BOULDER CANYON PROJECT — NELSON 447 



these were filing in a long procession up steep grades to dump 

 grounds in side canyons. 



An average blasting round broke 1,000 cubic yards of rock and 

 advanced the heading 17 feet. Work progressed at times from 

 eight headings. A total length of 256 feet of tunnels was driven in 

 24 hours, and 6,848 feet in 1 month. Removal of the million and a 

 half yards of rock in the 4 tunnels required 3,561,000 pounds of 

 powder, or 2.38 pounds per cubic yard. 



Lining the tunnels with concrete to prevent rock falls and furnish 

 a smooth surface for water flow, thus increasing their capacity, was 

 accomplished in the same capable way as were the excavations. Steel 

 forms were used, those for the side walls weighing 250 tons for an 

 80-foot length. Concrete was placed behind them from spout dump 

 buckets, through chutes, or forced into place by compressed air 

 guns. 



As soon as the two tunnels on the Arizona side were lined, the 

 river was turned from its ages-old channel and detoured through 

 the canyon walls for nearly a mile. The manner of its diversion is 

 one of interest. A pile trestle bridge was first built across the river 

 downstream from the diversion tunnel inlets, and the temporary 

 dams in front of the Arizona tunnel portals were blasted. Trucks 

 commenced dumping large rocks, then smaller ones, and finally silty 

 sand from the bridge into the channel, thus forming a dam in 24 

 hours which entirely turned the river flow. A dike of tunnel muck 

 was pushed across the river channel in the relatively still waters 

 upstream from the tunnel outlets, and the area between the two 

 temporary dams was then pumped dry. 



Behind the protecting barriers of the two temporary dams, con- 

 struction proceeded for the cofferdams, and excavations for the 

 main structure of the dam and the powerhouse. Earth for the per- 

 manent cofferdams was secured principally from two pits in Hemen- 

 way Wash, 4 miles from the dam site, and was hauled by train to 

 an interchange dump near the upper portals of the Nevada diversion 

 tunnels. Shovels loaded the material from the dump into trucks 

 which placed it in the dam. The earth was spread by caterpillar 

 tractors, moistened by hose, and then compacted by a sheep's foot 

 roller. Trains returning to the Hemenway pits were loaded with 

 rock and channel muck from the canyon workings, this material 

 being dumped at a suitable site 3 miles from the dam. Excavations 

 in the river channel for the dam and powerhouse were carried 

 downward a maximum depth of 135 feet below the old river bed. 

 A piece of 2- by 6-inch cut timber was uncovered 40 feet below 

 the river bed, indicating the depth to which the river had eroded its 

 channel during some flood or floods in the last 50 years. 



