206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



electric current to the works, a machine shop, a carpenter shop, ware- 

 houses for storing cement, warehouses for miscellaneous materials, 

 and various other structures. 



The methods of construction, if not unprecedented, may at least be 

 interesting to such as are not familiar with hydraulic engineering 

 work : 



A cofferdam consisting of a rectangidar timber crib structure, 

 loaded with stone and made water tight by means of clay puddle, 

 is built around a section of the dam about 1,000 feet long. The 

 water is then pumped out of this cofferdam. In this space thus 

 pumped dry is excavated a trench in the solid rock, on which the 

 dam is founded. The piers and arches forming the bridge and the 

 bottom part of the dam betAveen these piers are then built. After 

 this the cofferdam is removed and another section is cofferdammed 

 and the bridge built in the same way. This continues until the 

 bridge is extended all the way from the Illinois shore to the junc- 

 tion with the power house on the Iowa side. This will leave 119 

 large openings between the bridge piers, through which the water 

 passes unobstructed. These openings will finally be closed off, a 

 few at a time, by means of steel gates, and the balance of the 

 concrete part of the dam will be placed behind these steel gates, 

 gradually raising the crest of the dam imtil it has reached its full 

 height. 



The dam, including abutments, is being built 4,568 feet long, or 

 about seven-eighths of a mile. The spillway section is 4,278 feet in 

 length. The height above the river bed is about 32 feet and its base 

 is 42 feet wide. The upstream face is vertical. The downstream 

 face is an ogee curve, the upper portion a parabola OA^er which the 

 water will spill, the lower portion an arc of a circle which will throw 

 the water away from the toe of the dam. On the top of the spillway 

 are being placed the steel floodgates, one for each opening, 30 feet 

 wide and 11 feet high, supported by concrete piers. These piers are 

 6 feet thick and are built integral with the dam. The piers also 

 support an arched bridge, from which the gates will be operated by 

 electric hoists. By manipulating these gates the water above the 

 dam may be maintained at a nearly constant level at all seasons. 



The dam is being built entirely of massive concrete without reen- 

 forcement. It is being locked into the rock bed of the river by 

 potholes and other excavations and is practically a monolith. All 

 concrete, except at specially isolated places, is machine mixed, car- 

 ried from the mixing plant to the point of use in large buckets by 

 trains running on the completed portion of the dam, where a canti- 

 lever crane picks up the buckets of concrete from the cars, carries 

 them out and dumps the contents into the forms. 



