The history of masonry dam construction during the past generation offers several illustrations of radical departure from previously accepted standards. These changes, as well as the advance in the size of undertakings, have led to most significant alterations in construction methods. In fact, growth in size, departures in design and the various developments in methods have been simultaneous, interdependent and contributing each to the others; an effect of one may be the cause of another. The general advance will be analyzed and an attempt made to show what changes may be expected in the future.

The earlier dams were built of rubble masonry consisting of some large stone and a certain percentage of one-man stone and spalls. The large stone was bedded in mortar and the vertical joints were filled with small stone and mortar. Derricks were used, but often they were operated by hand or by teams. Advance in the practice resulted in the universal use of power-operated derricks with bull-wheel and swinging-gear attachments (eliminating the old-time tagman) in the self-contained or stiff-leg derrick which was more readily moved, and which from absence of guys interfered less with the cableway delivery of material which had accompanied the evolution of the derrick. More recent advance has been in the direction of increasing the power in order to operate at higher speeds. It is probable that in only a few of the larger and more recent rubble masonry dams, did the magnitude of the work point unmistakably to distribution of power from a central power station.

Increase Of Output

The unanimity in rates of progress on rubble masonry dams is most striking. Thus on Dunning's dam, August, 1887, to November, 1889, 35,700 total cu. yd. "On one occasion one foreman with eight masons and nine helpers, with double drum steam derrick laid nearly 500 cu. yd. in seventy-six hours." This equals about 6.6 cu. yd. per derrick hour, probably higher than average rate. The derrick was probably overmanned or supplied with a large percentage of one-man stone. "Another foreman with seven masons and eight or nine helpers laid 375 cu. yd. in seven days" equals about 5.35 cu. yd. per derrick hour.

On the Sodom dam, February, 1888, to October, 1892, totaling 35,887 cu. yd., said to be the first one upon which a cableway was employed. "The largest quantity laid per month was 3000 cu. yd. with twelve masons and three derricks;" which figures out about 5 cu. yd. per derrick hour. On the Titicus dam, 1890-1895 "Six derricks and thirty-six masons averaged 3240 cu. yd. per month, the maximum being 5700 cu. yd." Assuming twenty-six eight-hour days per month this would be an average of 2.6 cu. yd. and a maximum of 4.6 cu. yd. per derrick hour. Though not under comparable conditions, it may be interesting to observe that on the Tausa dam, "The maximum progress was during January, 1891, when 700 masons laid 2600 cu. yd." Assuming twenty-six working days, this performance equals 1.43 cu. yd., per ten-hour mason. At the New Croton dam the average was about 5 cu. yd. per derrick hour. On the Wachusett dam, while occasionally 6 cu. yd., the average was not over 5.5 cu. yd. per derrick hour.

The foregoing figures may be taken to indicate that 5 to 5.5 cu. yd. per derrick hour is the normal rate of this class of work; that total progress on a work of some magnitude would be this rate multiplied by the number of derricks which mere available area would permit being installed; and that improvements and elaborations of plant were to reduce one of the elements of cost (transportation) and to supply a larger number of derricks.

The Roosevelt dam, as previously explained, was built under a different specification, the large stone being bedded in mortar and the vertical joints being filled with concrete and spalls. Progress under this specification was about 16.5 cu. yd. per derrick hour, accompanied by a corresponding reduction in cost, although the situation particularly favored a low cost for some of the elements of the process. It seems probable that this is about the limit of output that may be expected of any method which utilizes such a percentage (50 per cent.) of stone. Although the method realized two-thirds of the advance in output possible under the change to cyclopean masonry, it effected no saving in cement (using 0.76 barrel per cu. yd.) over the requirement for that practice, the mortar beds requiring just about the amount saved by displacing concrete with stone. Altogether the method, while interesting, seems to offer no advantages which would tend to perpetuate it. It is believed that this was the first large dam where electric power was distributed from a central station and used with anything like the universality, satisfaction and economy which seems to characterize it as the method of the future.

Strictly cyclopean concrete was first used on the Boonton dam, and since then upon the Cross River, Olive Bridge, Kensico, Salmon River Idaho, Barker, Delta and others. The development of this method seems to have been in the direction of using a smaller percentage of stone and attaining a higher rate of progress. Thus the Boon-ton dam is stated to consist of about 50 per cent, stone, the Delta dam 43 per cent., Salmon River and Southern Power Company 40 per cent., Cross River 35 per cent., Olive Bridge and Kensico about 26 per cent. The rate of construction in cu. yd. per derrick hour for the earlier of these dams, if indeed ever noted, has not been stated; in the last two above mentioned an average rate of about 21 cu. yd. per derrick hour was attained.

Such differences in amount of stone and rate of building seem to be a perfectly natural development and are too great to be accounted for by differences in size and shape of stone available. Indeed the increase in rate was made possible by the decrease in stone, and the increase in rate should effect a much greater saving in cost than would offset the cost of extra cement due to decrease of stone.

The construction of a large masonry dam was until recently an art or problem, the solution of which centered upon the dam itself. Recently, however, it has become more and more a transportation problem; simply a question of how best to get the materials from one place to another. The materials and the specifications have changed so that now the quality of the masonry very largely takes care of itself. The masonry, barring some face work, is now dumped by laborers instead of being built by masons; coincident with this change is the much greater rate of progress, which may be considered partly as cause and partly as effect.