2019-01-18_REVISION - M1974086 (16)

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Ralston Quarry Slope Stability Prior to backfilling, a back drain would be installed at the heel of each stability embankment excavation to limit future saturation of the colluvial soils beneath the tailings. The lower portions of the slope, including the stability embankments then would be graded to a uniform slope angle of 3 : 1 (horizontal : vertical). This would include removal of the existing retention ponds. Our analyses indicate that this remedial approach would increase the calculated factors of safety against failure to 1.25 or higher. DISCUSSION For clarity, we have divided the study area into five zones generally from south to north: Zone 1 is the stockpile of relocated mine tailings constructed in 2017 and 2018. Zone 2 is the southern portion of the slope on which tailings were placed, and where additional filling appears to have buttressed the slope. No retention ponds were constructed at the toe of the slope in this zone. No slope failures have been reported in this zone. Zone 3 is the central portion of the slope where the 2017 slope failure developed. Zone 4 is the portion of the slope north of Zone 3 where the earliest, significant slope failure began to develop in 2003 and where remedial earthwork was performed by Asphalt Paving Company in 2010. Zone 5 is the portion of the west-facing ridge slope along which tailings were placed only on the top 25 feet (±) vertically, and is therefore remains in close to its pre- mining condition. No slope failures have been reported in this zone. These zones are shown on the Google Earth® image on the next page The results of the stability modeling and the remedial grading proposed by GROUND to yield factors of safety against slope failure in the future are addressed below for each zone. GROUND will be available to assist the Civil Engineer in preparing grading plans to implement the measures outlined. Job No. 18-3046 GROUND Engineering Consultants, Inc. Page 8