In mixing the concrete three parts of a standard grade of quartz sand, taken from the natural deposits of the Des Moines River, 2 miles south of Keokuk, is placed with one part of a standard grade of American Portland cement, and tempered with water. The stone to place with this mortar, available a few hundred feet from the end of the dam, is the run of crusher where the crusher jaws are set for standard 24-inch broken stone. The quantities of mortar and stone are such as to produce the maximum density and specific gravity. Mass rock is used in the body of the concrete work where any minimum dimension of the finished concrete is 3 feet or over. The cubiture of such mass rock may vary from one-half cubic foot to 60 cubic feet, but all such rock deposited is thoroughly embedded in the concrete so as to form a complete union with the surrounding concrete, and stones are separated from one another in dimension by at least 12 inches. No mass rock is placed within 12 inches of any finished surface.

POWER-HOUSE STRUCTURE.

At the Iowa end of the dam, slanting downstream toward the lock, is being built the power house, 1,616 feet long and about 123 feet wide, the location of which is charted opposite page 200. The substructure is being built of massive concrete, in which are molded the water passage and water-wheel chambers. On top of this is planned the superstructure, a house of concrete brick and steel. The superstructure will contain the electric generators, transformers, and switchboards. The height of the power house from foundation to roof will be about 133 feet.

In building the power house the method is to construct a cofferdam around the entire area in which the power house is built, inclosing approximately 37 acres. The water is pumped out of this inclosure, and the work of building proceeds in the space so unwatered. In connection with the power-house construction it is necessary to excavate a large amount of rock for the foundations. The rock is blasted out with dynamite and loaded on cars by steam shovels. It is then hauled to the crushing plant and after being crushed is mixed into concrete. The concrete is hauled to the point of use in buckets and deposited in the substructure of the power house by movable steel cranes. The molds or forms for this portion of the work, involving the water passages and wheel chambers, are complicated.

The concrete used in the power-house construction complies with the specifications mentioned above in connection with the dam. Further than this, where partitions in the superstructure are less than

5 inches thick the proportion of cement and sand is 1 to 2, respectively, instead of 1 to 3, and stone for walls of less than 10 inches thickness is screened and thoroughly separated, so that no stone in the mixture may have a greater dimension than 2 inches.

From the power-house end of the dam, as shown on the chart opposite page 200, there will be run an ice fender for protection against logs and floating ice. This will be built upstream, curving to a junction with the shore and will be 2,800 feet long. The material will be concrete. The general design will be similar to that used by Mr. Cooper in one of the Niagara Falls developments. To the eye the fender will appear as a solid wall fencing off the power house from the river. There will, however, be large arched openings below the water level through which the water will find its way to the power house.

INSTALLATION.

In the initial development it is planned to install 15 main water wheels of the Francis type pressure turbines with single runner mounted on vertical shaft, so providing for direct connection to main generators. These turbines are to have a normal output of 10,000 mechanical horsepower each at a speed of 57.7 revolutions per minute and head of about 32 feet. The maximum output is to be approximately 13,500 mechanical horsepower under a maximum head of about 39 feet. The design of governors, gate control, main step bearing, bucket design, and intermediate details incident thereto embody features usual in hydroelectric construction.

The main generators will likewise be 15 in number, of vertical shaft revolving field type, each having capacity of 8,000 kilowatts at normal rating with overload capacity of 25 per cent for two hours. These generators are to deliver three-phase alternating current at 11,000 volts and frequency of 25 cycles per second.

With the generators there will be installed initially two exciter turbines direct connected to the generators. These turbines will be of the same type as the main generator turbines, and will be mounted on concrete foundations and will have the same type of water inlet and discharge as provided for main units. The governors for main units and exciter turbines will be of standard construction for hydraulic regulation. Each turbine will be provided with an independent governor direct connected to the turbine gate control. The exciters direct connected with the exciter turbines are to deliver direct current to the generator fields at a suitable voltage.

Initially there will be installed step-up transformers of sufficient capacity to deliver to the transmission lines, over and above the line loss 60,000 electrical horsepower which has already been contracted