The effect of stope inclination and wall rock roughness on backfill free face stability
Rock Engineering 2009 - Rock Engineering in Difficult Conditions
In order to maximize the recovery of ore in variably dipping ore zones of moderate width, cemented backfill is normally placed to serve as structural support. With the intention of saving on costs, backfill of low cement content can be used, which are often supported by a sillmat of higher strength. The stability of the low cement backfill face, exposed during adjacent mining, must be carefully studied to provide very effective, safe and economic mining operations. Improper design of these stope support structures may result in fill mass failure resulting in relaxation and failure of the stope walls, with consequent losses of production, ore dilution, and in safety problems.
This paper presents a study conducted to assess fill performance during adjacent pillar mining and to provide a comprehensive understanding of backfill behaviour, the modes of failure that may occur and the consequences to production, ore dilution and to safety problems, and accurately predict their stability. A comprehensive understanding of the effects of arching is also provided. The design study investigated the effects of such parameters as stope width and height, closure strains, orebody geometry and inclination, and wall roughness on the stability of cemented backfill during adjacent pillar mining. Fill properties were based on paste fill specimens cured for 28 days. Paste fill performance has been assessed based on analytical and numerical modeling studies for the different mining conditions. Analytical modeling was carried out using limiting equilibrium analysis adapted from a method introduced by Mitchell et al (1982). Numerical modeling was carried out using FLAC3D. The modeling program suggested that, for stopes that are inclined with smooth wall rock conditions, backfill failure, driven by the fill self-weight, has minimum dependency on the binder content and is reduced by resisting forces developed on the footwall-fill contact. For inclined stopes with rough wall rock conditions, however, wall roughness contributes significantly to the stability of the backfill during adjacent pillar mining. Results also indicated that the degree of arching is a function of stope geometry and inclination, wall roughness, fill cohesion, and wall closure. Arching effects increased with decreasing stope width, increasing backfill cohesion and frictional resistance, increasing wall roughness and inclination, and with wall closure. Model results can be used to engineer stable backfill structures and to predict their behaviour and stability during mining.