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<br />' IV. ENGINEERING EVALUATION AND DESIGN <br />4-01. ASSESSMENT OF FAILURE <br />' Evaluation of site conditions reveals two separate stability problems; individual benches and the <br />overall pit slope. <br />A. Individual Bench Slability <br />The stability of individual benches is dependent primarily on the orientation of the slope <br />' relative to discontinuities within the rock mass, and on the character of the discontinuities. <br />Bench failure is possible when discontinuities dip out of the slope, thereby isolating a mass of <br />rock which can slide downwards. Similarly, combinations of discontinuities can isolate <br />' wedges and blocks of rock which can slide downwards. <br />Joint orientations were measured as part of field explorations (Subsection 3.01) These joint <br />' orientations combined with likely joint friction angles were compared with the orientation of <br />the pit slope. Results of this evaluation determined that it is kinematically possible f'or <br />isolated blocks and wedges to he dislodged from the rock mass. Additionally, within the <br />subject area and along the eastern pit wall to the north of the subject area, there is evidence of <br />' rock blocks and wedges isolated and dislodged from the main mass which agrees with results <br />of the analysis. <br />' It is our understanding that the benches were completely backfilled with soil from the outside <br />edge of each bench (break point) to the highest point on the highwall above the bench to <br />which soil would stand. Loading of benches with soil has the beneficial effect of stabilizing <br />' potential block failures by forming a buttress against the highwall above the bench. However, <br />the soil also generates an additional load on the bench thereby decreasing stability of blocks <br />below the bench. <br />' In summary, the orientation of discontinuities within the rock mass in the subject area relative <br />to the slope orientation results in a condition of which individual rock blocks and wedges are <br />kinematically free to dislocate. Loading the slope with soil near the outside edges of benches <br />worsened this condition. The combination of these mechanisms resulted in frequent although <br />localized bench failures which released debris downslope. <br />13. Overall Pit Slope Stability <br />' The overall pit slope is not stable as evidenced by the tension cracks and scarps at the top of <br />the slope. It is likely that a failure surface or shear zone extends from the tension cracks deep <br />through the rock mass exiting near the toe of the slope. <br />Evaluation of preliminary geologic information on the rock mass reveals that there are several <br />predominant sets of discontinuities and that they are closely spaced relative to the height of <br />the slope. Within the subject area, the rock mass can he thought of as a continuum of <br />discrete blocks exactly matched to each other. Wilh this condition, a shear surface defining <br />6 <br />1 ~''~_~` <br />