A feature in the design of several recent dams (Elephant Butte, Arrowrock, Olive Bridge, Kensico, Farnham,
Barker, and others) is a system of drainage in the masonry near the upstream face to intercept and remove any water which may enter the masonry. (See Figs. 22 and 23.) Leading to an open drainage and inspection tunnel are many channels made by building in the masonry vertical chimneys of very lean (and hence readily pervious) concrete blocks. Such a system, like the one in the foundations, establishes, just downstream from the tight zone, an open zone for facilitating the escape of water to an outlet under the refill at the toe.
Nearly all masonry dams have no such drainage system. While a dam with absolutely no leakage is so rare a thing that it is held by many to be non-existent, still very few of them show water upon their downstream face in any objectionable quantity. Even if the system is not expensive, it requires some attention, will probably interfere somewhat with progress on adjacent masonry and is certain to be regarded more or less as a device to hide inferior construction, all for the matter of an appearance which can be kept so slight as to be insignificant. Even if the appearance of the downstream face is such as to suggest the use to which the dam is being put, there is vastly more precedent for it than there can be objection to it.
Fig. 23. Cross section Kensico dam.
Previous to the advent of cyclopean concrete it could have been said of masonry dams that conditions are such, or can easily be made such, that no apprehension need be entertained regarding a possible uplift pressure on a horizontal joint in the masonry. The masonry of the upstream face usually is or can be made so much tighter than the remainder that any water which penetrates the dam can go on through without causing any pressure. The upstream face of a dam is 90 per cent, to 95 per cent, stone as compared with 50 per cent, for the interior, and the pointing of the upstream face should be much tighter than the mortar or concrete of the interior. Then through the mass of the masonry there should be nothing analogous to a continuous joint or seam such as may exist in the foundation.
It is significant that such drainage systems made their appearance very soon after the advent of Cyclopean concrete. To date they have been introduced only in dams of cyclopean concrete with faces of concrete or concrete blocks. It is quite possible that the first one or two cyclopean concrete dams, built while the method was somewhat experimental, may have been accompanied by results which indicated that the drainage system was a necessity. Whether or not the drainage system was indicated by experience, its desirability in connection with cyclopean concrete may be reasoned as follows:
Cyclopean concrete may and should be less permeable than the mortar formerly used.
The upstream face, if of concrete or concrete blocks, is more permeable than the stone formerly used.
Thus in both respects the difference between the upstream tight zone and the downstream relatively open zone is reduced; and while the total leakage may be no more, or even less, it is accompanied by conditions more conducive to the existence of an uplift pressure.
In addition to the foregoing it may be fairly said that the building of cyclopean concrete tends to the introduction of horizontal construction planes with less bonding across the planes, thus furthering the application of an uplift pressure and making it more effective.
One recent feature of dam design, the expansion joint, which will undoubtedly persist, seems to require that an intercepting drain be introduced immediately downstream from the upstream recess where the metal strip crosses the joint. While a drainage system in the masonry was entirely unnecessary for the dams built under the older specifications, still such a system appears to be a very desirable feature for future dams of cyclopean concrete or concrete, and it will undoubtedly become standard practice.