Item 23 - First One -Year Extension for Site Plan Review No. 2020-03 and Variance No. 2020-06

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4.2 Alluvium GEOTECHNICAL INVESTIGATION TABLE OF CONTENTS Holocene age alluvial deposits were encountered beneath the fill. As exposed in our borings, the upper 20 feet of alluvium consists of loose to medium -dense silty sand and poorly graded sand. These sediments are underlain by soft to firm silt and clay, in turn underlain by medium dense to dense gravely sand with cobbles. 5. GROUNDWATER The depth to first groundwater, as presented on Figure XVI.2d within the Orange County Technical Guidance Document (2013), is reported to be greater than 50 feet below the existing ground surface. The data depicted on this map is indicated as taken from Sprotte (1980) which indicates that the data is "biased towards the shallowest reported observations of groundwater". For the purpose of identifying a seasonally high groundwater level for infiltration design, we have reviewed publicly available data from https:Ugeotracker.waterboards.ca.gov/. The closest site with groundwater monitoring data is located approximately 1,000 feet to the southwest, near the intersection of Spurgeon and 1" Street. The site included 6 monitoring wells with groundwater measurements from 2002 to 2013, after which the wells were abandoned. The monitoring well readings indicate that groundwater has generally been at or below a depth of 55 feet from the ground surface during this time period. Groundwater was not encountered in our borings, drilled to a maximum depth of 55 VZ feet below the existing ground surface. Considering the depth of proposed construction, lack of groundwater in our borings, and the historic high groundwater level in the area (in excess of 40 feet deep), static groundwater is neither expected to be encountered during construction nor to have a detrimental effect on the project. p g � P J It is not uncommon for groundwater levels to vary seasonally or for perched groundwater conditions to develop where none previously existed, especially in impermeable fine-grained soils which are subjected to irri ation or precipitation. In addition recent requirements for stormwater infiltration could result in g p p � q shallower seepage conditions in the region. Proper surface drainage of irrigation and precipitation will be critical to future performance of the project. Recommendations for drainage are provided in the Surface Drainage section of this report (see Section 7.20). Geocon Project No. A9799-88-01 -3- June 7, 2022 6. GEOLOGIC HAZARDS 6.1 Surface Fault Rupture The numerous faults in Southern California. include active, potentially active, and inactive faults. The criteria for these major groups are based on criteria developed by the California Geological Survey (CGS, formerly known as CDMG) for the Alquist-Priolo Earthquake Fault Zone Program (CGS, 2018a). By definition, an active fault is one that has had surface displacement within Holocene time (about the last 11,700 years). A potentially active fault has demonstrated surface displacement during Quaternary time (approximately the last 1.6 million years) but has had no known Holocene movement. Faults that have not moved in the last 1.6 million years are considered inactive. The site is not within a state -designated Alquist-Priolo Earthquake Fault Zone (CGS, 2018b; CGS, 2014) for surface fault rupture hazards. No active or potentially active faults with the potential for surface fault rupture are known to pass directly beneath the site. Therefore, the potential for surface rupture due to faulting occurring beneath the site during the design life of the proposed development is considered low. However, the site is located in the seismically active Southern California region, and could be subjected to moderate to strong ground shaking in the event of an earthquake on one of the many active Southern California faults. The faults in the vicinity of the site are shown in Figure 3, Regional Fault Map. The closest trace of an active fault to the site is the Newport -Inglewood Fault Zone located approximately 9.6 miles to the southwest (Ziony and Jones, 1989). Other nearby active faults are the Whittier Fault, the Elsinore Fault, the offshore segment of the Palos Verdes Fault Zone, and the Chino Fault located approximately 9.8 miles northeast, 13 miles northeast, 18 miles southwest, and 18.5 miles northeast of the site, respectively (Ziony and Jones, 1.989). The active San Andreas Fault Zone is located approximately 42 miles northeast of the site. Several buried thrust faults, commonly referred to as blind thrusts, underlie the Los Angeles Basin (including the Orange County Coastal Plain) at depth. These faults are not exposed at the ground surface and are typically identified at depths greater than 3.0 kilometers. The October 1, 1987, Mw 5.9 Whittier Narrows earthquake and the January 17, 1994, Mw 6.7 Northridge earthquake were a result of movement on the Puente Hills Blind Thrust and the Northridge Thrust, respectively. These thrust faults and others in the greater Los Angeles area are not exposed at the surface and do not present a potential surface fault rupture hazard at the site; however, these deep thrust faults are considered active features capable of generating future earthquakes that could result in moderate to significant ground shaking at the site. Geocon Project No. A9799-88-01 -4- June 7, 2022 1. PURPOSE AND SCOPE This report presents the results of a geotechnical investigation of the proposed mixed -use development located on two blocks along East 4"' Street between French Street and North Minter Street in the City of Santa Ana, California (see Vicinity Map, Figure 1). The purpose of the investigation was to evaluate subsurface soil and geologic conditions underlying the site and, based on conditions encountered, to provide conclusions and recommendations pertaining to the geotechnical aspects of design and construction. The scope of this investigation included a site reconnaissance, field exploration, laboratory testing, engineering analysis, and the preparation of this report. The site was initially explored on May 29, 2018 by excavating eight 8-inch diameter borings to depths between 10 and 40'/z feet below the existing ground surface using a truck -mounted hollow -stem auger drilling machine. Additional site exploration was performed on March 20, 2022 by excavating four 8-inch diameter borings to depths between 45%2 and 55%Z feet below the ground surface for the purpose of percolation testing. The approximate locations of the exploratory borings are depicted on the Site Plan (see Figure 2A). A detailed discussion of the field investigation, including boring logs, is presented in Appendix A. Laboratory tests were performed on selected soil samples obtained during the investigation to determine pertinent physical and chemical soil properties. Appendix B presents a summary of the laboratory test results. The recommendations presented herein are based on analysis of the data obtained during the investigation and our experience with similar soil and geologic conditions. References reviewed to prepare this report are provided in the List of References section. If project details vary significantly from those described herein, Geocon should be contacted to determine the necessity for review and possible revision of this report. The subject site is located on two blocks on the north side of East 4`s Street between French Street and North Minter Street in the City of Santa Ana, California. Block A is occupied by a market building and associated parking lot; Block B is occupied by several single -story commercial structures and a vacant lot that was being used as a laydown yard for a nearby construction project at the time of our work. The overall site is bounded by East 5t' Street to the north, by East 4t' Street to the south, by North Minter Street to east, and by French Street to the west; the two parcels are separated by Mortimer Street. The site is relatively level with no pronounced highs or lows. Surface water drainage at the site appears to be by sheet flow along the existing ground contours to the city streets. Vegetation onsite consists of grasses, bushes and trees generally confined to planter areas. Geocon Project No. A9799-88-01 - 1 - June 7, 2022 It is our understanding that the proposed development on the west block will consist of a five- to seven - story mixed -use structure constructed around a four-story parking structure. The proposed development on the east block will consist of a five -story multi -family residential structure constructed around a three - to four-story parking structure. All structures will be constructed at or near the existing site grade. The proposed development is shown on the Site Plan and Cross Sections (see Figures 2A and 213). Based on information provided by the project structural engineer, DCI Engineers, anticipated loads (dead + live) for the proposed new footings are: • Parking Structures: between 203 kips and 405 kips; • 7-Story Mixed -Use Structure: up to 683 kips; • 5-Story Residential Structure: between 0.8 and 5.9 kips per linear foot. Once the design phase and foundation loading configuration proceeds to a more finalized plan, the recommendations within this report should be reviewed and revised, if necessary. Any changes in the design, location or elevation of any structure, as outlined in this report, should be reviewed by this office. Geocon should be contacted to determine the necessity for review and possible revision of this report. 3. GEOLOGIC SETTING The subject site is situated at the south-central portion of the Orange County Coastal Plain, a relatively flat -lying alluviated surface with an average slope of less than 20 feet per mile. The lowland surface is bounded by hills and mountains on the north and east and by the Pacific Ocean to the south and southwest (Department of Water Resources, 1967). Prominent structural features within the Orange County Coastal Plain include the central lowland plain, the northwest trending line of low hills and mesas underlain by the Newport -Inglewood fault zone along the coast (Newport Mesa, Huntington Beach Mesa, Bolsa Chica Mesa, and Landing Hill), and the San Joaquin Hills to the southeast (Department of Water Resources, 1967). 4. SOIL AND GEOLOGIC CONDITIONS Based on our field exploration and published geologic maps of the area, the site is underlain by young (Holocene age) alluvial fan deposits consisting predominantly of silty sand and sandy silt (USGS, 1999). Detailed stratigraphic profiles are provided on the boring logs in Appendix A. 4.1 Artificial Fill Artificial fill was encountered in our field explorations to a maximum depth of 5 feet below existing ground surface. The artificial fill generally consists of brown to dark brown silty sand. The artificial fill is characterized as dry to slightly moist and medium dense with trace brick fragments. The fill is likely the result of past grading or construction activities at the site. Deeper fill may exist between excavations and in other portions of the site that were not directly explored. Geocon Project No. A9799-88-01 - 2 - June 7, 2022 1. PURPOSE AND SCOPE.................................................................................................................1 2. SITE AND PROJECT DESCRIPTION........................................................................................... I 3. GEOLOGIC SETTING....................................................................................................................2 4. SOIL AND GEOLOGIC CONDITIONS.........................................................................................2 4.1 Artificial Fill..........................................................................................................................2 4.2 Alluvium................................................................................................................................ 3 5. GROUNDWATER ...........................................................................................................................3 6. GEOLOGIC HAZARDS..................................................................................................................4 6.1 Surface Fault Rupture............................................................................................................4 6.2 Seismicity ...............................................................................................................................5 6.3 Seismic Design Criteria.........................................................................................................5 6.4 Liquefaction Potential............................................................................................................7 6.5 Slope Stability ........................................................................................................................8 6.6 Earthquake -Induced Flooding................................................................................................8 6.7 Tsunamis, Seiches and Flooding............................................................................................8 6.8 Oil Fields & Methane Potential.............................................................................................9 6.9 Subsidence.............................................................................................................................9 7. CONCLUSIONS AND RECOMMENDATIONS......................................................................... to 7.1 General.................................................................................................................................10 7.2 Soil and Excavation Characteristics.....................................................................................12 7.3 Minimum Resistivity, pH, and Water -Soluble Sulfate........................................................12 7.4 Grading................................................................................................................................13 7.5 Shrinkage.............................................................................................................................15 7.6 Foundation Design...............................................................................................................16 7.7 Foundation Settlement.........................................................................................................18 7.8 Miscellaneous Foundations..................................................................................................19 7.9 Lateral Design......................................................................................................................20 7.10 Concrete Slabs-on-Grade.....................................................................................................20 7.11 Preliminary Pavement Recommendations...........................................................................22 7.12 Vehicular Rated Concrete Paver Recommendations...........................................................23 7.13 Retaining Wall Design.........................................................................................................25 7.14 Retaining Wall Drainage......................................................................................................26 7.15 Elevator Pit Design..............................................................................................................26 7.16 Elevator Piston.....................................................................................................................27 7.17 Temporary Excavations.......................................................................................................27 7.18 Slot Cutting ..........................................................................................................................28 7.19 Stormwater Infiltration.........................................................................................................31 7.20 Surface Drainage..................................................................................................................32 7.21 Plan Review.........................................................................................................................33 LIMITATIONS AND UNIFORMITY OF CONDITIONS LIST OF REFERENCES TABLE OF CONTENTS (Continued) MAPS, TABLES, AND ILLUSTRATIONS Figure 1, Vicinity Map Figure 2A, Site Plan Figure 213, Cross Sections Figure 3, Regional Fault Map Figure 4, Regional Seismicity Map Figures 5 and 6, Retaining Wall Drain Detail Figure 7 through 10, Percolation Test Results APPENDIX A FIELD INVESTIGATION Figures Al through Al2, Boring Logs APPENDIX B LABORATORY TESTING Figure B 1, Direct Shear Test Results Figures B2 through B7, Consolidation Test Results Figure B8, Laboratory Test Results Figure B9, Corrosivity Test Results 0- ROCC), W E S T, I N, GEOTECHNICAL. ENVIRONMENTA'Li. II MATERIALS I I p I 1 l11 �i II � ,l p I� I I� u: dIII I GEOTECHNICAL INVESTIGATION PROPOSED MIXED -USE DEVELOPMENT EAST 4TH STREET AND MORTIMER STREET SANTA ANA, CALIFORNIA APN: 398-325-01, 398-330-01 to 398-330-10 PREPARED FOR RED OAK INVESTMENTS, LLC IRVINE, CALIFORNIA PROJECT NO. A9799-88-01 J U N E 7, 2022 GEOCI'IO €`.1 %iC� 9 ■ E.1 18 V;M E' N T'A [ ■ M A,T E R I. A l S Project No. A9799-88-01 June 7, 2022 Mr. Andrew Nelson Red Oak Investments, LLC 4199 Campus Drive, #200 Irvine, California 92612 Subject: GEOTECHNICAL INVESTIGATION PROPOSED MIXED -USE DEVELOPMENT EAST O STREET AND MORTIMER STREET SANTA ANA, CALIFORNIA APN: 398-325-01, 398-330-01 to 398-330-10 Dear Mr. Nelson: In accordance with your authorization of our proposals dated April 3, 2018 and March 7, 2022, we have prepared this geotechnical investigation for the proposed mixed -use development located on two blocks along East 4th Street between French Street and North Minter Street in the City of Santa Ana, California. The accompanying report presents the findings of our study, and our conclusions and recommendations pertaining to the geotechnical aspects of proposed design and construction. Based on the results of our investigation, it is our opinion that the site can be developed as proposed, provided the recommendations of this report are followed and implemented during design and construction. If you have any questions regarding this report, or if we may be of further service, please contact the undersigned. Very truly yours, GEocoN WEST, INC. Petrina Zen PE 87489 (EMAIL) Addressee Jelisa Thomas Adams Gerald A. Kasman GE 3092 CEG 2251 2807 McGaw Avenue ■ Irvine, CA 92618 ■ Telephone (949) 491-6570 ■ oc@geoconinc.com Architecture + Planning 17911 Von Karman Ave. Suite 200 Irvine, CA 92614 ktgy.com 949.851.2133 KTGY Project No: 2017-0934 Project Contact: Debbie Holland Email: dholland@ktgy.com Principal: Wil Wong Project Designer: D. Schoolmeester Project Director: Debbie Holland Developer ED 0 A K INVES'I'NiEN I'S 4199 CAMPUS DRIVE IRVINE, CA 92612 PHONE NO. (714) 342-2502 U Q Z Q Q Z Q w U) W O 06 LO No. Date Description CH G m U) U Lu U Z Q J It is the clients responsibility prior to or during construction to notify the architect in writing of any perceived errors or omissions in the plans and specifications of which a contractor thoroughly knowledgeable with the building codes and methods of construction should reasonably be aware. Written instructions addressing such perceived errors or omissions shall be received from the architect prior to the client or clients subcontractors proceeding with the work. The client will be responsible for any defects in construction if these go r% v J procedures are not followed. ARCIIl LL G��SED CV No. C29795 * 0 RENEWAL DATE AUGUST 31, 2023 1 r 9jF of O C A�� License Stamp o 7 .m GEOTECHNICAL REPORT r L COPYRIGHT AO-80