Laserfiche WebLink
Pond 016 Slide Remediation Plan <br />Page 3 <br />5. The fifth full paragraph on NWCC page Sindicates that channel PM-2 should be excavated <br />into "competent bedrock material ". Test holes 12 and 13, which are near to the proposed <br />realignment of PM-2, are logged on NWCC Figure #2. In these holes, weathered claystone <br />was encountered at -10 feet beneath the surface, and claystone bedrock was present at 13 <br />to 17 feet below the ground level. Please provide additional information on SCC's plan to <br />ensure that PM-2 will be excavated into competent material. <br />Response: Excavation of Channel PM-2 will be observed by a representative of NWCC at the <br />time of construction to verify that the base of the channel/drain is installed into the competent <br />bedrock materials. Channel depth and grade will vary depending on conditions encountered at <br />time of construction. <br />6 Although not stated directly, the NWCC report seems to imply that relocated Channels PM- <br />2A and PM-2 will now serve the. dual purpose of transport of surface water runoff and <br />interception of subsurface water that is flowing into and out of the shrub plot landslide. <br />Please verify that this is or is not the intent of the proposed plan. <br />Response: The relocated Channels PM-2 and PM-2A have been designed and will be <br />constructed to transport surface water away from the shrub plot landslide area. The channels will <br />also serve the purpose of intercepting the subsurface flows coming from the shrub plot area. <br />7. Construction of PM-2 across the toe of the shrub plot slide may contribute to additional <br />loss of stability in the slide mass due to the removal of any materials now acting as a <br />buttress. Subsequent downhill movement of the mass could encroach upon the. PM-2 <br />channel and create a damming effect. Please address the possibility of this scenario <br />occurring. <br />Response: The construction of Channel PM-2 across the toe of the Shrub Plot slide area has <br />been designed to dewater the fill areas situated within the western edge of this area and to <br />eliminate the piping (subsurface erosion) of the soils in this area. The relocated PM-2 channel <br />will reduce the phreatic surface in the fill materials and significantly reduce the piping of the <br />soils in this area thus improving the overall stability of the remaining fill materials in this area. <br />In addition, the excavation of the channel within this area should result in the removal of the <br />majority of failed/weakened materials. The side slopes of the channel within the fill materials <br />will be constructed to at least a 3 (Horizontal) to 1 (Vertical) slope configuration. This slope <br />configuration should be stable if the fill materials are adequately dewatered. If the fill materials <br />are determined to be unstable as the excavations are completed, then the slide slopes will be <br />flattened to a more stable configuration. <br />8. The current application does not propose the construction of any ditches or channels to <br />divert surface or subsurface flow away from the eastern two-thirds of the shrub plot <br />landslide. While regarding the area is not yet feasible, would the installation of a cutoff <br />channel be possible with TR-73? <br />Seneca Coal Company • P.O. Box 670 a Hayden, Colorado 81639 <br />Telephone (970) 276-5219 • FAX (970) 276-5222