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

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recommendations outlined in this report may require reevaluation and should be based on the conditions encountered in the excavations at the time of construction. This remediation plan will require that the existing access road, which presently crosses the failed area, be reconstructed and placed in the general location of the previous roadway alignment. The overburden soils and weathered claystone materials that will be excavated from the failed area will be used to fill in Pond 016A. Pond 016A Landslide: We believe that the failure mechanism for this slope failure is similar to the mechanism causing the Pond 016 failure and that groundwater flowing within the weathered/highly fractured claystone materials is daylighting in the overburden soils and fills materials resulting in the subject slope failure. Based on our observations made at the site, the results of the recent subsurface investigations discussed previously in this report and our discussions with Seneca, we believe that the most effective plan for stabilizing the slope failure situated above Pond 016A will be to construct a well drained buttress fill at the toe of the landslide and immediately upslope of the existing Pond 016A embankment fill, as well as the construction of a drainage channel /interceptor ditch across the upper portion of the failed area. The above scenario will require that Pond 016A be filled with properly compacted and drained backfill materials. Based on our observations made in field, it appears that the embankment fill constructed as part of Pond 016A is currently stable and does not appear to be experiencing any downslope soil movement. We believe that the materials being excavated from the Pond 016 failure can be used to fill Pond 016A. We recommend that the fill materials be placed and compacted in 12 to 18 inch loose lifts to at least 90 percent of the maximum standard Proctor density and within +/- 4 percent of the optimum moisture content determined in accordance with ASTM D698. Prior to the placement of the fill materials, we recommend that the sediments in the base of Pond 016A be stabilized. Based on the amount of groundwater observed seeping from the edges of the head scarps and from test holes TH -13 and TH -14, we recommend that the subsurface drainage flowing through the weathered claystone materials, as well as the surface runoff flowing into the pond be directed away from the proposed fill areas prior to placement of fill materials. The subsurface and surface runoff should be collected in a new ditch/channel (PM -2) that is excavated into the competent bedrock materials and below the point at which the subsurface runoff is flowing into the failed area. The base of the ditch should be at least 10 feet in width and be properly lined with riprap or TRM. We also recommend that the overburden soils and highly weathered claystone materials exposed in the sides of the excavations be constructed with slope configurations of 3 (Horizontal) to I (Vertical) or flatter. The approximate location of the trenches are provided in Pond 016 Area Remediation Plan and Road Profile, Exhibit No. 20 -D. 2. Sheet 1 of 2 and the cross sections are provided Pond 016 Area Remediation Plan and Port Mine Cross Sections, Exhibit No. 20 -D. 2, .Sheet 2 of 2 in Appendix C. Shrub Plot Landslide: Based on our observations made at the site over the past 2 years and our discussions with Seneca regarding previous site grading operations and drainage improvements that have taken place in this area, we believe that this slope failure most probably consists of creep /downslope movement of the upper /reclaimed fill materials. Saturation by perched groundwater on the access ramp fill is the likely failure mechanism. Based on our observations made during the excavation of the test pits and drilling of the test holes, it appears that the upper fill . materials placed in this area were relatively loose and have most likely become saturated due to the groundwater source Job Number: 09 -8099 NWCC, Inc. Page 5