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3.5.2 SOIL PROPERTIES Subsurface soil properties have been evaluated in seven separate soil investigations. The dates and extent of the investigations are summarized in Table 5. The approximate locations of the borings are depicted on Figure 7. 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 distinctunits. However, based on a review of the seven 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 sirnplified°wd-is.mainly base&on-the.:1.992 CH2M HILL investigation. The 1992 CH2M HILL investigation was chosen to be representative of the subsurface soils at"the s te'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-ma iai-ranging from lean-ciay 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 BH-4 indicated an undrained shear strength of 5,145..npounds per square foot (psf). SPT blowcouts 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. • A 5- to 10-foot layer of very stiff yellowish-brown lean clay. This material is moist, with moisture contents between the liquid and plastic limits. A consolidation test indicates that this material is consolidated to approximately three times the effective overburden stress resulting in an overconsolidation ratio (OCR) of 3. The OCR is the ratio of the maximum effective stress the soil has been subjected to in the past (preconsolidation pressure) to the existing effective stress in the soil. A UU triaxial compression test at an approximate depth of 34 feet in CH2M HILL Boring BH-1 indicated a shear strength of 2,286 psf. Forward Landfill JTD 3-8 L:\Allied\2000.193\Reports\finaljtd:Sec-3.0:08/20/01 BRYAN A.STIRRAT 6 ASSOCIATES