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• 3.0 Corrective Measures <br />The immediate approach to stabilize the landslide was to slow the landslide using pins (micro- <br />piles) and an anchored RCC buttress at the toe and then, for long-term stabilization, remove the <br />primary triggers causing the landslide by dewatering. Dewatering the landslide is being <br />accomplished with dewatering wells, stone columns, lining the ponds, lining the ditches, and <br />relocating the underground water mains to above ground. Because the stone columns drain into <br />the alluvial deposits below the landslide, horizontal wells have also been installed into the alluvial <br />deposits to minimize excess hydrostatic pressures beneath the landslide. Figure 1 is a plan <br />showing the location of corrective measures. Figure 4 is a cross section of the landslide with the <br />corrective measures in place. Figure 5 is a time line of when the corrective measures were <br />installed. <br />Construction of additional corrective measures will be based upon the Observational Approach. <br />Additional corrective measures may include dewatering trenches and additional portal retaining <br />structures, such as buttresses or soldier pile walls, to further stabilize the head of the landslide. <br />• Also, depending upon the performance of the landslide, additional pins, stone columns, dewatering <br />wells, and the other corrective measures may be implemented. Some ongoing corrective measures <br />include maintenance of the lined ditches and capping the French drains above the landslide. The <br />following is a description of the existing and the potential additional corrective measures for <br />stabilizing the landslide. <br />3.1 Existing In-Place Corrective Measures <br />1. Pins (Micro-Piles) <br />Approximately 210 pins consisting of 30-inch diameter concrete filled steel pipe were installed <br />across the shear plane. The pins are 50 feet long with 20 feet above the shear plane and <br />30 feet below the shear plane. About one-third of the pins (70) are filled with gravel and <br />extend to the cobble layer beneath the colluvium for drainage. Three rows of pins at 12-foot <br />centers were placed near the toe of the landslide, as shown on Figures 1 and 4, to prevent a <br />progressive failure and protect the mine facilities. The pins are designed for plastic <br />deformation and will deform as the landslide slowly stabilizes. A detail of the pin design is <br />presented in Appendix B. <br />• <br />oszsoza~ssssi-vnBM 3 <br />