IV. ENVIRONMENTAL IMPACT ANALYSIS C. GEOLOGY/SOILS

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1 IV. ENVIRONMENTAL IMPACT ANALYSIS C. GEOLOGY/SOILS The following section is a summary of the preliminary geotechnical consultation conducted for the Proposed Project. The Report of Geotechnical Engineering Services, Proposed Wilshire Center, 650 to 680 South Vermont Avenue, 3150 to 3198 Wilshire Boulevard and 665 South Shatto Place, Los Angeles, California (the Geotechnical Report ) was prepared by GeoDesign, Inc., dated February 14, A copy of the Geotechnical Report can be found as Appendix D to this Draft EIR. ENVIRONMENTAL SETTING The Project Site is located at the southeast corner of Wilshire Boulevard and Vermont Avenue in the Wilshire Community Plan Area (CPA) of the City of Los Angeles. The Project Site is generally bounded by Wilshire Boulevard to the north, Vermont Avenue to the west, two multi-family residential buildings at 661 and 673 Shatto Place and surface parking uses to the south (on the same block), and Shatto Place to the east (see Figure II-1, Regional and Vicinity Map). Geologic Conditions and Topography The Project Site is located on a dissected apron of the La Brea Plain, formed along the south flank of the Elysian Hills. The La Brea Plain was formed by deposition of alluvial outwash material from the Santa Monica Mountains. Regionally, the site is located within the Peninsular Ranges Geomorphic Province, characterized by northwest-trending geologic structures such as the nearby Newport-Inglewood Fault Zone (located approximately 5.3 miles to the southwest). The San Andreas Fault bounds this geomorphic province on the east and the nearby Santa Monica and Hollywood Fault Zones bound the province on the north. Based on the Hollywood 7.5 Minute Quadrangle, the ground surface at the site ranges in elevation from approximately 255 to 243 feet above mean sea level (north to south). 1 (See Figure IV.C-1, USGS 7.5 Minute Quadrangle, Hollywood, CA). Soil Conditions and Groundwater The Project Site is located on the La Brea Plain, along the south flank of the Elysian Hills. Regionally, the Project Site is located within the Peninsular Ranges Geomorphic Province, which is bounded by the Newport-Inglewood Fault Zone to the southwest, the San Andreas Fault on the east, and the Santa Monica and Hollywood Fault Zones to the north. Ground surface at the Project Site slopes to the south 1 GeoDesign Inc., Preliminary Geotechnical Consultation, Proposed High-Density Development, South Side of Wilshire Boulevard Between Vermont Avenue and Shatto Place, April 29, Page IV.C-1

2 Figure IV.C-1, USGS 7.5 Minute Quadrangle, Hollywood CA Page IV.C-2

3 and southeast and ranges from elevations 255 to 243 feet above mean sea level (msl). 2 Subsurface conditions were evaluated by GeoDesign in January 2006 by drilling six borings on the Project Site to depths of 10.5 to 66 feet below ground surface (bgs). Figures IV.C-2 and IV.C-3 on pages IV.C-4 and IV.C-5 show the locations of borings B-1 through B-6. Artificial fill consisting of clay, silty sand, and sand with some concrete debris mantles the site. The fill materials ranged from approximately 0.5 feet to 10.5 feet thick, but were typically less than 4 feet thick. The artificial fill is underlain by older alluvial fan deposits (Dibblee, 1991). The upper 15 feet of alluvial deposits consist of predominately medium stiff to very stiff, clayey silt, silty clay, silt; and sandy clay; and dense, silty sand/sandy silt. The upper deposits are underlain by an alluvial sequence of dense, poorly graded sand and silty sand. Sedimentary bedrock of the Miocene age Puente Formation underlies the older alluvial deposits. The bedrock consists predominantly of massive to well-bedded, clayey siltstone and claystone with minor fine-grained sandstone interbeds. The bedrock is moderately to highly fractured and is locally wellcemented and was encountered at depths between approximately 19.5 and 30.3 feet bgs, at elevations between approximately 222 and 233 feet above msl. Naturally occurring asphalt, also referred to as tar, was present in fractures and joints in the bedrock materials. In addition, the soil and bedrock materials generally emitted a minor petrochemical odor. Ground water within the region and beneath the Project Site is anticipated to be shallow. According to maps published by the California Geological Survey (CGS), formerly known as the California Division of Mines and Geology (CDMG) (1998) the historic high groundwater level in the site vicinity is at a depth of approximately 16 to 20 feet beneath the existing ground surface. These levels were further confirmed by GeoDesign s site-specific investigation in January 2006, which encountered perched groundwater at depths of 16.5 to 33 feet bgs, at elevations between approximately to feet above msl. (See also Section IV.E, Hydrology and Water Quality). Based on a review of County of Los Angeles Division of Hydrologic Records, it was determined that there are no active groundwater monitoring wells near the Project Site. 3 The closest well to the Project Site is located approximately 1.9 miles to the east, near the intersection of Pico Boulevard and Figueroa Street. This well was last monitored on April 18, 1984 when the depth to water was feet beneath the existing ground surface. This depth corresponds to a water surface elevation of 95.4 feet above msl. 2 3 Geo Design, Report of Geotechnical Engineering Services, Proposed Wilshire Center, 650 to 680 S. Vermont Avenue, 3150 to 3189 Wilshire Boulevard, and 665 S. Shatto Place, Los Angeles, California, February 14, County of Los Angeles, Division of Hydrologic Records, Page IV.C-3

4 Figure IV.C-2, Locations of Geologic/Hydrologic Borings Page IV.C-4

5 Figure IV.C-3, Cross-Section of Geologic/Hydrologic Borings Page IV.C-5

6 Liquefaction Liquefaction is a process by which water-saturated unconsolidated sediments lose their strength due to increased pore pressure during or after an earthquake. Liquefaction potential is greatest where the groundwater level is shallow, and loose, fine sands occur within a depth of about 50 feet. Liquefaction potential decreases as grain size and clay and gravel content increase. Liquefaction potential increases as ground acceleration and shaking increases during earthquakes. According to the County of Los Angeles Seismic Safety Element (1990), the City of Los Angeles Safety Element (1996), and the California Geological Survey (CDMG, 1999), the Project Site is not located within a liquefaction hazard area. Based on materials encountered during GeoDesign s borings, the soils beneath the Project Site are primarily dense Pleistocene age alluvial deposits and Tertiary sedimentary bedrock and are not considered subject to liquefaction. Seismic Conditions The entire Southern California area is considered to be a seismically active region. The region has numerous active, potentially active, and inactive faults based on criteria developed by CGS. An active fault is defined as a fault that has had a surface displacement within Holocene times (about the last 11,000 years). A potentially active fault is a fault that has demonstrated surface displacement of Quaternary age deposits (within the last 1.6 million years). As depicted in Figure IV.C-4, the nearest fault to the Project Site is the Hollywood Fault, which is located 2.6 miles northwest of the Project Site. Although an Alquist-Priolo Zone has not yet been established for the fault, it is considered to be active by the City of Los Angeles and the State Geologist for the purposes of planning and development. The closest established Alquist-Priolo Zone to the Project Site is the Newport-Inglewood Zone, located approximately 5.2 miles southwest of the Project Site. Oil Wells and Methane Gas The Project Site is located approximately 0.25 miles south of the boundary of the City of Los Angeles Oil Field. As a result of the close proximity of the Project Site to the oil field, there is a potential for undocumented and/or abandoned oil wells to be encountered at the site during construction. Wells encountered during construction would be required to be abandoned according to the current requirements of the California Division of Oil and Gas. The Project Site is located within a methane zone as designated by the City of Los Angeles Department of Building and Safety Ordinance No , which was adopted on March 29, 2004 and establishes citywide methane mitigation requirements. Based on the findings of GeoDesign s subsurface explorations, the Project Site is classified as Design Level 1. Impacts associated with the accidental release of methane gas are addressed in further detail in Section IV.D, Hazardous Materials and Risk of Upset. Page IV.C-6

7 Figure IV.C-4, Major Regional Faults Page IV.C-7

8 ENVIRONMENTAL IMPACTS Thresholds of Significance In accordance with Appendix G to the State CEQA Guidelines, a significant impact to geology and soils may occur if the Proposed Project would result in any of the following conditions: a) Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: i) Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map or based on other substantial evidence of a known fault? ii) Strong seismic ground shaking? iii) Seismic-related ground failure, including liquefaction? iv) Landslides? b) Result in substantial soil erosion or the loss of topsoil? c) Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction, or collapse? d) Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property? e) Have soils incapable of adequately supporting the use of septic tanks or alternative waste water disposal systems where sewers are not available for the disposal of waste water? As discussed in the Initial Study (see Appendix A), the Proposed Project would have no impact with respect to Threshold (e) listed above. As such, no further analysis of this topic is required (see also Section V.C. Impacts Found to be Less Than Significant, of this Draft EIR). In addition, based on the City of Los Angeles L.A. CEQA Thresholds Guide, a project would result in a significant geotechnical impact if it exceeds the following threshold: A project would normally have a significant geologic hazard impact if it would cause or accelerate geologic hazards which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. Page IV.C-8

9 Project Impacts The Proposed Project would involve the construction of a one-to two-level below-grade parking structure and a five-level podium deck to support two high-rise residential towers. Construction of the Proposed Project would involve the excavation of approximately 49,675 cubic yards (cy) of soil assuming an excavation footprint of approximately 90,000 sf and a maximum excavation depth to approximately 30 feet bgs. Based on a review of available information, results of on-site explorations, and laboratory testing and analyses, the Geotechnical Report concluded the Proposed Project is feasible from a geotechnical perspective. The following analysis addresses the geotechnical impacts of the Proposed Project from a CEQA perspective in relation to the thresholds identified above. Seismic Hazards Fault Rupture The Proposed Project would result in a less-than-significant impact with respect to exposing people or structures to the risk of loss, injury, or death involving the rupture of a known earthquake fault. The Project Site is located in the seismically active region of Southern California. Numerous active and potentially active faults with surface expressions (fault traces) have been mapped adjacent to, within, and beneath the City of Los Angeles. However, there no active surface fault traces identified by the State as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map are known to be present beneath the Project Site. The nearest fault is the Hollywood Fault, which is located 2.6 miles northwest of the Project Site. Although an Alquist-Priolo Zone has not yet been established for the fault, it is considered to be active by the City of Los Angeles and the State Geologist for the purposes of planning and development. 4 The closest established Alquist-Priolo Zone is for the Newport-Inglewood Zone, located approximately 5.2 miles southwest of the Project Site. Therefore, the possibility of surface fault rupture affecting the Project Site would be considered remote. Therefore, development of the Proposed Project would not expose people or property to hazardous conditions resulting from rupture of a known earthquake fault on the Project Site and impacts would be less than significant. Seismic-Induced Ground Shaking The Project Site is located in a seismically active region and could be subjected to strong ground shaking in the event of an earthquake. In this respect, development of the Proposed Project would expose new residents, employees and visitors to the proposed dwelling units and commercial establishments to potentially significant adverse effects, including the risk of loss, injury, or death involving strong seismic 4 City of Los Angeles, Safety Element of the Los Angeles City General Plan, Exhibit A, Alquist-Priolo Special Study Zones & Fault Rupture Study Areas, March Active faults are classified by the State Division of Mines and Geology as faults showing evidence of surface displacement within the last 11,000 years. Page IV.C-9

10 ground shaking. However, such hazards are inherent to the region and the effects of ground shaking can be mitigated to a less-than-significant level by incorporating proper design and construction methods in conformance with current building codes and engineering practices. Modern, well-constructed buildings are designed to resist ground shaking through the use of shear walls and reinforcements. The proposed construction would be consistent with all applicable provisions of the City of Los Angeles Building Code, as well as the seismic design criteria contained within the Uniform Building Code. Although the Project Site is located within 2.6 miles of the active Hollywood Fault, and close to many other faults on a larger regional level, the potential for seismic hazards not be higher than in other areas of the City of Los Angeles or elsewhere in the region. Such risks have also been incorporated into the project specific seismic design and engineering plans for the Proposed Project and impacts would be less than significant. Seismic-Induced Settlement and Liquefaction The Proposed Project would not expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving seismic-induced ground failure associated with settlement and/or liquefaction. Based on the information presented previously in this Section, soils on the Project Site would not be susceptible to liquefaction. In addition, the Project Site is not within an area of known subsidence associated with fluid withdrawal (groundwater or petroleum), peat oxidation or hydrocompaction. Therefore, impacts associated with liquefaction and seismic-induced settlement would be less than significant. Landslides The Proposed Project would not expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving landslides. As discussed above, the Project Site ranges from elevation 255 to 243 feet above msl (from north to south). The Project Site is not located within a City-designated landslide area or an area identified as subject to seismic slope instability. Due to the relatively flat topography of the Project Site and surrounding area, potential impacts associated with landslides would be less than significant. Erosion and Topsoil The Proposed Project would not result in substantial soil erosion or the loss of topsoil. Although construction of the Proposed Project has the potential to result in the erosion of soil during site preparation and construction activities, erosion would be reduced by implementation of appropriate erosion controls during grading. Minor amounts of erosion and siltation could occur during project grading, which would be minimized through adherence to construction Best Management Practices (BMPs) identified in mitigation measures E-3 through E-8 in Section IV.E (Hydrology and Water Quality). The potential for soil erosion during the ongoing operation of the Proposed Project is relatively low due to the generally level topography of the area to be developed within the Project Site. Operational Page IV.C-10

11 erosion would be reduced through adherence to mitigation measures E-9 through E-22 in Section IV.E. All grading activities require grading permits from the Department of Building and Safety, which include requirements and standards designed to limit potential impacts to acceptable levels. In addition, all onsite grading and site preparation would comply with applicable provisions of Chapter IX, Division 70 of the Los Angeles Municipal Code which addresses grading, excavations, and fills. With implementation of the applicable grading and building permit requirements and the application of BMPs identified in Section IV.E, a less-than-significant impact would occur with respect to erosion or loss of topsoil. Expansive Soils The upper clayey soils on the Project Site are expansive and should not be used within two feet of the bottom of pavement or other flatwork. Nonetheless, with adherence to the geotechnical engineering recommendations provided in the Geotechnical Report and the mitigation measures identified in this Section, impacts with respect to expansive soils would be less than significant. Geologic Hazards A potentially significant adverse impact could occur with respect to causing or accelerating geologic hazards associated with the accidental discovery of undocumented and/or abandoned oil wells which could result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. Potentially adverse impacts associated with this hazard could be reduced to a less-than-significant level by abandoning accidentally encountered wells according to the current requirements of the California Division of Oil and Gas. A potentially significant adverse impact could occur with respect to causing or accelerating the accidental release or buildup of methane, which could result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. Potentially adverse impacts associated with this hazard can be reduced to a less-than-significant level through compliance with the City of Los Angeles Department of Building and Safety Ordinance No The City of Los Angeles Methane Ordinance requires specific field sampling and testing be performed to evaluate the methane gas concentrations on the Project Site and a suitable mitigation system be designed and completed in accordance with the City s requirements. Based on the findings of GeoDesign s subsurface explorations, the Project Site is classified as Design Level 1, which requires a passive sub-slab venting system and the installation of a soil-gas barrier. The mitigation plan will include an 80-mil high-density polyethylene (HDPE) barrier or Liquid Boot impermeable membrane between the floor slab and associated vent pipes beneath the barrier and risers to discharge gases. As compliance with the City of Los Angeles Methane Ordinance is mandatory, potential impacts associated with the accidental release of methane gas would be mitigated to a less-thansignificant level. Impacts associated with the accidental release of methane gas are addressed in further detail in Section IV.D, Hazardous Materials and Risk of Upset. Page IV.C-11

12 Groundwater Groundwater beneath the Project Site has the potential to impact the proposed development during construction and also during the life of the structure. During construction, it will be necessary to provide temporary groundwater control provisions in order to allow for the proposed excavation. Based on the borings conducted by GeoDesign in January 2006, groundwater is currently encountered at between and feet above msl in the location of the Wilshire Tower. The lowest finish floor level of the Wilshire Tower will be established at 234 feet above msl. Groundwater is currently encountered at between 226 and 229 feet above msl in the location of the Vermont Tower. The lowest finish floor level of the Vermont Tower will be established at 224 feet above msl. Temporary excavations required for foundations will extend several feet below the finish floor level for spread footings and likely five or more feet below the finish floor level for core mat foundations. Therefore the finished floor level for the Vermont Tower will encounter groundwater and the finished floor level for the Wilshire Tower may encounter groundwater. The temporary excavations for both the Wilshire Tower and the Vermont Tower will encounter groundwater. Groundwater control can be accomplished in several ways. One method which is typically practical for the type of conditions encountered at this site would include the installation of perimeter well points that are connected to collector pipes, which convey water to a suitable holding area. Typically a baker tank is used for this purpose. Initially it is anticipated that the groundwater flow rate and corresponding quantities would be relatively high, but would sharply decline as the perched water dissipates. The perched water will be recharged; however, once initial control is established, relatively low flow rates and quantities are anticipated for the duration of the construction process. Disposal of the collected groundwater is discussed in greater detail in Section IV.E (Hydrology and Water Quality). The groundwater level will also impact the design of the structures, as the building floor slab for the Vermont Tower must be designed to resist hydrostatic pressure on the order of four to five feet, which will result in additional concrete and steel, when compared to slabs not subject to hydrostatic pressure. At this time it does not appear that the slab for the Wilshire Tower will require design provisions to resist hydrostatic pressure. Similarly, the below grade walls in the lower four to five feet of the Vermont Tower will require permanent drainage (i.e., active dewatering) or must be designed to resist the hydrostatic pressure that would develop without the drainage. Due to the uncertainties in groundwater discharge rules and recommendations, many developments opt to strengthen the below grade walls over relying on a permanent dewatering system to mitigate this condition. In either scenario, compliance with the geotechnical recommendations provided by the project geotechnical engineer would effectively mitigate any adverse impacts associated with groundwater to-less than-significant levels. Page IV.C-12

13 CUMULATIVE IMPACTS Geotechnical impacts related to future development in the City of Los Angeles would involve hazards related to site-specific soil conditions, erosion, and ground-shaking during earthquakes. These impacts would be site-specific and would not be common to (nor shared with, in an additive sense) the impacts on other sites. Cumulative development in the area would increase the overall population for exposure to seismic hazards by increasing the number of people potentially exposed. However, with adherence to applicable State and Federal regulations, buildings codes and sound engineering practices, geologic hazards could be reduced to less-than-significant levels. Furthermore, development of each of the related projects and the Proposed Project would be subject to uniform site development and construction review standards that are designed to protect public safety. Therefore, cumulative geotechnical impacts would be less than significant. With regards to any geotechnical implications of excavating and developing the Project Site in proximity to the existing MTA Wilshire Vermont Red Line Station, no adverse impacts are anticipated. The Project Site is located at the southeast corner of Wilshire Boulevard and Vermont Avenue. The existing MTA Wilshire Vermont Station is located at a distance greater than 90 feet to the north (on the north side of Wilshire Boulevard) and the associated MTA tunnel is located approximately 330 feet to the north of the proposed development (the tunnel bottom is at an elevation of approximately 202 feet above msl). The potential for any adverse impacts associated with soil stability at the MTA station or tunnel is very low, since these structures are located a sufficient distance from the Project Site. MITIGATION MEASURES Code-Required Measures (C-1) The Project shall be designed and constructed in accordance with the requirements outlined in the latest edition of the City of Los Angeles Uniform Building Code, including all applicable provisions of Chapter IX, Division 70 of the LAMC, which addresses grading, excavations and fills. (C-2) The Project shall comply with the administrative procedures set forth in Ordinance No. 175,790 of the Los Angeles Municipal Code, which establishes citywide methane mitigation requirements and includes more current construction standards to control methane intrusion into buildings. (C-3) In the unlikely event any undocumented oil wells are encountered during the construction of the Proposed Project, the Project Applicant shall prove to the satisfaction of the Director of Building and Safety that all oil wells found within the subject property have been closed and abandoned in accordance with the most current abandonment standards required by the California Division of Oil and Gas. Page IV.C-13

14 Project-Specific Mitigation Measure (C-4) The Project shall be designed and constructed in accordance with the recommendations provided in the Report of Geotechnical Engineering Services, Proposed Wilshire Center, 650 to 680 South Vermont Avenue, 3150 to 3198 Wilshire Boulevard and 665 South Shatto Place, Los Angeles, California (the Geotechnical Report ) prepared by GeoDesign, Inc., dated February 14, (C-5) The Project Applicant shall ensure geotechnical testing and observation be conducted on-site by a state certified geotechnical engineer during any excavation and earthwork activities to ensure that recommendations provided in the Project Geotechnical Report are implemented where applicable. LEVEL OF SIGNIFICANCE AFTER MITIGATION With implementation of the mitigation measures recommended above, the Proposed Project s potential adverse impacts associated with geology and soils would be reduced to less-than-significant levels. Page IV.C-14