A 2-year test conducted by the Bureau of Mines Spokane (Washington) Mining Research Center (SMRC) has proven the feasibility of the yielding rockbolt, a concept which may prove very useful to the mining industry by providing ground support under a variety of conditions. The yielding rockbolt was originally developed in South Africa to overcome the limitations of conventional bolting in rock subjected to high-stress or rock burst conditions. The yielding effect is obtained by fitting the bolt prior to thread rolling with a smooth-bore die of internal diameter slightly larger than the shank diameter but smaller than the rolled thread diameter. Prior to field testing, the SMRC-designed bolt and the South African design were compared for ease of installation at the SMRC test adit. Both designs use the thread-die yielding concept, with the only difference being the position of the die. Although no difficulties were experienced in the installation of either type, the SMRC design was chosen because (1) installation was easier and faster with the kinds of equipment used in U.S. mines, (2) any type of standard or resin point anchor could be used, and (3) with the collar at the plate end, observation and measurement of the protruding threads gives an estimate of ground movement.
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