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statistical correction method for room and pillar partial <br />• extraction used is outlined by Abel and Lee (1960), which <br />(' added data to the original development by Wardell and <br />Eynon (1968). <br />This analysis was performed to evaluate effects if pillars <br />were left to protect surface structures, and the results are <br />presented in Table 2.0. <br />The subsidence produced by the proposed mining methods and <br />extraction rates at the Dorchester No. 1 Mine for retreat <br />extraction is expected to occur within a short period of time <br />(within a year), while subsidence, as a result of only <br />development mining, may take over 100 years to develop (Abel and <br />Lee, 1980; and NCB, 1975). <br />Maximum expected subsidence and horizontal surface strains, <br />presented in Table 1.0, represent conditions produced by the <br />present and proposed mining plan and methods. It does not <br />account for increased subsidence, due to existing mined seams <br />above the Dorchester Mine, which may not have undergone <br />subsidence, but does account for increased subsidence in areas <br />which have undergone subsidence as predicted by the NCB method. <br />Although increased subsidence could occur for mined seams which <br />have not subsided, insufficient information is available to <br />determine the magnitude of their impacts. Only monitoring of the <br />resultant subsidence can produce results which can be utilized <br />for further predictions. <br />Generally, studies conducted by the NCB have shown that <br />• maximum angle of draw measured from the vertical of the line <br />connecting the edge of the workings and the edge of the <br />15 <br />