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JanuurY 1 5. 2008 /'cw J 7 <br />An important step in monitoring will be making observations of the ground surface for <br />the appearance of new cracks. Open surface cracks are a hazard to personnel if they are large <br />enough tier truck wheels fall into the void spaces. Visual reconnaissance of surface cracks and <br />raveling is recommended for locating problem areas. Further location-specific characterization <br />can be performed to detect the size and potential hazard of such voids. <br />In G-Pit areas where the dragline has cut a highwall lace, similar types of monitoring <br />should be carried out as discussed for the K-Pit areas. Large-scale movement of the highwall <br />crest should be monitored by installing temporary survey benchmarks and monitoring <br />displacements. Visual observation of the highwall face condition should also be performed to <br />identify loose blocks and raveling potential. This monitoring should be performed similar to <br />other areas ofthe mine. <br />5.6 Groundwater Extraction and Monitoring <br />One of the critical conditions leading to the landslide was the recent rainfalls. Elevated <br />groundwater levels are thought to have resulted in saturating the weak L-Roofmudstone layer. It <br />is not well understood how subterranean groundwater flows through the bedding and open <br />fractures because of natural variations. Groundwater saturates the mudstone reducing its <br />strength due to soflening behavior. The rock mass itself appears weaker because pore water <br />pressure reduces the mean stress of the rock mass (referred to as effective stresses). The only <br />way to reduce this weakening is to drain the rock mass. <br />The locations of seven groundwater extraction wells have been identified, as shown in <br />Figure 39. These wells are located so as to drawdown groundwater across the entire landslide <br />toe and should not be disturbed by mining operations. These wells were recently installed to a <br />depth of.just below the Q-Floor; four wells penetrate the higher permeability spoils of the old <br />G-Strike Pit. <br />The stability analvses of the landslide assumed the groundwater level was 10 ft below <br />ground surface. This assumed a nearly full hydrostatic head on the weak L.-Roof mudstone <br />layer. Analysis results indicate that this was sufficient to cause instability of the hillside and <br />triggered shearing failure in l.-Roof mudstone layer. Placing dewatering wells downdip in high <br />permeability spoils to extract groundwater to below the 1.-Roof mudstone will improve drainage <br />of the landslide. The disturbed strata above the L-Roof should readily drain due to open <br />fractures. Dewatering increases the stability ofthe landslide. <br />Similarly, the drainage of the spoils piled on steep Q-Floor mudstone bedding is <br />recommended. The stability analyses ofthe spoils in the K-Pit area assumed I tl ofhead on the <br />weak Q-Flour mudstone. Analysis results indicate that groundwater pressure does have an <br />impact on stability of the spoils. Placing dewatering wells downdip of K-Pit spoils improves <br />their drainage and increases stability. <br />A.,aplto Associates, Inc.