An Empirical Solution for Estimating the Maximum Allowable Mass of Waste Rock to Prevent Non-Mixing Between Paste Backfill and Dumped Waste Rock

Traditionally, waste rock generated in underground mines is transported to the surface and stored in waste rock piles. This practice requires substantial energy consumption and incurs additional operational costs. In Canada, an alternative approach involves directly dumping waste rock into stopes being filled with paste backfill. This method eliminates the need to transport waste rock to the surface and avoid the crushing of large rock blocks into smaller particles, as well as the use of specialized equipment for mechanically mixing the two materials. Consequently, both energy consumption and greenhouse gas emissions are reduced. Furthermore, binder consumption decreases because a portion of the cemented paste backfill is replaced by uncemented waste rock. Despite these advantages, no practical tool is currently available to assist backfill engineers in determining the appropriate amount of waste rock to be dumped. As a result, excessive quantities of waste rock may be added to the paste backfill, leading to inadequate mixing between the two materials. When exposed during the excavation of an adjacent stope, the resulting fill mass may become unstable and fail, causing undesirable consequences. To address this issue, a series of laboratory experiments were conducted to evaluate the effect of several factors, including the solids content and thickness of the paste backfill, the dumping height, the maximum particle size of the waste rock, and the stope dimensions, on the maximum allowed mass of waste rock. Based on the experimental results, an empirical equation was developed to estimate the maximum allowed waste rock mass that can be dumped without causing non-mixing between waste rock and paste backfill. The predictive capability of the proposed equation was successfully validated using additional independent experimental data.