redrilling in comparatively short intervals of depth. Grout holes, not drilled by the successive method, were fitted with 2-inch pipes which were carried up through the mass concrete; then grouted after a substantial depth of concrete had been placed in the dam.

Two portable grout mixers and placers, each with 14. cubic feet capacity, were used in the foundation grouting. One machine was operated by using 100 pounds per square inch live pressure. The other machine was mounted on a wooden frame, together with a 2- by 5-foot air receiver and a booster compressor. Grout was carried through a 2-inch hose built to withstand a working pressure of 200 pounds per square inch.

FAULT ZONE

Rock walls surrounding the intersection of the fault and the cut-off trench were scaled preparatory to sinking a 12by 30-foot shaft under the upstream toe of the dam in the fault zone. To facilitate the shaft excavation, six grout holes, surrounding the shaft, were grouted by the successive method. Most of the shattered fault zone material was solidified by the grouting and had to be drilled and shot. The loosened material was loaded by hand into a 1⁄2-yard bucket, hoisted from the shaft, and dumped on cableway skips for disposal.

The excavation of the shaft extended entirely through the felsite, the agglomerate, and about 10 feet into the underlying tuff, to elevation 2,145. Opposite faces of the shaft, adjacent to the fault zone, were protected from movement by timbering. One deepwell turbine pump, working part time, kept the shaft free from water.

As soon as the excavation was completed the shaft was concreted, to shut off some of the seepage water. Concrete was mixed at the central plant and transported by rail to the railroad trestle built along the south canyon wall, thence down the incline chutes to the top of the shaft, and down the shaft through several drops of 16-inch diameter "elephant trunks" with radial gate controlled hoppers attached for regulating the amount of concrete placed and preventing segregation.

The crushed felsite underlying the large boulders in the transverse fault near the center of the river channel was hand excavated by open cut methods to elevation 2,254, simultaneously with the shaft excavation at the upstream toe of the dam. The sinking of the shaft, however, was continued well in advance of the remainder of the fault zone excavation. To protect the vertical walls adjacent to the fault zone from movement during the remaining excavation, a bulkhead was constructed at the downstream end of the 12- by 30-foot shaft, and solid concrete placed in the fault zone extending from elevation 2,254 to 2,308 and to the downstream face of the dam, leaving five shafts 12 feet square through which the excavation could

be extended below. The bottoms of the concrete blocks between shafts were constructed on 1:1 slopes so that good contact could be made when the balance of the fault zone was poured. Grout pipes and connections were embedded in the blocks for grouting.

The remaining material in the fault zone beneath the concrete blocks consisted of hard boulders embedded in crushed felsite and infiltrated clay. It was excavated by pick and shovel to satisfactory rock within about 15 feet of the agglomerate. The material was loaded by hand on cableway skips and hoisted through the five shaft openings for disposal. The south wall consisted of hard self-supporting rock, while the north wall was badly broken and shattered. To insure against movement in the foundation adjacent to the fault, the crevice was timbered. Some shattered materials were left in place and held by the lagging, to support overhanging blocks of foundation rock on the north wall, until concreted.

The excavation was sloped for drainage toward the downstream end, where the accumulated water was pumped with one deep well turbine pump. The depth of the excavation below the level of the adjacent foundation of the dam varied from 128 feet at the 12- by 30-foot shaft to 158 feet at the pump sump. The width of the crevice varied from 8 feet minimum to 32 feet maximum.

The fault zone concrete was placed through the five shaft openings with the same equipment used in concreting the upstream shaft. The heights of the concrete pours were controlled by the amount of shattered materials removed in advance of each pour, but averaged about 6 feet. A horizontal run of 10-inch pipe with vertical risers near each shaft opening was embedded in the concrete for discharging seepage and cleanup waters into the pump sump. The sump was eventually filled with select materials and the system was subsequently filled with grout through connections made to each 10-inch riser.

Foundation rock adjacent to the fault zone and shaft was grouted through 2-inch risers, after a substantial block of concrete had been placed in the dam over the river section, grout connections having been placed in holes drilled on approximately 10-foot centers. Eleven carloads of cement were forced into the fault zone area, with a maximum of over 1,100 cubic feet in one grout riser.

CONCRETE

Aggregates for concrete were obtained from gravel pits at Dunaway. The contractor hauled 800,000 tons of aggregates at a cost of $0.0033 per ton mile or $0.16 per cubic yard. Loading materials with a 2-yard dragline added $0.08 per cubic yard to the cost. Sand was washed at the screening plant and coarse aggregates divided into the following sizes: 4 to 4 inch, 4 to 11⁄2 inches, 1% to 24 inches, 234 to 8 inches.

drawn by a standard gage, 8-ton gas locomotive to the cableway loading dock at the dam, 1,500 feet distant. Under normal operations one load was delivered every 4 minutes. The concrete was chuted into place or delivered by gravity in an 8-cubic yard bottom dump cableway bucket, and spaded or puddled into place by workmen. Concrete next to the forms was spaded lightly to produce smooth surfaces.

Before placing concrete, the foundation rock was cleaned of all mud, excess water, and debris, and all splattered grout and rock scale removed by sand blasting. The time elapsing between completion of a pour and beginning of cleanup varied from 6 hours on hot days to 24 hours on cool nights. As soon as the part of the pour that had been placed first became hard enough, the soft laitance was removed by wire brushing. Exposed rock surfaces were hand blasted before concreting.

A system of wooden panel forms, 4 feet high by 8 feet long, was used to confine the concrete to 4-foot lifts in panel blocks

bounded by the contraction joints and the upstream and downstream faces of the dam. As soon as the irregularities of the bed rock were covered, a regular schedule of 4-foot lifts was started. The rate of placing was limited to not more than 4 feet in 72 hours, and not more then 28 feet in 30 days, except in panels 3 and 4, which were left at elevation 2,361 to provide landing platforms for the cableway bucket.

All horizontal concrete surfaces and foundation rock of the dam to which new concrete adhered was covered with a layer of mortar, mixed in proportion of 1 part cement and 2 parts sand. Following this, a wet mix, without cobbles, was placed to a depth of about 6 inches, forming a cushion to receive the regular cobble mix. Special care was taken to prevent the accumulation of excess grout around the copper seals and corners of the panels by keeping the concrete at these points higher than the remaining lift. All lifts were started at the downstream face to insure proper union with subsequent work. In the lower levels of the dam, the lifts were placed in successive horizontal layers, step fashion. Horizontal keys were formed by shoveling fresh concrete between 1- by 10-inch boards, held to shape by -inch iron rod brackets.

During cold weather, aggregates and mixing water were steam heated at the mixers, so that no concrete reached the forms at a temperature lower than 50° F. The fresh concrete at the forms was covered with canvas and protected at this temperature or higher for 72 hours by heating with. coke burning salamanders. Cold weather prevented placing during a part of each winter and retarded progress during the fall and spring.

Except in freezing weather, the upstream and downstream surfaces were kept moist by a continual spray of water. The concrete surfaces of the contraction joints were coated with water-gas tar soon after the forms were removed. The horizontal construction joints were wet down as soon as the laitance was removed and kept continually moist until covered with a subsequent lift, or until cured at least 2 weeks. The roadway and top of the parapets were covered with a layer of sand to hold the moisture and to protect the concrete. Concrete in the semicylindrical trashrack structures was placed monolithic with the dam.

SPILLWAY CREST STRUCTURE

The excavation for the spillway crest structure was done with a 1-yard gasoline dragline during November and December 1928. A wooden bulkhead, built in the 5- by 8-foot shaft about 5 feet below the required depth of excavation, kept debris out of the shaft. Broken rock was loaded on trucks and hauled upstream to a spoil bank at the edge of the bluff.

Each form for the spillway crest structure was built at the carpenter shop in one piece and cut in sections for transpor

tation and installation and to accommodate the concrete pours. The control chamber, pier, and bridge forms were built in place. Special grade, - by 4-inch shiplap was used to face the forms.

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With the reinforcement steel in place, concrete placed between construction joints. Most of the revetment lining was hand placed and screeded to conform to the specified shapes. The needle valves, drain, and vent pipes and miscellaneous metal work were embedded in the concrete as the placing progressed. A concrete mix of 1:1.55:3.56, with an average slump of 4 inches at the forms, was placed with the same equipment used in placing the shaft lining.

The foundation rock beneath the control chamber pier was grouted before the revetment paving was placed, whereas the foundation rock beneath the crest structure was grouted after the shaft lining had been grouted. Altogether, 300 sacks of cement grout with a water-cement ratio of 1:0 were forced into 24 grout connections in the foundation rock of the spillway-crest structure and control chamber, under 100 pounds pressure per square inch.

TUNNEL PLUG

Construction of the permanent concrete plug in the diversion and spillway tunnel, upstream from the spillway shaft, was begun on August 30 and completed, except for grouting, on September 16, 1932.

As a preliminary step to diverting the low summer flow of about 130 second-feet through the sluice gates, a wooden gate, 16 inches thick, 36 feet wide, and 30 feet high, was built in the concrete slot at the inlet portal of the diversion tunnel, being suspended from an overhead framework by a steel cable which was burned through, permitting the weighted gate to drop down the greased slot until it keyed into the slot across the bottom. Hay, sawdust, and other material, thrown into the water, effectively sealed the cut-off.

Preparatory to placing the concrete, the trestle, leading from the railroad to the tunnel outlet, was rehabilitated and extended into the tunnel where tracks were laid along the floor.

Placing of concrete was begun after the annual recesses for the plug and all other surfaces with which the concrete would come in contact had been cleaned and chipped to a depth of about 1 inch. Pours were made in 4-foot lifts, grout pipe being carried to contacts for grouting after shrinkage. An 8-inch pipe, through the plug, prevented accumulation of water against the upstream face during construction and was later grouted.

COSTS

The following tabulation indicates the quantities and costs involved in the construction of the dam and appurtenant works to September 30, 1936.