Abstract
Historically the limit equilibrium solutions by Mitchell and Roettger (1989) have been used when specifying backfill strengths for horizontal exposures. While these solutions have proven useful for small spans, where ground support is included (e.g. in underhand cut and fill mining), experience shows this approach to underestimate dilution in larger underhand stoping applications with cemented paste and hydraulic fill and overestimate the strength required to prevent catastrophic failure (i.e. achieve stability, while permitting some dilution).
Persistent, cohesionless, horizontal "cold/flow" joints, which form during the hydraulic deposition process are expected to be controlling the horizontal exposure mechanism. After incorporating the influence of these joints into dis-continuum numerical analysis, this model is shown to provide a more reasonable representation of the relationship between dilution and strength during horizontal exposures.
Based on the mechanics revealed from the numerical output two new analytical modes are proposed for representing the horizontal exposure behaviour. The first is based on Voussoir beam theory and relates depth of overbreak (or dilution) to backfill strength, while the second provides a model for estimating the strength below which catastrophic, uncontrolled caving would occur.
Finally, the analytical solutions suggest horizontal exposures to be largely independent of tensile strength. This is verified through a numerical sensitivity study.
Reference
Mitchell R.J. and Roettger J.J. (1989) Analysis and modelling of sill pillars. Innovations in mining backfill technology. Balkema, Rotterdam, pp 53–62