2012-01-23_PERMIT FILE - C1982057A (7)

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failure area will also probably occur. The original failure surface appears to be located near the toe of the original natural slope, which was undercut by the excavation for Pond 16. The removal of the soils from this area and the placement of the fill materials for the haul road, which was located uphill of the pond cut most likely initiated the failure by removing support from the toe of the slope and adding additional driving forces. The failure of the Series 24 wells in 2002/2003 indicates that the slide was active, even though no visible signs of failure were observed in the present failure area We also believe that the regrading that occurred in 2007, as the haul road was reclaimed, increased the driving forces on the upper portion of the slope failure thus causing the current slope failure. The failure of these overburden soils and weathered claystone materials is most likely a result of the groundwater seepage occurring within the overburden soils and highly weathered claystone materials, as well as the additional loads applied to the upper portion of the failure by the placement of fill materials over the marginally stable natural soils. Loss of support created by the formation of the head scarp and downslope movement of the debris flow has contributed to the uphill propagation of the failure. 0 Preliminary Stabilization/Reconstruction Recommendations Based on the bedrock and groundwater surface maps that have been developed with the information obtained from the test holes/wells and our observations of the bedrock and groundwater conditions encountered in test pits, it appears that the groundwater, which we believe is a major contributor to the slope failure, is flowing in a south to southwest direction. The thickness of the highly weathered claystone materials, in which the groundwater is flowing, is highly variable and does not appear to be dipping to the west- northwest as does the underlying bedrock materials. Based on the groundwater and bedrock conditions observed to this date, we believe that it is imperative that an underdrain/cutoff trench be constructed along the north and east sides of the failure to intercept and divert the water away from the failed area. We believe that if the groundwater is adequately diverted from the failure area, the amount of regrading and buttress construction required at the lower portion of the slide can be substantially reduced. We strongly recommend that two test trench/pits be constructed at the site at the approximate locations shown in Figure #10 to provide additional information on the depth and condition of the bedrock materials and • groundwater flows. We recommend that the base of the trenches be excavated at least 2 feet into the Job Number: 08-8099 NWCC, Inc. Page 8