2012-03-06_REPORT - M1974004 - Laserfiche WebLink

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Martin Marietta Materials Spec -Agg Project wedge failures will be controlled by the limited extent of the discontinuities mapped in the quarry, and by the limits on bench height. 5.2.2 North Walls Consistent with the previous Lachel annual reports, the north wall has been subdivided into two separate slopes for analysis: the northeast and the northwest walls. The instabilities observed in the eastern section of the north wall and the differing slope orientations necessitated the distinction between the two slopes. To take into account the shallower slope angle of the past three benches as well as the final mine plan design, a 35° overall dip angle for the slope was used in the analysis of each of the north walls. As previously documented along the northeast wall, and as described in this report, planar failures can occur when foliation planes that are intersected by two or more lateral boundary joints daylight in the exposed highwall. The stereonet plots shown for the northeast wall (Figure 12) and the northwest wall (Figure 13) indicate a possibility for planar failure along the foliation plane F -1. Planar failure can occur because the dip direction of the F -1 plane can daylight into the slope face and the trend of the line of intersection formed by F -1 and P -2 can act as a releasing surface. In addition, based on the stereonet plots, intersections between discontinuities F -1: P -2 and F -1: P -3 potentially exist for the northeast wall, and the plunge line of each of the intersections will fall very close to or within the daylighting envelope and at an inclination approximately equal to the angle of friction. For the northwest wall, intersection between discontinuity F -1: P -2 falls within the daylighting envelope and has potential for sliding. For the 2004 analysis (L&A, 2004), the long -term impact of foliation F -1 on the stability of both the northeast and northwest walls was further investigated using discontinuity data that includes elevation information for the foliation orientation. These data were plotted on a chart showing dip angle in degrees versus elevation in feet (for chart see L &A, 2004). The results resulted in an increase in the number of daylighting foliation planes that start at an elevation of 6,325 feet and continue with increased frequency to below the planned final pit elevation. Due to the possibility of a lower friction angle along the foliation plane (as low as 5 °, average 28 °) as determined from the 2003 Geotechnical Study (L&A, 2003), the potential exists for future instabilities to occur on the north walls as the pit is excavated to its final depth. These shallow dipping discontinuities along the foliation can have a significant impact on the stability of the north wall and should be monitored closely as excavation progresses. In addition, as discussed in the previous section, an east -west striking unnamed fault zone was previously exposed in the northwest corner of the main pit. With the combination of faulting and discontinuities on the north walls, the potential impact on the slope stability should be closely monitored as mining continues. 5.2.3 West Walls No major stability issues currently exist for the west walls of the main pit of the quarry. Previous analysis indicated that the west highwalls have approximately the optimum orientations for stability. Due to the change in orientation along the wall, two west wall orientations were analyzed in this study. The first had a dip direction of 091°, and using this orientation the stereonet plot (Figure 14) shows one pair of planes having a line of intersection that plunges less than the angle of the slope face and steeper than the failure envelope (P6:P1). The intersection of F1:P6 is located outside the daylight window, including the plotted variance of dip for the foliation plane therefore, failure along this intersection is unlikely. The plunge line for February 13, 2012 Page 11 Lachel & Associates, Inc. Project 11361017.00 ©2012 All Rights Reserved