Founding Masonry mode of founding the dam-body will depend on the character of the river-bottom. In Figures 14 and r5 the dam is founded upon piles, the spaces between the tops of which are filled in with masonry, forming a solid platform. The utmost care must be be stowed upon the finish of the exterior covering of the masonry of the dam, for, should a single stone become dislodged, the injury done by the impact of the water upon the exposed part is generally greatly increased. Abut ments, usually of masonry, unite the end of the dam with the banks. They should be high enough and carried far enough inland to prevent flood-water from overtopping them or finding its wav around their extremi ties, as else the stnicture will be liable to undermining and destruction.
Timber timber dams, the front or face may be either verti cal or sloping-, as above described. The body of such structures is formed of one or several walls of sheet-piling built across the stream and supported by lines of piles. The structure is made watertight by filling in the space between the walls of sheet-piling with earth or gravel. The whole is then covered with a platform of planks, which forms the surface of the dam. In the rear a backing of gravel, gravelly earth, or stone is disposed sloping gradually down to the stream-bed. As in the case of the above-described masonry dams, the ends of the dam must be connected with the shore in the most substantial manner, to avoid the danger of undermining. In building works of this kind, either the stream may be temporarily turned from its course while the work is being carried on or the structure may be erected in sections, the working- ground of each being protected from the entrance of the water by an enclosing- coffer-dam, as in the case of bridge pier fonndations; or other expedients well known to engineers (see p. 297) may be resorted to, as the circumstances of each case may indicate.
Thither DaMS in the United practice in the construction of tiinber dams in the United States is sufficiently interesting to warrant some attention to the subject. The following data are condensed from Trautwine (pi. 56, figs. i—s): " In the United States they (wooden dams) are usually of crib-work, of either rough round logs with the bark on, or of hewn timber—in either case, about a foot through. These timbers are merely laid on top of each other, forming in plan a series of rectangles with sides of about 7 to 12 feet. They are not notched together, but are simply bolted by one-inch square bolts (often ragged or jagged), about 2 to 2 Y, feet long, through two timbers at every intersection. These are not found to rust or wear seriously, even when exposed to a current Round logs are flattened where they lie upon each other. Experience shows that firmer but more expensive connections are unnecessary. The cribs are usually, but not always, filled with rough stone. triang,ular dams, disposed as in Figures to 3 (pi. 56), this stone filling is not so essential as in other forms, because the weight of the water and of the gravel backing- tends to hold the dam down on its base. Still, even in these, when the lower timbers are not bolted to a rock-bottoin or otherwise secured in place, some stone may be necessary to prevent the timbers from floating away while the work is unfinished and the gravel not yet deposited behind it. On rock the low
est timbers are often bolted to it, to prevent them from floating away dur ing construction; and when the water is some feet deep, this requires coffer dams. Or the cribs may be built at first only a few feet high, then floated into place, and sunk by loading them with stone, for the reception of which a rough platform or flooring will be required in the cribs a little above their lowest timbers The water may flow through the open crib-work as the building higher goes on, attention being paid to adding stone enough to prevent it floating away if a freshet should happen. Or cribs shown in plan in Figure 4, loaded with stones, may be sunk, leaving one or more intervals between them for the free escape of the water, and these openings may bt filially closed by floating into them closing-cribs like Figure 4n.
" The workmanship of a dam in deep water can, of course, be much bet ter executed in coffer-dams than by merely sinking cribs. The joints can be made tighter, the stone filling better packed, the sheet-piling more closely fitted, etc.
" When a very- uneven rock-bottom in deep water, or the introduction of sluices in the dam, or any other consideration, makes it expedient to build dams within coffer-dams, both should be carried on in seclions, so as to leave parts of the channel-way open for the escape of the water. Com mencing at one or both shores, the first section of the coffer-dam may reach say quarter-way or more across the stream. In the section of the dam itself built within this enclosing coffer-dam, ample sluices should be left for the water to flow through when we come to build the closing section of the coffer-dam. When the dam has been finished, these sluices may be closed by drop-timbers (timbers ready prepared for closing an opening through which water is flowing, and suddenly dropped into place by means of grooves or guides of some kind for retaining them in position). Before removing one section of the coffer-dam, the outer end of the enclosed dam itself must be firmly finished in such a manner as to constitute a part of the inner end of the next section of coffer-dam In some cases of shallow water mere mounds of earth may answer for coffer-dams, or rough stone mounds backed with earth or gravel." The illustrations (I./. 56) shown in connection with what has preceded, according to the same authority, are " sections drawn to a scale of existing dams in Pennsylvania that have stood successfully the force of heavy fresh ets for a long series of years." Thus, Figure r is a dam on the Schuyl kill Navigation; Figure 2 is a canal-feeder dam on the Juniata; Figure 6 is also on the Schuylkill Navigation; Figure S exhibits the form used on the Monongahela Slack-watcr Navigation. A usual precaution to pro tect the front of the dam against the impact of the overfall water in time of freshet is an apron of round logs or planks like that shown in Figures and 5. A modification of this is seen in Figure 7, which shows a Very effectual mode of breaking- the force of the falling water in the case of a soft bottom by building the front of the dam in the form of a series of steps, with an apron extending out for some distance below.