An Effective Stress Approach to Mine Backfill

Project Leader: Andy Fourie, The University of Western Australia

Professors Andy Fourie and Martin Fahey from the University of Western Australia and Dr Matthew Helinski, research associate, from the Australian Centre for Geomechanics are the contacts for this research project.

The use of backfill in underground mining is increasing due to the need for regional or local support, increased extraction and the environmental benefits that accrue from reduced volumes of tailings stored on surface.

In the past, mine backfill has generally been rockfill or (classified tailings based) high density hydraulic fill. Recent advancements have made the preparation, transport and placement of full plant tailings possible. This has required the material to be transported at relatively high solids contents to prevent segregation and separation, which has led to the adoption of the term pastefill to describe such backfill.

Pastefill is being increasingly used within the mining industry, although there remain a number of impediments to its widespread adoption. Gaps in the technology of placing fill and mining adjacent to filled stopes mean the geotechnical risks cannot currently be adequately quantified. Whereas pastefill overcomes many of the problems associated with previously used fill types, concerns remain regarding a number of issues relating to safety (particularly quantification of loads on barricades), production (issues such as fill mass failure) and costs (primarily binder and additives). It is believed that many of the geotechnical risks stem from a lack of understanding of the development of effective stresses within a fine-grained cementing fill mass, and can be eliminated or controlled by developing a framework based on effective stress theory. Development of such an understanding would enable issues such as time-dependent barricade loads, strength gain with time and the impact of moisture movement within a filled mass to be quantified for the first time.

In order to provide the mining industry with the necessary tools to evaluate the potential viability and minimise risks associated with using a pastefill system, it is essential that fundamental issues of effective stress development within a filled mass be understood.

This new project will address technology gaps that currently exist relating to the use of tailings based backfills, and which result in significant residual risks at present. The principles of effective stress will be used to analyse the mechanisms associated with placement mechanics as well as the post placement aspects associated with exposure stability.