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~' ~ ROAD Cl7S i" <br /> <br />LJ zol,s a iz NSl~e sixsm, <br />siMC55CS IN 16f <br />Figure 3. Minor principal s_ress con `_:u rs before and <br />after excava ~:i or, model wit-. joints. <br />The finite element analysis confirmed that the <br />design concept was correct. It indicated the poten- <br />tial hazard of a toppling-type failure of individual <br />benches. At the same time, it showed the improvement <br />of the overall stability of the cliff and the minor <br />importance of stress relief fractures already exist- <br />ing in the rock mass. <br />CONSTRUCTION E%PERIENLE <br />The construction of the road and the excavation <br />of rock cuts developed a number of interesting <br />observations. The final road design required rock <br />cuts up to 300 feet (91 m) high (Figure 4). During <br />excavation of pioneer roads close to the cut crest <br />and during excavation of the upper cut benches, <br />numerous vertical fractures parallel with the canyon <br />axis were found. They followed the strike of the <br />vertical joint set, and were open up to 1 foot <br />(0.30 m) with no infilling. They were encountered <br />only in the upper portion of the canyon at locations <br />remarkably close to the zones of tensile stresses <br />predicted by the finite element analysis of the <br />natural cliffs (Figure 3). When the contractor <br />encountered these large and deep fractures, he <br />usually requested that the overall cut slopes be <br />flattened so that the rock mass, including the <br />'roc t~_res, would be excavated. The request was <br />-efused as the fractures were expected and as the <br />cut excava=ion effectively reduced the tensile <br />:tresses ceeper in the rock mass. <br />_iast'.ng of vertical tut faces produced <br />excel;ent results, even if pre-sD1it techniques were <br />-ot svstern ti tally used. The cut faces, coinciding <br />.ith _ne vertical joint set, were smooth and free of <br />-arge loosened blocks. <br />-~e c:~tractor had problems with the recommended <br />.idtr• of i~'ividual benches of 10 feet (3 m). He <br />required a -inimum width of 12 feet (3.65 m) so that <br />-eav~ equ'=rent could be moved along the benches. <br />:ue *_= fav_rable experience with the cut construc- <br />tion, the ~. idth of some benches was increased to <br />:nis ~: alue, while the height of the benches was <br />;ncrc_sed troportionally to maintain the recommended <br />:ve ra :l a~: slope of 1(V):3(H). <br />.are`_i construction supervision did not reveal <br />.,y c=nger of a toppling-type failure or tilting of <br />=nd iv'dual benches and rock bolting was not applied. <br />.n s^_re cases, the blasting procedures caused dis- <br />=iaccrents of fairly large blocks of rock close to <br />.he to nth :rest. The blocks were displaced along <br />75 <br />