2012-01-23_PERMIT FILE - C1982057A (6)

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competent bedrock materials or until practical rig refusal is encountered. The test trench/pits should daylight into Pond 16 and will assist in providing temporary dewatering of the slope. We also recommend that the groundwater levels in the test holes/wells be monitored as the trench/pits are constructed and for at least six additional months after the trench/pits have been constructed. We believe that the information obtained from test trench/pits will allow us to design a cost effective underdrain/cutoff trench that should adequately dewater the slide area In addition, we believe that the test trench/pits can be incorporated into the final underdrain/cutoff trench design. We recommend that the drain system consist of a 6-inch diameter perforated PVC pipe that is surrounded with free draining gravels. The actual location and dimensions of the underdrain/cutoff trench will be determined after the bedrock and groundwater information from the test trench/pits is obtained. We believe that the most feasible method for stabilizing the failed slope is to construct a well drained earthen buttress fill at the toe of the slope failure and removing the fill materials that have been placed along the upper portions of the slope failure and in the old haul road alignment. However, this construction cannot occur until the failure area has had time to drain and the existing fill materials and overburden soils are dried out. is The earthen buttress fill materials should consist of the on-site soils and bedrock fill materials that are situated near the top of the failed area. The fill materials placed in the-buttress fill should be compacted to at least 95% of the maximum standard Proctor density and within 3 percent of the optimum moisture content. It appears that the existing fill materials and underlying natural soils presently range from 5 to 15 percent over the optimum moisture content and as noted above will have to be dried out before they can be used as fill materials. Ideally, the new embankment materials should be keyed into the underlying claystone-shale bedrock materials that are situated approximately 14 to 33 feet beneath the existing ground surface in the lower portion of the failed area. However, based on the depth to competent bedrock and the lack of stability of the existing fill materials and underlying natural soils, it may not be feasible to properly key in the new fill materials; therefore, the client should consider abandoning the existing roadway that crosses the slide area and consider constructing a new roadway along the southern side of Pond 16. If this alternate is opted for, the required site grading would consist primarily of the removal of the existing fill materials and a portion of the natural overburden soils. f, J Job Number. 08-8099 NWCC, Inc. Page 9