Laserfiche WebLink
5.9 DRAW ANGLES <br />• The angle of draw, or limit angle, is the inclination <br />(from horizontal) of the line connecting the edge of the <br />workings ~~ith the edge of the subsidence area, as shown on <br />Plate 11. Some ambiguity exists in defining exactly what <br />constitutes the limit of subsidence, but draw angles <br />measured within a specific mining region tend to be of a <br />similar magnitude. Draw angles are influenced by the <br />geological conditions. The potential for error in applying <br />an angle of draw measured in one area to another area is <br />considerable (Abel and Lee, 19II0). <br />Only limited experience is available for estimating <br />draw angles in the Mesaverde Forma*_ion. Dunrud (1976) <br />measured draw angles of 69° - 70° in weak to moderately <br />strong overburden strata consisting of 650 to 900 feet of <br />Mesaverde Formation. The limit angle steepened to 75° or <br />more in moderately strong overburden 900 to 1,000 feet <br />thick. Gentry and Abel (1977) measured an average angle of <br />draw of 74.5° in the Raton Formation at the York Canyon Mine <br />in New Mexico. Draw angles of 70° have been reported in <br />Pennsylvania (Brauner, 1973). <br />Based on the available data, we have assumed an ang)e <br />of draw of 69° for this study. This corresponds to the <br />• lowest measured draw angle reported for comparable material, <br />and so represents a conservative upper bound to the limits <br />of subsidence. <br />5.5 BREAK ANGLES <br />The angle of break is the inclination (from horizontal) <br />of the line connecting the edge of the working with the <br />point of maximum tensile strain, which approximately <br />corresponds to the cracks and fractures present at the <br />ground surface due to subsidence. <br />Break angles are less well documented than draw angle:;. <br />This is probably because surface Fracturing is less common <br />than subsidence, since it is generally limited to areas with <br />relatively thin overburden. Also, mining will frequently be <br />designed to prevent the formation of surface fractures. The <br />break angle can never be less than the draw angle. The <br />Subsidence Engineers' Handhook notes that the point of <br />maximum extension corresponds with the position of the <br />rihside where the width/depth ratio is greater than 1.3`_~, <br />but lies outside the rib when the ratio is smaller. This <br />indicates a break angle of 90° for relatively wide panels <br />• -37- <br />