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5.3 Geology and Soils 5.3 GEOLOGY AND SOILS SUMMARY The impact analysis presented in this section evaluates project impacts related to geologic hazards. The geotechnical analyses prepared for the project concluded that the project site is suitable for the proposed uses from a geotechnical perspective. Mitigation measures are recommended which would reduce project impacts associated with expansive and corrosive soils and dust and soil erosion to a less than significant level. INTRODUCTION This section summarizes the geotechnical study that was prepared for the proposed project. Refer to Appendix 5.3 for the following geotechnical report: Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, Environmental Impact Report (EIR)-Level Review of Site Conditions for the At-Grade Railroad Crossing for Lyons Avenue Extension. 2010. This report summarize findings regarding existing geology, existing surficial deposits, potentially significant geologic and surficial impacts, and recommended mitigation measures for these impacts. The focus of the geotechnical report was for the Lyons Avenue at-grade rail crossing component. This component would involve new construction and grading of this area of the proposed project sites. The existing 13th Street at-grade rail crossing has already been constructed and developed. With implementation of the proposed project this component would have changes that would prohibit crossing, however, these changes would not require new earthwork which would require geotechnical analysis. EXISTING CONDITIONS Regional Geology The City of Santa Clarita is located in the Transverse Range Geomorphic Province of California, which is characterized by east-west trending mountains and faults. Sedimentary basins within the Transverse Range Geomorphic Province include the Ventura, Soledad, and Ridge Basins, and the San Fernando Valley. The Ventura, Soledad, and Ridge Basins are the result of the interplay of the San Andreas Fault and the Transverse Range fault systems. Seismic activity along the San Andreas Fault is in response to differential movement between the Pacific geologic plate (west of the fault) and the North American geologic plate (east of the fault). Transverse Range faults generally reflect crustal (reverse) faults. Impact Sciences, Inc. 5.3-1 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR 0122.027 March 2010 5.3 Geology and Soils The project site is located within the central part of the Transverse Ranges geomorphic province of Southern California, in the eastern portion of the Ventura Basin. The Ventura Basin has been tectonically down-warped in the geologic past to produce a large-scale synclinal structure in which a thick sequence of Cenozoic sediments has accumulated. At shallow depths, the subject site is underlain by sub-horizontal alluvium deposited in the flood plain southwest of Newhall Creek. These deposits are underlain by the Plio-Pleistocene, nonmarine Saugus Formation at depth. No faults or folds have been identified at the site on any published geologic map of the area.1 A geologic overview map can be seen in Figure 5.3-1, Geologic Overview Map. Bedding within the alluvial deposits is nearly horizontal. The geologic structure of the underlying Saugus Formation bedrock strikes roughly east to west and dips gently to the north. Geomorphology The site topography is dominated by paved surfaces and two sets of existing railroad tracks approximately 75 feet west of Newhall Creek. Ground surface elevations range from about 1,258 feet at the northernmost portion of the site to about 1,267 feet at a high point along the existing set of railroad tracks at the easternmost portion of the site. Geologic Units The project site is underlain entirely by Quaternary alluvium at shallow depths. Artificial fill and railroad ballast have been placed below the railroad tracks. Pavement and aggregate base have been placed beneath existing roadways. Details of these units are provided below and can be seen in Figure 5.3-2, Project Site Geology. Quaternary Alluvium Quaternary alluvium (Qal) underlies the subject road improvement areas and surficial fills. Based on data obtained by Allan E. Seward Engineering Geology, Inc., (AESEGI) for the proposed adjacent Old Town Newhall Library, this alluvium generally consists of interbedded layers of loose to dense, poorly graded sand, silty sand, and gravely sand. Interbedded layers of sandy silts and clays are also present. These materials are generally medium dense to dense and uncemented in the upper 30 to 40 feet and locally loose in the upper 10 feet. 1 Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, (2010) 4. Impact Sciences, Inc. 5.3-2 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR 0122.027 March 2010 Project Site Legend: 2000 1000 0 2000 n APPROXIMATE SCALE IN FEET SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2006 FIGURE 5.3-1 Geologic Overview Map 112-027•01/10 MAIN STREET Legend: Project Site Boundary 80 40 0 80 n APPROXIMATE SCALE IN FEET SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2010 FIGURE 5.3-2 Project Site Geology 112-027•02/10 5.3 Geology and Soils Artificial Fill Artificial fill (af) was apparently placed below the existing railroad tracks to elevate the tracks above the Newhall Creek flood plain. The engineering characteristics of this material are currently unknown. Railroad Ballast Railroad ballast (RB) consisting of crushed natural rock was placed to support and elevate the two sets of railroad tracks at the site. Groundwater Review of historic groundwater data from the Seismic Hazard Map for the Newhall Quadrangle, Robson (1972) and Los Angeles Flood Control District (LACFCD) water well records indicates that historic-high groundwater at the proposed project site is between 75 and 100 ft below ground surface (bgs). The locations of nearby water wells, as seen on Figure 5.3-3, Water Well Location Map, show that the nearest water well is located approximately 750 feet to the north of the project site. The AESEGI review of historic water well records also determined that historic ground water levels ranged from 33 feet bgs to 103 feet bgs, see Appendix 5.3. Temporary, perched ground water conditions may exist below Newhall Creek following periods of significant rainfall and runoff. During subsurface explorations for the adjacent Old Town Newhall Library site, exploratory borings were drilled to a maximum depth of 50 feet. These explorations did not encounter groundwater. Seismicity The project site is located in the seismically active Southern California region. Earthquake-related hazards typically include ground rupture, ground shaking, and ground failure. Faults identified as active or potentially active in published geologic literature are not known to be present within or adjacent to the subject site. However, the project site is situated in the seismically active Transverse Ranges and can be expected to experience strong ground shaking from earthquakes generated on active regional faults, as evidenced by the strong ground shaking generated by the January 17, 1994, Northridge earthquake (magnitude 6.7). Ground Rupture and Shaking Review of the Alquist-Priolo Earthquake Fault Zone Map for the Newhall Quadrangle, the Seismic Safety Element of the Los Angeles County General Plan, and published geologic maps indicates that no active or Impact Sciences, Inc. 5.3-5 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR 0122.027 March 2010 5.3 Geology and Soils potentially active faults traverse the subject site.2 Review of the site topography and aerial photographs listed did not reveal any lineaments or other indicators suggestive of faulting at the site. The nearest known active fault is the San Gabriel Fault, which is 3.7 kilometers (km), or 2.22 miles,3 from the site at its nearest point, as seen in Figure 5.3-4, Fault and Earthquake Epicenter Location Map and in Table 5.3-1, Summary of Nearby Faults described below. Table 5.3-1, summarizes potential earthquake sources near the site, including estimates of maximum seismic magnitude that are considered geologically feasible for these sources, per the State of California.4 Table 5.3-1 Summary of Nearby Faults Closest Distance to Project Site (km) Surface Projection Maximum Fault Name Surface Trace of Rupture Area Magnitude Slip Rate (mm/yr) San Gabriel 3.7 3.7 7.0 1.0 Holser 4.3 3.1 6.5 0.4 Northridge (E. Oak 5.1 5.1 6.9 1.5 Ridge) Santa Susana 7.1 0.0 6.6 5.0 Sierra Madre (San 9.9 4.7 6.7 2.0 Fernando) Verdugo 16.7 15.8 6.7 0.5 Oak Ridge (on shore) 17.8 17.8 6.9 4.0 San Cayetano 22.2 22.2 6.8 6.0 Sierra Madre 24.5 21.4 7.0 3.0 Simi-Santa Rosa 26.7 26.7 6.7 1.0 San Andreas 33.1 33.1 7.8 34.0 mm/yr = millimeters per year; km = kilometers Source: Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, 2010. See Appendix 5.3. 2 Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, (2010) 6. 3 1.0 kilometer is equal to 0.6 mile. 4 Determined through use of the US Geological Survey program, “Seismic Hazard Curves and Uniform Hazard Response Spectra.” Impact Sciences, Inc. 5.3-6 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR 0122.027 March 2010 Project Site Legend: 2000 1000 0 2000 n APPROXIMATE SCALE IN FEET SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2006 FIGURE 5.3-3 Water Well Location Map 112-027•01/10 Project Site Legend: 15.0 7.5 0 15.0 n APPROXIMATE SCALE IN MILES SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report – January 2010 FIGURE 5.3-4 Fault and Earthquake Epicenter Location Map 112-027•01/10 5.3 Geology and Soils The California Department of Mines and Geology defines active faults as those that have had surface displacement within Holocene time (about the last 11,000 years).
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