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method of predicting subsidence stated that "in a virgin area the <br />prediction"..... "should be reduced by a multiplying factor of <br />0.9". Therefore the predicted maximum subsidence for areas at <br />the Dorchester No. 1 Mine which have not been previously mined <br />were reduced accordingly. <br />The NCB subsidence prediction model is recognized as the <br />most conservative reference available. Although the NCB <br />subsidence prediction model was based on different coal bearing <br />lithology than found in the United States, it was used to predict <br />the Dorchester Mine subsidence because of its conservatism. <br />To determine the worst case subsidence for room and pillar <br />mining, predictions of subsidence due to "development' only, were <br />determined based on Figure 2. The prediction of vertical <br />• subsidence and horizontal strain of the ground surface above and <br />adjacent to partially extracted room and pillar panels requires <br />modification of the NCB longwall method as follows: <br />• Previously unsubsided ground. The reduction of predicted <br />maximum subsidence by a factor of 0.9 was not used in the <br />case of room and pillar partial extraction. The data on <br />room and pillar related subsidence were taken from 13 <br />cases, only 4 of which had probably been previously <br />subsided (Abel and Lee, 1980). <br />• Pillar dimensions. The ratio of the pillar height to <br />pillar width in part determines the degree to which <br />either the pillar will flatten when it fails, or how far <br />it will punch into adjacent rock(s) in the case of <br />squeeze or pillar punching (Abel and Lee, 1980). The <br />14 <br />