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Engineering Analysis in Support of the Use of Crushed Rock in LCRS Drainage Layer Module 1, Foothill Sanitary Landfill San Joaquin County, California 1.0- Introduction San Joaquin County is seeking approval from the CVRWQCB for substituting a crushed rock drainage layer in place of the rounded gravel. Crushed rock with a uniform size of '/2 inch is proposed to be used for the LCRS drainage layer. This design will provide drainage characteristics equivalent to the gravel layer. To provide protection against puncture of the 60-mil HDPE geomembrane from the crushed rock, a geotextile cushion layer will be placed directly above the geomembrane before placing the crushed rock. The construction documents for the LCRS drainage layer within Module 1 of the Foothill Sanitary Landfill specify rounded or sub-rounded gravel placed directly over the 60-mil high density polyethylene (HDPE) geomembrane liner. During the design of Module 1, it was envisioned that on-site gravel deposits would be mined and screened to provide the rounded gravel. However, subsequent subsurface investigation and testing of the on-site gravel deposits determined that the on-site gravel is significantly larger in size than that required by the construction specifications. The use of the crushed rock and the geotextile cushion result in the following two design requirements that need to be addressed: 3. The minimum mass per unit area (oz/yd2 or g/m2) of the geotextile cushion required to protect the geomembrane from the crushed rock; and 4. The shear strength of the interface between the geotextile cushion and the 60-mil HDPE geomembrane should be adequate for slope stability of Module 1. This report addresses the above design requirements. The calculations provided below determine the mass of the geotextile cushion required to protect the geomembrane from the N:\sacDP\2003\Prj\83\835608.3ap.doc (FSL) 1