2011-09-27_REVISION - M1990057 (10)

Home Browse Search

optimum. Construction engineer will be on site to provide testing during fill placement and verify compaction of floor of pond by number of tests recommended by CTL Thompson. CTL /Thompson, Inc. has conducted slope stability analysis on the TSF embankments as shown in Exhibit TSF -02 and TSF -03. Samples were collected from drilling two (2) additional holes to verify and confirm the 1990 permeability test hole data and additionally confirm the embankment materials and underlying soils. Additionally, a sample of tailings materials generated from laboratory testing was analyzed. The embankment materials, native oils and tailings materials were tested for strength and classification, using direct shear tests for use in analyzing slope stability of the embankment. The results of the CTL Thompson Letter Report of July 8, 2011, are presented as Appendix A, Slope Stability Evaluation Tailings Pond Embankment and Future Tailing Slope, July 8, 2011. Subsequently, CTL Thompson issued an update to the July 8, 2011 Report, presented as Appendix B, Slope Stability Evaluation Tailings Pond Embankment and Future Tailing Slope, August 17, 2011. . Phase 1 TSF reconstruction and Phase 2 new construction will use similar construction techniques and methods. The Phase 3 expansion lifts, will additionally use typical cyclone placement of select coarse material on the inside of the expansion embankments for increased stability. Clay Liner A 12in minimum clay under liner is placed on the interior slopes and impoundment bottom, compacted as two -6in lifts with a sheepsfoot roller. The clay material is selected from available onsite impermeable soils borrowed from the tailings disposal area. The clay liner optimum moisture content is not less than 15% moisture. Leak Detection System Following installation of the clay liner, a seepage /leak detection system between the clay layer and the synthetic liner with the collection system located in the northwest corner of the impoundment, and the impoundment bottom sloping at ' /2% slope to the northwest. Exhibit TSF -05 shows additional details on the leak collection system and sump. The leak detection system is installed level with the top lift of the 2 -6in lifts of 1 x 10 cm/s clay, prior to the installation of the 45mi1 polypropylene geomembrane liner. The leak detection system is a wick drain system capable of intercepting liquid within the leak detection zone and conveying the liquid to a collection sump for storage. The wick drain is 12 "wide, 1 inch thick, trenched in the 12in clay liner on approximate 50ft centers and is laid out as shown on Exhibit TSF -05. The volume of the collection system is limited to the sump and is approximately 90 gallons. The sump is accessed through a 4in PVC pipe to the top of the impoundment. Collected leachate will be returned to the TSF and then recycled to the mill through the impoundment decant pump(s). The leak detection system requires a manual inspection of the sump through the 4in PVC pipe. If leachate is detected (indicating a leak in the 45mi1 polypropylene), the level will be monitored, a small pump will be installed in the PVC pipe and the Division notified of fluids from the leak detection system. If groundwater is detected (indicating a leak in the 12in clay liner), the level will be monitored, a small pump will be installed in the PVC pipe, the groundwater pumped to the impoundment and the Division notified of fluids from the leak detection system. Polypropylene Liner Following installation of the seepage /leak detection system, a synthetic pond liner of 45 -mil Polypropylene Geomembrane or equivalent will be installed as shown in Exhibit TSF -04. The M1990 -057 September 27, 2011