2011-08-31_INSPECTION - C1982057

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The following areas were identified as appropriate for repairs: Neck Pit / Haul Road B Slide. This appears to be a large rotational failure in an unmined area. Minor fill was placed to reduce the grade of a haul road leading to the Pond 12 area; fill placement may have contributed to instability by increasing loading at the head scarp area, but the width of the slide mass is much wider than the area that received fill. Scarps on nearby slopes of similar aspect indicate that the spring 2011 slope movement cannot be specifically attributed to mining activity with any degree of certainty. Toe bulges caused road damage due to vertical displacement, but no horizontal offset was observed along the road. The drainage ditch along the upslope side of the road remains functional. The failure area is located above a large, steep slope. The slope from the haul road to the drainage and beyond was mapped as a "Potentially Unstable Slope" by Miller, but is densely vegetated and does not exhibit evidence of instability. We discussed a possible repair and stabilization plan for the Neck Pit slide area in the field. The plan includes excavating several narrow trench drains to allow existing ponded water to drain from the surface of the slide mass toward the ditch and culvert. The culvert discharges under the haul road, north of the current slide area, and does not appear to contribute to instability. Minimal regrading of the slide surface is planned; the head scarp will be knocked down, possibly from above the slide area to avoid compacting the loose soil within the slide. The mine operator agreed to aerially seed this area to minimize surface compaction. We anticipate that the uncompacted soil will provide good conditions for rapid revegetation, thereby increasing stability of the slide mass. Aspens are a common pioneer species in landslide terrain. They seem to thrive in loose, uncompacted landslide soils, and their root systems and interconnectedness (aspen groves spread by sending out runners) contribute to soil mass stability. Pond 12. As noted above, the western mine boundary in the area of Pond 12 was mapped by Madole, prior to mining, as underlain by landslide deposits. The large, unstable slope to the north of the pond has exhibited ongoing instability, but appears to have been stabilized through recent excavation to less weathered rock and construction of a french drain along the base of the slope. In accordance with approved technical revision plans, the mine operator is placing additional rock in the drainage along the toe of the slope, and buttressing the lowermost toe of the constructed slope. Several smaller slumps developed to the south and east of Pond 12 this spring, and have been regraded by the mine operator. Pond 13. The Pond 13 area contains a large mapped mudflow. A slide occurred in Spring 2011 between a well cluster and Pond 13, as a result of low- strength soils, much higher than normal water content within the soil mass, and gravity. The slide continues to move as it approaches equilibrium. The toe of the slide has blocked several small drainages, and erosion or continued slope failure of the head scarp could threaten the well cluster. The drainages need to be cleaned out to restore capacity, and the head scarp should be knocked down in a controlled manner to protect the wells. The resulting slope should be revegetated to increase stability and reduce headward erosion. The surface of the slide mass between the toe and the head scarp could be left ungraded and uncompacted to promote more rapid revegetation. No action will be taken to repair or stabilize the other scarps that we observed at Yoast because no structures are threatened. These include the North Pit and South Pit scarps, and several other small, non- landslide scarps that can likely be attributed to settlement of fill placed against the highwall.