Laserfiche WebLink
and extraction zones. <br />• 1.35, the distance from <br />of maximum extension <br />break angle. <br />At width/depth ratios of less than <br />the center of the panel to the point <br />increases, indicating a decreasing <br />Dunrud (1975) notes nearly vertical break lines in <br />several instances, with break angles greater than 90° in <br />other cases. A break angle of 90° has been assumed for this <br />study. <br />5.6 ZONES OF POTENTIAL FRACTURING <br />Fracturing is likely to occur at the ground surface <br />when mining configurations produce zones of high tensile <br />strain. Tensile strains will generally decrease as the depth <br />of mining increases, so that the risk of surface fracturing <br />will decrease as mining depth increases. <br />Two mining configurations may potentially induce <br />surface fracturing. During the pillar recovery phase of <br />mining, cracks may form above the retreat line and <br />ultimately above the barrier pillar at the end of the panel <br />as the overlying beds cantilever over the pillar line. This <br />cantilever effect could occur above any substantial barrier <br />pillar which is too pride to fail, such as an outcrop barrier <br />• pillar or mains barrier pillars. Alternatively, when <br />adjacent panels are extracted, flexure of the overburden <br />beds as they pass over the barrier pillar between the panels <br />will result in tensile strains immediately above the pillar. <br />Some general guidelines are available for predicting <br />depths to which mining can be expected to induce surface <br />fracturing. Maximum tensile strains occur at the surface <br />when the pillar width is about 0.9 times the overburden <br />depth (Dunrud, 1976). This indicates that for 60 foot wide <br />barrier pillars surface cracks should be worst at an <br />overburden depth of about 150 feet, and should decrease in <br />severity with increasing overburden depths. tJhen the <br />overburden depth is sufficient to cause the barrier pillar <br />to yield, tensile strains above the pillar should be <br />substantially reduced and should prevent surface fractures <br />developing. It is expected that the barrier pillars will <br />yield at overburden heights of approximately 500 feet. <br />Field measurements of subsidence cracks in the <br />Mesaverde Formation by Dunrud (1975) provide an alternative <br />basis for estimating the overburden depths to which <br />subsidence cracks will occur. He found subsidence cracks <br />above a 90 foot wide barrier pillar gradually narrowed and <br />• - 38 - <br />