Prepared for Quarterra Multifamily Communities, LLC
1350 Ocean Avenue • Emeryville, CA 94608 • (510) 420-5738 www.rockridgegeo.com
PRELIMINARY GEOTECHNICAL INVESTIGATION
TO SUPPORT DUE DILIGENCE EVALUATION
PROPOSED RESIDENTIAL DEVELOPMENT
WILLOW AVENUE APARTMENTS
APN 406-070-042
HERCULES, CALIFORNIA
UNAUTHORIZED USE OR COPYING OF THIS DOCUMENT IS STRICTLY
PROHIBITED BY ANYONE OTHER THAN THE CLIENT FOR THE SPECIFIC
PROJECT
February 9, 2025
Project No. 24-2751
DRAFT
1350 Ocean Avenue • Emeryville, CA 94608 • (510) 420-5738 www.rockridgegeo.com
February 9, 2025
Project No. 24-2751
Mr. Tyler Wood
Vice President, Development
Quarterra Multifamily Communities
492 9th Street, Suite 300
Oakland, California 94607
Subject: Preliminary Geotechnical Investigation
to Support Due Diligence Evaluation
Proposed Residential Development
Willow Avenue Apartments
APN 406-070-042
Hercules, California
Dear Mr. Wood:
We are pleased to present the results of our preliminary geotechnical investigation in support of the due diligence evaluation of the property located at Assessor Parcel Number (APN) 406-070- 042, California. Our preliminary investigation was performed in accordance with our contract dated December 18, 2024.
The project site consists of a 6.65-acre irregularly shaped property located at the southeastern corner of the intersections of Highway 80 and Highway 4 on the north side of Willow Avenue.
The site is bounded by Highway 80 to the west, Highway 4 to the north, the Highway 4 off-ramp to the east, and Willow Avenue to the south. The subject property has multiple small hills throughout the site and a small creek at the northern end of the site. The site is currently vacant and covered with grass and some brush.
Based on our review of the conceptual drawings prepared by LPAS Architects dated October 18, 2024, we understand the proposed development being considered for the site consists of significant cuts and fills to create a more level site, construction of six 3-story residential buildings, designated Building 1 through Building 6, and construction of a 2-story clubhouse building. Proposed cuts will be as much as about 35 feet (southeast corner of site) and proposed fills will be as thick as about 17 feet (northeast corner of the site). Other improvements include asphalt-paved parking lots and drive aisles, site retaining walls, and landscaping. The proposed construction will be set back from the creek and associated FEMA flood zone in the northern portion of the site.
DRAFT
Mr. Tyler Wood
Quarterra Multifamily Communities
February 9, 2025
Based on the results of our preliminary geotechnical investigation, we conclude there are no major geotechnical issues that would preclude development of the site as proposed. The primary geotechnical issues affecting the proposed development include: 1) the presence of highly to very highly expansive soil and bedrock and 2) the proposed differential fill thicknesses below some of the building pads, which may result in differential settlement. For preliminary planning and budgeting purposes, based on previous experience, we preliminarily recommend limiting differential fill thicknesses to about 7 feet below individual building pads. Even with 7 feet of differential fill thickness, there will likely be differential settlement across the pads, although it is difficult to predict the magnitude and timeline for this settlement to occur. This will likely require overexcavation of existing soils/bedrock in some areas to reduce the differential fill thickness. If it is determined that limiting differential fill thicknesses to 7 feet below individual building pads is not cost effective, other means of controlling differential settlement may be considered, including: 1) installing aggregate piers beneath the zones of proposed fill, or 2) chemically treating the fill with lime to reduce its compressibility. We preliminarily conclude the proposed buildings should be supported on stiffened mat foundations or post-tensioned (P-T) slabs.
Our preliminary geotechnical investigation consisted of a limited subsurface exploration program. Prior to final design, a final geotechnical investigation should be performed to fill in data gaps of subsurface conditions 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. Should you have any questions, please call.
Sincerely,
ROCKRIDGE GEOTECHNICAL, INC.
Alex D. Limpert, P.E. Logan D. Medeiros, P.E., G.E.
Project Engineer Principal Engineer
Enclosure
DRAFT
TABLE OF CONTENTS
1.0 INTRODUCTION ...............................................................................................................1
2.0 SCOPE OF SERVICES .......................................................................................................2
3.0 FIELD INVESTIGATION AND LABORATORY TESTING ...........................................2
3.1 Test Borings .............................................................................................................3
3.2 Laboratory Testing ...................................................................................................3
4.0 SUBSURFACE CONDITIONS ..........................................................................................4
4.1 Groundwater ............................................................................................................4
5.0 SEISMIC CONSIDERATIONS ..........................................................................................5
5.1 Regional Seismicity .................................................................................................5
5.2 Geologic Hazards .....................................................................................................7
5.2.1 Ground Shaking ...........................................................................................8
5.2.2 Ground Surface Rupture ..............................................................................8
5.2.3 Liquefaction and Liquefaction-Induced Settlement .....................................8
5.2.4 Cyclic Densification.....................................................................................9
6.0 PRELIMINARY CONCLUSIONS AND RECOMMENDATIONS ..................................9
6.1 Expansive Soil .........................................................................................................9
6.2 Foundations, Settlement, and Grading ...................................................................11
6.2.1 Site Preparation and Grading .....................................................................12
6.2.2 Lime Treatment ..........................................................................................12
6.3 Permanent Retaining Walls ....................................................................................13
6.4 Construction Considerations ..................................................................................15
6.5 Excavation in Rock ................................................................................................16
6.6 Seismic Design.......................................................................................................16
7.0 ADDITIONAL GEOTECHNICAL SERVICES ...............................................................17
8.0 LIMITATIONS ..................................................................................................................17
REFERENCES
FIGURES
APPENDIX A – Logs of Borings
APPENDIX B – Laboratory Test Results
DRAFT
LIST OF FIGURES
Figure 1 Site Location Map
Figure 2 Site Plan
Figure 3 Proposed Cut and Fill Map
Figure 4 Regional Geologic Map
Figure 5 Regional Fault and Historic Seismicity Map
Figure 6 Earthquake Zones of Required Investigation Map
APPENDIX A
Figures A-1 Logs of Borings, B-1 through B-4
through A-4
Figure A-5 Classification Chart
Figure A-6 Physical Properties Criteria for Rock Descriptions
APPENDIX B
Figure B-1 Plasticity Chart
Figure B-2 Particle Size Distribution Report
DRAFT
24-2751 1 February 9, 2025
PRELIMINARY GEOTECHNICAL INVESTIGATION
TO SUPPORT DUE DILIGENCE EV ALUATION
PROPOSED RESIDENTIAL DEVELOPMENT
WILLOW A VENUE APARTMENTS
HERCULES, CALIFORNIA
1.0 INTRODUCTION
This report presents the results of the preliminary geotechnical investigation performed by Rockridge Geotechnical, Inc. to support the due diligence evaluation for the proposed residential development, referred to as Willow Avenue Apartments, to be constructed at APN 406-070-042 in Hercules, California. The site is located on the southeastern corner of the intersection of Highway 80 and Highway 4, on the north side of Willow Avenue, as shown in the Site Location Map (Figure 1).
The project site consists of a 6.65-acre irregularly shaped property accessed from Willow Avenue. The site is bounded by Highway 80 to the west, Highway 4 to the north, the Highway 4 off-ramp to the east, and Willow Avenue to the south. The subject property has multiple small hills throughout the site and a small creek at the northern end of the site. The site is currently vacant and covered with grass and brush.
Based on our review of the conceptual drawings for the project,1 we understand the proposed development being considered for the site consists of significant cuts and fills to create a more level site, construction of six 3-story residential buildings, designated Building 1 through Building 6, and construction of a 2-story clubhouse building. The approximate locations of the proposed buildings are presented on Figures 2 and 3. The approximate magnitudes of proposed cuts and fills throughout the site are shown on Figure 3. Proposed cuts will be as much as about 35 feet2 (southeast corner of site) and proposed fills will be as thick as about 17 feet (northeast corner of the site). Other improvements include asphalt-paved parking lots and drive aisles, site
1 Plan set titled Pre-App Design Package, dated October 18, 2024 and prepared by LPAS Architecture + Design
2 Drawing titled Rough Grading Earthwork Cut/Fill Map, Willow Ave. Apartments, dated November 1, 2024 and prepared by CBG Civil Engineers
DRAFT
24-2751 2 February 9, 2025
retaining walls, and landscaping. The proposed construction will be set back from the creek and associated FEMA flood zone in the northern portion of the site, as shown on Figures 2 and 3.
2.0 SCOPE OF SERVICES
Our preliminary investigation was performed in accordance with our contract dated December 18, 2024. Our scope of services consisted of exploring subsurface conditions at the site by drilling about six test borings, performing limited laboratory testing on select soil samples, and performing engineering analyses to develop preliminary conclusions and recommendations regarding:
• 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 buildings • preliminary design criteria for the recommended foundation type(s), including vertical and lateral capacities
• preliminary estimates of foundation settlement
• 2022 California Building Code (CBC) site class and mapped design spectral response acceleration parameters
• construction considerations.
3.0 FIELD INVESTIGATION AND LABORATORY TESTING
Our preliminary field investigation consisted of drilling four test borings and performing limited laboratory testing on select soil samples. Although we planned to drill as many as six test borings, soft ground conditions at the time of our field work resulted in equipment access issues and two of the planned borings were not drilled. Before performing the field investigation, we obtained a drilling permit from Contra Costa County Environmental Health Department (CCCEHD). We also contacted Underground Services Alert (USA) to notify them of our work, as required by law, and retained Subtronic Corporation of Martinez, California, a private utility locator, to check the boring locations to reduce the potential for encountering buried utilities during our field investigation. Details of our field investigation and laboratory testing are presented in this section.
DRAFT
24-2751 3 February 9, 2025
3.1 Test Borings
The test borings, designated as Boring B-1, B-3, B-5, and B-6, were drilled on January 7, 2025, by Taber Drilling of Sacramento, California, at the approximate locations shown in Figure 2 and Figure 3. Planned Borings B-2 and B-4 were not drilled due to access and time constraints associated with soft ground conditions but are shown in Figure 2. The borings were drilled to depths of 25 to 35 feet below the existing ground surface (bgs) using a CME-55 track-mounted drill rig equipped with 6-inch outside diameter hollow-stem flight augers. During drilling, our field engineer logged the soil and bedrock encountered and obtained representative samples for visual classification and laboratory testing. The boring logs are presented in Figures A-1 through A-4 in Appendix A. The soil and bedrock encountered in the boring were classified in accordance with the classification systems presented in Figures A-5 and A-6, respectively.
Soil samples were obtained using a Modified California (MC) split-barrel sampler with a 3.0- inch outside diameter and 2.5-inch inside diameter, lined with 2.43-inch inside diameter stainless steel tubes. The sampler was driven with a 140-pound automatic hammer falling about 30 inches per drop. The samplers were driven up to 18 inches, and the hammer blows required to drive the samplers were recorded every 6 inches and are presented in the boring logs. A “blow count” is defined as the number of hammer blows per 6 inches of penetration or 50 blows for 6 inches or less of penetration. The blow counts required to drive the MC sampler were converted to approximate SPT N-values using a factor of 1.0 to account for sampler type and approximate hammer energy. The blow counts used for this conversion were the last two blow counts. The converted SPT N-values are presented in the boring logs.
Upon completion of drilling, the boreholes were backfilled with neat cement grout in accordance with CCCEHD requirements. Soil cuttings generated during the drilling of the borings were spread onsite near the boreholes.
3.2 Laboratory Testing
We re-examined the soil samples obtained from our borings to confirm the field classifications and selected representative samples for laboratory testing. Laboratory tests were performed by B.
Hillebrandt Soils Testing, Inc. of Alamo, California to measure moisture content, dry density, DRAFT
24-2751 4 February 9, 2025
particle size distribution, and plasticity (Atterberg limits). The results of the laboratory tests are presented in the boring logs and in Appendix B.
4.0 SUBSURFACE CONDITIONS
Regional geologic information (Figure 4) indicates the site is underlain by Holocene-age alluvium (Qha), although the contact for Miocene-age sedimentary rocks (Tms) is within about 50 feet of the southern boundary of the site and mapped artificial fill (af) associated with the highway embankments is located just west and north of the site. The results of our field investigation indicate the site is underlain by alluvium and bedrock. Where encountered, the alluvium consists of very stiff to hard high-plasticity clay with variable sand content, except for the sample at 1 foot at B-3, which was stiff. We encountered bedrock below the alluvium at borings B-1, B-5, and B-6 at depths between about 6 and 21 feet bgs. Where encountered, the bedrock consists of siltstone and claystone. The bedrock is soft to low hardness, plastic to weak, and deeply to completely weathered. We did not encounter bedrock in B-3, which was drilled to the maximum depth explored of 35 feet bgs.
Atterberg limits tests performed on samples of the clay and claystone encountered near the proposed finished floor elevation of four proposed building locations indicate the clay is highly to very highly expansive4 with plasticity indices (PIs) of 36 to 46.
4.1 Groundwater
We did not encounter groundwater in any of our test borings. Based on the low permeability of the soil at the site, we judge the groundwater levels may not have been able to stabilize during our field investigation. Available historic high groundwater information presented in the Seismic Hazard Zone Report for the Richmond, Mare Island, and San Quentin 7.5-Minute Quadrangle prepared by the California Geological Survey (CGS, 2024) indicates a historic high groundwater level in the site vicinity to be roughly 10 feet bgs, however, given the significant amount of variability in existing ground surface elevation at the site, we conclude this mapped groundwater depth contour is not particularly meaningful. The creek at the site is at about elevation 35, which
4 Expansive soil undergoes large volume changes with changes in moisture content (i.e., it shrinks when dried and swells when wetted).
DRAFT
24-2751 5 February 9, 2025
is about 20 to 25 feet below the proposed finished grades of the site. We preliminarily recommend assuming a design groundwater depth to match the elevation of the creek. The depth to groundwater is expected to fluctuate several feet seasonally, depending on the amount of rainfall.
5.0 SEISMIC CONSIDERATIONS
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 Hayward, San Andreas, and Calaveras faults. These and other faults in the region are shown in Figure 5. For these and other active faults within a 50- kilometer radius of the site, the distance from the site and estimated characteristic moment magnitude5 [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).
5 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.
DRAFT
24-2751 6 February 9, 2025
TABLE 1
Regional Faults and Seismicity
Fault Segment
Approximate
Distance
from Site
(km)
Direction from
Site
Characteristic
Moment
Magnitude
Total Hayward + Rodgers Creek
(RC+HN+HS+HE) 7.4 Southwest 7.58
Hayward (North, HN) 7.4 Southwest 6.90
Green Valley 16 East 6.30
Concord 16 East 6.45
West Napa 17 North 6.97
Rodgers Creek - Healdsburg 23 Northwest 7.19
Mount Diablo Thrust North CFM 23 Southeast 6.72
Mount Diablo Thrust 25 Southeast 6.67
Hayward (South, HS) 27 Southeast 7.00
Clayton 27 East 6.57
Total Calaveras (CN+CC+CS+CE) 27 Southeast 7.43
Calaveras (North, CN) 27 Southeast 6.86
Great Valley 05 (Pittsburg - Kirby
Hills alt1) 32 East 6.60
Great Valley 05 (Pittsburg - Kirby
Hills alt2) 35 East 6.66
Total North San Andreas
(SAO+SAN+SAP+SAS) 37 West 8.04
North San Andreas (Peninsula, SAP) 37 West 7.38
North San Andreas (North Coast,
SAN) 37 West 7.52
Greenville (North) 38 East 6.86
San Gregorio (North) 39 West 7.44
Great Valley 04b (Gordon Valley) 42 Northeast 6.77
Mount Diablo Thrust South 42 Southeast 6.50
Hunting Creek (Berryessa) 45 North 6.69
DRAFT
24-2751 7 February 9, 2025
Damaging earthquakes have occurred along many of these faults in recorded history, as depicted in Figure 5 (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 West Napa Earthquake, Mw = 6.0
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 project site is in a seismically active region, we preliminarily evaluated the potential for earthquake-induced geologic hazards including ground shaking, ground surface rupture, liquefaction,6 lateral spreading,7 and cyclic densification.8 We used the results of our borings to preliminarily evaluate the potential of these phenomena occurring at the site.
6 Liquefaction is a phenomenon where loose, saturated, cohesionless soil experiences temporary reduction in strength during cyclic loading such as that produced by earthquakes. 7 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. 8 Cyclic densification is a phenomenon in which non-saturated, cohesionless soil is compacted by earthquake vibrations, causing ground-surface settlement.
DRAFT
24-2751 8 February 9, 2025
5.2.1 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, including the San Andreas, Calaveras, and Rodges Creek 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 strong to very strong 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 Fault Zoning Act, and no known active or potentially active faults exist on the site. Therefore, we preliminarily conclude the probability of fault offset at the site from a known active fault to be very low. In a seismically active area, the remote possibility exists for future faulting in areas where no faults previously existed; however, we preliminarily 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 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 majority of the site is not within a zone of liquefaction potential as shown on the map titled State of California Seismic Hazard Zones, Mare Island Quadrangle, Official Map, dated February 22, 2024 (Figure 6). Only the northern portion of the site, within the existing creek and outside the proposed development footprint, is mapped as a zone of liquefaction potential.
DRAFT
24-2751 9 February 9, 2025
The soil at the site generally consists of very stiff to hard cohesive soil that is not susceptible to liquefaction due to its cohesion. The soil is also underlain by bedrock, which is not susceptible to liquefaction. Therefore, we preliminarily conclude that the potential for liquefaction and liquefaction-induced ground failures, such as lateral spreading, to occur at the site during a major earthquake is very low.
5.2.4 Cyclic Densification
Cyclic densification (also referred to as differential compaction) of non-saturated sand (sand above the groundwater table) can occur during an earthquake, resulting in settlement of the ground surface and overlying improvements. The results of our borings indicate the soil above the groundwater at the site is not susceptible to cyclic densification due to its cohesion.
Therefore, we preliminarily conclude that the potential for ground surface settlement resulting from cyclic densification at the site is very low.
6.0 PRELIMINARY CONCLUSIONS AND RECOMMENDATIONS
From a geotechnical standpoint, we preliminarily conclude there are no major geotechnical issues that would preclude development of the site as proposed. The primary geotechnical issues affecting the proposed development include: 1) the presence of highly to very highly expansive soil and bedrock and 2) the proposed differential fill thicknesses below some of the building pads, which may result in differential settlement. Our preliminary conclusions and recommendations regarding these issues are presented in the following sections.
6.1 Expansive Soil
Atterberg limits tests performed on samples of the onsite clay at depths of 1.5 to 19.5 feet bgs (to account for future cuts within the building pads during mass grading) indicate the onsite clay and claystone is highly to very highly expansive. Expansive near-surface soil is subject to volume changes during seasonal fluctuations in moisture content. These volume changes can cause movement and cracking of foundations and slabs. Therefore, foundations and slabs should be designed and constructed to mitigate the effects of the expansive soil. In general, the effects of expansive soil can be mitigated by moisture-conditioning the expansive soil, providing non- expansive soil below slabs, and either supporting foundations below the zone of severe moisture DRAFT
24-2751 10 February 9, 2025
change or by providing a stiff, shallow foundation that can limit deformation of the superstructure as the underlying soil shrinks and swells.
We preliminarily recommend the upper 18 inches of soil subgrade beneath the proposed buildings be replaced with non-expansive fill. Similarly, the upper 12 inches of soil subgrade beneath exterior concrete flatwork should be replaced with non-expansive fill. The non- expansive fill may consist of lime-treated, on-site clay or select fill. Select fill should consist of imported or on-site soil that is free of organic matter, contain no rocks or lumps larger than 3 inches in greatest dimension, have a liquid limit less than 40 and plasticity index less than 12, and be approved by the Geotechnical Engineer.
Even with proper moisture conditioning and 12 inches of non-expansive fill, exterior slabs may experience some cracking due to shrinking and swelling of the underlying expansive soil.
Thickening the slab edges and adding additional reinforcement will control this cracking to some degree. In addition, where slabs provide access to buildings, it would be prudent to dowel the entrance to the building to permit rotation of the slab as the exterior ground shrinks and swells and to prevent a vertical offset at the entries.
At expansive soil sites, it is critical to manage surface and subsurface drainage to prevent water from collecting beneath pavements and slabs, where it can lead to swelling and shrinking of the subgrade soil and can cause subgrade instability under vehicular loads. If permeable pavements, tree wells, irrigated landscaped zones, and stormwater infiltration basins will be constructed close to the proposed buildings, they should incorporate design elements that prevent saturation of the soil adjacent to and below building foundations. While the objective of permeable pavement systems and infiltration basins is to allow for water storage and infiltration, we preliminarily conclude infiltration into the subgrade soil is not feasible at this site due to the low permeability of the highly expansive clay and bedrock, where encountered. Furthermore, from a geotechnical standpoint, water should not be allowed to collect alongside or beneath the building foundations, pavements, and flatwork. This can be achieved by providing subdrain systems and impermeable liners beneath permeable surfaces and installing vertical barriers between DRAFT
24-2751 11 February 9, 2025
permeable surfaces underlain by subdrains and non-permeable surfaces underlain by conventional aggregate base.
6.2 Foundations, Settlement, and Grading
As discussed in Section 4.0, the site is primarily underlain by very stiff to hard clay and bedrock, which are generally capable of supporting the proposed relatively light weight buildings without excessive settlement under static load conditions. However, the proposed buildings along the northern edge of the property are currently shown to be constructed at the edge of the existing slope and will be constructed on as much as about 15 feet of fill (northeast corner of Building 4).
Engineered fill will undergo settlement during placement and continue to settle for a period of time after grading, the duration of which is difficult to predict. Buildings 2, 3, and 4 will transition from cut to fill conditions, which will likely result in differential settlement across the building footprints. The settlement of compacted fills can be controlled to some degree by the moisture content and level of compaction achieved during grading, however, we conclude the settlement cannot be completely mitigated through these measures. The potential for compression of fills, as well as swelling potential due to the very highly expansive nature of the site soils, should be further evaluated in the laboratory during the final geotechnical investigation for the project.
For preliminary planning and budgeting purposes, based on previous experience, we preliminarily recommend limiting differential fill thicknesses to about 7 feet below individual building pads. Even with 7 feet of differential fill thickness, there will likely be differential settlement across the pads, although it is difficult to predict the magnitude and timeline for this settlement to occur. This will likely require overexcavation of existing soils/bedrock in some areas to reduce the differential fill thickness.
If it is determined that limiting differential fill thicknesses to 7 feet below individual building pads is not cost effective, other means of controlling differential settlement may be considered, including:
• Installing aggregate piers beneath the zones of proposed fill • Chemically treating the fill with lime to reduce its compressibility DRAFT
24-2751 12 February 9, 2025
We preliminarily recommend the buildings be supported on either stiffened mat foundations or post-tensioned (P-T) slabs using one of the mitigation measures discussed above to address differential fill thickness. Once a final investigation has been performed, detailed recommendations for design of mats or P-T slabs can be determined.
6.2.1 Site Preparation and Grading
Site demolition should include the removal of underground utilities, if any, where the new structures are proposed. In general, abandoned underground utilities should be removed to the property line or service connections and properly capped or plugged with concrete. Where existing utility lines are outside the proposed building footprint and will not interfere with the proposed construction, they may be abandoned in-place provided the lines are filled with lean concrete or cement grout to the property line. V oids resulting from demolition activities should be properly backfilled with compacted fill under the observation of the Geotechnical Engineer.
Detailed recommendations for grading and fill placement should be included in a final, design- level geotechnical investigation report once the project scope and design have been further developed.
In areas where fill wedges are planned on existing slopes, we preliminarily recommend the fills be benched into the existing slope with subdrains installed along the benches. During a final, design-level geotechnical investigation, additional borings and/or cone penetration tests (CPTs) should be performed in the proposed building footprints to better characterize depth to bedrock as it relates to proposed finished floor elevations.
6.2.2 Lime Treatment
The on-site soils may be chemically treated with lime in order to: 1) create the recommended non-expansive fill layer beneath the proposed foundations, 2) reduce the compressibility of thick fills beneath portions of the buildings, and/or 3) winterize the site and provide more stable working conditions, if grading work is performed during the rainy season. Lime treatment of fine-grained soils generally includes site preparation, application of lime, mixing, compaction, and curing of the lime-treated soil. Field quality control measures should include checking the DRAFT
24-2751 13 February 9, 2025
depth of lime treatment, degree of pulverization, lime spread rate measurement, and moisture content and density measurements, and mixing efficiency.
The lime treatment process should be designed by a contractor specializing in its use and who is experienced in the application of lime in similar soil conditions. Based on our experience with lime treatment, we judge that the specialty contractor should be able to treat the highly to very highly expansive on-site material to produce a non-expansive fill for the general site fill, building pad subgrades and, if desired, for exterior flatwork and pavement subgrades. For planning purposes, we preliminarily recommend assuming the lime treatment will consist of at least 5 percent of Quicklime by dry weight of soil. An average dry unit weight of 90 pounds per cubic foot (pcf) should be preliminarily assumed for design purposes. The specialty contractor should:
1) perform a lime demand test prior to treatment to determine the percentage of Quicklime required to achieve a pH of 12.4 or higher in the treated soil, 2) perform an Atterberg limits test to confirm the proposed percentage of Quicklime will reduce the plasticity index of the treated soil to 12 or less, and 3) prepare a treatment specification for our review prior to construction.
6.3 Permanent Retaining Walls
Permanent retaining walls should be designed to resist lateral earth pressures imposed by the retained soil, as well as a surcharge pressure from nearby foundations and vehicles where appropriate. In addition, because the site is in a seismically active area, retaining walls that retain more than 6 feet of soil should be designed for the more critical of static or seismic conditions.
For static conditions, we preliminarily recommend both restrained and unrestrained walls be designed using an equivalent fluid weight of 61 pcf for drained conditions and 93 pcf for undrained conditions, assuming the walls are backfilled with the on-site expansive soil. These pressures may be reduced if the walls are backfilled with lime-treated on-site soil.
Walls that will retain more than 6 feet of soil will need to be designed for the more critical of static (presented above) or the following seismic conditions. • Restrained Wall - Active earth pressure using an equivalent fluid weight of 40 pcf plus a seismic increment of 32 pcf for drained conditions; and 82 pcf plus a seismic increment of 16 pcf for undrained conditions.
DRAFT
24-2751 14 February 9, 2025
• Unrestrained Wall - Active earth pressure using an equivalent fluid weight of 40 pcf plus a seismic increment of 13 pcf for drained conditions; and 82 pcf plus a seismic increment of 6 pcf for undrained conditions.
Where there will be vehicle loading within 10 feet behind the wall, the wall should be designed for a vehicular surcharge of 100 psf over the upper 10 feet of the wall. If the traffic loading is limited to passenger vehicles only, the vehicular surcharge may be reduced to 50 psf. Where foundations will be supported above a “zone-of-influence” line extending up from the bottom of wall at an inclination of 1.5:1 (horizontal to vertical), the wall should be designed for a surcharge pressure. The magnitude of the surcharge pressure will need to be evaluated on a case-by-case basis.
To protect against moisture migration, all below-grade walls should be waterproofed and water stops should be placed at all construction joints. Although walls will be above the design groundwater level, water can accumulate behind the walls from other sources, such as rainfall, irrigation, broken water lines, etc. If the “drained” earth pressures presented above are used to design the walls, the walls will need to incorporate a drainage system and collector pipe as described below. Alternatively, the walls may be designed for the recommended “undrained” earth pressures presented above over their entire height, in which case the drainage system may be omitted.
One acceptable method for backdraining a retaining wall is to place a prefabricated drainage panel against the back of the wall. The drainage panel should extend down to a perforated PVC collector pipe at the base of the wall. The pipe should be surrounded by at least 4 inches of Caltrans Class 2 permeable material or 3/4-inch drain rock wrapped in filter fabric (Mirafi NC or equivalent). A proprietary, prefabricated collector drain system, such as Tremdrain Total Drain or Hydroduct Coil (or equivalent), designed to work in conjunction with the drainage panel may be used in lieu of the perforated pipe surrounded by gravel described above. The pipe should be connected to a suitable discharge point; a sump and pump system may be required to drain the collector pipes.
DRAFT
24-2751 15 February 9, 2025
If backfill is required behind below-grade walls, the walls should be braced, or hand compaction equipment used, to prevent unacceptable surcharges on walls (as determined by the Structural Engineer).
6.4 Construction Considerations
Significant grading and cuts of up to about 15 feet below existing grades within building pads and up to about 35 feet elsewhere onsite are currently anticipated. Analytical testing should be performed on all soil to be excavated and disposed of offsite to provide data to site(s) and/or the landfill to which the soil will be transported.
If site grading is performed during the rainy season, the soil will likely be wet and will have to be dried before compaction can be achieved. Heavy rubber-tired equipment, such as scrapers and vibratory rollers, could cause excessive deflection (pumping) of the wet soil and, therefore, should be avoided. If the project schedule or weather conditions do not permit sufficient time for drying of the soil by aeration, the subgrade can be treated with lime prior to compaction, or imported granular fill can be used. The appropriate amount of lime should be determined during construction based on a visual examination and, if necessary, laboratory testing of the soil to be treated. If the grading work is performed during the dry season, moisture-conditioning may be required.
Excavations that will be entered by workers should be sloped or shored in accordance with CAL- OSHA standards (29 CFR Part 1926). Where there is sufficient clearance from the property line, the excavation sides may be slope cut at a maximum inclination of 1:1 (horizontal: vertical), which is consistent with OSHA Type B soil. Cuts in portions of the site may potentially be sloped consistent with OSHA Type A soil; however, this should be evaluated on a case-by-case basis. The contractor should be responsible for the construction and safety of temporary slopes.
Where there is insufficient space to slope-cut the excavations, shoring may be required. The selection, design, construction, and performance of the shoring system (if needed) should be the responsibility of the contractor.
DRAFT
24-2751 16 February 9, 2025
6.5 Excavation in Rock
With current proposed cuts of up to about 35 feet below existing grades, excavations in rock will likely be required for construction. Where encountered in our borings, the bedrock is generally plastic to weak, with soft to low hardness, and deep to complete weathering, but grades to moderately strong and moderately weathered with depth. Note that reported SPT N-values within the rock, where explored, ranged from 24 to 71 blows for 12 inches of penetration and therefore, the rock varies substantially in hardness and degree of weathering. We anticipate highly weathered rock can be excavated with conventional excavators; and harder rock at depth may require the use of hydraulic breaking equipment (i.e., a hoe ram). Furthermore, because the bedrock was characterized by discrete, widely spaced borings during our preliminary investigation, it is possible harder rock and difficult excavation may be encountered. Therefore, the contractor should be prepared to excavate hard rock, including the possible use of hydraulic breaking equipment, and should bid the project accordingly. The material descriptions and SPT N-values presented in the boring logs should be evaluated by the contractor when bidding on the project and selecting appropriate equipment.
6.6 Seismic Design
The latitude and longitude of the site are 38.0132° and -122.2692°, respectively. For preliminary design in accordance with the 2022 CBC, we recommend the following:
• Site Class D (stiff soil, non-default)
• SS = 1.66g, S1 = 0.63g
The 2022 CBC is based on the guidelines contained within ASCE 7-16 (Supplement 3 revision), which stipulate that if S1 is greater than 0.2 times gravity (g) for Site Class D, a ground motion hazard analysis is required unless the long-period spectral design parameters (SM1, SD1) are increased by 50%. Therefore, we recommend the following seismic design parameters, which include the 50% increase as indicated by an asterisk:
• Fa = 1.0, Fv = 1.7
• SMS = 1.66g, SM1* = 1.60g
• SDS = 1.11g, SD1* = 1.06g
• Seismic Design Category D for Risk Categories I, II, and III DRAFT
24-2751 17 February 9, 2025
During the final geotechnical investigation, we recommend performing a geophysical survey to confirm the average shear wave velocity in the upper 100 feet, which will allow for a more accurate determination of Site Class. Due to the presence of shallow rock in portions of the site, a Site Class C designation may be more appropriate for some of the buildings.
7.0 ADDITIONAL GEOTECHNICAL SERVICES
The preliminary conclusions and recommendations presented within are based on a preliminary field investigation and not intended for final design. Prior to final design, we should be retained to provide a final geotechnical report based on a supplemental field investigation and the final proposed development. Additional borings and/or CPTs will be required to further evaluate the subsurface conditions beneath the site. Once our final report has been completed, the design team has selected a foundation system, and prior to construction, we should review the project plans and specifications to check their conformance with the intent of our final recommendations.
During construction, we should observe site grading, foundation installation, and the placement and compaction of fill. These observations will allow us to compare the actual with the anticipated soil conditions and to check if the contractors’ work conforms with the geotechnical aspects of the plans and specifications.
8.0 LIMITATIONS
This geotechnical investigation 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 our field investigation.
If any variations or undesirable conditions are encountered during construction, we should be notified so additional recommendations can be made. The preliminary foundation 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.
DRAFT
24-2751 18 February 9, 2025
REFERENCES
American Society of Civil Engineers (ASCE). (2017). ASCE/SEI 7-16: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. ASCE Publications. http://doi.org/10.1061/9780784414248
California Department of General Services. (2022). California Building Code. State Government of California. https://www.dgs.ca.gov/BSC/Codes
California Department of Transportation (Caltrans). (2022). Caltrans Standard Specifications.
State Government of California. https://dot.ca.gov/-/media/dot- media/programs/design/documents/2022_stdspecs-a11y.pdf
California Geological Survey. (2024, February 22). Earthquake Zones of Required Investigation, Mare Island Quadrangle. California Department of Conservation.
California Geological Survey. (2024). Seismic Hazard Zone Report for the Richmond, Mare Island, and San Quentin 7.5-Minute Quadrangle, Contra Costa County, California. California Department of Conservation.
Field, E.H., Biasi, G.P., Bird, P., Dawson, T.E., Felzer, K.R., Jackson, D.D., Johnson, K.M., Jordan, T.H., Madden, C., Michael, A.J., Milner, K.R., Page, M.T., Parsons, T., Powers, P.M., Shaw, B.E., Thatcher, W.R., Weldon, R.J., & Zeng, Y . (2013). Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3)—The Time-Independent Model (Open-File Report 2013- 1165). United States Geological Survey. http://doi.org/10.3133/ofr20131165 Graymer, R.W., Moring, B.C., Saucedo, G.J., Wentworth, C.M., Brabb, E.E., & Knudsen, K.L. (2006, March 6). Geologic Map of the San Francisco Bay Region (Scientific Investigations Map 2918). United States Geological Survey.
Petersen, M.D., Moschetti, M.P., Powers, P.M., Mueller, C.S., Haller, K.M., Frankel, A.D., Zeng, Y ., Rezaeian, S., Harmsen, S.C., Boyd, O.S., Field, N., Chen, R., Rukstales, K.S., Luco, N., Wheeler, R.L., Williams, R.A., & Olsen, A.H. (2014). Documentation for the 2014 Update of the United States National Seismic Hazard Maps (Open-File Report 2014-1091). United States Geological Survey. https://pubs.usgs.gov/of/2014/1091/
Thompson, E.M., Wald, D.J., Worden, B., Field, E.H., Luco, N., Petersen, M.D., Powers, P.M., Badie, R. (2016). Shakemap earthquake scenario: Building Seismic Safety Council 2014 Event Set (BSSC2014). United States Geological Survey. https://doi.org/10.5066/F7V122XD United States Geological Survey. (2021). Earthquake Hazards Program: Advanced National Seismic System (ANSS) Comprehensive Catalog of Earthquake Events and Products. United States Department of the Interior. https://earthquake.usgs.gov/earthquakes/search/
DRAFT
FIGURES
DRAFT
Project No. FigureDate 1
0 2,400 Feet
Approximate scale
1,200
Base map: Google Maps, 2024
SITE
SITE LOCATION MAP
ROCKRIDGE
GEOTECHNICAL 24-275112/30/24
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
DRAFT
0Approximate scale80 Feet02/05/2424-27512 Hercules, CaliforniaSITE PLANDateProject No.Figure Reference: Base map from a drawing titled "Conceptual Grading Plan", by cbg Civil Engineers, dated October 2024.ROCKRIDGEGEOTECHNICAL WILLOW AVENUE APARTMENTSHIGHWAY 80 & WILLOW AVENUE
B-2EXPLANATIONApproximate location of boring drilled byRockridge Geotechnical, Inc., January 7, 2025Approximate location of planned boring notdrilled due to site access constraintsB-1 B-2
B-4 B-1B-3
B-5 B-6
DRAFT
0Approximate scale80 Feet02/05/2524-27513 Hercules, CaliforniaPROPOSED CUT AND FILL MAPDateProject No.Figure Reference: Base map from a drawing titled "Rough Grading Earthwork Cut/Fill Map", by cbg Civil Engineers, dated November 1, 2024.ROCKRIDGEGEOTECHNICAL WILLOW AVENUE APARTMENTSHIGHWAY 80 & WILLOW AVENUE
B-2EXPLANATIONApproximate location of boring drilled byRockridge Geotechnical, Inc., January 7, 2025Approximate location of planned boring notdrilled due to site access constraintsB-1 B-2
B-4 B-1B-3
B-5 B-6
DRAFT
REGIONAL GEOLOGIC MAP
Approximate scale
2,000
Project No. FigureDate 4
0 4,000 Feet
ROCKRIDGE
GEOTECHNICAL 24-275102/04/25
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
Geologic contact:
dashed where approximate and dotted
where concealed, queried where uncertain
SITE
EXPLANATION
Base map: Google Earth with U.S. Geological Survey (USGS), Contra Costa County, 2024 Artificial Fill
Alluvium (Pleistocene)
Sedimentary rocks (Miocene)
af
Qpa
Tms
TmsTms
QpaQpa
Alluvium (Holocene)Qha
TmsTms
DRAFT
1906, M7.9
1838, M7.4
1868, M7.0
1989, M6.9
2014, M6.0
San Andreas (S
AN)
San
Andreas (SAP)
Hayward
(HN
)
Ha
yward (HS)
San Andreas (SAS)
Monte Vi
sta-Shannon
Ca
lave
ras (CN)
Calav
eras
(CC)
Calaveras (CS)
San Gregorio (Nor
th)
San Gregorio (So
uth)
Gre
enville
Green
Valley
Rod
gers Creek
Maacama
West Nap
a
Co
nco
rd
Great V
alley
(Midla
nd)
Reliz
Great Valley (Or
estim
ba)
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
01 0
32 Kilometers01 6
EXPLANATION
Faults (National Seismic Hazard Model)
ROCKRIDGE
GEOTECHNICAL 5
SITE
24-275102/04/25
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
DRAFT
0 4,000 Feet
Approximate scale
2,000
ROCKRIDGE
GEOTECHNICAL Project No. FigureDate 6
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
Mare Island Quadrangle
California Geological Survey
Released February 22, 2024
24-275102/04/25
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
SITE
DRAFT
APPENDIX A
Logs of Borings
DRAFT
MC
73 21.3 88
17 4
7 10
MC 34
9 14
20 MC 29
8 12
17 MC 31
8 15
16 MC 31
11 12
19 Sampler
Type
Sample
Blows/ 6"
SPT
N-Value
1 LITHOLOGY
DEPTH
(feet)
Boring location:
Date started:
Drilling method:
1 2
3 4
5 6
7 8
9 10
11 12
13 14
15 16
17 18
19 20
21 22
23 24
25 26
27 28
29 30
MATERIAL DESCRIPTION
LABORATORY TEST DATASAMPLES
Figure:
A-1
PROJECT:
Project No.:
24-2751
Type of
Strength
Test
Confining
Pressure
Lbs/Sq Ft
Shear
Strength
Lbs/Sq Ft
Fines
%
Natural
Moisture
Content,%
Dry
Density
Lbs/Cu Ft
1 Boring terminated at a depth of 27 feet below
ground surface.
Boring backfilled with cement grout.
Groundwater not encountered during drilling.
ROCKRIDGE
GEOTECHNICAL
MC blow counts for the last two increments were
converted to SPT N-Values using a factor of 1.0
to account for sampler type and hammer energy.
LL = 58, PI = 36; see Appendix B
Particle Size Distribution; see Appendix B
See Site Plan, Figure 2
01/07/2025 Date finished: 01/07/2025
D. Fernandez
Taber Drilling
CME-55 Track Rig
Hammer weight/drop: 140 lbs./30 inches Hammer type:
SANDY CLAY (CH)
olive-brown to dark brown, very stiff, moist, fine sand CH
Sampler:
Logged by:
Drilled by:
Rig:
PAGE 1 OF 1
Log of Boring B-1
olive-gray, hard, dry to moist
CLAY with SAND (CH)
olive-gray, hard, moist, fine sand, trace fine subangular gravel
CH
Modified California (MC)
very stiff
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
6-inch-diameter hollow-stem auger
SILTSTONE
olive-gray, moderately hard, weak to moderately strong
Automatic Safety Hammer
DRAFT
MC 91 38.0 8210
0 4
6 MC
17 4
7 10
MC 26
5 10
16 MC 27
4 12
15 MC 22
9 11
11 MC
26 5
10 16
MC 32
9 13
19 Sampler
Type
Sample
Blows/ 6"
SPT
N-Value
1 LITHOLOGY
DEPTH
(feet)
Boring location:
Date started:
Drilling method:
1 2
3 4
5 6
7 8
9 10
11 12
13 14
15 16
17 18
19 20
21 22
23 24
25 26
27 28
29 30
MATERIAL DESCRIPTION
LABORATORY TEST DATASAMPLES
Figure:
A-2a
PROJECT:
Project No.:
24-2751
Type of
Strength
Test
Confining
Pressure
Lbs/Sq Ft
Shear
Strength
Lbs/Sq Ft
Fines
%
Natural
Moisture
Content,%
Dry
Density
Lbs/Cu Ft
ROCKRIDGE
GEOTECHNICAL
See Site Plan, Figure 2
01/07/2025 Date finished: 01/07/2025
Hammer weight/drop: 140 lbs./30 inches Hammer type:
CLAY (CH)
olive-gray, stiff, moist, trace fine sand, trace rootlets CL
Sampler:
Logged by:
Drilled by:
Rig:
PAGE 1 OF 2
Log of Boring B-3
CLAY (CL)
olive-brown, very stiff, moist, trace fine sand
SANDY CLAY (CL)
olive-brown, very stiff, moist, fine sand
CL
Modified California (MC)
CLAY with SAND (CL)
olive-brown, very stiff, moist, fine to medium sand
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
6-inch-diameter hollow-stem auger
SANDY CLAY (CL)
dark brown, hard, moist, fine sand
CL
CL
dark brown and olive-brown
SANDY CLAY (CH)
olive-brown, very stiff, moist, fine to medium sand
CH
CH
D. Fernandez
Taber Drilling
CME-55 Track Rig
Automatic Safety Hammer
very stiff
LL = 69, PI = 46; see Appendix B
Particle Size Distribution; see Appendix B
DRAFT
Dry Density
Lbs/Cu Ft
Type of
Strength
Test
Shear Strength
Lbs/Sq Ft
Fines
%
Confining
Pressure
Lbs/Sq Ft
Natural
Moisture
Content, %
Sampler
Type
Sample
Blows/ 6"
SPT
N-Value
1 LITHOLOGY
DEPTH
(feet)
31 32
33 34
35 36
37 38
39 40
41 42
43 44
45 46
47 48
49 50
51 52
53 54
55 56
57 58
59 60
MATERIAL DESCRIPTION
LABORATORY TEST DATASAMPLES
Figure:
PROJECT:
Project No.:
PAGE 2 OF 2
Log of Boring B-3
ROCKRIDGE
GEOTECHNICAL
SANDY CLAY (CL) (continued)
gray
CL
MC
12 14
16 30
A-2b24-2751
1 Boring terminated at a depth of 35 feet below
ground surface.
Boring backfilled with cement grout.
Groundwater not encountered during drilling.
MC blow counts for the last two increments were
converted to SPT N-Values using a factor of 1.0
to account for sampler type and hammer energy.
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
DRAFT
MC
98 31.7 86
29 4
9 20
MC 22
4 9
13 MC 31
5 14
17 MC 28
8 12
16 MC 71
14 25
46 MC 54
14 25
29 Sampler
Type
Sample
Blows/ 6"
SPT
N-Value
1 LITHOLOGY
DEPTH
(feet)
Boring location:
Date started:
Drilling method:
1 2
3 4
5 6
7 8
9 10
11 12
13 14
15 16
17 18
19 20
21 22
23 24
25 26
27 28
29 30
MATERIAL DESCRIPTION
LABORATORY TEST DATASAMPLES
Figure:
A-3
PROJECT:
Project No.:
24-2751
Type of
Strength
Test
Confining
Pressure
Lbs/Sq Ft
Shear
Strength
Lbs/Sq Ft
Fines
%
Natural
Moisture
Content,%
Dry
Density
Lbs/Cu Ft
1 Boring terminated at a depth of 30 feet below
ground surface.
Boring backfilled with cement grout.
Groundwater not encountered during drilling.
ROCKRIDGE
GEOTECHNICAL
MC blow counts for the last two increments were
converted to SPT N-Values using a factor of 1.0
to account for sampler type and hammer energy.
See Site Plan, Figure 2
01/07/2025 Date finished: 01/07/2025
Hammer weight/drop: 140 lbs./30 inches Hammer type:
SANDY CLAY (CL)
light olive-gray, very stiff, dry to moist, fine sand
Sampler:
Logged by:
Drilled by:
Rig:
PAGE 1 OF 1
Log of Boring B-5
red-gray
olive-gray, deeply to completely weathered
CL
Modified California (MC)
CLAYSTONE
light brown, gray, and red, soft, plastic to friable
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
6-inch-diameter hollow-stem auger
olive-brown, soft to low hardness, friable
gray, low hardness, friable to weak
D. Fernandez
Taber Drilling
CME-55 Track Rig
Automatic Safety Hammer
LL = 66, PI = 44; see Appendix B
Particle Size Distribution; see Appendix B
DRAFT
MC
87 32.6 85
61 9
22 39
MC
24 4
9 15
MC 69
17 26
43 MC 38
12 16
22 MC 48
19 20
28 MC 16
5 8
8 MC 24
5 9
15 Sampler
Type
Sample
Blows/ 6"
SPT
N-Value
1 LITHOLOGY
DEPTH
(feet)
Boring location:
Date started:
Drilling method:
1 2
3 4
5 6
7 8
9 10
11 12
13 14
15 16
17 18
19 20
21 22
23 24
25 26
27 28
29 30
MATERIAL DESCRIPTION
LABORATORY TEST DATASAMPLES
Figure:
A-4
PROJECT:
Project No.:
24-2751
Type of
Strength
Test
Confining
Pressure
Lbs/Sq Ft
Shear
Strength
Lbs/Sq Ft
Fines
%
Natural
Moisture
Content,%
Dry
Density
Lbs/Cu Ft
1 Boring terminated at a depth of 25 feet below
ground surface.
Boring backfilled with cement grout.
Groundwater not encountered during drilling.
ROCKRIDGE
GEOTECHNICAL
MC blow counts for the last two increments were
converted to SPT N-Values using a factor of 1.0
to account for sampler type and hammer energy.
LL = 66, PI = 42; see Appendix B
Particle Size Distribution; see Appendix B
See Site Plan, Figure 2
01/07/2025 Date finished: 01/07/2025
Hammer weight/drop: 140 lbs./30 inches Hammer type:
SANDY CLAY (CH)
brown and olive-brown, very stiff, moist, fine to medium stiff, trace subangular to subrounded gravel
Sampler:
Logged by:
Drilled by:
Rig:
PAGE 1 OF 1
Log of Boring B-6
CLAY (CH)
dark brown and black with orange, very stiff, moist, fine to medium sand
red-yellow, low hardness, friable
CH
Modified California (MC)
yellow, soft to low hardness, plastic
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
6-inch-diameter hollow-stem auger
black
plastic to friable
CLAYSTONE
red-yellow, low hardness, friable weak
CH
D. Fernandez
Taber Drilling
CME-55 Track Rig
Automatic Safety Hammer
DRAFT
CLASSIFICATION CHART
Project No. Figure A-5Date 01/27/25
ROCKRIDGE
GEOTECHNICAL 24-2751
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
Major Divisions Symbols Typical Names
GW
GP
GM
GC
SW
SP
SM
SC
ML
CL
OL
MH
CH
OH
PT
Highly Organic Soils
UNIFIED SOIL CLASSIFICATION SYSTEM
Well-graded gravels or gravel-sand mixtures, little or no fines Poorly-graded gravels or gravel-sand mixtures, little or no fines Silty gravels, gravel-sand-silt mixtures
Clayey gravels, gravel-sand-clay mixtures
Well-graded sands or gravelly sands, little or no fines Poorly-graded sands or gravelly sands, little or no fines Silty sands, sand-silt mixtures
Inorganic silts and clayey silts of low plasticity, sandy silts, gravelly silts Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, lean clays Organic silts and organic silt-clays of low plasticity
Inorganic silts of high plasticity
Inorganic clays of high plasticity, fat clays
Organic silts and clays of high plasticity
Peat and other highly organic soils
Clayey sands, sand-clay mixtures
Range of Grain Sizes
Grain Size
in Millimeters
U.S. Standard
Sieve Size
Above 12"
12" to 3"
Classification
Boulders
Cobbles
Above 305
305 to 76.2
Silt and Clay Below No. 200 Below 0.075
GRAIN SIZE CHART
SAMPLER TYPE
Coarse-Grained Soils
(more than half of soil > no. 200
sieve size)
Fine -Grained Soils
(more than half of soil
< no. 200 sieve size)
Gravels
(More than half of
coarse fraction >
no. 4 sieve size)
Sands
(More than half of
coarse fraction <
no. 4 sieve size)
Silts and Clays
LL = < 50
Silts and Clays
LL = > 50
Gravel
coarse
fine
3" to No. 4
3" to 3/4"
3/4" to No. 4
No. 4 to No. 200
No. 4 to No. 10
No. 10 to No. 40
No. 40 to No. 200
76.2 to 4.76
76.2 to 19.1
19.1 to 4.76
4.76 to 0.075
4.76 to 2.00
2.00 to 0.420
0.420 to 0.075
Sand
coarse
medium
fine
C Core barrel
CA California split-barrel sampler with 2.5-inch outside diameter and a 1.93-inch inside diameter
D&M Dames & Moore piston sampler using 2.5-inch outside diameter, thin-walled tube
O Osterberg piston sampler using 3.0-inch outside diameter, thin-walled Shelby tube
PT Pitcher tube sampler using 3.0-inch outside diameter, thin-walled Shelby tube
MC Modified California sampler with a 3.0-inch outside
diameter and a 2.43-inch inside diameter
SPT Standard Penetration Test (SPT) split-barrel sampler with a 2.0-inch outside diameter and a 1.38- or 1.5-inch inside diameter (refer to text)
ST Shelby Tube (3.0-inch outside diameter, thin-walled tube) advanced with hydraulic pressure
SAMPLE DESIGNATIONS/SYMBOLS
Sample taken with California or Modified California split-barrel sampler. Darkened area indicates soil recovered
Classification sample taken with Standard Penetration Test sampler Undisturbed sample taken with thin-walled tube
Disturbed sample
Sampling attempted with no recovery
Core sample
Analytical laboratory sample
Sample taken with Direct Push sampler
Sonic
Unstabilized groundwater level
Stabilized groundwater level
DRAFT
I FRACTURING
Intensity Size of Pieces in Feet
Very little fractured Greater than 4.0
Occasionally fractured 1.0 to 4.0
Moderately fractured 0.5 to 1.0
Closely fractured 0.1 to 0.5
Intensely fractured 0.05 to 0.1
Crushed Less than 0.05
II HARDNESS
1. Soft - reserved for plastic material alone.
2. Low hardness - can be gouged deeply or carved easily with a knife blade.
3. Moderately hard - can be readily scratched by a knife blade; scratch leaves a heavy trace of dust and is readily visible after the powder has been blown away.
4. Hard - can be scratched with difficulty; scratch produced a little powder and is often faintly visible.
5. Very hard - cannot be scratched with knife blade; leaves a metallic streak.
III STRENGTH
1. Plastic or very low strength.
2. Friable - crumbles easily by rubbing with fingers.
3. Weak - an unfractured specimen of such material will crumble under light hammer blows.
4. Moderately strong - specimen will withstand a few heavy hammer blows before breaking.
5. Strong - specimen will withstand a few heavy ringing hammer blows and will yield with difficulty only dust and small flying fragments.
6. Very strong - specimen will resist heavy ringing hammer blows and will yield with difficulty only dust and small flying fragments.
IV WEATHERING - The physical and chemical disintegration and decomposition of rocks and minerals by natural processes such as oxidation, reduction, hydration, solution, carbonation, and freezing and thawing.
D. Deep - moderate to complete mineral decomposition; extensive disintegration; deep and thorough discoloration; many fractures, all extensively coated or filled with oxides, carbonates and/or clay or silt.
M. Moderate - slight change or partial decomposition of minerals; little disintegration; cementation little to unaffected.
Moderate to occasionally intense discoloration. Moderately coated fractures.
L. Little - no megascopic decomposition of minerals; little of no effect on normal cementation. Slight and intermittent, or localized discoloration. Few stains on fracture surfaces.
F. Fresh - unaffected by weathering agents. No disintegration of discoloration. Fractures usually less numerous than joints.
ADDITIONAL COMMENTS:
V CONSOLIDATION OF SEDIMENTARY ROCKS: usually determined from unweathered samples. Largely dependent on cementation.
U = unconsolidated
P = poorly consolidated
M = moderately consolidated
W = well consolidated
VI BEDDING OF SEDIMENTARY ROCKS
Splitting Property Thickness Stratification
deddeb-kciht yrev .tf 0.4 naht retaer
G evissaM
deddeb kciht .tf 0.4 ot 0.2 ykcolB
deddeb niht .tf 0.2 ot 2.0 ybbalS
deddeb-niht yrev .tf 2.0 ot 50.0 yggalF
Shaly or platy 0.01 to 0.05 ft. laminated
detanimal ylniht 10.0 naht ssel yrepaP
Project No. FigureDate A-6
PHYSICAL PROPERTIES CRITERIA
FOR ROCK DESCRIPTIONS
ROCKRIDGE
GEOTECHNICAL 01/27/25 24-2751
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
DRAFT
APPENDIX B
Laboratory Test Results
DRAFT
ML or OL
MH or OH
Symbol Source
Natural
M.C. (%)
Liquid
Limit (%)
CL - ML
0 10
20 30
40 50
60 70
0 1 02 03 04 05 06 07 08 09 0 1 0 0 1 1 0 1 2 0
LIQUID LIMIT (LL)
Description and Classification
% Passing
#200 Sieve
Plasticity
Index (%)
PLASTICITY CHART
Project No. FigureDate B-1
ROCKRIDGE
GEOTECHNICAL 02/04/25 24-2751
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
PLASTICITY INDEX (PI)
Ref erence:
ASTM D2487-00
B-1 at 19.5 feet
B-3 at 1.5 feet
B-5 at 14.0 feet
B-6 at 3.5 feet
CLAY with SAND (CH), olive-gray
CLAY (CH), olive-gray
CLAYSTONE, light brown, gray,
and red
CLAY (CH), dark brown and black
with orange
21.3 38.0
31.7 32.6
73 91
98 87
58 69
66 66
36 46
44 42
DRAFT
SYMBOL SOURCE DEPTH Material Description USCS(ft.)
SOIL DATA
PERCENT FINER
0 10
20 30
40 50
60 70
80 90
100
GRAIN SIZE - mm.
0.0010.010.1110100
% +3" Coarse
% Gravel
Fine Coarse Medium
% Sand
Fine Silt
% Fines
Clay
0.0 0.1 0.3 0.3 3.5 23.0 37.2 35.6
0.0 0.0 0.0 1.7 5.4 2.0 48.6 42.3
0.0 0.0 0.0 0.2 0.6 0.8 57.4 41.0
0.0 0.1 0.0 0.5 5.4 6.7 38.8 48.5
6 in.
3 in.
2 in.
1½ in.
1 in.
¾ in.
½ in.
3/8 in.
#4
#10
#20
#30
#40
#60
#100
#140
#200
B-1 19.5' CH
B-3 1.5' CH
B-5 14.0' CH
B-6 3.5' CH
PARTICLE SIZE DISTRIBUTION REPORT
ROCKRIDGE
GEOTECHNICAL 01/27/25 24-2751
WILLOW AVENUE APARTMENTS
HIGHWAY 80 & WILLOW AVENUE
Hercules, California
CLAY with SAND, olive-gray
CLAY, olive-gray
CLAYSTONE, light brown, gray, and red
CLAY, dark brown and black with orange
Project No. FigureDate B-2
DRAFT