The buttress would remain between the first cut in a new mining area, and would support existing spoil from previous mining. However, a low strength, partially altered ash parting in the lower coal seam remained a concern. There was also uncertainty regarding the location of the previously mined highwall. When sloping back the existing spoil was deemed too expensive, designers revised plans to incorporate the rock buttress.  This compressed the analysis schedule and left no time for coring and laboratory testing of the rock.

Leveraging previous regional experience, Shannon & Wilson performed an analysis to satisfy the tight schedule. Plans called for a temporary excess spoil fill on the pre-existing, weathered spoil. We evaluated the spoil fill stability and the potential influence on buttress stability. After developing and reviewing Factor of Safety charts with Western Fuels Association, we developed recommendations for minimum dimensions of the in-place rock buttress, along with separation requirements from the excess spoil fill.