IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY AND SOILS

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1 IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY AND SOILS The following analysis is based on the Geotechnical Investigation Report, Proposed Mid-Rise Multi- Family Residential Development Project Wetherly S. Wetherly Drive and S. Almont Drive, Beverly Hills Area of the City of Los Angeles, California, prepared by Petra Geotechnical, Inc., December 22, The Geotechnical Report is included as Appendix F of this Draft EIR. ENVIRONMENTAL SETTING The project site is located in the Wilshire community of the City of Los Angeles and is bound by S. Wetherly Drive to the west, W. 3 rd Street to the north, S. Almont Drive to the east, and an alley to the south (see Figures II-1 and II-2 in Section II. Project Description). The project site consists of a rectangular parcel and totals approximately acres. The topography of the project site is generally flat. Based on information shown on the published United States Geological Survey (USGS) topographic map for the area, the average elevation within the project site is approximately 194 feet above the mean sea level (msl) with area drainage generally directed to the south. Geologic Setting The project site is located within the northern portion of the La Brea Plain, which is situated near the boundary between the Peninsular Ranges geomorphic province on the south and Transverse Ranges geomorphic province on the north. The Santa Monica Mountains, located roughly 1.5 miles north of the project site, are part of the Transverse Ranges province. The Baldwin Hills, located several miles south of the project site, are part of the Peninsular Ranges geomorphic province. Northwest-southeast geologic structural blocks are separated by faults of a similar orientation and characterize the Peninsular Ranges. East west trending geologic structures, including faults and folds, characterize the Transverse Ranges. Geologic Materials The La Brea Plain is underlain by Quaternary alluvial deposits that were derived from the Santa Monica Mountains. North of the project area, the eastern Santa Monica Mountains expose bedrock that generally consists of granitic rocks, and Cretaceous and Tertiary sedimentary rocks. The project site is underlain by Quaternary alluvium derived from the Santa Monica Mountains. The alluvial soils of the project site consist generally of silty sand, clayey sand, silt, sand and sandy clay. Clayey soils are more prevalent at the southern end of the project site. The total depth of the alluvium exceeds 71 feet. Page IV.E-1

2 Groundwater During site investigations in December of 2006, groundwater was encountered at a depth of 53 feet below existing grade. Groundwater in the project area generally flows in a southeast direction. 1 As indicated by the 1998 California Geologic Survey (CGS) Seismic Hazard Evaluation Report, the project site may be subject to historic high groundwater levels of up to approximately 10 feet below existing grade. Fluctuations in groundwater levels may occur due to variations in rainfall, regional climate, and in response to landscape irrigation. Faults The project site and all of Southern California is considered to be a seismically active region. The region has numerous active, potentially active, and inactive faults. 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). Inactive faults have not moved in the last 1.6 million years. The closest active faults are the Hollywood fault, located approximately one mile north of the project site; the Santa Monica fault, located approximately 1.5 miles northwest of the site; and the Newport-Inglewood fault, located approximately three miles southwest of the site. Geologic-Seismic Hazards Surface Fault Rupture No evidence of active faulting was observed during the site investigation. Based on the review of published geologic maps, no active faults are known to traverse the project site, and it is not located within a fault rupture hazard zone as defined pursuant to the Alquist-Priolo Earthquake Fault Zoning Act. Ground Shaking Ground shaking from earthquakes is a seismic hazard that can cause damage to structures. The CGS assigns ground motion values the area of Los Angeles County wherein the project site is located. The Peak Ground Acceleration (PGA) 2 for the study area is 0.51 g (the acceleration due to gravity). As such, 1 2 Phase I Environmental Site Assessment, 321 & 323 South Almont Drive, Los Angeles, California, 90048, prepared by AEI Consultants, February 6, Included as Appendix G. The peak acceleration is the maximum acceleration experienced by the particle during the course of the earthquake motion. Page IV.E-2

3 during an earthquake event, severe ground shaking may be experienced and moderate to heavy damage could occur in the project area. 3 Liquefaction Loosely compacted granular soils located below the water table can fail through the process of liquefaction during strong earthquake-induced ground shaking. When solid particles in a saturated soil consolidate into a tighter package as a result of vibration due to an earthquake, the non-compressible pore water between the particles will be squeezed out. If the soil has a high permeability, a sufficient amount of water will drain out of the pores. However, if the permeability is relatively low, then the water will not be able to drain away quickly enough and excess pore water pressures will build up. When excess pore water pressures build up, they reduce the effective stresses acting on the soil and, in turn, reduce the shear strength of the soil. If the pore water pressure rises to a level such that the shear strength of the soil becomes zero, then liquefaction is said to have occurred. Factors known to influence liquefaction potential include soil type and depth, grain size, relative density, ground-water level, degree of saturation, and both intensity and duration of ground shaking. The project site is located within an established Seismic Hazard Zone for Liquefaction as established by the CGS pursuant to the Seismic Hazards Mapping Act. According to the Geotechnical Report, potentially liquefiable soils are located at approximate depths of 20 to 34 feet below the surface of the project site. Slope Stability/Landslides Landslides may be triggered by earthquakes, rainstorms, or construction-related activities (e.g., improper grading, structural design, landscaping, etc.). The project site is not located near any steep slopes and it is not located within a designated landslide area. 4 Seismically-Induced Settlement Settlement is often caused by loose- to medium-dense soils densified by building loads, wetting or seismic ground shaking. The project site is located in a seismically active region that could be subject to strong ground shaking. Subsequently, the site could be subject to seismically-induced settlement. 3 United States Geological Survey (USGS) website: May 1, City of Los Angeles Department of City Planning, Zone Information and Map Access System, website: accessed May 1, Page IV.E-3

4 Subsidence Subsidence is the gradual settling or sudden sinking of land due to movement or removal of underlying earth materials. Human-induced subsidence typically occurs due to activities such as extraction of groundwater, petroleum, and natural gas deposits. Groundwater and petroleum are not being extracted from the project site; therefore, subsidence as a result of such activities would not occur. However, the project site is located within the San Vicente Oil Field. 5 There is an oil well located just west of the project site, but it is a dry hole which has been plugged and abandoned. 6 It is not expected to represent a significant environmental concern, as no oil extraction activities are currently being undertaken or are planned for the project area. Expansive Soils Expansive soils are clay based soils that tend to expand (increase in volume) as they absorb water and shrink (lessen in volume) as water is drawn away. If soils consist of expansive clays, foundation movement and/or damage can occur if wetting and drying of the clay does not occur uniformly across the entire area. The project site is located in an area for which there is little or no specific data on the presence of expansive soils. 7 The soils underlying the project site are assumed to be moderately expansive. ENVIRONMENTAL IMPACTS Thresholds of Significance Appendix G of the State CEQA Guidelines In accordance with guidance provided in Appendix G of the State CEQA Guidelines, the Proposed Project could have a potentially significant impact if it were to: (a) Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: 5 Environmental and Public Facilities Maps: Areas Containing Significant Mineral Deposits and Oil Drilling & Surface Mining Supplemental Use Districts, Los Angeles City Planning Department, Citywide Division. 6 7 California Department of Conservation, Division of Oil, Gas, and Geothermal Resources, website: ftp://ftp.consrv.ca.gov/pub/oil/maps/dist1/117/map117.pdf accessed May 17, United States Department of Agriculture, Natural Resources Conservation Service Web Soil Survey, website: accessed September 28, 2006; and United States Geological Survey, Swelling Clays Map of the Conterminous United States website: accessed May 1, Page IV.E-4

5 i. Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area 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 identified in Table 18-1-B of the Uniform Building Code (1997), creating substantial risks to life or property; or (e) Have soils incapable of adequately supporting the use of septic tanks or alternative wastewater disposal systems where sewers are not available for the disposal of wastewater. With respect to Threshold (e) above, as discussed in the Initial Study for the Proposed Project, the project site is served by a wastewater collection, conveyance, and treatment system operated by the City of Los Angeles. No septic tanks or alternative disposal systems are necessary, nor are they proposed. No impact related to septic systems would occur and no further analysis is necessary. City of Los Angeles CEQA Thresholds Guide 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) 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. Project Impacts Seismic Hazards Surface Fault Rupture As discussed above, the project site is not located in an Alquist-Priolo Earthquake Fault Zone. The closest active fault is the Hollywood fault, located approximately one mile north of the project site. Page IV.E-5

6 Therefore, the possibility of surface fault rupture affecting the project site would be considered remote, and the Proposed Project would not present any adverse impacts with respect to exposing people or property to hazardous conditions resulting from rupture of a known earthquake fault on the project site. Therefore, a less than significant impact would occur with respect to fault rupture. Ground Shaking The Proposed Project could be subject to strong ground shaking from earthquakes produced by local faults. 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 and the recommendations of the Geotechnical Report. Although the project site is located within approximately one mile of the Hollywood Fault, and by many other faults on a regional level, the potential seismic hazard to the Proposed Project site would not be higher than in most areas of the City of Los Angeles or elsewhere in the region. Therefore, the risks from seismic ground shaking are considered to be less than significant. Erosion and Topsoil Construction During construction activities, particularly during excavation for the subterranean parking levels, there is a potential for erosion to occur during the grading process during periods of heavy precipitation. Regulatory measures are required to be implemented during construction periods to minimize wind and water-borne erosion (see Section IV.C, Air Quality). Project development would require excavation to a depth of roughly 35 feet. Approximately 125,000 cubic yards of soil would be excavated during construction. All earthwork and 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 the 2007 California Building Code; Chapter IX, Division 70 of the Los Angeles Municipal Code which addresses grading, excavations, and fills; and the recommendations of the Geotechnical Report. This type of excavation typically requires shoring, which consists of a system of soldier piles and wood, steel, or concrete lagging. Soldier piles consist of steel H beams or reinforcement cages installed within pre-drilled holes that are filled with concrete. The geotechnical report by Petra includes a number of guidelines for shoring. With implementation of the applicable grading and building permit requirements and the application of Best Management Practices, a less than significant impact would occur with respect to erosion or loss of topsoil. Page IV.E-6

7 Operation The project site is developed with seven multi-family residential structures which would be demolished under the Proposed Project. As the project has been previously graded and developed for the development of these residential uses, the presence of topsoil is not anticipated. The Proposed Project would develop the project site with pervious and impervious surfaces including structures, paved areas, and landscaping. As such, the proposed development would not leave soils exposed at the site or substantially increase the rate and amount of erosion occurring at the project site and impacts would be less than significant. Liquefaction The most significant geotechnical constraint to project development would be the presence of potentially liquefiable soils at approximate depths of 20 to 34 feet below the surface. However, the Proposed Project would excavate soils to a depth of approximately 35 feet bgs for the subterranean parking levels, thus removing this potentially liquefiable zone. The proposed excavation would be conducted with further recommendations from Petra Geotechnical s Investigation Report. Implementation of Mitigation Measure D-1 would ensure that the geotechnical recommendations are administered as part of the Proposed Project. As such, impacts would be less than significant. Seismically-Induced Settlement Based on the results of seismically-induced settlement analysis performed as part of the Geotechnical Report, the maximum total seismic settlement of onsite soils would be from roughly 1.1-inch to 1.3-inch with 35 feet of excavation. The differential settlement is estimated to be from roughly 0.56-inch to inch. These settlements ranges are within the acceptable limits set forth by the City of Los Angeles. Therefore, the risks from seismically-induced settlement would be less than significant. Nonetheless, special design considerations to accommodate potential settlement may be required for walls and flatwork, etc., at the edges of the project site where property line boundaries preclude the implementation of mitigation measures. Subsidence Groundwater and petroleum are not currently being extracted from the project site and would not be extracted as part of the Proposed Project; therefore, subsidence as a result of such activities would not occur. Thus, earth materials underlying the project site are not subject to hydrocompaction or subsidence. Therefore, the risks from subsidence would be less than significant. Expansive Soils As discussed above, the soils underlying the project site are assumed to be moderately expansive. Development of the Proposed Project would occur in accordance with the Los Angeles Building Code and the recommendations of the Geotechnical Report, which includes building foundation requirements Page IV.E-7

8 appropriate for site-specific conditions. As discussed in the Geotechnical Report, fill at the project site should be placed in six- to eight-inch thick maximum lifts, watered or air-dried as necessary to achieve near optimum moisture conditions and then compacted in-place to a minimum relative compaction of 90 percent. If the clay content of the fill soil on-site is found to be less than 15 percent, the compacted fill should be compacted to a minimum dry density 95 percent of the maximum dry density. The laboratory maximum dry density and optimum moisture content for each change in soil type should be determined in accordance with American Society for Testing and Materials (ASTM) methods. With compliance with the Los Angeles Building Code and implementation of the recommendations of the Geotechnical Report, as included in Mitigation Measure E-1, impacts related to expansive soils would be less than significant. CUMULATIVE IMPACTS Development of the Proposed Project in conjunction with the 53 related projects in Section III., Environmental Setting, would result in further infilling of various land uses in the City of Los Angeles. Geologic hazards are site-specific and there is little, if any, cumulative relationship between development of the Proposed Project and the related projects. As such, construction of the related projects is not anticipated to combine with the Proposed Project to cumulatively expose people or structures to such geologic-seismic hazards as earthquakes, ground shaking, liquefaction, landslides, and/unstable soils, expansive soils, or result in substantial soil erosion or the loss of topsoil. Therefore, no cumulatively considerable geological impacts are anticipated from the Proposed Project and the related projects. MITIGATION MEASURES E-1 The Proposed shall be designed and constructed in accordance with the recommendations provided in the Geotechnical Report, which shall be approved by the Department of Building and Safety prior to issuance of building and grading permits. LEVEL OF SIGNIFICANCE AFTER MITIGATION With implementation of the mitigation measure listed above, project impacts with regard to geology and soils would be less than significant. Page IV.E-8