2005-05-23_REVISION - M1977424 - Laserfiche WebLink

Home Browse Search

Logan Wash Mine TR No. 8 that water in the lower workings advances and recedes seasonally, the actual percolation rate of stored water in the lower workings is unknown. That is, if water is held in the mine and allowed to cumulate from season to season, at what rate will the stored water recede by percolation into the surrounding shale (Parachute Creek Member)? Additional storage would be available in the upper workings of the west drift. Any flooding of the upper portions of the west drift could result in additional head pressure on the mine adit plugs. To evaluate the integrity of the portal closure, OOSI prepared engineering calculations that analyze the stability of the concrete dam and earth plug (see TR No. 4, Attachment C}. Of primary importance is the integrity of the earth plug as it acts as the primary mass for retaining water in the mine. The calculations were performed without knowledge of alab-determined soil classification. After the calculations were performed, alab-determined classification was conducted resulting in a Unified Soil Classification System (USCS) classification of SM for the east portal earth plug material. Sliding calculations show that high safety factors can be expected for the soil plug in the case of maximum water depth of 9 ft, and that the safety factors are conservative because they do not include effects of friction along the sides of the soil plug. Internal erosion of the earth plug was estimated using the weighted creep ratio. Results of the calculations applying the SM soil type indicate that a potential for internal erosion at certain water depths may exist. Overtopping of the earth plug is a valid concern as construction methods did not provide for a compacted plug against the back (roof] of the adit. Please see TR No. 4, Attachment C for calculations and narrative on the Quickcrete® dam (see Section 2.3). 2.3 Current Closure Status As stated in TR No. 4, the initial Reclamation Plan did not discuss the details of portal closure at the Logan Wash Mine. The Plan simply stated that the "portals would be blocked and sealed." At the beginning of the summer of 2003 three portals remained open at the mine, the Research Mine Portal, the Daylight Portal, and the L-1 Portal. In September 2003, the Research Mine Portal and the Daylight Portal were sealed with 40-ft earth plugs. The L-1 Portal was sealed in November 2003. TR No. 4 presented the details of final closure for the L-1 and Daylight Portals and the interim closure for the Research Mine portal. Initial closure activities at the Research Mine took place in the spring of 2003. In order to reduce monitoring costs associated with the NPDES discharge permit for the mine, a concrete sack (Quickcrete®) dam was constructed to confine mine water to the mine workings and reduce or stem the flow from the portal. The dam was constructed over the existing earth/bentonite dike and discharge pipe, which had been installed in the mine approximately 45 ft from the portal and approximately 5 fr inside of the metal portal doors. Prior to construction of the dam, the drift floor at the base of the dam footprint was cleared of unconsolidated rock and fill, and a concrete floor was poured in the resulting shallow trench. The Quickcrete® dam consisted of 224 60-]b bags of Quickcrete cement product; a total of 13,444 Ibs of concrete (dry weight). The dam was constructed in the form of a wedge with a 5 ft base and a height of 4.5 ft above the drift floor. During installation, the cement sacks were constantly wetted to enhance curing of the concrete mix. Western Water 8 Land, Inc.