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1 <br />' but still appears to leave stresses dangerously close to failure in <br />the upper sandstone strata for the longer span conditions. A rock <br />bolt unit which would further increase the friction effects would <br />bring about an even more uniform distribution of stresses between the <br />layers and widen the gap between working stresses in these layers and <br />' potential failure. The resin bolt unit currently in use, being a full <br />contact fixture, does act to increase the friction effects over and <br />' above those considered in our analysis. <br />Another type of unit which might warrant possible consideration <br />in the future cvould be the relatively new friction rock stabilizers. <br />' These are, again, full contact fixtures capable of maximizing friction <br />effects. These units differ significantly, however, from resin bolts <br />in that they are capable of exerting radial loading for some distance <br />around the bore hole, actually placing a portion of the roof rock in <br />compression helping to overcome the tensile bending stresses. Although <br />s <br />the type of rock strata observed in this mine are not commonly asso- <br />ciated with large elastic deformations, it should be pointed out that <br />the friction rock stabilizers will permit a substantial amount of <br />: deformation and redistribution of stress within the roof without fail- <br />ing or diminishing their load carrying ability. In terms of cost, we <br />a <br />would anticipate that the friction rock stabilizers would be compar- <br />able to resin bolts or perhaps slightly less expensive in volume. <br />Once again, it must be emphasized that more rigorous analysis <br />techniques are available for roof design. Once mining has progressed <br />along far enough, the roof support systems may be re-evaluated using <br />more rigorous techniques. This may permit alteration of bolting <br />patterns to a cheaper configuration using the same units, or may <br />9 <br />