If the origin of the stone and concrete be at the elevation of the floor of the valley and delivery be made by tracks and cars, there enters the question of how to come within reach of derricks working near the ends of the dam. It seems to involve a long piece of track, switch-backs, etc., or else cable inclines at each end. If the origin of the materials is at one end of the dam, one of the same two expedients must be resorted to, with the additional disadvantage that the element of original elevation of materials is wasted (i.e., thrown away to be later expensively reacquired) or else high trestles must be built.
If locomotives are used to transfer the cars, it means (in addition to the first cost of plant) that much fuel is burned at very low efficiency, especially, if considerable range of elevation is overcome by grades in the system of tracks. The net load is a much smaller percentage of the total load that must be handled than is the case when delivery to the derricks is made by cableway.
Another very important difference in the operation of the two methods of supplying the derricks requires some explanation. The function of the cableway and the function of the derrick are two entirely different things; they do not conflict or overlap. Neither the cableway nor the derrick necessarily wastes any of the other's time. Although they work together the proper coordination of their duties does not require such coordination in point of time as does the other system. In other words, the cableway leaves a load at a certain point within reach of the derrick and picks up an empty which the derrick had previously left at that point. "That point" means some area accessible to both, which for the time is not being built upon, and which must be available under either system. The derrick, whenever its function of building masonry requires or permits, leaves an empty at that point and takes up the load left by the cableway.
Now under the system of delivery by cars on a system of tracks, the train or the car requires for the completion of its function the assistance of the derrick. The train cannot leave its load, pick up empties and return. If the assistance is rendered at a certain moment it may interfere with the function of the derrick, and if at some other moment it means that the train has waited; with a loss of economy in either case. Further, it means that there must be some unoccupied space upon the train, possibly upon each car, for otherwise the derrick must make its first trip to get the load and a later one to place the empty, making two trips instead of one. Such interference with the function of the derricks may not be serious on portions of the work. Very little time is required to dump a bucket of concrete or to set a large stone down in a concrete bed; but whenever the derrick is engaged in setting face work (as stone or concrete blocks) such operation requires more constant use of the derrick with consequent fewer intervals when it may attend to the train without delay to one or the other. In general, if for any reason a derrick devotes any considerable percentage of its time to other work than building masonry, it means a loss of output for the derrick, and if total yardage is attained it means a larger number of derricks.
Many of the foregoing disadvantages may be overcome by mere abundance of plant or by the use of more powerful machinery; but at the expense usually of high plant account and uneconomical use of power. It is not to be denied that there may exist situations where delivery of materials by cars on a system of tracks is the proper method rather than by cableway. The conditions indicating the advisability of such installation would be a valley too wide to be practically spanned by a cableway; a dam not so high but that derricks on it can easily reach the tracks, and consequently so low that its corresponding width would not permit enough cableways working over it to attain the desired progress; a comparatively small range of elevation through which the tracks must be built and the materials handled.
We have seen that 10-15 ton cableways 1200 ft. to 1500 ft. long have handled material for 9000 cu. yd. of masonry per month. Under the chapter on Probable Future Methods we shall inquire what if any feasible variations from existing cableway installations may be made in order to attain a larger monthly yardage either gross or per cableway.