A variation from the usual plan may be made, not only that more cableways may be used at any and all stages of the work, but also that if there is at some stages some inequality in performance still, they will be occupied till the dam is completed. The assumptions are the following: a dam 1500 ft. long; for 700 ft. in the center the masonry has a bottom width of 200 ft.; cableway towers may be placed 100 ft. from each end of the dam; span of cableways 1700 ft.; the towers (one at each end) to be such that the cables are spaced 25 ft. apart. The towers would be unique but not impossible structures.
Of the accompanying cuts, Fig. 35 shows an installation of eight cableways after the usual manner and how presently some of them become practically useless. Observe that for each 25-ft. decrease in thickness of masonry one cableway goes out of commission. However, if derricks are employed on the dam, the cableway may still be available through the device of building landing stages on the downstream side of the masonry; but there are several objections to such a method, and at best it would only render available about one cableway in addition to the number operating over the masonry. Fig. 36 shows a suggested layout to obtain the above-mentioned advantages. It gives 50 per cent, greater cableway capacity at the start, and maintains that original capacity up to near the top of the dam. In practical application the performances of the cable-ways can be kept more nearly equal than might be indicated from a casual inspection of the figure; the cableway spans are increased from 1700 ft. to about 1750 ft.
Fig. 35. Usual scheme of cableway installation.
Fig. 36. Possible installation scheme permitting more cableways.
Using previous records of actual performance let us see what such an installation might be expected to accomplish. At Olive Bridge dam four cableways delivered 35,300 cu. yd. in a month.
Then let us assume that it is only a question of getting the material to the cableways, and that twelve cableways could put 106, 000 cu. yd. per month onto the dam. Let us first see what this would involve if the present practice was followed of installing derricks on the dam to rehandle all the materials.
At the Medina dam, derricks set 27.2 cu. yd. per hour of masonry containing 10 per cent, of large stone. Assuming that rate per derrick (disregarding for the moment the question of how far it may be economical or necessary to sacrifice stone to gain yardage in output), about nineteen derricks would be required to set the masonry. Somewhat more than nineteen derricks could easily be placed on such a dam so as to work without interference. That the derricks could not maintain this rate in the upper 50 ft. of dam does not interfere with the general propositions that as many derricks should be employed as can be placed without interference, that they should do nothing but set masonry, that they should be supplied with materials up to their capacity, that it would undoubtedly be found economical to sacrifice much or all of the stone in the masonry if thereby the output per derrick hour could be raised from 21 cu. yd. or 22 cu. yd. to 27 cu. yd. and that it should be possible to operate such a plant so that 80 per cent, or more of the mass could be built at an average rate close to the above assumed maximum. It may be objected that the foregoing rate per derrick quoted for Medina was for masonry laid between forms, but it must be remembered that half or more of the material was delivered on cars along the toe of the dam. This method involves time of the derrick to reach its materials, and the moving of forms probably involves some delay as well.
If the materials were deposited directly by cableways, four-fifths of the derricks on the dam could be dispensed with and probably one-half of the labor. The working area would be unobstructed, conducing to rapid progress. Until the dam reached an elevation where it was comparatively thin, four derricks could set all of the face blocks. For that purpose the derricks could be smaller, lighter and more readily shifted from place to place.
The cableways could deliver in place, as masonry, as much material as they now deliver to derricks. Without being unhooked from the cableway, a bucket of concrete could be dumped directly in less time than is required at present in unhooking from the load and hooking onto an empty. If the quarry was such that a certain percentage of large stone could be produced at practically no extra cost or with no modification of quarrying methods, such stone could be placed as readily by the cableways as by the derricks. However, it would not be economical to do so unless the stone were large enough, say 3 cu. yd. and upward, to warrant a trip of the cableway, as it would not be practicable to handle skips containing several stones. In case it is found that (along lines 25 ft. apart) is not sufficient distribution of the materials, it would be simple and inexpensive so to mount the cableways that they could traverse 12 1/2 ft., and this would provide for amply covering the area. The additional cement required in case the stone is eliminated would be, say substituting 1-3-6 concrete for 25 per cent, of stone, somewhat less than 0.3 barrel of cement per cu. yd. of masonry at a cost which should often be much less than the savings in other directions resulting from the change.
While the cableway is a most admirable machine, suitable for a large variety of situations and operations, and while it will undoubtedly continue to be a very desirable item of plant for the construction of large masonry dams, it has one inherent feature which may tend to relegate it to the position of an accessory rather than of a principal machine. Briefly its limitation is that but one load can occupy it at a time. The cableway takes out a load, leaves it, takes an empty and returns, the round trip requiring three to five minutes. This involves motion in two directions, as well as raising and lowering at each end, hooking and unhooking, transmission of signals for most of the operations, and altogether an amount of lost motion, lost time, and breaks in continuity that are distressing to one who has analyzed the elements of the problem of transportation of materials.