In any system for transferring the materials to the dam, the points to be desired are the following:
That the motion be as direct and continuous as possible because change of direction or interruption to continuity involves multiplication of plant, loss of time, loss of power and increased expense.
That it shall interfere as little as possible with the operation of portions of the plant whose proper functions are something else.
That its capacity be ample for the rate of progress desired, that one installation may serve the entire work from beginning to end without renewal, moving, remodeling, or any radical change of methods.
Depending largely upon the length and height of the dam, and to some extent upon the point of origin of the materials, either one of two common methods may be employed, namely, cableways spanning the valley or a system of tracks along one or both sides of the dam. We will discuss the conditions determining the choice of either method and also the pertinent and allied matter of some conditions governing the capacity of derricks.
Cableways have been employed on spans exceeding 2000 ft., and they are the most economical machines possible in a large number and variety of cases. Besides handling the materials with celerity and a minimum consumption of power they are available for many incidental operations such as erecting, moving, loading or unloading heavy items of equipment or material. For moving derricks on the masonry the "sky hitch" is the acme of simplicity and efficiency; in fact, it pays for itself in performing that one class of service. It is needless to say that the road or railroad which gives access to the work should run under the cableway.
Cableways may have fixed anchorages with end supports or towers of a height depending on conditions. Again, if the service seems to require and the topography at the ends of the dam permits, they may be arranged so as to traverse up- and downstream, for which purpose the towers and anchorages are mounted on trucks running on several tracks. If it is possible with any reasonable height of end towers the cableway should be high enough to clear the top of the proposed work, as it is not only a nuisance and a delay, but also adds to the cost of the work, to let a cableway go out of commission and have to devise some other method to handle materials for the top of the dam.
The sag of a cableway is about 5 per cent, of the span. To this add about 25 ft. for carriage, fall blocks, hooks, extra sag under load and margin. From these data may then be figured the required height of towers. As the sag is 5 per cent, of the span, tower height can be saved by setting the tower further back when the ground rises steeper than 10 per cent. An accurate profile is necessary in order to figure intelligently on the desirable span and tower heights. In case it is desired to figure on cost of grading off a bench at each end of the dam for the purpose of installing a traversing cableway, it will be necessary to have, in addition to the profile, plans showing the topography for such area as might be affected by the benches.
A width of bench approximately equal to the tower height will be necessary. For the dam proper, traversing cableways will hardly be necessary. The number of cableways necessary to accomplish the desired rate of progress, placed side by side, will adequately cover the work. However, it will be well to figure on a traversing cableway when there are appurtenant structures like gate houses or other adjacent work involving the handling of large quantities, and if a saving sufficient to more than pay for grading the benches may be effected by handling those quantities by cable. This is especially true if there is a large amount of material to excavate from the foundation.
It may be mentioned that for the purpose of handling the ordinary materials of construction it is not practicable (although possible) to move the cableway laterally while engaged in transporting longitudinally; but when necessary this may be done in the case of moving plant or other special loads. In ordinary operation the traversing cableway can simply occupy one of a wide range of positions, and for any given destination of load the cableway occupies a position over it, and the stone, concrete, etc., are brought to a point under the cable in the same manner as to a fixed cable.
The matter of adequate anchorage should receive careful attention from an experienced man. The details and cost of course may vary widely with different material anchored in, for a rough estimate say $100 to $400 per anchorage. Anchorage in the case of traveling cable ways is of course to the rear of the moving base upon which the tower stands, which must be weighted to hold it. Details of such anchorage, together with the necessary weight, should be shown on a plan of tower usually furnished with the cableway. Obviously several towers may be mounted on the same set of tracks, so that the item of grading and tracks is the same for one cableway as for several. Usually the towers are framed laying on the ground, to be erected in assembled position later. (See Figs. 30 and 31.) While the framing and putting together may be done by any good carpenter, the erection of the towers, stringing and rigging the cableway should be under the direction of an experienced rigger; but it is a short and inexpensive operation. It is necessary to bear in mind the required spread of tower base in order to give lateral stability as this governs the minimum distance apart at which the cables can operate. In some cases involving tall towers it is conceivable that one tower must be placed behind an adjacent one in order to bring the two cables nearer together than would otherwise be possible. In special cases two or more cables may be mounted on one pair of towers.