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<br /> <br /> <br />Ignoring all other factors such as the presence of anomalously weak or strong strata, <br />general experience suggests that roof falls typically extend up to a height of between <br />0.8 to 1 times the roadway width or diagonal span of an intersection, as such it was <br />evident that a roof fall should not progress higher than 20 to 34 feet. <br /> <br />In those areas where shallow cover and weak strata or unconsolidated <br />alluvium/colluvium are present, it was determined that in areas that are deeper than <br />approximately 100 feet, the potential for a surface subsidence basin to develop due to <br />development mining is practically impossible. The possibility of a subsidence basin <br />developing as a result of a roof fall and a subsequent progressive “chimney-type” <br />failure, where the roof has no spanning ability, was investigated by Whittaker and <br />Reddish (1989). <br /> <br />The model, shown in Figure 1, indicates that the maximum height of a potential collapse <br />is based on a volume balance between the in situ rock and the caved/bulked material in <br />the “chimney”. Considering the variable roof lithology observed at the mine , it was <br />assumed that the natural angle of repose of the caved roof rock materials will be around <br />35 degrees and the bulking factor will vary between 1.33 and 1.5. To be conservative, <br />1.33 was assumed for this assessment (see Wittaker and Reddish (1989) and Canbulet <br />et al, 2002). <br /> <br />CONCLUSIONS <br /> <br />Using the conservatively assumed material properties and development roadway <br />dimensions, the maximum height that a chimney-type failure can progress in a <br />development mining section would range between 50 and 60 feet (see Appendix A for <br />detailed calculations). Further to this point, it is worth noting that the Wardell Guidelines <br />(1975) state that “…in general terms, collapse above a height of 5t (where t is the <br />thickness of the extracted seam) is unusual, although possible. Collapse above a height <br />of 10t would be quite exceptional”. In this case, that would equate to a roof fall height of <br />between 55 and 110 feet respectively. <br /> <br />Given that drillhole information indicates that the upper 25 to 50 feet of overburden <br />consists of alluvium and weathered rock, for the purpose of this assessment it was <br />assumed that the upper 50 f eet of overburden is weak and saturated in the area of <br />perennial streams. As such, this 50 foot depth was added to the above-calculated <br />maximum “chimney-type” failure height (i.e. of 50 to 60 feet) to ensure that the cave <br />does not extend into this zone, potentially allowing the bulked material in the “chimney” <br />to be washed away resulting in subsidence at depths greater than the calculated failure <br />height. <br /> <br />Considering the above, this assessment concludes that the potential for a subsidence <br />basin to develop above a development mining area, located under a perennial stream, <br />is practically impossible at depths greater than 110 feet. Therefore, no future <br />development mining will be conducted at overburden depths shallower than this <br />beneath a perennial stream. It should be emphasized that this assessment used all <br />worst-case conditions and if mining below a perennial stream would be required at