2018-07-13_INSPECTION - M2008082

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Mr. Joe Lamanna July 13, 2018 Page 2 average of 11.6 percent. The in-place density results are around 100 percent compaction (t10 percent) from the maximum dry density. At the base of the berm there is a sand and gravel layer just above bedrock for in borings B-1, and B-2. The berm fill was determined to be a sandy clay with gravel while drilling. The bedrock was most commonly a fine to medium grained, weathered sandstone. In B-3 there was claystone approximately 1.5 feet below the sandstone (Figure 3). Water was encountered within the boreholes at 17 to 19 feet below the surface. This is just above, or within the basal sand layer(Figure 3). This indicates that the sand layer is acting as a drain controlling the phreatic surface within the berm. SLOPE STABILITY ANALYSIS Slope stability analyses were performed for the Stage 6 using Slope/W computer software. Because a two to three-foot-thick sand and gravel layer was encountered in two of the bore holes, the slope stability analysis was modelled both with and without the sand and gravel zone. The stability analysis was conducted with static failure in both the north (upstream) slope, and the south (downstream) slope. Failure in the southern slope were evaluated for failure surfaces initiating in the upstream one-third of the dam crest, and across the entire dam crest. Each of these was performed for the typical (current) cross section (Figure 2). Modified sections were created by flattening the southern slope from current to 2:1 to 2.5:1 to 3:1 (horizontal to vertical) (Figures 4 through 11). These results are summarized in the table below: Factors of Safety Typical Existing 2:1 South Slope 2.5:1 South Slope 3.1 South Slope Cross-Section No Sand Layer Minor 1.10 1.40 1.65 1.95 Major 1.36 1.60 1.83 2.06 North Slope 2.09 2.08 2.08 2.08 Sand Layer Minor 1.08 1.44 1.67 1.96 Major 1.35 1.55 1.92 2.21 North Slope 2.07 2.07 2.07 2.07 CONCLUSIONS Following the slope stability analysis, the existing berm does not meet the DRMS 1.5 factor of safety value presented in their April 112, 2018 memorandum. This cad be rectified by flattening the southern slope of the berm. We plan to provide design recommendations on a follow up correspondence.