Cemented Paste Backfill Design under States of Low Confining Stress

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Modern underground mining methods rely on backfill to safely continue excavation under high rock stress conditions. Cemented Paste Backfill (CPB) is an engineered material that reuses “waste” products (mine tailings) and creates a thick consistency backfill that can be delivered at high rates to the underground voids, tightly filling all void spaces and curing with uniform material properties that can be tailored to specific mine design requirements. Two critical design scenarios involve assessing the CPB strength required when an adjacent void is excavated (sidewall stability), or when the CPB is undercut (sill mat stability). Design methods for both of these scenarios were developed in the 1980's and served the industry well in situations where backfill design was not critical. Increasingly, however, these methods are thought to be overly conservative and more detailed engineering approaches are sought. A major impediment to rational engineering design is the form of constitutive model used to simulate backfill behaviour, especially under these exposed-face scenarios where the confining stresses can be very low and even in the tensile range. The research program described in this proposal aims to address this problem and develop basic understanding as well as practical design tools that allow a better assessment of CPB strength requirements for sidewall stability or sill mat stability. New CPB laboratory techniques will be developed to allow testing under tensile states of “confinement”. Non-destructive wave techniques will be used to characterize progressive damage as the CPB is tested. A constitutive model incorporated elastic as well as brittle and ductile plastic mechanisms will be developed and implemented in a numerical code. Canadian industrial partners will be consulted to develop a database of sidewall and sill mat case histories for back-analysis purposes. The sophisticated modeling approaches used for research purposes will be critical to developing basic understanding of CPB behaviour under sidewall and sill mat exposure conditions, but these will not be practical for routine design; hence, additional work will be undertaken to develop simplified analysis and design approaches based on the more rigorous back-analyses, and they will be tested against ongoing design work with the partner mines. A total of 3 PhD and 3 MASc projects will be supported by the research. They will interact with the larger soil and rock mechanics group at UofT, as well as with industry partners in consulting and in mining operations. In this way the students will also become part of the technology transfer mechanism so that the fruits of the research can be brought to bear for the benefit of Canada.