"Cost of quarrying ($0,841 per cu. yd.) in above table applied to a period when only rubble masonry was laid; when face stones were obtained from same quarry the cost of quarrying was increased from $0,841 per cu. yd. to $1.80 per cu. yd. for entire output of quarry. This included splitting the face stone to within about 3 in. of required dimension".
The foregoing absolute costs are not particularly significant, as conditions obviously are so variable that they would rarely be applicable to another situation. The relative cost, however, is most interesting and pertinent. In another quarry, this relative cost might be more or less. It is not to be doubted also that there would be a similar if not as wide difference between the cost of quarrying for rubble and excavating for concrete material. This difference likewise would vary in amount according to the nature of the quarry.
The construction of a masonry dam usually requires a certain amount of stone before reaching the elevation where the face stone begins. This stone is naturally the first encountered after stripping, while a working face is being developed, and, as it is used for concrete material and rubble, the methods for getting it out maybe more of the nature of rock excavation than of quarrying. The character of the stone and of the quarry while thus being developed is naturally a matter of great interest. Prompted by his hopes that the quarry will be able to furnish face stone, one is apt to assign too much weight to two reasons why it may be better the further it is penetrated. These two reasons are: first, that stone is likely to improve in quality with distance penetrated from the surface; second, that less violent shooting than may have been used in development will result in a much more massive product. It is not to be doubted that there may usually be some basis for such reasoning and that there are occasional cases where quarrying for face stone is unmistakably indicated as the proper procedure, but it will more often be found economical to bring face stone from a quarry which does business in that class of stone, even if this means a big transportation cost.
A seemingly promising procedure for simplifying the methods and reducing the cost of production of the two classes of stone is to devote one portion of the quarry strictly to the production of one class and another portion to the other. In reality, this means the opening of two quarries. While this separation might accomplish something, it still implies that a large part of the quarry is of a character to warrant the quarrying for large well-shaped stone, and of sufficient promise of unchanging quality to warrant the risk of delaying the entire work on account of lack of face stone. The fact still remains that more quarry must be stripped, opened and faced up, that more plant must be installed and that much small stone must be wasted or used at greater cost for handling. In short, the entire matter is reduced to the previously stated proposition that the production of face stone should be unmistakably indicated, by an ample margin, before incurring the expense and chance of delay.
The following figures on cost of stone production in connection with the building of the Roosevelt dam illustrate a case where it was most desirable to introduce a large percentage of stone into the masonry, as the cement (manufactured at dam site in a mill erected for the purpose) cost $3.14 per barrel, and the quarry was such that large stone could be produced cheaply. The quarries were at each end of the dam at spillway level. It was necessary, in fact, to excavate the stone to make the waste channels. The stone was a very distinctly stratified, hard, fine-grained sandstone; on a dip of 29 deg. which favored the operation of quarrying. (See Plate VI, Fig. B).
In portions of the quarry the percentage of waste was not more than 15 per cent.; in other portions it was 75 per cent, or 80 per cent. The division between the cost of producing the stone which was used in the masonry, and of handling the stone that was wasted, is necessarily somewhat arbitrary, because the operations were not sharply differentiated in space or time. Of the 180,700 cu. yd. of waste whose cost is given, about 130,000 cu. yd. came from the quarries, and the remainder from elsewhere. Of the 209,800 cu. yd. of stone used in the masonry, 118,650 cu. yd. were derrick stone (which quantity includes 10,952 cu. yd. which was subsequently cut for the upstream face), 31,150 cu. yd. were spalls, and about 60,000 cu. yd. (solid measurement) were crushed for concrete material. The foregoing quantities represent the first 90 per cent. of the entire job. The wagon haul for the plant and supplies was partly from Globe 40 miles, and partly from Mesa 60 miles.
209,800 cu. yd. of stone used in masonry
180.700 cu. yd. of waste, of which 130,000 cu. yd. was quarry waste
Per cu. yd.
Per cu. yd.
Plant, assumed as 75 per cent, of first cost delivered on work.
Plant erection, hours labor..
Plant repairs, hours labor...
Power at 1/2 cent per h.p.-hr.
Repair parts and supplies...
Proportion of office and camp buildings.