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location) to measure porewater pressures within the buttress, refuse, and colluvium foundation <br />material underlying the refuse. <br />Finally, additional investigation work was performed upslope of the existing RPEE waste rock pile <br />working surface in the fall of 2013 to better characterize the colluvium that will form the foundation <br />of the future waste rock pile. This field campaign is discussed in more detail in Section 2.2.4. <br />2.2 Geotechnical Analysis and Design <br />The Effective Stress Stability Analysis (ESSA) is performed to account for slow loading or <br />unloading, or no external loading, in which the drained shear strength of the materials is mobilized <br />and no shear -induced pressures are developed. For example, a slowly moving landslide is best <br />analyzed using the ESSA method (albeit over time the rate typically increases, resulting in an <br />undrained condition). For this reason, the ESSA is often referred to as the "long term" case. <br />The ESSA was performed as part of the slope stability analysis for the RPEE waste rock pile. <br />2.2.1 Software <br />The software packages used in this exercise and modeling details are described below. <br />Analysis of the RPEE waste rock pile was performed using the deformation analysis approach. The <br />deformation analyses were conducted primarily using FLAC 7.0 (Fast Lagrangian Analysis of <br />Continua), though seepage and stability modeling in earlier phases of design and construction <br />utilized GeoStudio 2007 and FLAC31). FLAC 7.0 is a two-dimensional (21)) computer -modeling <br />program for engineering mechanics computation that simulates the behavior of structures built out of <br />soil, rock, or other geotechnical materials that may undergo plastic flow when their yield limits are <br />reached. Materials are represented by elements or zones, which form a grid to fit the shape of the <br />geometry modeled. Each element behaves according to a prescribed linear or nonlinear stress -strain <br />law in response to the applied forces or boundary restraints. The material can yield and flow, and the <br />grid can deform (in large -strain mode) and move with the material that is represented. <br />FLAC 7.0 is capable of modeling deformations and computing the factor of safety of a slope while <br />analyzing complex geometry, stratigraphy, and loading conditions. FLAC 7.0 can also model the <br />flow of groundwater through soil. The flow modeling may be done by itself, independent of the <br />5 <br />PAMpls\06 CO\26\06261003 MCC Refuse Pile Site Review & Permit\WorkFiles\Permit Application\2014 Revision\Exhibit 82 2014-05- <br />29.docx <br />