IV. ENVIRONMENTAL IMPACT ANALYSIS F. GEOLOGY & SOILS This Section describes the geologic conditions at and near the Project Site and identifies the types of geotechnical hazards associated with development of the site. Organization of this Section 1. INTRODUCTION 2. ENVIRONMENTAL SETTING a) Regulatory Setting i) State ii) City of Los Angeles iii) Anticipated Building Code Changes b) Existing Conditions i) Geologic Setting ii) Subsurface Geology at the Project Site (1) Artificial Fill (2) Alluvium (3) Bedrock iii) Groundwater iv) Faulting & Seismicity v) Liquefaction vi) Lateral Spreading vii) Landslides and Slope Instabilities viii) Expansive Soils ix) Subsidence and Settlement Page IV.F-1
3. ENVIRONMENTAL IMPACTS a) Threshold of Significance b) Project Impacts i) Seismic Fault Rupture ii) Strong Seismic Ground Shaking iii) Liquefaction and Liquefaction-induced Settlement and Lateral Spreading iv) Dewatering-Induced Settlement v) Expansive Soils 4. CUMULATIVE IMPACTS 5. MITIGATION MEASURES 6. LEVEL OF SIGNIFICANCE AFTER MITIGATION 1. INTRODUCTION This section contains information summarized from the following technical report, which can be found in Appendix IV.F: Report of Geotechnical Input for Environmental Impact Report Proposed Office Building and Parking Development, prepared by Van Beveren & Butelo, Inc., July 2008. 2. ENVIRONMENTAL SETTING a) Regulatory Setting i) State The State of California adopted the 2007 California Building Code (CBC), Volumes 1 and 2, which is based in part on the2006 International Building Code (IBC), on January 1, 2008. The State of California, Division of Mines and Geology (CMDG, now known as the California Geological Survey), adopted seismic design provisions in Special Publication 117 Guidelines for Evaluating and Mitigating Seismic Hazards in California on March 13, 1997. The Alquist-Priolo Geologic Hazards Zone Act was enacted by the State of California in 1972 to address the hazards and damage caused by surface fault rupture during an earthquake. The Act has been amended ten times and renamed the Alquist-Priolo Earthquake Fault Zoning Act, effective January 1, 1994. The Page IV.F-2
Act requires the State Geologist to establish earthquake fault zones along known active faults in the State. Cities and counties that include earthquake fault zones are required to regulate development projects within these zones. The Seismic Hazard Mapping Act of 1990 was enacted, in part, to address seismic hazards not included in the Alquist-Priolo Act, including strong ground shaking, landslides, and liquefaction. Under this Act, the State Geologist is assigned the responsibility of identifying and mapping seismic hazards zones. The State Seismic Safety Commission was established by the Seismic Safety Commission Act in 1975 with the intent of providing oversight, review, and recommendations to the Governor and State Legislature regarding seismic issues. The Commission has recently completed a review of the 1994 Northridge earthquake and has provided recommendations for changes in current laws. ii) City of Los Angeles The City of Los Angeles (the City) adopted the 2007 CBC, together with the 1997 UBC and a series of City amendments on January 1, 2008 as the City of Los Angeles Building Code (Building Code), Volumes 1 and 2. Volume 1 of the Building Code includes provisions for: 1) Foundations, Retaining Walls, and Expansive and Compressible Soils in Chapter 18; 2) Site Work, Demolition, and Construction in Chapter 33; and 3) Grading, Excavation, and Fills in a special Chapter 70 developed by and for the City. The City Grading Division of the Department of Building and Safety (LADBS) has also adopted Rules of General Application (RGAs), a series of Grading Standards that supplement the requirements of the Building Code. The RGAs include specific requirements of seismic design, slope stability, grading, foundation design, geologic investigations and reports, soil and rock testing, and groundwater. The LADBS is responsible for implementing the provisions of the Building Code and Grading Standards. The City requires that firms performing geotechnical investigations, sampling and testing have their laboratory certified by the LADBS Materials Control Section. The City s primary seismic regulatory document is the Safety Element of the City of Los Angeles General Plan, adopted November 26, 1996. The objective of the Safety Element is to protect occupants and equipment during various types and degrees of seismic events. In the Safety Element, specific guidelines are included for the evaluation of liquefaction, tsunamis, seiches, non-structural elements, fault rupture zones, and engineering investigation reports. The City s Emergency Operations Organization (EOO) helps to administer certain policies and provisions of the Safety Element. The EOO is a City department comprised of all City agencies, pursuant to City Administrative Code, Division 8, Chapter 3. The Administrative Code, EOO Master Plan and associated EOO plans establish the chain of command, protocols and programs for integrating all of the City s emergency operations into one unified operation. Each City agency in turn has operational protocols, as well as plans and programs, to implement EOO protocols and programs. A particular emergency or mitigation triggers a particular set of protocols that are addressed by implementing plans and programs. The City s emergency operations program encompasses all these protocols, plans and programs. Therefore, its programs are not contained in one comprehensive document. The Safety Element goals, objectives, and policies are broadly stated to reflect Page IV.F-3
the comprehensive scope of the EOO. As it pertains to tsunamis and other flood hazards, the Safety Element refers to the City s Flood Hazard Specific Plan, which addresses areas adjacent to hazards, agency involvement and coordination, and procedures to be implemented during an emergency. b) Existing Conditions i) Geologic Setting The Project Site is located in the southern San Fernando Valley, just north of the foothills of the Santa Monica Mountains at the northerly end of the Cahuenga Pass. The San Fernando Valley is an alluviumfilled basin, approximately 12 miles wide and 23 miles long. The alluvium is derived predominantly from bedrock materials comprising the Santa Monica Mountains to the south, the Santa Susana Mountains to the north, 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, which is characterized by east-west trending geologic structure including the nearby Santa Monica Mountains and the east-west trending San Fernando, Santa Susana, Simi, Santa Monica, and Hollywood faults. ii) Subsurface Geology at the Project Site The soil conditions at the Project Site were explored by drilling 11 borings to depths of 10 to 101 feet and 7 cone penetrometer tests to depths of 39 to 75 feet below the existing grade. Laboratory tests were performed on selected boring samples to aid in the classification of the soils. The Project Site is underlain by artificial (man-placed) fill and Pleistocene age alluvium that overlies Miocene age bedrock of the Topanga Formation. (1) Artificial Fill Artificial fill associated with previous Los Angeles County Metropolitan Transportation Authority (Metro) Red Line construction has been placed throughout the Project Site. The fill was observed and tested during placement based on the requirements set forth in the State of California, Department of Transportation (Caltrans) Standard Specifications. Construction of the Metro Red Line tunnel and station was not performed under the jurisdiction of the City 1. Although certification of the fill has not yet been provided by the City, because the Metro Red Line construction was not performed under City jurisdiction and was exempt from City ordinances, it is anticipated that the City would certify the fill for Project construction based on the prior compaction reports that were prepared during and after Metro Red Line construction. The fill is comprised of sand, silty sand, and sandy and silty clay. These materials appear to be derived from on-site natural soils and bedrock materials. The fill is firm and appears to be compacted. The fill soils encountered in the borings ranged in depths from 5 to 14 feet. Fill soils at other locations on the 1 Under State of California Public Utility Code Section 30100, Metro is exempt from local ordinances and is provided authority to do any act necessary, convenient or proper for the construction, improvement, maintenance, or use of all transit improvements that are under its jurisdiction, possession, or control. Page IV.F-4
Project Site may be deeper than encountered in the exploration performed for the referenced geotechnical report. (2) Alluvium Alluvial soils (alluvium) are natural, fluvial sedimentary deposits typically confined to stream channels or deposited on flood plains or alluvial fans. At the Project Site, these deposits are Quaternary age (Pleistocene and Holocene) and bury bedrock materials at the Project Site. The alluvium consists generally of all gradations from silty clay to sand and sandy gravel. (3) Bedrock The Project Site is underlain by sedimentary bedrock units of the Topanga Formation consisting of interbedded sandstone, siltstone, and shale. Bedrock was encountered during the site exploration at depths ranging from 31 to 79 feet beneath the surface. These deposits are marine in origin derived from offshore shoal, turbidite, and submarine fan deposits. The bedrock ranges generally from laminated to thickly bedded and moderately hard to moderately soft, but can be locally very hard and well cemented. The bedrock units are typically friable and moderately weathered. The cohesive bedrock is expansive. The Topanga Formation is intruded locally by mafic volcanic dikes in the region. The intrusives are generally of a massive diabase composition. The explorations and observations conducted by Van Beveren & Butelo, Inc. did not reveal volcanic rock units, although these units could be encountered at deeper depths than those explored for the purposes of the report. Sandstone bedrock units are considered non-expansive. iii) Groundwater The Project Site occupies the southerly limits of the San Fernando Valley Groundwater Basin, which consists of 112,000 acres and comprises 91 percent of the San Fernando Valley. Groundwater storage is generally within the coarse grained alluvial deposits that fill the valley floor under confined and unconfined conditions. An aquifer is a saturated geologic formation that yields usable quantities of water to a well or spring. A confining bed is a geologic unit that is relatively impermeable and does not yield usable quantities of water. Confining beds, also referred to as aquitards, restrict the movement of groundwater into and out of adjacent aquifers. Groundwater generally occurs in aquifers under confined or unconfined conditions. A confined aquifer is overlain by a confining bed, such as an impermeable layer of clay or rock. An unconfined aquifer has no confining bed above it and is usually open to infiltration from the surface. Wells were installed in four of the borings at the Project Site to permit measurements of the groundwater levels. Groundwater was observed within the alluvium and appears to be confined on top of the underlying bedrock. Fluctuations in the groundwater levels occur seasonally and following rainfall periods. The groundwater was measured in the wells at depths ranging from 21 to 31 feet below the ground surface, corresponding to elevations of 532 to 552 above mean sea level (msl). 2 In all of the 2 Fixed elevations are expressed in terms of feet above mean sea level (msl), where msl is defined as the level of the surface of the sea at its mean position midway between high and low tide. Page IV.F-5
wells, the groundwater gradient flows to the north towards the Los Angeles River Flood Control Channel (LARFCC). The LARFCC is a concrete-lined channel with a bottom elevation of approximately 530 above msl and is approximately 500 feet north of the Project Site at the point closest to the site. Based on a review of published information by the California Department of Mines and Geology, historically, the groundwater has been as high as 15 feet below existing grade. Groundwater level data measured in the wells at the Project Site on the dates indicated are presented in Table IV.F-1. Table IV.F-1 Groundwater Observation Well Readings Well (Boring) Depth Below Ground Surface Groundwater Elevation Number Date (ft) (ft-msl) 1 April 13, 2007 29.9 532 4 April 13, 2007 23.1 548 6 April 12, 2007 21.4 547 8 April 13, 2007 31.4 552 Source: Van Beveren & Butelo, 2008. iv) Faulting & Seismicity No known active faults are present through the Project Site, and the site is not located within a currently established Alquist-Priolo Earthquake Fault Zone for surface rupture hazard. The closest Alquist-Priolo Earthquake Fault Zone is approximately five miles to the northeast and is associated with the Verdugo fault. The numerous faults in Southern California include active, potentially active, and inactive faults. Classification for these major groups is based on criteria developed by the California Division of Mines and Geology (now known as the California Geologic Survey) for the Alquist-Priolo Earthquake Fault Zone Act Program. By definition, an active fault has ruptured within Holocene geologic time (about the last 11,000 years). Because no active faults are known to be located at the Project Site, surface rupture from fault plane displacement propagating to the surface is considered remote. Potentially active faults are those faults that display latest movement during Quaternary Geologic time where Holocene activity cannot be demonstrated. The Quaternary includes the Holocene and Pleistocene Ages and represents the last 1.6 million years of geologic time. Potentially active faults are not considered an imminent fault rupture hazard, but the potential cannot be completely dismissed. Inactive faults are those faults where the latest displacement is older than the Pleistocene and are not considered a surface rupture hazard to the Project Site. The closest active fault to the Project Site is the Hollywood fault located approximately 1.5 miles to the southeast at the southern base of the Santa Monica Mountains. The Hollywood fault is generally poorlydefined near the surface and has been located based on water well, oil well, and geophysical data, as well as near-surface trenching and drilling by numerous investigators. The Hollywood fault is considered Page IV.F-6
active, based on geomorphic evidence and fault trenching and drill hole correlation studies but is not included within an Alquist-Priolo Earthquake Fault Zone by the State Geologist. The Project Site is approximately 1.5 miles northwest of the boundary of the Elysian Park Fold and Thrust Belt. The Elysian Park fault is actually a blind fault (i.e., a buried fault that does not extend to the surface) capped by a fold and thrust structure. The axial trend of the fold extends approximately 12 miles through the Elysian Park-Repetto Hills from about Silver Lake on the west to the Whittier Narrows on the east. The 1987 Whittier Narrows earthquake (magnitude 5.9) has been attributed to subsurface thrust faults, which are reflected at the earth's surface by a west-northwest trending anticline known as the Elysian Park Anticline, or the Elysian Park Fold and Thrust Belt. The subsurface faults that create the structure are not exposed at the surface and do not present a potential surface rupture hazard. However, as demonstrated by the 1987 earthquake and two smaller earthquakes on June 12, 1989, the faults are a source for future seismic activity. As such, the Elysian Park Fold and Thrust Belt should be considered active features capable of generating future earthquakes. The active Mission Wells segment of the San Fernando fault zone is about 9 miles north of the Project Site. Surface rupture occurred along the Tujunga, Sylmar, and Mission Wells segments of the San Fernando fault zone during the February 9, 1971 San Fernando earthquake. The San Fernando fault zone comprises a number of left lateral/reverse frontal faults bounding the southern margin of the San Gabriel and Santa Susana Mountains. This fault slipped on February 9, 1971, causing an earthquake of magnitude 6.4. The Northridge Thrust fault is an inferred blind thrust fault that is considered the eastern extension of the Oak Ridge fault. This thrust fault is believed to be the causative fault of the January 17, 1994 Northridge earthquake. The Northridge Thrust fault is located beneath the majority of the San Fernando Valley. This thrust fault is not exposed at the surface and does not present a potential surface fault rupture hazard. However, the Northridge Thrust is an active feature that can generate future earthquakes. The Oak Ridge fault is a blind thrust fault located beneath the Santa Susana Mountains approximately 17 miles northwest of the Project Site. The fault associated with the 1994 Northridge earthquake is probably part of the Oak Ridge fault system, as it shares many of the characteristics of this fault. This blind thrust fault is known either as the Pico Thrust, named for the Pico Anticline (a geologic fold created by the fault), or as the Northridge Thrust. A list of known active and potentially active faults and their distances from the Project Site are shown on Tables IV.F-2 and Table IV.F-3, respectively. Several earthquakes of moderate to large magnitude (greater than 5.3) have occurred in the southern California area within the last 60 years. A list of these earthquakes is included on Table IV.F-4. Page IV.F-7
Table IV.F-2 Major Faults Considered to be Active in Southern California Maximum Credible Earthquake Distance From Site (miles) Slip Rate Fault (mm/yr) Hollywood 7.0 1.5 1.5 SSE Verdugo 6.75 0.5 5.0 NE Elysian Park Fold and Thrust Belt 7.1 1.7 1.5 SE Raymond 6.7 0.4 9.0 E San Fernando 6.8 5.0 9.0 N Northridge 6.9 1.5 >5.0 NW Newport-Inglewood Zone 7.0 1.0 7.0 S Santa Monica Mountains 7.2 4.0 1.5 S Whittier 7.1 3.0 18.0 SE Oak Ridge Pico Thrust 6.7 4.0 17 NW Sierra Madre 7.3 4.0 9.0 NE San Andreas (Mojave Segment) 8.2 30.0 30.0 NE Direction From Site Note: Site to fault distances measured using location of late Quaternary fault rupture map by Ziony and Jones, 1989 at a scale of 1:250,000. Source: Van Beveren & Butelo Inc., 2008. Table IV.F-3 Major Faults Considered to be Potentially Active in Southern California Maximum Credible Earthquake Distance From Site (miles) Slip Rate Fault (mm/yr) San Jose 6.7 0.5 28 ESE Chino 7.0 1.0 36 SE Duarte 6.7 0.1 30 NE Rialto-Colton 6.4 n/d 54 E Norwalk 6.7 0.1 22 SE Coyote Press 6.7 0.1 12 SE Los Alamitos 6.2 0.1 24 SE MacArthur Park 6.1 0.1 6 SW Overland 6.0 0.1 12 SW Charnock 6.5 0.1 10 SW Santa Susana 6.9 6.2 14 NW n/d = not determined Note: Direction From Site Site to fault distances measured using location of late Quaternary fault rupture map by Ziony and Jones, 1989 at a scale of 1:250,000. Source: Van Beveren & Butelo Inc., 2008. Page IV.F-8
Table IV.F-4 List of Major Historic Earthquakes in Southern California Fault Date of Earthquake Magnitude Distance From Site (miles) Direction to Epicenter Long Beach March 11, 1933 6.4 43 SSE San Fernando February 9, 1971 6.6 19 NW Whittier Narrows October 1, 1987 5.9 16 SE Sierra Madre June 28, 1992 5.4 20 E Big Bear June 28, 1992 6.4 86 E Landers June 28, 1992 7.3 98 E Northridge January 17, 1994 6.7 12 W Note: Site to fault distances measured using location of late Quaternary fault rupture map by Ziony and Jones, 1989 at a scale of 1:250,000. Source: Van Beveren & Butelo Inc., 2007. Peak ground accelerations were determined for three earthquake levels, those representing ground motions having a 10 percent probability of exceedance during a 50-year time period (the Design Basis Earthquake), a 10 percent probability of exceedance in a 100-year time period (the Upper Bound Earthquake), and a 2 percent probability of exceedance in a 50-year time period (the Maximum Considered Earthquake). The Project Site is not exposed to a greater than normal seismic risk than other areas of Southern California. However, based on the active and potentially active faults in the region, the Project Site could be subjected to significant ground shaking in the event of an earthquake. This hazard is common to Southern California. v) Liquefaction Soil liquefaction results from loss of strength during cyclic loading, such as imposed by earthquakes. When seismic ground-shaking occurs, the soil is subject to seismic shear stresses that may cause the soil to undergo deformations. If the soil undergoes virtually unlimited deformation without developing significant resistance, it is said to have liquefied. When soils consolidate during and following liquefaction, ground settlement occurs. Soils most susceptible to liquefaction are clean, loose, saturated, uniformly graded, fine-grained sands. Shallow groundwater is considered a factor as it creates the saturated condition of the soil. The Project Site is located within a State of California designated Liquefaction Hazard Zone. The liquefaction potential at the Project Site was evaluated for ground motion resulting from the Design Basis Earthquake. The liquefaction potential was evaluated using the results of the standard penetration test (SPTs) performed in the site exploration and laboratory tests. The results of the analyses indicate that liquefaction potential exists at the Project Site. Susceptible soils were encountered at various depths between the historic-high groundwater depth of 15 feet and a depth of up to approximately 60 feet. The total liquefaction-induced settlement of the existing alluvial soils would be on the order of 0.25 to 3.25 inches across the site (refer to Table IV.F-5). Page IV.F-9
Table IV.F-5 Summary of Estimated Seismic Settlements (Total Settlement) Existing Conditions Location Seismic Settlement Existing Conditions Site A 1 to 3 inches Site B 0.25 to 2.5 inches Site C 1.75 to 3.25 inches Source: Van Beveren & Butelo Inc., 2008. vi) Lateral Spreading Lateral spreading typically occurs as a form of horizontal displacement of relatively flat-lying alluvial material toward an open or free face such as an open body of water, channel, or excavation. Generally in soils, this movement is due to failure along a weak plane, and may often be associated with liquefaction. As cracks develop within the weakened material, blocks of soil displace laterally toward the open face. Cracking and lateral movement may gradually propagate away from the face as blocks continue to break free. Lateral spreading can occur within areas having potential for liquefaction, including those areas at the Project Site. vii) Landslides and Slope Instabilities The area of the proposed buildings is generally near level with surface elevations ranging from 562 feet above mean sea level in the northeasterly portion of the site to 585 feet in the southerly portion of the Project Site, south of the Hollywood Freeway, an overall relief of approximately 23 feet. With the exception of a 10-foot high descending 2:1 fill slope westerly of Site B and minor fill slopes along the Hollywood Freeway, existing slopes and bluffs are nonexistent. viii) Expansive Soils The more clayey soils within the natural alluvium and fill soils are subject to minor expansion and shrinkage resulting from changes in the moisture content. Tests on samples of the clay soils indicate that the Expansive Index can range up to approximately 38, which is a low expansion potential. ix) Subsidence and Settlement The Project Site is not located within an area of known subsidence associated with fluid withdrawal (groundwater or petroleum), peat oxidation, or hydrocompaction. Therefore, subsidence is not considered a significant hazard at the Project Site. However, the existing non-engineered fills at the Project Site may be weak and compressible, particularly with the addition of water. These fills are subject to settlement and are not suitable for support of foundations, slabs on grade, paving, or new compacted fills. Cut slopes in these fills are subject to sloughing and failure because of the low shear strength of these fills. Page IV.F-10
3. ENVIRONMENTAL IMPACTS a) Threshold of Significance The City of Los Angeles CEQA Thresholds Guide (2006) requires the geotechnical analysis to address the following areas of study: (1) geologic hazards; (2) sedimentation and erosion; (3) landform alteration; and (4) mineral resources. i) Geologic Hazards The City of Los Angeles CEQA Thresholds Guide (page E.1-4) states that 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. This threshold is applicable to the Project and as such is used to determine if the Project would have significant impacts related to geologic hazards. ii) Sedimentation and Erosion The City of Los Angeles CEQA Thresholds Guide (page E.2-3) states that a project would normally have significant sedimentation or erosion impacts if it would: Constitute a geologic hazard to other properties by causing or accelerating instability from erosion; or Accelerate natural processes of wind and water erosion and sedimentation, resulting in sediment runoff or deposition which would not be contained or controlled on-site. An analysis of impacts related to soil erosion is discussed in Section IV.M.2, Water Quality, of this EIR. This analysis demonstrates that the Project would not result in substantial erosion or deposition of materials off-site. iii) Landform Alteration The Project s Initial Study (see Appendix I-1 to this EIR) concluded that, with respect to landform alteration, no significant impact would occur and that further evaluation of this issue in an EIR is not required. iv) Mineral Resources The Project s Initial Study (see Appendix I-1 to this EIR) concluded that, with respect to mineral resources, no significant impact would occur and that further evaluation of this issue in an EIR is not required. Page IV.F-11
b) Project Impacts i) Seismic Fault Rupture (1) Sites A, B, and C As noted above, the closest active fault to the Project Site is the Hollywood fault located approximately 1.5 miles to the southeast at the southern base of the Santa Monica Mountains. The Hollywood fault is considered active, based on geomorphic evidence and fault trenching and drill hole correlation studies but is not included within an Alquist-Priolo Earthquake Fault Zone by the State Geologist. As such, the potential for surface fault rupture from the Hollywood Fault would be minimal and the Project would not cause or accelerate geologic hazards or expose people to substantial risk of injury. Impacts related to seismic fault rupture would be less than significant. (2) Sites D and E Sites D and E would be developed with surface parking lots and would not include structures that would expose people to substantial risk of injury. Impacts on Sites D and E related to seismic fault rupture would be less than significant. ii) Strong Seismic Ground Shaking (1) Sites A, B, and C The Project Site is located in a seismically active region, and development of the Project would expose future users of the Project Site to seismic groundshaking. Seismic groundshaking could damage the buildings, parking areas, and utility infrastructure. However, the Project Applicant would be required to design and construct the Project in conformance to the 2008 City Building Code. Conformance with current Building Code requirements would minimize the potential for structures on the Project Site to sustain damage during an earthquake event. The Project would thus not cause or accelerate geologic hazards or expose people to substantial risk of injury. Therefore, Project impacts related to groundshaking would be less than significant. (2) Sites D and E Sites D and E would be developed with surface parking lots and would not include structures that would expose people to substantial risk of injury. Impacts on Sites D and E related to groundshaking would be less than significant. iii) Liquefaction and Liquefaction-induced Settlement and Lateral Spreading (1) Sites A, B, and C As discussed previously, the Project Site is located in an area designated as potentially liquefiable on the Seismic Hazard Maps of the State California Burbank Quadrangle. Van Beveren & Butelo, Inc. analyzed the potential for seismic settlement resulting from liquefaction at the Project Site. Based on this analysis, the potential seismic settlement at the Project Site would vary from 0.25 inch to 2.75 inches. The seismic settlement estimates for each exploration, grouped by Sites A, B, and C are presented on Table IV.F-6. Page IV.F-12
Table IV.F-6 Summary of Estimated Seismic Settlements (Total Settlement) Existing and Post-Project Conditions Location Seismic Settlement Existing Conditions Seismic Settlement Proposed Construction Site A 1 to 3 inches 0.25 to 1 inch Site B 0.25 to 2.5 inches 0.25 to 2.5 inches Site C 1.75 to 3.25 inch 1.75 to 2.75 inch Source: Van Beveren & Butelo Inc., 2008. Based on anticipated Project Site conditions, the potential for ground failures associated with liquefaction (i.e., lateral spreading, differential settlement, post-liquefaction reconsolidation, and sand boils) would be present on the Project Site. The excavation for the Project s subterranean levels would remove at least some of the potentially liquefiable deposits, but some may remain. As such, the Project could potentially cause or accelerate geologic hazards or expose people to substantial risk of injury and Project impacts related to liquefaction would be significant. (2) Sites D and E Sites D and E would be developed with surface parking lots and would not include structures that would expose people to substantial risk of injury. Impacts on Sites D and E related to liquefaction would be less than significant. iv) Dewatering-Induced Settlement (1) Sites A, B, and C (a) Temporary Dewatering Groundwater was measured in borings at depths of 21 to 31 feet below the ground surface. Historically, the groundwater level has been as shallow as 15 feet. Excavations of up to 70 feet deep are planned under the Project and, therefore, dewatering of the excavations that would extend below groundwater would be required. The dewatering would need to be performed prior to excavation to avoid disturbance of the soils at foundation level. Pump tests would be performed prior to design of the temporary dewatering system. Pump tests performed as part of Metro Red Line construction and presented in the 1993 report prepared by Converse Consultants are applicable and would be reviewed as part of the final design. Construction of the underground tunnel and station for the Metro Red Line station lowered the groundwater to a maximum depth of up to 70 feet below the surface. The dewatering would be performed around the existing Metro Red Line tunnel and adjacent to the existing Campo De Cahuenga and would be close to Weddington Park (South) and City View Lofts. The dewatering is likely to cause some settlement, and the possible effect of that settlement on the existing structures must be considered in the design of the dewatering program. Dewatering can cause settlement of the ground surface as the soils below groundwater level transition from buoyancy to a drained state. Since the drained soils have a greater density than the submerged Page IV.F-13
(buoyant) soils, the stresses in the ground below the groundwater level increase, resulting in settlement. The Project Site was previously dewatered to depths of up to 70 feet for the construction of the existing Metro Red Line tunnel and station. In addition, seasonal fluctuations in the groundwater levels over several thousands of years have already subjected the Project Site to such settlement. Therefore, additional settlements are in the rebound range of the soil consolidation, meaning that the settlement would be significantly less than when it was first subjected to the load (stress) technically referred as the virgin consolidation. Depending on the spacing and depth of the dewatering wells, the quantity of water being pumped, and the shape of the drawdown curve, the groundwater levels on sites adjacent to the Project Site could also be affected, resulting in settlement on the adjacent Campo De Cahuenga historic site, Weddington Park (South), and City View Lofts including the frontage streets of Campo De Cahuenga Way, Lankershim Boulevard, Valleyheart Drive, and Bluffside Drive. The Metro Red Line tunnel is within the bedrock, and therefore, settlement of this structure would be negligible. Although the settlement would be largely limited to the area where the groundwater is lowered, the lateral extent of the settlement could occur to about five to six times the depth of the dewatering. The estimated maximum total settlement is approximately 1.5 inches within the Project Site and would decrease beyond the property lines. The precise groundwater profile (drawdown curve) would be determined by the pump tests. With this consideration, if the groundwater is lowered from 20 to 70 feet, the estimated total settlement of the adjacent sites can vary up to about 1.5 inches. The settlement would occur primarily within the alluvium; very little settlement is anticipated within the bedrock. The settlement of the Metro Red Line tunnel is expected to be less than 0.25 inch. This limited amount of settlement would not adversely affect the tunnel. The Campo de Cahuenga historic building is located approximately 100 feet from Site A, where dewatering would occur. If the dewatering wells are located close to the edge of Site A and if the depths of the dewatering wells are restricted so the groundwater is not lowered at the wells more than about 10 feet below the excavation, it is estimated that the settlement of adjacent properties would range from about 0.0 to 1.5 inches. The settlement would decrease to zero within approximately 300 feet of the Project Site; the resulting differential settlement would be approximately 0.5 inch within 100 feet, which should be acceptable to most structures, and adjacent public streets and facilities. It is estimated the differential dewatering settlement of the Campo de Cahuenga structure would be less than 0.5 inch, which is relatively small and would not result in any major structural distress. Implementation of mitigation measures set forth below would reduce this impact to less than significant. The quantity of water to be pumped from each site has been estimated, assuming that a permanent dewatering system is installed at each of the buildings. The estimated quantities are presented in Table IV.F-6 under Permanent Dewatering, below. The pumping quantities for the temporary system would vary from the permanent system because the dewatering wells are not continuous and each well lowers the groundwater to a greater depth to achieve a dry site between the wells. The temporary dewatering quantities could be up to two times the values presented in Table IV.F-7. Page IV.F-14
Disposal of groundwater pumped during dewatering activities and the potential effects of dewatering activities on the movement of water ground pollutants are discussed in Section IV.L.2, Water Quality of this EIR. Table IV.F-7 Estimated Pumping Quantities Location Depth Groundwater Lowered Estimated Pumping Quantity Site A 45 feet 200 gallons per minute Site B 15 feet 100 gallons per minute Site C 5 feet 50 gallons per minute Source: Van Beveren & Butelo Inc., 2008. (b) Permanent Dewatering Two options are under consideration for design of the permanent structures: 1. Design the subterranean portions of the Project to resist the hydrostatic pressure, 3 or 2. Install a subdrain behind the basement walls. Under Option 1, the subterranean portion of the structures would be designed to resist the hydrostatic pressures. For design purposes, the groundwater level is assumed to be equal to the historic high of 15 feet below existing grade. The pressure would be equal to that developed by a fluid with a density of 62.4 pounds per cubic foot and would be in addition to the earth pressure. The basement walls and floor slabs would be thoroughly waterproofed, and the waterproofing behind the basement walls would extend to the adjacent ground surface. Under Option 2, a subdrain would be required behind the basement walls and below the lowest level floor slab to prevent damage to the basement walls and floor slab resulting from hydrostatic pressures. Because of the anticipated quantities of water to be pumped and the desire to limit the depletion of groundwater beneath the Project Site, the subterranean portion of the structures at Site A would be designed to resist hydrostatic pressure. However, subdrains are being considered as an option beneath Sites B and C. The method of draining the basement walls would depend on the method of constructing the basement walls. If the walls are constructed directly against a shored embankment, a flat proprietary drainage product could be placed directly against the shoring. In this scenario, the water collected at the base of the walls would be collected in a gravel-surrounded perforated pipe and directed into a subdrain below the lower level floor slab. The City does not allow the use of flat drainage products alone, and thus the use of gravel-filled pockets with weep holes draining through the wall below the slab would also be required. 3 The pressure at any given point of a non-moving (static) fluid is called the hydrostatic pressure. The pressure at a given depth in a static liquid is a result the weight of the liquid acting on a unit area at that depth plus any pressure acting on the surface of the liquid. Page IV.F-15
The floor subdrain would consist of at least six inches of drainage material with drain lines about 40 feet on centers extending about one foot below the drainage material. The drainage material would meet the requirements of Class 2 Permeable Material as defined in the current Caltrans Standard Specifications. The subdrain would connect to a sump equipped with automatic pumping units. The flow rate of water to be pumped from each of the sites has been estimated, assuming that permanent subdrain systems are installed. The Converse Report indicated that the permeability of alluvium within the Metro Red Line station site varies from about three feet per day to seven feet per day. Assuming an average permeability of about five feet per day, the estimated infiltration rates for each of the sites, including Site A, are presented in Table IV.F-6. Quantities for Site A are included only for purposes of estimating pumping quantities from Site A during construction, as discussed above under Temporary Dewatering. The above flow rates are estimates only. The actual flow rates would need to be determined prior to construction after performing pump tests at each of the sites, in order to accurately reflect the soil and groundwater conditions present at the site at the time pumping would begin. In addition, the estimated quantities would be reassessed after reviewing the construction dewatering performed to permit the excavation for each of the sites, also to reflect the most current conditions at the time pumping commences. As the Project site would have already been dewatered using the temporary dewatering, any dewatering-inducing settlement would particularly occur during the construction phase. No additional settlement is anticipated due to permanent dewatering. 1. Sites D and E Sites D and E would be developed with surface parking lots and would not include construction activities requiring dewatering. Impacts on Sites D and E related to dewatering induced settlement would be less than significant. ii. Expansive Soils 1. Sites A, B, and C As discussed previously, the clayey soils within the natural alluvium and fill soils at the Project Site are subject to minor expansion and shrinkage resulting from changes in moisture content. However, expansion potential that could affect the proposed structures and infrastructure would be mitigated through compliance with the recommendations in existing and future geotechnical reports prepared for the Project and approved by the Department of Building and Safety as part of the normal building permit process. These recommendations include excavation and replacing the upper one foot of clay soil beneath the lightly-loaded slabs on grade with a one-foot thick layer of non-expansive soil. Therefore, Project impacts related to expansive soils would be less than significant. 2. Sites D and E Sites D and E would continue to be used as surface parking lots and would not include structures that would expose people to substantial risk of injury. Impacts on Sites D and E related to expansive soils would be less than significant. Page IV.F-16
4. 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. Therefore, cumulative geology and soils impacts would be less than significant. 5. MITIGATION MEASURES Because the Project would be subject to liquefaction and settlement, the following mitigation measures are required: F-1: Site-specific geotechnical reports shall be prepared for each phase of the Project prior to construction. The reports shall address site preparation, fill placement, and compaction, foundations, pavement design, footings, and pile foundations. F-2 During construction of the Project, a qualified representative of the Project s geotechnical engineer shall be present on-site to ensure implementation of all relevant requirements outlined in the final geotechnical reports. F-3: F-4: F-5: Geotechnical observation and testing shall be completed during the placement of new compacted fills, foundation construction, buttresses, stabilization fills, ground improvement, and any other geotechnical-related construction. The geotechnical firm performing these services shall be approved by the City. To mitigate potential settlement impacts on the Campo de Cahuenga historic site, structural monitoring, including surveys, shall be performed during dewatering to determine the condition and susceptibility of the building to settlement distress. If during construction, the settlement is observed to cause distress to the building, the building shall be repaired to mitigate the distress. The design of the temporary dewatering system shall be performed by a qualified dewatering contractor and be based on pump tests performed from wells installed at the Project Site. 6. LEVEL OF SIGNIFICANCE AFTER MITIGATION With implementation of the mitigation measures listed above, Project impacts related to geology and soils would be less than significant. Page IV.F-17