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Ralston Quarry Slope Stability <br /> Groundwater was encountered in the test holes at the time of drilling and then re- <br /> measured several times. Water levels and measurement dates are shown on the test <br /> hole logs. At the upper test holes (Test Holes 1 and 4) depth to groundwater typically <br /> was on the order of 30 feet whereas at the test holes at lower elevation (Test Holes 2 <br /> and 3) groundwater depths were more variable, but generally on the order of 18 feet. <br /> Groundwater levels can be expected to fluctuate, however, in response to annual and <br /> longer-term cycles of precipitation and surface drainage. Transient, local zones of <br /> perched groundwater likely develop commonly within the tailings and at the contact <br /> between the tailings and the native soils. Transient seeps and springs have been <br /> reported on the slope at various times, particularly in the areas where slope failures have <br /> occurred. <br /> SLOPE STABILITY MODELING <br /> The slope stability analysis was performed by using the Slope-W computer program. <br /> The Morgenstern-Price method was used to estimate strength parameters for the <br /> assumed factor of safety. <br /> To supplement the laboratory strength data, slope stability analysis was performed on a <br /> critical slope through the area of the 2017 slope failure. This allowed the strength <br /> parameters of the mine tailings and colluvium to be back calculated, assuming a factor <br /> of safety of just below 1.0 for that case. Precipitation and snowmelt were assumed to <br /> have saturated the colluvial soils. (These assumptions are supported by previous slope <br /> failures, where groundwater seepage was reportedly observed near the toe of the <br /> failures shortly prior to the slope failures.) The results of this analysis yielded values for <br /> the angles of internal friction similar to those obtained from laboratory testing. <br /> Based on those combined findings, the parameters tabulated below were used to in the <br /> further stability modeling. Note that the unit weights listed are un-saturated (not <br /> buoyant)weights. <br /> Job No 18-3046 GROUND Engineering Consultants, Inc. Page 6 <br />