The term "Quarrying" as applied to the process of producing stone for use in the construction of a masonry dam should properly be explained and qualified somewhat. In many cases this process would be more aptly described as rock excavation and crushing. Methods employed in large permanent quarries for the commercial production of stone for general building purposes are very seldom applicable to the kind of quarries under consideration, because stone within practicable distance of a dam seldom occurs in such deposits or is of such quality as to warrant the opening and working of a first-class quarry.
Assuming that there are a certain limited number of dam sites and also of stone deposits admirably adapted for quarrying, the mere matter of coexistence in space would preclude the likelihood of their occurring together frequently. A possible further reason why they should not is that the location of many streams was fixed by distortions, ruptures, or faults in the earth's surface, and stone is not apt to be found in regular unbroken masses, adjacent to such distortions. However that may be, stone of some kind and quality is necessarily to be found at any possible site for a masonry dam. To obtain stone suitable for the bulk of the masonry is merely a matter of stripping off the top soil and disintegrated rock. It is often impracticable, even where possible, to obtain the necessary percentage of stone suitable for cutting into face stone, coping, etc. (See Plate VI, Figs. A and B.) A failure to appreciate the true inwardness of this fact has occasionally resulted in a much higher cost for the output of the quarry. A brief study will show that it is often advisable to import the cut and dimension stone at a cost apparently high per cu. yd. rather than to employ in the rubble stone quarry the methods which are necessary to save the cut and dimension stone from it. On account of the yardage rather than the quality involved, the same principle operates also in the matter of desired percentage of large rubble stone as distinguished from spalls and concrete material. In a word the entire difference lays in the methods that may be employed for quarrying and handling. For instance, when 60 per cent, to 70 per cent, of the useful output of the quarry must be large stone and a portion of them be suitable for cutting, considerable care must be exercised in the amount of explosive used. The shooting must be hard enough so that the stone may be subsequently barred out of the face or lifted by derricks, but still it must not shatter the rock or open seams unnecessarily.
The methods must at all times be suitable for the production of large stone. They cannot be relaxed upon any apparent deterioration of the quality or formation of the stone because any overshooting may shatter the stone in the next lift or that to be moved by the next line of holes. In this event, much small stone will be produced at the same expense as the massive stone. If loosened in large masses the stone must subsequently be plug drilled and split to a working size. Consequently more men per derrick are required, and the yardage output of the derrick is limited. The recovery and utilization of the large stone interfere with or preclude the use of the most economical methods of loading and removal of crusher material and waste. The reduced output per derrick requires that a larger number of derricks be provided, consequently a more extensive working face and probably more stripping to develop the quarry. On the other hand, different quarrying methods may be adopted if we eliminate the necessity for saving stone to cut for face and dimension work. If, in addition, we reduce by one-half the amount of large rubble stone by substituting concrete, the difference in methods and cost may be revolutionary.
The quarry may then be shot much harder. This will throw the rock down and clear from the face to where it may be more easily handled; and will also break it up so that little or no subsequent work is necessary to reduce it to a size that may be handled. If the methods are determined by the condition that the largest possible percentage of large and shapely stone must be obtained, a certain cost and output result; whereas if the production of large stone is purely incidental to securing an immense output of concrete material, a much lower cost and larger output follow. To anticipate somewhat a proposition which will be developed in the chapter on Probable Future Methods, it may be said that many of the dams of the future will probably be of concrete with no added large stone. In such a case the production of the concrete material will be reduced to the pure and simple proposition of excavating and crushing. The consequent simplification of plant and methods will produce marked reductions in the extent of working face of the quarry, and in plant and operating cost far below anything yet attained. Records of actual differences in cost per cu. yd. between quarrying for large stone and excavating for concrete material might be misleading except in the probably rare case of the two being based on experience under identical conditions in the same quarry. Rock excavation might be closely comparable to average records, but we have seen that the usual quarry contiguous to a masonry dam is apt to be an indifferent or a poor one, and that the quarrying costs consequently are apt to be higher than average quarrying costs. The following quoted quarrying costs in connection with the construction of the New Croton dam perfectly illustrate the principle above set forth.