Draft Environmental Impact Report - California Off Highway Vehicle ...

3.7 Geology and Soils
3.7 Geology and Soils
This section presents details about the existing setting and the regulatory setting for geology and
soils. It also presents an analysis of the geology and soils impacts that would result from
implementing the Clay Pit SVRA General Plan, including constructing and operating the
headquarters facilities.
3.7.1 Existing Setting
This section describes additional setting information to supplement the geology and soils setting
information provided in Section 2.3.1, “Physical Resources,” of the General Plan. The General Plan
describes the topography of the project site as characterized by a large basin and describes the
disturbed clay and cobble soil characteristics.
Soils
The primary geotechnical constraint identified on the site is the presence of moderate to highly
expansive near‐surface clay soils (Geocon 2010; Appendix D of the General Plan). These types of
soils can cause differential movement (shrinking or swelling) and damage to overlying structures.
Percolation test results also indicate that soils on‐site have very slow infiltration properties, which
would affect the design of leach fields. Refer to General Plan Section 2.3.1, “Physical Resources,”
for additional details regarding soils.
Seismicity and Surface Rupture
The project site is not located near any known “active” earthquake fault trace, according to the
California Geologic Survey (CGS 1997). The Alquist‐Priolo Earthquake Fault Zone maps show that
the only fault in Butte County considered active and within the fault zone is the Cleveland Hills
fault. This fault is shown on the Bangor 7.5‐minute quadrangle map (Gay 1977). The fault runs in a
nearly north‐south orientation directly south of Lake Oroville and approximately 5 miles east‐
southeast of Clay Pit SVRA. This fault last ruptured in 1975 (Butte County 2010:4.6‐7).
According to the California Geological Survey’s Probabilistic Seismic Hazard Assessment Program,
Butte County is considered to be within an area that is predicted to have a 10% probability that a
seismic event would produce horizontal ground shaking at a level that correlates to a Modified
Mercalli Intensity of V to VII, light to strong. As a result, the California Geological Survey has
defined the entire county as a seismic hazard zone (Butte County 2010:4.6‐9).
Liquefaction
Liquefaction is a process in which uniform, clean, loose, fine sandy, and silty sediments below the
water table temporarily lose strength during an earthquake and behave as a viscous liquid rather
than a solid. Liquefaction is restricted to certain geologic and hydrologic environments, primarily
recently deposited sand and silt in areas with high groundwater levels. In Butte County, areas of
Clay Pit State Vehicular Recreation Area
Draft EIR 3.7-1 February 2012