The number of cu. yd. built per derrick per hour is the only really logical and enlightening unit by which to compare intelligently progress under one method with progress under another in the analysis of conditions as they apply to particular dams, foremen and derrick locations. The construction plant for any large dam should be so designed that the materials will be assembled, prepared and brought onto the dam as rapidly as the masonry can be built by working all the derricks that can economically be brought to work upon it. In short, the limit to masonry progress should be and usually is the number of derricks that can readily work without mutual interference, and the cu. yd. per hour due to the particular specification under which the work is being done. Obviously some such unit is the only logical basis of comparison as to efficiency of plant and personnel between two dams where the same specification is being followed. A monthly estimate of so many thousand cu. yd. is meaningless unless accompanied by a statement of the number of derricks engaged and the total number of derrick hours.
It is to be hoped that all engineers in charge of such work will keep and publish records of output on the basis of cu. yd. per derrick hour. Monthly estimates should show accurately the total cu. yd. per month. This monthly total may be easily divided into totals for each derrick in proportion to the amount of stone concrete and mortar used by each derrick, .which figures can be easily kept by the inspectors, and these totals per derrick should be divided by number of hours. Such analysis and comparison are most entertaining and instructive. Of most interest to the engineer is the monthly estimate divided by derrick hours as this illustrates the different efficiencies possible under various specifications and also at various stages of the construction of the dam. Of more interest to the superintendent is the number of cu. yd. per hour for each derrick as this shows the efficiencies of the different foremen and discloses whether certain derricks are working under some temporary handicap of position, or whether some conditions require remedy.
On one large dam it was the practice for the superintendent to keep similar figures. These figures were published each week and were accompanied by a bonus for the crew standing at the head of the list. Out of about nine foremen the bonus almost invariably went to one of four, which of the four would depend upon what derrick happened to be working in the most favorable position during the week. Thus this record showed both the efficiencies of different foremen and the effect of favorable location.
In addition to the number of cu. yd. per derrick hour, figures showing the relative amounts of large and small stone, mortar and concrete can easily be kept for each derrick. They are enlightening as to character of work being done and should be frequently brought to the attention of the foreman and inspector of each derrick. As an illustration, the following is an imaginary record of two derricks for six ten-hour days.
Stone, cu. yd..............
Spalls, cu. yd.............
Concrete, cu. yd................
Motar, cu. yd..........................
Cu. yd. masonry built.......................
Cu. yd.per derrick hour.....................
Cement used, barrels........................
According to the foregoing figures Derrick A has built 60 cu. yd. more than Derrick B, or 1 cu. yd. per hour, and without further scrutiny it would seem that the foreman of Derrick A was the more efficient. On examination of the figures, however, it will be seen that Foreman A used more mortar for bedding his large stone than did Foreman B; also that in the space (vertical joints) filled with concrete and spalls, Foreman A put slightly less than 22 per cent, spalls, while Foreman B put slightly less than 24 per cent, spalls. It is not argued that a comparison like the above should be taken to indicate that the faster rate and the extravagance in the use of cement necessarily accompany each other. There is a presumption, however, that to a certain extent they may be cause and effect; only numerous comparisons between records of different foremen and different records of the same foreman could determine this.
Now let us assume reasonable values for the materials and labor and see how the masonry costs compare.
408.0 at 0.80
190.8 at 0.75
broken stone ....
304.6 at 0.50
135.9 at 0.75
871.0 at 1.75
A built 1020 cu. yd. at a cost of $2.46 per cu. yd. B built 960 cu. yd. at a cost of $2.41 per cu. yd. Then it becomes a question of amount of overhead and interest charges plus any other value such as bonus or penalty attached to time, whether the performance of Derrick A or Derrick B should be considered the better. Thus, if five derricks are working and have 300,000 cu. yd. to build, then performance B requires twenty-two more working days to complete, and makes 300,000 cu. yd. at 5 cents or $15,000 less.
Plant has become standardized so that a derrick on one dam means practically the same thing as a derrick on another. They can for similar positions cover the same area under any masonry specification. A derrick crew consists of about the same number of men, viz.: foreman, engineer, and eight or nine men. A less number of men would mean that the derrick was not being worked to its capacity and more men would be in each other's way.