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• and 6. Planar failures were observed at the erosional re-entrants where the moderate dip of <br />joint set 6 is daylighted in re-entrant (Hazard Zones Sc and 6b). The intersection of the <br />conjugate set of shear joints produce wedge failures (Figure 4d). L,ocally, bedding planes <br />were observed to provide a constraint on the size of these failures by forming a weak plane <br />at the top of the wedge. Such failures were observed in Hazard Zones 3, 4 and 5. <br />Field observations indicate that the base of the cliff has been eroded locally with <br />the result that the rocks above the voids have lost support. The overlying rocks have the <br />potential for failure. The origin of the erosion is not known but may be related to water <br />seeping through the cliff and through the case-hardened rinds, and also to freeze-thaw <br />effects. <br />The potential for slope failure will be excerbated by longwall mining at a depth of <br />approximately 650 ft below the cliff. The relationship of the cliff to the existing and <br />projected underground workings can be seen in Figure 5. <br />• 3.6 TooplinE. Arcuate. Planar. and WedPe Failures <br />This section discusses the types of failures observed at the site and some geologic <br />factors that contribute to the failures. <br />T~,pplinE Failure. Joint or fracture planes close to and approximately parallel to <br />the free face of the cliff are subject to opening. Opening of the joint characteristically begins <br />at the crest of the cliff and close to its face. Propagation of the opening with depth depends <br />in part on the lateral tensile stresses and the cohesive strength of the rock. The potential <br />instability is related to the height of the cliff face and the depth to which the tensile stresses <br />act. Opening of parallel joints at progressively greater distances from the cliff edge develop <br />in this same way. <br />• 11 <br />