Laserfiche WebLink
concentrations associated with intersection areas, one may see in <br />this same table that maximum safe spans are approximately cut in half, <br />Next, we looked at the relative effect on theoretical bending <br />stresses in each roof strata for different roof spans and different <br />types of reinforcement. Each layer was investigated for roof spans <br />ranging from 15 to 20 feet. Further, we investigated the effect of <br />both friction reinforcement and suspension reinforcement, as well as <br />the combined effects of both. Two different levels of friction <br />reinforcement were investigated - one where a 3/4-inch standard rock <br />bolt unit was post tensioned to 13,600 psi (approximately 6000 pounds <br />plate load), and the other where tension in the rock bolt unit was <br />only 3400 psi (approximately 1500 pounds plate load). This latter <br />bolt tension approximates what might be expected in a passive rock <br />bolt unit upon deformation of the roof strata where no active post <br />tensioning is applied during installation. <br />The first layer is the thin shale layer immediately above the <br />coal. In this layer, friction reinforcement was found to reduce <br />bending stresses slightly (approximately 168 to 258, depending upon <br />the degree of post tensioning). However, the effect of suspension <br />on this thin shale layer was very great with a reduction in bending <br />stresses by some 868. The combined effects of both friction and <br />suspension affected an even further reduction in bending stresses. <br />The second layer is a siltstone of moderate thickness. The <br />effect of friction reinforcement once again decreased bending stresses <br />by some 168 to 258, depending upon the degree of post tensioning. <br />The effect of suspension type reinforcement decreased stresses by <br />approximately 328 in this layer. The combined effects of friction <br />and suspension yield a 438 to 498 reduction in bending stresses. <br /> <br />6 <br />b <br />