30. Structural Details. This section of the report describes structural details of the spillway elements discussed in the preceding section. Drawing numbers of related detail drawings are given but the drawings are not included in this report.

(a) Approach Channel. --The approach channel is in open cut approximately 75 feet wide, with side slopes of 1/2 to 1, and about 100 feet long. It is actually the continuation of the diversion channel extended at high level to the inlet of the spillway gate structure. It was excavated in hard rock to elevation 4690.

(b) Left Inlet Wall. --The left inlet wall, a reinforced concrete counterforted retaining wall structure, is 154.6 feet long by about 42. 5 feet average overall height. By means of metal-sealed contraction joints, the structure was divided into five sections. Stop-log slots are provided at the downstream end of both inlet walls, upstream from the gate structure, to accommodate stop logs which may be placed to permit servicing the spillway gates when the water level is above the gate sills. (Drawing 285-D-372.)

(c) Right Inlet Wall. --The right inlet wall is a gravity wall made up of a combined mass concrete and counterforted wall section. It is 125.5 feet long with an average overall height of 46 feet and a base width of about 23 feet. The average height of the lower mass concrete portion is about 21 feet and that of the upper or counterforted portion about 25 feet. The wall is divided, by means of vertical metal-sealed contraction joints, into four independent sections.

This wall has three purposes:

(1) To direct the flow in a uniform pattern to the structure;

(2) To retain the fill which forms the parking area at the level of the dam crest; and

(3) To serve as an abutment for the access bridge to the outlet works trashrack structure.

(Drawings 285-D-119, 399, 400, 403, 404, and 405.)

(d) Inlet Apron. --The inlet apron is a 12-inch-thick concrete lining slab in the bottom and on the side slopes of the approach channel immediately upstream from the spillway gate structure. It extends laterally from the face of the left inlet wall to the face of the right inlet wall, and longitudinally from a line tangent to the face of the curved right inlet wall to the upstream edge of the gate structure. By means of parallel longitudinal metal sealed joints, the inlet apron lining slab is divided into seven parts. At the upstream edge of the slab a 5- by 2.5-foot concrete cutoff wall, placed monolithically with the slab, extends across the approach channel from the left inlet to the right inlet wall.

The function of the inlet apron is that of providing a concrete-lined transition between the unlined approach channel and the gate structure, which would improve the flow pattern and tend to prevent leakage, scouring, or undercutting of the gate structure.

(e) Gate Structure (see also sec. 29(b)). --The gate structure is composed of five main parts and four appurtenant structures. The main parts are an ogeeshaped overflow weir, center pier, the left side wall, the right side wall, and the head wall. The appurtenant structures are a compressor house, a hoist bridge, a highway bridge and radial control gates.

(1) Overflow weir. --The spillway overflow or weir portion of the gate structure is 66 feet wide and 35 feet long, and has its foundation at elevation 4689 and crest at elevation 4700.00. It was constructed in three separate blocks 22 feet wide, 35 feet long, 14.5 feet thick at the crest and 3.5 feet thick at the downstream end. The shape of the ogee cross section upstream from the crest has been previously described.

Downstream, the parabolic curve x2 = 200y is blended to the tangent slope of the chute floor at the downstream end, by a 35-foot-radius curve.

Two 30 by 25-foot radial regulating gates are installed on the spillway crest, resting on steel plates anchored and grouted at the crest, and supported in slots formed on the center pier and side walls of the gate structure.

The vertical joints formed between the weir blocks and between the blocks and the side walls were keyed, metal-sealed, and later grouted to form a monolithic structure.

The center block serves as a foundation for the center pier. At the upstream edge of the weir structure, a cutoff wall 10.5 feet deep, 6.0 feet wide at the top and 3.5 feet wide at the bottom, extends the full width of the structure. Dowels extend from the top of the cutoff wall, at each face, into the weir structure above.

Below the cutoff wall, there is a 15-foot-deep grout curtain placed through drill holes at 10-foot centers.

(Drawings 285-D-116 and 450.)

(2) The center pier (see also sec. 29(b)). --The center pier, located on the centerline of the spillway, rises from the center block of the overflow weir. It extends from a point 7. 34 feet upstream from the edge of the weir to the downstream end of the same, an overall distance of 67.65 feet. It is 6.0 feet thick to elevation 4731 and continues on to elevation 4755.5 at 3.0 feet thick. This pier provides anchorage for one end of each 30- by 25-foot radial gate and acts as a support for one end of each section of the head wall, stop logs, and the hoist and highway bridges. The upstream end of the pier is streamlined for its full height and the downstream end below high-water line is tapered and rounded to provide smooth flow through the gate openings. (Drawings 285-D-119 and 352.)

(3) Head wall (see also sec. 29(d)). --The head wall is a 3-foot-thick vertical cantilever wall 21 feet high, 30.83 feet long, placed integrally with and extending from each wall of the gate structure. The outer end of each portion of the head wall rests in a slot and is supported vertically and laterally by the center pier. At the bottom and downstream edge of the head wall, an attached steel head apron provides a continuous partial seal with the upstream face of the radial gates.

When the reservoir water surface is between the normal and maximum elevations, it is possible by means of the head wall and its seal to confine the flow to the permissible gate openings.

One-inch preformed joint filler was placed in the slots in the center pier between the ends of the head wall and the pier in order to allow for expansion or contraction and yet maintain a reasonable seal.

(Drawings 285-D-119 and 372.)

(4) Gate structure walls (see also sec. 29(c)). --The right and left walls of this structure are gravity walls made up of a combined lower mass concrete and an upper counterforted wall section. Each wall is 67.5 feet long and has six counterforts spaced at 11. 12-foot centers. The base is 5 feet wide with a back slope excavated on a 1/2 to 1 slope to the top of the sandstone. The average heights of the mass concrete and counterforted portions are about 35 and 34 feet, respectively, and the average base width at the top of the sandstone is approximately 24 feet. The relative heights of the mass concrete and counterforted portions were determined by economic considerations. The functions of the

walls are:

To retain the dam embankment on the left and the fill of the parking area on the right;

To serve as abutments for the hoist bridge and highway bridge;

To support integrally, the ends of the head wall;

To provide anchorage and a seal for one end of the radial gates; and

To provide support for the ends of the stop logs.

In addition, a portion of the right wall provides an integrally constructed support for the compressor house.

(Drawings 285-D-119, 371, and 372.)

(5) Hoist bridge (see also sec. 29(e)). --The hoist bridge is a two-span simply supported reinforced concrete bridge which spans the spillway at the upstream edge of the gate structure. The deck is composed of three 1.58- by 3-foot girders placed monolithically with a floor slab 15 feet wide and 10-3/4 inches in average thickness. One-inch expansion joints with cork filler are provided between the ends of the bridge and each abutment, and between the ends of adjoining spans. The bearing surface and the face of the abutment walls are painted, and the bearing surface at the top of the center pier is covered with two sheets of 1/16-inch asbestos packing. These provisions tend to allow expansion and contraction under temperature variations without introducing horizontal forces at the top of the pier and the abutments.

Hoists, enclosed in individual metal housings, are mounted on the deck of each span, near the abutments and the center pier, for raising and lowering the radial gates.

(Drawings 285-D-119 and 387.)

(6) Highway bridge (see also sec. 29(f)). --The highway bridge is a twospan simply supported reinforced concrete structure which spans the spillway near the downstream end of the gate structure. The downstream edge of the bridge is flush with the line of the downstream edge of the dam crest. The deck is composed of two 1.58- by 3. 25-foot girders placed monolithically with a floor slab of 10-3/4-inch average thickness.

The expansion joints and bearing surfaces at the abutments and at the center pier are identical to those previously described for the hoist bridge.

(Drawings 285-D-118 and 368.)

(7) Compressor house (see sec. 29(g)). --The compressor house is a small reinforced concrete building located on the right side of the gate structure at elevation 4758 between the hoist and highway bridges.

It is flush with and rests upon a special foundation constructed integrally with the right side wall of the gate structure, and houses the compressors used in the ice prevention system for the regulating gates.

(Drawings 285-D-119 and 398.)

(f) Spillway Chute (see also secs. 29(i) and (j)). --The spillway chute is a sloping concrete-lined structure which extends from the downstream end of the gate structure to the stilling basin, a distance of 225. 65 feet.

The floor slab, which changes in elevation from 4690.06 at the upper end to elevation 4594.00 at the stilling basin, slopes at increasing grades: 34.5 feet at S = 0.02; 99.6 feet of parabolic vertical curves (x2 = 200y); and 91.5 feet at a 1 to 1 slope as it enters the stilling basin. The slab thickness generally increases from the upper to the lower end of the chute. Thickness through most of the length is 15 or 19 inches, increasing in the last 61 feet to 5 feet. One section of floor slab about 30 by 50 feet, adjacent to the left wall and the upstream end of the stilling basin was omitted, and placed after diversion as a solid concrete diversion plug the downstream face of which was flush with the chute floor.

At the junction of the chute and stilling basin floors, eight dentated apron teeth, 5.5 feet wide, 4 feet high and triangular in section, were placed monothically with the floor slab, at 8. 25-foot centers.

At one section of the chute, approximately 60 feet long (station 7+44 to station 8+04) by the full width of the structure, it was necessary to remove poor rock foundation material and replace it with free-draining material. In addition, it required the installation of two bearing walls on the lines of the interior longitudinal keyed joints, 3 feet wide, 40 feet long and an average of 7 feet deep. In this stretch, the wall footings were extended through the strata of poor rock to firm rock foundations.

With the exception of the above area, which is provided with special keys to hold the slab in place, the chute floor is anchored into the foundation by 1-1/8 square anchor bars spaced at about 6-foot centers and grouted 4 feet into the underlying rock.

A system of drains is placed under the floor slab to relieve excessive uplift pressures which might develop between the slab and the foundation.

The chute walls are of a gravity design similar in type and general description to the walls of the gate structure previously described. By means of contraction joints, the walls are divided into independent units 34.5 feet long. The average wall heights are:

Mass concrete portion 17.0 feet.

Counterforted portion 19. 5 feet.

Overall height about 36.5 feet.

The base width is 5 feet with the back slope excavated on a 1/2 to 1 slope to the rock surface. The width at the top of the mass concrete portion averages about 13 feet.

Near the top of the 1/2 to 1 slopes, the walls are anchored to the rock walls by one, two, or three rows of 1-1/8-inch square anchor bars spaced at about 6-foot centers and grouted 4 feet into the rock.

A 4- by 4-foot reinforced concrete shaft, placed integrally with the left side wall, provides access to the gallery in the downstream portal plug.

(Drawings 285-D-119, 320, 333, 335, and 346.)

(g) Stilling Basin (see also secs. 29(k) and (1)). --The stilling basin, into which the spillway empties, is 66 feet wide and 151 long. The level reinforced concrete floor slab has an average thickness of 4 feet and is anchored into the foundation by 1-1/8-inch square anchor bars spaced at 6-foot centers, and grouted 14 feet into the underlying rock foundation. The floor slab terminates with a cutoff wall excavated 4 feet into the rock.

A row of dentated sills, with teeth 3 feet wide, 19 feet long, and 8.75 feet high, located 84 feet from the upstream end, extends across the basin at 8. 25-foot centers. By means of longitudinal and transverse keyed contraction joints, the floor slab is divided into nine rectangular blocks. A system of drains is placed under the slab to relieve excessive uplift pressure which might develop between the slab and the foundation.

The walls of the basin are of mass concrete design, having vertical front faces and back slopes of 1/2 to 1. Generally, they extend 41 feet above the basin floor, except for 60.5 feet of the left wall and 48 feet of the right wall at the downstream end of the structure. This portion of the left wall decreases in height uniformly from 41 feet to zero at the lower end; and for 48 feet immediately upstream from the powerplant the right wall was raised 3-1/2 feet and topped by a seawall lip which extends 3 feet inward. This seawall feature, which was tested and proven in the

hydraulic laboratory, will confine the waves to the stilling basin, for discharges of 20,000 second-feet and greater. The west wall of the powerplant is flush with and serves as the right wall of the basin for 34 feet at the downstream end.

The stilling basin as constructed was proven by model tests to be capable of providing a satisfactory hydraulic jump for all flows up to and including 35, 000 second-feet, the maximum capacity of the spillway. If operated according to plan, within the limits of the design flood, the stilling basin will seldom, if ever, be required to handle more than 20,000 second-feet.

(Drawings 285-D-117, 118, 119, 326, 332, and 360.)

(h) Outflow Channel. --The outflow channel, which extends from the end of the stilling basin floor to the edge of the Bighorn River, is an open-cut, riprap-lined canal approximately 100 feet long. It is about 66 feet wide with side slope on the left of 1/2 to 1. The west wall of the powerplant forms the right side of the channel.

31. Modifications to Conform to Field Conditions. - After award of the construction contract had been made and excavation had begun, subsequent information regarding the foundation conditions at the left inlet wall, right inlet wall, and a portion of the spillway chute revealed the need of the following design changes.

At the right abutment of the dam, for reasons of safety to the dam and spillway structure, all the Gros Ventre shale, and thin-bedded sandstone above the Flathead sandstone, were removed to the firm surface of the Flathead sandstone. As a result of this decision, both the left and right inlet walls were redesigned to fit the new topographic conditions.

After the excavation for the spillway structure was completed, the profiles and sections showed a depression in the excavated rock between approximately stations 7+40 and 8+25 for the full width of the spillway, with a maximum depth of 20 feet near station 7+50. In this stretch it became necessary to extend the foundations of the side wall to the diorite, and to construct two parallel bearing walls at the location of the longitudinal contraction joints in the chute floor. In the space between walls, under the floor slab, free-draining material was used. The floor slab was supported by the footings of the side walls and by the two bearing walls. Some special contraction joints and a system of drainage pipes were provided as a protection against possible uplift pressures.