October 21, 1949.

The bridge girders are reinforced with longitudinal bars and U-shaped bars to resist moments and shear stresses. The deck slab and diaphragm walls are reinforced with bars placed in both faces. The center span is reinforced in its top and bottom faces with bars placed in both directions, with the main stress steel placed in the bottom face between the pier abutments.

53. Chute Floor. The chute floor, having a thickness of about 18 inches, extends from the downstream end of the gate structure, station 10+43.5, to the upstream edge of the stilling basin slab, station 12+75.57; and extends laterally to the toes of the left and right side walls. In order to facilitate concrete placement and to minimize the effects of temperature and shrinkage, the floor is divided longitudinally and laterally by unsealed contraction joints into slabs ranging in size from approximately 30 by 30 feet to 30 by 50 feet. At the upstream end along the lateral joint of each slab, a cutoff, cast integrally with the slab and about 2.5 feet wide at the top and 1 foot wide at the bottom, extends about 3 feet into the foundation. Cutoffs, 16 inches high, are cast integrally with the floor slab along the longitudinal joints. The cutoffs provide support and prevent displacement of the slabs, and serve to confine the filter blanket under the slabs.

The floor slabs are reinforced for temperature and shrinkage stresses with 3/4-inch-round bars spaced on 12-inch centers each way on the top face. The lateral cutoffs are reinforced to resist stresses caused by expansion and contraction of the floor slab.

54. Chute Walls.

The chute walls are located on the left and right extremities of the chute floor, and extend from the downstream end of the spillway gate structure at station 10 +43.50 to the upper end of the stilling basin at station 12+75.57, a distance of about 232 feet.

The walls from station 10+43.50 to station 11+11.50 are of the counterfort type with the counterforts placed at 12-foot centers and normal to the spillway centerline. The range of dimensions from the upstream end to the downstream end of this section of wall is as follows:

(1) Height, 54.5 to 18.6 feet.

(2) Overall base width, 40.25 to 12.4 feet.

(3) Toe length, 15.25 to 5.25 feet.

(4) Toe thickness, 6.75 to 1.5 feet.

(5) Thickness of heel slab, 18 to 12 inches.

The face wall is 12 inches thick at the top, and the back face has a batter of 1/8 inch in 12 inches. The counterforts are also 12 inches thick on top but have a batter of 1/4 inch in 12 inches on each side. A shelf support is provided for the chute floor slab at the edge of the toe of the wall. At locations where the toe thickness is less than 3-1/2 feet, a cutoff extends below the footing to provide a seat for the floor slab.

The walls from station 11+11.50 to station 12+75.57 are of the cantilever type and consist of five sections, two of which are each 34.75 feet long, and three of which are each 31.5 feet long. The walls are 12 inches at the top, and the back face has a batter of 1/2 inch in 12 inches. The range of dimensions from the upstream end to the downstream end of this section of wall is as follows:

(1) Height, 23.8 to 18.6 feet.

(2) Overall base width, 15.5 to 12.5 feet.

(3) Toe length, 6.67 to 5.25 feet.

The thickness of the toe of the cantilever wall is about 18 inches, and the heel thickness ranges from 18 inches at the vertical stem to 12 inches at the outside edge of the heel. A toe cutoff, cast integrally with the toe of the wall, extends 24 inches below the wall footing and provides a shelf for supporting the floor slab. The profile for the two upstream sections is level and the profile for the three downstream sections is on a 0.44 slope. The profile of the bottom of the wall matches that of the adjacent floor slab.

A tabulation of loading conditions and other data used in the wall design is

given as appendix E.

55. Stilling Basin. --The stilling basin extends from station 12+75.57 to the end of the spillway structure at station 14+40.5 and consists basically of a floor and right and left walls. The floor upstream from station 13+25.00 has a slope of 0.5, and is level downstream from that station. The floor thickness increases uniformly from 2.0 feet at station 12+75.57 to 4.5 feet at station 13+25.50 and then decreases uniformly to 2 feet at the end of the basin. A 3-foot-wide by 10-foot-deep cutoff wall is provided at the downstream end of the basin. At the junction of the sloping and level portions of the floor, chute blocks 5 feet wide, 6 feet high, and 12 feet long are placed integrally with the floor on 5-foot centers; and at the downstream end of the basin a Rhebock dentated sill, 12 feet high, was also placed integrally with the floor and extending across its full width. The floor is divided into blocks by a transverse contraction joint at station 13+95.50 and by a number of parallel longitudinal keyed contraction joints spaced about 30 feet apart. Thus the blocks are about 30 feet wide and either 44.5 or 120 feet long.

The stilling basin floor was designed for stability against flotation, and is reinforced to resist temperature and shrinkage stresses as well as stresses resulting from the most critical combination of water load, live and impact loads, and uplift pressures.

The assumed loading conditions analyzed for the design of the stilling basin floor slab were as follows:

(1) Downward water load on slab resulting from a depth determined by a hydraulic model test for a discharge of 200,000 second-feet.

(2) Underdrains effective in reducing the uplift pressure to that caused by the normal tailwater depth, elevation 3038.

(3) Horizontal hydrostatic load on inclined portion of slab caused by uplift pressure from normal tailwater depth, elevation 3038.

(4) Dynamic forces acting on top of the dentated sills and on the horizontal portion of the floor will be compensated by negative pressures induced in areas just downstream from the dentated sill teeth.

The stilling basin was not designed to be stable during a sweepout of the basin when uplift is permitted to build up beyond elevation 3038, or when the basin is unwatered and the underdrains are not effective for relieving the uplift.

A tabulation of loading conditions and other data used in the design of the stilling basin is given in appendix D. Internal moments, resulting from the application of various loads, and the uplift force were computed for use in determining concrete stresses and reinforcement requirements.

In the upstream section of the stilling basin, 1-1/4-inch reinforcement bars were placed longitudinally and spaced at 12-inch centers. A second layer of bars was placed in the 0.5 slope and level portion of the stilling basin floor. Transverse reinforcement consisted of 3/4-inch round bars spaced at 12-inch centers. For the cutoff walls at stations 12+75.57 and 13+00.50, 3/4-inch round stirrup-type bars were placed transversely and spaced at 12-inch centers. For reinforcing the stilling basin floor downstream from station 13+95.50 and dentated sill and downstream cutoff wall, also 3/4-inch round bars spaced at 12-inch centers were used.

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(a) Walls. The walls from station 12+75.57 to station 13+25.00 are of the counterforted type and consist of two 25-foot-long sections. (For detail of these walls the reader is referred to drawings 328-D-385 and -631, not included in this technical record.) The counterforts are spaced at 13.25-foot centers. The range of dimensions from the upstream end to the downstream end of this section of wall is as follows:

(1) Height, 23.3 to 50.5 feet.

(2) Overall base width, 15.5 to 44.5 feet.

(3) Toe length, 6.5 to 15 feet.

The top of the wall is level and the profile of the bottom matches that of the floor slab. The face wall and counterforts are each 12 inches wide at the top. The back face of the face wall has a batter of 5/16 inch in 12 inches, and the counterforts have a batter of 5/16 inch in 12 inches on each side. A cutoff is also provided as in the preceding wall section for support of the floor slab.

From station 13+25.00 to station 14+00.50, the walls are counterforted and divided into two sections, each 37.5 long. The counterforts are spaced at 13.25-foot centers and placed normal to the face with a 5.5-foot cantilever extension at each end of the wall section. The range of dimensions for this section of wall from the upstream and downstream ends is as follows:

(1) Height, 50.5 to 48.8 feet.

(2) Overall base width, 44.5 to 42.0 feet.

(3) Toe length, 15.0 to 12.5 feet.

The wall is level along the top. The face wall and counterforts are 12 inches wide on the top. The back face of the face wall and the sides of the counterforts have a batter of 5/16 inch in 12 inches. Backfill was placed behind the wall to elevation 3058.5.

The downstream portion of each stilling basin wall from station 14+00.5 to station 14+40.5 is of the counterforted type. At the latter station, the wall turns 90° away from the spillway centerline to form the wing wall face. This wall extends 29.5 feet to the back edge of the main wall footing. This portion of the stilling basin wall is provided with two counterforts spaced at 17.25-foot centers placed normal to the wall face. The wing wall stem serves as the end counterfort for the stilling basin wall stem. An integrally placed 3-foot-wide cutoff is provided below the downstream end of the wall, the outside face being made flush with the downstream face of the wing wall at station 14+40.5. The cutoff wall extends across the full width of the footing and into the foundation to a depth of 8 feet below the base slab. From station 14+14.50 to station 14.+40.50, the base slab is thickened and forms a portion of an integrally placed dentated sill..

(b) Wing Wall Extensions.-- There are two extensions to the wing walls of the spillway stilling basin. (For detail of these walls the reader is referred to drawings 328-D-515 and -632 not included in this technical record.)

The right wing wall extension extends from the right stilling basin wing wall to the right abutment and consists of three sections having lengths of 2, 29.2, and 31.25 feet, respectively. The 2-foot-long section is about 48 feet high and rests atop a sheetmetal piling cutoff. The 29.2-foot section is provided with three counterforts spaced at 10.3-foot centers and placed normal to the wall face. Cantilever extensions of 4.25 are provided at the ends of the wall. The third section of the wing wall extension is provided with two counterforts spaced at 17.25-foot centers and about 7 feet from the end of the wall. The wall height ranges from 16.8 to 48 feet. The face wall is 12 inches thick at the top and has a back face batter of 3/16 inch in 12 inches. A 3-foot-wide cutoff wall, whose face is flush with the downstream side of the face wall, extends 8 feet into the foundation.

The left wing wall extension extends from the left stilling basin wing wall to the right wall of the outlet works stilling basin, a distance of about 71 feet. This wall extension consists of three sections having lengths of 2, 36.5, and 32.5 feet, respectively. The 2-foot-long section is similar to the corresponding section of the right wing wall extension. The 36.5-foot-long section is provided with three counterforts spaced at 13.67-foot centers. Cantilever extensions on the side nearest to the stilling basin and on the side away from the stilling basin are 3.58 and 5.58 feet, respectively.

The top of the left wing wall extension is at elevation 3062 for the first 12.5 feet, then it slopes downward on a 2 to 1 slope to elevation 3050. This wall has a height ranging from 47.4 to 24.7 feet and a base width ranging from 46.75 to 24.7 feet. The thickness of the top of the wall and the wall batter are the same as for the corresponding wall section of the right wall extension. The 32.5-foot-long section is provided with two counterforts spaced at 18-foot centers at the face wall. The counterfort near the spillway channel is normal to the face wall, and the other counterfort is turned so that its centerline is about 16 feet away from the other counterfort at its extreme end away from the face wall. Cantilever extensions of 7.25 feet are provided at both ends of the wall section. A 3-foot-wide cutoff wall, whose outside face is flush with the downstream side of the face wall, is extended 6.3 feet below the wall into the foundation.

56. Drainage System and Filter Blanket. A drainage system was provided under the spillway for draining any seepage from the foundation or any leakage through the spillway joints. This system consists of a grid of sewer-pipe drains embedded in a pervious blanket under the spillway chute and stilling basin. This filter blanket also insulates the structure from the foundation, which could be subjected to frost action if sufficient moisture accumulated in the foundation material.

The filter blanker is a minimum of 16 inches thick, consisting of two 8-inch layers. The upper layer consists of graded gravel, and the lower layer consists of a graded mixture of sand and pea gravel. In areas where extremely fine foundation material was encountered, an additional 8-inch layer of graded sand was placed beneath the layer of sand and gravel. The selection of material for the filter blanket was based on the following:

(1) A ratio from 12 to 40 between 15 percent grain size of coarser layer and the 15 percent grain size of finer layer, and

(2) A ratio from 12 to 58 between 50 percent grain size of coarser layer and 50 percent grain size of finer layer. In the above, the percent grain size of the material is the abscissa of the mechanical analysis curve for each material at the percentage of finer material indicated. Also gradation curves of materials for adjacent zones were required to be approximately parallel.

Material obtained from the vicinity of Wauneta, Nebr, and screened so that 100 percent passed a 1-1/2-inch screen and was retained on a No. 4 screen was used for the top layer of the blanket. Pit-run material obtained from the same source, and gravel from the vicinity of Enders Dam, were used for the middle layer. The bottom layer (required only in areas where extremely fine foundation material was encountered) was very fine sand from the Republican River and was acceptable provided the grading was approximately that specified for concrete sand. To prevent leaching of fine material through open joints of the drain pipe, a layer of screened gravel graded from 3/4 to 1-1/2 inch in size was placed around the pipe.

The sewer-pipe drains were divided into two separate systems, one for the upper portion of the spillway chute and one for the lower portion of the chute and the stilling basin floor. The upstream system consists of 4-inch-diameter, open-jointed, bell-and-spigot sewer pipe placed laterally just upstream from the transverse floor slab cutoff walls, with their ends extending under the spillway wall toes into the backfill behind the walls. These drains are placed at 28-foot centers. The lateral drains are intersected by longitudinal drains spaced at 30-foot centers and placed midpoint between the longitudinal spillway floor joints. The longitudinal drains are 6-inch diameter for the upper 55 feet, and 8-inch diameter for the remaining length. The 8-inch drains empty into a 12-lateral collecting drain which is provided with an outlet in the spillway channel above the tailwater level. The downstream system of drains was placed under the stilling basin floor and lower portion of the spillway chute floor. This system consists of a grid of 8-inch, open-jointed, bell-and-spigot sewer pipe placed laterally and longitudinally at about 30-foot centers. The longitudinal drains of the system lead to a 12-inch collecting drain, placed just upstream from the intersection of the spillway slope and the basin floor. The drain system discharges through outlets in the downstream faces of the basin chute blocks.

For draining water away from behind the spillway walls, a continuous twozoned gravel drain was placed under the backfill adjacent to the face wall and toe, and for a length of spillway walls opposite the upstream system of underdrains. Water from this system discharges through the 4-inch lateral drains which are ended in the gravel filter. Strainers are provided in the openings of these drains to keep gravel from entering the drainage system.

57. Sheet Steel Piling Cutoffs. Because the stilling basin foundation is composed of permeable materials, the stilling basin was provided with sheet steel cutoffs. These consist of a row of 30-foot-long sheet steel piling driven at the downstream end of the basin and extended along the outer edges of the stilling basin walls parallel to the spillway centerline. The cutoffs prevent rapid percolation and probable piping resulting from a high differential between tailwater and a reduced pressure under the stilling basin floor, and also tend to confine and protect the foundation. The line of piles at the downstream edge of the s tilling basin and those under the 2-foot-wide wing wall extension are capped with concrete. Piles at other locations are not capped.