Page images
PDF
EPUB

ALCOVA DAM

KENDRICK PROJECT (FORMERLY CASPER-ALCOVA PROJECT), WYOMING

BY W. E. COLLINS, ASSISTANT ENGINEER, BUREAU OF RECLAMATION

THE KENDRICK PROJECT (formerly the CasperAlcova project), located in east central Wyoming, will initially bring under irrigation 35,000 acres of land lying in the North Platte Valley near Casper, Wyo. The main canal, however, is being constructed of sufficient size to permit future extensions of the project. The project comprises three main construction features, namely, Seminoe Dam, which will create a reservoir with a capacity of 1,020,000 acre-feet, together with a power plant having an electrical installation of 37,500 kilo volt-amperes; Alcova Dam, which will permit diversions of water from North Platte River; and the main canal, which is 62 miles long, including six tunnels and numerous siphons and culverts.

The diversion features of the project consist of Alcova Dam, an earth-fill structure which will raise the river water surface 170 feet; a spillway of 55,000 second-feet maximum capacity; an outlet works designed to discharge 5,300 second-feet under the maximum reservoir head; and headworks for the canal system, designed to divert 1,200 second-feet.

Alcova Dam is located on the North Platte River, 32 miles upstream from Casper, Wyo. The river at this point has eroded through a high ridge, forming a narrow canyon. The formations at the dam site are Tensleep sandstone underlain by an Amsden limestone dipping slightly downstream. Numerous hot-water springs with an estimated total flow of 6 second-feet enter the river in the vicinity of the dam. The hot water has eroded solution channels in the limestone which required special consideration in the design of the dam and outlet tunnel. The sandstone is extremely resistant to weathering and to the hot water that is found in the vicinity.

The reservoir area back of the dam is 2,200 acres. During the irrigation season the water surface will be maintained at the normal elevation of 5,500, to permit diversion into the main canal.

THE DAM

The dam is an earth and rock-fill embankment rising 256 feet above the foundation excavation. The crest is at elevation 5,510 and is 40 feet wide and 700 feet long. The dam is constructed of clay, sand, and gravel backed on the

downstream slope with heavy gravel and rock and protected on the upstream slope with rock riprap. The clay, sand, and gravel section of the dam is divided into three zones. Zone 1, forming the center of the clay, sand, or gravel section, is constructed of the most impervious material available. Zone 2, on each side of zone 1, is constructed of material grading from impervious at the contact with zone 1 to pervious at the outer limits. Zone 3, near the downstream rock fill, is constructed of sand and gravel and is relatively pervious. Material for zones 1 and 2 was selected in the borrow pits located about 4,000 feet to the north of the dam. Suitable stripping from the required foundation excavation was used in zone 3.

To contact bedrock under the impervious section of the dam, an area about 400 feet long was stripped upstream from the axis of the dam. Rock was exposed over most of the foundation area after 30 feet of material had been removed. Further excavation revealed a deep, narrow channel from 30 to 40 feet wide which varied in depth below the general level of the rock from 60 feet in the upstream portion of the excavation to 6 feet at the drainage gallery. The sides were nearly vertical, with many overhangs and water-worn potholes. Dr. C. P. Berkey, geologist, described the chasm as a plunge basin caused by a waterfall during the erosion of Alcova Canyon. Excavation of the chasm was very difficult, due to the rough character of the rock and the confined limits of the excavation. The abutments were stripped of badly jointed and loose rock down to sound rock which could be grouted.

Three main concrete cut-off walls traversed the canyon under the impervious section of the dam. Auxiliary walls were constructed at points where they were deemed necessary. The purpose of the walls is to prohibit a continuous seepage of water along the contact line between the rock foundation and the embankment. The walls extend an average of 12 feet into the embankment. Footings for the walls were constructed in rock and are 3 feet wide and a minimum of 2 feet deep. The rock for the footings was excavated by line drilling and light shooting.

It was thought that hot water might seep through the foundation downstream from the grouted area and saturate the embankment. Therefore a drainage gallery was constructed near the downstream extremity of the grouted area. The gallery, 500 feet long, is located 85 feet upstream and approximately parallel to the axis of the dam. The gallery drains into a shaft directly below the valve and gate chamber. Drain holes were drilled through the floor of the gallery into the foundation. The finished inside section is egg-shaped and is 7 feet high by 5 feet wide. The gallery is lined with 2 feet of concrete. A 12-inch pipe is embedded in the concrete to provide ventilation for inspection of the gallery. Pumps, located in the valve and gate chamber, will be provided to unwater the gallery. The gallery also offers a means of regrouting the foundation if excessive leakage develops.

[graphic][merged small]

Earth materials for the construction of the dam, as found in the borrow pits, were more or less stratified, with clay on

top and sand and gravel underneath. The proportion of clay, sand, or gravel excavated was regulated by varying the depth of the shovel cut. Due to the stratified condition of the material, special mixing methods were necessary. Mixing was done by making extra passes of the shovel in the borrow pits and by dozing and disking on the embankment. The borrow pits were irrigated to moisten the material so that maximum compaction could be secured. Sprinkling was done on the embankment, when necessary.

The material was excavated with a 2%-cubic-yard shovel, and hauled with eleven 8-cubic-yard trucks and two 80cubic-yard trac-trucks. The material was dumped in piles on the embankment, dozed into a uniform layer, disked and rolled 12 times with a sheepsfoot roller weighing about 14,000 pounds. The layers were about 6 inches thick after

[graphic][merged small]

compaction. Pneumatic tampers were used around the concrete cut-off walls and near the abutments where it was impossible to use the sheepsfoot roller.

Laboratory and field tests for control of moisture, density, permeability, and proportioning of the materials were carried on continuously with the placing of the embankment.

Strippings in the river channel were sand, gravel, and boulders, most of which could be used in zone 3 of the dam. All stones over 12 inches in diameter were placed in the rock-fill section of the dam. The sand and gravel was spread in 18-inch layers and rolled about 12 times. The stripping was excavated with a 2-cubic-yard shovel and hauled with four 5-cubic-yard trucks and one to four 2cubic-yard trucks. A 1-cubic-yard machine, operating either as a dragline or a shovel, was used for general purposes, such as digging ditches, stripping bedrock, and excavating around large boulders.

The rock fill was placed in approximately 3-foot layers and sluiced with water to produce as much settlement and consolidation as possible during construction. Rock for the rock-fill and riprap sections of the dam was obtained from large boulders in the stripping excavation, stripping of the canyon walls, and from the spillway and tunnel excavations.

SPILLWAY

A spillway designed to pass 55,000 second-feet is located on the left abutment. Three counterbalanced, 25-foot 8inch by 40-foot fixed-wheel gates will be provided to control the water surface in the reservoir. The inlet to the gate structure is a partially lined channel about 400 feet long and 102 feet wide with one-fourth to 1 sloping sides. The gate structure will be 127 feet wide, 60 feet long, and 104.5 feet high. The sill of the gates will be at elevation 5,460. The

gates will be raised by an electrically powered hoist mechanism. Since the reservoir surface must be closely regulated during the irrigation season, automatic control will be provided. An electrical switch, actuated by a float, will open and close the gates as the reservoir surface varies. A 22-foot roadway will traverse the spillway at the gate structure.

A spillway channel will extend about 760 feet downstream from the gate structure. The lower 258 feet are designed as a stilling basin. The width of the channel will vary from 95 feet at the gate structure to 150 feet at the stilling basin. The floor of the stilling basin is at elevation 5,309, which is 21 feet below the natural ground surface. Vertical retaining walls with contraction joints at 56-foot centers will form the sides of the channel. The height of the retaining walls varies, the maximum being 55 feet.

The floor of the channel is a reinforced concrete slab with an average thickness of 15 inches. Contraction joints will be provided to divide the slab into panels of about 25 by 28 feet. Each panel will be anchored to the rock with four 1^-inch square anchor bars. The floor and the rctainingwall footings will be drained by vitrified sewer pipe laid with open joints in porous concrete, generally placed under contraction joints. Part of the drainage empties into the canyon to the right of the spillway, and the remainder empties into the stilling basin.

Spillway rock is blasted, excavated with a 1 %-cubic-yard shovel, and hauled to the rock-fill section of the dam.

OUTLET WORKS

A circular tunnel, 20 feet in diameter and 1,381 feet long, was constructed through the right abutment and lined throughout with an average thickness of 18 inches of concrete. The river was diverted through the tunnel during the construction of the dam. The designed diversion capacity is about 8,000 second-feet.

At the end of the diversion period, the tunnel will be plugged with concrete at the valve and gate chamber. Two 96-inch conduits will pass through the plug, and each conduit will contain a 96-inch ring follower gate and an 84-inch internal differential needle valve which will discharge directly into the tunnel below the valve and gate chamber. The ring follower gates will be used for emergency operation and the needle valves for control.

The valve and gate chamber is 56 feet high, 33 feet 6 inches long, and 43 feet wide. The gates and valves are serviced by a 20-ton traveling crane. An elevator to the left of the chamber offers a means of hoisting the heavy valve and the gate parts to the ground surface. The elevator shaft is 12 by 16 feet with a 6- by 8-foot air shaft on the downstream side. The air shaft introduces air into the tunnel directly below the valve and gate chamber. Adjacent to the elevator shaft a pump chamber is constructed in which is located the drainage shaft to the drainage gallery. A two-story operating house 24 by 24.5 by 39 feet high is constructed over the elevator shaft. The first floor of the building contains the elevator, a loading dock, a small office, and the intake for the air shaft. The second floor contains the elevator machinery, transformers for the power supply, blower for ventilating the valve and gate chamber, and a small storage space.

A trashrack structure, made of a framework of concrete beams and columns, was constructed at the upstream portal of the tunnel. The structure is semicircular in plan, with a radius of about 23 feet and a height of about 47 feet. The water enters the structure through 18 openings 8 feet 8 inches by 10 feet in size. Trash bars, 6 inches by % inch at 6-inch centers, were placed in the openings.

A concrete-lined channel with a 25-foot floor width and 1 to 1 sloping sides was constructed for a distance of 314 feet from the outlet portal of the tunnel. The vertical height of the side walls is variable, with a maximum height of 35 feet. The rock was excavated so that the concrete for the walls could be placed directly against the rock. The concrete has a minimum thickness of 12 inches and an average thickness of 18 inches.

No timbering or steel liner plates were necessary in excavating the tunnel. The first 300 feet were excavated through Amsden limestone formation and the remainder through the Tensleep sandstone. Several hot-water springs were encountered with temperatures as high as 130° F., which made working conditions very disagreeable.

An 8-foot pioneer bore was excavated at the arch of the tunnel and kept at least 20 feet ahead of the main heading. The tunnel muck was loaded by a %-cubic-yard air shovel into \ 2-, and 3-cubic-yard dump trucks and hauled to the upper and lower cofferdams or rock-fill section of the dam. Compressed air for the drilling and for the air shovel was furnished by a battery of five 300-cubic-foot-capacity air compressors. Gases and smoke were forced from the tunnel by bleeding the compressed-air line near the heading and from the exhaust of the air shovel and jackhammers.

The elevator and air shaft were constructed by first excavating the air shaft from the top down. Muck from the air shaft was hoisted in a ^-cubic-yard bucket attached to a shovel cable. The air shaft was then enlarged to form the elevator shaft. Muck from the elevator shaft was dropped into the air shaft and trapped at the bottom. It was then dumped into trucks and hauled out through the outlet tunnel to the dump piles. Excavation of the valve and gate chamber was started at the top and carried down with the elevator shaft. Muck was handled in the same manner as in the elevator shaft.

CONCRETE MIXING PLANTS

The concrete mixing plant for the diversion and outlet tunnel and the elevator and air shaft was located near the

outlet portal of the tunnel. The plant consisted of a 1-cubicyard mixer, 5,000-pound, visible-dial scales, three 25-cubicyard aggregate storage bins, and a 3-car cement storage bin. All concrete except for the elevator and air shaft, was placed by a conciete pump. Two set-ups of the machine were made, one at the mixing plant and one just downstream from the valve and gate chamber. Concrete was delivered to the forms through a 7-inch, jointed, cast-steel pipe pumping a maximum, horizontal distance of 850 feet. When the concrete pump was set up at the mixing plant, the concrete was delivered directly into the hopper of the machine; when at the valve and gate chamber the concrete was delivered by truck into the hopper. The concrete for the elevator and air shaft was hauled by truck to the top of the shaft and dropped through 7-inch, jointed, cast-steel pipe into a hopper from which it was distributed through a chute into the forms.

The mixing plant for the cut-off walls, drainage gallery, trashrack, and operating house is located on the right side of the canyon rim, about 200 feet downstream from the axis of the dam. Equipment in the mixing plant consists of a horizontal-drum-type mixer driven by a 20-horsepower electric motor, a manually operated batcher with a 2,000pound scale, a batchmeter, three aggregate storage bins, an 18- by 36-foot cement storage shed, and a water-storage tank.

Concrete was transported from the mixer to the forms in 1-cubic yard, side-dump hoppers mounted in an iron frame which was chained to the frame of a light truck. Four such hoppers were available, but three was the maximum number used at any one time. The number used depended on the desired rate of placing and the length of time required to make a round trip.

Concrete was either dumped from truck hoppers into a stationary hopper from which it was distributed into the forms with ^-cubic-yard buggies, or dumped into a 1-cubicyard bottom-dump hopper bucket attached to a ^-cubicyard dragline used as a crane. On several occasions the concrete was dumped directly from the truck hopper into the forms.

All concrete was poured in open forms, except in the drainage gallery where it was placed with an air gun. The concrete was vibrated with air vibrators. In complicated form work, two riveting hammers were used to vibrate the outside of the forms. The concrete was placed in horizontal layers not more than 18 inches in thickness.

No definite plans have been made for placing concrete in the spillway. It is planned, however, to move the present mixing plant to some location near the stilling basin for use in such consiruction. The plant may be moved again for pouring the gate structure and chute section. Alternate use of the bottom-dump hopper bucket handled by a crane and the concrete pump are being considered for placing the spillway concrete.

Concrete aggregates were obtained about 5,000 feet north of the dam. The pit was stripped with a 2^-cubic-yard shovel and the material hauled in dump trucks. The screening plant consisted of a crusher, belt conveyors, and a 15-foot by 40-inch diameter screen driven by a 50-horsepower electric motor. The material was moved to the belt conveyor by a dragline equipped with a bottomless bucket and driven by a 50-horsepower electric motor. The screened aggregate was hauled to stock piles with two 1 %-ton trucks having 2-cubic-yard dump bodies.

GROUTING

Grouting the foundation was one of the most important features of construction. Large solution channels existed in the limestone formation and had to be filled with grout. Grouting of the diversion and outlet tunnel was done first. When the tunnel was excavated, numerous springs and seeps of hot water were encountered. The grouting was done before the concrete lining in the tunnel was placed. One hundred and seventy-five holes with a total length of 1,751 feet were drilled. The depth of the holes varied from 4 to 20 feet. These holes took 3,248 cubic feet of grout at pressures of from 40 to 100 pounds per square inch. The maximum amount of grout taken by one hole was 2,000 cubic feet.

When the tunnel lining was placed, grout pipes were placed at 20-foot centers longitudinally in the arch. After the concrete had set at least 30 days, grout was forced into the pipes to fill any cavities between the lining and the rock. The pipes took 5,396 cubic feet of grout at 100 pounds per square inch pressure.

The first problem in the foundation grouting was to decrease the flow of water into the stripping excavation. A grout curtain was constructed around the area to be stripped in the river channel, which proved very effective in decreasing the flow of hot water. The drilling and grouting work closely followed the stripping operation as the inflow of hot water had a tendency to increase with depth of stripping. The entire foundation under the impervious section of the dam was "blanket grouted" through 2-inch diamond-drill holes, mostly between 30 and 50 feet deep and at approximately 20-foot centers. In addition, grout curtains were constructed under the three main cut-off walls and drainage gallery by drilling and grouting holes 60 to 100 feet deep at 5-foot centers. Additional grouting was done in badly creviced areas where the need was apparent. The blanket and curtain grouting extended up the abutments just ahead of the earth-fill construction.

The grout holes for each type of grouting were extended through the Tensleep sandstone into the Amsden limestone. The sandstone was generally somewhat seamy and faulty and took a medium amount of grout. The limestone was

generally tight but had many seams and large solution channels which, when tapped, resulted in a strong flow of hot water. The solution channels took large quantities of grout, one hole near the upstream tow of the dam taking 38,000 cubic feet.

Most of the holes were both shallow and deep grouted. For the shallow grouting, the hole was .first drilled to a depth of from 15 to 35 feet and grouted at pressures between 30 and 60 pounds per square inch. Genei ally, the hole was washed out before the grout had set, to avoid redrilling for the deep grouting. In some cases where the rock was very seamy, a backflow of grout occurred when the hole was washed out. In these cases the grout was allowed to set in the hole, and the hole was redrilled for further grouting. For deep grouting the hole was extended to its full depth of from 50 to 60 feet and grouted at pressures between 100 and 200 pounds per square inch.

There were a few seeps at the base of the right abutment which special grouting failed to stop. These seeps were controlled during earth placing by gravel pockets and drains. When the fill was constructed high enough to give sufficient weight for fair grouting pressures, the gravel pockets and drains were grouted through grout pipes. Abutment seams, crevices, and overhangs were treated similarly.

Neat cement was used for grout in ordinary holes where the amount of grout taken was small. A mixture of one part cement to two parts fine dune sand was used where a large amount of grout was required. It was found that the amount of grout taken per linear foot of hole gradually decreased as the grouting progressed. The first grouting operation, the grouting of the curtain around the stripping excavation, required an average of 9.6 cubic feet of grout per linear foot of hole. The last grouting operation required only 0.7 cubic feet per linear foot of hole. For the foundation, 1,013 holes were drilled having a total length of 67,034 linear feet. The total amount of grout used was 211.70" cubic feet.

HEADWORKS

The headworks for the canal system are located a mile west of the dam, at the inlet portal of a 14-foot diameter horseshoe tunnel, 2,810 feet long. A 14- by 12-foot radial gate with an electrically operated hoist regulates the flow into the canal. The sill of the gate is at elevation 5,487. The first 35 feet of the headworks are a transition with a floor width of 14 feet. The side walls are warped from l?s to 1 slopes at the entrance to vertical at the gates.

The headworks and tunnel were constructed under the same contract. The headworks and inlet portal excavation were made with a 1 !£-cubic-yard shovel. The concrete mixing plant was located at the outlet portal of the tunnel, and the tunnel concrete was transported in cradle dump cars hauled by a 4-ton dinky locomotive.

« PreviousContinue »