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Geotechnical Evaluation of Mine-Induced Seismicity on Monument Dam <br />Dorsey & Whitney, LLP <br />Section 5 -Stability of the Earthfitl Dam <br />5.1 General <br />The earthfill dam is constnicted against and on top of an active landslide that forms the ]eft <br />abutment, GEI evaluated the stability of the earthfill dam using the following procedure: <br />Used estimated material properties obtained from previous studies to confirm the <br />results of that study and to evaluate the static stability of the downstream slope of the <br />earthfill dam. The results of this analysis were further investigated for capacity to <br />withstand seismic loads using pseudo-static analysis to determine the yield <br />acceleration of the downstream slope. <br />5.2 Data Available for Analysis <br />Armstrong and Associates performed a stability analysis of Monument Dam in 1983 <br />(Armstrong, 1983). Armstrong and Associates developed material properties for several <br />layers identified within the dam embankment based on a limited field exploration (boreholes <br />installed for piezometers P-1 and P-2) and a limited laboratory testing program. The resulting <br />factor of safety for the downstream slope with the reservoir at 10 feet below the dam crest <br />(coincident with the current emergency spillway elevation) was reported as 0,96 (Armstrong, <br />1983), among other results. The recommended minimum factor of safety for this type of <br />analysis would be 1.5 (SEO, 1988). <br />5.3 Liquefaction Potential <br />A liquefaction analysis was not performed for this study because of the lack of subsurface <br />data and material properties that would form the basis for analysis. Armstrong identified the <br />soil in the earthfill embankment as clayey sand (SC) and low plasticity clay (CL). The SC <br />soil has a minimum fines content of 15 percent, and the CL has a minimum of 50 percent. <br />Based on fines content alone, the CL materials would be considered non-liquefiable. The <br />soils in the landslide are a mixture of clayey and granular materials. Because of the mixed <br />deposition of the soils and rock boulders in the landslide, it may include local lenses or zones <br />of liquefiable materials, however the potential for these lenses or zones to be large or <br />continuous is low. The earthfill dam and the landslide should be further investigated with an <br />appropriate boring and laboratory testing program to identify potentially liquefiable soils. <br />GEI~ 17 0603500503-16Monummf DamGw~cdwulEvil <br />March 2006 <br />