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Southern limits of the proposed mine coincide closely <br />with the projected line of the burn. This line has been <br />based on geologic interpretations and supported by the <br />existing mining experience. We understand that it was <br />generally assumed that the thickness of the burnt out zone <br />is about 300 feet. The projected burn line has been <br />extrapolated according to this condition and is based on the <br />knowledge of the outcrop of the coal seam along the southern <br />end of the lease boundary. <br />It should be expected that the interpretation of the <br />burn line may not be very accurate. There are no data <br />available other than those obtained from mining within the <br />existing lease. There are no drilling results available to <br />support this evaluation. The accuracy of the projected burn <br />line is important if a safe and economic outcrop barrier <br />pillar should be designed along the southern boundary of the <br />future mine. The dimensions of the outcrop barrier pillar <br />are very important for the subsidence evluation; because of <br />Iow overburden thickness along the southern part of the <br />;,line, subsidence can be expected to be quite severe, and it <br />could be greatly influenced by potential failures of the <br />outcrop pillar, as will be discussed later in this report. <br />The subsidence within the proposed long-term lease area <br />will very much depend on the underground development. While <br />• subsidence has to be expected above the mined-out panels or <br />blocks of panels, limited subsidence will occur above the <br />pillars, specifically above the wide protective zones that <br />include the mains and submains of the mine. Both the mains <br />and submains may eventually be mined out, but at a certain <br />period of time the situation will be encountered where the <br />panels are mined out but mains and submains are still <br />present. It is very probable that the consequences of <br />subsidence may be severe at that time; the relative <br />subsidence between the zones above the pillars and above the <br />mined out panels nay reach a maximum at that time. Under <br />unfavorable conditions, such as log: overburden, subsidence <br />cracks may dev=lop. <br />In order to facilitate the evaluation of subsidence <br />under such conditions, the mine development conditions <br />planned by CWI were simplified to a scheme which can be used <br />for the subsidence predictions. Such an underground <br />~9evelopment scheme is presented on Plate 10. In this scheme, <br />the areas where the subsidence hazard is low and whey? <br />either no subsidence or limited subsidence can be expected <br />are defined. Such zones are represented by zones outside <br />proposed mining areas, zones =hove protective pillars, and <br />zones above the outcrop barrier pillar. .411 other areas, <br />• - 23- <br />