4.6 GEOLOGY AND SOILS

Transcription

1 LSA ASSOCIATES, INC. MARCH 2008 FINAL ENVIRONMENTAL IMPACT REPORT YUCCA VALLEY RETAIL SPECIFIC PLAN The purpose of the Geology and Soils section is to evaluate whether the proposed project would create a physical change in surface or subsurface soil or rock characteristics, or would expose people or structures to major geotechnical hazards. Changes could also include the damage or destruction of unique geologic/physical features. The following discussion of geotechnical issues is based on Geotechnical Engineering Investigation Proposed Wal-Mart Store # Gas Station and Future Retail Outparcels (Krazan & Associates, Inc., December 22, 2003). This report can be found in Appendix I Existing Setting The project site is located in the Yucca Valley, bounded by the San Bernardino Mountains to the northwest and the Little San Bernardino Mountains on the south. The San Bernardino and Little San Bernardino Mountain ranges are part of the Transverse Ranges Geomorphic Province. The portion of the San Bernardino Mountains located in the vicinity of Yucca Valley consists of quartz monzonite of late Mesozoic age. The Little San Bernardino Mountains are made up of gneissic rock of uncertain age. Quaternary alluvium covers a majority of the Yucca Valley floor. The geology in the vicinity of the project site consists of Holocene undivided alluvium made up of poorly consolidated gravel and sand derived from the mountain ranges to the east and south. The project site slopes gently to the north with the elevation on-site ranging from approximately 3,190 to 3,230 feet above mean sea level (AMSL). The project site is located in a seismically active region (Uniform Building Code Seismic Zone 4) and subject to strong to severe ground shaking. The local geology in the general vicinity of the project site includes the San Andreas, Johnson Valley, Burnt Mountain, Eureka Peak, and Pinto Mountain Faults. The project site is located approximately 15 miles from the San Andreas-Southern Fault, 2 miles from the Johnson Valley Fault, 1.5 miles from the Burnt Mountain Fault, 1 mile from the Eureka Peak Fault, and 0.43 mile from Pinto Mountain Fault. As indicated in Figure 4.6.1, no mapped faults are located within the limits of the project site. The site is, therefore, not in an Alquist-Priolo Zone. Soils on the proposed project site consist of primarily Hanford coarse sandy loam. Borings, ranging in depth from 10 to 50 feet below existing grade, were conducted at 79 locations throughout the project site. Free groundwater was not encountered in any of these borings Policies and Regulations State Regulations Alquist-Priolo Fault Zoning Act. Surface rupture is the most easily avoided seismic hazard. The Alquist-Priolo Special Studies Zones Act was signed into law in In 1994, it was renamed the Alquist-Priolo Earthquake Fault Zoning Act (A-P Act). The primary purpose of the A-P Act is to mitigate the hazard of fault rupture by prohibiting the location of structures for human occupancy across the trace of an active fault. Passage of this law was a direct result of the 1971 San Fernando Earthquake. Extensive surface fault ruptures during this earthquake damaged numerous homes, commercial buildings, and other structures. The A-P Act only addresses the hazard of surface fault rupture and not other earthquake hazards. R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc 4.6-1

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3 PAXTON AVENUE }þ 62 TWENTYNINE PALMS HIGHWAY SUNNYSLOPE DRIVE AVALON AVENUE S!N ,000 Feet FIGURE PROJECT AREA CALIFORNIA MAJOR FAULTS EUREKA PEAK FAULT PINTO MOUNTAIN FAULT SOURCE: AirPhotoUSA, 2006; California Geological Suvey, 2000 R:\YUC330\GIS\EIR\faults.mxd (07/05/07) Yucca Valley Retail Specific Plan Draft Environmental Impact Report California Major Faults

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5 LSA ASSOCIATES, INC. MARCH 2008 FINAL ENVIRONMENTAL IMPACT REPORT YUCCA VALLEY RETAIL SPECIFIC PLAN The A-P Act requires the State Geologist (Chief of the California Geological Survey) to delineate Earthquake Fault Zones along faults that are sufficiently active and well defined. Sufficiently active faults show evidence of Holocene surface displacement along one or more or their segments. Well-defined faults are clearly detectable by a trained geologist as a physical feature at or just below the ground surface. The boundary of an Earthquake Fault Zone is generally about 500 feet from major active faults and 200 to 300 feet from well-defined minor faults. The A-P Act dictates that cities and counties withhold development permits for sites within an Alquist-Priolo (A-P) Zone until geologic investigations demonstrate that the proposed structures are not threatened by surface displacements from future faulting. A-P Zone maps are distributed to all affected cities, counties, and State agencies for their use in developing planning policies and controlling renovation or new construction. Local agencies must regulate most development projects within the A-P Zones. Projects include all land divisions and most structures constructed for human occupancy. While State law exempts single-family woodframe dwellings and steel-frame dwellings that are less than three stories and are not part of a development of four units or more, local regulations may be more restrictive than State law. Before a project can be permitted within an identified Earthquake Fault Zone, cities and counties must require a geologic investigation to demonstrate that proposed buildings would not be constructed across active faults. A site-specific evaluation and written report site must be prepared by a licensed geologist. If an active fault is identified, a structure intended for human occupancy cannot be placed over the trace of the fault and must be set back, generally no closer than 50 feet from the fault. The Seismic Hazards Mapping Act. Passed in 1990, the Seismic Hazards Mapping Act (SHMA) addresses non-surface fault rupture earthquake hazards, including strong ground shaking, liquefaction, and seismically induced landslides. The California Geological Survey (CGS) is the principal State agency charged with implementing the 1990 SHMA. Pursuant to the SHMA, the CGS is directed to provide local governments with seismic hazard zone maps that identify areas susceptible to amplified shaking, liquefaction, earthquake-induced landslides, and other ground failures. The goal is to minimize loss of life and property by identifying and mitigating seismic hazards. The seismic hazard zones delineated by the CGS are referred to as zones of required investigation. Site-specific geotechnical hazard investigations are required by SHMA when construction projects fall within these areas. Natural Hazards Disclosure Act. Effective June 1, 1998, the Natural Hazards Disclosure Act requires that sellers of real property and their agents provide prospective buyers with a Natural Hazard Disclosure Statement when the property being sold lies within one or more State-mapped hazard areas. If a property is located in a Seismic Hazard Zone as shown on a map issued by the State Geologist, the seller or the seller s agent must disclose this fact to potential buyers. R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc 4.6-5

6 FINAL ENVIRONMENTAL IMPACT REPORT LSA ASSOCIATES, INC. YUCCA VALLEY RETAIL SPECIFIC PLAN MARCH 2008 Local Regulations Town of Yucca Valley Comprehensive General Plan Policies. The General Plan defines goals and policies related to geology and soils. The specific policies of these elements that are relevant to the proposed project are as follows: Seismic Safety Element Policy 7 In areas of high seismicity, the Town shall encourage the location of septic tank leach fields, seepage pits, and heavily irrigated areas away from foundations and other structural supports to minimize the creation of a localized liquefaction hazard. Slopes, Sediment Control and Soil Conservation Element Policy 5 In order to control soil disturbance and erosion, grading associated with all development plans shall be kept to the minimum necessary to provide for planned improvements, while maintaining maximum natural and undisturbed vegetation s of Significance According to CEQA Guidelines Appendix G, the proposed project would result in a significant impact if it would do any of the following: Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: o o o o Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area or based on other substantial evidence of a known fault; Strong seismic ground shaking; Seismic-related ground failure, including liquefaction; and/or Landslides. Result in substantial soil erosion or the loss of topsoil. Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction, or collapse. Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property. Have soils incapable of adequately supporting the use of septic tanks or alternative wastewater disposal systems where sewers are not available for the disposal of wastewater R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc

7 LSA ASSOCIATES, INC. MARCH 2008 FINAL ENVIRONMENTAL IMPACT REPORT YUCCA VALLEY RETAIL SPECIFIC PLAN Impacts and Mitigation Measures Less Than Significant Impacts The following impacts were determined to be less than significant. In each of the following issues, either no impact or a less than significant impact would occur (and, therefore, no mitigation would be required) or adherence to established regulations, standards, and policies would reduce potential impacts to a less than significant level. Rupture of a Known Earthquake Fault Would the proposed project expose people or structures to potential substantial adverse effect including the risk of loss, injury or death involving the rupture of a known earthquake fault? Primary ground damage due to earthquake fault rupture typically results in a relatively small percentage of the total damage in an earthquake, but being too close to a rupturing fault can cause profound damage. It is difficult to reduce this hazard through structural design. The primary mitigation technique is to set back from, and avoid, active faults. While the Town of Yucca Valley is located within one of the most seismically active regions of the State, the project site is not located within an Earthquake Hazard Zone established for the San Andreas Fault, Pinto Mountain Fault, or any other fault. Because the proposed project site is not located within an Earthquake Hazard Zone, and no known faults are present on the project site, the potential for on-site ground rupture is very low. Impacts associated with this issue are considered to be less than significant. Seismic-Related Ground Failure, Including Liquefaction Would the proposed project expose people or structures to potential substantial adverse effect including the risk of loss, injury or death involving seismic-related ground failure, including liquefaction? Liquefaction occurs when loose, unconsolidated, water-laden soils are subject to shaking, causing the soils to lose cohesion. The possibility of liquefaction occurring at a project site is dependent upon the occurrence of a significant earthquake in the vicinity; sufficient groundwater (within 50 feet of the ground surface) to cause high pore pressures; and on the grain size, plasticity, relative density, and confining pressures of the soil at the project site. On-site surface soils predominantly consist of medium dense to silty sand and sand with gravel. On-site surface soils are disturbed, are moderately strong, and are slightly to moderately compressible. Groundwater was not encountered within a depth of 50 feet during on-site boring operations. The potential for seismic-induced liquefaction within the limits of the project site is very low; therefore, potential impacts related to this issue are considered less than significant. Ground subsidence is the settling or shrinking of the ground or near surface soils with little or no horizontal movement. This can result from the extraction of oil, gas or water from beneath the earth s surface or from natural forces such as earthquakes (when loose unconsolidated soils settle) or hydrocompaction (when unsaturated soils become saturated and reorient into a more compact form). R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc 4.6-7

8 FINAL ENVIRONMENTAL IMPACT REPORT LSA ASSOCIATES, INC. YUCCA VALLEY RETAIL SPECIFIC PLAN MARCH 2008 No ground subsidence has been identified in the project vicinity; therefore, impacts are less than significant. Landslides Would the proposed project expose people or structures to potential substantial adverse effect including the risk of loss, injury or death involving landslides? On-site elevations range from approximately 3,190 to 3,230 feet AMSL. The project site slopes gently to the north. There are no landslides documented within the Town of Yucca Valley and its immediate vicinity; however, a maximum credible earthquake occurring along any of the faults within the Town can create the potential for seismically-induced rockfalls to occur. These rockfalls would occur within the mountain and hilly areas of the Town. The proposed project site is not located within a mountain or hilly area of Yucca Valley. As indicated in the Yucca Valley Comprehensive General Plan EIR, the proposed project site is located within an area with low susceptibility to rockslides/landfalls and other slope stability problems. The potential for landslides within the limits of the project site is very low; therefore, potential impacts related to this issue are considered less than significant. Soil Erosion or Loss of Topsoil Would the proposed project result in substantial soil erosion or the loss of topsoil? The construction of the proposed on-site uses would require the alteration of the site s existing topography and the removal or relocation of existing topsoil, which may facilitate or hasten the erosion of on-site soils. Construction projects resulting in the disturbance of 1 acre or more are required to obtain a NPDES permit issued by the Regional Water Quality Control Board (RWQCB) to control soil erosion due to stormwater. Construction project proponents are also required to prepare a Storm Water Pollution Prevention Plan (SWPPP). As discussed in the Section 4.3, Air Quality, the project would be required to comply with Mojave Desert Air Quality Management District (MDAQMD) Rule 402 and MDAQMD Rule 403 to control fugitive dust. As the requirements of Rules 402 and 403 have already been discussed in Section 4.3, Air Quality, the following is a summary of Rules 402 and 403. MDAQMD Rule 402 requires implementation of dust suppression techniques to prevent fugitive dust from creating a nuisance off-site. Rule 403 requires that fugitive dust be controlled so that the presence of such dust does not remain visible in the atmosphere beyond the property line of the emission source. Implementation of these dust suppression techniques, along with implementation of Best Management Practices (BMPs) required of all new development projects (i.e., mulching, watering, and vegetating of the project site) as specified by the NPDES permit and project-specific SWPPP, would reduce potential impacts associated with soil erosion and loss of topsoil to a less than significant level R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc

9 LSA ASSOCIATES, INC. MARCH 2008 FINAL ENVIRONMENTAL IMPACT REPORT YUCCA VALLEY RETAIL SPECIFIC PLAN Expansive Soils Would the proposed project be located on expansive soil, creating substantial risks to life or property? Expansive soils are those soils with a significant amount of clay particles that have the ability to give up water (shrink) or take on water (swell). When these soils shrink or swell, the change in volume exerts significant pressures on loads (such as buildings) that are placed on them. Expansive soil conditions (as defined in Table 18-1-B of the UBC, 1994), if not properly mitigated by site preparation and/or foundation design, can cause substantial damage to structures and other improvements over time. Soils located on-site consist of Hanford coarse sandy loam. These soils have a low shrink-swell (expansive) potential. Since soils on the project site have a low shrink-swell potential, impacts associated with expansive soils are considered less than significant. Potentially Significant Impacts The following impacts were determined to be potentially significant. In each of the following issues, a potential impact would occur and mitigation would be required. Impact Strong Seismic Ground Shaking Would the proposed project expose people or structures to potential substantial adverse effect including the risk of loss, injury, or death involving strong seismic ground shaking? Like most of southern California, the project site is located in a seismically active region (Uniform Building Code Seismic Zone 4) and subject to strong to severe ground shaking. The amount and intensity of ground shaking at a particular site is a function of both the distance to the active fault and the local geology. The San Andreas, Johnson Valley, Burnt Mountain, Eureka Peak, and Pinto Mountain faults lie in the general vicinity of the project site. The project is located approximately 15 miles from the San Andreas-Southern fault, 2 miles from the Johnson Valley fault, 1.5 miles from the Burnt Mountain fault, 1 mile from the Eureka Peak fault and 0.43 mile from Pinto Mountain fault. Strong ground shaking during a major earthquake can potentially damage buildings and result in physical injuries to individuals in such buildings. Physical injuries to individuals could occur during a seismic event if items stored on shelves were to fall. While the potential for items falling off shelves would be present within the proposed supercenter structure during a seismic event, such potential is present in typical retail environments. The project does not include any aspect which would cause a greater than normal risk of injury during a seismic event. Compliance with building conditions that would be required within Seismic Zone 4 would reduce the significance of risk to patrons from falling items to an acceptable level. The proposed project would place people and structures at risk from strong seismic ground shaking, due to its close proximity to several faults; therefore, the impact is significant. R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc 4.6-9

10 FINAL ENVIRONMENTAL IMPACT REPORT LSA ASSOCIATES, INC. YUCCA VALLEY RETAIL SPECIFIC PLAN MARCH 2008 Mitigation Measure. The following mitigation measure shall be implemented: 4.6.1A The design and construction of the proposed on-site uses shall adhere to the recommendations identified in the geotechnical investigation prepared for the proposed project; engineering standards detailed in the UBC for development within Seismic Zone 4; and/or other design requirements established by the Town. The geotechnical investigation prepared for this project identifies the potential for some degree of structural damage during seismic shaking, but the risk can be reduced through recommendations contained in the geotechnical investigation. These design and construction measures, from the geotechnical investigation, shall include the following: To minimize post-construction soil movement and provide a uniform support for the buildings, overexcavation and recompaction within the proposed footings shall be performed to a minimum depth of 3 feet below existing grades or 2 feet below the bottom of the proposed footings, whichever is deeper. The overexcavation and recompaction within the interior slab-on-grade areas shall be performed to a depth of 2 feet below existing grades or 1 foot below the bottom of the proposed slab sections, whichever is deeper. The overexcavation and recompaction shall also extend laterally 10 feet beyond the edges of the proposed footings. Within the proposed exterior flatwork and pavement areas, overexcavation and recompaction shall be performed to at least 12 inches below existing grade or finish grade, whichever is lower. Prior to placement of fill soils, the upper 8 inches of native soils shall be scarified, moisture-conditioned to near optimum moisture content, and recompacted to a minimum of 95 percent of maximum dry density based on ASTM D1557 Test Method. The proposed structures shall be supported on a shallow foundation system bearing on at least 2 feet of engineered fill. Shoring or sloping back trench sidewalls shall be required within these sandy soils. The proposed structure footings shall be designed utilizing an allowable bearing pressure of 2,500 pounds per square foot (psf) for dead-plus-live loads. Footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is deeper. Level of Significance after Mitigation. Adherence to recommendations contained in the geotechnical investigation along with the requirements of the UBC and Town/State development code would reduce potential impacts associated with damage to structures caused by earthquake to a less than significant level. The mitigation measure would ensure that building footings and foundations are designed to current seismic standards, which in turn would withstand an accepted magnitude of earthquake R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc

11 LSA ASSOCIATES, INC. MARCH 2008 FINAL ENVIRONMENTAL IMPACT REPORT YUCCA VALLEY RETAIL SPECIFIC PLAN Impact Unstable Soils Would the proposed project be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction, or collapse? One of the most common phenomena during earthquakes is the settlement of loose, unconsolidated soils. On-site soils are slightly to moderately compressible under saturated conditions. Compressible soils are expected to have a low bearing capacity and are subject to differential settlement. Loose fill materials present on-site may be vulnerable to hazards associated with settlement. Measures are identified below to mitigate the potentially significant impacts associated with settlement. Mitigation Measure. The following mitigation measure shall be implemented: 4.6.2A The design and construction of the proposed on-site uses shall adhere to the recommendations identified in the geotechnical investigation prepared for the proposed project. The geotechnical investigation prepared for this project identifies the potential for some degree of soil settlement, but the risk can be reduced through adherence to recommendations contained in the geotechnical investigation. These design and construction measures, from the geotechnical investigation, shall include the following: To minimize post-construction soil movement and provide a uniform support for the building, it is recommended that overexcavation and recompaction within the proposed footings be performed to a minimum depth of 3 feet below existing grades or 2 feet below the bottom of the proposed footings, whichever is deeper. The overexcavation and recompaction within the interior slab-on-grade areas shall be performed to a depth of 2 feet below existing grades or 1 foot below the bottom of the proposed slab sections, whichever is deeper. The overexcavation and recompaction shall also extend laterally 10 feet beyond the edges of the proposed footings. Within the proposed exterior flatwork and pavement areas, it is recommended that overexcavation and recompaction be performed to at least 12 inches below existing grade or finish grade, whichever is lower. Level of Significance after Mitigation. Adherence to recommendations described above and the requirements of the UBC and Town Seismic Design Code would reduce potential impacts associated with unstable soils and settlement caused by an earthquake to a less than significant level. R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc

12 FINAL ENVIRONMENTAL IMPACT REPORT LSA ASSOCIATES, INC. YUCCA VALLEY RETAIL SPECIFIC PLAN MARCH 2008 Impact Expansive and Collapsible Soils Would the proposed project be located on expansive soil, creating substantial risks to life or property? When saturated, collapsible soils undergo a rearrangement of their grains and the water removes the cohesive (or cementing) material. The result is rapid, substantial settlement. An increase in surface water infiltration, such as from irrigation or a rise in the groundwater table, combined with the weight of a building or structure, can initiate settlement and cause foundations and walls to crack. Soils located on-site consist of Hanford coarse sandy loam. The upper native soils on-site are moisture-sensitive and moderately compressible under saturated conditions. Excessive postconstruction settlement has occurred with structures in the vicinity of the project site. Measures are identified below to mitigate the potentially significant impacts associated with collapsible soils. Mitigation Measure. The following mitigation measure shall be implemented: 4.6.3A The design and construction of the proposed on-site uses shall adhere to the recommendations identified in the geotechnical investigation prepared for the proposed project, which identifies the potential for some degree of post-construction settlement. The risk can be reduced through adherence to recommendations contained in the geotechnical investigation. These recommendations include (but shall not be limited to) the following: Prior to the placement of fill soils, the upper 8 inches of native soils shall be scarified, moisture-conditioned to near optimum moisture content and recompacted to a minimum of 95 percent of maximum dry density based on ASTM D1557 Test Method. Native silty sand or silty sand soils/sand soils are suitable for reuse as engineered fill. Fill material shall be compacted to a minimum of 95 percent of maximum density based on ASTM D Test Method. Level of Significance after Mitigation. Adherence to recommendations described above and the requirements of the UBC and Town Seismic Design Code would reduce potential impacts associated with expansive and collapsible soils to a less than significant level. Impact Alternative Wastewater System Soil Capacity Would the proposed project have soils incapable of adequately supporting the use of septic tanks or alternative wastewater disposal systems where sewers are not available for the disposal of wastewater? The Town of Yucca Valley does not currently maintain a sewage collection system or wastewater treatment facility. On-site septic systems have been historically utilized by residences and businesses throughout the Town. Due to the amount of wastewater generation attributable to the project in conjunction with the high nitrogen levels in the local groundwater supply, the project will include on R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc

13 LSA ASSOCIATES, INC. MARCH 2008 FINAL ENVIRONMENTAL IMPACT REPORT YUCCA VALLEY RETAIL SPECIFIC PLAN site secondary effluent treatment with nitrogen removal. There will be no septic system included as part of the project. Therefore, the project will create no impact associated with inadequate soil permeability, depth to water table, and susceptibility to flooding in relation to use of septic tanks or alternative wastewater disposal. Level of Significance after Mitigation. Because there is no impact associated with this issue, mitigation measures are not required Cumulative Impacts Cumulative projects are shown in Chapter 2, Table 2.A and Figure 2.1. The cumulative impact area for geology and soils resources is the Town of Yucca Valley. Implementation of the proposed project would not result in potential impacts associated with ground failure, the rupture of a known earthquake fault, landslides, expansive soils, or soil erosion. There are no projects that would, in combination with the proposed project, result in any significant impact to seismic related ground failure, rupture of a known earthquake fault, landslides, expansive soils, or soil erosion. Although the proposed project would create potentially significant impacts associated with ground shaking, unstable soil conditions, collapsible soils, and the capability of soils to support a septic system, such impacts are reduced to a less than significant level with the implementation of mitigation measures. There are no other projects that would, in combination with the proposed project, produce significant impacts associated with ground shaking, unstable soil conditions, or collapsible soils. The project will include on-site secondary effluent treatment with nitrogen removal as part of its design. Because there will be no wastewater discharges from a septic system, there would be no cumulative project impact associated with septic discharges to groundwater and/or soils. In addition, the Hi-Desert Water District (HDWD) along with the Town of Yucca Valley is examining long-term solutions to its wastewater treatment needs, such as provision of a municipal wastewater treatment facility. Plans for a municipal wastewater treatment facility are currently being developed. According to the preliminary map provided on the HDWD website, the Phase I service area includes the project site. The Town would condition the project to install dry sewer facilities and connect to the wastewater treatment facility when it becomes available for use in the future. Implementation of such a facility would address long-term cumulative wastewater impacts. Therefore, there are no significant cumulative impacts associated with geology and soils. R:\YUC330\Final EIR\March 2008 version\eir Sections\4.6 Geology and Soils.doc

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