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HomeMy WebLinkAbout02160029_Sec05-05 Geology and Soils Project Impacts West Ming Specific Plan - Draft EIR Geology and Soils Michael Brandman Associates 5.5-1 H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc 5.5 - Geology and Soils 5.5.1 - Introduction This section evaluates the potential impacts on the proposed project from the existing geology and soils conditions on the project site. Information in this section is based on the following documents. • Geotechnical Feasibility Study / Geological Hazard Study, Soils Engineering Inc., February 5, 2004. The complete report is contained in Appendix F of the Technical Appendices of the Draft EIR. • Addendum #1 to Geotechnical Feasibility Study and Geological Hazard Study for the West Ming Specific Plan (Sections 11, 13, 14 & portions of 10 &15) In Bakersfield, California. The complete report is contained in Appendix F of the Technical Appendices of the Draft EIR. • Metropolitan Bakersfield General Plan, December 2002. This document is available for review at the City of Bakersfield Planning Department at 1715 Chester Avenue in Bakersfield, CA 93301. 5.5.2 - Environmental Setting Soils and Geology The majority of the project site is relatively flat and there is no evidence of historic landslides. There are no bedrock outcrops present within 1 mile of the site. Some lower areas in the western portion of the project site are utilized as recharge areas. The elevation change across the site is approximately 10 to 15 feet with a slight downhill slope to the southwest. A portion of the Kern River Canal and recharge basins also traverse the northern portion of the site. The project site rests on a considerable thickness of alluvium identified as Recent Quaternary Fan Deposits (Qf). Surface soils within the project site have been classified according to the Unified Soils Classification System based on 43 test borings drilled at the project site to a maximum depth of 52 feet below ground surface. Soils at the Project site consist of interbedded silty sand, clayey sand, sandy clay, poorly graded sand, and well-graded sand overlying bedrock. The surface soils are in a loose to medium-dense condition and are typical of the south west area of Bakersfield. Soils encountered in the top 5 feet on site indicate that highly expansive soil will not be encountered on the project site. Faulting and Seismicity The project site is not within an Alquist-Priolo Earthquake Fault Zone. No active faults are located within the site boundaries or adjacent properties. A subsurface oil field defined fault is located within the southeast portion of the project site and is considered inactive. Active faults which have the most potential to produce ground shaking at the Project site are listed in Table 5.5-1 below with the closest fault being: Kern Front Fault (15.4 kilometers (9.6 miles) northwest). Estimated peak ground accelerations at the project site are based mainly on fault distance and magnitude. Therefore, faults which are closest to the site or have large maximum credible magnitudes or both are included in Project Impacts Geology and Soils West Ming Specific Plan - Draft EIR 5.5-2 Michael Brandman Associates H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc Table 5.5-1 below. The White Wolf and San Andreas Faults have produced the majority of the seismic activity in the area of the project site, and the largest estimated historical site acceleration at the project site is 0.235g from a 7.7 magnitude earthquake on the White Wolf Fault in July 1952. The closest earthquake to the site was approximately 19 kilometers (12 miles) away at a magnitude of 5.2 in May 1993. It is not likely that ground rupture would occur at this site since it is not located within 500 feet (0.095 mile) of a known active fault trace. No Seismic Source Type A faults are within 5 kilometers (3.1 miles) of the project site which are capable of producing large magnitude events of greater than or equal to earthquakes of 7.0 magnitude and a slip rate of 5 millimeters (0.2 inches) or less per year. Likewise, no Seismic Source Type B earthquake faults are located within 10 kilometers of the site which would produce earthquake events greater than or equal to 7.0 magnitude and a slip rate of less than 5 millimeters annually, or an earthquake magnitude of less than 7.0 and a slip rate greater than 2 millimeters (0.08 inches) annually, or a magnitude greater than or equal to 6.5 and a slip rate less than 2 millimeters annually. The nearest Seismic Source Type B fault is the White Wolf Fault located approximately 26.7 kilometers (16.6 miles) away. The nearest Seismic Source Type A fault is the San Andreas Fault which is approximately 47.4 kilometers (29.5 miles) away. The Project site is in Seismic Zone 4, pursuant to the California Building Code, based on its location to the other know active faults. Table 5.5-1 shows that the estimated maximum peak ground acceleration at the site would be 0.24g from a 6.3 magnitude earthquake on the Kern Front Fault. A 7.2 magnitude earthquake on the White Wolf Fault would result in an estimated ground motion of 0.233g at the site. A maximum probable earthquake of magnitude 7.8 on the San Andreas Fault would create a peak site ground acceleration of 0.168g at the site. Table 5.5-1: Active Faults Within the Project Area Fault Approximate Distance (Kilometers/Miles) Maximum Earthquake Magnitude (Mw)1 Maximum Peak Ground Acceleration Gravity (g) Estimated Site Intensity (MM)2 Seismic Source Type Kern Front 15.4/9.6 6.3 0.246 IX C White Wolf 28.4/17.6 7.2 0.233 IX B Pleito Thrust 41.9/26 7.2 0.152 VIII B San Andreas (1857 Rupture & Carrizo) 47.4/29.5 7.2 to 7.8 0.110 to 0.168 VII to VIII A Big Pine 57.6/35.8 6.7 0.059 VI B Garlock (West) 59.2/36.8 7.1 0.078 VII A San Gabriel 72.4/45 7.0 0.056 VI B San Juan 76.2/47.4 7.0 0.053 VI B 1 Richter Scale Magnitude 2 Mercale Scale Intensity Source: Soils Engineering, Incorporated, 2004 Project Impacts West Ming Specific Plan - Draft EIR Geology and Soils Michael Brandman Associates 5.5-3 H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc Liquefaction According to the Kern County Water Maps (Kern County Water Agency 2003), the depth to the unconfined aquifer in the fall of 2000 was approximately 80 feet below ground surface. Shallow groundwater was not encountered in the top 51 feet below ground surface in the 43 test borings conducted in 2004 by SEI. Limited perched water can be present in areas of heavy irrigation, septic systems and recharge areas. Based on the soil types present and lack of shallow groundwater, the liquefaction potential for most of the site is considered low. Due to the presence of water in the northwestern portion of the site in 2005, SEI conducted , a liquefaction analysis on soil samples collected from soil boring B12-A, which was taken from the northwest corner of the project site. Groundwater was encountered at a depth of 15 feet below ground surface at that location, and subsurface soils were classified as sandy silt. SEI determined that the soils in the northwestern portion of the site are potentially liquefiable between 15 feet and 17.5 feet below ground surface. SEI indicated that normal groundwater levels are greater than 50 below ground surface, however, groundwater was present at higher elevations due to the substantial amount of surface water in the Kern River and in the recharge ponds in that location. Settlement A settlement analysis was prepared by SEI in April 2005. The analysis states that the total dry sand settlement calculated for the top 15 feet of soil is 0.441 and settlement was 0.34 inches for the saturated zone between 15 feet and 40 feet for a total settlement of 0.781 inches. According to SEI, 0.781 inches of settlement is considered very minor. In addition, the amount of settlement in the potentially liquefiable zone between 15 and 17.5 feet was 0.06. 5.5.3 - Thresholds of Significance According to Appendix G of the State CEQA Guidelines, a project would normally have a significant effect on the environment if it would: • Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: - 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; Refer to Division of Mines and Geology Special Publication 42. - Strong seismic ground shaking; - Seismic-related ground failure, including liquefaction; - Landslides; • Result in substantial soil erosion or the loss of topsoil; Project Impacts Geology and Soils West Ming Specific Plan - Draft EIR 5.5-4 Michael Brandman Associates H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc • 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; or • 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 waste water. 5.5.4 - Project Impacts and Mitigation Measures The proposed project would have the potential to expose people and structures to seismic ground shaking, ground failure, soil erosion, and unstable soils, Following is a discussion of the project impacts that correspond to the thresholds previously identified in Section 5.5.3. Impact 5.5.A: The proposed project could expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: 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 landslides. The project will change the use of the project site from agricultural, vacant, and oil production uses to urban uses. During periods of construction, there will be temporary increases in human activity on the project site from the presence of construction crews. Over the long-term, there will be a permanent increase in the level of human activity on the project site. The increased levels of human activity of the project site will increase the potential exposure of persons living and working on the project site to seismic events including risk of loss, injury, and death related to earthquakes and related hazards which are described as follows: Fault Rupture Although the project site is within the vicinity of several active faults, it is not located within an Alquist Priolo Earthquake Zone, and no Seismic Source Type A or B earthquake faults are within proximity to the project site which could produce potentially significant impacts on site. The project site is located within Seismic Zone 4 of the 2001 California Building Code, and the California Building Code contains criteria for building design related to Seismic Zone 4 which can be incorporated into project design and will reduce impacts to onsite structures related to earthquake events. These criteria include setbacks from the high pressure lines currently located on the project site. The nearest Seismic Source Type A fault is the San Andreas Fault, located 29.5 miles from the site, and would be considered potentially significant if located within 15 miles of the project site. Likewise, the nearest Seismic Source Type B fault is the White Wolf Fault, located 17.6 miles from the project site, and would be considered potentially significant if located within 10 miles of the project site. Furthermore, the nearest Alquist-Priolo Earthquake fault zone is approximately 15.4 kilometers (9.6 miles) northeast and will not significantly affect the use of the project site. Therefore, Project Impacts West Ming Specific Plan - Draft EIR Geology and Soils Michael Brandman Associates 5.5-5 H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc the adverse effects from a rupture of a known active earthquake fault at this site are less than significant. Seismic Ground Shaking In the event of an earthquake on one of the nearby faults (see Table 5.5-1), it is likely that the project site will experience ground shaking and exposing people and structures associated with the project to ground shaking, as well as the existing high pressure petroleum and gas lines. The Design-basis Earthquake ground motion for the project site is estimated at 0.27g based on the proximity and potential earthquake magnitude of the closest known active fault. An estimated ground motion of 0.235g occurred at the site from a 7.7 magnitude earthquake on the White Wolf Fault in July 1952; and, the White Wolf Fault has produced most of the historical earthquakes in the vicinity of the project site. Based on the estimated ground motion, the site is in a Seismic Zone 4 and will require structural design incorporated in building plans for individual projects meeting the requirements of the California Building Code related to Seismic Zone 4. With these requirements, the site preparation and structural foundations will be built to withstand 0.27g at the project site from an earthquake on the nearest faults. Improvement plans will be reviewed by the City of Bakersfield during site plan review for individual projects built within the Specific Plan to ensure that the recommendations of the Geotechnical Engineer (SEI 2004) are included in project design. Likewise structural setbacks from the high pressure lines present on the project site will be enforced during the City of Bakersfield site plan review process. This will ensure that the structures will withstand most earthquake events and the existing high-pressure lines located on the project site will not impact the proposed uses. Therefore, seismic ground shaking on the project site is considered less than significant. Seismically-Induced Ground Failure and Landslides There is a low potential for rock fall and landslides to impact the site in the event of a major earthquake (SEI 2004). Based on the predicted maximum horizontal accelerations at the project site and the soil types identified by SEI (2004), minor subsurface settlement may occur on site during a major earthquake, and this is considered less than significant. Ground failure from liquefaction could occur in the northwestern area of the project site where the recharge areas are currently located. This is due to the granular (non-cohesive) consistency of the soils and the presence of a potentially high groundwater table. The results of the liquefaction analysis for this area (SEI 2005) indicates that the soils in the northwestern area are potentially liquefiable between 15 feet and 17.5 feet below ground surface and are non-liquefiable below a depth of 17.5 below ground surface. Therefore, implementation of the project in the northwestern portion of the site could be significantly affected by liquefaction. The remainder of the site would not be subject to liquefaction impacts due to the depth of groundwater of more than 50 feet below ground surface. Seismically-Induced Flooding The potential for earthquake-induced flooding at the site appears to be very low (SEI 2004). In addition, the project includes the extension of the existing levee along Ming Avenue to eliminate the Project Impacts Geology and Soils West Ming Specific Plan - Draft EIR 5.5-6 Michael Brandman Associates H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc potential for flooding to occur on the project site during a 100-year flood event. The implementation of the proposed levee will reduce the potential for seismically-induced flooding to less than significant. Mitigation Measures 5.5.A.1 Concurrently with submittal of a tentative subdivision map application or other development plan in the northwestern corner of the project site, whichever occurs first, the project applicant shall conduct additional liquefaction studies in the northwestern corner of the project site (i.e., in the vicinity of the existing recharge ponds) during recharge periods to fully evaluate liquefaction impacts on specific development projects in this area of the site. Based on the findings of these studies, site specific designs shall be incorporated in the grading and building plans to reduce onsite liquefaction impacts. The scope of the liquefaction studies, findings, and recommendations to reduce liquefaction shall be reviewed and require approval by the City of Bakersfield Public Works and Building Departments prior to grading and building plan approvals. Level of Significance After Mitigation Less than significant. Impact 5.5.B: The proposed project could result in substantial soil erosion or the loss of topsoil. During the construction phase of the project, activities such as grading and construction will disrupt surface vegetation and soils and will expose these disturbed areas to erosion by wind and water. The proposed project will be developed in phases and portions of the project site will have disturbed areas while other portions will be under current agricultural and oil activities or will have already been converted to the land uses proposed in the West Ming Specific Plan. Erosion during construction activities could be significant. In the long-term and after construction activities have been completed on the project site, the ground surface will have impermeable surfaces as well as permeable surfaces. The impermeable surfaces will include roadways, driveways, parking lots, pavement treatment surrounding homes, as well as the home and building sites. The permeable surfaces will include landscape areas as well as the detention/retention basins. Landscaping will stabilize the permeable areas while the detention/retention basins are designed to retain soils. Overall, development of the project would not result in conditions where substantial surface soils would be exposed to wind and water erosion. Therefore, the project site would experience less than significant long-term impacts from water and wind erosion and loss of top soil. Mitigation Measures 5.5.B.1 Prior to grading plan approval, an erosion control plan for construction activities that describe the best management practices (BMPs) that will be used to reduce the Project Impacts West Ming Specific Plan - Draft EIR Geology and Soils Michael Brandman Associates 5.5-7 H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc potential for soil erosion and loss of top soil. The erosion control plan shall be submitted to the City of Bakersfield Public Works Department for review and approval. The BMPs could include soil stabilizers and silt fencing as well as other measures. Level of Significance After Mitigation Less than significant. Impact 5.5.C: The proposed project is not 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. There is no evidence of landslides on the project site, and the project site is not located in an unstable geologic unit or on soil that is considered unstable. A settlement analysis was prepared by SEI in April 2005. The analysis states that the total dry sand settlement calculated for the top 15 feet of soil is 0.441 and settlement was 0.34 inches for the saturated zone between 15 feet and 40 feet for a total settlement of 0.781 inches. According to SEI, 0.781 inches of settlement is considered very minor and less than significant. In addition, the amount of settlement in the potentially liquefiable zone between 15 and 17.5 feet was 0.06 which is also considered less than significant. The potential for hydro-collapse was also evaluated by SEI. According to SEI’s findings, the potential for hydro-collapse is low when land has be irrigated. Because the project site has been farmed since the 1970s, the potential for hydro-collapse is low and considered less than significant. Mitigation Measures No mitigation measures are required. Level of Significance After Mitigation Less than significant. Impact 5.5.D: The proposed project is not located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994) and would not creating substantial risks to life or property. Based on the type of soils encountered in the top 5 feet of soil at the 43 exploratory borings conducted on the project site, SEI determined that it is likely that no significant areas of highly expansive soils will be encountered. Therefore, implementation of the proposed project would result in a less than significant impacts related to expansive soils. Mitigation Measures No mitigation measures are required. Level of Significance After Mitigation Less than significant. Project Impacts Geology and Soils West Ming Specific Plan - Draft EIR 5.5-8 Michael Brandman Associates H:\Client (PN-JN)\0216\02160029\DEIR 9-1\02160029_Sec05-05 Geology and Soils.doc Impact 5.5.E: The project does not include the use of septic tanks or alternative wastewater disposal systems; therefore, the proposed project would not be affected by the soil capability of adequately supporting the use of septic tanks or alternative wastewater disposal systems. Septic tanks or alternative wastewater disposal systems are not proposed. The project will include lateral connections to City of Bakersfield sewer mainlines. Therefore, no impacts would occur. Mitigation Measures No mitigation measures are required. Level of Significance After Mitigation No impacts.