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• 15 <br />However, because of a variety of pillar sizes aril depths, there <br />is a possibility of pillar squeeze ar ~llapse whidi could induce <br />limited surface subsidence. If the pillars do fail ttie amount of <br />ground movement will depend upon the heading space available into <br />which the pillar can cave into the entry space. Futherrmre, if the <br />roof ar floor is soft or becomes wet the pillars m~,y be fozoed <br />through either or both. <br />3.1 Determination of the angle of draw <br />Normally, the surface aces affected by the subsidence ~.s <br />greater than the area mined out (Figure 8). The Limit of the s,ir- <br />• fare subsidence trough depends m the angle of draw az limit angle. <br />The ang~e of draw is defined as the angle referenced to the vt~,tical <br />made by drawing a line from the margins of the mined out Dieu .2us- <br />ing the subsidence. <br />In inclined and steep gal seams the angle of draw has a dpi <br />ferent value (Figure 9). It has bees observed (Kratzsch, X983) t1ti~t <br />the angle of draw at the lower face bearnes flatter with increasing <br />dip, whereas the angle at the upper face became:, steeper. <br />As it has been already mentioned the anyle of draw deperuti on <br />the cartp~sition and strength of the strata. The strata at the Harvey <br />Gap area are mr~osed of siltstones, shales arn' tiandstor~es. Such a <br />strata is comparatively strong canpared to strata cc~osed of h~it- <br />• tle shale and mudstones as in the case of bntish mini roof <br />