3.5 Geology and Soils

Transcription

1 3.5 GEOLOGY AND SOILS The following reports have been prepared to analyze the geological impacts of the proposed project and are included in their entirety in Appendices F1 through F7 of this Draft Environmental Impact Report (DEIR): Geotechnical Feasibility Investigation. Prepared by Geotechnical Exploration, Inc. for Sea Breeze Properties, project No , September 23, 2008 (Appendix F1). Geotechnical Investigation, University Place Development, San Marcos, California. Prepared by Leighton and Associated for Lennar Communities, Leighton Project No , September 21, 2004a (Appendix F2). Geotechnical Investigation, University Center, San Marcos, California (bound report). Prepared by Geotechnics Incorporated for American Assets, ICG Job No , Log No R, December 31, 1990 (Appendix F3). Limited Rippability Study (letter report), University Center Site (various parcel numbers), Southwest of State 78 and Twin Oaks Valley Road, San Marcos, California. Prepared by Geotechnical Exploration, Inc., for Sea Breeze Properties, GEI Job No , June 20, 2005 (Appendix F4). Limited Rippability Study (letter report), University Center Site (various parcel numbers), Southwest of State Route 78 and Twin Oaks Valley Road, San Marcos, California. Prepared by Geotechnical Exploration, Inc., for Sea Breeze Properties, GEI Job No , July 20, 2005 (Appendix F5). Preliminary Geologic Reconnaissance (letter report), 17-Acre Property, East Barham Drive, San Marcos, California. Prepared by Southern California Soil & Testing, Inc. (SCS&T) for The McDonald Group, SCS&T No , Report No.1, October 1, 2003 (Appendix F6). Reconnaissance Rippability Study, University Place Development, San Marcos, California (letter report). Rubenfeld Commercial Parcels, West of Shubin Lane, San Marcos, California. Prepared by Geotechnical Exploration, Inc. (GEI), for Scottsdale Investment Management Group, GEI Job No , September 15, 2004 (Appendix F7) Environmental Setting Topography The proposed project site ranges from generally flat, to gently rolling ground to moderately steep hillsides. Elevations range from 550 to 710 feet above mean sea level (AMSL), with the highest point being on a single hill which occupies the southwest portion of the site. Figure depicts the existing topography on the project site and in the project vicinity. University District Specific Plan City of San Marcos

2 Soils The project site is located in the Peninsular Ranges geomorphic province of southern California, which is characterized by granitic crystalline rocks of the Cretaceous-age southern California batholith. Based on site surveys, field mapping, subsurface excavations, and review of existing literature and published San Diego soil survey. The project site substrata includes: Undocumented Artificial Fill (Afu), Quaternary Colluvium (Qal/Qcol) deposits on top of Cretaceous Granite (Kgr) bedrock. Soils are shown on Figure Undocumented Artificial Fill (Afu) The majority of the undocumented fill soils are in a small area in the central portion of the site, along Barham Drive, and in minor amounts at various other locations within site. The undocumented fill soils were likely placed on the site during previous site grading associated with construction of Barham Drive. Based on exploratory borings, these undocumented fill soils are expected to be up to approximately 8 feet thick in the central and southern portions of the site and range from 1 to 4 feet thick in other locations on the site. Colluvium (Qal/Qcol) Colluvial soils encountered overlying the entire site. Colluvium generally consists of red-brown to orange-brown, damp to moist, medium dense to very dense, silty sand to silty clayey sand that appeared to be massive and residual highly weathered bedrock. Geotechnical analysis indicates that with the exception of near-surface soils, the colluvium encountered in can be considered competent. The upper four feet of the colluvial soils were slightly porous and dry and are not considered suitable for support of improvements or additional fill. Cretaceous Granitic Rock (Kgr) The western portion of the project site is underlain by bedrock material consisting of Cretaceous-aged Granitic rock encountered at depths ranging from 7 to 23 feet. This material generally excavated to an orange brown to grayish brown, silty medium to coarse sand with clay. Sample drillings indicated granitic material at 2 to 4 feet thick, which could indicate localized residual boulders that may exist within the decomposed granite mass. Soils derived from the underlain material and bedrock on the project site include: Placentia sandy loam, (2 to 9 percent slopes), Grangeville fine sandy loam (0 to 2 percent slopes), Cieneba coarse sandy loam, (5 to 15 percent slopes, eroded), Exchequer rocky silt loam (9 to 30 percent slopes), Escondido very fine sandy loam, (9 to 15 percent slopes), Escondido very fine sandy loam, (15 to 30 percent slopes). These soils range from slow to rapid for runoff and slight to high for erosional qualities. Cieneba coarse sandy loam (C1D1), 5 to 15 percent slopes, eroded: The Cieneba series consists of excessively drained, very shallow to shallow coarse sandy loams. This soil form in material weathered in place from granitic rock, on rolling to hilly areas. Runoff is slow to medium, the erosion hazard slight to moderate, and the shrink-swell behavior is low. University District Specific Plan City of San Marcos

3 Project Area ExE Exchequer Rocky Silt Loam CID2 Offsite Area Cieneba Course Sandy Loam FaC Fallbrook Sandy Loam Grangeville Fine Sandy Loam San Marcos Blvd EsE2 Escondido Very Fine Sandy Loam PeC Placentia Sandy Loam EsC Escondido Very Fine Sandy Loam VsC Visalia Sandy Loam EsD2 Escondido Very Fine Sandy Loam PeC 78 San Marcos Creek ClD2 ClD2 Source: SSURGO; 2008 Eagle Aerials; 2008 G:\Projects\200595_SANMARCOS\67907_HEART\map_docs\mxd\EIR\ \Fig3.5-1Soils.mxd Last Updated : ² ExE EsE ,600 Feet EsD2 EsD2 EsC ExE ClD2 Twin Oaks Valley Rd. VsC PeC VsC FaC Soils FIGURE University District City of San Marcos Environmental Impact Report

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5 Escondido very fine sandy loam (EsD2), 9 to 15 percent slopes: This soil series in consists of moderately deep to deep, well-drained very fine sandy loams that form in material weathered in place from metamorphosed sandstone. The depth to hard metamorphic rock ranges from 20 to 35 inches. Permeability is moderate, with the average holding capacity 3 to 5.5 inches. Runoff is medium, the hazard of erosion moderate, and the shrink-swell behavior is low. Escondido very fine sandy loam (9EsE2), 15 to 30 percent slopes. This soil series in consists of moderately deep to deep, well-drained very fine sandy loams that form in material weathered in place from metamorphosed sandstone. This soil is moderately steep and is 20 to 34 inches deep over rock. Runoff is medium to rapid, the erosion hazard is moderate to high, and the shrink-swell behavior is low. Exchequer rocky silt loam (ExE), 9 to 30 percent slopes: This soil series consists of shallow and very shallow, well-drained silt loams that formed in material weathered from hard metabasic rock. This soil is on ridges and foot slopes, is strongly sloping to moderately step. These soils are on mountainous uplands and have slopes of 9 to 70 percent, with elevation ranges from 400 to 3,000 feet. Runoff is medium to rapid, erosion hazard is moderate to high, and the shrink-swell behavior is low. Grangeville fine sandy loam (), 0 to 2 percent slopes: This soil series consists of somewhat poorly drained, very deep fine sandy loams derived from granitic alluvium, formed on alluvial fans and alluvial plains with slope averaging 2 percent. Runoff is very slow, erosion hazard slight, and the shrink-swell behavior is low. Placentia sandy loam (PeC), 2 to 9 percent slopes: This soil series consists of moderately well drained sandy loams that have a sandy clay subsoil, formed in granitic alluvium. Permeability is very slow in the subsoil. Runoff is slow to medium, erosion hazard slight to moderate, and the shrink-swell behavior is high. Seismicity Seismic hazards that occur as a result of the presence of faults capable of generating earthquakes are: (1) strong ground motion/shaking; (2) ground rupture; (3) liquefaction; and (4) landslides. Ground motion occurs when faults rupture at depth where pressures are high and result in earthquakes. Ground rupture occurs when the fault ruptures at depth and movement along the fault propagates to the ground surface resulting in vertical and/or lateral displacement. Liquefaction and landslides are the result of ground motions where localized surface and subsurface ground unit conditions are susceptible to collapse or flow. The site can be considered to lie within a seismically active region, as can all of Southern California. Most seismic hazards on a site are a consequence of ground shaking caused by events on distant active faults. Seven active faults are located within 63 miles (100 kilometers) of the site. The nearest active regional fault is the offshore segment of the Rose Canyon Fault Zone located approximately 13.2 miles west of the site. There are no known active faults either on or adjacent to the project site Thresholds of Significance As defined in Appendix G of the CEQA Guidelines, project impacts to geological resources are considered significant if any of the following occur: University District Specific Plan City of San Marcos

6 a) Expose people or structures to potential substantive adverse effects, including the risk of loss, injury, or death involving: i) 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. ii) Strong seismic ground shaking? iii) Seismic related ground failure, including liquefaction? iv) Landslides? b) Result in substantial soil erosion or the loss of topsoil? c) 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 onsite or offsite landslide, lateral spreading, subsidence, liquefaction or collapse? d) 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? e) Have soils incapable of adequately supporting the use of septic tanks or alternative water disposal system where sewers are not available for the disposal of waste water? Project Impacts Seismicity The proposed project site is located in a seismically-active area, but there are no known active faults crossing immediately adjacent to the project site, as illustrated in Figure Most seismic hazards at the site are a consequence of ground shaking caused by events on distant, active faults. Shallow ground ruptures are considered unlikely as there are no active faults mapped crossing the site, and the site is not located within a mapped Alquist-Priolo Earthquake Fault Zone as shown on United States Geological Survey (USGS) and California Geological Survey (CGS) maps. Lurching and ground cracking are a possibility as a result of a significant seismic event on a nearby active fault. Future development on the project site would be subject to the requirements of the Uniform Building Code (UBC) for Zone 4 for resistance to seismic shaking. In addition, the proposed project will be constructed in accordance with other UBC criteria, current seismic design specifications of the Structural Engineers Association of California, other applicable regulations, ongoing site-specific geotechnical investigations, and all applicable requirements of the State of California Occupational Safety and Health Administration (Cal/OSHA). Therefore, impacts from seismic ground shaking would be less than significant. Liquefaction Liquefaction is a process in which soil grains in a saturated sandy deposit lose contact due to earthquakes or other sources of ground shaking. Granular soils underlain by near surface ground water table are most susceptible to liquefaction, while the stability of most clayey material are not adversely affected by vibratory motion. Based on the geological studies the liquefaction hazard of the relatively thin and dense University District Specific Plan City of San Marcos

7 Fallbrook 5 Vista Valley Center Oceanside Carlsbad 15 San Marcos Escondido Source: Data Source; YEAR G:\Projects\200595_SANMARCOS\67907_HEART\map_docs\mxd\EIR\ \Fig3.5-2Faults.mxd Last Updated : $ 5 Encinitas Solana Beach Del Mar Miles Project Location Poway La Mesa 78 Fault by Age Historic Santee Holocene Late Quaternary Quaternary Pre-Quaternary Ramona Fault by Type El Cajon Known, w/ Surface Expression Known, w/o Surface Expression Assumed Faults FIGURE University District City of San Marcos Environmental Impact Report

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9 colluvial soils, Santiago Formation and granitic rock underlying the site are typically not susceptible to liquefaction due to such factors as density and grain size distribution. Based on the soil type on the project site and lightly occurring groundwater impacts from liquefaction would be less than significant. Landslides Landslides are mass movements of the ground that include rock falls, relatively shallow slumping and sliding of soil, and deeper rotational or transitional movement of soil or rock. The proposed project site ranges in elevation from approximately 550 to 710 feet AMSL. Topography throughout the site is generally hilly with flatter areas associated with San Marcos Creek in the western portion and with a swale located in the eastern portion. The eastern half of the site is developed and slopes upward to the east. The project site is located in the Peninsular Ranges geomorphic province of southern California. The site is characterized by bedrock material consisting of Cretaceous-aged granitic rock encountered at depths ranging from 7 to 23 feet. The soils derived from the underlain material and bedrock includes: very fine to coarse sandy loams, rocky silt loams. Based on the underlying bedrock and the soil composition of the project site, impacts due to landslides would be considered less than significant. Erosion/Loss of Top Soil As identified above, the six different soil materials on the project site range from slight to high potential for erosion. The majority of the site occupies soil with slight to moderate erosional qualities. Since the slopes would be constructed using fill material generated from the cut portions of the site, many of the constructed fill slopes would be prone to superficial erosion, especially during periods of rain. The implementation of a project-specific Erosion Control Plan and a Storm Water Pollution Prevention Plan (SWPPP) in accordance with the California State Water Resources Control Board Order No DWQ, NPDES General Permit No. CAS as identified in the Preliminary Geotechnical Investigation. The SWPPP would comply with Best Available Technology (BAT) and Best Conventional Pollutant Control Technology to reduce or eliminate storm water pollution from areas of construction activity. During construction, soil erosion would be controlled and would reduce any erosional impacts to less than significant. Alternative Wastewater Disposal The proposed project would not be serviced by either septic tanks or an alternative wastewater disposal system. Vallecitos Water District provides water, wastewater, and water reclamation services to the city of San Marcos. Sewer infrastructure within the proposed project area would connect to existing Vallecitos Water District sewer facilities. Therefore, no impacts to the project site would occur from the addition of alternative wastewater disposal systems. Soil Instability - Expansive Soils The geologic units located on the project site consist predominantly of very-fine to course, well-drained sandy loams. These soils have a low potential for expansion. While the surface soils on the site may have relatively lower levels of cohesion, the underling materials (consisting of granitic bedrock) are substantially more stable. The project site occupies soils that have low shrink-swell qualities that would indicate a low potential for soil expansion. The addition of compacted engineered fill to developed portions of the site, lateral spreading, liquefaction, subsidence, or collapse are not expected to occur. University District Specific Plan City of San Marcos

10 Development on this type of soil would not create substantial risks to life or property. Therefore, impacts would be less than significant. Soil Instability Compressible soils A portion of the site area is underlain by potentially compressible soils. A portion of the site area is underlain by potentially undocumented fill, compressible alluvial, and colluvial soils which will require removal and replacement with properly compacted fill during the construction process as defined by the UBC/California Building Code (CBC). This represents a potentially significant impact CUMULATIVE IMPACTS Potential impacts related to geologic, seismic, and soils hazards are all site specific. All cumulative development would be subject to similar requirements to those imposed and implemented for the proposed project and would be required to adhere to applicable regulations, standards, and procedures. The proposed project would not result in significant cumulative impacts in regards to geology and soils Mitigation Measures GEO-1 For the portions of the project site that are underlain by potentially compressible soils, the soil shall be removed and replaced with properly compacted fill during the construction process as defined by the UBC/CBC. The fill material removed from the project site shall be tested and evaluated to meet design requirements Conclusion Based upon the analysis presented in Sections and 3.5.4, impact associated with seismicity, liquefaction, landslides, erosion/loss of topsoil, and expansive soils were determined to be less than significant. Impacts related to alternative disposal (e.g., septic) was determined to not have an impact. A potentially significant impact related to compressible soils was identified. A portion of the site area is underlain by potentially compressible soils. A portion of the site area is underlain by potentially undocumented fill, compressible alluvial, and colluvial soils which will require removal and replacement with properly compacted fill during the construction process as defined by the UBC/CBC. This will reduce the impact to below a level of significance. University District Specific Plan City of San Marcos