TIMBER IN FOUNDATIONS. The free use of timber in founda tions is the chief difference between American and European methods of founding masonry in deep water. The consideration that led to the introduction of timber in foundations was its cheapness. Many of the more important bridges built before 1870 rest upon crib work of round logs notched at their intersection and secured by drift-bolts; but at present, cribs are always built of squared timber. Until about the beginning of this century, there was not much difference between the cost of timber and masonry in foundations, the principal advantage in the use of timber being the facility with which it was put into place; but with the rapidly decreasing supply of timber and its consequent rapid increase in cost, and with the marked decrease in the cost of concrete, it is probable that in this country in the future not much timber will be used in foundations. Soft wood or timber which in the air has comparatively little durability, is equally as good for this purpose as the hard woods. It has been conclusively proved that any kind of timber will last practically forever, if completely immersed in water.
When a pier is to be founded in a sluggish stream, it is necessary only to excavate a hole in the bed of the stream, in which the crib (or the bottom of the caisson) may rest. The excavation is usually made with a dredge, any form of which can be employed. The dipper dredge is the best, but the clam shell or the endless chain and bucket dredge is sometimes used. If the bottom is sand, mud, or silt, the soil may be removed (1) by pumping it with the water through an ordinary centrifugal pump 823),—the suction hose of which is kept in contact with, or even a little below, the bottom,—or (2) by the Eads mud-pump (0 877). With either of these methods of excavating, a simple frame or light coffer-dam may be sunk to keep part of the loose soil from running into the excavation.
If the stream is shallow, the current swift, and the bottom soft, the site may be excavated or scoured out by the river itself. To make the current scour, construct two temporary wing-dams, which diverge up-stream from the site of the proposed pier. The wings can be made by driving stout stakes or small piles into the bed of the stream, and placing solid panels—made by nailing ordinary boards to light uprights—against the piles with their lower edge on the bottom. The wings concentrate the current at the location of
the pier, increase its velocity, and cause it to scour out the bed of the stream. This process requires a little time, usually one to three days, but the cost of construction and operation is comparatively slight.
When the water is too deep for the last method, it is sometimes possible to suspend the caisson a little above the bed of the stream, in which case the current will remove the sand and silt from under it. At the bridge over the Mississippi at Quincy, Ill., a hole 10 feet deep was thus scoured out. However, if the water is already heavily charged with sediment, it may drop the sediment on striking the crib and thus fill up instead of scour out. Notwithstanding the hole is liable to be filled up by the gradual action of the current or by a sudden flood before the crib has been placed in its final position, this method is frequently more expeditious and less expensive than using a coffer-dam.
If the crib should not rest squarely upon the bottom, it can sometimes be brought down with a water jet 0 757-58) in the hands of a diver. However, the engineer should not employ a diver unless absolutely necessary, as the expense is very great.
If the soft soil extends to a considerable depth, or if the necessary spread of foundation can not be obtained without an un desirable obstruction of the channel, or if the bottom is likely to scour, then piles may be driven, upon which the crib or caisson may finally rest. Before the introduction of the compressed-air process, this was a very common method of founding bridge piers in our Western rivers; and it is still frequently employed for small piers. The method of driving and sawing off the piles has already been described—see Chapter XV.
The mud over and around the heads of the piles may be sucked off with a pump, or it may be scoured out by the current ($ 840). The attempt is sometimes made to increase the bearing power of the foundation by filling in between the heads of the piles with broken stone; but this is not good practice as the stone does but little good, is difficult to place, and is likely to get on top of the piles and prevent the crib from coming to a proper bearing.