The last of the important dams of this country to be built by the method which has been described, i.e., by bedding the large stones in mortar and the filling of vertical joints with spalls and mortar laid up by masons, were the New Croton, 1892 to 1906, and the Wachusett, 1900 to 1905. Beginning with the Boonton dam, 1900 to 1906, a new method was employed which has since been used on the Cross River, Croton Falls, Olive Bridge, Shoshone and other dams. This method seems certain to entirely supersede the former practice. Briefly it is to bed all large stone in concrete, also to do ail filling of vertical joints with concrete into which spalls may be rammed.
This radical change is the natural result of the development of concrete practice during the last twenty years; which development has been due to the gradual recognition of the fact that a wet mixture of concrete results in better work than the once universal dry mix. This modified concrete practice, in addition to improving the quality of the concrete, effected a material saving in the labor of ramming the concrete, and also effected a saving in cement on account of the spalls that could be rammed into the concrete after it had been placed. Another important cause of the change to a wet mixture was the development of reinforced concrete work and the practical impossibility of satisfactorily ramming dry concrete in connection with reinforcement.
This new method of construction of masonry dams simplifies and cheapens the work immensely besides resulting, if properly done, in a more impervious structure than the old method. The large stone and spalls must of course be subjected to the same careful inspection and cleaning. Fairly wet concrete is dumped down on the wall and the large stone simply set down in the mass, no elaborate process of making the bed and bedding the stone being required. One precaution should be carefully and may be easily observed, namely, have an ample depth of concrete under the large stone which is being placed. One may otherwise be apt to extend the setting of large stone out too near the edge of the batch of concrete where it has run down too thin.
If the concrete has spread out so thin that the bed of the large stone is partly above the concrete, any subsequent batch of concrete dumped near by is practically certain to trap air under the large stone and result in leaving an open space. Except for this simple precaution the setting of the large stone is a comparatively casual operation which may be as well done by reasonably intelligent laborers as by expert masons. It is not necessary or even desirable that the large stone should hang in the hooks with level bed, the same purpose (of expelling all the air as it is lowered) may be accomplished even better by lowering the stone in such a position that a corner or an edge will enter the concrete first and then tilting the stone to its final position as it is lowered.
The large stone may be set so that the joints between them will be anywhere from 6 in. to i ft. or more in width, depending on circumstances. Stone say 3 ft. high should be set with the wider joint, and stone 1 ft. high may be set with a much narrower joint. An approximate rule might be that a joint should have a width equal to one-half of the height of the stone, the point being to leave joints that can be readily and satisfactorily filled later. As each large stone is set, spalls may be rammed into the concrete in the joint between it and adjacent stone without waiting for the joint to be filled to the top; as beyond a certain depth spalls cannot be rammed or pushed at one operation and otherwise the bottom foot or so of the joint would contain no spalls. After a considerable area has been covered by large stone set in this manner, the filling of the vertical joints may be completed by dumping in concrete and ramming spalls into the concrete.
If any of the large stone have been set for such a length of time that their concrete bed has begun to harden, it will be necessary to exercise care that the process of dumping the concrete does not break the bed. Usually, however, the area will be covered with large stone and the joints will be filled before enough set has taken place to make it a matter of concern whether the large stone are hit and slightly disturbed or not.
When the concrete is dumped and as it is flowing into and along the joints, men with shovels or bars should assist the process seeing that the joint is completely filled and that no nesting of the coarser aggregate occurs - in short, that the concrete does not unmix. It may often be needful to shovel some concrete into a high, narrow or tortuous joint to be certain that it is properly filled. In order that the maximum of spalls may be introduced into the concrete it may be necessary to make two operations of filling the joints, i.e., not dump at one time or place such a depth of concrete that spalls cannot readily be pushed to the bottom. There should always be a depth of concrete sufficient to bury practically the entire bulk of the spalls for if the spalls protrude to any appreciable extent they interfere with the proper deposition of subsequent concrete. Under no circumstances is it allowable to introduce spalls first and then pour concrete over and around them. It is not economical to try to introduce spalls beyond a certain percentage, as then the labor cost of doing it properly more than offsets the saving of cement. On the Roosevelt dam it was found that of the space thus filled by concrete and spalls, the economical limit was about 22 per cent, spalls to 78 per cent, concrete.
Both of the above described methods were employed for about three years, say 1903 to 1906, lapping over each other as it were during that period while several dams were being finished according to the old method under which they had been begun; since then the new method has prevailed. During this three-year period at least two important dams were begun, embodying in their specifications a method best described as being a combination of the two. In the Roosevelt and Pathfinder dams it was specified that the large stone should be bedded in mortar, as by the old method, and the vertical joints filled with concrete and spalls as by the new. This transitional variation, though it looks at first sight like a compromise between the two methods, is in reality practically the same as the new one. The rate of progress, cost and quality of resulting masonry are practically the same. By bedding in mortar and securing thinner bed joints a somewhat greater percentage of stone is secured in the dam, but as the mortar requires more cement the result is no difference in the amount of cement per cu. yd. of total masonry.
Although so far the results from the transitional and new methods do not warrant the statement that one is any better than the other, still, as will be shown in the chapter on Probable Future Methods, it is probable that the new method or straight cyclopean concrete will be the standard practice, but modified by a tendency toward the use of less large stone.
To compare figures regarding each method, take several prominent examples of dam construction. The Wachusett and New Croton dams were built under the old classic specification, namely, large stone bedded in mortar, vertical joints hand laid up with spalls and mortar; the Pathfinder and Roosevelt dams were built under the combination method, large stone bedded in mortar, vertical joints filled with concrete and spalls while the Olive Bridge dam embodies the cyclopean rubble method, large stone bedded in concrete, vertical joints filled with concrete and spalls.
Large stone, percent............
Mortar, per cent...............
Barrels cement per cu. yd........
Rate of construction in cu. yd. per hour per derrick.
3.8 to 6.0
20 to 27
Average rate when conditions were favorable for maximum progress.
Total cu. yd..................
488,300 including 56,200 cu. yd. of face blocks.