The total number of machines required of each kind, such as derricks, cableways, pumps, crushers, mixers, etc., should be estimated as closely as possible. A further estimate should be made as to the maximum number which it will be necessary to have in operation at any one time in order to figure on the size of the central station outfit. Bear in mind the sequence of the various operations, namely how they may or may not overlap or be made to dovetail with other operations, the machines required for each in their various stages, etc. Thus stripping and opening of quarries and sand pits can be done at a time when only a small amount of power is required, as on river diversion works. The power required to handle excavation probably decreases as the depth of the pit increases (not because of increased depth but because of slower progress) and is partially offset by the greater amount of water to pump and the higher lift. On starting the masonry in the bottom of the pit, more and more room is available for, and increasing power is required for, the masonry as it comes up at the same time that the pumping is decreasing. It may not be possible to estimate closely the amount of power required for pumping. The number of derricks required in the quarry is also more difficult to predict than the number which can be employed on laying the masonry. The number of derricks required in the quarry depends largely on the character of the quarry and is ascertained only when the quarry is well opened up. The number of derricks also depends upon the masonry specifications as affecting the percentage of large stone it is expected to introduce into the masonry, and also whether or not the quarry is to be worked in such a way as to save considerable stone to be cut for the face work.
If a dam is to be constructed of Cyclopean masonry containing 25 per cent, or 30 per cent, of large stone and with faces of concrete blocks, a much smaller number of quarry derricks will be required than in the case of a dam containing 50 per cent, or more of large stone and with cut stone faces. In general it may be said that the 137 number of derricks required in the quarries will be between one and one and one-half times the number employed in laying masonry. Unless there are some special and remarkable conditions it will be safe to assume that a central power installation sufficient for the purpose of maximum rate of progress on masonry construction will be sufficient for any stage of the entire operation. Now the requirement for this stage can be estimated with sufficient accuracy.
If we assume as necessary for the construction of a certain dam eight derricks on masonry and ten in the quarry with engines of 30 h.p., two cableways at 75 h.p., crushing and mixing plant at 200 h.p., transportation of materials to dam site 100 h.p., for drills, shop purposes, and miscellaneous 100 h.p., it does not follow that the central power plant should be the sum of the above. Indeed it will be surprisingly less on the same principle that forty rock drills may be run on about twenty times the air necessary for one. For example, a derrick is actually engaged in lifting loads for but a very small fraction of the time. Say the derrick sets 15 cu. yd. of masonry (weighing 30 tons) per hour and that the work performed in handling materials and setting masonry is equivalent to lifting that weight three times to a height of 20 ft., plus an equivalent amount for weight of boom, swinging the loads passing empty skips, friction, etc. Then the derrick has done work equivalent to its nominal capacity (30 h.p.) for but seven and one-fourth minutes during the hour. Assume that one cableway rated at 100 h.p. serves four such derricks engaged on masonry, that the work performed is equivalent to raising 120 tons of materials plus 30 tons for skips to a height of 75 ft., that the conveying is equivalent to raising it 15 ft. more and that returning with empty skips is equivalent to 50 per cent, of the work of taking the loads out. Then the cableway has been employed equivalent to its rated capacity for but twelve and one-fourth minutes. Some portions of the plant, as the crushing and mixing plant, screens, etc., undoubtedly do work equivalent to their rated capacity for a much larger percentage of the time.