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portion of this particular layer would actually fail into the mine <br />opening. Rather, we would anticipate that stresses would be re- <br />distributed in compression into this layer through wedging along <br />pre-existing fractures and discontinuities. The effect would be, <br />however, to permit large deflections in the lower layers which might <br />precipitate a failure and collapse in these lower layers. Considering <br />both the combined friction and suspension effects, the same general <br />ordering in factors of safety by layer persists. <br />CONCLUSIONS AND RECOMMENDATIONS: <br />To summarize the results of this analysis in terms of any <br />indicated action or change in the current roof bolting program, we <br />caould offer the following. Firstly, it is hard to argue with success. <br />If the current program using resin bolt units on a 4-foot by 4-foot <br />to 4-foot by 5-foot pattern is a proven system, then we would recom- <br />mend starting the new mine by continuing cvith the existing program. <br />The cheapest rock bolting unit in the market today is the common <br />mechanical bolt, having a mechanical anchor at the back of the unit <br />and a steel plate at the head of the unit. If the mechanical bolt <br />is not post tensioned during installation, then it serves almost <br />solely as a suspension type unit. Our analysis has shown that this <br />type of unit would have the effect of transferring abnormally high <br />stresses into the stiff sandstone layer well into the roof area. <br />This ~oould appear to be an undesirable condition that may lead to an <br />eventual failure initiating in this sandstone layer. If the mechanical <br />unit is post tensioned during installation, then it does result in a <br />better and more uniform distribution of stresses within the roof strata, <br />8 <br />