The efficiency of the masonry derricks depends greatly upon an intelligent plan for placing them and coordinating their subsequent movements. On starting masonry construction a plan should be made showing the limits of the masonry at the bottom. This plan may be extended and corrected from time to time as additional foundation becomes available at the ends or as the thickness of the dam is reduced. To lay out and preserve a system for placing derricks, it will be found convenient to use cardboard figures cut to show the space occupied by a derrick and covered by the sweep of its boom. The plan should indicate any areas, outside of the masonry lines that may at some stage be occupied by a derrick. If there is considerable depth of refill against the masonry on either or both sides of the dam, and if this refill can come up with the masonry, it may be desirable to place the derricks outside of the dam while such masonry is being built. (See Plate VII, Figs. A and B.) If the derricks are to be served by fixed cableways, their lines should be shown on the plan to see how they can reach each derrick in order to set it, move it or supply it with material. If the cables are traversing it is sufficient simply to know that they cover the entire situation. As much of the masonry must be built by derricks setting upon the masonry itself, it is even more important that these should be properly spaced and their movements systematized.
Obviously with the booms working in segments of a circle there must be some overlapping in order to cover the entire area. For certain thicknesses of wall down near the bottom, a layout might be a row of derricks just outside of each face with another row in the center (Plate VIII, Fig. A), or conditions might indicate two or more rows of derricks on the masonry. (See Plate IV, Fig. A.) A certain less thickness would require the two rows to be staggered as a transition to the one row that must be arrived at with a thickness of 100 ft. or less. (See Plate VIII, Fig. A; Plate X, Fig. D; Plate XI, Fig. A), The foregoing is not meant as an argument that the position and movement of a derrick must be studied and determined as minutely or as far in advance as if it were engaged in erecting the steelwork of a skyscraper. It is an argument, however for the necessity of a fairly definite plan at any stage and an appreciation of how one scheme of setting must merge into another. It is neither necessary nor possible to plan each position in advance to the end of the work. The superintendent, nevertheless, should have very definitely in mind what the situation will be a week hence and have a close approximation to what it will be a month in advance.
After covering what may be called the bottom, the dam as it comes up grows longer and narrower so that the area is not materially affected for some time. The number of derricks that can be advantageously worked does not vary very much from one stage to another. In addition to the normal number of derricks it is well to provide one or two as substitutes in the case of repairs or other contingencies. It is of no advantage, otherwise, to have idle derricks on the ground or to erect them far in advance of the time when they can be used. It is much cheaper to move a derrick than to pay for one. Adequate cableways, particularly if they are traversing, make it a very short and simple matter to move a derrick. It was no uncommon occurrence at the Wachusett dam to pick a derrick out of the line at one end of the dam, set it in at the other and lose less than an hour actual working time of the derrick. Of course, this operation is not always so short and simple. If the loss in efficiency when building in the bottom of a depression or on the top of a pinnacle were properly appreciated, the derricks would be moved oftener than they are. If instead of starting say 15 ft. below its level and building to 15 ft. above, these heights should be halved, the gain in efficiency would more than pay for the larger number of moves.
The accompanying illustrations show the mounting of derricks and their position with respect to the work for quite a number of dams and variety of stages. A peculiar mounting of derricks was resorted to during the latter part of the work on the New Croton dam, where cyclopean masonry was substituted for a portion of the core wall and embankment section, as elsewhere described. Within the working area two square steel towers were erected, a derrick was mounted upon each corner of their tops and the towers were built into the masonry as the work progressed. (See Plate VIII, Figs. C and D.) This method does not appear to possess any startling advantages under ordinary circumstances. The specifications for the Cross River and the Croton Falls dams, which were built soon after, each contained an item calling for a bid price per ton for steel towers built thus into the masonry. It is interesting to note that in neither case did the contractor avail himself of that method of mounting his derricks.
At the Kensico dam, most of the derricks are mounted in pairs on travelers moving upon I-beam stringers, which are in turn supported by concrete piers. The illustrations in Plate X, Figs. A, B and C, show the method perfectly. This system, as well as the manner of transporting materials to the derricks, seems to indicate a penchant on the part of the contractor for seeing things move on wheels, and an inappreciation of the celerity with which cableways can accomplish the same ends. (See Fig. 32).
Here two cableways are available and are used for nothing but to handle plant, but instead of using them to the limit for that purpose cumbersome alternatives are resorted to with no apparent accompanying advantage. In one detail the Kensico derricks are to be highly commended, namely, the stiff legs are weighted with concrete blocks instead of boxes of spalls.