When we contemplate the development of cyclopean concrete, the tendency (too pronounced and persistent to be accidental) toward a smaller percentage of stone, and the saving in cost of both plant and labor which would result from frankly discarding stone altogether, the conclusion is irresistible that the important dams of the future will be of concrete. It will be interesting to inquire how far present appliances and methods will be modified by this change.
The entire matter of putting in or leaving out stone is a matter of economy, of balancing value of cement saved against cost of production of large stone in a particular quarry. In other words, stone for crusher material can be produced, crushed and ground to concrete aggregate and the concrete can be mixed and placed in the dam for a certain price per cu. yd. The question is, will the cost of a certain percentage of the concrete be more or less than the cost of producing the large stone (at a modification of entire quarry methods) transporting and placing it; bearing in mind the additional plant required to handle the stone?
The quarry plant will be simplified and reduced in amount, quarry labor will be reduced, transportation from quarry to crusher and mixer, and thence to the dam will be developed to permit a much higher rate of progress, derricks on the dam will be eliminated except possibly for setting face work, and labor on the dam will be reduced very materially. In general, plant and methods will be devised strictly with the end in view of handling concrete. If any stone is introduced it will be because it can be produced in the particular quarry at no additional cost and can be readily put into the dam with a small additional amount of plant.
The incongruity in present practice is the immense number of derricks and attendants employed on the dam for no purpose at all except to move buckets of concrete and a small amount of stone a few feet from the place where left by the cableway, and to dump them or set them down. They perform no function that could not be as well performed by cableways except for the mere matter of a few feet in location, and that detail can be arranged. Masonry construction has been reduced simply to leaving the materials in a certain spot, and the derrick is simply a relic not yet discarded.
A very common cableway capacity is 10 tons, with a hoisting speed of 300 ft. per minute and a conveying speed of 1800 ft. per minute. Such speeds are high enough for every practical purpose; the time necessary for a trip depends principally or largely upon the time required for hooking and unhooking the loads. The possible gain in actual yardage handled that might be attained by increasing the speed is so small that it seems very doubtful if it will ever be found practicable to attempt it. There is a probability, however, that considerable gain may be made by increasing the average weight of load.
Thus for illustration take the previously mentioned eight-hour record for the cableways at the Olive Bridge dam. The loads of concrete were not more than 5 1/2 tons including the containing skip.
It would seem not impossible to design the plant and the operating methods so as to be able to convey nearly or quite double the net load with slight if any decrease in number of trips per day.
In the case of a dam of such length as to at first sight seemingly preclude the use of cableways, and still of such a height, and in a valley whose side slopes are such, that the problem will not be satisfactorily solved by a system of tracks, the difficulty may be overcome by the installation of cableways with an intermediate tower, and by the sending out of materials from a terminal at each end. Of course, this would involve two points for assembling the materials, but such a dam would contain yardage enough so that the saving by use of cableways might easily be much more than enough to pay for the two terminals as compared with one terminal and any other system of delivery. The intermediate tower could be of steel and be left in the masonry. This tower would more than double the capacity of the cableway, as, in addition to working from two ends instead of one, the average length of trip would be reduced. The most striking example of an intermediate tower is in connection with the Tunkhannock viaduct now under construction on Delaware, Lackawanna and Western Railroad, at Nicholson, Pa. An intermediate tower 230 ft. high (later raised to 290 ft.) containing 101 M. ft. of timber supports two 2 1/4-in tandem cableways which are spaced 25 ft. apart. The span to one end tower is 1510 ft. and to the other one 1525 ft. It is not necessary to discuss here variations in operating procedure which may be followed in the case of the construction of a viaduct rather than a dam.
It has been previously pointed out that soon after the dam is started, from the time when the whole area of what might be called the bottom of the dam is under way, the available working area may not be materially reduced till some elevation near the top of the dam is reached. The area changes shape, becoming narrower and longer. Suppose an original installation of parallel cableways sufficient in number to cover the area fully. Then as the area grows narrow some of the cableways are put out of commission as they are no longer over the working area. To maintain the output more work must therefore come upon the remaining cableways.