A New Solution to Assess the Required Strength of Mine Backfill with a Vertical Exposure

Abstract

Backfilling is commonly applied to aid pillar recovery in open stope mines. The backfill must then remain self-standing in the primary stope when mining an adjacent secondary stope. The design of exposed fill is often based on a solution proposed by Mitchell and coworkers over 30 years ago. Several modifications of this solution have later been reported. These solutions have been partly validated against a few physical model test results, and they have also been compared with numerical simulations. The analyses have shown that these solutions cannot completely capture the response of the exposed backfill, particularly when the required cohesion of the cemented backfill is relatively large. New simulation results presented here indicate that the sliding plane tends to change from planar at small fill cohesion to spoon shaped [three-dimensional (3D)] at larger cohesion. In the former case, the failure mechanism is controlled by shear stress along the sliding plane near the base of the stope; this corresponds well to existing analytical models. In the latter case, however, failure of the backfill is controlled by both shear (near the base) and tensile (near the top) stresses. This feature is not represented by existing analytical models. In this paper, a new analytical solution is proposed for assessing the stability of exposed cemented backfill with a vertical face. An instability criterion is also introduced for estimating the critical strength of side-exposed backfill. Additional numerical simulations are then conducted to validate the proposed analytical solution. The results show that the newly developed solution correlates well with the numerical simulations for representative stope geometry and backfill strength. This new solution is thus regarded as an improvement over existing solutions. Further calculations are made to illustrate the effect of key parameters on the stability of side-exposed fill. A discussion follows on the limitations and effect of various influence factors on the design of exposed cemented backfill.