2010-05-07_PERMIT FILE - C1994082A

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The weathered claystone materials were nil to slightly sandy, moderately to highly plastic, blocky, highly fractured, moist to very moist and brown to gray in color. A coal layer, approximately 9 to 10 feet in thickness was encountered below the weathered claystone materials in TH -13. Claystone -shale bedrock was encountered below the coal layer and extended to the maximum depth investigated, 80 feet. Claystone -shale bedrock materials were encountered in TH -14 below the weathered claystone materials and these bedrock materials were encountered from approximately 55 to 95 feet bgs. Coal was encountered beneath the claystone -shale bedrock materials in TH -14 at a depth of approximately 95 feet and extended to the maximum depth investigated, 100 feet. Groundwater was encountered in TH -13 and TH -14 at depths ranging from 58 to 70 feet bgs at the time of drilling. The groundwater levels were encountered at depths ranging from 38 to 53 feet bgs when measured 3 to 73 days after drilling was completed. Depths at which groundwater was encountered, along with the number of days after drilling that measurements were completed are shown on the test hole logs (Figure # 1). Conclusions and Recommendations: Based on the results of measurements and observations made over the last 18 months at the project site and our discussions with Seneca regarding possible solutions for dewatering the slide area and stabilizing the slide area, NWCC has developed the following plans for reducing the groundwater seepage into the failed areas, as well as site grading recommendations for bringing the failed areas to a more stable configuration. NWCC believes that groundwater flowing within the weathered claystone and claystone -shale bedrock materials is daylighting within the overburden soils and fill materials resulting in the subject slope failure. This groundwater must be intercepted upslope of the failure areas to improve stability. Based on the information obtained from the test pit/trenches excavated in 2009, it does not appear feasible to construct trench drains within the existing fill materials and overburden soils without laying/constructing these materials to a flatter and more stable configuration. NWCC believes that the competent bedrock materials at the site can be safely excavated to a 0.5 (Horizontal) to 1 (Vertical) slope configuration; however, the existing fill materials, overburden soils and highly weathered bedrock materials should be excavated with slope configurations on the order of 2 (Horizontal) to 1 (Vertical) or flatter. Based on our discussions with Seneca, a plan has been developed that will consist of construction of a permanent/open dewatering trench (Trench) along the upper portions of the failed area, and construction of a channel drain (Channel) constructed at the toe of the failure area and within the existing drainage channel (YPM -6 -3). The approximate locations of the proposed Trench and Channel are shown in the attached plans (Sheet 1 of 2). It is NWCC's opinion that removal of saturated fill materials and overburden soils from the upper portion of the failure during the construction of the Trench will significantly reduce the driving forces causing the failed area to slide downhill. In addition, we believe that a well drained buttress (Channel) constructed at the toe of the failed area will assist in drainage and stabilization of the failed materials. Due to the amount of groundwater seepage that has been encountered at the site, NWCC recommends drainage collection systems be installed in the base of the Trench and Channel. The drainage systems should consist of slotted, 4 to 6 -inch diameter PVC pipe (SDR -35) surrounded by relatively free draining gravels. NWCC recommends free draining gravels consist of screened gravels ranging from 3/8 to 4 inches Job Number: 07 -7600 NWCC, Inc. Page 3