This system is increasingly common in ordinary building operations. The concrete is elevated at some centrally-located tower, dumped into a hopper and then distributed by means of chutes, the chutes being suspended from guys or from an arm or boom revolving in a horizontal plane about the tower as a center. The advantages of the method are apparent to the most casual consideration. The cost of central tower and chutes and the greater height to which the concrete must be elevated are vastly more than repaid by the facility of lateral transportation, above and removed from other operations, requiring no runways or gang with wheelbarrows and readily reaching the tops of column or wall forms. Delivery by chutes has recently been applied with entire success to the construction of a large masonry dam.
The Lake Spaulding dam being built by the Pacific Gas & Electric Company on Bear River, California, is to be 900 ft. long on top with a maximum height of 305 ft. and is to contain 304,000 cu. yd. Fig. 33 and Plate XII illustrate the situation perfectly.
The main chute from the mixer is 24 in. wide, of wood, lined on the bottom with cast-iron plates and on the sides with sheet steel. It has an inclination of 4 1/2 in. on 1 ft. On the dam auxiliary chutes distributed the concrete as desired. In the lower portion of the dam the elevation was such that the chutes could reach every part of the work unassisted. Later as the masonry climbed up, reaching the minimum inclination at which the chutes could operate, it became necessary to boost the concrete in transit. This was done not by vertical elevators but by belt conveyors which accomplished both distance and elevation. Plate XII, Figs. B and C, show this operation, while Plate XII, Fig. D, shows a near view of one of them looking in the direction of the travel. Although the inclination of these belts has not been stated, the views seem to show it to be nearly as much as that of the chutes. By the belt system concrete has been delivered at the rate of 2000 cu. yd. per ten hours on a 24-in. belt traveling 400 ft. per minute.
Fig. 33. Lay-out of construction plant, Lake Spaulding dam.
The tendency of the concrete to adhere to and accumulate upon the belt instead of dumping clean is very ingeniously overcome by means of jets of compressed air. Just underneath the belt where it turns back over the last roller, after dumping, are several fan-shaped nozzles which deliver a continuous sheet of compressed air against the face of the belt. This device is entirely successful in removing practically all of the adhering concrete and maintaining the belt in good working condition. (Four 1-yd. mixers were employed.) That this system not only permits of rapid progress but is also very economical, is shown by the fact that in thirty-one ten-hour days (August, 1913) 40,485 cu. yd. of concrete were mixed and poured in the dam; and that this was accomplished with an average force of 313 men including not only those directly handling the materials, but all others as carpenters, engineers, derrick men, etc.
This system, as will be further shown in the chapter on Probable Future Methods, should be entirely applicable to almost any situation, even with much less advantage of natural elevation in favor of the inclines. In connection with such a system cableways are practically indispensable as a means of handling and shifting the chutes and conveyors together with their supports, also the face forms. When not engaged on such work the cableways could be utilized in putting large stone into the masonry.
On the Big Creek hydro-electric development in southern California concrete was delivered by chutes for the construction of several small dams. Dam No. 1, the highest, has a crest length of 800 ft., a maximum height of 135 ft. and contains 58,700 cu. yd. of concrete which was placed in sixty days by twelve mixers. A construction trestle (Plate XIII, Fig. C) the full height and length of the dam was built in twenty-six days just outside the upstream face, and coinciding with the line of the face so that the trestle timbers also served to support the forms. From cars running on the top of the trestle concrete materials were delivered to the twelve mixers installed within the trestle below; from the mixers wooden chutes conveyed the concrete to the dam. (Plate XIII, Fig. D.) The chutes were given a minimum slope of 16 deg. and alongside of each was a runway. Nine stiff-leg derricks with 125-ft. and 140-ft. booms handled the 81,000 cu. yd. of rock excavation for the foundation, and later put plums into the concrete to the extent of 15 per cent, of the total volume. Each 8-ft. lift of concrete was given twelve hours to set before again proceeding with the work. Hence the twelve mixers were operated in two batteries of six each. Two eleven-hour shifts per day were worked, and no cableways were employed.
This system seems more cumbersome and costly than the one employed at Lake Spaulding, and the performance though creditable was not nearly as good. The twelve mixers mounted on a trestle built to the full height and length of the dam was a costly plant for a 58,700-cu. yd. dam when compared with the Lake Spaulding system of chutes, conveyors and four mixers for a 304,000-cu. yd. dam. The four mixers at Lake Spaulding were capable of a much greater yardage output than the system at Big Creek permitted the twelve mixers to accomplish there.