Engineering and Contracting . the scow. This material was later cast over by adredge, to form the top of the dam. The dams were builtup to about 5 ft. above the normal stage of water; theirwidth at top was about 5 ft. and their sides assumed aslope of about 2;; to 1 in water varying in depth from8 to 12 ft. The amount of material used in the construc-tion of dams was about 433,000 cu. yd. Pumping.—The work of unwatering the central area of2,800 ft. length was begun Oct. 5, 1908, by the use of two12-in. centrifugal pumps and an air lift consisting of 488-in. pipes. These pipes were suspended at
Engineering and Contracting . the scow. This material was later cast over by adredge, to form the top of the dam. The dams were builtup to about 5 ft. above the normal stage of water; theirwidth at top was about 5 ft. and their sides assumed aslope of about 2;; to 1 in water varying in depth from8 to 12 ft. The amount of material used in the construc-tion of dams was about 433,000 cu. yd. Pumping.—The work of unwatering the central area of2,800 ft. length was begun Oct. 5, 1908, by the use of two12-in. centrifugal pumps and an air lift consisting of 488-in. pipes. These pipes were suspended at an inclinationof about 30 degrees from the vertical and extended fromabout 1 ft. above the water surface nearly to the bottomof the river. Compressed air admitted at the lower endof the pipes caused the water to be discharged from thepipes into a sluice-way, through which it flowed into theriver. This air lift was very efficient until the water hadbeen lowered from 12 ft. depth to about 7 ft. depth, when Grasse Isle, M:ch. DetroU Spoil Bank< Coffer Dam- Sketch Showing Location of Dams and Channel. the air was forced through the water without raising use of the air lift was then discontinued. The workof unwatering this area was so far completed that theoperation of channelers and rock drills was begun , 1908. Further pumping was done as required bythe progress of the work of excavation. The leakagethrough the dams was small, and after the initial un-watering the amount of pumping was mainly that due torains and melting snow. The number of pumps in usevaried from time to time. There were in use for a partof each daj two stationary pumps of 12 and 18 in. in size,one at each end of the inclosure, discharging directlyinto the river; also, there were a number of smallerpumps placed where needed for various short periods oftime, discharging into sluice-ways or ditches throughwhich the water flowed to either end of the inclosure,where it was discharged into the river
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