Laserfiche WebLink
Seneca II-W Mine <br />2006 Neckpit Slide Remediation Plan <br />SEDCAD4 Demonstration Model Assumptions <br />Purpose: <br />The intent of this design is to increase the Factor of Safety (FS) against continued slope failure at <br />the 2006 Neckpit Landslide by modifying surface water drainage. Infinite slope analyses indicated <br />that by decreasing groundwater in the soil, the FS would increase. By adding drainage channels <br />above and through the existing slide mass, infiltration into the soil will be decreased and the FS will <br />increase. <br />Description: <br />Above the slide mass, a postmine (PM) trapezoidal channel (PM-4C) will be constructed. PM-4C <br />will have a cross section similar to the existing PM-4 series channels. Channel dimensions aze <br />presented in the channel design schedule (see Attachment 20-1, Figure 20-1.2). Channel PM-4C has <br />channel gradients that vary between one (1) and 12 percent; runoff collected in the channel will be <br />routed into culvert PM-10, an existing 24-inch culvert and then into an existing natural drainage <br />course. <br />A second postmine channel will be constructed through the center of the slide body. This channel, <br />PM-4D, will be a v-ditch channel with SH:1 V side slopes. The wetted perimeter and side slopes <br />will be protected with a woven geosynthetic turf reinforcement matt (TRM). The matt will be a <br />ENKAMAT 7020, or equivalent, and will increase the surface roughness and minimize flow <br />velocities. The slide body has been regarded to approximate original contour. The upper portion of <br />the channel has a slope gradient of approximately 1.SH:1V, or about 66 percent. A grade break <br />occurs in the channel at approximately the 7,677 elevation contour and the slope changes to <br />approximately 2H:1V, or about 50 percent. An energy dissipater will be constructed with rock <br />riprap below the channel outfall. Below the energy dissipater, runoff will flow Freely into the <br />existing natural drainage course. <br />Assumptions: <br />• Channel and culvert designs were determined using a 10-year, 24-hour storm event in <br />accordance with Rule 4.05.3(3). <br />• Runoff water wil- flow as overland sheet flow. <br />• Flow through the postmine channels is uniform and fully turbulent, allowing the use of <br />Manning's equation. <br />TR-56 20-B.B-1 Revised 12/06 <br />