2011-07-01_PERMIT FILE - C1982057A (3)

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Conclusions and Recommendations: Based on the groundwater measurements and observations made at the site during 2008, 2009 and 2010, and our discussions with Seneca regarding possible solutions for dewatering and stabilizing the landslide areas, NWCC has developed the following plans for reducing groundwater seepage into the failure /landslide areas, as well as site grading recommendations for reconstructing/regrading the failed areas to more stable configurations. Pond 016 Landslide: As stated in our original report, NWCC believes that groundwater flowing within the weathered/highly fractured claystone materials is daylighting within the overburden soils and fills materials resulting in the slope failure. This groundwater must be intercepted and directed away from the failed areas to improve slope stability. Based on the information obtained from the test pit/trenches excavated in 2009, it does not appear that it will be feasible to construct trench drains within the existing fill materials and overburden soils without laying back/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 I (Vertical) slope configuration; however, the overburden soils and highly weathered bedrock materials should be excavated to slope configurations of 2 (Horizontal) to I (Vertical) or flatter. Based on our discussions with Seneca, a plan has been developed that will consist of the removal of the failed materials along the northern half of the slide area and then constructing rock drains at the base of the excavation and in the competent bedrock materials. Fill materials will be placed over the rock drains so that adequate road grade can be maintained for the property owners. The approximate location of this excavation and drains (A -A') is shown in the Pond 016 Area Remediation Plan and Road Profile, Exhibit No. 20 -D.2, Sheet I of 2 and the cross sections for this area are shown in the Pond 16 Area Remediation Plan and Post Mine Cross Section, Exhibit No. 20 -D.2, Sheet 2 of 2, which are provided in Appendix C. It is NWCC's opinion that removal of saturated fill materials, overburden soils and weathered/highly fractured claystone materials from the upper portion of the failure will significantly reduce the driving forces contributing to the failure condition. We also believe that a rock drains and open channels constructed within the competent bedrock materials should significantly improve the subsurface drainage conditions at the site and help stabilize the lower portion of the failed area. Due to the amount of groundwater seepage that has been encountered at the site, NWCC recommends that the drainage collection system be installed at the base of the cut slope excavated into the competent bedrock materials. The base of the drain should be at least 5 feet in width. The drainage system should consist of free draining gravels and cobbles. NWCC recommends that free draining gravels consist of screened gravels and cobbles ranging from 3 to 6 4 inches in size and containing less than 5 percent passing the No. 200 sieve. Details for the proposed drainage system are shown on the plan provided in Pond 016 Area Remediation Plan and Road Profile, Exhibit No. 20 -D.2. Sheet I of 2 in Appendix C. Depending on the amount of groundwater flow encountered in the base of the excavation, the use of perforated, 6 -inch diameter PVC pipe (SDR -35) surrounded with gravels ranging from 3/8 to 3 inches in size may be required. The actual depths and finished configurations for the cut slopes and drains, and trenches should be determined in the • field as the groundwater and bedrock conditions are exposed and can be observed. In addition, the details and Job Number: 09 -8099 NWCC, Inc. Page 4