ARCHIVED REPORTS_2001_2 - Laserfiche WebLink

Home Browse Search

clay. The cohesive soils can be characterized as very stiff to hard, and non -cohesive sands are dense to very dense. The stability conditions of the native subgrade beneath anticipated refuse loads are considered non -problematic. Assuming a refuse height of approximately 200 feet above the subgrade, CH2MHill calculated that the factor of safety for failure of a lean clay layer that is 10 feet below the liner grade would be 2.2 to 2.3. 3.3 Gross Stability Results As shown on the attached stability analyses, it is anticipated that a static factor -of -safety of 1.5 or more can be achieved for a range of liner interface shear strength values (Figure 3). While these shear strength values are within the range of values that have been obtained in many past liner installation projects, manufacturers may not provide certification for some of the upper -bound strengths (e.g., the manufacturer -listed internal strength for hydrated GCL may not exceed 5 degrees phi and 400 psf cohesion). Recognizing this condition, adchtional.,,analyses.,were,.�ornOeted to simulate liner geometries that include a GCL layer. Because both clay and GCL are currently permitted for use in the liner system, GCL was integrated in the analyses because its internal hydrated strength is typically the lowest of soil and geosynthetic components that may be included in geocomposite-linex systems which twill present a worst-case condition. _ The first case that was considered involves the southern border of the western extension of the existing Austin Road Landfill -and the western portions of the existing Forward Landfill (Figure 10 of the JTD). In these areas, existing refuse abuts unimproved areas where native soils will be excavated and new liner sections will be installed. The existing border of refuse in these areas is generally 600 feet or greater from the planned toe of slope for the landfill expansion. As shown on the stability cross-sections, for these areas, in order to achieve an adequate factor of safety in this area, a 350 -foot wide section of 40 -mil textured HDPE would need to be included in the liner system if a GCL layer is included in the design. This secondary HDPE section would sandwich the GCL layer along the outside (toe side) of the calculated failure surface, and would minimize the potential for sore -pore waters to "wick up" into the GCL. This configuration would therefore allow for use of non -hydrated GCL strengths (i.e., use of a typical interface strength between textured HDPE and the GCL [14 degrees phi and 0 psfJ). For the case where a GCL layer is used and existing refuse slopes are closer than 600 feet to the landfill toe, the analyses indicate that the GCL will need to be "sandwiched" between 40 -mil and 60 -mil textured HDPE layers for the entire length of the liner to achieve a static factor of safety of 1.5 or better. Again, use of the 40 -mil HDPE layer would eliminate the need to assume hydrated GCL shear strengths and allows the use of interface strengths between the GCL and textured HDPE layers. L:Allied\2000.193\Reports\finaljtd:Appendix E GLA:08/16/01