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Geotechnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey 8 Whitney, LLP <br />Section 6 -Stability of the Landslide Component of <br />the Dam <br />6.1 General <br />We evaluated the stability of the existing landslide using the following procedure: <br />• Performed static stability analyses to estimate the likely shear strength and pore <br />pressures along the probable failure surface at the time of failure, called the `pre- <br />failure' stability analysis case. Determination of the likely strength on the failure <br />surface is referred to as a "back calculated" shear strength procedure. <br />• Estimated the shear strength of the materials along the probable failure surface based <br />on information from borings in the landslide (Western Engineers, 1993). <br />• Performed a static stability analysis of the existing landslide (`post-failure' case) <br />using the strengths estimated from the above analyses and lower water pressures. The <br />• results of this analysis were further investigated for capacity to withstand seismic <br />loads using pseudo-static analysis to determine the yield acceleration of the slope. <br />The overall stability analysis for the landslide was similar to that for the main dam except we <br />used block-type failure surfaces for the landslide analysis. <br />6.2 Pre-Failure Static Stability Analysis <br />-. i <br />6.2.1 Representative Cross Section <br />The landslide has moved a relatively short distance down slope in recent years as compared <br />to the original mass landslide that moved to the valley bottom from much higher on the left <br />valley slope. Therefore, the existing topography is considered representative of the `pre- <br />failure' condition for the most recent past movement. Using the existing topography around <br />the landslide (Shipman, 1987) and information from the borings drilled in the landslide <br />(Westem Engineers, 1993), we developed a section to represent the pre-failure conditions. <br />We represented the subsurface stratigraphy with two layers: upper landslide material, which <br />includes an underlying 3-foot-thick "failure zone" which is considered to be reworked <br />landslide material, over lower landslide material that is in a less disturbed condition, as <br />shown on Figure 6.1. <br />GEI <br />20 <br />060150 0603-Ib Monammt Dam Gwtclmical Eva] <br />March 2006 <br />