4.8 GEOLOGY AND SOILS

4.8-1

This chapter describes the existing geologic and soil conditions of the project site and evaluates the potential impacts to geology and soils associated with the project. This chapter also includes a discussion of cumulative impacts to geology and soils. Unless otherwise noted, existing conditions information in this chapter is from the Martial Cottle Park Final Resource Inventory report prepared in July 2009 by Wallace, Roberts and Todd; LSA Associates; and Design, Community & Environment. A. Regulatory Framework

The following section discusses geology and soils policies from regulatory agencies that have jurisdiction over the project site. 1. State Policies and Regulations a. Alquist-Priolo Earthquake Fault Zoning Act The Alquist-Priolo Earthquake Fault Zoning Act was passed by the Califor-nia Legislature in 1972 to mitigate the hazard of surface faulting to structures. The Act’s main purpose is to prevent the construction of buildings used for human occupancy on the surface trace of active faults. The Act addresses only the hazard of surface fault rupture and is not directed toward other earthquake hazards. According to the Act, local agencies must regulate most development in fault zones established by the State Geologist. Before a pro-ject can be permitted in a designated Alquist-Priolo Earthquake Fault Zone, the city or county with jurisdiction must require a geologic investigation to demonstrate that proposed buildings would not be constructed across active faults.1 b. California Seismic Hazards Mapping Act The California Seismic Hazards Mapping Act of 1990 (California Public Re-sources Code Sections 2690 through 2699.6) addresses seismic hazards other than surface fault rupture, such as liquefaction and seismically-induced land-

1 California Department of Conservation’s website, http://www.consrv.ca. gov/cgs/rghm/ap/Pages/index.aspx, accessed on March 31, 2010.

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slides. The Seismic Hazards Mapping Act specifies that the Lead Agency for a project may withhold development permits until geologic or soils investiga-tions are conducted for specific sites and mitigation measures are incorporated into project plans to reduce hazards associated with seismicity and unstable soils.2 c. California Building Code The California Code of Regulations (CCR), Title 24, is also known as the California Building Standards Code. The California Building Standards Code combines three types of building standards from three different origins:

♦ Building standards that have been adopted by State agencies without change from building standards contained in the International Building Code.

♦ Building standards that have been adopted and adapted from the national model code standards to meet California conditions.

♦ Building standards, authorized by the California legislature, that consti-tute extensive additions not covered by the model codes that have been adopted to address particular California concerns.

Part 2 of Title 24 is the California Building Code (CBC), which is based on the 2006 International Building Code. The International Building Code was developed by the International Conference of Building Officials to provide a set of consistent standards for building structures. The Code requires strict building standards for essential facilities and structures on soft soil where shaking intensity from a potential earthquake is high. Section C3-1 of the County of Santa Clara Ordinance Code adopts the 2007 CBC as the building code of the County.

2 California Department of Conservation’s website, http://www.consrv.

ca.gov/CGS/shzp/Pages/article10.aspx, accessed on March 31, 2010.

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d. California Public Resources Code California Public Resources Code Section 5097.5 prohibits excavation or re-moval of any “vertebrate paleontological site […] or any other archaeological, paleontological or historical feature, situated on public lands, except with express permission of the public agency having jurisdiction over such lands.” Public lands are defined to include lands owned by or under the jurisdiction of the State or any city, county, district, authority, or public corporation, or any agency thereof. Section 5097.5 states that any unauthorized disturbance or removal of archaeological, historical, or paleontological materials or sites located on public lands is a misdemeanor. 2. Local Policies and Regulations a. Santa Clara County General Plan The County’s General Plan contains several goals and policies relevant to geology and soils. Goals and policies relevant to the project are listed in Ta-ble 4.8-1. b. County of Santa Clara Grading Ordinance The County’s Grading Ordinance is contained in Title C, Chapter III of Di-vision C12, Subdivisions and Land Development, of the County Ordinance Code. The Grading Ordinance establishes minimum requirements for grad-ing work within the county and outlines procedures to enforce these re-quirements. The purpose of this Ordinance is to protect surface water quality by preventing grading that would cause soil erosion and soil sediment trans-port. B. Existing Conditions

The project site is located within a region of significant seismic activity and geotechnical instability. This area is included in the Coast Ranges Geomor-phic Province, which extends from south of the Oregon border to central coastal California. Santa Clara County and the City of San Jose are located in the southern coastal ranges within the Santa Clara Valley. The project site is located approximately 1 mile to the west from the Guadalupe River, and

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TABLE 4.8-1 SANTA CLARA COUNTY GENERAL PLAN POLICIES RELEVANT TO GEOLOGY AND SOILS

Strategy/Policy Number Strategy/Policy Content Health and Safety Chapter/Natural Hazards

Policy C-HS 28

Countywide strategies for reducing the threat of natural hazards to life and property should include: a. Inventory hazards and monitor changing conditions. b. Minimize the resident population within high hazard areas. c. Design, locate and regulate development to avoid or withstand

hazards. d. Reduce the magnitude of the hazard, if feasible. e. Provide public information regarding natural hazards.

Strategy #1 Inventory Hazards and Monitor Changing Conditions

Policy C-HS 29 Inventories and mapping of natural hazards should be adequately maintained for use in planning and decision-making.

Strategy #2 Minimize the Resident Population Within High Hazard Areas

Policy C-HS 30

Local jurisdictions’ urban development and land use policies should minimize the resident population within areas subject to high natural hazards in order to reduce: a. The overall risk to life and property; and b. The cost to the general public of providing urban services and

infrastructure to urban development.

Policy C-HS 31 Cities should not expand Urban Service Areas into undeveloped areas of significant hazards.

Policy C-HS 32

Areas of significant natural hazards shall be designated in the County’s General Plan as Resource Conservation Areas with low development densities in order to minimize public exposure to avoidable risks.

Strategy #3 Design, Locate and Regulate Development to Avoid or Withstand Hazards

Policy C-HS 33

Development in areas of natural hazards should be designed, lo-cated, and otherwise regulated to reduce associated risks, by regu-lating the type, density, and placement of development where it will not: a. Be directly jeopardized by hazards; b. Increase hazard potential; and c. Increase risks to neighboring properties.

Source: Santa Clara County General Plan, 1994, http://www.sccgov.org/portal/site/dpd/, ac-cessed on January 6, 2010.

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1 mile to the northeast of Coyote Creek. These two fluvial systems are the source of alluvial deposits at the project site. 1. Fault Zones The major faults in the project region trend northwest/southeast. Within the vicinity of the project site the major faults are the San Andreas Fault, a right-lateral strike-slip fault near the crest of the Santa Cruz Mountains to the west, and the Hayward and Calaveras faults, both right-lateral strike-slip faults in the Diablo Range to the east. These faults have exhibited significant tectonic motion in recent times and in the distant geological past. In 1979, the Calav-eras Fault produced a 5.9-magnitue earthquake yielding a rupture of 1 centi-meter along a 39 kilometer long fault scarp within Santa Clara and San Benito Counties. In 1984, the Calaveras Fault ruptured again at a magnitude of 6.1, resulting in a surface rupture of 20 centimeter along a 1.2 kilometer-long fault scarp. This second rupture triggered an afterslip in a 15 kilometer-long creep-zone to the south. In 1989, the Loma Prieta earthquake occurred along the San Andreas Fault near the Loma Prieta Peak in the Santa Cruz Mountains at a magnitude of 6.9. The average strike-slip displacement of this rupture was 1.2 meters while the average reverse-slip displacement was 1.6 meters. The Loma Prieta was the largest earthquake to occur on the San Andreas Fault since the San Francisco earthquake in April 1906. Numerous other faults are located in the surrounding hills and throughout the Santa Clara Valley. The 1972 Quaternary Geologic Map of San Jose East Quadrangle locates the Piercy and Silver Creek faults just north of the project site in the adjacent hills. These faults run parallel to the San Andreas Fault and have the poten-tial to produce seismic activity. Geologic formations are shown in Figure 4.8-1. 2. Liquefaction Because the project is situated in a region of significant seismic activity and geotechnical instability, there is the potential for earthquakes to occur and produce severe ground shaking and result in ground failure or damage to structures.

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As shown in Figure 4.8-2, the project site is located within a liquefaction haz-ard zone. Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. The poorly drained soils associated with the project site are prone to liquefaction. 3. Soils Soils in the Santa Clara Valley primarily consist of clay in the low-lying areas, loam and gravelly loam in the upper portions of the valley, and eroded rocky clay loam in the hills. The clayey soils that make up the majority of the val-ley floor, including the project site, are derived from alluvial deposits from the surrounding and upstream geological formations. The valley is filled by as much as 1,950 feet of primarily alluvial sediment largely accumulated within the last 780,000 years. These deposits are essentially flat-lying. Prime farmland is located throughout the valley floor, including the project site. See Chapter 4.3, Agricultural Resources, for more information on soils as they pertain to agriculture. Figure 4.8-3 shows the soil types existing on the pro-ject site. 4. Other Geologic Hazards Other hazards associated with earthquakes include surface rupture, differen-tial settlement, seismically-induced landslides, and seismically-induced inunda-tion. Additional hazards related to soil and geologic conditions include com-pressible soils (subject to shrink and swell behavior), weak soils (subject to failure), lateral spreading, and liquefaction or collapse. 5. Paleontological Resources A fossil locality search was conducted by the University of the California Museum of Paleontology (UCMP), Berkeley, for a previous study that in-cluded the San Jose metropolitan area. No fossil localities are recorded within or adjacent to the project site.

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C. Standards of Significance

Geology and soils impacts associated with the project would be considered significant if the project would:

1. Expose people or structures to potential substantial adverse effects, in-cluding the risk of loss, injury, or death involving: a. Strong seismic ground shaking. b. Seismic-related ground failure, including liquefaction.

2. Result in substantial soil erosion or siltation or the loss of topsoil.

3. Be located on a geologic unit or soil that is unstable, or that would be-come unstable as a result of the project, and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction, collapse, shrink/ swell potential, soil creep or soil erosion.

4. Be located on expansive soil, as defined in the report, Soils of Santa Clara County, or the California Building Code, creating substantial risks to life or property.

5. 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.

6. Cause substantial compaction or over-covering of soil either on-site or off-site.

7. Cause substantial change in topography or unstable soil conditions from excavation, grading, or fill.

8. Be located in an area designated as having a potential for major geological hazard.

9. Be located on, or adjacent to, a known earthquake fault.

10. Directly or indirectly destroy a unique paleontological resource or site or unique geologic feature.

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D. Impact Discussion

All potential impacts described below would be the same for Phase I and sub-sequent project phases. As such, project-level and program-level components are not distinguished below. 1. Exposure of People or Structures to Potential Substantial Adverse

Effects, Including the Risk of Loss, Injury, or Death Involving: a. Strong Seismic Ground Shaking Large earthquakes could generate strong to violent ground shaking at the pro-ject site and could cause damage to structures and threaten public safety. The project site lies within a seismically active region that includes much of west-ern California. Several active faults are present in the region, including the San Andreas, Hayward, and Calaveras faults. These faults are capable of gen-erating large earthquakes that could produce strong to violent ground shaking at the project site. The United States Geological Survey (USGS) has esti-mated that there is a 70 percent chance of a large earthquake (magnitude 7 or greater) in the Bay Area by the year 2030.3 At present, it is not possible to predict precisely when or where earthquakes will occur on these faults. During an earthquake, seismic risk to a structure would depend on the dis-tance to the earthquake epicenter, the characteristics of the earthquake, the subsurface conditions underlying the structure and its immediate vicinity, and the characteristics of the structure. The project site is located approximately 7 miles from the Hayward Fault, 9 miles from the Calaveras Fault, and 14.5 miles from the San Andreas Fault.4 Additionally, the project site is on rela-tively thick, clayey alluvial deposits that could cause amplification of ground shaking.

3 Working Group on California Earthquake Probabilities, 2003, Earthquake

Probabilities in the San Francisco Bay Region 2002 – 2031: A summary of Findings, U. S. Geological Survey Open File Report 03-214.

4 USGS Google Earth Fault Mapper, http://earthquake.usgs.gov/regional/ nca/haywardfault/, accessed March 22, 2010.

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Like most parks, development under the project would be minimal. Facilities allowed under the project would be limited to: ♦ Entry kiosk ♦ Visitor center ♦ Visitor pavilion ♦ Restrooms ♦ Rain shelters ♦ Agriculture packaging, processing, and storage facilities ♦ Café ♦ Catering facilities ♦ Caretaker residence or site host ♦ Produce stands ♦ Greenhouses ♦ Shade houses ♦ Classrooms ♦ Miscellaneous small structures associated with ongoing agricultural and

cooperative management activities As required by the County of Santa Clara, these buildings would be required to meet the CBC regulations for seismic safety (i.e. reinforcing perimeter and/or load bearing walls, bracing parapets, etc.). In addition, all project-related grading, trenching, backfilling and compaction operations would be conducted in accordance with the CBC, and conform to regulations for seis-mic safety contained therein. Compliance with the CBC for development at the project site would reduce potential impacts associated with strong seismic ground shaking to a less-than-significant level. b. Seismic-Related Ground Failure, Including Liquefaction During large earthquakes, ground failure can occur along fault traces. Ground failure associated with earthquakes generally occurs along faults that have been recently active. Active faults are not known to be present at the site. Therefore, ground failure associated with fault movement is considered very unlikely at the site.

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The project site is located within a liquefaction hazard zone.5 Soils at the pro-ject site are considered to be poorly drained, with potential to lose strength and stiffness resulting from earthquake shaking or other rapid loading. Mod-ern geotechnical engineering procedures such as soil testing, proper design, and quality construction controls can identify and mitigate for liquefiable soils during site development. By applying knowledge about the kinds of soils, their strengths, and groundwater conditions, and by properly designing and constructing fills and foundations, modern soil engineering practices have improved greatly. These standards reduce the potential for liquefaction to levels that are generally considered acceptable. Since the project would allow only a limited amount of development, and any constructed buildings would be subject to standard geotechnical engineering, the risk of liquefaction is low. The impact related to seismic ground failure and liquefaction is considered to be less than significant. 2. Substantial Soil Erosion or Siltation or the Loss of Topsoil Soil erosion, siltation, and the loss of topsoil could result from a variety of activities. These impacts commonly occur during construction, when vegeta-tion is removed from a building site and the soil is left out to dry. However, development at the project site would be minimal. The only buildings al-lowed for under the project would be minimal, as noted in Section D.1. Con-struction of buildings on the site would be subject to the County of Santa Clara Grading Ordinance, which includes standards for erosion control such as adequate slope protection with constructed dikes, swales, and ditches.6 The minimal amount of development expected on the project site would not pro-duce substantial soil erosion, siltation, or contribute to the loss of topsoil. The ongoing agricultural activities on the project site do have the potential to create long-term, substantial soil erosion, siltation, and contribute to the loss of topsoil. In an agricultural setting, these issues commonly occur when soils

5 County of Santa Clara Parks and Recreation Department, 2009, Martial Cottle Park Final Resources Inventory, page II-6.

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lack proper nutrient and water management. Conditions in most California soils do not favor the buildup of organic matter; therefore, they require regu-lar additions of organic matter or the use of cover crops to increase soil aggre-gate stability, soil tilth, and diversity of soil microbial life.7 Without proper management, soil particles become dry and are easily swept up with the wind or loaded into stormwater runoff. The project incorporates guidelines that focus on building soil health to pre-vent erosion, siltation, and topsoil loss. Guideline SOIL.3 requires agricul-tural land lessors at the project site to employ sustainable soil practices for building soil health, such as using cover crops, maintaining vegetated soils, mulching, and composting. Additionally, Guideline SOIL.5 requires that farmers receive education on reducing soil erosion and Guideline SOIL.6 re-quires that specific erosion limiting Best Management Practices (BMPs) are utilized. The proposed project guidelines, in combination with the County Grading Ordinance, would reduce potential erosion impacts to a less-than-significant level. 3. Location on a Geologic Unit or Unstable Soil that Could Result in

On- or Off-Site Landslides, Lateral Spreading, Subsidence, Liquefac-tion, Collapse, Shrink/Swell Potential, Soil Creep, or Erosion

As discussed previously, soils on the project site are considered to be poorly drained. These unstable soils would be subject to potential landslides, lateral spreading, subsidence, liquefaction, collapse, shrink/swell, soil creep, and ero-sion. The limited amount of development allowed under the project could occur in such hazard areas. However, new construction would be required to comply with the CBC, which contains building criteria and standards that are designed to reduce geologic risks to acceptable levels, and the County of Santa

6 County of Santa Clara Grading Ordinance, 2001, page 24. 7 University of California Agriculture Research and Education Program,

What is Sustainable Agriculture? http://www.sarep.ucdavis.edu/Concept.htm, ac-cessed March 23, 2010.

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Clara Grading Ordinance, which establishes requirements for grading work that reduces the potential for soil erosion and soil sediment transport. Development under the project would be subject to the CBC and the County Grading Ordinance, which would ensure that potential unstable soil impacts would be less than significant. 4. Location on Expansive Soil, Creating Substantial Risks to Life or

Property As previously discussed, soils on the project site are considered to be poorly drained. The project site also has expansive soils. These unstable, poorly drained soils, and expansive soil, are a potential hazard. Modern geotechnical engineering procedures including soil testing, proper design, and quality con-struction controls can identify and mitigate for expansive soil during site de-velopment. These standards reduce the potential risk of expansive soil to lev-els that are generally considered acceptable. Since the project would allow only a limited amount of development, and any constructed buildings would be subject to standard geotechnical engineering, the risk to life or property from expansive soil is low. Impacts related to expansive soil are therefore considered to be less than significant. 5. Soils Incapable of Adequately Supporting the Use of Septic Tanks or

Alternative Wastewater Disposal Systems The project would not employ septic tanks or alternative wastewater disposal systems, as described in Chapter 3, Project Description. Thus, there would be no impact related to soils incapable of adequately supporting such facilities. 6. Substantial Compaction or Over-Covering of Soil either On- or Off-

Site There are few development projects that do not result in some form of com-paction and/or over-covering of soil. The development allowed for under the project would be entirely on-site and limited to roads and trails that would require soil over-covering as well as constructing several new buildings, as noted in Section D.1, above.

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The amount of soil that would be over-covered in this project is insubstantial in comparison to the amount of farmland that would be preserved. The pro-posed project would not impact any soils off-site. Therefore, the impact related to substantial compaction or over-cover of soil either on- or off-site would be less than significant. 7. Substantial Change in Topography or Unstable Soil Conditions from

Excavation, Grading, or Fill As previously discussed, the project site has unstable soils. However, the in-stability results from the type of soil that exists on the project site and its characteristics. Unstable soils on-site do not result from a substantial change in topography, excavation, grading, or fill. The project would not involve changes in topography due to excavation, grading, or fill. Therefore, the impact related substantial change in topography, excavation, grading, or fill is considered to be less than significant. 8. Location in an Area Designated as Having a Potential for Major Geo-

logical Hazard The project site, as discussed previously, is not at risk of seismic-related ground failure; however, the site may experience strong seismic ground shak-ing. Additionally, the existing soil on-site is known to be unstable and is lo-cated in an area with a potential risk for liquefaction.8 The limited amount of development allowed under the project could be im-pacted by such a geologic hazard; however, development would be required to adhere to the CBC and the County Grading Ordinance. The CBC and the Grading Ordinance are designed to reduce the risks of geologic hazards to acceptable levels. Therefore, impacts would be less than significant.

8 County of Santa Clara Parks and Recreation Department, 2009, Martial

Cottle Park Final Resource Inventory, page II-6.

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S T A T E P A R K G E N E R A L P L A N / C O U N T Y P A R K M A S T E R P L A N E I R

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9. Location On, or Adjacent To, a Known Earthquake Fault As previously mentioned, there are no known earthquake faults on the pro-ject site. The project site is located approximately 7 miles from the Hayward Fault, 9 miles from the Calaveras Fault, and 14.5 miles from the San Andreas Fault.9 The likely risks associated with proximity of these faults to the pro-ject site are limited to strong seismic ground shaking since surface rupture at the project site is considered very unlikely. The impact related to proximity to a known earthquake fault is considered to be less than significant. 10. Direct or Indirect Destruction of a Unique Paleontological Resource

or Site or Unique Geologic Feature The entire project site is composed of Holocene (10,000 years B.P. to present) alluvial deposits. The depth of these Holocene deposits in the project site is unknown, but these may extend 25 to 35 feet below the ground surface.10 Holocene deposits are too recent to contain paleontological resources (fossils), and it is not anticipated that ground-disturbing project activities in the project site will impact buried paleontological resources. Therefore, impacts would less than significant. E. Cumulative Impacts

Because impacts related to geology and soils tend to be site-specific, this sec-tion analyzes potential impacts relating to geology and soils that could occur from a combination of the proposed project with other reasonably foresee-able projects in the project vicinity. The proposed project would bring Park visitors and staff into a zone of high seismic hazard than would have been the case without the project or other development projects in the vicinity of the site. However, application of

9 USGS Google Earth Fault Mapper, http://earthquake.usgs.gov/regional/

nca/haywardfault/, accessed March 22, 2010. 10 County of Santa Clara Parks and Recreation Department, 2009, Martial

Cottle Park Final Resource Inventory, page II-7.

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relevant engineering standards is considered sufficient to reduce risks to visi-tors and staff to less-than-significant levels. The proposed project in combina-tion with other development projects in the site’s vicinity would therefore not cause any significant cumulative impacts.