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We have assumed an angle of draw of 25 degrees for this study. This <:orresponds <br />to the draw angle widely accepted as a conservative number in the North Fork Valley. <br />1.5 BREAK ANGLES <br />The angle of break is the inclination (from horizontal) of the line connecting the edge <br />of the working with the point of maximum tensile strain, which corresponds to the location <br />of maximum residual cracking and fracturing present at the ground surface. <br />Break angles are less well documented than draw angles. This is probably because <br />surface fracturing is less common than subsidence, since it is generally limited to areas <br />with relatively thin overburden. Also, mining will frequently be designed to prevent the <br />formation of surface fractures. The break angle can never be less than the draw angle. <br />The Subsidence Engineer's Handbook notes that the point of maximum extension <br />corresponds with the position of the ribside where the width/depth ratio is greater than <br />1.35, but lies outside the rib when the ratio is smaller. This indicates a break angle of 90 <br />degrees for relatively wide panels and extraction zones. At width/depth ratios of less than <br />1.35, the distance from the center of the panel to the point of maximum extension <br />increases, indicating a decreasing break angle. <br />Dunrud (1976) notes nearly vertical break lines in several instances, with break <br />angles greater than 90 degrees in other cases. Such high break angles are probably due <br />to tension rather than shear. A break angle of 90 degrees has been assumed for this <br />study. <br />1.6 ZONES OF POTENTIAL FRACTURING <br />Fracturing is likely to occur at the ground surface when mining configurations <br />produce zones of high tensile strain. Tensile strains will generally decrease as the depth <br />of mining increases, so that the risk of surface fracturing will decrease as mining depth <br />increases. <br />During longwall mining and the pillar recovery phase of mining, cracks may form <br />above the retreat line and ultimately above the barrier pillar at the end of the panel as the <br />overlying beds cantilever over the pillar line. This cantilever effect could occur above any <br />- i 0 - 10!00 <br />PR-a y <br />