IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY AND SOILS

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1 IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY AND SOILS INTRODUCTION This section evaluates potential impacts related to geology, including seismicity, and soils associated with development of the proposed project. 1 ENVIRONMENTAL SETTING Geologic Setting The project site is located in the north central portion of the San Fernando Valley. The topography of the project site and surrounding area is relatively flat. The San Fernando Valley is an elliptical-shaped alluvium-filled basin, approximately 23 miles wide and 12 miles long. This valley is formed by deposition from streams and rivers that have transported sediments from the surrounding upland areas. The alluvium is mainly derived from the Santa Monica Mountains to the south, the Santa Susana Mountains to the northwest, the Simi Hills to the west, the San Gabriel Mountains to the northeast, and the Verdugo Mountains to the east. Regionally, the project site is located in the Transverse Ranges geomorphic province. 2 The project site is located in the Transverse Ranges Province. Most of the province is mountainous; many of the higher ridges and peaks rise above 5,000 feet, and the highest mountains rise to elevations of more than 10,000 feet. The backbone of the province, in its central and eastern parts, is formed by the San Gabriel and San Bernardino Mountains. The Province extends eastward about 275 miles from Point Arguello into the Mojave Desert. The project site is more specifically located in the Northwestern Block of the Los Angeles Basin. The Northwestern Block includes parts of the east trending Santa Monica Mountains, the Verdugo Mountains, and the San Fernando Valley. This province is characterized by east-west trending geologic structures that include the east-west trending Santa Monica Mountains and the east-west trending active San Fernando Fault Zone. The trend of the San Fernando Valley reflects the overall trend of the Transverse Ranges, where major structural features exhibit an east-west orientation in contrast to the northwest-southwest trend that dominates the rest of California. The San Fernando Valley is an area 1 2 See the Report of Geotechnical Evaluation for Environmental Impact Report, Proposed Panorama Place Project through West Roscoe Boulevard, Panorama City District, Los Angeles, California (the Geotechnical Report ), dated July 31, 2006, prepared by MACTEC Engineering and Consulting, Inc. The Geotechnical Report is incorporated herein by this reference, and included in its entirety as Appendix D to this Draft EIR. Regional geologic information is based on the August 11, 2006 Phase I ESA for the proposed project. Page IV.E-1

2 of compression between the San Gabriel Mountains on the northeast and the Santa Monica Mountains on the south. Subsurface Geology and Soils A previous geotechnical investigation was conducted at the project site in association with the Montgomery Ward s store. 3 During this investigation, seven borings were drilled at the project site to a maximum depth of 60 feet. Based on these prior explorations, the project site is predominately underlain by Holocene age alluvial fan deposits and wash deposits. The alluvial materials consist of alternating layers of silty sand, sandy silt, and poorly graded and well-graded sand with varying amounts of gravel and few cobbles. The upper soils underlying the site are variable in strength and compressibility, with the silty soils being subject to substantial weakening and compression with an increase in moisture. The deeper alluvial soils are considered dense and would be unaffected by moisture. The Holocene age alluvial materials and the underlying Pleistocene age materials are estimated to be 800 to 1,000 feet thick and are underlain by Tertiary age sedimentary rocks. (Refer to Figure IV.E-1, Geologic Map.) Groundwater The project site is located in the San Fernando Hydrologic Subarea in the Upper Los Angeles River Hydrologic Unit. The historic high water level is over 100 feet below ground surface (bgs). The closest known water well to the project site is Well No located 2,000 feet west of the site. The shallowest depth to groundwater measurement was 195 feet in Depth to groundwater was 254 feet in the most recent measurement taken in Groundwater was not encountered within the 60 foot depth explored by previous borings at the project site. Seismicity and Seismic Hazards The project site is located in a seismically active region of California. Based on criteria established by the California Geological Survey (CGS), faults may be categorized as active, potentially active, or inactive. Active faults are those which show evidence of surface displacement within the last 11,000 years. Potentially-active faults are those that show evidence of last displacement within the last 1.6 million years. Faults showing no evidence of displacement within the last 1.6 million years may be considered inactive for most purposes. 3 Report of Foundation Investigation, Proposed Retail Store, Roscoe Boulevard and Tobias Avenue, Panorama City, California, Leroy Crandall and Associates, Page IV.E-2

3 Figure IV.E-1, Geologic Map Page IV.E-3

4 In 1972, the Alquist-Priolo Special Studies Zones Act (the Act ) was passed into law. This Act (now known as the Alquist-Priolo Earthquake Fault Zoning Act) defines active and potentially-active faults utilizing the same aging criteria established by CGS, described above. However, the policy under the Act is to zone only those potentially active faults that have a relatively high potential for ground rupture. Therefore, not all faults termed potentially-active by the CGS are zoned under the Alquist-Priolo Act. The project site is not located within a currently established Alquist-Priolo Earthquake Fault Zone for surface fault rupture hazards. The closest Alquist-Priolo Earthquake Fault Zone to the project site is the San Fernando fault zone, located approximately four miles to the north in the San Fernando Valley area. Active and potentially active faults in the project area are described below under separate headings. Active Faults The closest active fault to the project site with the potential for surface fault rupture is the Verdugo fault zone, located four miles to the east. The following is a list of other active faults in the project area: The Santa Susana Fault, located 6.6 miles northwest of the project site; The Sierra Madre Fault, located roughly 8.6 miles to the northeast; The San Gabriel Fault, located about nine miles north of the project site; The Hollywood Fault, located 10 miles south of the project site; The Santa Monica Fault, located approximately 12 miles southwest of the project site; The Raymond Fault, located approximately 13 miles southeast of the project site; The Malibu Coast Fault Zone, located approximately 14 miles southwest of the project site; The Newport-Inglewood Fault Zone, located roughly 14 miles south of the project site; The Simi-Santa Rosa Fault, located approximately 14 miles northwest of the project site; and The San Andreas Fault Zone, located 28 miles northeast of the project site. Blind Thrust Faults A thrust fault is a fault that does not rupture at the surface as it is buried under the uppermost layers of rock in the earth s crust. The Northridge Thrust Fault underlies much of the San Fernando Valley including the project site. The 1994 Northridge earthquake occurred along the Northridge Thrust Fault. The Puente Hills Blind Thrust Fault system extends eastward from downtown Los Angeles to Brea (in northern Orange County). The Puente Hills Blind Thrust Fault system includes three north-dipping segments, named from east to west as the Coyote Hills segment, the Santa Fe Springs segment, and the Page IV.E-4

5 Los Angeles segment. The closest vertical surface projection of the Puente Hills Blind Thrust Fault system to the site is approximately 11 miles to the southeast. The Upper Elysian Park fault is a blind thrust fault that overlies the Los Angeles and Santa Fe Springs segments of the Puente Hills Thrust Fault. The eastern edge of the Upper Elysian Park fault is defined by the northwest-trending Whittier Fault Zone. The vertical surface projection of the Upper Elysian Park fault is approximately 14 miles southeast of the site at its closest point. These faults are considered to be active and have the possiblility to generate future earthquakes. However, as with other blind thrust faults in the Los Angeles area, these faults are not exposed at the surface and do not present a potential surface rupture hazard. Potentially Active Faults The closest potentially active fault to the project site is the Northridge Hills fault located approximately five miles to the northwest. The above-mentioned faults are shown in relation to the project site on Figure IV.E-2, Regional Faults Map. ENVIRONMENTAL IMPACTS Methodology The environmental impacts of the proposed project with respect to geology and soils were determined based on the findings of the Geotechnical Report. Thresholds of Significance The proposed project would have a significant geology and soils impact if: (a) It would cause or accelerate seismic hazards which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury; (b) It would constitute a geologic hazard to other properties by causing or accelerating instability from soil erosion or loss of topsoil; or (c) One or more distinct and prominent geologic or topographic features would be destroyed, permanently covered or materially and adversely modified. Such features may include, but are not limited to, hilltops, ridges, hill slopes, canyons, ravines, rock outcrops, water bodies, streambeds, and wetlands. Page IV.E-5

6 Figure IV.E-2, Regional Fault Map Page IV.E-6

7 Project Impacts Seismic Hazards In general, risks associated with seismic hazards at the project site are typical of those throughout the Southern California region. Possible sources or causes of seismically-induced hazards are addressed below. Rupture of a Known Earthquake Fault The project site does not lie within an Alquist-Priolo Special Studies Zone, nor is the project site within a State of California Earthquake Fault Zone. Therefore, the proposed project would not expose people or structures to potential substantial adverse effects involving rupture of a known earthquake fault, and no impact would occur. Seismic Ground Shaking Due to the seismically active nature of the Southern California region, the project site is susceptible to ground shaking during a seismic event within the region. Potential impacts from seismic ground shaking are present throughout Southern California and would be of comparable intensity at the project site as it would be for large parts of the City and the region. However, the proposed project would comply with the Los Angeles Building Code to reduce seismic risks to an acceptable level. Therefore, the proposed project would not expose people or structures to potential substantial adverse effects relating to strong seismic ground shaking, and the associated impact would be less than significant. In addition, implementation of the Mitigation Measures E-1 and E-2, listed below, would further reduce potential impacts associated with seismic ground shaking. Settlement Settlement is often caused by loose to medium-dense soils densified by building loads, wetting or seismic groundshaking. The upper portions of the alluvial deposits underlying the project site are generally relatively loose and are considered to be susceptible to settlement. However, as the proposed project would implement the recommendations of the Geotechnical Report and design building foundations in accordance with the Los Angeles Building Code, potential impacts associated with settlement would be less than significant. Liquefaction Liquefaction is the process in which loose granular soils below the ground-water table temporarily lose strength during strong ground shaking as a consequence of increased pore pressure and, thereby, reduce effective stress. The vast majority of liquefaction hazards are associated with sandy soils and silty soils of low plasticity. Potentially liquefiable soils (based on composition) must be saturated or nearly saturated to be susceptible to liquefaction. Significant factors that affect liquefaction include water level, soil type, Page IV.E-7

8 particle size and gradation, relative density, confining pressure, intensity of shaking, and duration of shaking. Liquefaction potential has been found to be the greatest where the ground water level is shallow and submerged loose, fine sands occur within a depth of about 50 feet or less. Liquefaction potential decreases with increasing grain size and clay and gravel content, but increases as the ground acceleration and duration of shaking increase. According to the County of Los Angeles Seismic Safety Element, the City of Los Angeles Safety Element, and the California Division of Mines and Geology, the site is not within an area identified as having a potential for liquefaction. Although near surface soils may be relatively loose, the groundwater table is at too great a depth for liquefaction to be a potential impact. Therefore, no impact would occur with respect to liquefaction. Slope Stability The project site is not within an area identified as having a potential for seismic slope instability (slope instability resulting from seismic ground shaking). According to the City of Los Angeles Safety Element and the County of Los Angeles Seismic Safety Element the site is not within an area identified as having a potential for slope instability. There are no known landslides at the project site, nor is the project site in the path of any known or potential landslides. The alluvial-deposits underlying the project site are generally uncemented and susceptible to erosion. Due to their relatively loose nature, the upper alluvial deposits are susceptible to sloughing and failure if temporary cut slopes are constructed at angles greater than approximately 2:1. This would result in a potentially significant impact. However, implementation of the recommendations in the Geotechnical Report would address the potential impact associated with slope instability during the construction of the proposed project. Expansive and Corrosive Soils Expansive soils generally contain clay minerals that absorb moisture and cause the material to swell or increase in volume. Likewise, when expansive soils dry out, they tend to shrink or decrease in volume. Volume changes associated with changes in the moisture content of near surface soils can cause uplift or heave of the ground surface. The alluvial deposits present at the project site typically have a low expansion potential, however, the silts and local clays could have medium to high expansion potential. Site soils could be corrosive to ferrous metals and deleterious to copper and concrete. This would result in a potentially significant impact. However, the proposed project would implement the recommendations of the Geotechnical Report and design building foundations in accordance with the Los Angeles Building Code to address the potential impact associated with expansive and corrosive soils. Landforms There are no distinct or prominent geographic features, such as hilltops, hilltops, ridges, hill slopes, canyons, ravines, rock outcrops, water bodies, streambeds, or wetlands on or near the project site. In addition, there are no unique geologic features on or in the vicinity of the project site. Therefore, no Page IV.E-8

9 unique geologic features will be modified or destroyed as a result of the proposed development and no impact would occur. CUMULATIVE IMPACTS Geotechnical impacts related to future development in the City would involve hazards related to sitespecific soil conditions, erosion, and ground-shaking during earthquakes. The impacts on each site would be specific to that site and its users and would not be common or contribute to (or shared with, in an additive sense) the impacts on other sites. In addition, development on each site would be subject to uniform site development and construction standards that are designed to protect public safety. The projects identified on the related projects list would not be exposed to greater than normal seismic risk as compared to other areas in Southern California. The proposed project would in no way compound the effects of the related projects. Therefore, cumulative geotechnical impacts would be less than significant. MITIGATION MEASURES (E-1) The project shall be designed and constructed in accordance with the recommendations provided in the Report of Geotechnical Evaluation for Environmental Impact Report, Proposed Panorama Place Project through West Roscoe Boulevard, Panorama City District, Los Angeles, California, dated July 31, 2006, prepared by MACTEC Engineering and Consulting, Inc (included as Appendix D to this DEIR). (E-2) The project shall be designed and constructed in accordance with requirements of the Los Angeles Building Code. (E-3) The project developer shall complete a grading plan that conforms to the City s Landform Grading Manual guidelines, subject to approval by the Advisory Agency and the Department of Building and Safety s Grading Division. 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. UNAVOIDABLE ADVERSE IMPACTS The proposed project would not have an unavoidable adverse impact with respect to geology and soils. Page IV.E-9