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<br />'• The open joints aze conduits of water and aggravate the meta-stable condition by: <br />1 (1) reducing the effective strength along the joint faces, (2) increasing the overturning <br />moment in the tension crack, and (3) increasing wedging and overturning moment by <br />freezing and thawing. These processes also contribute to extending the fracture with depth. <br />Toppled blocks generally fall dose to the cliff face and do nut roll very faz down <br />the talus slope. If a toppled block breaks near the crest of the cliff and falls to the base of <br />the cliff, it can bounce and roll for several hundred feet down the talus slope; these are the <br />blocks of most concern for the rock-fall hazards to the road. Blocks as large as 1Ox10x10 <br />feet were observed near the road in the Hazard Zones 4, 5, and 6. These Hazard Zones also <br />contain the highest elevations of the Twentymile Sandstone. Future toppling of blocks in <br />1 these zones is expected to continue. <br />1 Earthquake shaking can dislodge blocks in the cliff or along its base and cause <br />them to roll downslope. Because strong earthquake ground motion is rare in the area, such <br />l• an event is unlikely to occur in the neaz future. <br />Arcuate Failure. In locations where intersecting joints and fractures are closely <br />spaced, the rock is relatively weak. An arcuate failure results because intersecting joints <br />dominate. The development of this type of failure is in part a result of the tensile stresses <br />along the cliff face but the role of water has an important role because of many conduits <br />caused by the closely spaced joints. <br />1 Arcuate failures generally remain fairly close to the cliff face in part because the <br />water and pore water pressure that triggered the movement dissipates rapidly as the rock <br />mass moves and breaks apart. Although the entire rock mass that constitutes the failure may <br />1 not be a major hazard to the road, individual blocks may move down the talus slope toward <br />J the road. This type of failure was observed in Hazard Zones 2 and 6. The arcuate failure in <br />Zone 2 is resting on the talus slope; the rock mass is extensively broken. However, <br />individual blocks may topple and or migrate downslope. The arcuate failure in Zone 6 is at <br />• 12 <br />