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Mining Methods and Slope Stability Evaluation for Expanded Development of the Walstrum Aggregate Quarry The overall shape of drill patterns will affect rock movement relief, which in turn influences fragmentation and the stability of temporary mining benches. Blast relief for bench blasts is improved when hole patterns are angled to reduce corner confinement. When square or rectangular patterns are used, the rock moving out of tight corners often tears away large chunks of material beyond the planned end of the blast. This "End Break" can create large boulders and excessive toe burdens are inherited by the next shot. Figures 4.4 and 4.5 show pattern layout examples illustrating the typical effects of square versus angled blasthole patterns. Tight Corner Causal excessive j • • • • • • • • • • • • End Break and j Boulders at Crest ` • • • • • • • • ♦ • • • Last Shot L `• • • • • • • • • • Exceaslve Burden from Previous End-break Causes Boulders at Toe Rectangular Shots Can Cause End-Break Problems Figure 4.4—Excessive end-break caused by designing blasts with tight corners Reduced End ,..-.._.._,�•.;�.c-,_......--..___,•-- ,.___.~-_.._..�_.�.._. Break When • • • • • • • • • • • Shot Follows \rirud anh�np• Natural Break, • • • • • • • • • • • • // • • • • • • • • • • • • Shoot Proper Hde Placemanty F Less Crest Backbreak For Previous Shot Reducing End-Break with Angled Shot Ends. Figure 4.5 —Reducing end-break by designing blasts with angled corners Blasts should also be designed to avoid causing excessive vibrations, which can damage or weaken rock slopes. Blast timing sequences should also provide adequate delay time between successive rows of holes. Experience and controlled studies have proven that damage to the final REVEY Associates,Inc. Page I I of 14 October,2001