in the filled marshland area below the marsh deposits, as well as at or near the surface near the northern edge of the project site. At Boring B-4, drilled at the northwest corner of the site, the 3-foot thick surficial fill layer was underlain by brown, stiff sandy clay, becoming very stiff toward the maximum depth of exploration at 35 feet. CPT-1, advanced near the northeast corner of the site, encountered what is interpreted as generally stiff to very stiff silty clay grading to clayey silt in portions of the soil profile, to the depth of exploration of about 70 feet, with occasional lenses of dense sands, including a dense sand layer between depths of 5.5 and 7.5 feet. Marshland deposits in Boring B-1 were underlain by medium dense, dark gray to dark brown clayey sand and clayey gravel to the depth of exploration of 50 feet, and in Boring B-5 by very stiff, gray brown sandy clay to the maximum depth of exploration of 60 feet. In CPT-2 and CPT-3, the marshland deposits were interpreted to be underlain by a 1 to 2.5-foot thick layer of medium dense to dense sand, in turn underlain by stiff to very stiff silty clay with occasional dense sand layers, and ultimately, below respective depths of 59 and 78 feet, by hard clay to the maximum depth of exploration. In CPT-4, soils below a depth of about 10 feet were interpreted to consist of primarily medium dense sand to silty sand to a depth of about 19 feet, underlain by stiff to very stiff silty clay to a depth of about 54 feet, below which the soils consisted of very stiff to hard clay to the maximum depth of exploration of 70 feet. An Atterberg limit test performed on a native sample recovered from a depth of about 4.5 feet in Boring B-4 resulted in a measured Liquid Limit (LL) of 33, and a corresponding respective Plasticity Index (PI) of 15, indicative of medium plasticity and low expansion potential. 4.2 Groundwater Free groundwater was encountered in the five new borings during drilling at depths ranging between 6 and 13.5 feet, with depths between 6 and 6.5 feet encountered in Borings B-2, B-3 and B-5. Groundwater depths interpreted from the CPTs ranged between 4 and 5 feet. Stabilized groundwater depths were also observed in open 11

15 archaeological trenches near the center of the site in April 2014 to be on the order of 5 to 6 feet. Water depths of 13 and 17 feet were encountered in July 1999 during drilling of two test borings in the northeast quadrant of the site. The borings were backfilled with a neat cement grout in accordance with City of Berkeley Environmental Health drilling permit requirements shortly after drilling. We note that the borings may not have been left open for a sufficient period of time to establish equilibrium groundwater conditions. Groundwater levels can vary in response to time of year, variations in seasonal rainfall, tidal influence, well pumping, irrigation, and alterations to site drainage. 5.0 GEOLOGIC HAZARDS 5.1 Seismic Induced Hazards Seismic hazards resulting from the effects of an earthquake generally include ground shaking, liquefaction, lateral spreading, dynamic settlement (densification), fault ground rupture and fault creep, and tsunamis and seiches. The site is not necessarily impacted by all of these potential seismic hazards. Applicable potential seismic hazards are discussed and evaluated in the following sections in relation to the planned construction Ground Shaking The site will likely experience severe ground shaking from a major earthquake originating from a number of significant faults in the San Francisco Bay Area, including the Hayward, Calaveras, and San Andreas faults. Earthquake intensities vary throughout the Bay Area depending upon the magnitude of the earthquake, the distance of the site from the causative fault, the type of materials underlying the site and other factors. In addition to shaking of the structure, strong ground shaking can induce other related phenomena that may have an effect on structures, such as liquefaction or dynamic densification settlement; adjacent seismic slope failure, lurching or lateral spreading, or seismically induced waves; and inundation due to upslope dam failure Liquefaction Induced Phenomena The site is mapped as within a CGS Seismic Hazard Zone requiring liquefaction investigation (CGS, 2003). The site has also been mapped as in a zone of moderate liquefaction hazard (i.e., 3% of zoned area predicted to liquefy for M=7.1 earthquake) based on Holzer et al. (2005). Using the interactive liquefaction susceptibility map by the Association of 12

16 Bay Area Governments (ABAG), the old Strawberry Creek channel was mapped as in a zone of very high liquefaction susceptibility, and other portions of the site as in a zone of moderate liquefaction susceptibility. Research and historical data indicate that soil liquefaction generally occurs in saturated, loose granular soil (primarily fine to medium-grained, clean, poorly-graded sand deposits) during or after strong seismic ground shaking and is typified by a loss of shear strength in the affected soil layer, thereby causing the soil to flow as a liquid. Typically, liquefaction potential increases with increased duration and magnitude of cyclic loading. However, because of the higher intergranular pressure of the soil at greater depths, the potential for liquefaction is generally limited to the upper 40 feet of the soil. Potential hazards associated with soil liquefaction below or near a structure include loss of foundation support, lateral spreading, sand boils, and areal and differential settlement. Lateral spreading is lateral ground movement, with some vertical component, as a result of liquefaction. The soil literally rides on top of the liquefied layer. Lateral spreading can occur on relatively flat sites with slopes less than two percent under certain circumstances, generally when the liquefied layer is in relatively close proximity to an open, free slope face such as the bank of a creek channel. Lateral spreading can cause surficial ground tension cracking (i.e., lurch cracking) and settlement. The soils encountered in the subsurface investigation included isolated zones of medium dense sands and silts below the groundwater table of varying fines content which are conducive to liquefaction. Such soils were encountered in CPT-4. Therefore, a screening evaluation of liquefaction and resulting quantitative analysis of potential liquefactioninduced settlement was conducted for this site. Our methodology of liquefaction evaluation essentially followed the proceedings from the NCEER workshop on liquefaction (Youd and Idriss, 2001) and CGS Special Publication 117A (2008). This methodology compares a critical Cyclic Shear Stress (CSR) against the field Cyclic Resistance Ratio (CRR). When the CSR exceeds the CRR, the factor of safety falls below 1.0 and liquefaction can occur. The initiation of liquefaction settlement occurs when threshold ground acceleration is exceeded. The California Building Code specifies the use of a Peak Ground Acceleration (PGA M ) for use in liquefaction analyses. This resulted in a PGA used in our analysis of We also used a design Richter Moment Magnitude of 6.4 applicable to the northern segment of the Hayward fault. Groundwater was encountered in borings and exposed exploratory trenches at about 5 feet during the geotechnical and archaeological field investigations in April 2014, which was 13

17 assumed for analysis. A Factor of Safety (FS) of 1.0 was assumed to initiate liquefaction. We utilized the software Liquefy Pro, developed by CivilTech Software to perform our liquefaction analysis for exploration CPT-4. The following table presents a summary of our analysis results. Calculation spreadsheets and graphic printouts of our analyses are presented in Appendix C of this report. CPT Designation CPT-4 Summary of Calculated Liquefaction Settlement (inches) 1.2 (total); 0.55 to 0.7 (estimated differential) All layers calculated to liquefy were below the high groundwater table assumed for analysis, as the calculated settlement occurred in isolated, discontinuous, saturated sandy silt and silty sand layers between depths of 7 and 8 feet, and 11 and 16 feet. The site is not considered to be susceptible to lateral spreading due to the lack of a nearby free slope face. Therefore, the potential for future seismic settlement due to lateral spreading is judged to be very low Dynamic Densification (Settlement) Dynamic densification or settlement is a process in which unsaturated, relatively clean sands and silts are densified by the vibratory motion of a strong seismic event. At the project site, groundwater depths were encountered as shallow as about 5 feet from existing ground surface. Surficial fill materials of varying composition were generally encountered during our field explorations that included loose to medium dense to dense sands. It is our opinion that such site soils are susceptible to dynamic settlement; however, these surficial soils are generally unsuitable for building and slab support and are anticipated to be removed during site grading primarily for the partially belowgrade garage level Fault Ground Rupture and Fault Creep The State of California adopted the Alquist-Priolo Earthquake Fault Zone Act of 1972 (Chapter 7.5, Division 2, Sections , California Public Resources Code), which regulates development near active faults for the purpose of preventing surface fault rupture hazards to structures for human occupancy. In accordance with the Alquist-Priolo (A-P) Act, the California Geological Survey established boundary zones or Earthquake Fault Zones surrounding faults or fault segments judged to be sufficiently active, well-defined and mapped for some distance. Structures for human occupancy within designated Earthquake Fault Zone boundaries are not permitted unless 14

18 surface fault rupture and fault creep hazards are adequately addressed in a site-specific evaluation of the development site. The site is not currently within a designated Earthquake Fault Zone as defined by the State (Hart and Bryant, 1997). The closest Earthquake Fault Zone is associated with the Hayward Fault, located about 2.4 miles to the northeast of the site. Based on our evaluation, the potential for fault ground rupture or creep at the site is very low to nil Seismic Slope Failure The site is not mapped within a CGS Seismic Hazard Zone for earthquake-induced landslides. The site is not located near a significant open slope face; therefore, the potential for earthquake-induced slope failure to affect the project is judged to be very low to nil. 5.2 Consolidation Settlement Consolidation occurs as a result of water being squeezed out from a saturated soil as internal pore water pressures induced by an external load are dissipated over time. As the water moves out from the soil, the solid particles realign into a more dense configuration with settlement resulting. Consolidation typically occurs as a result of new buildings or fills being placed over them, but consolidation can also occur from groundwater withdrawal. Consolidation of clayey soils is usually a long-term process, where-by the water is squeezed out of the soil matrix with time. Sandy soils consolidate relatively rapidly with an introduction of a load. Highly compressible, soft to medium stiff marshland fat clay soils were found to underlie the southern four-fifths of the site. Additionally, some soft, compressible clays and silts were encountered within overlying fill, and the upper portion of stiffer clays underlying softer clay deposits may also be expected to undergo consolidation settlement, although to a significantly smaller extent. Applied new building loads inducing pressure on these compressible clays, or new fill placement, including replacement of existing loose fill soils by denser engineered fills, would be expected to induce consolidation of the underlying compressible clays, resulting in site and foundation settlements that could be damaging to new construction. Highly compressible clays were encountered primarily between the bottom of fill soils and a depth of about 30 feet, at which the clays become stiffer and less compressible. In general, site soils deeper than 50 feet are hard to dense, and relatively incompressible. Consolidation settlement analyses were conducted on a generalized filled marshland soil profile assuming compressible material present between depths of 8 and 40 feet. Analysis results indicate that 15

19 site settlements induced by placement of each foot of new site fill may induce an ultimate settlement on the order of inches. Consolidation settlements would be expected to occur relatively slowly, with 50% and 90% of the calculated ultimate settlement estimated to occur over a span of and years, respectively, after completion of fill placement. Similarly, building loads such as induced by a new, shallow spread footing foundation may be expected to induce significant settlement below the footing. Assuming a new 10-foot square footing is bottomed three feet below existing grade at a dead plus live load bearing pressure of 2,500 psf (i.e., 250 kip load), an ultimate settlement on the order of inches was calculated. 5.3 Expansive Soils Soils observed and/or sampled from alluvial soils in the upper five feet of the northern portion of the site were observed to be of low to medium plasticity. These soils typically exhibit a low to moderate expansion potential with moisture variation. Fill soils underlying the site were found to be highly variable, ranging from non-plastic sands to highly expansive clays. Highly expansive clays, including the marshland compressible clays, are not suitable for slab or near-surface structural support and are expected to be removed by site or remedial grading. Therefore, special measures to mitigate the potential effect of expansive soils are not expected to be required for the project. 16