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3-7 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 Clear Lake clay (CL) soil type is common along the North Branch and South Branch of Littlejohn's Creek. 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. 3.5.2 Soil Properties Subsurface soil properties have been evaluated in several geotechnical investigations are summarized in Table 5. 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. The cohesionless materials can be characterized as dense to very dense. Because the sediments at the site were deposited as fluvial floodplain and channel deposits, the soil deposits at the site were found to be heterogeneous and laterally and vertically discontinuous. It is therefore difficult to characterize the subsurface soils into distinct units. However, based on a review of the previous investigations listed in Table 5, the subsurface soils can be divided into six general layers. It should be noted that the following generalized subsurface profile has been greatly simplified and is mainly based on the 1992 CH2M HILL investigation. The 1992 CH2M HILL investigation was chosen to be representative of the subsurface soils at the site because it is consistent with the other investigations and it also provides the most extensive geotechnical test data. Beginning at the existing ground surface (approximately elevation 39 feet amsl), the six layers are as follows: ♦ A 10- to 15 -foot layer of light brown material ranging from lean clay to silt and silty clay. This layer is dry and very stiff to hard in consistency, with moisture contents typically at or below the plastic limit. A UU triaxial compression test performed on a sample at a depth of 15 feet in CH2M HILL Boring BHA indicated an undrained shear strength of 5,145 pounds per square foot (psf). SPT blowcounts in this layer are typically greater than 50. ♦ A 16- to 20 -foot layer of material ranging from silty sand to clayey sand. This layer is dry to moist and typically dense to very dense. Forward Landfill SWT Engineering Joint Technical Document - April 2014 z:\projects\allied waste\forward\five year permit rvw 2013\jtd-5 yr pr 2013\text\sec 3—final.doc