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~. . • of this region are Drimarily magnesium marlstone con- taining.an organic substance called kerogen, which when heated, breaks down to oil and gas). The upper portions of the shales contain a low oil content and form most of the cliffs, while the lower shales of higher oil content are locally slope-forming and covered with extensive talus deposits. Structurally, the rocks include two sets of vertical joints, perpendicular to each other. the strike of one set coincides with the direction of the canyon (north-south), while the other set is per- pendicular to the canyon axis (east-west). Non- vertical joints are less frequent; since they strike east-west or perpendicular to the direction of the road cuts, they are of little importance to the cut stability. The Evacuation Creek rocks have unconfined com- pressive strengths Qf between 5000 and 15,000 psi (350 and 1050 Kg/cm ). For the Parachute Creek shales, unconfined compressive streng tFi varies between 12,000 and 22,000 psi (840 and 1550 Kg/cm2), and depends largely on the oil content (Sellers, et al., 1971). Most of the rock cuts were designed in the competent and hard cliff-forming shale of this member. The road is constructed in a zone close to the southern rim of the Roan Plateau. It would be expected that fairly low horizontal stresses would exist in the rock mass at such a location and that the stresses would be further dec reas ecf by the erosion of deep canyons. Stress field measurements carried out by AgaDito (1974) close to the Parachute Creek Canyon tend to confirm this assumption. He found that the in-situ vertical stresses are slightly higher (18 percent) than the weight of the over- burden. The horizontal stresses reached values equal to 0.25 of vertical stress in the east-west direction and 0.16 in the north-south direction. It can be assumed that the values of horizontal stresses decrease as the canyon face is approached. As a result of these stress conditions, vertical fractures, roughly parallel with the canyon walls, can be observed in the Canyon. These fractures separate large blocks of rock from the valley side, some of which have failed into the canyon (toppling- type failure). DESIGN CONSIDERATIONS The Parachute Creek member comprises a remark- ably simple rock mass with three systems of dividing planes -- horizontal bedding planes, wnich are con- tinuous for large distances and spaced several feet apart and two sets of vertical joints, spaced at approzi ma tely the same distance but less continuous than the bedding planes. The direction of most rock cuts coincides with the strike of one vertical joint set. Several considerations were taken into account during the design of the cuts. The overall stability was obviously of primary importance and protection against rockfall comprised an important factor for cuts several hundred feet deep. The cuts were designed so that the excavation methods (dri lting and blasting) are technically and economically favorable. ROAD CUTS r~ Vertical cut faces interrupted by horizontal benches appeared to be a logical solution. Benches can significantly reduce or eliminate the rockfall hazard. 1.'hile in modern highway construction prac- tice, rock slopes without benches are preferred and the rockfall hazard is solved by a single, wide catching bench at the toe of the cut (Ritchie, 1963), due to the severe fracturing of the rock strata and the potential for toppling failures, intermediate benches were believed to be necessary in this instance. The benches also comprise an important means of future access to the cut slopes if any maintenance is required. Since all benches were to be ezca va ted in sound, weathering-resistant rock, there appeared to be no danger that the crests of the benches would fail during the lifetime of the cut. Excavation procedures are favorable for this cut configuration. It is well known (Trudinger, 1973) that a clean cut face can be obtained when the plane, given by a line of drill holes, coincides with the plane of mechanical discontinuity (vertical joint in this case). If this rule is not respected and the holes are drilled at a small angle with the discontinuity planes, the final cut face is not smooth and small wedges limited by both planes usua]ly fail after blasting. Drilling of shot holes is always easier when their direction coincides with the joints and the consumption of explosives is lower. q vertical cut face, interrupted by horizontal benches, was a logical solution to this case. The height of individual tut sections and the width of the intermediate benches (or the overall cut slope) rena fined unknown. They are of great importance to the overall cut sta bi 7ity and analytical considera- tions were necessary. ANALYTICAL CONSl DERATIONS A cut with 10-foot (3 m) wide benches and 30- foot (9.10 m) high vertical faces was designed which means an overall cut slope of 7(H):3(V), or 72 degrees. Since the slope of the natural canyon sides has the same value or is steeper, the excava- tion of the rock cuts maintained or decreased the overall slope and improved the overall stability conditions in most cases. it was evident that the strength of the rock was high enough to prevent any failure that would be caused by breakage of the rock substance. Because of the favorable orientation of the dividing planes, there was little danger of a block or wedge-type failure (John, 1968; Hoek and Bray, 1974). The toppling-type failure was the most probable type and, in fact, was well documented both on natural cliffs of the canyon and on some previously-excavated tuts with vertical faces. In order to evaluate the potential for this type of failure, finite element analysis was per- fomed for the designed cut configuration. The prira ry purpose of this analysis was to locate zones of tensile stresses behind the cut or cliff face and to analyze these zones so that they could be elimi- nated during construction by applying appropriate lateral bolting forces. 73