Abstract
Backfill makes up of 10-20% of the total mining costs and 75% of that is cement. Backfill (paste) is poured into an open stope (or drift in the case of a drift and fill operation) where it is retained by a structural barricade that blocks the draw point until the backfill undergoes strength gain. Typically, barricades are designed to withstand the initial placement of backfill, which is termed the “plug”. The plug is allowed to cure and subsequently is assumed to isolate the barricade from the load applied by the “main” pour. A safe and practical barricade design requires an appreciation of rock mechanics and the limitations of underground construction.
Typical design work consist of utilizing analytical or numerical analysis to find the capacity of the barricade, with an applied factor of safety between 4 and 6 depending on the application. These high factor of safety are typically rationalized by potential variability of the design due to underground work environments. However, such a high factor of safety provide a false sense of safety. An alternative approach is to conduct analysis that considered the typical worse case scenarios for all of the design components to validate stability. This type of approach is more in-line with limit state design analysis. The problem with this approach is quantifying the typical worse case scenarios for the design components. There is a limitation of data to provide a statistical representation to quantify the inputs.
The proposed approach is enhanced by specialized backfill experience gained from an international, multi-operation research program and subsequent related work. Such work has provided a pragmatic understanding of various underground barricade construction techniques and variabilities. In this paper, the Authors will review and summarise these design components and how they can be implemented in a limit state design approach.