DAMS.
Where earth cannot be used, the choice of materials until quite recently has been between timber, timber and loose stone, and masonry. Within recent years a few dams of steel or of steel reinforced by masonry have been erected. Of course the greatest care must be taken to provide against dam failures, for which there are the following common causes: (I) By sliding on the base or on some horizontal joint; (2) by over turning; (3) by fracture due to tension; (4) by crushing, in the case of masonry dams; (5) by erosion, in the case of earth, or, though rarely, by breaking up and washing away, from the top downward, in the ease of masonry structures. After a good site has been chosen and the utmost care devoted to the construction of the founda tions and the supervision of the material and workmanship,'the chief factor of safety in dam construction is obtained by placing a sufficient volume and weight of material in the damn itself to withstand the pressure upon it. This pressure is directly proportioned to the height of water behind the dam and not to the total volume, as is sometimes supposed. In well-designed earth dams the cross-section is so great, for other reasons, as to give a weight far in excess of that which could be removed by the pre sure of the retained witty t. Ilut, in masonry dams the cross section may be proportioned to resist the pres sure with mathematical nicety, allowing, of course, the factor of safety common to all good engineering work. In the new type of steel dams questions of volume and weight yield place to the tensile and compressive strength of the material.
A most essential feature in the design of dams of all classes is ample provision fur passing waste or flood water. Otherwise the increased, pressure against the up-streaurface of the dam due to the excessively high water in the reservoir, or else the force of the current in passing over the top of the dam may cause a serious rupture. In overfall dams relief may be obtained, in some cases, by providing flood-gates at one end of the structure, either connected with or detached from the main dam; by having a crest to the whole dam which can he dropped in time of floods (see MOVABLE DAMS, below) ; or, in connection with one or both of the foregoing precautions, there may be an artificial overflow or waste channel leading from a spillway above or at one side of the dam down to the natural channel of the stream some distance below. Such a spillway and overflow channel are essential to all earth dams. It should also be noted that waste gates, or under sluices, are sometimes provided beneath the crest of masonry dams, particularly in India. These may be placed near the bottom of the reservoir to permit washing out deposits of silt. Where no other means are feasible, waste water may be carried to a point below the damn through a tunnel cut in the solid rock at one side and beyond the structure itself.