2009-09-21_PERMIT FILE - C1981010 (13)

Home Browse Search

January 15, 2008 Page 60 6.0 SUMMARY This report has documented characterization efforts for the G-Pit landslide and surrounding mining areas. The laboratory testing and borehole characterization program have been described and results have been summarized as they impact future mining. Results from previous stability analyses were summarized and new analyses of the spoil stability have been presented. Our understanding of mechanisms leading to the landslide and expected current conditions, as applicable to mining this area, are discussed. Recommendations for cut slope angles and spoil pile angles are provided and comments on the mining plan are given from a geotechnical perspective. The results of this work are summarized as follows. The slide plane of the landslide is approximately 96 ft below ground in the weak L-Roof mudstone layer. The slide plane undulates with bedding and sheared through high points in the beds disturbing the L-Seam reserves. The rock mass within landslide block is disturbed to various degrees. The most disturbance is updip in the tensile failure region and along the perimeter where shearing occurred over the large movement. This disturbance will manifest during mining as loose blocks. Flatter cut-face slopes will be required in unfavorable areas to ensure local stability. The ground below the landslide is essentially undisturbed. The exception is near the old G-Strike Pit area where disturbance on the weak Q-Floor mudstone was observed. • The post-landslide rock mass strengths are lower than pre-landslide due to open fractures and loss of confinement. Rock mass strengths below the slide plane are similar to pre- landslide estimates. • Estimates of recoverable reserves are provided in Table 4. There is a high likelihood of recovering most reserves in the L-Seam and below. • Stability of the spoils on the weak Q-Floor mudstone bedding is dependent on the local bedding dip. It is recommended that the spoil pile height not exceed 68 ft in areas where the average bedding dip 13°. Steeply dipping areas of K-Pit will require a reduction in the maximum spoil pile height to 30 ft and the outer slope face of the pile must be sufficiently compacted to maintain stability and prevent creep failure (see Section 4.4). • It is expected that the ground below the landslide can be mined using conventional dip-pit dragline methods. In the landslide, a truck and shovel mining method is recommended due to the disturbed ground. • Piling spoils as high as practical in the old G-Strike Pit area will help to stabilize the landslide prior to mining. • The risk of similar landslide events occurring in other areas of the mine, especially in Z-Dip Pit area and East-Panel area are low given the current knowledge of conditions. • Dewatering wells should be pumped to remove groundwater from landslide and spoil piles. This will significantly improve stability. • Monitoring should be continued throughout mining of the K-Pit and G-Pit area and data should be analyzed. Sudden changes will require additional monitoring or immediate operational action. Monitoring should include: regular visual cut-face inspection, large- Agapito Associates, Inc.