Abstract
Effective and efficient use of binders is critical to optimizing underground backfill performance while minimizing operating costs. Controlled laboratory studies demonstrate the strong influence of binder efficiency (i.e., strength development) on bulk properties (e.g., density or porosity). While it might be hoped that backfill self-weight consolidation during placement will increase binder efficiency, there are several high-quality field studies that indicate this self-weight consolidation mechanism may be inconsequential. The underlying causes for the lack of consolidation are studied in detail in the laboratory using Cemented Paste Backfill (CPB) from Williams mine. A specialized hydraulic conductivity test quantifies permeability reductions due to binder hydration on samples with as-prepared bulk properties. A servo-controlled oedometer quantifies stiffness changes with variable time delays to the onset of effective stress development and with variable effective stress loading rates. A 1.8 m high column test with carefully controlled boundary conditions and representative backfilling rates is used to simulate the full-scale response expected in the topmost layers of deposited backfill. The column test results are interpreted using Biot-type analysis for accreting sediments, modified to incorporate time-dependent material properties. The backfill’s void ratios determined from the column tests are consistent with field observations, and the integrated interpretation of laboratory physical and numerical test results is that the backfill’s enhanced strength and stiffness due to hydration occurs faster than the onset and rate of effective stress development as pore water pressures dissipate. While the results are specific to the Williams mine CPB, the result probably have broad implications for other mines because the Williams fieldwork demonstrated that its CPB is one of the fastest to develop effective stresses during placement. Therefore, all mines should carry out fieldwork to quantify any self-weight effects that may occur at their mine if they intend to rely on such effects to increase binder efficiency.