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LJ~ ' 3.1.2 The crown pillar ' The crown pillarZ above the mined disposal area is designed to be a minimum of 125 feet thick, to ensure segregation from the mined ' room above. This pillar is 2.5 times the thickness of the seam, which would appear to be conservative with respect to crown p illar ' design in coal mines3. The computed factor of safety for this pillar against sliding under gravity is over 10. A further level of conservatism results from the following considerations: ' 1. only a total of 200 feet of workings are located in the seam above the disposal area; and ' 2. as the seam dips at 53°, this working area is not actually directly above the proposed disposal area in any case. 1 ' 3.1.3 The barrier pillar ' The disposal area is separated from the mined area by a 120 d l i d f i i i i i foot l llar. Th s thick barrier p se ne coa s compr o unm , p l ar and is sized to ensure that the pillar could withstand the full pressure head exerted if the entire disposal void were filled with a liquid with the density of the backfilled material, and the ' material were flowing out of the vent pipes at the surface4. The ' ZThe term "crown pillar" is here used to describe the pillar separating the disposal area from the nearest overlying working ' area. It is appreciated that this differs slightly from the normal use of this term, but as the function of this pillar is to ensure separation of the disposal area from the workings, it is considered that the pillar's function is identical to that of a crown pillar. ' 35ee attached computation sheet for a simple evaluation of the stability of the crown pillar. t 45ee attached computation sheet for an evaluation of the factor of safety of the barrier pillar. ' Adrian HroNII Consultants -13- 1145/900820