5.11 Geology and Soils

Transcription

1 5.11 Geology and Soils

2 5.11 GEOLOGY AND SOILS This section evaluates the geologic and seismic conditions within the City of Azusa and evaluates the potential for geologic hazard impacts associated with implementation of the proposed project. Information in this section is based on the following documentation: Geotechnical Investigation for the Proposed Material Recovery Facility, Transfer Station, and Entrance Improvements (Geotechnical Investigation), prepared by Geocon West, Inc., dated May 17, 2010 (refer to Appendix 15.11, Geotechnical Investigation); Azusa General Plan Update Analysis of Existing Conditions and Trends Document for the Azusa General Plan Update (December 2001); and Azusa General Plan Geologic Hazards Element EXISTING SETTING GEOLOGIC SETTING The project site is located in the central portion of the San Gabriel Valley. The San Gabriel Valley is an alluvial filled valley bounded by the Sierran Madre Fault Zone and San Gabriel Mountains on the north, the Puente Hills on the south, Covina and Indian Hills on the east, and by the Raymond Basin on the west. The alluvial deposits have been derived from erosion of the San Gabriel Mountains to the north and subsequent deposition by the San Gabriel River, San Antonio Creek, and other drainages. The alluvium is estimated to be approximately 200 feet thick at the base of the mountains, and over 900 feet thick in the City of Irwindale. Regionally, the site is located at the northern boundary between the Peninsular Ranges and Transverse Ranges geomorphic provinces. 1 Geology conditions in and around Azusa are reflected in the nature of the steep mountains, low foothills, and relatively flat valleys that make up the area. There are some geologic conditions that benefit the community and some that can result in hazardous conditions if proper precautions are not taken. In the north and northwest mountainous areas of the City, the land is generally too steep and bedrock is too unstable for most construction. Construction is possible in selected areas of the mountains. However, many precautions must be taken to avoid land sliding, severe erosion, and earthquake shaking hazards. Azusa s foothills just northeast and east of the central City are less steep and more stable than the mountains. While there is greater potential for safe development in these areas, some slope stability, erosion and mudflow potential remains in certain areas. Potential movement on some earthquake faults provides challenges for certain types of construction. Flatter valley areas make up most of the City (including the project site). Potential earthquake fault hazards also exist for the valley area and must be considered carefully for safe future growth of educational, health care, commercial, industrial, and residential development. 2 1 Geocon West, Inc., Geotechnical Investigation for the Proposed Material Recovery Facility, Transfer Station, and Entrance Improvements, dated May 17, City of Azusa, Azusa General Plan Geologic Hazards Element, April Public Review Draft March Geology and Soils

3 Locally, the site is situated along the southern and southeastern edge of the Azusa Land Reclamation landfill. The landfill consists of an approximate 300 acre, steeply-excavated landfill pit that was partially backfilled with artificial fill consisting of soil mixed with municipal solid waste. The southwestern edge of the landfill limit encroaches within the boundary of the subject site based on geologic maps, chronologically dated aerial photos, and site explorations. 3 GEOLOGIC MATERIALS Based on the Geotechnical Investigation, the majority of the proposed construction area is mantled by artificial fill soils underlain by Holocene Age alluvial fan and stream channel deposits. However, the area within the limits of the landfill is underlain by a cap of fill which is underlain by municipal solid waste and/or landfill debris. The artificial fill and landfill debris are underlain by Holocene Age alluvial fan and stream channel deposits at depth. Following is a discussion of the soil and geologic units at the project site. Artificial Fill According to the Geotechnical Investigation, varied amounts of artificial fill are located throughout the subject site. The depth of artificial fill outside the limits of the buried landfill varies between 1 and 5 feet in depth below the ground surface (bgs). The depth of the fill cap encountered within the limits of the buried landfill varies between 3 and 17 feet in depth bgs. The fill consists of light gray to reddish brown sand with silt, silty sand, and sandy silt with varied amounts of gravel, cobbles and traces of debris. The artificial fill is generally loose to dense or soft to stiff, and slightly moist to moist. The fill is believed to be the result of past grading and/or construction activities at the site. Deeper fill may occur between excavations and on other parts of the site that were not directly explored during the Geotechnical Investigation. Landfill Debris The Geotechnical Investigation notes that landfill debris exists to a maximum depth of 45.5 feet bgs in the area explored (in the vicinity of the Bale Storage area) and is expected to be deeper in the northern portion of the site. The landfill materials generally consist of a large amount of solid waste mixed in with sand, silty sand, and silt, with varied amounts of gravel, cobbles and boulders. Alluvial Fan/Stream Channel Deposits The artificial fill and landfill debris is underlain by Holocene Age alluvial fan and stream channel deposits. The alluvial deposits encountered during the Geotechnical Investigation consist primarily of brown to light olive brown, poorly graded sand, and well graded sand, with varied amounts of gravel, cobbles and boulders throughout the depth of exploration. The soils are generally dry to slightly moist, and dense to very dense. The alluvial deposits are derived from the nearby San Gabriel Mountains and the San Gabriel River. 3 Geocon West, Inc., Geotechnical Investigation for the Proposed Material Recovery Facility, Transfer Station, and Entrance Improvements, dated May 17, Public Review Draft March Geology and Soils

4 REGULATORY FRAMEWORK FEDERAL The purpose of the Federal Soil Protection Act is to protect or restore the functions of the soil on a permanent sustainable basis. Protection and restoration activities include prevention of harmful soil changes, rehabilitation of the soil of contaminated sites and of water contaminated by such sites, and precautions against negative soil impacts. If impacts are made on the soil, disruptions of its natural functions as an archive of natural and cultural history should be avoided, as far as practicable. In addition, the requirements of the Federal Water Pollution Control Act (also referred to as the Clean Water Act [CWA]) through the National Pollution Discharge Elimination System (NPDES) provide guidance for protection of geologic and soil resources. STATE Uniform Building Code Development standards require projects to comply with appropriate seismic design criteria in the Uniform Building Code (UBC), adequate drainage facility design, and preconstruction soils and grading studies. The UBC also includes regulations regarding building near fills containing rubbish or other decomposable material. California Building Code California building standards are published in the California Code of Regulations, Title 24, known as the California Building Code (2007 CBC). The 2007 CBC applies to all applications for building permits. The 2007 CBC consists of 11 parts that contain administrative regulations for the California Building Standards Commission and for all State agencies that implement or enforce building standards. Local agencies must ensure that development complies with the guidelines contained in the 2007 CBC. Cities and counties have the ability to adopt additional building standards beyond the 2007 CBC. LOCAL Building Code Chapter 14, Article II of the City s Municipal Code contains the City s Building Codes. Division 2 of Article II includes Uniform Building Code amendments and modifications. Section includes regulations pertaining to building on fills containing decomposable material. This section states the following: a. Permits shall not be issued for buildings or structures regulated by this Code within 1,000 feet of fills containing rubbish or other decomposable material unless the fill is isolated by approved natural or manmade protective systems or unless designed according to the recommendation contained in a report prepared by a licensed civil engineer. Such report shall contain a description of the investigation, study Public Review Draft March Geology and Soils

5 and recommendation to minimize the possible intrusion and to prevent the accumulation of explosive concentrations of decomposition gases within or under enclosed portions of such building or structure. At the time of the final inspection, the civil engineer shall furnish a signed statement attesting that the building or structure has been constructed in accordance with his recommendations as to decomposition gases required in this subsection. b. Buildings or structures regulated by this Code shall not be constructed on fills containing rubbish or other decomposable material unless provision is made to prevent damage to the structure, floors, underground piping and utilities due to uneven settlement of the fill. One-story light-frame accessory structures not exceeding 400 feet in area nor 12 feet in height may be constructed without special provision for foundation stability. Azusa General Plan Geologic Hazards Element The Geologic Hazards Element addresses natural hazards in the City. Potential risks to residents and the local environment associated with hazards such as liquefaction, soil failure, earthquakes, and flooding, are considered. The Geologic Hazards Element provides background information related to each issue, and identifies goals and policies regarding hazardous structures, flooding and drainage, emergency preparedness and education, and post-disaster reconstruction. The following policies are those related to seismic issues. Refer to Section 5.9, Hazards and Hazardous Materials for policies regarding hazardous conditions within the City, and Section 5.10, Hydrology and Water Quality for drainage issues. Require that earthquake survival and efficient post-disaster functioning are primary concerns in the siting, design and construction standards of essential facilities. (Geo 1.1) Require that proposed essential facilities apply the most current professional standards for seismic design and be subject to seismic review, including detailed site investigations for faulting, liquefaction, ground motion characteristics, and slope stability. (Geo 1.2) IMPACT THRESHOLDS AND SIGNIFICANCE CRITERIA CEQA SIGNIFICANCE CRITERIA The environmental analysis in this section is patterned after the Initial Study Checklist recommended by Appendix G of the CEQA Guidelines, as amended, and used by the City of Azusa in its environmental review process, and is contained in Appendix 15.1 of this EIR. The Initial Study Checklist includes questions relating to geology and seismic hazards. The issues presented in the Initial Study Checklist have been utilized as thresholds of significance in this section. Accordingly, a project may create a significant environmental impact if it would: Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: Public Review Draft March Geology and Soils

6 - 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 (refer to Section 10.0, Effects Found Not To Be Significant). - Strong seismic ground shaking (refer to Section 10.0, Effects Found Not To Be Significant). - Seismic-related ground failure, including liquefaction (refer to Section 10.0, Effects Found Not To Be Significant). - Landslides (refer to Section 10.0, Effects Found Not To Be Significant). Result in substantial soil erosion or the loss of topsoil (refer to Impact Statement GEO-1). 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 (refer to Impact Statement GEO-2). Be located on expansive soil, as defined in Table 18-1-B of the California Building Code (2004), creating substantial risks to life or property (refer to Section 10.0, Effects Found Not To Be Significant). Have soils incapable of adequately supporting the use of septic tanks or alternative waste water disposal systems where sewers are not available for the disposal of waste water (refer to Section 10.0, Effects Found Not To Be Significant). Based on these standards, the effects of the proposed project have been categorized as either a less than significant impact or a potentially significant impact. Mitigation measures are recommended for potentially significant impacts. If a potentially significant impact cannot be reduced to a less than significant level through the application of mitigation, it is categorized as a significant unavoidable impact IMPACTS AND MITIGATION MEASURES LOSS OF TOPSOIL GEO-1 THE PROPOSED PROJECT MAY RESULT IN SUBSTANTIAL EROSION AND LOSS OF TOPSOIL. Impact Analysis: According to the Geotechnical Investigation, the subject site is characterized by alluvial fan and stream channel deposits consisting of granular soils. The permeability is expected to be high for the alluvial fan and stream channel deposits. Small-scale localized erosional activity has been observed along the various minor slope faces of the project site. The fill and alluvial soils are slightly prone to erosion. Public Review Draft March Geology and Soils

7 Project construction would involve grading activities over the entire project site. There is a potential for erosion to occur during the grading process during periods of heavy rainfall. Some portions of the subject site have not been subject to development nor have been covered by pavement and impervious surfaces. Runoff at the site is expected to increase during development as portions of the site are graded and paved. Grading and excavation operations may result in substantial soil erosion or the loss of topsoil in the absence of mitigation. Therefore, Mitigation Measure GEO-1 would require the project to adhere to the recommendations contained within the Geotechnical Investigation regarding erosion and runoff. On-site grading would be performed in such a manner that alteration of runoff or erosion of graded areas would not occur. All areas of construction would be fine-graded to direct water away from foundations and direct water to the nearest available storm drain or to the street. The project would also be required to comply with all requirements set forth in the NPDES permit for construction activities, as enforced by the Los Angeles RWQCB. Additionally, erosion and loss of topsoil as a result of wind (fugitive dust) would be minimized with implementation of Mitigation Measure AQ-2. With implementation of Mitigation Measures GEO-1 and AQ-2, and compliance with NPDES requirements, erosion is not expected to be a significant impact to development and impacts would be less than significant. Also, the proposed project would result in reduced impervious surfaces as compared to existing conditions. Therefore, operational impacts to erosion and loss of topsoil would be less than the existing condition. Impacts in this regard are less than significant. Mitigation Measures: Refer to Mitigation Measure AQ-2. GEO-1 The project shall incorporate all engineering recommendations contained within the Geotechnical Investigation for the Proposed Material Recovery Facility, Transfer Station, and Entrance Improvements (Geotechnical investigation), prepared by Geocon West, Inc., dated May 17, 2010, during project site design, construction, and operations to reduce any potential geotechnical hazards at the project site. These recommendations shall be stipulated in the construction contracts and specifications. The Geotechnical Investigation is included in Appendix 15.11, Geotechnical Investigation of this EIR and is incorporated by reference into this mitigation measure. Level of Significance: Less Than Significant Impact. UNSTABLE GEOLOGIC UNITS GEO-2 THE PROJECT SITE WOULD BE SUSCEPTIBLE TO SETTLEMENT, DISTRESS, AND CONSOLIDATION AS A RESULT OF THE HISTORIC LANDFILL. Impact Analysis: The project site is partially located within the limits of a known buried landfill. Portions of the Bale Storage loading dock, access road, parking lots, and scale facility that overlie the historic landfill could be subject to significant settlement and distress. The continued consolidation, and decomposition of the buried waste materials could result in a corresponding loss of volume, creating a potential for settlement in areas where structures or compacted fill are planned within the limits of the buried landfill. Based on these considerations, the probability of hazards associated Public Review Draft March Geology and Soils

8 with consolidation of these waste materials within the limits of the buried landfill is considered very high. Mitigation Measure GEO-1 would require the project to incorporate all engineering recommendations contained within the Geotechnical Investigation to reduce impacts related to settlement, distress, and consolidation. These recommendations include, but are not limited to, periodically placing paving overlays on the roadway to recreate a level surface, construct the scales in a manner that would allow for mechanical leveling, utilizing a reinforced concrete mat foundation to support the scales or use drilled piers. Additionally, Municipal Code Section prohibits buildings constructed within 1,000 feet of fills containing rubbish or other decomposable material unless the building is designed according to the recommendations of a licensed civil engineer. Although construction over portions of the landfill would increase maintenance costs, implementation of Mitigation Measure GEO-1 would reduce potential settlement hazards. Therefore, with implementation of Mitigation Measure GEO-1, impacts regarding settlement, distress, and consolidation at the project site would be reduced to less than significant levels. Mitigation Measures: Refer to Mitigation Measure GEO-1. Level of Significance: Less Than Significant Impact CUMULATIVE IMPACTS THE PROPOSED PROJECT, IN COMBINATION WITH OTHER CUMULATIVE PROJECTS, COULD RESULT IN SIGNIFICANT GEOLOGICAL IMPACTS. Impact Analysis: Threshold: Result in substantial soil erosion or the loss of topsoil. Portions of the City and surrounding areas contain soils that have erosion potential. Construction of planned and future cumulative projects would facilitate soil erosion and loss of topsoil. Grading activities leave soils exposed to rainfall and wind conditions that results in erosion. The geotechnical characteristics of each cumulative project site would be evaluated on a project-by-project basis, and appropriate mitigation measures would be required, as necessary, in addition to Federal and State requirements for mitigating erosion. Therefore, assuming cumulative projects implement project specific mitigation measures, cumulative soil erosion and loss of topsoil impacts would be less than significant. Project construction would involve grading activities over the entire project site. According to the Geotechnical Investigation, there is a potential for erosion to occur during the grading process of the proposed project during periods of heavy rainfall. Grading and excavation operations may result in substantial soil erosion or the loss of topsoil in the absence of mitigation. The project would be required to comply with all requirements set forth in the NPDES permit for construction activities, as enforced by the Los Angeles RWQCB. Therefore, in addition to the State and Federal requirements regarding erosion, Mitigation Measure GEO-1 would require the project to adhere to the engineering recommendations contained within the Geotechnical Investigation regarding erosion and Public Review Draft March Geology and Soils

9 runoff. Additionally, erosion and loss of topsoil as a result of wind (fugitive dust) would be minimized with implementation of Mitigation Measure AQ-2. Impacts in this regard are less than significant. With implementation of Mitigation Measures GEO-1 and AQ-2, as well as compliance with NPDES requirements, the proposed project s cumulative contribution to soil erosion and loss of topsoil would be less than significant. Level of Significance Before Mitigation: Overall Cumulative Impact Potentially Significant Impact. Project Cumulative Contribution Potentially Significant Impact. Mitigation Measures: Refer to Mitigation Measures GEO-1 and AQ-2. Level of Significance After Mitigation: Overall Cumulative Impact Less Than Significant Impact. Project Cumulative Contribution Less Than Significant Impact. 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. At this time, no future development is proposed in the vicinity of the historic landfill boundaries. Thus, overall cumulative impacts are not considerable. Although the project site would be susceptible to settlement, distress, and consolidation due to the soil components at the project site, the project would conform to applicable City criteria, adhere to standard engineering practices, and incorporate standard practices of the UBC. Additionally, Mitigation Measure GEO-1 would require the project to incorporate all engineering recommendations contained within the Geotechnical Investigation to reduce impacts related to settlement, distress, and consolidation. Therefore, the project would not contribute to cumulative impacts and impacts in this regard are not cumulatively considerable. Level of Significance Before Mitigation: Overall Cumulative Impact No Impact. Project Cumulative Contribution Less Than Significant Impact. Mitigation Measures: Refer to Mitigation Measure GEO-1. Public Review Draft March Geology and Soils

10 Level of Significance After Mitigation: Overall Cumulative Impact No Impact. Project Cumulative Contribution Less Than Significant Impact SIGNIFICANT UNAVOIDABLE IMPACTS No unavoidable significant impacts related to geology and soils have been identified following implementation of mitigation measures referenced in this section. Public Review Draft March Geology and Soils

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