Page images
PDF
EPUB
[graphic][merged small]

debris is relatively small, the maximum depth being about 42 feet.

That the region has been subjected to successive compressional forces is evidenced by folding and displacement in the rock horizons. A short distance above the dam site a prominent synclinical fold is visible in the sedimentary rock on the right abutment. The axis of the syncline trends at an acute angle with the direction of river flow, crossing the river about 500 feet below the axis of the dam. As would be expected the shortening of the beds due to folding, and the displacement of the bedrock, was accompanied by thrust faulting. Such a fault, roughly paralleling the axis of the syncline, occurs along the lower limits of the right abutment. The overriding movement of the quartzite on the less competent diorite produced considerable fracture and crushing in the latter. The foundation investigations disclosed a crushed or "soft-rock" zone in the upstream portion of the dam foundation, varying in width from 10

to 100 feet. This crushed-rock zone and the corresponding contact fault constituted a major problem in the dam foundation.

THE DAM

The concrete-arch dam for which alternate bids were received was designed as a constant-thickness type with relatively long fillets. The upstream face was vertical with a radius of 300 feet. The crest, 12 feet wide and 850 feet long, had a maximum height above bedrock of about 200 feet. The base thickness at the crown section was approximately 66 feet, and the dam was divided into sections or blocks by vertical radial contraction joints at 30foot centers. An embedded system of grout pipes and outlets was provided for pressure grouting the contraction joints. A free-crest, ogee spillway, 180 feet long, was located in the central portion of the dam. The outlet works consisted of two steel conduits, one 57-inch diameter and one 72-inch diameter, placed in the concrete of the dam. Flow through the conduits, after entrance through a semicircular trashrack, was controlled by two 48-inch diameter, internal differential needle valves, and two 57-inch diameter, hydraulically operated, ring-follower gates.

The earth and rock fill dam, now under construction, is a rolled clay, sand, and gravel embankment, with a heavy rock fill or blanket on the downstream face. The earth embankment consists essentially of an impervious core supported by relatively pervious sections both up and downstream. When completed the dam will have a crest width of 35 feet, a crest length of 610 feet, and a maximum height of 168 feet above the river bed. The upstream face is sloped 3:1 and protected with a 3-foot layer of dumped rock riprap. The downstream face of the earth embankment has a slope of 1 %: 1 beneath the rock fill which increases in thickness from top to bottom. Beginning with a thickness of 5 feet at the crest, the rock fill has a slope of 2:1 through a vertical distance of 104 feet, where the slope changes to 4:1 for the next 50 feet vertically. From this point it is carried horizontally for a distance of approximately 100 feet and terminated in a 1:1 slope. Its maximum thickness at the toe of the earth section is about 60 feet.

A 3-foot high, concrete parapet wall and a 9-inch concrete curb will be constructed along the upstream and downstream edges of the crest, respectively. The crest elevation of 9,344 will allow a 14-foot free board above the normal water surface. The completed dam will contain approximately 833,000 cubic yards of rolled earth embankment, 130,000 cubic yards of downstream rock fill, and 26,000 cubic yards of upstream riprap.

Three reinforced concrete cut-off walls extend across the foundation and up the abutments to various elevations. These walls are built to heights of 10 and 15 feet above the bedrock surface. The footings were excavated by line drilling to a minimum depth of 3 feet, except across the crushed or soft-rock zone where the excavation was deepened to 10 or 12 feet in order to reach sound rock. Where the lower or axis cut-off wall crosses the fault, a 50-foot stoped section was carried back along the contact zone, and keyed into the sound rock of the hanging wall and foot wall.

The river bed was stripped to bedrock over an area beginning about 80 feet below the dam axis and extending upstream a distance of approximately 400 feet above the axis. The average depth of excavation was 30 feet and the maximum depth about 42 feet. The excavated material was placed in the downstream portion of the earth embankment in 18-inch layers, and compacted by sluicing, after rolling proved ineffectual. The abutments were stripped of topsoil and unsuitable foundation materials to rock or satisfactory natural ground.

Both curtain and blanket grouting was done to insure against excessive leakage through the foundation and abutments. The main curtain grouting, consisting of alternate

20- and 50-foot-deep holes at 20-foot centers, follows the location of the cut-off walls. Under the downstream, or axis, cut-off wall the holes on the right abutment are drilled to depths of 125 feet, to penetrate well into the fractured zone and to effect continuity with the abutment grouting placed from within the tunnel gate chamber. Three additional grout curtains, at intervals of approximately 35 feet, are placed on the right abutment a short distance above the upstream cut-off wall. A fourth grout curtain is located on the left abutment, in a draw below the downstream cut-off wall. This curtain extends across the foundation between the stoped cut-off wall in the fault zone and the grouting below the spillway crest.

The blanket grouting was laid out to fit particular foundation conditions, rather than to follow a definite coordinate pattern. Grout was first placed through shallow holes, 15 to 20 feet in depth. This operation was followed by a second placement through holes varying from 15 to 50 feet in depth. The fault grouting along the cut-off walls is supplemented by a line of holes paralleling the fault line and extending upstream from the stoped cut-off wall. These holes, located in the sedimentary rock above the contact, are drilled sufficiently deep to cross the fault zone and penetrate the underlying diorite.

SPILLWAY

The spillway is of the side-channel type, concrete lined, and is located on the left abutment. It has a free, ogeeshaped crest, 180 feet long, set at elevation 9,330. The depth of the inlet channel is 16.5 feet and 29 feet at the upstream and downstream ends, respectively. The bottom width is 20 feet and the side slopes )'2:1. An 8-foot-high check, or weir, is placed across the bottom of the channel near the lower end of the crest, to effect better flow conditions in the channel. The designed capacity of the spillway is 10,000 second-feet with a crest head of 6 feet.

The spillway discharges into a 755-foot-long, open trapezoidal section chute, excavated in the abutment rock and concrete lined throughout. The chute has a bottom width of 20 feet and %: 1 side slopes. The average depth of the lined section is 10 feet. The total drop from normal water surface to tail water is 149 feet; but the rock forming the river bed is of such stable character that no stilling basin is required to dissipate the energy of the spillway discharge before entering the river. A 14-foot roadway, concrete deck, and steel girder bridge is placed across the spillway inlet channel to connect the roadway on the left abutment with the crest of the dam. A second bridge, of like design, crosses the outlet channel about 700 feet below the axis of the dam, to permit access to the valve house.

The estimated quantities involved in the spillway construction are: Common excavation, 20,000 cubic yards; rock excavation, 61,500 cubic yards; and concrete, 4,300 cubic yards.

OUTLET WORKS

Withdrawal of reservoir storage will be effected by an outlet tunnel and two 48-inch balanced needle valves. The tunnel, located in the right abutment, is used for river diversion during construction of the dam. It has a total length of 953 feet, and is concrete lined throughout. Above an emergency gate chamber, located at the axis of the dam, the tunnel is a 10-foot horseshoe section, 535 feet long, and is provided with a reinforced "bear cage" trashrack at the inlet end. The trashrack bars are % by 6 inches in section and placed at 6-inch centers on the top and sides. The upper section of the tunnel operates under pressure at all times.

The gate chamber will be provided with a concrete tunnel plug in which will be placed one 4- by 5-foot, and one 5- by 6-foot emergency gates. From these gates two riveted steel pipes, one 57-inch diameter and one 72-inch diameter, will conduct water through the remaining 418 feet of tunnel, which is an 11.5- by 16-foot horseshoe section, to the 48inch balanced needle valves housed in a reinforced concrete valve house at the downstream portal.

The valve house is flat-roofed and is 20 by 36 feet in plan. In addition to the needle valves, it will contain the emergency gate controls, a 6-ton traveling crane, a gas-enginedriven 15-kilowatt generator for furnishing current to light the tunnel and gate chamber and to operate the emergency gates, and an oil heater and fuel tank. Access to the gate chamber is by a metal walkway that leads up the tunnel from the valve house. The different sized outlet pipes result from consideration of future power installation, in which event the 72-inch diameter will be used as the penstock.

The tunnel was driven from both portals. Below the inlet portal the first 100 feet are in the diorite, and the next 100 feet in quartzite. This section is followed by a short stretch of fractured limestone, after which the tunnel enters a hard limestone that continues to the gate chamber. Below the gate chamber the excavation is again in quartzite, except for about 100 feet above the downstream portal where a blocky porphyry is encountered and through which steel liner plates were required for a distance of 80 feet. Except for about 35 feet in this section no timbering was necessary. The tunnel muck was placed in the downstream rock fill of the dam.

Following the placement of the concrete lining the tunnel was pressure grouted along the section between the gate chamber and the inlet portal. Neat cement grout, with water-cement ratios of 0.5 to 4.0, was placed by means of opportunely located holes, drilled through the tunnel lining 8 to 50 feet into the surrounding rock. Holes for the abutment grout curtain were also drilled and grouted from within the gate chamber. Placing pressures varied from 20 to 100 pounds per square inch.

ROAD

Construction of the dam necessitated relocating that part of the existing Taylor Park Highway that will be submerged by the reservoir. The relocated section, which begins some 2,800 feet below the dam, traverses the left abutment and runs along the northwest shore of the reservoir. The first 3,200 feet were built by the Bureau of Reclamation, and the balance of the road, including a bridge over Taylor River was constructed by the Forest Service. The original road between the dam site and the relocated section will be maintained to provide access to the valve house.

CONSTRUCTION

Bid' were received February 18, 1935, and the contract awarded April 22, 1935, to the Utah-Bechtel-MorrisonKaiser Construction Co. Before construction was started the Frederickson & Watson Construction Co. were included as managing contractors.

Excavation was started at the tunnel inlet May 1, and at the tunnel outlet, May 23, 1935. Difficult excavation was encountered at the outlet portal because of its location in a heavy talus deposit with frozen ground a few feet below the surface. The tunnel was holed through August 7, 1935, and the work of trimming the section and excavating the gate chamber followed immediately. Placement of the concrete lining was begun September 30, and completed December 8, 1935. Work was suspended during the winter months and resumed early in the spring of 1936. The tunnel grouting was finished May 11, and the river diverted through the tunnel on May 13.

Stripping the dam foundation was begun in June 1935, and completed in July 1936. The river-bed gravel and alluvium was excavated with a 70-horsepower dragline and an 80-horsepower shovel, and loaded into 6- and 8cubic yard trucks for transportation and placement in the downstream semipervious section of the embankment. About 34,000 cubic yards were so placed. Final clean-up of the foundation bedrock was made by pick and shovel combined with compressed-air and water jets. The stripping operations were finished in July 1936.

Excavation of the cut-off wall footings progressively followed the removal of the bedrock overburden. Line drilling was required throughout. Placing of the concrete in the cut-off walls was begun in July 1936. Wheelbarrows and gravity methods were used across the river bottom, and later placement up the abutments was made with an aerial cableway and buckets. The cut-off walls were completed in September 1936.

Grouting of the foundation and abutments was done along with the exposure of the foundation rock. It was expected that much of the foundation drilling could be done by jackhammer and wagon-drill methods. However, the nature of the fractured rock limited the usefulness of these methods to shallow depths. Core drilling proved the most effective, and three diamond-drill rigs worked three shifts daily at the peak of the operation in the early summer of 1936. A total of 15,000 linear feet was drilled, and approximately 25,000 cubic feet of neat cement grout placed at pressures varying from 50 to 200 pounds per square inch, depending on the particular condition of placement. Grouting operations were completed the latter part of July 1936.

[graphic][merged small]

The first earth embankment was placed in the upstream area of the foundation on August 8, 1936. The borrow pit, located on the south side of the river about 3,500 feet above the dam site, was stripped ahead of the excavation with a 12-cubic-yard carry-all scraper and a caterpillar-driven dozer. This single borrow-pit area contains the required material gradations for the embankment. No irrigation was necessary as the natural moisture content was close to the optimum, in fact exceeded the optimum at times, necessitating changes in the location of the shovel cuts.

Material was excavated with two 80-horsepower and one 70-horsepower 2-cubic-yard shovels and transported to the embankment by 15 trucks of 5- to 8-cubic-yard capacity, where it was spread with three 11-foot blade dozers, caterpillar operated, and compacted by rolling in 6-inch layers. Two 2-drum and one 3-drum sheep's foot rollers, drawn by 50- and 75-horsepower caterpillar tractors, were used for rolling. Twelve roller trips, at an average speed of 2}/2 miles per hour, were made over each 6-inch layer. The roller loading was such as to effect unit compaction pressures of 200 to 300 pounds per square inch.

Work was suspended for the winter on October 31, 1936.

Up to this time approximately 320,000 cubic yards of embankment from borrow-pit excavation had been placed; and the dam, including the spillway and outlet works, was 59 percent complete.

CONCRETE

The concrete-aggregate plant, located adjacent to the aggregate borrow pit, is in the reservoir area about 4,000 feet above the dam site. The material is delivered to the plant by means of a scraper operated by a stationary gasoline engine and a caterpillar-drawn, 12-cubic-yard carryall scraper. The pit run material is dumped on a bar grizzly with 3-inch openings for the removal of oversize. The aggregate passing the grizzly is received on a belt conveyor and delivered to a series of revolving screens for classification in coarse gravel (l]£-inch maximum), fine gravel, and sand. A 2-inch stream of water, pumped 800 feet from the river, is applied to the aggregates at entrance into the screens. The screened gravel is deposited in overhead bunkers, and the sand is washed down an inclined chute into a washing tank. From the tank a chain-drag conveyor carries it up through clear-water jets to a set of rolls for controlling the fineness modulus. After passing the rolls the sand is delivered to a loading bunker. A single gasoline engine furnishes the mechanical energy for operating the plant.

The prepared aggregates are delivered from the bunkers to adjacent stock piles. During cold weather the aggregates are heated by piling over a tubular heater, made of open metal barrels placed end to end, through which is run a continuous length of 1-inch pipe provided at intervals with flame-feeding spray nozzles. Fuel oil is supplied to this 1-inch line from a storage tank, about 50 gallons being required for heating periods of 6 to 8 hours.

The batching plant consists of three overhead storage bins, provided with hoppers equipped with beam scales for weighing to the batch trucks which deliver to the mixer with four-batch loads.

Except for the cut-off walls and a small quantity in the spillway, the concrete placement to date has consisted of the trashrack structure and the tunnel and gate-chamber lining. The tunnel concrete was mixed near the upstream tunnel portal. Mixing water was first pumped from the river, and later from a sump formed by the trashrack excavation. A 100-gallon tank heater was installed on the supply line, and a large gasoline torch was placed in the mixer for cold-weather operations. The concrete was discharged from the mixer into %-cubic-yard, bottom-dump, buckets, carried on a monorail suspended from the tunnel roof. The buckets were propelled by man power.

The tunnel lining was placed in successive 40-foot panels, employing wood forms made of %-\nch, 5-ply, plyboard facing, braced with timber and light rails, rolled to the shape of the tunnel section. The invert was placed separately and ahead of the side walls and arch. The sidewall concrete was placed to a height of 5 or 6 feet above the invert by hand shoveling through removable windows in the forms. The remaining concrete in the arch was placed by pneumatic methods. Air vibrators were used for consolidating and compacting the pours. Keyed construction

joints are provided between the invert and side walls and between successive panels.

CONTRACTOR'S POWER PLANT

The contractor's power plant, located on the right side of the river a little below the tunnel outlet, consists principally of an air compressor and two electric generating units. The compressor is a two-cylinder unit belt, driven by a 200horsepower 6-cylinder Diesel engine. It has a capacity of 800 to 1,000 cubic feet of air per minute, delivered to a receiving tank from which distribution lines run to the points of use. Both generating units are alternating current 3-phase, 480-volt. The larger, 150 kilowatts, is belt driven by a second 200-horsepower 6-cylinder Diesel engine; the smaller, 90 kilowatt, is operated by a portable caterpillarmounted engine. The generating equipment is controlled from a central switchboard and the electric current is distributed through two main transmission lines, amply protected by lightning arrestors. These lines are carried along the right abutment to the upstream tunnel portal, from whence secondary feeders run to various points of the work.

COST

The estimated cost of the dam and appurtenant features is $1,173,582. Of this amount a total of $831,316 had been expended up to June 1, 1937.

« PreviousContinue »