In the specification of expensive details or methods to secure some real or fancied effect which the constructor could sufficiently approximate at less cost.
The designer, not knowing the constructor, or perhaps being chary about trusting to his experience and fidelity, not unnaturally produces a conservative design. It is one intended to survive considerable laxity or departure from high standards of construction - and the foolproof design must be expensive.
Certain features or methods of construction seem to be overlooked in some designs because the designer often possesses a very inadequate appreciation of their adequacy as compared with that possessed by the constructor.
The past few years have seen revolutionary changes in both the design and construction of masonry dams, and the next few will see further changes in construction methods if not so much in design.
The older dams were of rubble masonry, containing 60 per cent, to 70 per cent, of stone, laboriously laid up by hand with mortar and spalls. The upstream faces were of cut stone with comparatively thin well-pointed joints. Although some leakage or seepage is almost invariably present, it is probably under such conditions that no harmful pressures exist, and it is rarely sufficient in amount to present a scandalous appearance.
The present type of dam is of cyclopean concrete containing far less large stone (down to 25 per cent, or less) with concrete or concrete block faces. The resulting prominent cracks are cared for by expansion joints; and the leakage (whether more or less than in the rubble type) is intercepted and removed by a drainage system.
In future dams the stone will be frankly discarded on account of the plant cost of equipping to handle it. The process will be reduced to the very simple one of manufacturing a certain amount of concrete and transporting it a certain distance. Much higher rates of progress and much lower unit costs will be realized.
Changes in design will probably consist largely in the development of the expansion joint and drainage systems; also in making greater use of the possibilities of concrete for architectural effect.