5.3 GEOLOGY AND SOILS

Transcription

1 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 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 13 th 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 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

2 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 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

3 Project Site Legend: n APPROXIMATE SCALE IN FEET SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report January 2006 FIGURE Geologic Overview Map /10

4 MAIN STREET Legend: Project Site Boundary n APPROXIMATE SCALE IN FEET SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report January 2010 FIGURE Project Site Geology /10

5 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 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

6 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 Summary of Nearby Faults Closest Distance to Project Site (km) Fault Name Surface Trace Surface Projection of Rupture Area Maximum Magnitude Slip Rate (mm/yr) San Gabriel Holser Northridge (E. Oak Ridge) Santa Susana Sierra Madre (San Fernando) Verdugo Oak Ridge (on shore) San Cayetano Sierra Madre Simi-Santa Rosa San Andreas mm/yr = millimeters per year; km = kilometers Source: Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, See Appendix Allan E. Seward Engineering Geology, Inc., Geologic and Geotechnical Report, (2010) 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 Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

7 Project Site Legend: n APPROXIMATE SCALE IN FEET SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report January 2006 FIGURE Water Well Location Map /10

8 Project Site Legend: n APPROXIMATE SCALE IN MILES SOURCE: Allan E. Seward Engineering Geology, Inc., Geological and Geotechnical Report January 2010 FIGURE Fault and Earthquake Epicenter Location Map /10

9 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). Potentially active faults are ones that have had surface displacement during the last 1.6 million years; inactive faults have not had surface displacement within the last 1.6 million years. Seismically induced ground acceleration is the shaking motion that is produced by an earthquake. Probabilistic modeling is done to predict future ground accelerations. Review of the Seismic Hazard Evaluation Report for the Newhall Quadrangle indicates that the peak horizontal ground acceleration (PHGA) with a 10 percent chance of exceedance in 50 years for alluvial conditions at the project site is about 0.8 gravity (g). The potential PHGA at the project site equal to two-thirds of the peak acceleration with a 2 percent chance of exceedance in 50 years is 0.54 g (as defined in the 2007 California Building Code [CBC]). Ground Failure Ground failure is a general term that refers to secondary, permanent ground deformation caused by strong earthquake shaking, including liquefaction of saturated granular deposits or fine-grained soils with low plasticity, lateral spreading, ground lurching, seismic settlement (dynamic densification) of loose, poorly consolidated materials, differential materials response, slope failure, sympathetic movement on weak bedding planes or non-causative faults, and shattered ridge effects. Liquefaction is defined as the transformation of a granular material from a solid state to a liquefied state as a consequence of increased pore water pressure. During ground shaking, the alluvial grains are packed into a tighter configuration. Pore water is squeezed from between the grains, increasing the pore pressure. As the pore pressures increases, the load bearing strength of the material decreases. There are areas within the City of Santa Clarita that overlie unconsolidated alluvium with a high groundwater table. These areas are primarily found near the Santa Clarita River and its tributaries. The project area is underlain primarily by alluvium, which is found throughout the canyon bottoms and river areas of the City. Alluvium is silt deposited by creeks and rivers. The major hazard associated with alluvium is liquefaction. Alluvium tends to be unconsolidated, meaning there is space between the grains of soil. If alluvium is sufficiently saturated, and an earthquake occurs, the soil can take on more liquid characteristics, damaging structures. According to the Seismic Hazard Map for the Newhall Quadrangle, the project site is not located in a zone in which investigation of liquefaction potential is required. The depth to historic high ground water at the site is greater than 50 feet. The potential for liquefaction and associated seismic settlements and lateral spreading is therefore considered very low. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

10 BasedonreviewoftheseismichazardmapfortheNewhall Quadrangle, the subject site is not located in a zone in which investigation of potential for earthquake-induced landslides is required. Potential at the site for slope failures and shattered ridge effects is considered nonexistent due to the flat nature of the site. Potential at the site for differential materials response and slippage along weak, inclined bedding planes is considered to be negligible because the site is underlain by consistent, flat-lying alluvial deposits. Slope Stability Earthquake-induced slope failures include activation and reactivation of landslides, rock falls, debris falls, and surface failures. No slopes greater than 10 feet in height exist at the project site or are proposed on the current site plan. A 4-foot-high slope exists on the southwestern margin of elevated alignment for the railroad tracks, which will be eliminated at Lyons Avenue to construct the proposed project. The bank of Newhall Creek located approximately 75 feet northeast of the railroad tracks is approximately 10 feet high. Soil Compressibility/Consolidation Based on explorations for the adjacent Newhall Library site, the alluvium in this area is typically medium dense to depths of 30 to 40 feet, with local loose zones in the upper 10 feet. A maximum of 5 feet of fill is currently proposed above this material. The density of the artificial fill placed below the railroad tracks is currently undefined. Dam Inundation No dams currently exist in the Newhall Creek Drainage and the site is not in a dam inundation area per the Flood and Inundation Hazard Map (Plate 6) of the Los Angeles County Safety Element of the General Plan. REGULATORY FRAMEWORK Federal Regulations No specific federal regulations were identified that impact the geology and soils considerations. State and local regulations (e.g., building codes) reflect national and international building codes; this is discussed below. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

11 State Regulations Alquist-Priolo Earthquake Fault Zoning Act The Alquist-Priolo Special Studies Zones Act 5 seeks to mitigate the hazard of fault rupture by prohibiting the location of structures for human occupancy across the trace of an active fault. The main purpose of the act is to prevent the construction of buildings used for human occupancy on the surface trace of active faults. The law requires the State Geologist to establish regulatory zones (known as Earthquake Fault Zones or Special Studies Zone) around the surface traces of active faults and to issue appropriate maps. The act was renamed in 1994 to the Alquist-Priolo Earthquake Fault Zoning (APEFZ) Act. The nearest known active fault is the San Gabriel Fault, which is 3.7 km from the site at its nearest point. The APEFZ Act mandates that cities and counties (lead agencies) require that within an Earthquake Fault Zone (EFZ) geologic investigations be performed to demonstrate that potential development sites are not threatened by surface fault displacements from future earthquakes. To aid the various jurisdictions that function as lead agencies for project approvals in California, the California Geological Survey (CGS) must delineate Earthquake Fault Zones on standard U.S. Geological Survey topographic maps (1-inch-equals feet scale) along faults that are "sufficiently active and well defined" as defined in the APEFZ Act. Quoting from the implementation guide, Special Publication 42, 6 Zone boundaries on early maps were positioned about 660 feet (200 meters) away from the fault traces to accommodate imprecise locations of the faults and possible existence of active branches. The policy since 1977 is to position the EFZ boundary about 500 feet (150 meters) away from major active faults and about 200 to 300 feet (60 to 90 meters) away from well-defined, minor faults. Exceptions to this policy exist where faults are locally complex or where faults are not vertical. Lead agencies are responsible to regulate most development projects within the Earthquake Fault Zones as described in the APEFZ Act, but may enact more stringent regulations. Certain smaller residential developments can be exempt. 5 Details regarding the Alquist-Priolo Earthquake Fault Zone Act can be found at /rghm/ap/index.htm. Accessed in January Details regarding the Alquist-Priolo Earthquake Fault Zone Act can be found at /rghm/ap/index.htm. Accessed in January Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

12 Seismic Hazards Mapping Act The Seismic Hazards Mapping Act 7 (SHMA) addresses the primary earthquake hazard, strong groundshaking, as well as the secondary hazards of liquefaction, earthquake-induced landslides, and, in some areas, zones of amplified shaking. As with the APEFZ Act, the CGS is the primary state agency charged with implementing SHMA, and CGS provides local jurisdictions with seismic hazard zone maps that identify areas susceptible to liquefaction, earthquake-induced landslides, and amplified shaking. Site-specific hazard investigations are required by the SHMA when a development project is located within one of the Seismic Hazard Mapping Zones (SHMZ) defined as a zone of required investigation. Lead agencies with the authority to approve projects shall ensure that: The geotechnical report shall be prepared by a registered civil engineer or certified engineering geologist, having competence in the field of seismic hazard evaluation and mitigation. The geotechnical report shall contain site-specific evaluations of the seismic hazard affecting the project, and shall identify portions of the project site containing seismic hazards. The report shall also identify any known off-site seismic hazards that could adversely affect the site in the event of an earthquake. And Prior to approving the project, the lead agency shall independently review the geotechnical report to determine the adequacy of the hazard evaluation and proposed mitigation measures and to determine the requirements of Section 3724(a), above, are satisfied. Such reviews shall be conducted by a certified engineering geologist or registered civil engineer, having competence in the field of seismic hazard evaluation and mitigation. CGS Special Publication (SP) and companion volumes for implementation of the SP 117 process (one volume for liquefaction and one volume for earthquake-induced landslides) 9 provide detailed guidance for lead agencies to review SHMA reports. The overall goal is to protect the public by minimizing property damage and the loss of life. 7 Details regarding the Seismic Hazards Mapping Act can be found at /SHMPpgminfo.htm. 8 California Geological Survey, Special Publication (SP) 117: Guidelines for Evaluating and Mitigating Seismic Hazards in California; 9 Recommended Procedures for Implementation of SP 117; /LiquefactionproceduresJun99.pdf and Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

13 California Building Code The 2007 CBC 10 includes additions to the previous building code that make it more stringent, in particular with regard to seismic and earthquake conditions for critical structures such as essential facilities, public schools and hospitals. The CBC, which is included in Title 24 of the California Administrative Code, is a compilation of three types of building standards derived from three different sources: Those adopted by state agencies without change from building standards contained in national model codes (e.g., the International Building Code [IBC]) Those adopted and adapted from the national model code standards to meet California conditions (e.g., most of California is Seismic Design Categories D and E) Those authorized by the California legislature, which constitute extensive additions not covered by the model codes that have been adopted to address particular California concerns (e.g., the specification of Certified Engineering Geologist rather than engineering geologist) International and national model code standards adopted into Title 24 apply to all occupancies in California except for modifications adopted by state agencies and local governing bodies. Facilities and structures such as power plants, freeways, emergency management centers, and dams are regulated under criteria developed by various California and federal agencies. Local City of Santa Clarita Unified Development Code All grading and excavation must comply with Chapters to (Division 3) of the City of Santa Clarita Unified Development Code (UDC). Rules and regulations contained within these chapters provide for the control of excavation, grading, and earthwork construction, including fills or embankment activities. During the grading permit application process, the City Engineer may require engineering geological and soil reports, as well as seismic hazard zone studies be prepared for proposed developments. The engineering geological report would require an adequate description of the geology of the site, along with conclusions and recommendations regarding the effect of geologic condition of any proposed development. Soil reports would be required to characterize the existing soil resources on a site, and provide recommendations for grading and design criteria. Development in seismic hazard zone will require studies that evaluate the potential for seismically induced liquefaction, soil instability, and earthquake induced landslides to occur on a site. 10 California Building Code, Title 24, Part 2. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

14 The City of Santa Clarita enforces structural requirements of the building code, the Alquist-Priolo Special Studies Zones, and sound engineering and geotechnical practices in evaluating structural stability of proposed new development. PROJECT IMPACTS Significance Threshold Criteria According to Appendix G of the State CEQA Guidelines, the 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; 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 wastewater. The proposed project is a public improvement project that will not utilize wastewater facilities or septic tanks. Consequently, no further analysis is required for this threshold. The City of Santa Clarita Local CEQA Guidelines (Resolution 05-38) adopted on April 26, 2005, also serve as the basis for identifying thresholds to determine the significance of the environmental effects of a project on this resource area and have been included for analysis. There will be a significant impact if the project were to include movement or grading of earth exceeding 100,000 cubic yards. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

15 The proposed project would grade approximately 6,000 cubic yards of fill. As this quantity is less than 100,000 cubic yards, no further analysis is required for this threshold. Impact Analysis The following geotechnical aspects of the proposed project were evaluated in this impact analysis. The grades at the northeast end of Lyons Avenue and adjacent portions of Railroad Avenue will need to be raised in order to tie into the existing grade at the railroad crossing. Retaining walls may be used to accommodate the changes from the proposed grades to adjacent properties proposed to remain at existing grades. Threshold 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; or landslides Rupture is primarily of concern where a project site overlies or is immediately adjacent to a known fault. No known faults are located within the project area with the nearest known fault 3.7 km from the project site. Therefore, the impacts related to the rupture of a known earthquake fault would be less than significant. The proposed project is located in Southern California, a geologically and tectonically active region, where large magnitude, potentially destructive earthquakes are common. Therefore, ground motions from moderate or large magnitude earthquakes could affect the project site during the design life of the project. The nearest fault is the San Gabriel Fault located 3.7 km northeast of the project area. Other faults in the vicinity include the Holser and Santa Susana. The most likely significant event in the area could occur along the San Andreas Fault, located 16 miles northeast of the City. As discussed above in Seismicity, the PHGA with a 10 percent chance of exceedance in 50 years for alluvial conditions at the project site is about 0.8 g. The potential PHGA at the project site equal to two-thirds of the peak acceleration with a 2 percent chance of exceedance in 50 years is 0.54 g (as defined in the 2007 CBC). Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

16 The current standards for construction provided in the CBC are designed to safeguard against major failures and loss of life, but are not intended to prevent damage, maintain function or provide for easy repair. Conformance to code standards does not constitute any kind of guarantee or assurance that significant structural damage will not occur in the event of a maximum level of earthquake ground motion. However, it is reasonable to expect that a well-planned and constructed structure will not collapse in a major earthquake and that protection of life will be reasonably provided, but not with complete assurance. Therefore, potential impacts would be less than significant as the project is an infrastructure project and contains no habitable structures and would be constructed to the most current CBC standards. Ground failure is a general term for seismically induced, secondary, permanent ground deformation caused by strong ground motion. This includes liquefaction, lateral spreading, ground lurching, seismic settlement of poorly consolidated materials (dynamic densification), differential materials response, sympathetic movement on weak bedding planes or non-causative faults, slope failures, and shattered ridge effects. The project area is underlain primarily by alluvium, which is found throughout the canyon bottoms and river areas of the City. Alluvium is silt deposited by creeks and rivers. The major hazard associated with alluvium is liquefaction. Alluvium tends to be unconsolidated, meaning there is space between the grains of soil. If alluvium is sufficiently saturated, and an earthquake occurs, the soil can take on more liquid characteristics, damaging structures. However, as identified on the City of Santa Clarita, North Newhall Seismic Hazard Zone Map (2005) and the USGS, State of California Seismic Hazards Zones, Newhall Quadrangle, the project site is not designated to contain areas for potential liquefaction. As there was no encounter of groundwater at 50 feet bgs, potential impacts from seismically induced liquefaction would be less than significant. BasedonreviewoftheseismichazardmapfortheNewhall Quadrangle, the subject site is not located in a zone in which investigation of potential for earthquake-induced landslides is required. Potential at the site for slope failures and shattered ridge effects is considered nonexistent due to the flat nature of the site. Potential at the site for differential materials response and slippage along weak, inclined bedding planes is considered to be negligible because the site is underlain by consistent, flat-lying alluvial deposits. Consequently, seismically induced landslides on the project site would have a less than significant impact. As seen in Figure 5.3-3, no water wells or oil wells have been drilled at the project site. Therefore, the potential for the proposed project to compromise the integrity of wells is non-existent. As a result, potential impacts would be less than significant. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

17 No dams currently exist in the Newhall Creek Drainage and the site is not in a dam inundation area per the Flood and Inundation Hazard Map (Plate 6) of the Los Angeles County Safety Element of the General Plan. The potential for dam inundation is therefore considered nonexistent and impacts would be less than significant. Mitigation Measures None required. Residual Impacts Impacts would be less than significant. Threshold Result in substantial soil erosion or the loss of topsoil During construction of the proposed project, the soils on-site may become exposed, and thus subject to erosion. However, the project is required to comply with existing regulations that reduce erosion potential. The proposed project will comply with South Coast Air Quality Management District (SCAQMD) Rule 403, which would reduce the potential for wind erosion. Similarly, water erosion during construction would be substantially reduced by complying with the National Pollution Discharge Elimination System (NPDES). As further detailed in the Hydrology and Water Quality section, NPDES requires the construction of the project to incorporate Best Management Practices (BMPs) to reduce erosion and prevent eroded soils from washing off site. According to the Geologic/Geotechnical Report prepared by Allan E. Seward Engineering Geology, Inc., the project site consists of quaternary alluvium, artificial fill, and railroad ballast. Potential operational impacts due to erosion would be less than significant as the proposed improvements would cover the area with asphalt or concrete. Mitigation Measures No mitigation measures are required. Residual Impacts Impacts would be less than significant. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

18 Threshold 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 Based on explorations for the adjacent Newhall Library site, the alluvium in this area is typically medium dense to depths of 30 to 40 feet, with local loose zones in the upper 10 feet. A maximum of 5 feet of fill is currently proposed above this material. The density of the artificial fill placed below the railroad tracks is currently undefined. As a result, potential impacts may be significant. Mitigation Measures The following mitigation measures shall be implemented. MM MM Prior to issuance of a grading permit, additional hydro-compression or consolidation testing shall be conducted to aid in evaluation of settlement within identified geologic units during future geotechnical investigations for Grading Plans. Possible mitigation of settlement of project soils would include removal and recompaction of loose or soft material or by increasing the thickness of the pavement section. During construction activities, excavated topsoil shall be salvaged, stockpiled, and subsequently placed over fill areas to assist in revegetation and to minimize erosion and loss of topsoil. The use of any excavated soils must be deemed appropriate by the contracted Geotechnical Consultant for use as backfill material. Residual Impacts Subject to implementation of mitigation impacts would be less than significant. Threshold 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 The site alluvial materials are generally granular and are not typically expansive in nature. The expansion index of the site materials should be verified with laboratory testing at the Grading Plan stage. If expansive materials are encountered, options to mitigate potential adverse affects include modification of the base and pavement section. Non-expansive soil can be placed behind retaining walls to mitigate potential distress. Potential removal and recompaction of shallow, loose soils may be required at the site. The native soils are expected to shrink in volume when placed as compacted fill. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

19 AESEGI s past experience with similar soils on nearby sites suggests that the on-site soils likely have a low concentration of sulfate and chloride, and low acidity. This indicates a low potential for corrosion of concrete and, therefore, it would be anticipated that Type I or II Portland Cement will be satisfactory for use at the site. The resistivity of similar soils near the site tested by AESEGI indicates that they are typically moderately corrosive to ferrous metals. Therefore, there may be potential corrosive impacts to the proposed project. Mitigation Measures The following mitigation measures shall be implemented. MM MM MM Expansive materials at the site shall be evaluated by the Project Geotechnical Engineer during the grading plan stage of development. Expansion potential of site soils can be mitigated by controlling the water content and density of fill soils, by specifying embedment and reinforcement of structures, and by removing the expansive materials and replacing them with compacted material with low expansion potential. Other potential mitigation would include modification of the base and pavement section. Soils shall be evaluated by the Project Geotechnical Engineer prior to issuance of grading permits for corrosive characteristics of the site soils. If corrosive soils are encountered, options to mitigate potential corrosive soils include protective wraps and coatings for buried metal pipes and special types of cement that are resistant to corrosion. Specifications for retaining walls shall be provided on grading plans. MM MM Any oversized material that may be encountered during construction shall not be incorporated into potential compacted fill for the at-grade crossing. Specifications and guidelines for handling and disposal should be addressed by the project Geotechnical Engineer prior to the issuance of grading permits. The percent shrinkage of this material shall be verified prior to the issuance of grading permits so that the potential volume of import fills can be accurately anticipated. The shrinkage characteristics of the existing fills shall also be evaluated if removals are required. Residual Impacts Subject to implementation of mitigation measures, impacts would be less than significant. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR

20 MITIGATION MEASURES ALREADY INCORPORATED INTO THE PROJECT Recommendations provided by the geotechnical engineer identified in Geologic and Geotechnical Report; EIR-Level Review of Site Conditions, prepared by Allan E. Seward Engineering Geology, Inc., and dated January 7, 2010, shall be incorporated as standard conditions of approval for the proposed project. CUMULATIVE IMPACTS Geotechnical impacts are site specific in nature and each development site is subject to, at minimum, uniform site development and construction standards relative to seismic and other geologic conditions that are prevalent within the locality and/or region. Because the development of each site would have to be consistent with City of Santa Clarita requirements for projects in the City and the Unified Development Code as they pertain to protection against known geologic hazards, impacts of cumulative development would be less than significant given known geologic considerations. CUMULATIVE MITIGATION MEASURES No significant cumulative geotechnical impacts would occur; therefore, no cumulative mitigation measures are recommended. UNAVOIDABLE SIGNIFICANT IMPACTS With implementation of the above-identified mitigation measures, project-specific impacts associated with geology and soils would be reduced to less than significant. Therefore, no unavoidable significant project-specific impacts are anticipated. Impact Sciences, Inc Lyons Avenue At-Grade Rail Crossing: Stage I Draft EIR