The U.S. Bureau of Mines, in cooperation with Hecla Mining Co., has monitored back- fill performance and ramp stability during mining of several cuts of an experimental underhand longwall stope in the Lucky Friday Mine, Mullan, ID. This mining method was designed to increase productivity by introducing mechanized mining equipment and reduce rock burst hazards associated with excavating isolated pillars. The data were also used to calibrate a three- dimensional, finite-element model of the lower portion of the Lucky Friday Mine and allowed the refinement of rock mass properties from an earlier two-dimensional model. The data collected, in conjunction with observations, on-going mining experience, and the calibrated model, allowed researchers to demonstrate the geomechanical soundness of the method and led to its adoption through- out the mine. This case study demonstrated the desira- bility of building a model prior to specifying and placing instruments.
Rock bursts have a long history in the Coeur d'Alene Min- ing District of northern Idaho (McMahon, 1988) and are threatening the future of district mines. The Bureau of Mines has long been involved in developing alternatives to the traditional overhand cut-and-fill stoping method that would reduce rock burst hazards and be amenable to mechanization. One of these methods, the underhand longwall cut-and-fill method, was chosen for testing at the Lucky- Friday Mine, Mullan, ID (figure 1). The experimental stope, dubbed the Lucky Friday underhand longwall (LFUL) stope, was tested under a cooperative agreement among the Bureau, Hecla Mining Co. of Coeur d'Alene, ID, and the University of Idaho at Moscow, ID.
Underhand cut-and-fill is a method in which a block of ore is mined by cutting and filling in sequence from the top of the block to the bottom, rather than from the bottom to the top as in the overhand cut-and-fill method. The intact vein thus forms the stope floor instead of the stope back. As in the overhand cut-and-fill method, the vein is accessed through crosscuts from laterals, and mining is conducted with a conventional drill, blast, and muck cycle.
The underhand longwall method uses the principle of a single advancing face to reduce rock burst hazards, an idea that is not new. The South African High-Level Committee on Reck Bursts and Reck Falls (1977) recommended longwall mining as a means of reducing rock burst hazards associated with mining remnants (or sill pillars) as early as 1924; this method is now standard practice in South Africa.
In the underhand longwall method, the open stope is backfilled following each cut with a reinforced, cement-stabilized fill material that provides a safe stope back or roof. Reinforcing members typically include wire mesh, timber, and rock bolts. Maintenance of a safe back is critical, a point underscored by a tragic backfill collapse that killed four miners at the Falconbridge Mine, Ontario, on June 20, 1984 (Hedley and Wetmiller, 1985). However, a well-designed fill back should have safety advan- tages over the burst-prone vein rock that it replaces. Although vein destressing (in which long blast holes drilled ahead of mining are shot to relieve stress concentrations) can be used to prevent bursting in the back, the resulting fractured rock can pose slab- bing and caving hazards. Destressing the floor of an underhand stope avoids this problem entirely.