ARCHIVED REPORTS_2008_29 - Laserfiche WebLink

Home Browse Search

2.4 SOILS 2.4.1 SURFACE SOILS The surface soils near the site are mainly comprised of two soil types typical of Victor Formation sediments. The two soils, as mapped by the U.S. Soil Conservation Service, are known as Clear Lake clay (CL) and Jacktone clay (ST). These soils are similar in composition, being formed from alluvium. The Jacktone and Clear Lake clays are equally pervasive at the site with the Clear Lake clay more prevalent in the northern and southern portions of the site and the ST more prevalent through the middle portion. The ST covers the nearly flat terrain 100 to 200 feet from the North and South Creeks. Typically, this soil type is found on 0 to 2 percent slopes forming a moderately deep hardpan of somewhat poorly drained soil made up of generally dark gray clay about 2 inches thick, underlain by light gray clay loam, followed by hardpan of 3 inches. Permeability is low, the available water capacity is moderate, the shrink-swell potential is high, water may be perched on top of the hardpan following heavy rain, runoff is slow, and the water erosion hazard is slight. The typical uses of this soil type are irrigated cropland, orchards, vineyards, and sometimes irrigated pasture and urban development. The CL soil type is common along the North and South Creeks. The surface layer is dark gray clay about 29 inches thick followed by another 8 inches of dark gray clay, then 5 inches of mixed light brownish gray clay loam, then variegated dark grayish brown, weakly to strongly cemented hardpan to a depth of 60 inches. The permeability of this soil is low, available water capacity is moderate, the shrink- swell potential is high, water erosion hazard is low, and the soil is subject to rare flooding during abnormally high rainfall events. This soil type is typically used for irrigated cropland, orchards, and sometimes irrigated pasture and urban development. 2.4.2 SOIL PROPERTIES Subsurface soil properties have been evaluated in a number of separate soil investigations, including the most recent. The soils encountered during the geotechnical investigations generally consist of interbedded layers of sand, silt, and ® lean clay. The cohesive materials are generally of very stiff to hard consistency. Forward Landfill-Project Description 2-5 BRYAN A.STIRRAT&ASSOCIATES J:\Allied\Forward\2007.0018 Lateral Expansion\Reports\Prof Descrip\021308 Final Proj Des\Sec 2.doc:2/13/2008)