IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY/SEISMIC HAZARDS

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1 IV. ENVIRONMENTAL IMPACT ANALYSIS E. GEOLOGY/SEISMIC HAZARDS 1. ENVIRONMENTAL SETTING The following analysis of geology and seismic hazards for the Middle School Project is based on the Report of Geotechnical Investigations for the Proposed Middle School Project, prepared by Van Beveren & Butelo, Inc., dated October 13, 2003, and the subsequent report entitled the Applicability of Prior Report, Revisions to Middle School Project, August 27, Supporting information regarding the project setting is based on the Technical Memorandum Geotechnical Input to Environmental Documentation Proposed Middle School Project, prepared by Van Beveren & Butelo, Inc., dated May 8, The analysis for the Athletic Fields site is based on the Report of Geotechnical Investigations for the Proposed Athletic Fields Site, prepared by Van Beveren & Butelo, Inc., dated September 1, a. Existing Conditions (1) Geologic Conditions (a) Consolidated Middle School/High School Site The Consolidated Middle School/High School site is located near the crest of the northern flank of the central Santa Monica Mountains and approximately 7 miles from the Pacific Ocean. This mountain range trends east-west along the southern boundary of the San Fernando Valley and was formed during broad, regional uplift. The elevations vary across the pad from about 1,250 to 1,265 feet above sea level. The site is generally mantled by shallow fill deposits ranging from about 1 to 2 feet over bedrock of the Modelo Formation. The Modelo Formation bedrock is composed of thinly to thickly bedded siltstone, shale, and sandstone units. The bedrock contains expansive bedrock, which occurs over the site. The expansive bedrock can be potentially expansive when oxidized, which can be harmful to structures. The bedrock is corrosive to ferrous metals, copper and concrete. The topography of the entire project site is diverse with a combination of natural, cut and fill slopes inclined at about 0.75:1 to 2:1 (horizontal to vertical) in the natural portions and as steep as 0.5:1, in the cut portions of the slope. The fill slopes were constructed at 1.5:1 to 2:1. The fill slopes occupy two pre-existing canyons that were backfilled during prior construction. A third canyon located on the southern portion of site has been graded with non-engineered fill placed over colluvium. Page 169

2 Fill was also placed on the northern portion of the site for the construction of Mulholland Drive. The resulting fill slopes are up to 30 feet in height. An 80-foot high cut-slope, east of Mulholland Drive was graded at approximately 1:1 for the construction of Mulholland Drive. This slope exposes units of the Modelo Formation similar to those units encountered beneath the proposed classroom site. (b) Athletic Fields Site The proposed Athletic Fields site is also located near the crest of the northern flank of the central Santa Monica Mountains. The topography at the site is characterized by a large, nearlevel pad, an ascending cut slope to the east and a small descending cut slope to the north along Mulholland Drive. The site was excavated to create a relatively level pad for the existing nursery school facility, with elevations varying across the pad from 1,310 to 1,320 feet above sea level. The west-facing cut slope was graded at an inclination of about 1.5:1 (horizontal to vertical). The slope varies in height from approximately 60 to 100 feet. A slope varying from 5 to 10 feet descends from the pad to Mulholland Drive to the north. There is an existing 10-foothigh retaining wall on the north side of the cut slope. Fill soils cover the majority of the site in depths ranging from 0.5 to 25 feet below the existing grade. The fill consists of silty sand and sandy silt, derived from the nearby sandstone bedrock. As with the Consolidated Middle School/High School site, some of the bedrock contains expansive bedrock, which is a potentially highly expansive bedrock. (2) Seismic Hazards (a) Faulting and Ground Shaking The proposed project sites are both located within seismically active southern California. In order for geologic impacts relating to earthquakes to be significant, they must exceed the typical hazard risk for the region. According to geologic investigations conducted for the project, no faults have been identified on, or are known to trend through or toward the site. The sites are not located within an area identified to have the potential for seismically induced landsliding on the State of California Seismic Hazard Zones map (CDMG, Seismic Hazard Zone, Van Nuys Quadrangle, 1998) and are not located within a City or County of Los Angeles Landslide Inventory Area. 55, County of Los Angeles, Technical Appendix to the Safety Element of the Los Angeles County General Plan, 1990., Safety Element of the Los Angeles City General Plan, Page 170

3 Faults are classified as active, potentially active, or inactive. Faults that have historically produced earthquakes or show evidence of movement within Holocene time (i.e., the past 11,000 years) are known as active faults. Faults from past geologic periods of mountain building that do not display any evidence of Holocene activity are considered potentially active. The distances to the respective faults are approximately the same given the proximity of the Consolidated Middle School/High School site and the Athletic Fields site. The nearest active fault to the project sites is the Hollywood fault, which is located approximately 4.2 miles to the southeast of the site. Other active faults include the Malibu Coast fault located approximately 7 miles south-southwest and the Newport-Inglewood fault zone located about 7.9 miles southeast. The closest potentially active faults are the Overland fault 7.9 miles south-southeast, and the Charnock fault located 8.6 miles south-southeast. The regionally extensive Benedict Canyon fault is located approximately 2.6 miles southeast of the site. This fault is considered inactive but significantly offsets various geologic units and influences geologic structure along the fault trace. Both sites are located within the seismically active southern California Region and would likely experience the affects of strong ground motion from earthquakes occurring on neighboring active faults. However, the potential for ground rupture at the sites due to faulting is considered remote. (b) Liquefaction Liquefaction is a form of earthquake-induced ground failure that occurs primarily in relatively shallow, loose, granular, water-saturated soils. Liquefaction potential decreases as grain size and clay and gravel content increase. The materials beneath both the Consolidated Middle School/High School site and Athletic Fields site consist of fills, siltstone and shale with some sandstone. The materials are very dense and very stiff, and ground water is not present. The project is not located within an area designated by the State of California or by the City of Los Angeles for potential liquefaction. 57, 58 The risk of liquefaction is considered remote since the project is not located within a regional groundwater basin. (c) Seismically Induced Landslides Seismic settlement is caused by loose to medium-dense granular soils densifying during ground shaking. Dry and partially saturated soils, as well as saturated granulated soils are subject to seismically induced settlement. The project sites are both underlain by dense, cemented bedrock materials. Therefore, the potential for seismically induced settlement is remote California Department of Conservation Division of Mines and Geology, Seismic Hazard Zones Van Nuys Quadrangle, dated February 1, Los Angeles, City of, Safety Element of the Los Angeles City General Plan, Page 171

4 (d) Seiche and Tsunami Seiches are waves formed from oscillations in enclosed or restricted bodies of water (i.e., harbors, lakes). Seiches can cause water to overtop reservoirs and lakes. According to the Los Angeles County Seismic Safety Element, the site is not located downslope of a large body of water that could adversely affect the site in the event of earthquake-induced failures or seiches. 59 Therefore, seiches are not considered a significant hazard on the project site. Tsunamis are large ocean waves generated by large-scale, short duration submarine earthquakes. Tsunami waves are capable of traveling great distances (over 1,000 miles) and damaging low-lying coastal regions. The project sites are approximately seven miles from the Pacific Ocean at an elevations ranging from approximately 1,250 to 1,320 feet above mean sea level. Therefore, tsunamis, or seismic sea waves, are not considered a significant hazard at the project site. (3) Slope Stability (a) Consolidated Middle School/High School Site As discussed above, the project is not shown to be within an area identified to have the potential for seismically induced landsliding on the State of California Seismic Hazard Zones map and is not located within a City or County of Los Angeles Landslide Inventory Area. However, a small surficial failure was observed on the slope, south of the lower field during field investigations. Locally, the slope areas covered by colluvium, defined as a loose deposit of rock and soil debris accumulated through the action of gravity at the base of a cliff or slope, are unstable and undergoing downhill creep and crumbling. This condition occurs where the slope is steeper than about 1.5:1. The steep, west-facing slope along Sepulveda Boulevard on the Middle School site generates loose debris from the weathering of bedrock materials. This slope presents a hazard due to rockfall, erosion and mudflows. Mudflows occur when surface water runoff is allowed to overtop a slope or drainage and infiltrate upper soils. The increased moisture adds weight to the mass resulting in a loss of shear strength. However, no evidence of distress from lateral spreading was observed during field investigations. 59 Los Angeles County, Technical Appendix to the Safety Element of the Los Angeles County General Plan, Page 172

5 (b) Athletic Fields Site Available geologic maps of the project area do not indicate landslides on or near the Athletic Fields site. As stated above, the site is not located within an area identified to have the potential for seismically induced landsliding and is not located within a City or County of Los Angeles Landslide Inventory Area. Evidence of surficial failures or large-scale landslides was not observed during our reconnaissance at the site. The site is relatively level and is bounded by a west-facing cut slope on the east. The cut slope has an average gradient of 1.5:1 (horizontal to vertical) and a maximum height of approximately 100 feet. Bedding within the Modelo Formation dips into the slope or is neutral to the slope face. Additionally, joint patterns generally dip approximately 70 degrees to vertical, which is steeper than the existing cut slope. These conditions are considered favorable for gross stability from a geologic standpoint. Distress from lateral spreading was not observed during our investigation activities. (4) Expansive Soils (a) Middle School Site As stated above, the Modelo Formation bedrock underlies the fill soils on the Middle School site. Modelo Formation units at the site are characterized generally by thinly to thickly bedded, cemented, siltstone and shale with some thinly bedded sandstone beds. Portions of the bedrock, referred to as the black shale, contain sulfides resulting in the material being potentially highly expansive when oxidized, as well as acidic and highly corrosive to ferrous metals, copper and concrete. The expansion could cause heaving of foundations and slabs on grade. (b) Athletic Fields Site The fill is underlain by sandstone, siltstone and shale bedrock of the Modelo Formation. The bedrock is not consistent across the site, with the northern half of the site consisting of cemented bedrock and the southern half underlain by less cemented bedrock. Expansive bedrock was only encountered on the south side of the Athletic Fields site at depths of approximately 20 feet. b. Regulatory Framework (1) California Geologic Survey The California Geologic Survey (CGS) provides guidance with regard to seismic hazards. Under the CGS s Seismic Hazards Mapping Act, seismic hazard zones are to be identified and Page 173

6 mapped to assist local governments in planning and developing. The intent of this publication is to protect the public from the effects of strong ground shaking, liquefaction, landslides, or other ground failure, and other hazards caused by earthquakes. In addition, the CGS s Special Publications 117, Guidelines for Evaluating and Mitigating Seismic Hazards in California provides guidance for evaluation and mitigation of earthquake-related hazards for projects within designated zones of required investigation. (2) Specific grading requirements and regulations for hillside areas are governed by the Los Angeles Municipal Code (LAMC). Specifically, Section provides grading provisions for hillside areas, including regulations pertaining to building foundations, removal of ground cover, and hazardous soil and earth conditions. Additionally, Section , Conditions Precedent to Issuing a Grading Permit, details submittal information required for hillside grading. Specifically, Section includes requirements regarding import and export of material. Section , Planting and Irrigation of Cut and Fill Slopes in Hillside areas, provides ground cover requirements for erosion control and Section includes regulations pertaining to erosion control and drainage devices. Finally, Section , Construction Requirements and Limitations, includes general construction requirements as well as requirements regarding flood and mudflow protection, and Section includes regulations for areas that are subjected to slides and unstable soils. 2. ENVIRONMENTAL IMPACTS a. Methodology An assessment of grading, site design, and seismicity was performed to identify potential impacts associated with geology and soils. As indicated above, the analysis is based on sitespecific geotechnical reports prepared by Van Beveren & Butelo, Inc. The exploration was prepared based on review of existing documentation, field investigations (subsurface exploration), and laboratory testing. b. Thresholds of Significance (1) Geologic Hazards The following threshold of significance would be applied to the proposed project as set forth in the L.A. CEQA Thresholds Guide, (page C.1-4) which states that a project would normally have a significant geologic hazard impact if: Page 174

7 The project would cause or accelerate geologic hazards, which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. (2) Sedimentation and Erosion The following threshold of significance would be applied to the proposed project as set forth in the L.A. CEQA Thresholds Guide, (page C.2-3) which states that a project would normally have a significant sedimentation or erosion impacts if it would: Constitute a geologic hazard to other properties by accelerating instability from erosion. (3) Landform Alteration The following threshold of significance would be applied to the proposed project as set forth in the L.A. CEQA Thresholds Guide, (page C.3-2) which states that a project would normally have a significant impact on landform alteration if: One or more distinct and prominent geologic or topographic features would be destroyed, permanently covered, or materially and adversely modified as a result of the project. Such features may include, but are not limited to, hilltops, ridges, hillslopes, canyons, ravines, rock outcrops, water bodies, streambeds, and wetlands. c. Analysis of Project Impacts (1) Project Design Features The Consolidated Middle School/High School site would develop the site with institutional uses associated with a Middle School that would comprise approximately 30,000 square feet of program floor area. The second component of the project includes athletic fields that would provide physical education facilities for both the Middle School and High School students within the site formerly used for the Stephen S. Wise Nursery School. The only building on the Athletic Fields site would be a small field house located along the western edge of the site. In addition, retaining walls would be required on both sites. A series of geotechnical investigations has been undertaken to ensure that all of the structural elements would be constructed in a manner that provides safety for project occupants and that avoids adverse impacts regarding the geologic conditions of the project sites. The geotechnical reports provided several recommendations that would be incorporated into the Page 175

8 project. The recommendations for the proposed project address geotechnical considerations such as the location of retaining walls, placement of fill, excavation, and soil engineering. The recommendations most relevant to the preparation of documentation pursuant to CEQA are listed below. In addition to the measures listed, the project would also be required to incorporate all geotechnical specifications site provided in Appendix E of this EIR. Middle School Site Building Foundations: The classroom buildings would be supported on drilled castin-place concrete piles and the floor slabs would be structurally supported with a void (crawl space) beneath the floor. Retaining Walls: Walls greater than about 10 feet in height would be supported on spread footing in the bedrock. Lower retaining walls would be supported on drilled piles in a manner similar to the classroom buildings. The low wall adjacent to the slope at the building designated as building B would be pile supported. Where the depth to bedrock is great, drilled cast-in-place concrete pile foundations would be utilized. To avoid surcharging the existing Sepulveda Boulevard tunnel, foundations for retaining walls would be constructed below a 1:1 plane extending upward from the base of the tunnel. Grading: Deposits of uncertified fill would be excavated prior to placing new compacted fill on-site. Permanent fill slopes would be constructed at 2:1. Slope Stability: Sepulveda Boulevard would be protected by construction of a slough wall with a rock catchment fence at the toe of the slope. Athletic Fields Site Site Preparation: After removing the existing buildings and paving, the following site preparation would be made within the areas of the athletic field and field house: 1. All existing vegetation would be removed. 2. The upper one foot of the existing fill soils would be excavated. 3. The exposed natural soils would be scarified to a depth of 6 inches, brought to within 2 percent of optimum moisture content, and compacted to at least 95 percent of the maximum dry density obtainable by the ASTM Designation D1557 method of compaction. Page 176

9 Excavations and Temporary Slopes: Where excavations are deeper than approximately 4 feet, the sides of excavations within the existing fill would be sloped back at 1:1 (horizontal to vertical) or shored for safety. Excavations would be observed a qualified geotechnical engineer so that any necessary modifications based on variations in the soil conditions can be made. All applicable safety requirements and regulations, including OSHA regulations, would be met. Fill Compaction: Any required fill would be placed in loose lifts not more than 8 inches thick and compacted. Material for Fill: The on-site soils, less any debris or organic matter, would be used in required fills. Cobbles and boulders larger than 4 inches in diameter would not be used in the fill. All proposed import materials would be approved by a qualified geotechnical engineer prior to being placed at the site. Foundations: The retaining walls would be supported on spread footings and constructed using conventional sloped excavations and backfill. As an option to conventional construction, the wall could be designed and constructed using cantilevered soldier pile shoring. The soldier piles could consist of either reinforced concrete piles or steel beams installed in drilled holes and backfilled with concrete. Foundations for the field house would be supported on spread footings in either the existing fill or new properly compacted fill. (More detailed specifications regarding the foundations are provided in the geotechnical reports.) d. Potential Impacts (1) Seismic Hazards Both the Consolidated Middle School/High School and the Athletic Fields sites are not located within an Alquist-Priolo Earthquake Fault Rupture Hazard Zone or a Seismic Hazard Zone established by the California Geological Survey (CDMG Special Publication 42, 1997). Although located within the seismically active southern California region, the sites are at no greater risk than other sites in the region with respect to strong ground motion and the proposed Middle School buildings should perform satisfactorily when designed and constructed according to Los Angeles Building Code. In addition, the potential for ground rupture due to faulting is considered remote. Therefore, the project would not cause or accelerate geologic hazards, which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury and the impacts would be less than significant. Page 177

10 (2) Slope Stability (a) Consolidated Middle School/High School Site As stated above, there is no evidence of large-scale landslides or unstable slope conditions on the Consolidated Middle School/High School site. However, the west-facing slope along Sepulveda Boulevard generates loose debris from the weathering of bedrock materials. This slope would continue to generate soil and rock fragments downslope and would likely regress upslope into the building pad unless measures are implemented to stabilize the slope. As described above, the project would incorporate a retaining wall with a catchment fence as recommended in the geotechnical report. Drainage has not been well controlled at the tops of some slopes resulting in water flowing over the slopes. These slopes are subject to rockfalls, erosion, and local mudflows primarily during periods of heavy rainfall. These events result in an accumulation of rocks, mud and debris on Sepulveda Boulevard. According to the geotechnical report, this potential slope stability hazard would be addressed by correcting the drainage at the top of the slopes. Impacts related to water runoff are further analyzed in Section IV.F, Hydrology, of this EIR. With the implementation of the recommendations provided in the site-specific geotechnical report, the Middle School project would not cause or accelerate geologic hazards, which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury or constitute a geologic hazard to other properties by accelerating instability from erosion. Therefore, impacts related to slope stability would be less than significant. (b) Athletic Fields Site As stated above, the project site is relatively level and is bounded by a west-facing cut slope on the east. The cut slope has an average gradient of 1.5:1 (horizontal to vertical) and a maximum height of approximately 100 feet. The geologic conditions of this slope are considered favorable for stability. The site does not exhibit evidence of distress from lateral spreading and groundwater was not encountered during site reconnaissance. The geotechnical report for the proposed Athletic Fields site recommends construction methods to promote slope stability as well as specifies foundation types, excavation procedures, and location of piles. In addition, drainage would be provided behind the retaining walls in accordance with the recommendations outlined in the geotechnical report. With the incorporation of recommendations provided in the geotechnical report, the proposed Athletic Fields site would not cause or accelerate geologic hazards, which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury Page 178

11 or constitute a geologic hazard to other properties by accelerating instability from erosion. Therefore, impacts related to slope stability would be less than significant. (3) Expansive Soils (a) Consolidated Middle School/High School Site As stated above, the proposed Consolidated Middle School/High School site is underlain by bedrock of the Modelo Formation beneath the shallow fill soils. The bedrock is composed of siltstone, sandstone and shale. Portions of the bedrock contain sulfides resulting in the materials being potentially highly expansive when oxidized, acidic and highly corrosive to ferrous metals, copper and concrete. Therefore, bedrock and fill soils may be expansive and could cause distress to building foundations, slabs, walkways and pavement at the site. To resist the potentially harmful expansion, the geotechnical report recommends that the classroom buildings be supported on drilled cast-in-place concrete piles and the floor slabs structurally supported with a void (e.g., crawl space) beneath the floor. With the implementation of the geotechnical recommendations for expansive soils, the proposed Middle School project would not cause or accelerate geologic hazards, which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury. Therefore, such impacts would be less than significant. (b) Athletic Fields Site The planned excavation of the Athletic Fields site would expose bedrock over most of the site. The bedrock is very stiff to hard and cemented and would offer foundation support. However, the presence of black shale creates a potential for expansion which could cause heaving of lightly-loaded foundations and slabs on grade. As with the Consolidated Middle School/High School site, the Athletic Fields site is also underlain by potentially expansive soils. However, the proposed structure, the fieldhouse, would not extend close to the expansive shale. Therefore, development of the proposed Athletic Fields would not cause or accelerate geologic hazards, which would result in substantial damage to structures or infrastructure, or expose people to substantial risk of injury and impacts would be less than significant. (4) Grading and Site Design (a) Consolidated Middle School/High School Site Implementation of the project would require some modification of the existing topography. Earthwork for the Consolidated Middle School/High School development would Page 179

12 involve 29,000 cubic yards of cut material, 5,800 cubic yards of fill material and an additional 6,000 cubic yards of removed foundation material resulting in a total of 29,200 cubic yards of exported material. Enclosure of the balconies and the construction of a canopy over the High School entrance and adjacent walkways would require no earthwork at the High School site. As stated above, the Middle School site consists of compacted fill soils at the northeast corner of the site. The project design features for the proposed project would include procedures and specifications for the placement of fill. In addition, activities associated with the export of soil would occur in accordance with requirements, as specified in the Los Angeles Municipal Code and grading plan review and approval. With the incorporation of the site-specific geotechnical recommendations, the proposed project would not destroy, permanently cover, or materially or adversely modify a prominent geologic or topographic feature. As such, project implementation is not anticipated to result in significant impacts associated with landform alteration or the exportation of soil. Therefore, the Consolidated Middle School/High School project would not adversely modify a prominent geologic or topographic features and impacts related to landform alteration or the exportation of soil would be less than significant. (b) Athletic Fields Site Earthwork for the development of the Athletic Fields would require 2,600 cubic yards of cut material, and 1,200 cubic yards of fill material resulting in removal from the site of 1,400 cubic yards of material to be exported. The project design features for the proposed athletic fields include specifications for fill material and the compaction of fill. As with the Consolidated Middle School/High School site, activities associated with the export of soil would occur in accordance with requirements, as specified in the Los Angeles Municipal Code and grading plan review and approval. With the incorporation of the sitespecific geotechnical recommendations, the proposed project would not destroy, permanently cover, or materially or adversely modify a prominent geologic or topographic feature. As such, project implementation is not anticipated to result in significant impacts associated with landform alteration or the exportation of soil. 3. CUMULATIVE IMPACTS Cumulative impacts are evaluated by considering the effects of a project along with related projects that may contribute to a combined impact. Impacts associated with geologic and seismic issues are typically confined to a project site or a very localized area and do not affect off-site areas associated with the related projects or ambient growth. Six related projects are identified in Section III, Environmental Setting. The closest related project is located at the current Stephen S. Wise Middle School site, west of the proposed athletic fields site. The increase in the number of students at the Curtis School would occur opposite the project site, Page 180

13 north of Mulholland Drive. The two sites are distinct parcels, separated by vegetation and a short entry drive and Mulholland Drive, respectively. Therefore, this related project is sufficiently distant from the proposed project sites that cumulative geologic hazards would not occur. The proposed project would not contribute to a cumulative increase in the overall potential for exposure to seismic hazards since the proposed project would not result in an increase in faculty or students within the project sites. This includes elements, activities, or operations that would cause or accelerate geologic hazards extending off-site. However, the related projects would subject additional population to potential impacts. All projects, including the proposed project, are subject to established guidelines and regulations pertaining to seismic hazards. Activities associated with the export of soil would occur in accordance with City of Los Angeles requirements, as specified in the Los Angeles Municipal Code and grading plan review and approval. In addition, related projects would be evaluated on a case-by-case basis and subject to mitigation measures to reduce environmental impact to levels below significance. Therefore, the proposed project would not contribute to a cumulative impact related to geologic hazards. As such, adherence to applicable building regulations and standard engineering practices would ensure that cumulative impacts would be less than significant. 4. MITIGATION MEASURES The following mitigation measure is proposed to ensure that potential geotechnical impacts would be less than significant: Mitigation Measure E-1: The Applicant or its contractor shall incorporate the recommendations detailed in the site-specific geotechnical investigations prepared for the proposed project, as approved by the. 5. LEVEL OF SIGNIFICANCE AFTER MITIGATION With implementation of the proposed mitigation measure, significant impacts associated with geology and seismic hazards would not occur as a result of the proposed project. Page 181