The Kensico dam now being constructed by the New York Board of Water Supply will contain about 1,000,000 cu. yd. of cyclopean concrete, of which about 200,000 cu. yd. containing about 27 per cent, of plums have been laid. Here all of the stone and much of the concrete are delivered by cars and locomotives via tracks on both sides of the dam. (See Plate X, Figs. A and B; also see Fig. 32.) There is a mixing plant at each end. The stone (including all crushed stone) and sand originate at one end. The two 10-ton, 1861-ft. span traveling cableways are used solely for handling plant. The masonry record to date is 53,000 cu. yd. in twenty-seven days, or equivalent to the output of six cableways at previous records of performance.
This amount of masonry was laid in 1913, between Aug. 25 and Sept. 24 inclusive, or in twenty-seven eight-hour days. No account was kept of the actual working hours of the derricks; thirteen derricks were installed on the masonry, and assuming that they all worked full time they would have shown a performance of about 19 cu. yd. per derrick hour. It is probable, however, that the number of derrick hours of actual work was less than the above assumption. The materials were all passed to the derricks by means of cars running on tracks upon each side of the dam. From the mixers, some concrete was also run out via tracks extending out a short distance along the center of the dam from each end.
While in this particular case this method of handling the materials has resulted in a large monthly total yardage, still it is open to some general objections which it may be well to analyze.
If the materials, stone and concrete, originate at the level of the bottom of the valley they may indeed be easily brought on tracks to one side of the dam, and while the dam is in its earlier stages may be brought through or around one end to the other side, so as to be available to derricks reaching over either side. As the masonry grows in height, however, the tracks upon one side must usually be discontinued unless one of several objectionable expedients are resorted to, namely:
(a) - Leaving an opening through the masonry.
(b) - Climbing around the end by a longer route, probably requiring switch-backs, and in any case involving a heavy labor and fuel cost, as well as a heavy investment in track on which the salvage is practically nothing.
(c) - Establishing two quarries with crushing and mixing plants, one above and one below the dam. Even if such a procedure is possible it would be a division of forces and multiplication of plant that could hardly fail to result in a sacrifice in economy.
In many cases it is desirable or necessary that water be stored in the reservoir long before the dam is finished, thus precluding tracks upon the upstream side.
When the masonry is started the tracks may have to be some distance away on account of the excavation slopes. As the masonry is brought up and the refill made they may be shifted up near the masonry lines. If the refill is of considerable depth the tracks may have to be shifted laterally or in elevation several times, and they may interfere with the operation of refilling. The change of elevation may involve much work on the tracks for some distance in order to preserve feasible grades.
Fig. 32. General lay-out of construction plant Kensico dam.
After the masonry has been built to an elevation above the refill and above the tracks, the amount of work and time involved in reaching over with the derricks and lifting the loads is a consideration which becomes more and more serious as the masonry progresses in elevation above the tracks. On the downstream side, owing to the batter of the face, a derrick on the masonry (i.e., in a position to build masonry) will soon find itself at a considerable lateral distance from a track on the refill. With the usual batter, a practicable length of boom and some allowance for clearance of track, etc., this method would be possible until the masonry was some 75 ft. above the track level. To continue the masonry above that elevation by that method involves one of two expedients: either a trestle along and upon the downstream face of the masonry upon which the cars can be run, and which may require the remodeling of a large part of the track system; or the interposition of derricks upon the downstream face for the purpose of passing material and skips back and forth between the cars and the masonry derricks. Either expedient is expensive and a nuisance, and is open to the objection that it might seriously mar the appearance of the face masonry through rust stains or grease drippings. On the upstream side the masonry derricks may indeed reach over to the tracks at any stage of the work up to completion, although the lift may become such that considerable time of the derrick is thus occupied to the loss of time in setting masonry. Thus for a lift of 100 ft. and a load of 8 tons, a 30-h.p. derrick will require about two minutes for the lift; or say it is desired to build during the day masonry requiring 250 tons of stone and concrete, and assume that the containing skips or buckets weigh another 50 tons (a moderate assumption). This would equal thirty-seven and one-half 8-ton loads and one hour and fifteen minutes actual hoisting time. Hence 16 per cent, of an eight-hour day for the derrick has been devoted to overcoming a difference of elevation easily to be encountered in the construction of a large dam.