In addition to questions of pure utility, the architectural appearance should be given careful thought in connection with any important structure. This is as true of dams as of buildings. The amount of justifiable additional expenditure is as variable in one as in the other and depends on location, surroundings, client and other circumstances. The matter will not be discussed here except to say that entirely adequate and pleasing effects can be produced at small expense.
Clearly, an overflow dam affords comparatively very little opportunity for esthetic treatment, and this almost entirely on terminal or connected structures. Until recently the exposed faces have been of stone with more or less work bestowed on them to produce the desired courses, joints and faces. Some recent important dams have faces of concrete blocks which can be readily molded of uniform or any required dimensions and appearance. As concrete blocks are artificial stone, the only difference to the builder is the detail of whether blocks are molded or stone cut. Faces formed of concrete deposited against forms are considerably cheaper, but at best they present an appearance somewhat incongruous in a structure with pretensions to beauty. On smaller, and particularly on overflow dams, they may with care be so used as to present a not unpleasing appearance.
Joints in the face work have often been specified to be 1/2 in. in thickness. Apparently this specification is based on two assumptions: first, that a thin joint presents a better appearance; second, that it is better that the largest possible percentage of face area be stone rather than joint. In general masonry work, and for the sake of appearance, the thickness of the joint may possibly vary somewhat with the size of stone, i.e., rise of courses; but for such massive work where the courses are usually from 18-in. up to 3-ft. rise, and particularly where the stone are left with pitched faces, it will be found that 2-in. joints present a much better effect than thinner ones. Joints are a proper as well as necessary feature of masonry faces, and require no apology. Large stone with bold faces require for congruity that the joint be not obscure.
It is admitted at once that the upstream face should be as impermeable as possible and that stone is more impermeable than mortar; but in the actual every-day exigencies of construction it will be found that a thin joint is very apt to defeat its own purpose. A joint 1 in. or 2 in. thick is more certain to be completely filled than a 1/2-in. joint, besides being easier both to cut and to build. Another argument will be discussed later. (See Settlement of Masonry.) The preparation of stone and bed and the process of setting are practically the same as has been described for the interior stone, except that one face of the stone must be placed to coincide with the face of the dam. The settling of the stone into the mortar is usually accomplished by pounding with a heavy wooden maul instead of floating it with bars.
One detail of masonry practice, probably all right in ordinary building construction, but which should be looked upon with grave suspicion when applied to the face of a dam is the use of wedges which are inserted in the bed joint at the face to hold the stone to proper line and grade while being set. True the mortar is piled up higher than the wedges and the stone is settled in the mortar until it rests upon the wedges; but the point is that it does rest upon the wedges (else they are useless) and may or may not rest upon the mortar throughout the whole bed. In fact, subsequent slight adjustments of the position of the stone, or other adjacent operations, often operate to break the intimate contact which should exist between stone and mortar. Again, the wedges must be removed. As they may enter several inches in from the face, and as the loosening is usually effected by rapping the outer ends with a hammer, thus rotating the wedge upon the bearing point, the mortar thereby displaced may or may not be adequately replaced by the subsequent pointing. Care should be taken that the mortar is so stiff that it will not fall away from the stone for a greater depth than will certainly be reached by the pointing.
Most of the stone showing on either face of a dam is generally of the class of work known as pitch-faced work, in which the face of the stone has had no work bestowed upon it and shows no tool marks. The line bounding the face is at the intersection of two planes, one of which has been cut down to the uniformity demanded by the specified joint; and the other (the face) shows a rough rock face, i.e., a quarry face modified only by certain spalling off which was accomplished by the pitching tool working at the boundary line. Thus the cutting is done to form the four planes bounding the face, and the area of those planes is usually (assuming average length of stones and average ratio of rise to depth) about four times the area of the face.
Obviously the thinner the joint the more work is required to cut the surface to a plane of commensurate uniformity. For whatever thickness of joint required, it is customary to specify a corresponding accuracy of the planes for a certain distance back from the face. This distance should be one and one-half to two times the depth specified for the pointing. A thicker and less uniform joint is allowable at a greater depth from the face. Of course, the stone should not project beyond the plane enough to interfere with the setting of the stone or the adjacent one.