6. Relocations and Rights-of-Way. (a) Highway Relocation. --U. S. Highway No. 20 and State Highway No. 320 were both relocated for a distance of about 5 miles each because of the construction of the dam and the creation of the reservoir. Some heavy cuts and high fills were required for U. S. Highway No. 20 because of the alinement and the terrain.

(b) Railroad Relocation. --The construction of nearly 14 miles of roadbed, a 7,131-foot tunnel with two ventilating fanhouses, and seven new bridges was involved in the relocation of the roadway of the Chicago, Burlington, and Quincy Railroad, which passes along the eastern side of the Boysen Reservoir. The tunnel begins above the reservoir water surface and descends to emerge from the right abutment of the dam, and below the water surface of the reservoir.

Slightly over 3 miles of the Chicago and North Western Railway was relocated along the southeast side of the reservoir but no major structures were involved.

(c) Access Roads. --Access roads from U. S. Highway No. 20 to the dam and the powerplant are very short. The road to the top of the dam is about 1,100 feet long and takes off from the new highway in a westerly direction to the top of the dam.

The road to the powerplant follows the old highway about 3,400 feet, then connects to the former railroad grade and follows this alinement for about 2,200 feet to the powerplant.

7.

Cost.

Total cost of Boysen Dam and Powerplant and relocation of the two railroads, based on actual costs to July 1, 1956, plus estimated costs through June 1959, is $33,969,647, as shown in the tabulation below. A detailed cost summary is shown as appendix A, and breakdowns of costs for the two principal specifications, No. 1890 and No. DC-3430, are presented as appendixes B and C, respectively.

PART II--DESIGN

8. Description.

PART II--DESIGN

CHAPTER II--EARTH DAM

Boysen Dam (fig. 4) is an earth- and rockfill structure with a maximum height of about 150 feet above the bed of the river and a crest length of about 1, 100 feet. The crest is 30 feet wide, gravel surfaced, and lined with wooden posts at 10-foot centers.

The dam has an upstream slope of 3 to 1 between the crest (elevation 4758) and the 20-foot berm at elevation 4660, and a slope of 4 to 1 between the berm and the riverbed. The downstream slope is 2 to 1 from the crest to elevation 4650, 5 to 1 between elevations 4650 and 4625, and 2 to 1 below elevation 4625.

The embankment consists of a selected sand, gravel, and clay core covered on each side by a sand, silt, and gravel zone. These zones are covered on the upstream side by riprap 3 feet thick above elevation 4660 and on the downstream side by rockfill increasing in thickness from the crest to the toe of the embankment. The upstream portion of the embankment below the berm was constructed of impervious material to make it suitable for use as a cofferdam.

A cutoff trench is provided with a 50-foot bottom width, 1-1/2 to 1 side slopes, and a 70-foot maximum depth. A concrete cutoff wall with footings extending into bedrock is located along the centerline of the cutoff trench.

9. Foundation Design. Ten diamond core drill holes were completed by Government forces between January 15 and March 12, 1943. Forty additional diamond core drill holes were drilled between August 28, 1945, and September 1, 1946, during which time four test trenches were dug. In all, a total of eight test trenches were dug. In April and May 1948, 124 Denison samples from six drill holes were submitted for testing.

The Bighorn River has, in recent geologic times, carved a narrow, deep gorge in hard rock. It subsequently filled this gorge with silt, sand, and gravel to a depth of 60 to 80 feet. A dense sound diorite with a weathered zone near the contact forms the base of the foundation under the river alluvium, and Cambrian sandstones and shales form the abutments. The diorite is exposed in the lower portions of the abutments, and unconformably overlying it is 110 feet of Flathead sandstone. Overlying that are several hundred feet of Gros Ventre shales, sandstones, siltstones, and claystones.

The principal fault which exists at the dam site has a vertical displacement of between 300 and 500 feet, strikes approximately east-west, and dips 40° to 60° north or downstream.

The diorite is jointed, the tight joints being discontinuous and filled with calcite and pyrite. It is mostly unweathered, but even the weathered portion is sound enough to carry the load of an earth dam. The Flathead sandstone is moderately hard, brittle, and mostly unweathered, but contains open joints. Many blocks slipped prior to excavation and created large seams and caves. The Gros Ventre formation with its soft shale and soft sandstone is made even less competent in the vicinity of the dam site by faulting and folding.

The major part of the left abutment is formed by the stream-cut face of the sandstone, forming a triangular mass of which the upstream edge is the dip slope and the downstream edge is the major fault that crosses the dam site. The apex of the sandstone block is at about elevation 4644, some 8 feet below the maximum water surface. The fault line parallels the axis across the sandstone block at elevation 4644. The surface trace of the fault down the face of the abutment is in the downstream direction at about the line where a plane through the axis with a downstream 1 to 1 slope would intersect the abutment. The sandstone is generally hard and competent except that the face which is almost vertical has weathering cracks to a depth of about 10 feet. The left abutment downstream from the sandstone mass consists of fault gouge, crushed shale, sandstone, and limestone.

On the upstream side of the right abutment ridge there exists a slab of material composed of alternating layers of sandstone and shaly micaceous clay. The slab rests on a 150 slope and is unsupported either on the lower edge or the sides. The right abutment was the subject of a stability analysis and is discussed in a later section of this report.

A previous dam, located about 1-1/2 miles downstream from the site of Boysen Dam, caused the deposition of lake sediments which were not eroded subsequent to the breaching of that dam.

The top layers of the lake sediments consist of clay which is highly plastic, and contain some organic matter. Below the clay layers are layers of uniform sand interbedded with thin clay layers. The clay is unconsolidated and saturated in its natural state and very impervious. By the unified soil classification system, it is classified as a highly plastic inorganic clay (CH). Results of laboratory tests are as follows:

(1) In the unconfined compression test the material had maximum compressive strengths of 3. 4 to 4. 8 pounds per square inch.

(2) In the one-dimensional consolidation test the consolidation was 25. 9 to 29.4 percent under a pressure of 200 pounds per square inch.

(3) The effective shearing resistance of the clay material was low and could not be measured by the standard triaxial shear test for the samples at their natural water content.

(4) Gradation tests showed 48 to 67 percent clay and 0 to 3 percent sand.

(5) Because of the high degree of saturation and low rate of permeability, consolidation takes place rather slowly.

(6) The diorite has been tested to 20,000 pounds per square inch compressibility.

The cutoff trench was designed to prevent losses of water through the overburden and to reduce the velocity of percolation so that no danger of piping through the foundation or uplift at the downstream toe of the embankment will exist. It also increases the stability of the foundation and decreases the settlement under the center of the dam as the earth backfill is greatly superior to the foundation clay it replaces.

In depth the cutoff trench extends through the overburden to the sound rock surface and has a maximum depth of about 70 feet. The cutoff trench has a bottom width of 50 feet and side slopes of 1-1/2 to 1. A cutoff wall of unreinforced concrete is provided along the centerline of the cutoff trench. The wall is 5 to 10 feet in height and extends about 3 feet into rock.

Grouting was done after the cutoff wall footing was placed. As the rock on the bottom of the gorge contains small and discontinuous joints, grout holes on 10-foot centers were sufficient. The grouting on the right abutment extends back along the ridge and connects with and extends beyond the grouting around the outlet works tunnel. Initial spacing of grout holes was 20 feet and depths of 20 to 30 feet were used to penetrate into the Flathead sandstone. The grouting was extended to include the abutment area between the outlet works tunnel and the new railroad tunnel. Grout holes in the left abutment were spaced about 5 feet apart to insure adequate coverage in the open and sand-filled joints. The holes were 15 to 25 feet deep. The grout curtain was extended to maximum water surface elevation.

Toe drains were provided in the specifications design to prevent percolating waters from accumulating at the base of the more pervious zones of the embankment and saturating the foundation overburden downstream from the dam. It was decided during construction that the overburden material was sufficiently pervious to permit eliminating the drains.

The sandstone in the left abutment was closely jointed and slightly porous. It was strong enough but contained paths of leakage. In order to fill the seams and cracks the embankment next to the abutment was puddled. Puddling had previously been used successfully at Alcova, Taylor Park, Anderson Ranch, Deer Creek, and Jackson Gulch Dams. It provides satisfactory compaction and good sealing of open joints.