Rock Mechanics Investigations at the Lucky Friday Mine. Part 2. Evaluation ofUnderhand Backfill Practice for Rock Burst Control

摘要

The U.S. Bureau of Mines has been conducting a series of rock mechanicsinvestigations at the Lucky Friday Mine in the Coeur d'Alene Mining District of northern Idaho. In the present study, the energy release raate (ERR) index was used to evaluate three underhand stope backfill parameters--cement content, density, and placement gap--to determine their influence on rock bursts. The index provides a relative measure of the likelihood of rock bursting for a given rock mass. It is known that the elastic idealization of the rock mass inherent in the method causes underestimation of stope closure and hence underestimation of the influence of backfill on rock bursting. However, the evaluation still showed that (1) changing from present practice to one in which backfill was omitted altogether would increase the ERR by 42 pct and (2) perfect placement of backfill would reduce the ERR below present practice levels by 28 pct. The easiest step towards perfect placement would be to eliminate the placement gap, which accounted for half of the difference in ERR between present practice and ideal placement conditions. There was no indication that increasing initial fill strength through increased cement content would affect the ERR.