11. GEOLOGIC

INVESTIGATIONS. Extensive geological explorations of the Yellowtail damsite were started in the summer of 1946. Through 1950, 190 diamond drill holes were completed. These explorations were conducted for Yellowtail Dam and appurtenant structures, two afterbay damsites, a pumping plant site, and a bridge site. Four 6.5- by 8.5-foot tunnels, aggregating 1,487 feet, were driven into the damsite abutments. A preliminary geologic map, superimposed on topography sheets, was prepared of the damsite area. The U.S. Geological Survey prepared a map which covers the geology of the damsite and reservoir areas.

During 1951 and 1952 three water level observation holes were drilled adjacent to the left abutment. These holes were drilled because some of the first investigations indicated the ground water level on the left abutment was at a lower elevation than the river. After these holes were completed, investigations were suspended, except for a few field trips, for several years after finding the ground water gradient sloped to the river.

Investigations were resumed in October 1960 and continued throughout the construction and postconstruction period. Several holes were drilled to provide answers for specific design questions such as bedrock elevations, rock quality, and rock types. Numerous holes were drilled in connection with the grouting program both as exploration holes to determine the zones which required grouting and also as a check to determine adequacy of the grouting program after it was completed. From 1960 through 1965, 115 core holes were drilled.

Several water level observation holes have been drilled adjacent to the abutments. These deep holes were drilled prior to, during, and after filling the reservoir and were used to check the ground water table in its static condition and changes in the water table as influenced by the rising reservoir.

12. REGIONAL GEOLOGY. Yellowtail Dam and the reservoir lie on the eastern edge of the Middle Rocky Mountain Physiographic province. The Bighorn River, master stream in the area, originates in the Wind River Mountains of west-central Wyoming. Flowing northward across the Wind River and the Bighorn Basins in Wyoming into Montana, it cuts across the northwest flank of the Bighorn Mountains in a 44-mile gorge before joining the Yellowstone River approximately 80 miles downstream from the dam.

Yellowtail Dam and Powerplant, the reservoir, appurtenant structures, and the afterbay dam are located entirely in sedimentary rocks. These strata, predominantly of marine origin, lie in a nearly conformable sequence and represent every system from Cambrian to Cretaceous, except the Silurian. The major construction activities were situated in the Madison, Amsden, and Tensleep formations.

The Madison limestone in geologic history was subjected to severe erosion and solution activity. The solution activity penetrated the Madison limestone to various depths, but only the upper 140 to 160 feet were severely affected. The solution cavities were generally filled during early Amsden deposition with red mud which is now a variably calcareous, red, shaly siltstone containing angular fragments of limestone ranging up to boulder size. This material displays a conglomerate effect, depending on the siltstonelimestone rubble proportion, and is locally referred to as "solution breccia."

Based on the topographic development of the canyon walls at the damsite area, the Madison limestone has been divided into three members-lower, middle, and upper. The upper member is defined as the top 140 to 160 feet of the Madison which were subjected to this severe solution activity and the cavities filled with solution breccia. The middle and lower units are essentially all limestone.

Solution breccia and the siltstone masses are especially prevalent at the contact of the upper and middle members but less prevalent above this zone. This material weathers more readily than the durable limestone, explaining some of the reentrants and pockets in the upper member.

Solution cavities lined with calcite crystals were found in the upper 20 feet of the middle Madison member. These cavities are the result of solution during a much later time than the breccia-filled cavities found in the upper member. Project tunnels and grout holes intersected some of these cavities. A few of these cavities were several cubic yards in size. These cavernous zones took considerable quantities of grout during grouting operations.

The Amsden formation, deposited in early Pennsylvanian time, consists of sandstones, shales, and limestones which were progressively deposited as the waters increased in depth. The Tensleep formation consisting of the fine- to coarse-grained sandstone of shallow

water or dune origin was deposited on the Amsden formation.

The Bighorn and Pryor Mountains, though distinct structural units, are essentially parts of one great uplift which extends from Wyoming northward into southcentral Montana. The northern end of the Bighorn Mountains is a northward-plunging anticline with a rather broad top and steep limbs.

The axis of the Bighorn Mountains swings from N.40° W. in Wyoming to N.60° W. in Montana. The foothills of this anticlinal arch are composed of a series of hogbacks which dip from 45° to 70° and form a narrow belt that surrounds the mountains on nearly all sides. In the vicinity of the damsite, the Tensleep sandstone, emerging at a steep angle from beneath the younger sediments, flattens out to form the capping rim of the Bighorn Canyon.

Few faults are found in the area. One major fault crosses the Bighorn Canyon at nearly right angles about halfway between Dryhead and Devils Canyons. This fault which has a displacement of about 225 feet is tight and does not affect the water-holding capability of the reservoir.

Several slip planes were found in the left abutment foundation of the dam. A major slip plane crosses the spillway stilling basin, bears through the left abutment, and intersects the river immediately upstream of the dam. These slip planes are located in the Madison limestone but appear to die out in the overlying Amsden formation. A fault with several hundred feet of displacement exists in the hogbacks about a mile downstream of the damsite. This fault and the major slip planes have the same general trend, but there is no evidence to indicate that they connect.

13. YELLOWTAIL DAMSITE GEOLOGY. Depths of overburden ranged from a maximum of 96.5 feet on the left abutment to 39.1 feet on the right abutment. The greatest depths of overburden were found below elevation 3300 on the left abutment and elevation 3280 on the right abutment. Above these elevations only a thin veneer, less than 6 or 7 feet thick, was found. Overburden in the river channel averaged about 10 feet thick and consisted of silt, sand, gravel, and limestone boulders.

The static water table was tributary to the river. The foundation rock below the water table was reasonably tight, though some bedding planes were water passages. The contractor had very little trouble keeping the foundation dry.

The lower and middle members of the Madison limestone provided an excellent foundation. Final excavation lines followed the designed cuts, as interpreted from the investigation drilling and mapping, very closely. There were only a few small localized areas where extra rock had to be removed to obtain a satisfactory foundation.

The pockets of siltstone in the upper Madison on both abutments tended to air slack and deteriorate within a short period of time after excavation. The following final cleanup procedures were used to insure that concrete was placed against the best possible rock: The outer 12 to 18 inches of the siltstone were removed a few days prior to concrete placement, and then another 3 or 4 inches were trimmed off on the day that particular lift was placed.

Major faults do not cross the foundation, but a series of slip planes are found on the left abutment. Jointing throughout the foundation was well developed, but tight, and was not a factor in sloping the keyway. Sizes of rock fragments were determined mostly by the bedding plane spacing, as the joints were tight enough to have had only negligible effect.

A series of stress relief joints cross the downstream toe of block 21 at nearly right angles. The rock shoulder above the toe was shattered by this jointing, so considerable rock had to be removed to protect the underlying powerplant parking area and river outlet stilling basin. The loss of this rock shoulder would have endangered the toe, so the remaining rock was anchored with 20- to 40-foot-long groutable rock bolts. In lieu of removing small fragments, some surface pinning with standard rock bolts was also done.

A description of the basic grouting plan is contained in section 26. B-hole grout takes on the left abutment in the lower and middle Madison formations were much higher than those on the right abutment. These takes can be attributed to the series of slip planes which cross the left abutment. Takes in the foundation area were attributed to the grout traveling and filling openings along bedding planes, since this area is practically devoid of major joints or slip planes. Actual B-hole grout takes are discussed in section 260.

Actual B-hole grout takes on both abutments in the upper Madison formation were small, so it is assumed that the relatively shallow holes did not encounter the open cavities which are known to exist at various locations. Several slip planes crossed the left abutment, but all were lined with siltstone and proved to be fairly tight. These slip planes become more open at depth as

indicated by the higher grout takes in the lower and middle Madison formation.

The large takes in the left abutment A-holes in the lower and middle Madison formation can be attributed to grouting slip planes. The right abutment holes took very little grout. Some large takes also occurred below 200 feet in the foundation area. A number of these deep holes flowed water under pressure and also developed considerable back pressure while grouting. Actual A-hole grout takes are discussed in sections 261 and 262.

See section 27 for a discussion of the A-hole grouting in the upper Madison formation from the grouting and inspection tunnels. A description of the drainage of the upper Madison formation is contained in section 29.

The spillway intake channel proper is located in the upper member of the Madison limestone, and with the exception of a few cavity areas, the rock quality is very good to excellent. Rock at this location is chiefly limestone (tin to massive bedded) with a few pockets of solution breccia and many siltstone stringers. The Amsden-Madison contact is essentially near the top of the intake channel proper, so the entire backslope, about 200 feet high, is located in the Amsden formation. The basal Amsden member, which is a poor-quality shaly siltstone, was cut on a 1.4 to 1 slope and the rest of the Amsden on a 3/4 to 1 slope. The cavities which were found in the walls of the spillway intake channel were backfilled with concrete, and the rock around the structure was grouted.

The spillway tunnel passes through all three members of the Madison limestone. The three members displayed excellent stability, and no unusual construction problems developed.

Rock quality in the spillway stilling basin was generally very good, even though the rock was crossed by a series of slip planes. These slip planes seeped water, and a drain system was required under the structure. One slip plane crossed the stilling basin and passed through the outlet end of the spillway tunnel. Since this slip plane was a significant water passage, the rock around the end of the spillway tunnel was not grouted in order to eliminate any chance for grout to get into the big slip, follow along it to the stilling basin drains, and grout them shut.

The pilot bore up the vertical shaft of the highvoltage and control cable tunnel remained open and unprotected for eight or nine months, at which time final excavation procedures were initiated. Severe air

slacking and sloughing of the basal Amsden had occurred. Measures were taken to prevent additional air slacking by completely lagging the openings between the ring beams and backfilling with concrete as soon as possible.

14. RESERVOIR GEOLOGY. At elevation 3657.0 Yellowtail Dam forms a reservoir about 71 river miles long, of which the first 42 miles is in Montana and the remainder in Wyoming. Downcutting by the Bighorn. River kept pace with the Bighorn uplift to form a nearly inaccessible, narrow, winding canyon that extends from about three-quarters of a mile below the dam to 14 miles upstream near the mouth of Dryhead Creek. A distinct monoclinal flexure between Dryhead and Big Bull Elk Creeks lowers the top of the Madison limestone about 1,500 feet to the general level of the Bighorn Basin. The river from the monocline south (upstream) for 30 miles is contained in a shear-walled gorge. The remaining upstream length of the reservoir is contained in a gently sloping valley.

To evaluate seepage losses from the reservoir, several water level observation holes were drilled from the grouting and inspection tunnels and in the area adjacent to Yellowtail Dam. The water levels plus water temperatures at various elevations of these holes have been monitored.

The springs that developed downstream from the left abutment during reservoir filling were also checked for flow and temperature. A grouting program from the left abutment grouting and inspection tunnel (see sec. 275) was initiated in 1967 and had essentially completed its objectives in early 1969. A period of monitoring observation holes began again at that time.

15. AFTERBAY DAMSITE GEOLOGY. The afterbay damsite was first investigated in 1949. Five holes were drilled to determine the depth and type of overburden and the character of the bedrock. A series of seismic tests were run to obtain additional depth-tobedrock points.

Additional explorations were conducted in 1963. Three 6-inch Denison holes were drilled at the damsite and selected samples sent to the Bureau of Reclamation laboratory in Denver for testing. Four test pits were dug at points on the proposed axis for overburden samples and to determine depth to bedrock.

All local sources of riprap were rejected in favor of the Madison limestone because they were either not suitable or not accessible. The limestone quarry was located on the west side of the Bighorn Canyon about one-third of a mile below Yellowtail Dam.

The foundation of the dam consists of dark-gray Thermopolis shale. This shale is generally hard and very thin bedded. The shale makes an excellent foundation, but noticeable deterioration occurs when the shale dries and then is rewetted. Because of this adverse characteristic, the shale was protected within 24 hours after excavation to prevent excessive deterioration. The rebound or stress relief effect was negligible, though locally some drumminess of the shale was observed.

16. SPECIAL GEOLOGIC PROBLEMS. A large landslide along the east side of the Yellowtail reservoir and extending from the confluence of Big Bull Elk Creek upstream for approximately 4,500 feet was activated by the initial reservoir filling. The slide apparently was a reactivation of an ancient slump

caused by failure of weak shales which underlie the massive limestone walls in this reach of reservoir.

A slide near the right abutment of Yellowtail Dam was first noticed on November 27, 1961, which was shortly after the road to the top of the dam was completed. The backslope excavation for the road cut the top part of the Madison formation and the bottom part of the shaly siltstone which comprises the basal member of the Amsden formation. The shaly siltstone which is about 60 feet thick has low stability, and during excavation of the access road to the top of the dam, toe support was removed, triggering the slide. The slide was caused by a combination of poor-quality, unstable material and subsurface water. Stabilization of the slide area is discussed in section 25.