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Ralston Quarry Slope Stability <br /> Unit Weight Friction Angle Cohesion <br /> Material Y' 0 c <br /> (pc (degrees) (W <br /> Tailings 125 35 100 <br /> Colluvium 125 22 135 <br /> Shale Bedrock 130 20 1,000 <br /> We understand that the tailings were placed on approximately the natural ground <br /> surface that had received little preparation for filling and had not been provided with <br /> benches or drains. Subsequently, water accumulated at the contact between the tailings <br /> and the underlying clayey, colluvial soils, wetting those underlying soils. <br /> Our observations and the results of our analyses suggest that the most likely failure <br /> mode of the mine tailing slope is large-radius, rotational movements through both the <br /> tailings and colluvial soils. The stability of the tailings and colluvial soils appear to be <br /> controlled by the locations and degree to which perched groundwater affected the <br /> tailings and colluvium shear strength. (The failed areas largely corresponded to areas <br /> where retention ponds had been constructed, suggesting that the presence of those <br /> ponds was a significant factor in the development of the slope failures at the site.) <br /> Additional analyses were performed to model conditions following remedial construction. <br /> From those analyses, we have concluded that the stability of vulnerable slopes identified <br /> in the Discussion section of this report can be improved by the construction of upper and <br /> lower stability embankments on the slopes. <br /> Embankment construction will include cutting near horizontal benches through the <br /> tailings and the underlying colluvial soils to reach the underlying shale. Relatively <br /> coarse, granular materials (the tailings or similar) would be placed as compacted fill to <br /> backfill the excavation to approximate original grade. <br /> Job No. 18-3046 GROUND Engineering Consultants, Inc. Page 7 <br />