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-5- <br />• <br />rocks, i.e. lower strengths accompany higher moisture contents and (or) lower densities. <br />• - The strength of the Cameo "B" seam coal increase at a decreasing rate with <br />„ increased confinement, as previously described by other investigators for other coals <br />' - (Wilson, 1972). The angle of internal friction (~) for Cameo "B" seam coal is 59.5° <br />. - for fhe range of confining pressures from 0 to 425 psi, 47.2° from 0 fo 1000 psi and <br />36.4° from 0 to 2000 psi. At depths beyond 1000 ft the angle of intemal friction <br />` should be reduced to the 36.4° value. <br />- The effect of using the recommended 0 to 1000 psi strength values for design <br />of the pillars between Z00 and 300 ft of depth will be to underestimate their actual <br />strength. <br />Two samples of-floor shale were tested under unconfined cmmpression. Their <br />~ 1960 psi specimen compression strength represents the lowest'of the roof, coal seam <br />I' <br />• and floor sequence. Previous Testing of the near-roof rock indicated an uncronfined <br />compression strength of nearly 9000 psi. <br />The coal mean unconfined compression strength is 2780 psi. Given these <br />conditions pillar failure would be expected to develop first in or at the floor and <br />progress up toward the roof. If possible the thickness of the floor shale should be <br />•- determined. The floor shale did not deteriorate during the wet coring process. This <br />implies that the floor should remain relatively stable during mining, even if wet or <br />under water. <br />-• <br />