GEOLOGY, SOILS, AND SEISMICITY / MINERAL RESOURCES

Transcription

1 4.6 GEOLOGY, SOILS, AND SEISMICITY / MINERAL RESOURCES INTRODUCTION The Geology, Soils, and Seismicity / Mineral Resources chapter of the Draft EIR describes the geologic and soil characteristics of the project site and evaluates the extent to which implementation of the South of Woodward Avenue Project (proposed project) could be affected by seismic hazards such as ground shaking, liquefaction, and expansive soil characteristics. Informational sources for this evaluation include the Manteca General Plan, 1 the Manteca General Plan EIR, 2 the Natural Resources Conservation Service (NRCS) Soil Survey for San Joaquin County (Soil Survey), 3 and the Preliminary Geotechnical Reports conducted by Advanced GeoEnvironmental, Inc. (see Appendix F) EXISTING ENVIRONMENTAL SETTING The following setting information focuses on the seismic, geotechnical, and soil conditions of the proposed project site. In addition, existing mineral resources in the area are described. Regional Seismicity A fault is defined as a fracture or zone of closely associated fractures along which rocks on one side have been displaced with respect to those on the other side. A fault zone is a zone of related faults that commonly are braided and subparallel, but may be branching or divergent. Movement within a fault causes an earthquake. When movement occurs along a fault, the energy generated is released as waves which cause ground shaking. Ground shaking intensity varies with the magnitude of the earthquake, the distance from the epicenter, and the type of rock or sediment the seismic waves move through. The regional seismicity near the proposed project is dominated by the San Andreas, Hayward, and Calaveras faults; with the Rescue Lineament-Bear Mountains fault zone, Clayton-Marsh Creek-Greenville fault, O'Neil fault system, and Ortigalita faults having less of a potential impact. The known faults nearest to the City are the Tracy-Stockton Fault crossing southwest near Tracy to the northeast near Linden, and a small buried fault running southeast from the Tracy area. Strong ground shaking is described as a motion of sufficient strength to affect people and their environment or any ground movement recorded on a strong motion instrument or seismograph. The common way to describe ground motion during an earthquake is with the motion parameters of acceleration and velocity in addition to the duration of the shaking. The project site is located in an area rated as a low-intensity earthquake zone (Seismic Zone III). A low-intensity zone is defined by the United States Geological Survey (USGS) as an area that is 4.6-1

2 likely to experience an earthquake measuring a maximum of in magnitude on the Richter scale, and a maximum intensity of VII or VIII on the Modified Mercalli scale. The Richter scale measures the amplitude of seismic waves recorded by a seismograph. The Modified Mercalli scale measures the intensity of an earthquake by the way the shaking is felt and responded to by humans, and by the amount of damage the earthquake causes to buildings and structures. The Modified Mercalli scale is shown in Table Table Modified Mercalli Scale of Earthquake Intensity Scale Effects I. Earthquake shaking not felt. II. Shaking felt by those at rest. III. Felt by most people indoors; some can estimate the duration of shaking. Felt by most people indoors. Having objects swing, windows and doors rattle, IV. wooden walls and frames creak. Felt by everyone indoors; many estimate duration of shaking. Standing autos rock. V. Crockery clashes, dishes rattle, and glasses clink. Doors close, open, or swing. Felt by everyone indoors and most people outdoors. Many now estimate not only the duration of the shaking, but also its direction and have no doubt as to its cause. VI. Sleepers awaken. Liquids disturbed, some spilled. Small unstable objects displaced. Weak plaster and weak materials crack. Many are frightened and run outdoors. People walk unsteadily. Pictures thrown off VII. walls, books off shelves. Dishes or glasses broken. Weak chimneys break at roofline. Plaster, loose bricks, unbraced parapets fall. Concrete irrigation ditches damaged. Difficult to stand. Shaking noticed by auto drivers, waves on ponds. Small slides and VIII. cave-ins along sand or gravel banks. Stucco and some masonry walls fall. Chimneys, factory stacks, towers, elevated tanks twist or fall. General fright. People thrown to the ground. Steering of autos affected. Branches IX. broken from trees. General damage to foundations and frame structures. Reservoirs seriously damaged. Underground pipes broken. General panic. Conspicuous cracks in ground. Most masonry and frame structures X. destroyed along their foundations. Some well-built wooden structures and bridges are destroyed. Serious damage to dams, dikes, and embankments. Railroads bent slightly. General panic. Large landslides. Water thrown out of banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flatland. General destruction XI. of buildings. Underground pipelines completely out of service. Railroads bent greatly. General panic. Damage nearly total, the ultimate catastrophe. Large rock masses XII. displaced. Lines of sight and level distorted. Objects thrown into air. Source: California Division of Mines and Geology, Regional Geotechnical Setting The regional geology is influenced by the Great Valley, an elongated lowland, which extends 500 miles north and south, separating the Sierra Nevada from the Coastal Ranges. This elongated asymmetric structural basin was formed by the westward tilting of the Sierra Nevada block against the eastern flank of the Coastal Ranges. The basement rock complex of the Sierra extends westward, beneath the valley, on a gentle slope reaching points near the Coastal Ranges

3 Elevation in the valley is generally several hundred feet above sea level, but ranges from a low point below sea level to approximately 1,000 feet above sea level. Generally, slopes are nearly level across the City of Manteca with the elevation ranging from approximately 10 to 50 feet above sea level, gently rising from the San Joaquin River on the west toward the east and the Sierra Nevada. Drainage into the San Joaquin Valley is mainly from the Sierra Nevada. The Great Valley is filled with thick sedimentary rock sequences or strata, which began deposition approximately 200 million years ago. Large alluvial fans that consist of unconsolidated to semi-consolidated gravel, sand, silt, and clay have developed on each side of the Valley. The larger and more gently sloping fans are located on the east side of the Valley and overlie metamorphic and igneous basement rocks. This basement rock is exposed in the Sierra Nevada Foothills and consists of metasediments, volcanics, and granites. The sediments that form the Valley floor were largely derived by erosion of the Sierra Nevada. The smaller and steeper slopes on the west side of the Valley overlie sedimentary rocks more closely related to the Coastal Ranges. Project Site Soil Conditions The proposed project is comprised of three sites (Atherton Homes at Woodward Park I, Atherton Homes at Woodward Park II, and DeJong Property) located in the San Joaquin Valley. Geotechnical borings and samples were conducted for the project site. Although Geotech Reports were prepared for Atherton Homes at Woodward Park I and II only, the soil composition for DeJong is similar to the surrounding sites. In fact, the soil types identified in the Geotech Reports comprise approximately 99.7 percent of the on-site soils. Based on the Geotech Reports and the NRCS web soil survey, (see Figure 4.6-1) the following are the soils located on the project site: Delhi fine sand, 0 to 5 percent slopes; Delhi loamy sand, 0 to 2 percent slopes; Tinnin loamy coarse sand, 0 to 2 percent slopes; and Bisgani loamy coarse sand, partially drained, 0 to 2 percent slopes. Soil characteristics may or may not make the soils suitable for accommodating uses such as shallow excavations, local roads and streets, and foundations associated with residential development. Soil limitations can include slow or very slow permeability, limited ability to support a load, high shrink-swell potential, moderate depth to hardpan, low depth to rock, and frequent flooding. Each soil type identified above has characteristics that affect soil behavior, and each is described in further detail below

4 Figure Project Site Soils Project Site San Joaquin County, California (CA077) Map Unit Symbol Map Unit Name Bisgani loamy coarse sand, partially drained, 0 to 2 percent slopes Delhi fine sand, 0 to 5 percent slopes Delhi loamy sand, 0 to 2 percent slopes Tinnin loamy coarse sand, 0 to 2 percent slopes Acres in AOI Percent of AOI % % % % Totals for Area of Interest % 4.6-4

5 Delhi fine sand, 0 to 5 percent slopes The Delhi series consists of very deep, somewhat excessively drained soils. They formed in wind modified material weathered from granitic rock sources. Delhi soils are on floodplains, alluvial fans and terraces. Slopes are 0 to 15 percent. The mean annual precipitation is approximately 13 inches and the mean annual temperature is approximately 62 degrees Fahrenheit. Delhi soils are somewhat excessively drained, negligible to slow runoff, and demonstrate rapid permeability. Delhi loamy sand, 0 to 2 percent slopes The Delhi loamy sand, 0 to 2 percent slopes soil type is a very deep soil formed on nearly level and gently sloping surfaces of former sand dunes complexes, in aeolian sediment derived from granitic sources. The surface layer typically consists of grayish brown and light brownish gray loamy sand approximately 16 inches deep, overlying approximately 10 inches of grayish brown loamy sand, which in turn overlies approximately 34 inches of pale brown sand. The surface layer locally consists of loamy fine sand or fine sand. The soil is somewhat excessively drained, and permeability is rapid, runoff is slow, and water erosion hazard is slight. Tinnin loamy coarse sand, 0 to 2 percent slopes The Tinnin loamy coarse sand, 0 to 2 percent slopes is a very deep, well drained, nearly level soil found on alluvial fans. The surface layer is typically grayish brown loamy, coarse sand approximately 28 inches thick, overlaying approximately 25 inches of brown, mottled loamy coarse sand. The lower part to a depth of 75 inches is pale brown, mottled loamy coarse sand. Permeability is rapid and water capacity is low. Runoff is slow and the hazard of water erosion is slight. In areas used for homesite development, the main limitation is the rapid permeability. Bisgani loamy coarse sand, partially drained, 0 to 2 percent slopes The Bisgani loamy coarse sand, partially drained, 0 to 2 percent slopes is a very deep, poorly drained, nearly level soil found on low alluvial fans. Typically, the surface layer is gray loamy coarse sand about 11 inches thick. The underlying material to a depth of 60 inches is mottled light gray and light brownish gray loamy coarse sand. Permeability is rapid and available water capacity is low. Runoff is very slow and the hazard of water erosion is slight. In areas used for homesite development, the main limitations are the high water table and the rapid permeability. Subsidence Potential Subsidence is the settlement of soils. Settlement can result from either desiccation (dehydration) and shrinkage, or oxidation of organic material, or both, following drainage. As noted in the Manteca General Plan EIR, the Soil Survey for the area found that subsidence is not a characteristic of the soils that occur within the City, which includes those at the proposed project site

6 Expansive Soils Expansive soils are those that increase in volume when they absorb water and shrink when they dry out, commonly referred to as shrink-swell potential. Soil surveys generally rate shrinkswell potential in soils on a low, medium, and high basis. If the shrink-swell potential is rated moderate to high, shrinking and swelling can cause damage to buildings, roads, and other structures. Mineral Resources According to the Manteca General Plan and General Plan EIR, the California Division of Mines and Geology has identified one location within the General Plan study area as a Zone MRZ-2, Significant Mineral Resources Zone. The zoning designation refers to sand deposits that are considered to be of regional significance located near the San Joaquin River. Brown Sand and Gravel, Incorporated, has produced processed sand at the Oakwood Lake Pit within the study area; however, mining operations have been completed and the Oakwood Lake Resort has been built on the reclaimed mined lands. In addition, a residential project has been approved by San Joaquin County on the site of the former quarry. As a result, mineral resources were found not to be a significant issue for the City and further environmental analysis was not required in the Manteca General Plan EIR REGULATORY SETTING The following section is a brief summary of the regulatory context under which geology, soils, and seismic hazards are managed at the federal, state, and local levels. Federal Federal Earthquake Hazards Reduction Act Passed by Congress in 1977, the Federal Earthquake Hazards Reduction Act is intended to reduce the risks to life and property from future earthquakes. The Act established the National Earthquake Hazards Reduction Program (NEHRP). The goals of NEHRP are to educate and improve the knowledge base for predicting seismic hazards, improve land use practices and building codes, and to reduce earthquake hazards through improved design and construction techniques. State Alquist-Priolo Earthquake Fault Zoning Act The 1972 AP Zone Act was passed to prevent the new development of buildings and structures for human occupancy on the surface of active faults. The Act is directed at the hazards of surface fault rupture and does not address other forms of earthquake hazards. The locations of active faults are established into fault zones by the AP Zone Act. Local agencies regulate any new developments within the appropriate zones in their jurisdiction

7 Seismic Hazards Mapping Act Passed in 1990, the Seismic Hazards Mapping Act addresses non-surface rupture earthquake hazards, which may include liquefaction and subsidence. A mapping program is also established by this Act, which identifies areas within California that have the potential to be affected by such non-surface rupture hazards. California Building Standards Code / Uniform Building Code The State of California provides minimum standards for building design through the California Building Standards Code (California Code of Regulations (CCR), Title 24). The California Uniform Building Code (CUBC) is based on the federal Uniform Building Code (UBC) used widely throughout the United States. The CBC includes specific safety and design standards for new structures to resist the forces of strong winds and seismic activity. California Surface Mining and Reclamation Act The primary State law concerning conservation and development of mineral resources is the California Surface Mining and Reclamation Act (SMARA) of 1975, as amended. California Public Resources Code (PRC), Division 2, Chapter 9, Section 2710, et. seq. includes the SMARA. In 1975, SMARA was enacted to limit new development in areas with significant mineral deposits and to prevent or minimize the negative impacts of surface mining to public health, property, and the environment. According to SMARA, a State Geologist must classify the lands within California based on mineral resource availability. Local Manteca General Plan The Manteca General Plan establishes the following goals and policies applicable to geology, soils and seismicity. Goal S-1 Goal S-2 Prevent loss of lives, injury, and property damage due to geological hazards and seismic activity. Prevent loss of lives, injury, and property damage due to the collapse of buildings and critical facilities, and to prevent disruption of essential services in the event of an earthquake. Policy S-P-1 The City shall require preparation of geological reports and/or geological engineering reports for proposed new development located in areas of potentially significant geological hazards, including potential subsidence (collapsible surface soils) due to groundwater extraction

8 Policy S-P-2 The City shall require new development to mitigate the potential impacts of geologic hazards through Building Plan review. Policy S-P-3 The City shall require new development to mitigate the potential impacts of seismic induced settlement of uncompacted fill and liquefaction (water-saturated soil) due to the presence of a high water table IMPACTS AND MITIGATION MEASURES Standards of Significance The following thresholds of significance related to geology, soils, and seismicity are derived from the criteria listed in Appendix G of the State CEQA Guidelines. Impacts resulting from the project would be considered significant if the project would: Expose people or structures to substantial adverse effects, including the risk of loss, injury, or death involving: o Rupture of a known earthquake fault; o Seismic ground shaking; o Seismic-related ground failure, including liquefaction; and o Landslides Result in substantial soil erosion or 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 landslides, lateral spreading, subsidence, liquefaction or collapse; Be located on expansive soil, as defined in Table B of the Uniform Building Code (1994), creating substantial risks to life or property; 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; or Result in the loss of availability of a known mineral resource that would be of value to the region and the residents of the State or of a locally-important mineral resource recovery site delineated on a local general plan, specific plan or other land use plan. Method of Analysis The Geology, Soils, and Seismicity / Mineral Resources chapter identifies any impacts to and from geological, seismic, or soils-related impacts that could occur during the construction and/or operation of the proposed project. Evaluation of potential geological and soil impacts were based on a review of the Atherton Homes at Woodward Park I and Atherton Homes at Woodward Park II Geotechnical Reports. Although a Geotechnical Report was not prepared for the DeJong Property site, the soil characteristics are similar to the adjacent Atherton Homes at Woodward Park I and II sites. Impacts related to mineral resources were evaluated based on the Manteca 4.6-8

9 General Plan. The standards of significance listed above will be used to delineate the significance of any potential impacts associated with the geology, soils, and seismicity, as well as mineral resources of the site. It should be noted that impacts associated with erosion can be found in the Hydrology and Water Quality Chapter of the Draft EIR (Chapter 4.8). Project-Specific Impacts and Mitigation Measures If significant impacts are identified for the construction and operational phases of the proposed project, recommended mitigation measures have been included to reduce the identified impacts to less-than-significant levels, as feasible Risks to people and structures associated with earthquakes, including seismic activity, liquefaction, fault rupture, and landslides. As described above in the Existing Environmental Setting, the proposed project would be located in an area that is seismically active. Given the known faults in the region, the project area can be expected to experience earthquakes ranging from 5.0 to 5.9 in magnitude on the Richter scale, and a maximum intensity of VII or VIII on the Modified Mercalli scale. In addition, significant earthquakes from regional fault systems have affected all of San Joaquin County in the past; therefore, the possibility of some level of regional ground shaking in the future is likely. During earthquakes, ground shaking may cause a loss of strength in cohesionless saturated soils. This process is called liquefaction and occurs most commonly in loose sands associated with a high water table. At the Atherton Homes at Woodward Park I and II sites, sands are located below the groundwater table; however, according to the Geotechnical Reports a low potential exists for liquefaction to occur due to the relatively low ground accelerations expected at the site and the density of the sands below the groundwater table. Additionally, the State regulates development in California through a variety of tools that reduce hazards from earthquakes and other geologic hazards. The 2013 California Building Code (CBC) contains provisions to safeguard against major structural failures or loss of life caused by earthquakes or other geologic hazards. The City of Manteca s building regulations are included in the City s Municipal Code as chapter The proposed project would be required to adhere to the provisions of the 2013 CBC, which would reduce hazards from strong seismic ground shaking and other seismic-related effects, including liquefaction. Therefore, because strong seismic ground shaking and seismic-related ground failure, including liquefaction, would not be expected to occur, and because the project would be required to comply with the 2013 CBC requirements, impacts would be less than significant. Mitigation Measure(s) None required

10 4.6-2 Risks associated with structural damage from unstable or expansive soils. Expansive soils are those that increase in volume when they absorb water and shrink when they dry out, commonly referred to as shrink-swell potential. Soil surveys generally rate shrink-swell potential in soils on a low, medium, and high basis. If the shrink-swell potential is rated moderate to high, shrinking and swelling can cause damage to buildings, roads, and other structures; as a result, special design is often needed. According to the Manteca General Plan EIR, four of the 22 General Plan Study Area soils have been identified as expansive soils: Egbert (152), Egbert (153), Guard (169), and Galt (160). As indicated in Figure above, none of these soil types are found on the SOWA project site. In addition, according to the Preliminary Geotechnical Reports prepared for AHWPI and AHWPII, the on-site soil conditions are conducive to the type of residential development anticipated. Notwithstanding the above considerations, the preliminary Geotechnical Reports recommend that design-level geotechnical investigations be prepared to formulate site-specific recommendations for the proposed project improvements, as appropriate. Therefore, a potentially significant impact could occur to the proposed residential structures and associated infrastructure systems from unstable soils if said structures are not properly designed from a geotechnical engineering perspective. Mitigation Measure(s) Implementation of the following mitigation measures would reduce the above impact to a less-than-significant level (a) Prior to any final map approval, the applicant shall submit to the Engineering Division, for review and approval, a design-level geotechnical engineering report produced by a California Registered Geotechnical Engineer. The report shall address and make recommendations on the following: Road, pavement, and parking area design; Structural foundations, including retaining wall design (if applicable); Grading practices; Erosion/winterization; Special problems discovered on-site, (i.e., groundwater, expansive/unstable soils, etc.); and Slope stability (if applicable to any required trenching activities) (b) All grading and foundation plans shall be reviewed and approved by the Engineering Division and the Building Safety Division, respectively, prior to issuance of building permits to ensure that all geotechnical recommendations specified in the geotechnical report(s) are properly incorporated and utilized in the design

11 4.6-3 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 wastewater. The proposed project would connect to the existing City wastewater collection infrastructure and be served by the City s Wastewater Quality Control Facility. Therefore, the proposed project would not utilize a septic tank system and no impact would occur. Mitigation Measure(s) None required Impacts related to mineral resources. The Manteca General Plan identified one area within the vicinity of the City that has been designated by the California Division of Mines and Geology as an area with mineral resources of regional significance, which is located approximately five miles west of the project site down Woodward Avenue. However, mining operations have since been completed and the mined lands have been reclaimed. The General Plan notes that a residential project has been approved by San Joaquin County on the site of the former quarry. Known mineral resources are not located on the project site or in the immediate vicinity, and land designated or zoned for mineral resources is not within the City limits. As mineral resources are not located in the vicinity of the proposed project or the City, implementation of the proposed project would not result in the loss of availability of a known mineral resource or of a locally-important mineral resource recovery site. Therefore, the proposed project would have no impact related to mineral resources. Mitigation Measure(s) None required. Cumulative Impacts and Mitigation Measures The continuing buildout of developments in the City of Manteca and surrounding areas would be expected to increase the need for surface grading and excavation and, consequently, increase the potential for impacts related to soil erosion, unforeseen hazards, and exposure of people and property to earthquakes. The following discussion of impacts is based on the implementation of the proposed project in combination with other proposed and pending projects in the region

12 4.6-5 In combination with existing and future developments, increased potential impacts related to geological impacts and hazards. Due to regional earthquake activity, the potential exists for earthquake-related ground shaking during the life of the proposed project. The proposed project would increase the number of people and structures that could be exposed to potential effects related to seismic hazards. Development of the proposed project would also increase the number of structures that could be subject to the effects of shallow depth to rock or expansive soils, and site preparation would result in temporary and permanent topographic changes. However, potentially adverse environmental effects associated with seismic hazards, as well as those associated with geologic or soils constraints and topographic alteration, are usually site-specific and generally would not combine with similar effects that could occur with other projects in Manteca. Furthermore, all projects would be required to comply with the CBC and other applicable safety regulations. Consequently, the proposed project would generally not be affected by, nor would the project affect, other development approved by the City of Manteca. Therefore, the impact would be considered less than significant. Mitigation Measure(s) None required In combination with existing and future developments, increased potential impacts related to mineral resources. As discussed above, the sole known mineral resource recovery site in the vicinity of the City has ceased mining operations and the Oakwood Lake Resort has been built on the reclaimed mined lands. In addition, the Manteca General Plan notes that a residential project has been approved by San Joaquin County on the site of the former quarry. As a result, mineral resources were found not to be a significant issue for the City and further environmental analysis was not required in the Manteca General Plan EIR. For similar reasons, as mineral resources are not located in the vicinity of the proposed project or the City, development of the proposed project would not result in any impacts to mineral resources. Similarly, because a known mineral resource or a locally-important mineral resource recovery site does not exist within or in the vicinity of the City, the proposed project, in conjunction with other existing and future development in the City of Manteca, would not result in a cumulatively considerable impact associated with mineral resources. Therefore, cumulative impacts related to mineral resources would be considered less than significant. Mitigation Measure(s) None required

13 Endnotes 1 City of Manteca. Manteca General Plan 2023 Policy Document. October 6, City of Manteca. Manteca General Plan 2023 Environmental Impact Report. October 6, Natural Resources Conservation Service. Web Soil Survey for San Joaquin County. Accessed July 29,