Determining fill fence stability and backfill characteristics during increased backfill rates at an underground hard rock mine

9th International Symposium on Mining with Backfill
Rimas Pakalnis, Grant Corey,
Abstract Increasing production when using backfill techniques involves many aspects including decreasing backfill cycles of stopes to their most efficient state. This study focused on determining the stability of thin shotcrete fill fences at a hard rock mine in northwestern Ontario during increased pour rates of paste backfill. Approved mine standards dictate that fill rates may not exceed 1.5 meters of paste depth per 10-hour shift when pouring behind a standard fill fence. One production cut is generally 3 meters to 4.5 meters high and filled to within 0.3 meters of the back. Approved fill fences at the mine are a slender design of 10.5 centimeters of shotcrete applied to a rebar framework with a burlap lined ‘bedspring’ backing. To decrease the backfill cycle time the study looked at fence stability during increased pour rates of 2.5 meters paste depth poured within a 10-hour shift. The faster pour rates result in greater pressure applied to the fill fence. Determination of the stability of the fence is needed for both personnel safety and to avoid failure of a fence. Failure of a fill fence during backfill activities would have severe impacts on production and potential hazards to nearby workers. An instrumentation design was developed including pressure cells, strain gauges, and tilt-meters was built into experimental fences to observe the effects of the increased pour rates on lateral wall pressures, strain development in the rebar and shotcrete reinforcement, and physical deformation patterns along the face of the fence. Results included determination of factors of safety for the fences, lateral pressures, active earth pressure coefficients for future design work, and recommendations for more stable fence designs with increased pour rates. A secondary study of field verification through pull tests has evolved from this study providing a greater understanding of paste loading on thin walls.
Keywords: Paste backfill, Fill Fence, Fence Stability, Paste Characteristics
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