When of masonry, the stones should be bonded, not merely as they would be in an ordinary vertical wall, where the direction of the stress is perpendicular, but each course should be knit in with that above and below it in a somewhat similar manner to what is termed “random” work.  And lastly, if hydraulic mortar be used, a sufficient time should elapse after construction before being subjected to strain, or in other words, before water is allowed to rise in the reservoir.  For this latter reason, and also the liability to damage by sudden floods during the progress of the works, dams of Portland cement concrete, on account of their quick consolidation, possess advantages over those of hydraulic masonry apart from the necessity in the latter instance of constant supervision to prevent “scamping” by leaving chinks and spaces vacant, especially where large masses of stone or Cyclopean rubble are used.

Again, should the dam be drowned by flood during its erection, no harm would accrue were it composed of Portland cement concrete, whereas should it be of hydraulic mortar masonry, the wall would probably be destroyed or, at all events, considerably injured by the mortar being washed out of the joints.  Portland cement, however, is only suitable for situations where the foundation is absolutely firm, as, should there be the slightest settlement, fissures would certainly be produced.

As regards foundations, the dam of the Puentes reservoir in Spain is somewhat remarkable—­see Fig. 12.  Its height is 164 ft., and the profile or cross section is of precisely the same character as that of the Alicante dam, the latter being 135 ft. in height, 65 ft. wide at the crest, and 65 ft. at the base, and erected about 300 years ago.  At the Puentes dam the flanks of the valley were reliable, but, as must be frequently the case in such situations, the bed of the valley was composed to a great depth of gravel, debris, and shaky strata.  The difficulty was overcome by throwing an arch, or arches, across the valley, the abutments being formed by the solid rock on each side, and building the dam upon this arching and filling in below the latter down to a sufficient depth with walling.

Bearing in mind the sudden and great floods to which dams constructed in such situations must be subjected, and, if the valley be very narrow, the probability that sufficient space at the side for a by-wash will be difficult to obtain, it would seem reasonable that in the calculation for their section allowance should be made for the possible condition of the whole length of the dam being converted into a weir, over which the waters may flow without risk of injury to the dam, to a depth of, say, at least twice that ever probable.

The topping of dams by floods is not uncommon, and if the extra strain thus induced has not been allowed for, their destruction is nearly certain, as instanced in more than one case in Algeria, where, although the average rainfall is only 15 in. yearly, a depth of 61/4 in., or more than one-third of the annual total, has been known to fall in twenty-four hours.