The presence of shale seams in the keyway areas indicated that additional geological information was required in order to consider problems of percolation through these seams, rapid deterioration of these seams when exposed to large quantities of water, and weakness of the stratigraphic column of the keyway. Project survey personnel collected data and prepared maps and drawings to show location of all major shale seams in both abutments. These maps and drawings were furnished to the Denver office for consideration in preparation of designs. It was decided that treatment of the shale seams would consist of placing concrete cutoff tunnels in the shale beds at the heel and toe of the dam in order to increase the percolation path of seepage water through the shale and to confine the shale and prevent it from squeezing out under load when it became wet. Additional details of the treatment for shale seams are covered in sections 13 through 16. B. Geology 10. Regional Geology. The dam and reservoir area is near the east end of the Uinta Mountains. These mountains are a broad anticlinal fold with the older rocks exposed on the crest. Rocks ranging from pre-Cambrian to Tertiary in age occur on both the north and south flanks of this range, dipping away from the crest and exposed in colorful sequence. All are sedimentary in origin and include sandstone, shale, quartzite, and limestone. Faulting is quite prevalent along the northern flank of these mountains and can be observed in several places. A fault of large displacement occurs along the north edge of Hideout Flat, 18 miles upstream, where the Weber sand stone of Carboniferous age rests flush against the much older pre-Cambrian Uinta quartzite. The course of the Green River as it cuts its way across the eastern end of the Uinta Mountains is one of unusual interest. It meanders in and out of hard formations regardless of structure. On several occasions, it cuts itself out of the hard rock into relatively soft shales, and when it seems it would have had a much easier path to continue in the shale it turns abruptly back into the hard, resistant formations which comprise the center of these rugged mountains. Its location was undoubtedly determined by a much different topography in the past. The present meandering course was doubtless established in a softer uniform sediment, and after cutting down hundreds of feet the river found itself partly astride the resistant formations and partly in the softer sediments. Subsequent erosion has removed all the upper sediments, leaving the river in Red Canyon entirely in the resistant Uinta formation. The dip of the strata is to the northeast and the range from 1° to 75°. Two maps published by the U. S. Geological Survey (1955 and 1956)--the Manila quadrangle and the Flaming Gorge quadrangle -- show detailed structure and areal extent of the formation in the areas north of the dam site. Two other quadrangles--the Dutch John Mountain and Goslin Mountain areas--provide excellent data on the general geology of this region. 11. Reservoir Geology. The reservoir, with a maximum water surface elevation of 6045, rests on sedimentary rocks ranging in age from pre-Cambrian to Tertiary. These formations are predominantly shale and sandstone with minor amounts of limestone, and all are fairly impervious to percolating water. Since the Green River forms the low drainage of this entire area and all ground water is tributary to this stream, there seems no possibility that reservoir seepage will be of any consequence. 12. Damsite Geology. Green River, in its course through the reservoir basin, meanders in a large half circle. At the damsite, its general direction is southeast but it makes a 40° curve to the south at the axis. This curve provides an attractive inlet and outlet for the diversion works and a good slope for the spillway. The profile along the axis of this site is unusually good for a high dam. A shoulder extends out from the right side which makes an excellent location for an arch dam. Except for a small patch of Tertiary conglomerate on the high right abutment, bedrock at this site all belongs to the Uinta formation of pre-Cambrian age. It consists of quartzite, quartzite conglomerate, quartzose sandstone, and a few thin beds of shale. The general quality of this rock is excellent for all types of construction purposes. The shale occurs in lenticular beds usually less than 2 feet thick and widely spaced so they are insignificant when contrasted with the massive quartzite. Grain size in the sandstone and quartzite ranges from medium sand to pebbles 2 inches in diameter. The sand stones and quartzites differ only in the degree of cementation. Both are cemented with silica, and where the cementing action has completely filled the pore space, they have formed the quartzites. Where the pore space is only partially filled, they are called quartzo se sand stone. Beds of conglomerate are interspersed in all the strata and are well cemented and equally as hard as the quartzite. Bedding ranges from 2 to 20 feet apart, with many thick, massive ledges 20 to 40 feet high. Structurally, the dip and strike of the strata are ideal to receive the thrust of the dam. The dip is 16-1/2° Ñ. and the strike N. 52° E. Folding in the area has produced some shearing in the brittle rock. A prominent system of joints parallel to the axis and located on the right abutment is evident. The joints are nearly vertical and are spaced 2 to 4 feet apart. Drilling disclosed that these joints did persist at depth but they were fairly tight. Some healing action has taken place and the crevasses are partially filled by secondary cementation. Two faults or shear zones occur in the dam site area. Neither of these is involved in the dam itself, but the diversion tunnel crosses one of them. One is located upstream about 82 feet from the face of the dam. It is vertical and trends N. 73° E. The shear zone is 8 to 10 feet wide and consists of broken blocks of quartzite. There is little or no gouge associated with the movement where the fault can be observed. The other shear zone is located 1, 100 feet downstream from the face of the dam and follows a shallow draw up the right abutment. There is no well-defined topographic expression on the left. A sharp change in dip and strike on each side of the draw, together with the talus covering in otherwise hard, well-exposed rock, was strong evidence of its position. It was explored by two core drill holes both of which were angle holes across the shear zone. Both encountered fractured rock. One drill hole was drilled on the spillway tunnel line and showed some fracturing but this was not severe enough to result in heavy ground or serious construction problems. The fractures range from 6 inches to 2 feet apart and the sand stone is leached or ground up to small fragments near the fractures. (a) Diversion Tunnel. --The diversion tunnel crossed the upstream fault between tunnel stations 2+25 and 2+30. Tunnel excavation in the shear zone of the fault area disclosed that the rock was shattered but tight and dry. The tunnel lies in bedded hard quartzite and sandstone with a few thin beds of red ferruginous shale. This rock dips upstream at 16-1/2° and the major part of the tunnel is approximately along the line of maximum dip. Except for jointing, this rock is well adapted to tunnel construction. Where jointing is more severe, steel supports were placed in the tunnel at 5-foot centers. These supports were composed of 8-inch wide-flange beams and were located between tunnel stations 1+25 and 2+55, stations 5+75 and 6+29, and stations 9+33 and 9+63. The rock was brittle and shot well, but it was hard and resistant which dulled drill bits quickly. Several shale beds were encountered during tunnel excavation, but they were so thin and widely separated that they did not influence the conditions in the tunnel to any significant degree. Little or no water was encountered except in the invert of the tunnel and these small seeps were directly tied to river stages. (b) Spillway. --Because of its superior angle for the stilling basin, the left abutment was selected for the spillway. Geologic conditions were suitable for a tunnel-type spillway. The rock was generally of good quality except near the outlet end of the tunnel where it crossed a downstream fault, and in this area there was a zone of broken rock. The left abutment where the spillway is located was exposed to more weathering than the right abutment and contained many unstable boulders and ledges. This unstable material presented no problem for successful operation of a tunnel-type spillway; however, it did present a problem of potential danger to the powerplant area in the river bottom should it come loose and fall. It was therefore removed during construction operations. (c) River Channel Overburden. --It was determined that the foundation rock in the river channel was hard and unweathered. The river fill was essentially sand and well-rounded gravel and boulders of hard quartzite. A few large rock slabs occurred intermingled with the gravel. These slabs represented rockfalls from the cliff adjacent to the river. CHAPTER III. FOUNDATION TREATMENT A. Shale Zones 13. General. The foundation rock for the dam (see chap. II) has an approximate dip of 15° upstream and toward the reservoir. The rock ranges from a moderately hard sandstone cemented with silica to a very hard quartzite. Interspersed at various elevations throughout the foundations are zones of varying thicknesses composed of interbedded lenslike layers of shale, siltstone, and sandstone in varying proportions. These zones range from 1 foot or less to 10 or 12 feet in thickness. Individual shale layers within the zones and at other levels in the bedrock sequence are generally of minor thickness and range from paper thin to about 4 feet. For convenience, the more important shale zones are numbered for identification, beginning at 16 in the foundation excavation beneath the river channel and decreasing to 9L and 5R on the left and right abutments, respectively. See figures 4 and 5 for typical identification of shale zones in both abutments. 14. Concrete Cutoffs for Shale Zones. It was deemed advisable to excavate, and backfill with concrete, upstream and downstream cutoff drifts into the abutments for the larger size shale zones. The object of these cutoffs was to increase the path of possible reservoir percolation through the zones and to confine the shale zone materials between the cutoffs. Eight zones were protected by cutoff drifts, as indicated by the following tabulation: *Only one cutoff drift, due to minimum reservoir head and reduced The required depths for the cutoff drifts were determined by the character of the materials encountered as the excavation for the drifts progressed, by the thickness of the zones, and by the reservoir head to which they will be subjected. 15. Concrete Protective Walls For Shale Zones. For some distance downstream of the toe of the dam the shale zones are exposed, due to scaling and sluicing activities on the abutments during construction of the dam. Immediately above several of the shale zones were massive layers of rock with a predominate near-vertical jointing system. This jointing system, together with the dip of the bedding planes toward the river, would |