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091024 - 05.1 ATTACHMENT E
GEOTECHNICAL ENVIRONMENTAL WATER RESOURCES CONSTRUCTION SERVICES COASTAL/MARINE GEOTECHNICS 2010 Crow Canyon Place, Suite 250 San Ramon, CA 94583 (925) 866-9000 Fax (888) 279-2698 www.engeo.com Project No. 5393.000.000 January 26, 2024 Revised August 23, 2024 Mr. Othmar Van Dam Tassajara Holdings, LLC 6033 Laurelspur Loop San Ramon, CA 94582 Subject: Kent and Van Dam Properties Danville, California GEOTECHNICAL REVIEW OF TENTATIVE MAP PLANS References: 1. Milani & Associates. 2023. Vesting Tentative Map, Kent & Van Dam Properties, Danville, California. August 2024. Job No. 270019. 2.ENGEO. 2022. Geotechnical Report, Kent and Van Dam Properties, Danville, California. February 10, 2022. Project No. 5393.000.000. Dear Mr. Van Dam: As requested, we reviewed the geotechnical aspects of the Vesting Tentative Map Plans prepared by Milani & Associates (Reference 1) for the Kent and Van Dam Properties project in Danville, California. The purpose of our review was to confirm that the plans are in general conformance with our recommendations as presented in our geotechnical report (Reference 2) and to provide supplemental recommendations, as necessary. Milani & Associates prepared the plans. Our review included the following sheets. SHEET DESCRIPTION SHEET NUMBER Cover Sheet and General Notes 1 Overall Site Plan and Landslide Plan 7 and 8 Grading and Drainage and Utility Plan 9, 10, 11, 12 Preliminary C.3 Exhibit and Notes and Details 13 and 14 Site Sections and Details 18 Based on our review, it is our opinion from a geotechnical viewpoint that the sheets reviewed in the Vesting Tentative Map Plans were prepared in general conformance with the intent of our recommendations. The site is overlain by numerous landslides with significant depth which will require extensive corrective grading measures such as keyways, subdrains , and subexcavation of unstable material. Preliminary corrective grading recommendations are provided in our report (Referenced 2); however, ENGEO should be allowed to review and revise the recommendations based on the final development plan. In addition, we suggest that the grading work be contracted with an experienced grading contractor, well versed in hillside grading. ATTACHMENT E Tassajara Holdings, LLC 5393.000.000 Kent and Van Dam Properties January 26, 2024 GEOTECHNICAL REVIEW OF TENTATIVE MAP PLANS Revised August 23, 2024 Page 2 We make no representations as to the accuracy of dimensions, measurements, calculations , or the design by others. If you have any questions regarding the content of this letter, please do not hesitate to contact us. Sincerely, ENGEO Incorporated Jacob White, CEG Robert H. Boeche, CEG Andrew Firmin, GE jw/af/rhb/cb Copyright © 2022 by ENGEO Incorporated. This document may not be reproduced in whole or in part by any means whatsoever, nor may it be quoted or excerpted without the express written consent of ENGEO Incorporated. KENT AND VANDAM PROPERTIES 2449 AND 2451 TASSAJARA LANE APN 207-061-008 AND 207-061-009-8 DANVILLE, CALIFORNIA GEOTECHNICAL REPORT SUBMITTED TO Mr. Othmar Vandam 6033 Laurelspur Loop San Ramon, CA 94582 PREPARED BY ENGEO Incorporated February 10, 2022 PROJECT NO. 5393.000.000 GEOTECHNICAL ENVIRONMENTAL WATER RESOURCES CONSTRUCTION SERVICES COASTAL/MARINE GEOTECHNICS 2010 Crow Canyon Place, Suite 250 San Ramon, CA 94583 (925) 866-9000 Fax (888) 279-2698 www.engeo.com Project No. 5393.000.000 February 10, 2022 Mr. Othmar Vandam 6033 Laurelspur Loop San Ramon, CA 94582 Subject: Kent and Vandam Properties 2449 and 2451 Tassajara Lane Danville, California GEOTECHNICAL REPORT Dear Mr. Vandam: With your authorization, we conducted a geotechnical exploration for the proposed residential development at the Kent and Vandam properties in Danville, California. The accompanying report presents the results of our site exploration and geotechnical conclusions and recommendations for the project. It is our opinion that the proposed development is feasible from a geotechnical standpoint provided that the geotechnical recommendations presented in this report are incorporated into design and implemented during construction. If you have any questions or comments regarding this report, please call and we will be glad to discuss them with you. Sincerely, ENGEO Incorporated Siobhan O’Reilly-Shah, PE Phil Stuecheli, CEG Josef J. Tootle, GE sos/jjt/ps/dt Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report i of ii February 10, 2022 TABLE OF CONTENTS LETTER OF TRANSMITTAL 1.0 INTRODUCTION .................................................................................................. 1 1.1 PURPOSE AND SCOPE .................................................................................................... 1 1.2 PROJECT LOCATION AND DESCRIPTION ..................................................................... 1 1.3 PROPOSED DEVELOPMENT ........................................................................................... 1 1.4 EXISTING GEOTECHNICAL DATA ................................................................................... 2 1.5 2019 FIELD EXPLORATION AND LABORATORY TESTING ........................................... 2 1.5.1 Borings ................................................................................................................... 2 1.5.2 Test Pits ................................................................................................................. 2 2.0 FINDINGS ............................................................................................................ 3 2.1 SITE BACKGROUND ......................................................................................................... 3 2.2 GEOLOGY .......................................................................................................................... 3 2.2.1 Regional Geology .................................................................................................. 3 2.2.2 Site Geology ........................................................................................................... 3 2.3 GROUNDWATER CONDITIONS ....................................................................................... 4 2.4 SEISMICITY ........................................................................................................................ 4 3.0 ANALYSIS AND CONCLUSIONS ....................................................................... 5 3.1 SEISMIC HAZARDS ........................................................................................................... 5 3.1.1 Ground Rupture ..................................................................................................... 5 3.1.2 Ground Shaking ..................................................................................................... 5 3.1.3 Liquefaction ............................................................................................................ 6 3.1.4 Seismically-Induced Landslide Deformation .......................................................... 6 3.2 2019 CBC SEISMIC DESIGN PARAMETERS ................................................................... 6 3.3 LANDSLIDE HAZARDS ...................................................................................................... 6 3.3.1 Landslides and Earthflows ..................................................................................... 6 3.3.2 Off-site Landslides and Debris Flows .................................................................... 7 3.4 SLOPE STABILITY ANALYSIS .......................................................................................... 8 3.4.1 Geometry and Assumed Soil Parameters ............................................................. 8 3.4.2 Method of Analysis ................................................................................................. 8 3.4.3 Results of Slope Stability Analyses ....................................................................... 8 3.5 SOIL CORROSION POTENTIAL ........................................................................................ 9 3.6 EXPANSIVE SOIL............................................................................................................... 9 4.0 EARTHWORK RECOMMENDATIONS ............................................................... 9 4.1 GENERAL SITE CLEARING .............................................................................................. 9 4.2 REMOVALS ...................................................................................................................... 10 4.2.1 General Subexcavation ........................................................................................ 10 4.2.2 Existing Fill ........................................................................................................... 10 4.2.3 Landslide Removals ............................................................................................. 10 4.3 SLOPE GRADING ............................................................................................................ 10 4.3.1 Toe Keyways and Benching ................................................................................ 10 4.3.2 Subsurface Drainage ........................................................................................... 11 4.3.3 Cut Slopes ............................................................................................................ 11 Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report TABLE OF CONTENTS (Continued) ii of ii February 10, 2022 4.4 REMEDIAL GRADING EVALUATION/ACCEPTANCE .................................................... 11 4.5 FILL PLACEMENT ............................................................................................................ 11 4.6 SELECTION OF MATERIALS .......................................................................................... 12 4.7 UNDERGROUND UTILITY BACKFILL ............................................................................. 12 4.8 EROSION CONTROL ....................................................................................................... 13 4.9 STORMWATER BIORETENTION AREAS ....................................................................... 13 5.0 FOUNDATION RECOMMENDATIONS ............................................................. 14 5.1 PIER AND GRADE BEAM ................................................................................................ 14 5.2 CRAWL SPACE MOISTURE CONTROL ......................................................................... 15 5.3 PAD AND FOUNDATION DRAINAGE ............................................................................. 16 5.4 EXTERIOR SLAB-ON-GRADE ......................................................................................... 16 6.0 RETAINING WALL RECOMMENDATIONS ...................................................... 17 6.1 SHORING/CATCHMENT WALL ....................................................................................... 17 6.2 CONVENTIONAL RETAINING WALLS ............................................................................ 17 6.2.1 Retaining Wall Surcharge .................................................................................... 18 6.3 MECHANICALLY STABILIZED EARTH (MSE) WALLS................................................... 18 6.4 RETAINING WALL DRAINAGE AND BACKFILL ............................................................. 19 7.0 PRELIMINARY PAVEMENT DESIGN ............................................................... 19 7.1 CUT-OFF CURBS ............................................................................................................. 20 8.0 LIMITATIONS AND UNIFORMITY OF CONDITIONS ....................................... 20 PROJECT REFERENCES TECHNICAL REFERENCES FIGURES APPENDIX A – Exploration Logs APPENDIX B – Laboratory Test Data APPENDIX C – Previous Exploration Logs (ENGEO, 2005) APPENDIX D – Previous Exploration Logs (HJA, 2007) APPENDIX E – Previous Laboratory Test Data (ENGEO, 2004 - 2007) APPENDIX F – Slope Stability Analysis APPENDIX G – Geotechnical Contract Standards Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 1 February 10, 2022 1.0 INTRODUCTION 1.1 PURPOSE AND SCOPE The purpose of this report is to develop geotechnical recommendations for the design and construction of the proposed project. Our scope of services included the items described below. Review previous borings and laboratory data from adjacent projects. Review previous reports by other consultants listed in the References, available literature, historic aerial images, and published geologic maps covering the study area. Subsurface field exploration and laboratory testing. Interpretation of subsurface field exploration data. Evaluation of potential geotechnical concerns. Data analysis and conclusions. Prepare a report summarizing our conclusions and recommendations. This report was prepared for the exclusive use of our client and their consultants for design of this project. In the event that any changes are made in the character, design or layout of the development, we must be contacted to review the conclusions and recommendations contained in this report to evaluate whether modifications are recommended. 1.2 PROJECT LOCATION AND DESCRIPTION The proposed development includes the Kent and Vandam properties totaling approximately 8.5 acres, located in Danville, as shown on Figures 1 and 5. The 4.6-acre Kent property, designated as Assessor’s Parcel Number (APN) 207-061-008, is bordered by Tassajara Lane on the northwest, the existing Subdivision 9014 on the north, private parcels on the east and south, and the Vandam property on the west. The 3.9-acre Vandam Property, designated as APN 207-061-009-8 is bordered on the north by Tassajara Lane and by private properties on the west and south. Elevations across the site vary from approximately 508 feet along the east property line on the Kent property to about 670 feet at the southwest corner of the Vandam property, as shown on Figure 5. Existing structures on the properties include a barn and outbuildings. The site topography has been modified by grading, including small cuts and fills on the Kent property. The eastern side of the Kent property was re-graded to create the existing east-facing slope in 2007 as part of the construction of Subdivision 9014. 1.3 PROPOSED DEVELOPMENT Based on preliminary grading plans prepared by Milani & Associates, Inc. dated March 2009, it is proposed to construct seven single-family residential lots on Kent and Vandam Properties as shown on Figure 5. Building pads will be created by making cuts of up to about 20 feet, generally around the southern and western portions of the site, and fills of up to about 15 feet on the northern portion of the site. It is proposed to construct a new 15-foot-high 3:1 (horizontal:vertical) slope along the north margin of the project. Tassajara Lane will be widened along the property’s western boundary. The proposed building pads will be split-level, with 10-foot-high 2:1 slopes between the Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 2 February 10, 2022 upper and lower level. The development of the subject site will also include construction of interior driveways and utilities to service the development. Structural loads and other details related to the proposed residential structures and site improvements have not been developed at this time. However, we understand that the proposed residential houses are anticipated to consist of one- and two-story wood-framed structures. 1.4 EXISTING GEOTECHNICAL DATA ENGEO completed a geotechnical exploration in 2005 on the adjacent Subdivision 9014 (formerly the Gates property), that included drilling three borings on the Gates property and two borings on the Kent property. Henry Justiniano & Associates (HJA) prepared a series of geotechnical reports for the Kent and Vandam properties in 2008 and 2009 (listed in the References). The HJA exploration included drilling eight borings, laboratory testing and preparation of geotechnical recommendations for the proposed seven-lot residential development depicted on Figure 5. Logs of the previous ENGEO borings and laboratory test results are contained in Appendix A. HJA boring and laboratory test results are presented in Appendix B. 1.5 2019 FIELD EXPLORATION AND LABORATORY TESTING Our field exploration included drilling 9 continuously cored borings and 21 test pits at the locations shown on Figure 6. An explanation of our field exploration methods and the exploration logs are presented in Appendix A. To measure shear strength, soil gradation, plasticity index, dry density, and moisture content, we tested samples recovered during drilling activities. Our laboratory test results are presented in Appendix B, and select test results are also presented on the boring logs in Appendix A. 1.5.1 Borings We drilled eight borings with a CME dry core sampling system to obtain relatively continuous soil profiles at each boring location. The cores were photographed and logged in the field by our engineering geologist. The recovered cores were returned to our lab for additional observation and testing. We used our field logs combined with examination of core photos and laboratory testing to develop the report logs in Appendix A. The logs depict subsurface conditions at the exploration locations for the date of exploration; however, subsurface conditions may vary with time. 1.5.2 Test Pits We excavated 21 test pits to a maximum depth of 14 feet. An ENGEO geologist observed the test pit excavation and logged the subsurface conditions at each location. We retained a backhoe to excavate the test pits using a 2-foot-wide bucket and logged the type, location, and uniformity of the underlying soil/rock. We obtained bulk soil samples from the test pits using hand-sampling techniques. The test pit logs present descriptions of the subsurface conditions encountered. We used the field logs to develop the report logs in Appendix A. The logs depict subsurface conditions at the exploration locations for the date of exploration; however, subsurface conditions may vary with time. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 3 February 10, 2022 2.0 FINDINGS 2.1 SITE BACKGROUND To characterize and understand the site use history and the geomorphology, we reviewed stereographic-paired aerial photographs dating to 1939 (References). The Kent property includes a barn and corrals and has previously been used for agricultural purposes. The Vandam property is open, undeveloped grassland. There have been several episodes of minor grading on the Kent property, occurring from the mid-1960s through the 1980s, resulting in placement of fill as described below in Section 2.2.2. 2.2 GEOLOGY 2.2.1 Regional Geology As shown on Figure 2, The Tassajara Valley is underlain by folded upper Miocene-age nonmarine sedimentary rocks generally referred to as the Green Valley/Tassajara Group (Tgvt) (Graymer, 1994). The Tgvt typically consists of interbedded claystone, clayey siltstone, sandstone and conglomerate. The Tgvt bedrock contains a high proportion of fine-grained and expansive clay. Similar folded Miocene/Pliocene-age rocks extend from the south edge of the Mount Diablo uplift to the Livermore Valley. Regional landslide mapping by Nilsen (1975) is depicted on Figure 3. Nilsen identified earthflows in the swale areas south and east of the Kent property, as well as on the western portion of the Vandam property. For this project, landslide mapping was refined based on detailed examination of aerial photographs and a geologic field reconnaissance. 2.2.2 Site Geology Artificial Fill (Qaf, Qef) – portions of the Kent and Vandam properties were previously graded in the 1960s through the 1980s to fill in old incised drainages and locally create more level areas. The older fill is considered undocumented and are labeled on Figure 6 as Qaf. Undocumented fill up to about 12 feet thick was encountered in borings along the north side of the Kent property. In 2007, Braddock & Logan Services constructed the adjacent Subdivision 9014. Part of the site grading included a buttress key that extended up to about 40 feet below the original ground surface. The southern edge of the buttress extended on to the Kent property, as shown on Figures 6 and 7. Grading on Subdivision 9014 was observed and tested by ENGEO, and the extent of corrective grading and exposed geologic conditions were documented in the ENGEO report of July 5, 2007. The limits of documented engineered fill are depicted on Figure 5. The engineered fill typically consisted of clay and rock fragments derived from buttress excavations and from fill imported from the nearby Tassajara Lane project. Surficial Landslides (Qls) – most of the area of the Kent property and swale areas on the Vandam property are underlain by surficial landslides consisting of stiff, high plasticity clay. The surficial landslide deposits typically contain numerous internal slip surfaces and are generally bounded at the base by well-defined slip planes. A landslide on the west-central portion of the Vandam property has exhibited several recent episodes of activity, including as Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 4 February 10, 2022 recently as the winter of 2016-2017. This landslide extends off the property to the west on to the adjacent Subdivision 9014 property. Deep-seated Bedrock Landslides (Qls) – Borings completed by ENGEO for the current and previous projects encountered evidence of landslide slip planes within weathered bedrock underlying surficial landslide deposits at depths of about 25 to 45 feet as described in the boring logs and depicted on the Cross Sections, Figure 7. In addition, landslide slip planes in the upper portions of the bedrock were previously observed in the back cut of the existing buttress key at the north end of the Kent property. Dormant Landslide Lobe – The east side of the Kent property is bordered by an approximately 700-foot-long dormant landslide lobe that extends off the property to the east. Test pits excavated along the east property line encountered landslide deposits to the maximum depth explored, over 14 feet. Adjacent Borings 2-B3 and 2-B7 encountered landslide deposits to depths of about 30 to 40 feet. The geomorphology of the landslide lobe is subdued and it appears to be inactive. Tassajara Green Valley Group Bedrock (Tgvt) – Bedrock deposits encountered in site explorations consist mainly of claystone and clayey siltstone with minor amounts of interbedded friable sandstone. The bedrock is weak, closely fractured and the claystone and siltstone are highly expansive. The bedrock strikes west-northwest and dips at steep inclinations generally between 40 and 60 degrees to the north. 2.3 GROUNDWATER CONDITIONS Groundwater was encountered in Boring 2-B-4 at 24 feet but was not encountered in any other previous borings or test pits completed for this study. During grading of the buttress key on the adjacent Subdivision 9014, the 40-foot deep excavation did not encounter significant free groundwater. 2.4 SEISMICITY Numerous small earthquakes occur every year in the San Francisco Bay Area and larger earthquakes have been recorded and can be expected to occur in the future. Figure 3 shows the approximate location of faults and epicenters of significant historic earthquakes recorded within the Greater Bay Area Region. We provide a list of active faults within 25 miles of the site and their estimated maximum earthquake magnitudes in the following table. The California Geological Survey defines an active fault as one that has had surface displacement within Holocene time (approximately the last 11,000 years) (Bryant and Hart, 2007). TABLE 2.4-1: Nearby Active Faults, Latitude: 37.80557 Longitude: -121.95365 FAULT NAME DISTANCE FROM SITE (MILES) MAXIMUM MOMENT MAGNITUDE Mount Diablo Thrust South 6.3 6.6 Calaveras 5.8 7.1 Mount Diablo Thrust North 2.3 7.2 Concord 11.2 6.6 Hayward 17.3 2.4 The United States Geologic Survey evaluated California seismicity through a study by the 2014 Working Group on California Earthquake Probabilities (WGCEP) (Field, 2014) which led to Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 5 February 10, 2022 development of Uniform California Rupture Forecast (Version 2) (UCERF3). The 2014 WGCEP evaluated the 30-year probability of MW 6.7 or greater earthquake occurring on the known active fault systems in the San Francisco Bay Area, and the 2014 WGCEP estimated an overall probability of 72 percent for this area. The site is not located within a currently designated Alquist-Priolo Earthquake Fault Zone and no known surface expression of active faults is believed to exist within the site; therefore, fault rupture through the site is not anticipated. 3.0 ANALYSIS AND CONCLUSIONS It is our opinion that the proposed seven-lot subdivision is feasible from a geologic and geotechnical standpoint, provided the recommendations contained in this report are incorporated into the development plans. Based on our explorations and analysis as well as a review of available published maps and reports for the site, the main geotechnical concerns for the proposed development are critically expansive site soil and bedrock, the extensive existing landslide deposits, existing undocumented fill, and the fact that landslide deposits identified on the site cross property boundaries where corrective grading excavations will be required. Landslide and expansive soil hazards exist throughout the Danville area. Landslides can be effectively mitigated during site grading by removing the landslides and replacing the slide debris with engineered fill, or buttressing the landslides and providing debris catchment areas. Expansive soil can be mitigated by proper foundation design and grading measures. These items and other geotechnical issues are discussed in the following sections of this report. 3.1 SEISMIC HAZARDS Potential seismic hazards resulting from a nearby earthquake can generally be classified as primary and secondary. The primary effect is ground rupture, also called surface faulting. The common secondary seismic hazards include ground shaking, liquefaction, and seismically induces landslide deformation. The following sections present a discussion of these hazards as they apply to the site. Based on topographic and lithologic data, the risk of regional subsidence or uplift, tsunamis, flooding or seiches is considered low to negligible at the site. 3.1.1 Ground Rupture Since there are no known active faults crossing the property and the site is not located within an Earthquake Fault Special Study Zone, it is our opinion that ground rupture is unlikely at the subject property. 3.1.2 Ground Shaking An earthquake of moderate to high magnitude generated within the San Francisco Bay Area region could cause considerable ground shaking at the site, similar to that which has occurred in the past. To mitigate the shaking effects, structures should be designed using sound engineering judgment and the 2019 California Building Code (CBC) requirements, as a minimum. Seismic design provisions of current building codes generally prescribe minimum lateral forces, applied statically to the structure, combined with the gravity forces of dead-and-live loads. The code-prescribed lateral forces are generally considered to be substantially smaller than the Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 6 February 10, 2022 comparable forces that would be associated with a major earthquake. Therefore, structures should be able to: (1) resist minor earthquakes without damage, (2) resist moderate earthquakes without structural damage but with some nonstructural damage, and (3) resist major earthquakes without collapse but with some structural as well as nonstructural damage. Conformance to the current building code recommendations does not constitute any kind of guarantee that significant structural damage would not occur in the event of a maximum magnitude earthquake; however, it is reasonable to expect that a well-designed and well-constructed structure will not collapse or cause loss of life in a major earthquake (SEAOC, 1996). 3.1.3 Liquefaction Soil liquefaction results from loss of strength during cyclic loading, such as imposed by earthquakes. The soil considered the most susceptible to liquefaction is clean, loose, saturated, uniformly graded fine sand below the groundwater table. Due to the soil classifications of the on-site soil/bedrock and the fines content, the risk of liquefaction at the site is low. 3.1.4 Seismically-Induced Landslide Deformation Existing landslides on the Kent and Vandam properties could be subject to seismically induced deformation, and will require corrective grading to improve slope stability as discussed below in Section 3.3. 3.2 2019 CBC SEISMIC DESIGN PARAMETERS The following table provides seismic design criteria utilizing ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures that will be incorporated into the 2019 California Building Code (CBC) criteria. TABLE 3.2-1: 2019 CBC Seismic Design Parameters, Latitude: 37.8056 Longitude: -121.9537 PARAMETER VALUE Site Class C Mapped MCER Spectral Response Acceleration at Short Periods, SS (g) 2.21 Mapped MCER Spectral Response Acceleration at 1-second Period, S1 (g) 0.72 Site Coefficient, FA 1.2 Site Coefficient, FV 1.4 MCER Spectral Response Acceleration at Short Periods, SMS (g) 2.65 MCER Spectral Response Acceleration at 1-second Period, SM1 (g) 1.01 Design Spectral Response Acceleration at Short Periods, SDS (g) 1.77 Design Spectral Response Acceleration at 1-second Period, SD1 (g) 0.68 3.3 LANDSLIDE HAZARDS 3.3.1 Landslides and Earthflows As depicted on Figures 6 and 7, most of the Kent property and a large portion of the Vandam property are underlain by existing landslides ranging in depth from several feet to up to about 40 feet. These landslides occur within highly expansive surficial clay soil and the upper portions of the weathered bedrock. The landslides can generally be categorized and earthflows and Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 7 February 10, 2022 rotational slumps, which are generally viscous and slow-moving, typically triggered by seasonal rainfall events. The landslide deposits along the northern portion of the Kent property were stabilized as part of the grading of the Subdivision 9014 project by construction of a 30- to 40-foot-deep, 80- to 100-foot-wide buttress key. However, the upslope portion of these landslides, as well as landslides on the Vandam property, will require additional corrective grading measures to stabilize the proposed cut and fill slopes depicted on the grading plans. The proposed stabilization measures are depicted on the Cross Sections, Figure 7, and on the corrective grading plan, Figure 8. 3.3.2 Off-site Landslides and Debris Flows Landslide deposits extend off the west and east sides of the Kent property and on to neighboring parcels. There are additional landslides in the existing swale area south of the Kent property. Based on these conditions, it will be necessary to provide some form of debris catchment along the west and south project boundaries. The most significant off-site debris flow risk occurs on the Kent property where the existing topographic swale extends up slope and to the south for several hundred feet, directly behind the proposed Lot 4. In our opinion, the project plans should be modified to provide a debris catchment basin with a minimum capacity of approximately 1,000 cubic yards. Along the western boundary of the Vandam property, some form of debris catchment will also be required, such as a debris catchment wall. Both debris catchment basins and catchment should be provided with a means of equipment access to allow periodic removal of debris. Regular maintenance of the debris catchment should be anticipated and some form of funding should be provided for yearly inspection and debris removal as needed. Because existing landslide deposits on the Vandam property extend across property lines, proposed corrective grading excavations are likely to result in off-site mobilization of landslide deposits on the adjacent property to the west. Corrective grading measures along the east property line are configured to minimize construction mobilization risks. The risk of off -site landslide mobilization can be accommodated by: Obtaining permission to grade on adjacent properties, and extending corrective grading to completely remove and replace existing landslide deposits in conjunction with on-site corrective grading. This option is recommended for the western side of the Vandam property. If access to the adjacent property west of the Vandam parcel cannot be obtained, construct a temporary shoring wall designed to support open excavations and to increase long-term slope stability. The shoring wall can also be designed with a minimum of 5 feet of exposed freeboard to also act as a debris catchment. Perform corrective grading in narrow sections to reduce off-site landslide risks. This option could be applicable along the western side of the Vandam property. Configure corrective grading to minimize the risk of off -site landslide mobilization by limiting the width of open exaction and by limiting maximum temporary slope inclination to 1½:1. This option could be applied along the eastern property boundary at the Kent property. Along the western property boundary, HJA recommended a tangent pier wall with exposed freeboard. We provide recommendations for the shoring/catchment wall in Section 6.1. Based on landslide mapping provided on Figure 6, we recommend that the shoring/catchment wall extend for a minimum length of 300 feet. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 8 February 10, 2022 3.4 SLOPE STABILITY ANALYSIS 3.4.1 Geometry and Assumed Soil Parameters We analyzed three cross sections showing the geologic conditions and proposed grading at the site. Prior to performing slope stability analyses, we evaluated the strength of native soil and proposed engineered fill based on index testing, Consolidated Undrained (CU) Triaxial Compression Tests and Unconsolidated Undrained (UU) Triaxial Compression Tests. In our evaluation, we used previous testing for this project site and from the adjacent Tassajara Lane as well as additional test performed for this study. Based on our data review, we developed the idealized soil profiles shown on our slope stability results. TABLE 3.4.1-1: Slope Stability Analysis Material Properties SOIL MATERIAL LAYER UNIT WEIGHT (PCF) COHESION (PSF) FRICTION ANGLE (DEGREE) Engineered Fill - Undrained 125 2000 0 Engineered Fill - Drained 130 200 23 Landslide 125 0 12 Tygt 125 1000 25 3.4.2 Method of Analysis We performed a simplified deformation analysis using the computer program SLIDE, which is a limit equilibrium program that allows the user various search routines to locate the minimum factor of safety and critical slip surface. We used circular searching methods and the Spencer’s method for our analyses (Spencer, 1973). We used the design peak ground acceleration (PGA) of 0.71g for liquefaction analysis based on ASCE 7-16 seismic parameters. We used a moment magnitude (Mw) of 7.1, based the maximum possible earthquake on the Calaveras fault. We performed a “pseudo-static” screening analysis as recommended in the California Geological Survey’s (CGS) SP117A “Guidelines for Evaluating and Mitigating Seismic Hazards in California.” For this screening analysis, we selected a seismic coefficient of 0.26g for an assumed displacement threshold of about 15 centimeters or approximately 6 inches. 3.4.3 Results of Slope Stability Analyses TABLE 3.4.3-1: Summary of Slope Stability Analyses FACTOR OF SAFETY LOCATION STATIC PSEUDO-STATIC (0.15g) Cross Section 2-2’ 1.2 1.2 Cross Section 4-4’ 2.4 1.3 Cross Section 5-5’ 2.1 1.1 Our analyses indicate a factor of safety greater than 1.5 for the static analysis, except for Cross Section 2-2’, and 1.0 for the Pseudo-Static analysis. Based on SP117, the lateral displacement for a design seismic event is anticipated to be less than 6 inches. According to SP117A, this amount of lateral displacement is generally considered small enough that residential structures can be designed with foundations stiff enough to allow for the movement without serious damage. Appendix D presents select printouts of our analyses. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 9 February 10, 2022 Cross Section 2-2’ cuts through an existing landslide partially located on an adjacent property that is discussed in Section 3.3.2. If remedial grading cannot be performed on the adjacent property and the upper portion of the landslide were to remain in place, then the static factor of safety is less than 1.5 as shown in our slope stability analysis. In addition, this area would likely be subject to failure during construction, as the remedial grading for the lower portion of the landslide is performed. We recommend that a shoring/catchment wall be constructed to stabilize the upper portion of the landslide. We present recommendations for this retaining wall in Section 6.1. 3.5 SOIL CORROSION POTENTIAL Soil corrosion testing performed on the adjacent Subdivision 9014 indicated a moderate sulfate exposure for concrete. For planning purposes, it should be an assumed that soil sulfate levels at subject project will be similar. The final soil conditions on graded pads will not be known until the completion of site grading. We recommend that additional sulfate testing be performed at the completion of finished grading. 3.6 EXPANSIVE SOIL Expansive soil shrinks and swells because of moisture changes. This can cause heaving and cracking of slabs-on-grade, pavements, and structures founded on shallow foundations. Site soil and bedrock soil encountered in current and previous explorations consisted of highly expansive clay with PIs generally ranging from 31 to 55, indicating high to critically expansive soil conditions. It will therefore be necessary to design improvements to mitigate expansive soil conditions. 4.0 EARTHWORK RECOMMENDATIONS Based on the findings of our subsurface exploration, geologic mapping, laboratory testing, and field experience, we provide the following geotechnical recommendations for use during grading and construction. We should be notified at least 48 hours prior to grading to coordinate our schedule with the grading contractor. All grading operations should meet the requirements of our Geotechnical Contract Standards presented in Appendix G and must be observed and tested by our field representatives. Ponding of stormwater, other than within engineered detention basins, should not be permitted at the site, particularly during work stoppage for rainy weather. Before the grading is halted by rain, positive slopes should be provided to carry surface runoff to storm drainage structures in accordance with the approved Storm Water Pollution Prevention Plan (SWPPP). 4.1 GENERAL SITE CLEARING Areas to be developed should be cleared of surface and subsurface deleterious materials, including existing building foundations, slabs, buried utility and irrigation lines, septic systems, pavements, debris, and designated trees, shrubs, and associated roots. Following clearing, the site should be stripped to remove surface organic materials. Strip organics from the ground surface to a depth of at least 2 to 3 inches below the surface. Tree roots should be removed to a depth of at least 5 feet below finished grade in cut lots and 3 feet below original Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 10 February 10, 2022 grade in fill lots. Our field representative shall determine actual depths of stripping and tree root removal during grading. Within the development areas, excavations resulting from demolition and stripping, which extend below final grades, should be cleaned to firm undisturbed soil as determined by our field representative. No loose or uncontrolled backfilling of depressions resulting from demolition and stripping is permitted. The surface should then be scarified, moisture conditioned, and backfilled with compacted engineered fill. The requirements for backfill materials and placement operations are the same as for engineered fill. 4.2 REMOVALS 4.2.1 General Subexcavation The soil/bedrock should be excavated to a minimum depth of 5 feet below existing grade in areas to receive fill and 5 feet below finish grade in areas of cut, and replaced with uniformly mixed fill compacted in accordance with Section 4.5. 4.2.2 Existing Fill Our supplemental exploration and the previous studies encountered existing undocumented fill material up to 12 feet thick on the project site. The existing fill generally overlays landslide deposits and will be removed and replaced during corrective grading for landslide stabilization. Existing engineered fill on the northern portion of the Kent property will be suitable for supporting improvements, but localized keying and benching will be required to support proposed fill slopes as shown on Figures 7 and 8. 4.2.3 Landslide Removals Landslide removals are depicted on the Cross Sections, Figure 7, and the corrective grading plan, Figure 8. The removal depicted on the Figures are intended to provide general guidelines for corrective grading to increase site stability to acceptable static and seismic factors of safety. Landslide removals should be observed in the field by ENGEO so that we can make appropriate recommendation for adjusting the depth and extents of removals based on revealed conditions. 4.3 SLOPE GRADING Graded slopes should be constructed at inclinations no steeper than 2:1 (horizontal:vertical) for height of up to 8 feet. Slopes higher than 8 feet should be inclined no steeper than 3:1. The contractor is responsible to construct temporary construction slopes in accordance with CALOSHA requirements. 4.3.1 Toe Keyways and Benching After stripping and clearing, mass grading should begin with construction of toe keyways. We recommend constructing a keyway at the toe of fill slopes and reconstructed cut and cut-fill transition slopes. The keyways should be a minimum of 25 feet wide and extend at least 5 feet below original grade or proposed grade into firm competent soil or rock, as determined by our field representative during grading. The bottom of the keyway should slope at least 2 percent downward toward the heel of the key. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 11 February 10, 2022 Typical keyway and benching details are shown on Figure 9. We may recommend deeper keyways and/or benches based on actual soil/rock conditions observed during construction. The area beyond the toe of fill slope shall be sloped for sheet overflow or other drainage provisions. We should observe and approve the keyways and benches in the field prior to placement of engineered fill. 4.3.2 Subsurface Drainage Subsurface drainage systems should be installed in keyways and in swales or natural drainage ways that are to be filled, as shown on our Corrective grading plan (Figure 8). Additional subdrains should be added, where seepage, springs, or wet conditions are encountered during excavation as recommended by our field representative. Place subdrains at bottom of keyway where possible to gravity flow. The anticipated locations, approximate elevations and recommended outfall points for subdrains are depicted on the corrective grading plans, Figure 8. Subdrain systems should consist of a minimum 6-inch-diameter perforated pipe encased in an 18-inch minimum thickness of Caltrans Class 2 permeable material or clean crushed rock wrapped in geotextile filter fabric, as shown on the typical detail on Figure 10. We recommend that subdrains have a minimum fall of 1 percent. The subdrain pipe and filter fabric should meet the requirements contained in our Geotechnical Contract Standards presented in Appendix G. Discharge from the subdrains will generally be low but in some instances may be continuous. Subdrains should outlet into the storm drain system or other approved outlets, and their locations should be surveyed and documented by the project Civil Engineer for future maintenance. Subdrains should gravity flow to the approved outlet. Not all sources of seepage are evident during the time of grading because of the intermittent nature of some of these conditions and their dependence on long-term climatic conditions. Furthermore, new sources of seepage may be created by a combination of changed topography, manmade irrigation patterns, and potential utility leakage. Since uncontrolled seepage is one of the major causes of detrimental soil movements, it is of utmost importance that we be advised of any seepage conditions so that remedial action may be initiated, if necessary. 4.3.3 Cut Slopes Cut slopes on the proposed project will expose either existing landslide deposits of relatively weak and expansive Tassajara Formation bedrock. As shown on Figure 7, we recommend that the proposed cut slope be overexcvated and reconstructed as a drained fill buttress. 4.4 REMEDIAL GRADING EVALUATION/ACCEPTANCE We will observe and/or document remedial grading areas, including keyways, benches, and subexcavation areas, prior to receiving fill. The areas receiving fill and subdrains should be surveyed. 4.5 FILL PLACEMENT The contractor should perform subgrade compaction prior to fill placement to provide adequate bonding with the initial lift of fill. The contractor should first scarify the exposed soil to a depth of at least 12 inches then moisture condition and compact the subgrade in accordance with the requirements listed in the table below. The lift thickness should not exceed 12 inches or the depth Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 12 February 10, 2022 of penetration of the compaction equipment used, whichever is less. Track rolling to compact faces of slopes is usually not sufficient; typically, slopes should be overfilled a minimum of 2 feet and cut back to design grades. The following compaction recommendations should be used for the placement and compaction of subgrade and fill: TABLE 4.5-1: Subgrade and Engineered Fill Compaction and Moisture Content Requirements MATERIALS* MINIMUM RELATIVE COMPACTION (%) MINIMUM RELATIVE COMPACTION (%) – UPPER 6 INCHES OF FILL IN PAVEMENT AREAS MINIMUM MOISTURE CONTENT (percentage points above optimum) Expansive On-site Material* 87 to 92 90 5 Non-Expansive Import Fill* 90 95 2 Pavement AB** 95 -- 0 *Expansive material defined as PI>12 **As specified in Section 7.0 The relative compaction and optimum moisture content of soil and aggregate base referred to in this report are based on the most recent ASTM D1557 test method. We recommend that the fill be compacted at higher than optimum moisture contents, as shown above, to minimize the effects of swell and/or hydrocompression. The term “moisture condition” refers to adjusting the moisture content of the soil by either drying if too wet or adding water if too dry. Compacted soil is not acceptable if it is unstable. It should exhibit only minimal flexing or pumping, as observed by our field representatives. 4.6 SELECTION OF MATERIALS On-site soil material is suitable as fill material provided it is processed to remove concentrations of organic material (soil which contains more than 2 percent organic content by weight), and debris. Imported fill materials should meet the above requirements and the Geotechnical Contract Standards in Appendix G. 4.7 UNDERGROUND UTILITY BACKFILL The contractor is responsible for conducting trenching and shoring in accordance with CALOSHA requirements. We recommend that utility trench backfilling be done under our observation. Pipe zone backfill (i.e., material beneath and immediately surrounding the pipe) may consist of a well-graded import less than ¾ inch in maximum dimension. Trench zone backfill (i.e., material placed between the pipe zone backfill and the ground surface) may consist of native soil. Trench zone back fill should be compacted according to the recommendations in Section 4.5. We recommend the import material used for pipe zone backfill consist of fine- to medium-grained sand or a well-graded mixture of sand and gravel and that this material not be used within 2 feet of finish grades. In general, uniformly graded gravel should not be used for pipe or trench zone backfill due to the potential for migration of: (1) soil into the relatively large void spaces present in this type of material and (2) water along trenches backfilled with this type of material. Where utility trenches pass under a building perimeter, they must be provided with an impervious seal consisting of native materials or concrete. The impervious plug should extend at least 3 feet to each side of the crossing. This is to reduce surface-water percolation into the material under Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 13 February 10, 2022 foundations and pavements where such water would remain trapped in a perched condition, allowing clay to develop its full expansion potential. Care should be exercised where utility trenches are located beside foundation areas. Utility trenches constructed parallel to foundations should be located entirely above a plane extending down from the lower edge of the footing at an angle of 45 degrees. Utility companies and Landscape Architects should be made aware of this information. Compaction of trench backfill by jetting should not be allowed at this site. If there appears to be a conflict between The City or other agency requirements and the recommendations contained in this report, this should be brought to the Owner’s attention for resolution prior to submitting bids. 4.8 EROSION CONTROL The tops of fill or cut slopes should be graded in such a way as to prevent water from flowing freely down the slopes. Due to the nature of the site soil and bedrock, graded slopes may experience severe erosion when grading is halted by heavy rain. Therefore, before work is stopped, a positive gradient away from the tops of slopes should be provided to carry the surface runoff away from the slopes to areas where erosion can be controlled. It is vital that no completed slope be left standing through a winter season without erosion control measures having been provided. Because the existing bedrock is relatively nutrient poor, it may be difficult for vegetation to become properly established, resulting in a potential for slope erosion. Revegetation of graded slopes can be aided by retaining the organic-rich strippings and spreading these materials in a thin layer (approximately 6 inches thick) on the graded slopes prior to the winter rains and following rough grading. All landscaped slopes should be maintained in a vegetated state after project completion. The use of drought-tolerant vegetation requiring infrequent drip irrigation during summer is recommended. No pressurized irrigation lines should be placed on or near the tops of graded slopes. 4.9 STORMWATER BIORETENTION AREAS If bioretention areas are implemented, we recommend that, when practical, they be planned a minimum of 5 feet away from structural site improvements, such as buildings, streets, retaining walls, and sidewalks/driveways. When this is not practical, bioretention areas located within 5 feet of structural site improvements can either: 1. Be constructed with structural side walls capable of withstanding the loads from the adjacent improvements, or 2. Incorporate filter material compacted to between 85 and 90 percent relative compaction and a waterproofing system designed to reduce the potential for moisture transmission into the subgrade soil beneath the adjacent improvement. In addition, one of the following options should be followed. 1. We recommend that bioretention design incorporate a waterproofing system lining the bioswale excavation and a subdrain, or other storm drain system, to collect and convey water to an approved outlet. The waterproofing system should cover the bioretention area Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 14 February 10, 2022 excavation in such a manner as to reduce the potential for moisture transmission beneath the adjacent improvements. 2. Alternatively, and with some risk of movement of adjacent improvements, if infiltration is desired, we recommend the perimeter of the bioretention areas be lined with an HDPE tree root barrier that extends at least 1 foot below the bottom of the bioretention areas/infiltration trenches. Site improvements located adjacent to bioretention areas that are underlain by base rock, sand, or other imported granular materials, should be designed with a deepened edge that extends to the bottom of the imported material underlying the improvement. Where adjacent site improvements include buildings greater than three stories, streets steeper than 3 percent, or design elements subject to lateral loads (such as from impact or traffic patterns), additional design considerations may be recommended. If the surface of the bioretention area is depressed, the slope gradient should follow the slope guidelines described in earlier section(s) of this document. In addition, although not recommended, if trees are to be planted within bioretention areas, HDPE Tree Boxes that extend below the bottom of the bioretention system should be installed to reduce potential impact to subdrain systems that may be part of the bioretention area design. For this condition, the waterproofing system should be connected to the HPDE Tree Box with a waterproof seal. Given the nature of bioretention systems and possible proximity to improvements, we recommend that we be retained to review design plans and provide testing and observation services during the installation of linings, compaction of the filter material, and connection of designed drains. It should be noted that the contractor is responsible for conducting all excavation and shoring in a manner that does not cause damage to adjacent improvements during construction and future maintenance of the bioretention areas. As with any excavation adjacent to improvements, the contractor should reduce the exposure time such that the improvements are not detrimentally impacted. 5.0 FOUNDATION RECOMMENDATIONS 5.1 PIER AND GRADE BEAM The proposed residential structures are proposed to be constructed on split-level pads. We recommend that these structures be support by a pier-and-grade-beam foundation. Isolated piers should be avoided and individual piers should be connected with grade beams. Support on drilled friction piers can be designed using the following criteria. Minimum Pier Diameter: 16 inches Minimum Pier Depth: 15 feet (for piers located within 10 feet from an adjacent downslope, the pier depth should be increased by 3 feet for a 3:1 slope and 5 feet for a 2:1 slope to account for the sloping foreground.) Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 15 February 10, 2022 Maximum Allowable Friction Value: 500 psf (pounds per square foot). This value may be increased by one-third to allow for short-term seismic or wind loads. The upper 36 inches of soil should be ignored in the pier-load computation. Lateral Load: Uniform pressure of 20 psf over the upper 10 feet Pier Spacing: Minimum three pier diameters on center. Where closer spacing is unavoidable, the pier depth should be increased. We should make specific recommendations for increased pier depths in each specific case during our foundation plan review. Lateral passive resistance: 250 pcf acting over two pier diameters, The upper 5 feet of the pier should be neglected for passive pressure resistance. A Structural Engineer should design the pier reinforcement. The pier reinforcement should be tied to the grade beams as recommended by the Structural Engineer. Expansive soil may exert upward pressure on the base of grade beams. Therefore, the subgrade soil should be well over optimum moisture at the time of concrete placement. Under no circumstance should concrete be cast upon dry, desiccated soil. We recommend a 2-inch void under grade beams to reduce the uplift pressure on grade beams. Expansive soil may exert upward pressure on over-poured concrete collars at the tops of the piers or piers constructed in an excavation that has an inverted bell-shape in the upper portions of the pier. To avoid these conditions, the top of all piers should be constructed with the use of Sonotube®, or equal, concrete forms with the same nominal diameter as the pier excavation. Pier-drilling operations and concrete placement should be coordinated so that pier holes are left open a minimum amount of time. We should observe the pier-hole drilling operations and make appropriate recommendations in the field to accommodate required depth and embedment criteria. We should review structural plans and calculation prior to construction to check for conformance with these recommendations and to verify that subdrains will be avoided. 5.2 CRAWL SPACE MOISTURE CONTROL For the portions of the structures constructed with raised floors and underlying crawl space areas, there is an inherent risk of excessive ground moisture and water vapor leading to wood damage, mold, mildew, etc. To reduce the potential for ground moisture in crawl spaces, we recommend that measures be implemented to control moisture below and around the structures. It is important that the crawl space designer provides necessary measures to properly and liberally ventilate crawl space areas to reduce adverse effects of high water vapor conditions. The crawl space ground surface should be covered with either: (1) a concrete slab with a thickness of at least 2 to 3 inches, often called a rat slab, placed directly over a polyethylene membrane, or (2) a durable vapor retarder/liner conforming to Class A of ASTM E 1745. Vapor retarders shall be installed in accordance with manufacturer’s recommendations including sealing seams, pipe penetrations and attached to perimeter concrete stemwalls. As a minimum, we recommend that ventilation Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 16 February 10, 2022 openings be provided through foundation walls or exterior walls for the under-floor space, between the bottom of the floor joists and the earth under the building, in accordance with 2019 California Building Code. Additionally, locations of the ventilation openings shall be around all sides of foundation perimeters, and crawl-space airflow shall allow for adequate evacuation of excessive water vapor. 5.3 PAD AND FOUNDATION DRAINAGE Due to the split-pad configuration of the proposed lots, surface and shallow subsurface drainage will require close attention during the design and construction of pad improvements. Subsurface drains should be constructed beneath and around the building perimeter to limit subsurface seepage under foundations for buildings with a raised floor. The subsurface drain trench should be at least 12 inches wide and a minimum of 18 inches deep. All perimeter trenches and pipes should have a minimum slope of 1 percent and must be within 12 inches of the foundation. The closed roof downspout collector pipe and the perimeter subdrain can be constructed in a single trench if desired; however, the closed collector pipe must be placed above the perimeter subdrain pipe, and in no case may the subdrain pipe be connected to the closed drainpipe system. A schematic of a typical foundation drain system and details for drain construction are presented in Figure 10. Finished grades established following fine grading should provide positive surface gradients away from foundations to provide for rapid removal of surface water. Surface water flows on the finished graded lot should be directed to appropriate surface drainage collection facilities such as area drains or drainage swales. All surface drainage should be collected and conveyed to the stormwater drainage system in a manner that is approved by the project Civil Engineer. Ponding of stormwater must not be permitted on the building pads during prolonged periods of inclement weather. As a minimum requirement, finished grades should have slopes of at least 5 percent within 10 feet from the exterior walls, at right angles to them, to allow surface water to drain positively away from the structure. For paved areas, the slope gradient can be reduced to 2 percent. All roof stormwater should be collected and directed to downspouts. Stormwater from roof downspouts should be directed to a solid pipe that discharges to an approved outlet. 5.4 EXTERIOR SLAB-ON-GRADE Exterior flatwork includes items such as concrete sidewalks, steps, and outdoor courtyards exposed to foot traffic only. Provide a minimum section of 6 inches of concrete over 4 inches of aggregate base. Compact the aggregate base to at least 90 percent relative compaction (ASTM D1557) at a minimum 2 percentage points above optimum moisture content. Thicken flatwork edges to at least 10 inches to help control moisture variations in the subgrade and place reinforcement within the middle third of the slab to help control the width and offset of cracks. The Structural Engineer should design the exterior slab-on grade reinforcement, but as a minimum, we suggest any recommended slab reinforcement consist of steel bars in lieu of welded wire mesh. In our experience, welded wire mesh may not be sufficient to control slab cracking in an expansive soil environment. Minor cracking and distress should also be anticipated in slabs-on-grade as a result of concrete shrinkage and the highly expansive nature of the on-site soil. Frequent control joints should be provided to control cracking as recommended by the American Concrete Institute (Publication ACI 302.1R-89). Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 17 February 10, 2022 The subgrade material under exterior slabs-on-grade should be uniform. Where exterior slab-on-grade construction is planned, care must be exercised in attaining a near -saturation condition of the subgrade soil before concrete placement, and the subgrade should not be allowed to dry prior to concrete placement. Exterior slabs-on-grade should be constructed structurally independent of the foundation system. An expansion joint material should be provided between structural elements and slabs to allow for each element to move independently and with a reduced potential for distress to the adjacent element. Slabs-on-grade should slope away from the building to prevent water from flowing toward the foundations. 6.0 RETAINING WALL RECOMMENDATIONS 6.1 SHORING/CATCHMENT WALL As discussed in Section 3.3.2, the uppermost portions of existing landsides along the western boundary of the Vandam property extend across the property line. If it is not possible to obtain permission to access the adjacent property and perform earthwork repairs, it will be necessary to construct a retaining to support the existing landslide that will remain in place on the adjacent property. If groundwater will be able to drain through the retaining wall, such as for a soldier pile and lagging wall, then a lateral equivalent fluid pressure of 125 pcf should be used for wall design. If water won’t be able to freely drain through the wall, such as for a secant pile wall, then a lateral equivalent fluid pressure of 155 pcf should be used for design. Passive pressure of 250 pcf acting over two pier diameters can be used for design. The passive pressure should start below 3 feet for a 3:1 slope and 5 feet for a 2:1 slope. The shoring catchment wall should include a minimum of 5 feet of catchment height at the top of the wall and the piers should extend a minimum of 10 feet below the landslide base. 6.2 CONVENTIONAL RETAINING WALLS Retaining walls should be designed using the earth pressures in the table below. Retaining walls greater than 6 feet in height should be analyzed for seismic stability by adding the seismic increment to the active earth pressure. TABLE 6.2-1: Retaining Wall Design Earth Pressures BACKFILL ACTIVE PRESSURE (UNRESTRAINED) (PCF) SEISMIC INCREMENT (PCF) Level 50 pcf 30 3:1 70 pcf 40 2:1 90 pcf 50 Retaining walls should be supported on drilled piers designed in accordance with the recommendations in Section 5.1. Drainage should be provided behind the retaining walls as recommended below to prevent any build-up of hydrostatic pressures from surface water infiltration and/or a rise in the groundwater level. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 18 February 10, 2022 6.2.1 Retaining Wall Surcharge If a sidewalk or traffic is planned behind the wall, a minimum surcharge load of 50 psf or 100 psf, respectively (rectangular distribution), should be added to the top 10 feet of the wall. Design unrestrained walls to resist an additional uniform pressure equivalent to one-third of any surcharge loads applied at the surface. Design restrained walls to resist an additional uniform pressure equivalent to one-half of any surcharge loads applied at the surface. 6.3 MECHANICALLY STABILIZED EARTH (MSE) WALLS MSE walls may be considered for landscape area retaining walls that are 4 feet or less in exposed height and located at least 10 feet away from building foundations. Due to the highly expansive nature of the on-site soil, MSE walls backfilled with native material can be expected display some amount of lateral movement over time. We expect that this movement could be up to 2 inches. Alternatively, the anticipated lateral movement can be reduced if the MSE walls are backfilled with non-expansive import soil. The following general assumptions and design guidelines should be incorporated into MSE wall design. Keystone (Standard 21½-inch) blocks, or pre-approved equivalent, with positive connection (fiberglass pins) should be used. Site soil may be used as the foundation soil, retained soil, and reinforced fill soil. For level foreground, the base of the lowest block should be embedded at least 1 foot below lowest adjacent grade. For downsloping foreground, the walls should be embedded to achieve at least 8 feet of horizontal distance to the nearest free face from the base of the lowest block. For select, non-terraced front and side yard landscape walls with a total retained height less than 5 feet and without building, flatwork or other improvements within 5 feet behind the top of wall, a reduced embedment depth to achieve at least 5 horizontal feet from the front base edge of the lowest block to the nearest slope face may be feasible. Surcharge loads from vehicles should be included in the wall design if the surcharge loading is situated above a 1:1 line of projection extending up from the rear base edge of the bot tom block. For tiered wall systems, the lower retaining wall(s) should consider surcharge loads from walls and soil backfill located above. In addition, the upper wall should be embedded to at least 1 foot below the top of the lower wall and global stability analyses should be performed. We recommend that following soil criteria be used in the MSE wall design. TABLE 6.3-1: Soil Material Parameters SOIL MATERIAL COHESION (C’) (pcf) FRICTION ANGLE (’) (degrees) UNIT WEIGHT () (pcf) Reinforced Fill 0 25 125 Retained Soil 0 25 125 Foundation Fill 0 25 125 Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 19 February 10, 2022 We recommend that the following minimum factors of safety be used in the MSE wall design. TABLE 6.3-2: Stability Factors of Safety STABILITY CASE SAFETY FACTOR (STATIC/SEISMIC) External Sliding 1.5 / 1.2 Bearing Capacity 2.0 / 1.5 Overturning 2.0 / 1.5 Internal Pull-out Resistance 1.5 / 1.2 6.4 RETAINING WALL DRAINAGE AND BACKFILL All retaining walls should be provided with drainage facilities to prevent the build-up of hydrostatic pressures behind the walls. Wall drainage may be provided using a 4-inch-diameter perforated pipe embedded in Class 2 permeable material or free-draining gravel surrounded by synthetic filter fabric. The width of the drain blanket should be at least 12 inches. The drain blanket should extend to about one foot below the finished grades. The upper 1 foot of wall backfill should consist of on-site clayey soil. Collector perforated pipes should be directed to an outlet approved by the Civil Engineer. All backfill should be placed in accordance with recommendations for engineered fill provided in the referenced report. Light equipment should be used during backfill compaction to reduce possible overstressing of the walls. 7.0 PRELIMINARY PAVEMENT DESIGN Using estimated traffic indices for various pavement loading requirements and an assumed R-value of 5 for a clayey subgrade, we developed the following recommended pavement sections using Chapter 630 of the Caltrans Highway Design Manual (including the asphalt factor of safety), presented in the table below. The civil engineer should determine the appropriate traffic indices based on the estimated traffic loads and frequencies. TABLE 7.0-1: Preliminary Flexible Pavement Design TRAFFIC INDEX (TI) PAVEMENT SECTION AB (inches) AC (inches) 4.0 12 3 5.0 13 3½ 6.0 14 4 7.0 16 4½ Notes: AB is aggregate base Class 2 Material with minimum R = 78 AC is asphalt concrete These sections are for estimating purposes only; actual sections should be based on R-Value tests performed on samples of actual subgrade materials recovered at the time of grading. Pavement construction and all materials should comply with the requirements of the Standard Specifications of the State of California Department of Transportation, Civil Engineer, and appropriate public agency. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 20 February 10, 2022 The contractor should compact finish subgrade and aggregate base in accordance with Section 5.6. Aggregate Base should meet the requirements for ¾-inch maximum Class 2 AB in accordance with Section 26-1.02a of the latest Caltrans Standard Specifications. 7.1 CUT-OFF CURBS Due to the high plasticity soil at the project site, pavement cracking and distress can be expected, especially in pavements adjacent to unirrigated slopes. We recommend that a cutoff curb be constructed to limit the moisture variation in the pavement subgrade and reduce cracking and distress. 8.0 LIMITATIONS AND UNIFORMITY OF CONDITIONS This report presents geotechnical recommendations for design of the improvements discussed in Section 1.3 for the Kent and Vandam Properties project. If changes occur in the nature or design of the project, we should be allowed to review this report and provide additional recommendations. It is the responsibility of the owner to transmit the information and recommendations of this report to the appropriate organizations or people involved in design of the project, including but not limited to developers, owners, buyers, architects, engineers, and designers. The conclusions and recommendations contained in this report are solely prof essional opinions and are valid for a period of no more than 2 years from the date of report issuance. Whenever the words “supervision”, “inspection”, “approve”, “certify”, or “control” are used, they shall mean observation of the work and/or testing of the compacted fill by our field representative to assess whether substantial compliance with plans, specifications and design concepts has been achieved, and does not include direction of the actual work of the contractor or the contractor’s personnel. We strived to perform our professional services in accordance with generally accepted geotechnical engineering principles and practices currently employed in the area; no warranty is expressed or implied. There are risks of earth movement and property damages inherent in building on or with earth materials. We are unable to eliminate all risks; therefore, we are unable to guarantee or warrant the results of our services. This report is based upon field and other conditions known to us at the time of report preparation. We developed this report with limited subsurface exploration data. We assumed that the subsurface exploration data are representative of the actual subsurface conditions across the site. Considering possible underground variability of soil, rock, stockpiled material, and groundwater, additional changes may be required to complete the project. Such changes may result in additional costs. We recommend that the owner establish a contingency fund to cover such costs. If unexpected conditions are encountered, we must be notified immediately to review these conditions and provide additional and/or modified recommendations, as necessary. Our geotechnical exploration did not include work to determine the existence of possible hazardous materials. If any hazardous materials are encountered during construction, the proper regulatory officials must be notified immediately. This document must not be subject to unauthorized reuse that is, reusing without our written authorization. Such authorization is essential because it requires us to evaluate the document’s applicability given new circumstances, not the least of which is passage of time. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report Page | 21 February 10, 2022 Actual field or other conditions will necessitate clarifications, adjustments, modifications, or other changes to our recommendations and documents. Therefore, we must be engaged to prepare the necessary clarifications, adjustments, modifications, or other changes before construction activities commence or further activity proceeds. If our scope of services does not include on-site construction observation, or if other persons or entities are retained to provide such services, we cannot be held responsible for any or all claims arising from or resulting from the performance of such services by other persons or entities, and from any or all claims arising from or resulting from clarifications, adjustments, modifications, discrepancies, or other changes necessary to reflect changed field or other conditions. It should be noted that we relied on data compiled from multiple sources, including subsurface data collected by others. We are unable to make any representation as to the accuracy of those data. For exploration performed by us, we determined the lines designating the interface between layers on the exploration logs using visual observations. The transition between the materials may be abrupt or gradual. The exploration logs contain information concerning samples recovered, indications of the presence of various materials such as clay, sand, silt, rock, existing fill, etc., and observations of groundwater encountered. The field logs also contain our interpretation of the subsurface conditions between sample locations. Therefore, the logs contain both factual and interpretative information. Our recommendations are based on the contents of the final logs, which represent our interpretation of the field logs. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report February 10, 2022 REFERENCES American Concrete Institute, 2005, Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05). Bryant, W.A., and Hart, E.W., 2007, Fault-Rupture Hazard Zones in California – Alquist-Priolo Earthquake Fault Zoning Act with Index to Earthquake Fault Zones Maps: California Geological Survey Special Publication 42, 42 p. California Building Code (CBC), 2016. Crane, R. C., 1995, Preliminary Geologic Map of the Diablo Quadrangle, Contra Costa County, California, Draft available from H. & L. Hendry, Concord, California. Dibblee, Thomas W. Jr., 1980, Preliminary Geologic Map of the Diablo Quadrangle, Contra Costa County, California, United States Geological Survey. ENGEO, 1999, Preliminary Geotechnical Reconnaissance, Tassajara Lane, Subdivision 8389, Danville, California; November 24, 1999; Project No. 4674.5.001.01. ENGEO, 2000a, Preliminary Geotechnical Exploration, Tassajara Lane, Subdivision 8389, Danville, California; January 10, 2000; Project No. 4674.5.001.01. ENGEO, 2000b, Preliminary Geotechnical Exploration Proposed Tassajara Lane Improvements, Danville, California; March 21, 2000; Project No. 4674.5.002.01. ENGEO; Supplemental Geotechnical Exploration, Tassajara Lane, Subdivision 8389, Danville, California; February 20, 2002; Project No. 4674.1.003.02. ENGEO; Geotechnical Exploration, Gates and Kent Properties, Danville, California; July 12, 2005; Project No. 5393.1.100.01. ENGEO; Geotechnical Corrective Grading Plans, Gates Property, Danville, California; January 2007; Project No. 5393.1. 100.01. ENGEO; Final Report of Testing and Observation Services Provided during Rough Grading, Gates Property, Danville, California; July 5, 2007; Project No. 5393.1.100.02. Field, E.H., and 2014 Working Group on California Earthquake Probabilities, 2015, UCERF3: A new earthquake forecast for California’s complex fault system: U.S. Geological Survey 2015–3009, 6 p., https://dx.doi.org/10.3133/fs20153009. Google Earth; program accessed September 2019. Graymer, R.W., et al, 1994, Preliminary Geologic Map Emphasizing Bedrock Formations in Contra Costa County, USGS, Open File Report 94-822. Mikola, R.G, Candia, G., Sitar, N., 2014, Seismic Earth Pressures on Retaining Structures and Basement Walls, Tenth U.S. National Conference of Earthquake Engineering., Frontiers of Earthquake Engineering, July 2015, Anchorage Alaska. Mr. Othmar Vandam Kent and Vandam Properties 5393.000.000 Geotechnical Report REFERENCES (Continued) February 10, 2022 Nilsen, Tor H., 1975, Preliminary Photointerpretation Map of Landslide and other Surficial Deposits of the Diablo 7½ Quadrangle, Contra Costa County, California. Slide v8.024[Computer Software], 2018, RocScience. Spencer, E., 1967, A method of analysis of the stability of embankments assuming parallel inter- slice forces, Geotechnique, v. 17(1), pp. 11–26. Structural Engineers Association of California (SEAOC), 1996, Recommended Lateral Force Requirements and Tentative Commentary, (“Blue Book”), 6th Edition, Seismology Committee, Structural Engineers Association of California, Sacramento, California. Unruh, J. R., and T. L. Sawyer, 1997, Assessment of Blind Seismogenic Sources, Livermore Valley, Eastern San Francisco Bay Region: Final Technical Report, U.S. Geological Survey, National Earthquake Reduction Hazards Program. Wagner, J. R., 1978, Late Cenozoic History of the Coast Ranges East of San Francisco Bay, Ph.D. Dissertation, UCB. TECHNICAL REFERENCES Aerial Photographs: BUU-279-110,111 flown 7-25-1939 Cartwright Aerial Surveys, CC 9-200,201, flown 5-15-1965 Pacific Aerial Surveys, AV-253-23-29,30, flown 5-23-1957 Pacific Aerial Surveys, AV-1860-09-15,16, flown 5-15-1980 Pacific Aerial Surveys, AV-2131-09-15,16, flown 4-27-1982 FIGURES FIGURE 1 – Vicinity Map FIGURE 2 – Regional Geology Map – Graymer FIGURE 3 – Regional Geologic Map – Nilsen FIGURE 4 – Regional Faulting and Seismicity FIGURE 5 – Proposed Development FIGURE 6 – Geologic Map FIGURE 7 – Cross Sections FIGURE 8 – Corrective Grading Plan FIGURE 9 – Typical Keyway Detail FIGURE 10 – Typical Subdrain Detail FIGURE 11 – Typical Perimeter Subdrain and Underfloor Drain Pier and Grade Beam 0 FEET 80 PROPOSED DEVELOPMENT KENT AND VANDAM PROPERTIES DANVILLE, CALIFORNIA 5393.000.000 AS SHOWN 5 SITE 1 2 3 4 5 6 ' T g v t T g v t T g v t T g v t T g v t T g v t Q l s Q l s Q l s Q l s Q l s Q l s Q l s Q l s Q l s Q l s o Q l s o Q l s Q l s Q l s Q l s Q e f Q a f Q a f 1 ' 2 ' 3 ' 4 ' 5 ' 6 2 - T P - 1 2 2 1 - B - 5 1 - B - 4 2 - B - X 4 0 2 - B - 6 3 7 2 - B - 7 4 2 2 - B - 1 2 2 - B - 5 3 7 2 - B - 4 3 5 2 - B - 2 2 4 2 - B - 8 3 0 2 - B - 9 2 5 2 - B - 3 3 2 2 - T P - 1 6 2 2 - T P - 1 5 2 2 - T P - 1 7 1 2 - T P - 1 8 2 2 - T P - 2 2 > 1 4 2 - T P - 7 7 2 - T P - 8 4 2 - T P - 6 2 2 - T P - 2 9 2 - T P - 1 > 1 4 2 - T P - 9 7 2 - T P - 1 0 5 2 - T P - 1 1 3 2 - T P - 1 3 3 2 - T P - 1 4 6 2 - T P - 3 4 2 - T P - 1 9 > 1 4 2 - T P - 2 0 > 1 2 2 - T P - 2 1 > 1 3 2 - T P - 5 > 1 2 2 - T P - 4 2 H J B - 1 3 5 H J B - 2 1 H J B - 5 2 6 H J B - 4 7 H J B - 3 2 0 H J B - 4 H J B - 6 1 0 H J B - 7 3 H J B - 8 3 4 Q a f Q a f Q l s T g v t Q l s T g v t Q l s T g v t Q l s Q l s T g v t Q a f G E O L O G I C M A P K E N T A N D V A N D A M P R O P E R T I E S D A N V I L L E , C A L I F O R N I A 5 3 9 3 . 0 0 0 . 0 0 0 A S S H O W N 6 0 F E E T 5 0 E X P L A N A T I O N P R O P E R T Y B O U N D A R Y G E O L O G I C C O N T A C T B O R I N G W I T H D E P T H I N F E E T ( E N G E O , 2 0 1 9 ) B O R I N G W I T H D E P T H T O B E D R O C K ( H J A , 2 0 0 7 ) B O R I N G W I T H D E P T H I N F E E T ( E N G E O , 2 0 0 5 ) T E S T P I T ( E N G E O , 2 0 1 9 ) C R O S S S E C T I O N L O C A T I O N U N D O C U M E N T E D F I L L E N G I N E E R E D F I L L ( 2 0 0 6 ) L A N D S L I D E L A N D S L I D E , D E E P S E A T E D D O R M A N T T A S A J A R A / G R E E N V A L L E Y F O R M A T I O N L A N D S L I D E , E A R T H F L O W L A N D S L I D E D E E P - S E A T E D L A N D S L I D E , D E E P S E A T E D D O R M A N T S T R I K E A N D D I P O F B E D D I N G S T R I K E A N D D I P O F S H E A R P L A N E T g v t 2 - B - 7 2 H J B - 7 3 2 - B - 1 2 2 - T P - 8 4 Q e f Q a f Q l s Q l s Q l s o 6 6 ' Q l s Q l s o SUBDRAIN (TYPICAL) SUBDRAIN (TYPICAL) SUBDRAIN (TYPICAL) SUBDRAIN (TYPICAL) 480 520 560 600 640 1' 480 520 560 600 640 2' 480 520 560 600 640 5' 480 520 560 600 640 4' 480 520 560 600 6' 480 520 560 600 640 1 480 520 560 600 640 2 520 560 600 640 3 4 480 520 560 600 640 5 480 520 560 600 6 520 560 600 640 3' Qls Qls Qls Qls Qls Qls SUBDRAIN (EXISTING) Qef Qls Qls 2006 BUTTRESS KEY Qls Tgvt GEOLOGIC CONTACT (TYPICAL) GEOLOGIC CONTACT (TYPICAL) GEOLOGIC CONTACT (TYPICAL) GEOLOGIC CONTACT (TYPICAL) PROPOSED GROUND SURFACE (TYPICAL) PROPOSED GROUND SURFACE (TYPICAL) PROPOSED GROUND SURFACE (TYPICAL) PROPOSED GROUND SURFACE (TYPICAL)PROPOSED GROUND SURFACE (TYPICAL) RECOMMENDED CORRECTIVE GRADING (TYPICAL) RECOMMENDED CORRECTIVE GRADING (TYPICAL) RECOMMENDED CORRECTIVE GRADING (TYPICAL) RECOMMENDED CORRECTIVE GRADING (TYPICAL) RECOMMENDED CORRECTIVE GRADING (TYPICAL) RECOMMENDED CORRECTIVE GRADING (TYPICAL) EXISTING GROUND SURFACE EXISTING GROUND SURFACE EXISTING GROUND SURFACE EXISTING GROUND SURFACE EXISTING GROUND SURFACE EXISTING GROUND SURFACE Tgvt Tgvt Tgvt Tgvt Tgvt Tgvt Tgvt 2006 BUTTRESS KEY Qef 480 520 560 600 640 2-TP-11 2-TP-12 2-B-9 2-B-2 2-B-1 2-B-3 2-B-7 2-B-5 2-B-6 SUBDRAIN (EXISTING) Qaf Qaf 2-B-5 Qaf Qls Qlso Tgvt Qls 1"=40' SECTION 3-3' 1"=40' SECTION 1-1' 1"=40' SECTION 2-2' 1"=40' SECTION 4-4' 1"=40' SECTION 5-5' 1"=40' SECTION 6-6' PROPOSED SUBDRAIN (TYPICAL) SUBDRAIN (TYPICAL) Qls Qaf PROPOSED GROUND SURFACE (TYPICAL) Qaf Qaf CROSS SECTIONS KENT AND VANDAM PROPERTIES DANVILLE, CALIFORNIA 5393.000.000 AS SHOWN 7 Qef Qaf Tgvt UNDOCUMENTED FILL ENGINEERED FILL (2006) LANDSLIDE LANDSLIDE, DEEP SEATED DORMANT TASAJARA/GREEN VALLEY FORMATION BORING (ENGEO, 2019) EXPLANATION 0 0 40 40 2-B-7 Qls Qlso B E N C H @ 5 1 5 ' K E Y W A Y @ 5 0 5 ' BENCH @ 530' B E N C H @ 5 4 5 ' B E N C H @ 5 5 0 B E N C H @ 4 9 0 1 2 3 4 5 6 ' T g v t T g v t T g v t T g v t T g v t T g v t Q l s Q l s Q l s Q l s Q l s Q l s Q l s Q l s Q l s Q l s o Q l s o Q l s Q l s Q l s Q l s Q e f Q a f Q a f 1 ' 2 ' 3 ' 4 ' 5 ' 6 2 - T P - 1 2 2 1 - B - 5 1 - B - 4 2 - B - X 4 0 2 - B - 6 3 7 2 - B - 7 4 2 2 - B - 1 2 2 - B - 5 3 7 2 - B - 4 3 5 2 - B - 2 2 4 2 - B - 8 3 0 2 - B - 9 2 5 2 - B - 3 3 2 2 - T P - 1 6 2 2 - T P - 1 5 2 2 - T P - 1 7 1 2 - T P - 1 8 2 2 - T P - 2 2 > 1 4 2 - T P - 7 7 2 - T P - 8 4 2 - T P - 6 2 2 - T P - 2 9 2 - T P - 1 > 1 4 2 - T P - 9 7 2 - T P - 1 0 5 2 - T P - 1 1 3 2 - T P - 1 3 3 2 - T P - 1 4 6 2 - T P - 3 4 2 - T P - 1 9 > 1 4 2 - T P - 2 0 > 1 2 2 - T P - 2 1 > 1 3 2 - T P - 5 > 1 2 2 - T P - 4 2 H J B - 1 3 5 H J B - 2 1 H J B - 5 2 6 H J B - 4 7 H J B - 3 2 0 H J B - 4 H J B - 6 1 0 H J B - 7 3 H J B - 8 3 4 C O R R E C T I V E G R A D I N G P L A N K E N T A N D V A N D A M P R O P E R T I E S D A N V I L L E , C A L I F O R N I A 5 3 9 3 . 0 0 0 . 0 0 0 A S S H O W N 8 0 F E E T 5 0 E X P L A N A T I O N P R O P E R T Y B O U N D A R Y G E O L O G I C C O N T A C T B O R I N G W I T H D E P T H I N F E E T , ( E N G E O , 2 0 1 9 ) B O R I N G W I T H D E P T H I N F E E T , ( H J A , 2 0 0 7 ) B O R I N G W I T H D E P T H I N F E E T , ( E N G E O , 2 0 0 5 ) T E S T P I T , ( E N G E O , 2 0 1 9 ) C R O S S S E C T I O N L O C A T I O N U N D O C U M E N T E D F I L L E N G I N E E R E D F I L L ( 2 0 0 6 ) T A S A J A R A / G R E E N V A L L E Y F O R M A T I O N L A N D S L I D E , E A R T H F L O W L A N D S L I D E D E E P - S E A T E D L A N D S L I D E , D E E P S E A T E D D O R M A N T S T R I K E A N D D I P O F B E D D I N G S T R I K E A N D D I P , O F S H E A R P L A N E P R O P O S E D K E Y W A Y , S H O W I N G B A S E E L E V A T I O N L A N D S L I D E R E M O V A L B A S E O F L A N D S L I D E R E M O V A L W I T H E S T I M A T E D E L E V A T I O N L I M I T O F C O R R E C T I V E G R A D I N G S O L D I E R P I L E W A L L A L T E R N A T I V E 1 O F F S I T E L A N D S L I D E S A L T E R N A T I V E 2 S U B D R A I N T g v t 2 - B - 7 2 H J B - 7 3 2 - B - 1 2 2 - T P - 8 4 Q e f Q a f Q l s Q l s Q l s o 6 6 ' R E M O V E A N D R E P L A C E O F F S I T E L A N D S L I D E S A L T E R N A T I V E 2 S O L D I E R P I L E W A L L A L T E R N A T I V E 1 5 4 0 5 5 0 5 6 0 2 % S L O P E 2 4 ' 5 ' M I N I M U M 2 4 ' 1 8 " M I N I M U M O R I G I N A L G R O U N D S T R I P P I N G A S R E Q U I R E D S U B D R A I N (T Y P I C A L ) (S E E F I G U R E 1 0 ) E N G I N E E R E D F I L L P L A C E D I N A C C O R D A N C E W I T H P R O J E C T S P E C I F I C A T I O N SPROPOSED G R A D E D E P T H A T T O E T O B E D E T E R M I N E D I N T H E F I E L D B Y T H E G E O T E C H N I C A L E N G I N E E R B E N C H I N T O F I R M M A T E R I A L A S R E C O M M E N D E D B Y T H E G E O T E C H N I C A L E N G I N E E R D U R I N G G R A D I N G M I N I M U M M I N I M U M T Y P I C A L K E Y W A Y D E T A I L K E N T A N D V A N D A M P R O P E R T I E S D A N V I L L E , C A L I F O R N I A 5 3 9 3 .0 0 0 .0 0 0 N O S C A L E 9 3 . 1 % F A L L ( M I N I M U M ) O N A L L T R E N C H E S A N D D R A I N L I N E S 2 . A L L P E R F O R A T E D P I P E P L A C E D P E R F O R A T I O N S D O W N 1 . A L L P I P E J O I N T S S H A L L B E G L U E D N O T E S : K E Y W A Y S U B D R A I N - O P T I O N 2 FI E L D B Y T H E G E O T E C H N I C A L E N G I N E E R H EI G H T T O B E D E T E R M I N E D I N T H E 1 8 " M I N I M U M 6 " P E R F O R A T E D P I P E 2 " F I L T E R M E D I U M * KEYWAYSUBDRAIN-OPTION1 18"MINIMUM FILTERMEDIUM*18"MINI M U M *FILTERMEDIUM1"3/4"3/8"#4#8#30#50#200 18-33 25-40 40-100 90-100 1005-150-70-3 SIEVESIZE%PASSINGSIEVE ALTERNATIVEACLASS2PERMEABLEMATERIALALTERNATIVEBCLEANCRUSHEDROCKORGRAVELWRAPPEDINFILTER F A B R I C ALLFILTERFABRICSHALLMEETTHEFOLLOWINGMINIMUMAVERAG E ROLLVALUESUNLESSOTHERWISESPECIFIEDBYENGEO:180lbs6oz/yd70-100U.S.STD.SIEVE80gal/min/ft80lbsPUNCTURESTRENGTH(ASTMD-4833)FLOWRATE(ASTMD-4491)APPARENTOPENINGSIZE(ASTMD-4751)MASSPERUNITAREA(ASTMD-4751)GRABSTRENGTH(ASTMD-4632)MATERIALSHALLCONSISTOFCLEAN,COARSESANDANDGRAVELO R CRUSHEDSTONE,CONFORMINGTOTHEFOLLOWINGGRADINGREQU I R E M E N T S : 2 F I L T E R M E D I U M * C O M P A C T E D F I L L S W A L E S U B D R A I N 4 8 " M I N I M U M M I N I M U M 1 8 " 6 " P E R F O R A T E D P I P E 2 " 2 % M I N I M U M S L O P E B A S E O F K E Y W A Y 6"PERFORATED P I P E P E R SPECIFICATIONS . P L A C E D PERFORATIONS D O W N DRAINAGECOMPOSITEWITH6OZ.DRAINAGEFABRICONBOTHSIDES,SUCHASSKAPSTRANSNETTN220OREQUIVALENTMATERIALPRE-APPROVEDBYTHEGEOTECHNICALENGINEER CO M P A C T E D F I L L 2%MINIMUMSLOPEBASEOFKEYWAY C O M P A C T E D F I L L T Y P I C A L S U B D R A I N D E T A I L K E N T A N D V A N D A M P R O P E R T I E S D A N V I L L E , C A L I F O R N I A 5 3 9 3 . 0 0 0 . 0 0 0 N O S C A L E 1 0 UNDERFLOOR DRAIN 4" SOLID PIPE COMPACTEDSOIL BACKFILL 6" MINIMUM (VARIES) 4" SOLID AREADRAIN RISER COMBINED SUBDRAIN AND PERIMETER DRAIN TYPICAL FOUNDATION SUBDRAIN PLAN STRUCTURE TO APPROVED OUTLETS SOLID OUTLET PIPES *FILTER MEDIUM SIEVE SIZE % PASSING SIEVE 2 SOLIDUNDERFLOOR DRAIN PIPE LATERALS AS NEEDED TO DRAIN ALL UNDERFLOOR AREAS SOLID COLLECTOR PIPE PERFORATED SUBDRAIN PIPE MATERIAL SHALL CONSIST OF CLEAN, COARSE SAND AND GRAVEL OR CRUSHED STONE, CONFORMING TO THE FOLLOWING GRADING REQUIREMENTS: 1" 3/4" 3/8" #4 #8 #30 #50 #200 100 90-100 40-100 25-40 18-33 5-15 0-7 0-3 ALL FILTER FABRIC SHALL MEET THE FOLLOWING MINIMUM AVERAGE ROLL VALUES UNLESS OTHERWISE SPECIFIED BY ENGEO: GRAB STRENGTH (ASTM D-4632) MASS PER UNIT AREA (ASTM D-4751) APPARENT OPENING SIZE (ASTM D-4751) FLOW RATE (ASTM D-4491) PUNCTURE STRENGTH (ASTM D-4833) 180 lbs 6 oz/yd 70-100 U.S. STD. SIEVE 80 gal/min/ft 80 lbs ALTERNATIVE A ALTERNATIVE B CLASS 2 PERMEABLE MATERIAL CLEAN CRUSHED ROCK OR GRAVEL WRAPPED IN FILTER FABRIC 2 UNDERFLOOR CRAWLSPACE REFER TO SOILS REPORT FOR SURFACE PROTECTION REFER TO SOILS REPORT FOR SURFACE PROTECTION NOTES: 1. ALL PIPE JOINTS SHALL BE GLUED. 2. ALL PERFORATED PIPE PLACED PERFORATIONS DOWN. 3. 1% FALL (MINIMUM) ON ALL TRENCHES AND DRAIN LINES. 4. THE CLOSED COLLECTOR AND THE PERIMETER SUBDRAIN CAN BE CONSTRUCTED IN A SINGLE TRENCH, IF DESIRED. HOWEVER, THE CLOSED COLLECTOR PIPE MUST BE PLACED ABOVE THE SUBDRAIN PIPE, AND IN NO CASE SHOULD THE TWO SYSTEMS BE INTERCONNECTED. 5. V ENTILATION OPENINGS SHALL BE AROUND ALL SIDES OF FOUNDATION PERIMETERS, AND CRAWL-SPACE AIRFLOW SHALL ALLOW FOR ADEQUATE EVACUATION OF EXCESSIVE WATER VAPOR (VENTILATION SPECIALIST SHALL BE CONSULTED AS NECESSARY. COMPACTED NATIVE SOIL GRADE BEAM 4" PERFORATED PIPE 3%TO 5%WITHIN 5'OF HOUSE EXTERIOR WALL FILTER MEDIUM* PIER (WHERE APPLICABLE) VAPOR RETARDER SHEET (10 MIL MINIMUM) 4" SOLID COLLECTOR PIPE 6" MINIMUM (VARIES) 12" MINIMUM 6" MAXIMUM TYPICAL PERIMETER SUBDRAIN & UNDERFLOOR DRAIN PIER & GRADE BEAM KENT AND VANDAM PROPERTIES DANVILLE, CALIFORNIA 5393.000.000 NO SCALE 11 APPENDIX F SLOPE STABILITY ANALYSIS 1. 2 1. 2 W W 1. 2 1. 2 Ma t e r i a l Na m e Co l o r Un i t We i g h t (l b s / Ō 3) Co h e s i o n (p s f ) Ph i (d e g ) En g i n e e r e d Fi l l ‐ Un d r a i n e d 12 5 20 0 0 En g i n e e r e d Fi l l ‐ Dr a i n e d 12 5 20 0 23 La n d s l i d e De p o s i t s 12 5 0 12 Tv g t 13 0 10 0 0 25 7 5 0 7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 0 50 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 0 P r o j e c t N o . 53 9 3 . 0 0 0 . 0 0 0 M e t h o d Sp e n c e r A u t h o r SO S S c a l e 1: 7 0 0 D a t e 10 / 0 8 / 2 0 1 9 P r o j e c t Ke n t a n d V a n d a m P r o p e r t i e s C o n d i t i o n Pr o p o s e d G r a d e s A n a l y s i s St a t i c S e c t i o n Se c t i o n 2 . 1. 2 1. 2 W W 1. 2 1. 2 0 . 2 6 Ma t e r i a l Na m e Co l o r Un i t We i g h t (l b s / Ō 3) Co h e s i o n (p s f ) Ph i (d e g ) En g i n e e r e d Fi l l ‐ Un d r a i n e d 12 5 20 0 0 La n d s l i d e De p o s i t s 12 5 0 12 Tv g t 13 0 10 0 0 25 8 0 0 7 5 0 7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 0 50 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 0 P r o j e c t N o . 53 9 3 . 0 0 0 . 0 0 0 M e t h o d Sp e n c e r A u t h o r SO S S c a l e 1: 7 0 0 D a t e 10 / 0 8 / 2 0 1 9 P r o j e c t Ke n t a n d V a n d a m P r o p e r t i e s C o n d i t i o n Pr o p o s e d G r a d e s A n a l y s i s Ps e u d o - S t a t i c S e c t i o n Se c t i o n 2 . 2. 4 2. 4 W W 2. 4 2. 4 Ma t e r i a l Na m e Co l o r Un i t We i g h t (l b s / Ō 3) Co h e s i o n (p s f ) Ph i (d e g ) En g i n e e r e d Fi l l ‐ Un d r a i n e d 12 5 20 0 0 En g i n e e r e d Fi l l ‐ Dr a i n e d 12 5 20 0 23 La n d s l i d e De p o s i t s 12 5 0 12 Tv g t 13 0 10 0 0 25 7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 0 60 0 65 0 70 0 P r o j e c t N o . 53 9 3 . 0 0 0 . 0 0 0 M e t h o d Sp e n c e r A u t h o r SO S S c a l e 1: 7 0 0 D a t e 10 / 0 8 / 2 0 1 9 P r o j e c t Ke n t a n d V a n d a m P r o p e r t i e s C o n d i t i o n Pr o p o s e d G r a d e s A n a l y s i s St a t i c S e c t i o n Se c t i o n 4 . 1. 3 1. 3 W W 1. 3 1. 3 Ma t e r i a l Na m e Co l o r Un i t We i g h t (l b s / Ō 3) Co h e s i o n (p s f ) Ph i (d e g ) En g i n e e r e d Fi l l ‐ Un d r a i n e d 12 5 20 0 0 La n d s l i d e De p o s i t s 12 5 0 12 Tv g t 13 0 10 0 0 25 0 . 2 6 7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 0 60 0 65 0 P r o j e c t N o . 53 9 3 . 0 0 0 . 0 0 0 M e t h o d Sp e n c e r A u t h o r SO S S c a l e 1: 7 0 0 D a t e 10 / 0 8 / 2 0 1 9 P r o j e c t Ke n t a n d V a n d a m P r o p e r t i e s C o n d i t i o n Pr o p o s e d G r a d e s A n a l y s i s Ps e u d o - S t a t i c S e c t i o n Se c t i o n 4 . 2. 1 2. 1 W W 2. 1 2. 1 Ma t e r i a l Na m e Co l o r Un i t We i g h t (l b s / Ō 3) Co h e s i o n (p s f ) Ph i (d e g ) En g i n e e r e d Fi l l ‐ Un d r a i n e d 12 5 20 0 0 En g i n e e r e d Fi l l ‐ Dr a i n e d 12 5 20 0 23 La n d s l i d e De p o s i t s 12 5 0 12 Tv g t 13 0 10 0 0 25 7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 0 60 0 65 0 P r o j e c t N o . 53 9 3 . 0 0 0 . 0 0 0 M e t h o d Sp e n c e r A u t h o r SO S S c a l e 1: 7 0 0 D a t e 10 / 0 8 / 2 0 1 9 P r o j e c t Ke n t a n d V a n d a m P r o p e r t i e s C o n d i t i o n Pr o p o s e d G r a d e s A n a l y s i s St a t i c S e c t i o n Se c t i o n 5 . 1. 1 1. 1 W W 1. 1 1. 1 Ma t e r i a l Na m e Co l o r Un i t We i g h t (l b s / Ō 3) Co h e s i o n (p s f ) Ph i (d e g ) La n d s l i d e De p o s i t s 12 5 0 12 Tv g t 13 0 10 0 0 25 En g i n e e r e d Fi l l ‐ Dr a i n e d 12 5 20 0 23 0 . 2 6 7 5 0 7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 10 0 15 0 20 0 25 0 30 0 35 0 40 0 45 0 50 0 55 0 60 0 65 0 P r o j e c t N o . 53 9 3 . 0 0 0 . 0 0 0 M e t h o d Sp e n c e r A u t h o r SO S S c a l e 1: 7 0 0 D a t e 10 / 0 8 / 2 0 1 9 P r o j e c t Ke n t a n d V a n d a m P r o p e r t i e s C o n d i t i o n Pr o p o s e d G r a d e s A n a l y s i s Ps e u d o - S t a t i c S e c t i o n Se c t i o n 5 .