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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 /> 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 /> 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 /> CONCLUSIONS <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 /> 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 feet 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 /> 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 <br />