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for slopes: the Undrained Strength Stability Analysis (USSA) and the Effective Stress Stability <br />Analysis (ESSA). The USSA is performed to analyze the case in which loading or unloading is <br />applied rapidly and excess porewater pressures do not have sufficient time to dissipate during <br />shearing. This scenario typically applies to loading from, for example, embankment construction <br />where the loading takes place quickly relative to the permeability of the soils. Loading from <br />floodwaters also meets these qualifications. Unloading, such as dragline excavation during strip <br />mining, falls into this category, as well. This 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 pressures are <br />developed. For example, a slowly moving landslide is usually analyzed using the ESSA method. For <br />this reason, the ESSA is often referred to as the "long term" case. <br />Both the USSA and ESSA cases were examined as part of the Haul Road HR -1 stabilization project. <br />2.1.1 Geometry <br />A cross - section through the East Slide was used for the design seepage and stability analysis. This <br />cross - section, shown on Figure 5, includes all known bedrock layers and overlying colluvium. The <br />slope stability model developed for the design case used design finished grade for the colluvium <br />surface in the model geometry. The contact between the bedrock and the colluvium was estimated in <br />most locations, based on available data from surveys conducted as the slide was excavated. In <br />addition to these materials, a granular drainage blanket and a granular interceptor trench were also <br />included as shown in Figure 5. It should also be noted that a layer of Carbonaceous Shale was <br />included at the top of the bedrock bench which was exposed during construction. This layer was <br />deemed to be important for stability as a potential failure surface could penetrate this material. <br />It should be noted that the ground surface geometry used in this modeling exercise was based on <br />limited survey data during construction. A final as -built survey and associated final modeling are <br />discussed in Section 2.4 below. <br />2.1.2 Groundwater <br />Groundwater was incorporated into the slope stability analyses by first performing a finite element <br />seepage analysis. An assumption that there is one 7.5 -gpm seep every 50 feet along the length of the <br />slide area was made to calibrate a pre -slide seepage model. This resulted in a total head boundary <br />condition on the upslope side of the model of 6,350 feet. Potential seepage face review nodes were <br />placed along the entire ground surface of the model. <br />P Mpls 06 CO 26 06261003 Haul Road Slide Repair WorkFiles Report MCC Slump Repair - Instrumentation- DrainRev_ -05 -25 docx <br />4 <br />