FINISHING THE FOUNDATION. There are two cases: (1) when the heads of the piles are not under water; and (2) when they are under water.
1. When the piles are not under water there are again two cases: (a) when a timber grillage is used; and (b) when concrete alone is used.
2. When the piles are sawed off under water, the timber structure which intervenes between the piles and the masonry (in this case called a crib) is put together first, and then sunk into place. The construction is essentially the same as when the piles are not under water, but differs from that case in the manner of getting the timber into its final resting place. The methods of constructing foundations under water, including that by the use of timber cribs, will be dis cussed in Art. 2 of the next chapter.
The timbers which • rest upon the heads of the piles, called caps, are usually about 1 foot square, and are fastened by boring a hole through each and into the head of the pile and driving into the hole a plain rod or bar of iron having a slightly larger cross section than the hole. (A rod so used is called a drift bolt—see 4 795.) Old bridge timbers, timbers from false works, etc.. are frequently used in constructing the grillage and are ordinarily as good for this purpose as new. As many courses may be added as is necessary, each per pendicular to the one below it. The timbers are sometimes laid close together and sometimes with spaces between them. Some times the spaces are left open to cheapen the construction, and some times they are filled with gravel or broken stone to aid in sinking the grillage. The timbers of the top course are laid close together, or a floor of thick boards is added on top to receive the masonry. Of course no timber should be used in a foundation, except where it will always be wet.
Objection is sometimes made to the platform or grillage as a bed for a foundation because, owing to the want of adhesion between wood and mortar, the masonry might slide off from the platform if any unequal settling should take place. However, there is but
slight probability that a foundation will ever fail on account of the masonry's sliding on timber, since ordinarily this could take place only when the horizontal force is nearly half of the downward pres sure.* This could occur only with dams, retaining walls, or bridge abutments, and rarely, if ever, with these. Any possibility of slipping can be prevented also by omitting one or more of the timbers in the top course—the omitted timbers being perpendicular to the direction of the forces tending to produce sliding,—or by building the top of the grillage in the form of steps, or by driving drift bolts into the platform and leaving their upper ends projecting.
Drift Bolts. Drift bolts are rods of steel driven into a hole slightly smaller than the rod, the difference in the diameter of the rod and the hole being called the drift. Drift bolts are frequently used in engineering construction for holding large timbers together, the case mentioned in 4 793 being a very common one. Formerly Square rods were used as drift bolts, the corners being jagged or 3arbed; but universal experience shows that smooth round rods hold much better than either plain or barbed square ones. The ends of drift bolts, are usually rounded a little with a hammer—only enough to remove any burr or sharp edge.
The holding power of drift bolts varies with the amount of the drift, the diameter of the rod, and the kind of timber. According to experiments made under the author's direction,t the average holding power of a 1-inch round rod driven into a Winch hole in pine, per pendicular to the grain, is 501 pounds per linear inch (3 tons per linear foot); and under the same conditions the holding power of oak is 1,300 pounds per linear inch (7.8 tons per linear foot). The holding power of a bolt driven parallel to the grain is almost exactly half as much as when driven perpendicular to the grain. If the holding power of a 1-inch rod in a --inch hole be designated as 1, the holding power in a winch hole is 1.69, in a 44-inch hole 2.13, and in a -4-inch hole 1.09. The holding power decreases very rapidly as the bolt is withdrawn.