For a small dam the assembling, handling and placing of the materials may well be merged into what might be called two operations, hauling materials to site, and building the dam. To elucidate: Several derricks may cover the entire work, unload the materials from teams, transfer to and from mixer and crusher, pass from one to another along the work and do all the placing in final position. In fact the small yardage, and the limitation thus placed on the amount of advisable expenditure for plant, points to some very similar procedure. However, with increase in size of dam it becomes successively questionable, then advisable and finally imperative that these various operations should be sharply differentiated, and that each should have its plant and methods suitable for the particular case and for the yardage involved.
Whatever the origin of the materials (stone, sand and cement) entering into the construction of the dam, they must be assembled at one point before being sent onto the wall. It is true that the large stone may be sent direct from the quarry to the dam irrespective of the manner of transportation of the other ingredients. However, it will be found almost invariably that economy of plant and operation require that the large stone join the concrete and mortar at some point of assembly, and that it take the same route thence to the dam.
At some point between the quarry and the dam a certain portion of the quarry product must be crushed for concrete. At the same or another point this crushed stone must be joined by the (pit or manufactured) sand and the cement for the purpose of mixing concrete. While the details of crushing and mixing plants may vary widely, and indeed not be identical upon any two pieces of work, still certain general rules and considerations may be borne in mind in laying out the routes for the materials and the details of the process.
First as to the point of assembly of the materials: This should be kept up at such an elevation relative to the dam that gravity can play a large part in the passage of the materials through crushers,
149 screens, storage bins, measuring hoppers, mixers, etc., and that the emerging concrete or mortar will require the least subsequent elevation on its way to the dam.
One factor which naturally enters into the problem is the location of the points of origin of the materials, i.e., of the quarry, sand pit and cement storage, and of the routes from such points. The assembly point might conceivably be a deciding factor in the choice between alternate quarries or sand pits; a longer distance might be economical in order to obtain easy or descending grades. If at the point of assembly, or between it and the dam, the materials must be elevated there is little choice as to the exact stage at which it may be done. Thus stone brought on cars is dumped onto the crusher platform - the crushed stone may be screened and elevated or elevated and screened. If the former, an elevator is required for each size product; if the latter, one elevator may have sufficient capacity and the screen is light enough to be easily mounted on top of any storage bin.
Again, if the introduction of a bucket elevator between the crusher and the mixer will reduce the height through which a cableway must hoist the concrete before conveying it, introduce it by all means as the time of the cableway it too valuable, to say nothing of the weight of the concrete buckets or skips.
If considerations of available room render it necessary, the operations of crushing, screening and storage may be performed at the quarry. In this case, however, there enters the question of probable duplication of bin capacity, as there must be storage below the crushers and also storage above the mixers. But one storage bin is required if the two operations are performed at one point with or without intermediate elevation. The performance of the two operations at two points means an interruption to the continuity of the motion of the material, the interposition say of an additional train service between the storage at the crusher and the storage at the mixer. The actual lengths of haul involved probably are so short that such an interposed train service would be equivalent to doubling the transportation cost of the material thus handled.
A point that is occasionally overlooked is that the crushing and mixing plant is the point of restricted channel in the flow of the material. The product of many units along an extensive working face in the quarry converges at this plant to be later distributed among many units over the working area on the dam. The disablement of a quarry or masonry unit is a local matter, insignificant in its effect upon the progress of the work, but a breakdown at the crushing and mixing plant is liable to be a more serious matter. Ample crushing and mixing capacity is indeed usually provided, but the provision of this capacity in one unit is apt to lead to the weakness that a breakdown stops the entire work. Pertinent to this situation is the old adage "Don't put all your eggs in one basket." Assume that the estimated required crusher capacity is 50 tons per hour. Then it may be better to provide two crushers of 35 tons or 40 tons capacity each than one of 60 tons or 70 tons. Important elements of the problem are the amount of storage capacity, the design of the plant with regard to rapid repairs and the completeness of the stock of repair and duplicate parts kept on hand. If the material must be elevated between the crusher and the mixer, and from economy in bin capacity the storage is all upon one side of the elevator, then the continuous operation of the elevator is as vital as any more prominent feature of the plant.
On pages 204 and 205 is discussed the manner in which unit costs increase with decrease of output, and how the cost of certain operations is nearly in proportion to the time instead of the output. As this is peculiarly the case with the operation of a crushing and mixing plant, the aim in the design of the plant should be to make the fullest use of mechanical devices and to reduce the number of attendants to the lowest practicable point.
Gates controlling the flow of material, measuring and dumping devices should be made, as far as possible, to operate automatically; or else bring the various indicating and controlling mechanisms together so that they may easily be handled by the minimum number of operators. The possible saving may be easily figured thus: Assume that at the various estimated rates of progress the operating force will be required for say 700 working days and that their labor may cost $2.00 per man per day; then a plant expenditure for automatic or labor-saving devices of $1000 for each man displaced is easily justifiable. The design and arrangement of such a plant particularly in the minor features of conveyors and elevators depend upon so many circumstances, like capacity, topography, access to and between different parts of the work, design and location of other plant, etc., that one design could never be copied for another job. Still one very extensive plant may be here illustrated and described, particularly as its cost is given.