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The ESSA is performed to account for much slower loading or unloading, or no external loading, in <br />which the drained shear strength of the materials is mobilized and no shear - induced pressures are <br />developed. For example, a very slowly moving landslide is best analyzed using the ESSA method <br />(albeit over time the rate typically increases, resulting in an undrained condition). For this reason, the <br />ESSA is often referred to as the "long term" case. <br />Both the USSA and ESSA were performed as part of the slope stability analysis for the RPE East <br />refuse pile. <br />2.2.1 Software <br />Both seepage and stability modeling were performed on the RPE East refuse pile. The software <br />packages used in this exercise and modeling details are described below. <br />2.2.1.1 Seepage Modeling <br />Modeling of the seepage was conducted using SEEP /W, a computer modeling program developed by <br />GEO -SLOPE International, Ltd. SEEP /W uses the finite- element analysis technique to model flow <br />and pore -water pressure distribution within porous materials such as soil and rock. This method was <br />chosen because comprehensive formulation makes it possible to analyze both simple and highly <br />complex seepage problems. It can formulate saturated and unsaturated flow, steady -state and <br />transient conditions, and a variety of boundary conditions. <br />SEEP /W generates an output file containing the heads at the nodes of the finite element mesh. <br />Integration of GEO -SLOPE products allows the use of a SEEP /W head file in the slope stability <br />program to compute effective stresses. In this manner, the impact of seepage on stability can be <br />evaluated. <br />The mesh for the analysis consisted of triangular /rectangular unstructured finite elements and <br />rectangular structured elements for some zones. The mesh was made fine enough to capture the <br />effects of any areas of high hydraulic gradient and to ensure that the boundary conditions were <br />applied effectively to the interior of the model. Additionally, infinite elements were placed on the left <br />(downstream) boundary to minimize the boundary effects in this area. <br />P. \Mpls \06 CO\26`.06261003 RPE E Permit Mod Support \WorkFiles\Permit Application \Text \Exhibit 82 RPEE design 2011 -07 -21 TR 127 doc <br />