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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 />2.2.1.2 Stability Modeling <br />The slope stability analyses were conducted using SLOPE/W, a computer- modeling program <br />developed by GEO -SLOPE International, Ltd. SLOPE /W uses limit equilibrium theory to compute a <br />factor of safety for earth and rock slopes. It is capable of using a variety of methods to compute the <br />factor of safety of a slope while analyzing complex geometry, stratigraphy, and loading conditions. <br />As previously mentioned, SLOPE/W allows the user to import the groundwater head file from the <br />seepage analysis to compute effective stresses. <br />Spencer's method was used as the search technique to determine the factor of safety of the dikes in <br />this stability analysis. This method is considered the most adequate because it satisfies all conditions <br />of static equilibrium and provides a factor of safety based on both force and moment equilibrium. <br />2.2.2 Model Parameters <br />Standard penetration tests (SPTs) were conducted on in -place materials in the LRP in June 2009, as <br />well as a limited number of laboratory tests. Data from the SPTs were analyzed to establish the <br />range of potential friction angle and undrained shear strength values to be modeled. Figure 2 -1 <br />provides a plot of the friction angles determined from a blow count correlation developed by <br />P:\Mpls \06 CO\26 \06261003 MCC Refuse Pile Site Review & Permit \WorkFiles\Permit Application \Text \2010- 04- 19_RPE East Permit Application.doc 4 <br />