2023-09-20_BONDING - M1980047 - Laserfiche WebLink

Home Browse Search

Colony Reclamation Cost Estimate, Update to Exhibit L Rev. Sept. 2023 work required to reclaim this 4 acres, there are no costs included in the reclamation cost estimate for this small area. For permanent reclamation, the 27 acres of the Mine Bench surface, Area 6B, will require cutting an open channel to replace the existing 6-foot pipe that is buried in the Mine Bench. This pipe conveys flows from the upper Middle Fork valley through the Mine Bench to the concrete flume in the Mine Bench face. Those flows are essentially the runoff from the upper Middle Fork valley, which now pass through Middle Fork Dam. In addition to the annual spring melt which can approach 30 cfs, there is a more or less continuous flow of about 2 cfs over the dam's spillway except in drought years. For permanent reclamation, the buried steel conduit in the Mine Bench will be removed and replaced with an open channel, whose plan and profile are shown on Figures L-2 and L-3. Since the open channel may experience extremely large flows up to 880 cfs during intense storms, the steeper portion of the channel from Station 6+50 to Station 12+00 will be slush grouted with concrete to reduce the possibility of channel damage. Middle Fork Dam, which is now a permanent facility, meters all storm flows from the upper Middle Fork valley. The dam attenuates all large storm flows, and will reduce a 500-year storm to a routed outflow of about 880 cfs, per the 1984 design report for the dam. The channel will therefore be designed to convey that flow to the upper end of the Mine Bench flume at the southern end of this Area 6 B. There is already a concrete junction box where the 72-inch buried steel pipe meets the upper end of the Mine Bench flume. It has been assumed for this cost estimate that this junction box can be located and used as the outlet works of the proposed open channel on the Mine Bench surface. The permanent plateau access road cuts across the Mine Bench in two places and culvert crossings will need to be provided. Near the upper end of the Mine Bench, the cut channel will be fairly deep (about 60 feet cut into the existing Mine Bench rockfill). This crossing will require use a 300-ft long 84-inch diameter reinforced concrete pipe (RCP) to reduce the need for future maintenance of this deep crossing. Although an 84- inch diameter pipe would be satisfactory from a hydraulic perspective, it appears that a 90-inch pipe will be slightly less expensive due to shipping costs from where the 84-inch pipe is cast (Denver). Consequently a 90-inch RCP pipe has been assumed for this cost estimate. Similarly, at the roadway crossing at lower end of the bench, the cut depth is about 40 feet and 200 feet of a 90-inch diameter reinforced concrete pipe will be used for that crossing. It is anticipated that some repairs to the existing Mine Bench flume would be needed. Due to differential settlement of the Mine Bench face in the first few years after construction in 1982, there is a sagged section of the flume about 250 feet long. The existing 60-inch CMP anchored into the existing flume was installed in 1984 to convey flows past the cracks and separations in this sagged area. It is reasonable to assume that the differential settlement is now complete, and probably has been complete for several years. Accordingly, for final reclamation the CMP will be removed and dental concrete will be formed and placed in the sagged and cracked segment, allowing the flume to perform as a simple high speed open channel as it was originally designed. The remainder of the Mine Bench is a flat lying crushed rock surface that is proposed to remain as is to serve as a staging area for road and dam maintenance. The rockfill material produced by excavating the open channel will simply be spread around on the Page 7