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To: Mike Peacock, Kathy Welt <br />From: Paul Swenson, Jed Greenwood <br />Subject: Summary of Additional 2D Stability Modeling for RPE East; Modified Phase 1 B Buttress <br />Date: October 12, 2012 <br />Page: 2 <br />Project: 06261003 <br />c: File <br />Stability Analysis <br />Two types of stability analyses are typically performed for dams and slopes: the Undrained Strength <br />Stability Analysis (USSA) and the Effective Stress Stability Analysis (ESSA). The USSA is performed to <br />analyze the case in which loading or unloading is applied rapidly and excess pore water pressures do not <br />have sufficient time to dissipate during shearing. This scenario typically applies to loading from <br />embankment construction, for example, where the loading takes place quickly relative to the permeability <br />of the soils. It is often referred to as the "end -of- construction" case. <br />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 pore pressures are <br />developed. For this reason, the ESSA is often referred to as the "long term" case. <br />Both the USSA and ESSA were performed as part of this slope stability analysis for the modified Phase <br />1B buttress. Furthermore, static and seismic analyses were also performed as part of this exercise. <br />Software <br />Computerized seepage and stability modeling were performed on the modified Phase 1 B buttress <br />configuration. The software packages used in this exercise and modeling details are described below. <br />Seepage Modeling <br />Seepage modeling was conducted using SEEP/W, a computer modeling program developed by GEO- <br />SLOPE International, Ltd. SEEP/W uses the finite - element analysis technique to model flow and pore - <br />water pressure distribution within porous materials such as soil and rock. This method was chosen <br />because comprehensive formulation makes it possible to analyze both simple and highly complex seepage <br />problems. It can formulate saturated and unsaturated flow, steady -state and transient conditions, and a <br />variety of boundary conditions. <br />SEEP/W generates an output file containing the heads at the nodes of the finite element mesh. Integration <br />of GEO SLOPE products allows the use of a SEEP/W head file in the slope stability program to compute <br />effective stresses. In this manner, the impact of seepage on stability can be evaluated. <br />P:\Mpis \06 CO\26 \06261003 MCC Refuse Pile Site Review & Permit \WorkFiles\Slope Modeling\9-27-12\RPE_East_Modeling_Summary_10- <br />12-2012.docx <br />