Laserfiche WebLink
• encountered in the test holes and pits, and our observations made of the site, we believe that the failure is <br />likely to propagate further to the northeast and uphill. Additional downslope movement of the debris flow <br />at the base of the failure azea will also probably occur. <br />Preliminary Stabilization/Reconstruction Recommendations <br />Based on the subsurface conditions encountered at the subject site, our understanding of the history of the <br />subject slope failure and development of the features at the site, our review of the photographs taken during <br />the construction of Pond 12 and shortly after, and the results of this investigation, we believe that there are <br />a several alternatives for stabilizing/reconstructing the subject slope failure. However, the type and degree <br />of slope reconstruction/remediation will be highly dependent on the feasibility of site dewatering. A <br />number of piezometers were installed across the failed area; however, these piezometers were installed in <br />late June and early July and after the peak seasonal runoff had occurred. We strongly recommend that the <br />groundwater levels be monitored until next summer so that the amount and direction of the groundwater <br />flow can be better evaluated and the dewatering system can be designed. <br />We believe that the most feasible method for stabilizing the failed slope is to construct a well drained <br />• earthen buttress fill at the toe of the slope failure and removing the fill materials that have been placed <br />along the upper portions of the slope failure, as well as in the old haul road alignment. The earthen buttress <br />fill materials should consist of the on-site soils and bedrock fill materials that are situated at or above the <br />top of the failed area. The fill materials placed in the buttress fill should be compacted to at least 95% of <br />the maximum standard Proctor density and within 3 percent of the optimum moisture content. <br />Ideally, the new embankment materials should be keyed into the underlying claystone-shale bedrock <br />materials that are situated approximately 21 to 27 feet beneath the existing ground surface in the lower <br />portion of the failed area. However, based on the depth to competent bedrock and the lack of stability of <br />the existing fill materials and underlying natural soils, it may not be feasible to properly key in the new fill <br />materials; therefore, the client should consider using the natural hillside on the south side of the drainage as <br />a buttress. This alternative would require that the existing drainage channel be filled in with properly <br />compacted fill materials and would most likely require that the existing drainage channel be realigned or a <br />culvert pipe be placed at the base of the channel. In addition, a substantial portion of Pond 12 will <br />probably have to be filled in and the point of dischazge at the existing dam may have to be reconstructed or <br />relocated. <br />In addition to the construction of the earthen buttress fill, a relatively deep subsurface cutoff trench drain <br />• system will most likely need to be installed at or above the failed area to intercept and divert the <br />Job Number: 07-7600 NWCC, Inc. Page 7 <br />