446 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 19 3 5 



set with its web horizontal. The hoisting equipment was placed 

 in the head towers on the Nevada side, and directions for movement 

 of the load on the cableway were phoned to the operator by signal- 

 men located at the loading and unloading sites. Winches in the 

 head towers of each cableway for moving the towers on their tracks 

 were electrically operated from the same circuit. 



Perhaps the most important equipment of the contractor, Six 

 Companies, Inc., was the sand and gravel screening and washing 

 plant and the two concrete mixing plants. Pit material was hauled 

 ■by rail to the screening plant and there separated into sand, cobbles, 

 and three sizes of gravel. In its 34 months of operation, the plant 

 classified 7,700,000 tons of concrete aggregates, and on many occa- 

 sions processed 800 tons in 1 hour. 



The Lo-Mix concrete plant was located 4,000 feet upstream from 

 the dam and at the crest elevation of the upper cofferdam. Con- 

 crete produced by this plant amounted to 2,097,000 cubic yards. 

 Its record for 1 day was 7,013 cubic yards and 182,784 in 1 

 month. The Hi-Mix plant was situated near the Nevada canyon 

 rim, 650 feet downstream from the dam abutment. Placed in oper- 

 ation on March 1, 1933, this plant had mixed 2,324,000 cubic yards 

 of concrete by October 1, 1935. As many as six 4-cubic-yard mixers 

 were manufacturing concrete simultaneously, and on December 27, 

 1934, the plant produced 3,000 cubic yards in one 8-hour shift. 

 Records established by both plants were 10,417 cubic yards in 1 

 day and 261,847 yards in 1 month. Cement was brought to the 

 project in bulk, as many as 35 cars being used in 1 day. After 

 July 7, 1933, all cement passed through a blending plant in order 

 to blend the products from the various mills. 



Excavation of the tunnels to carry the entire river flow while the 

 dam and powerhouse were built was the first work undertaken at 

 the dam site. The speed and dispatch with which the tunnels were 

 driven by the contractor forecast the efficient and expeditious man- 

 ner in which the entire construction program was to be completed. 

 The canyon walls were first pierced with small bores along the tun- 

 nel line with the usual mining equipment of compressed-air drills, 

 electric locomotives, and dump cars, but tunneling history was made 

 in the enlargements to the full 56-foot circular section. An espe- 

 cially designed drilling jumbo, mounted on a truck and equipped with 

 30 drills, was backed up to the face, and a deafening roar filled the 

 tunnel as the numerous drills ate their way 10, 15, 20 feet into solid 

 rock. A ton of dynamite was loaded into the drill holes, all of the 

 machinery moved away from the heading, and the following elec- 

 trically fired blast shook the canyon walls. Power shovels moved 

 to the tunnel face, the broken rock was loaded into trucks, and soon 



