When guide piles cannot be driven to firm bearing, timber frames are sometimes used to act as guides and support the sheet-piling against lateral pressure. These frames may be built upon the ground, floated to the site and sunk into position. The sheet-piles are then driven around the frames so as to inclose it.
Interlocking steel piling is often employed for single wall work because of its greater strength and tightness. Timber piling for such use should he tongued and grooved. Wakefield piling has most frequently been used.
Some leakage is always to be expected in cofferdams, and in many instances special precautions are necessary to exclude water. The possibility of meeting difficulty in preventing leakage is the prin cipal objection to this method of construction. Banking clay against the outside of the cofferdam is a common expedient to prevent leak age through or immediately under the dam. When it is feared that channels may open under the piling, gravel may be deposited around the base of the dam to close such incipient openings. Tarpaulins are often employed to cover the outside of the dam, or spread out upon the bottom outside the base of the dam and weighted with gravel. When the bottom is rock, it is sometimes necessary to cover the whole area inside the cofferdam with a layer of concrete to prevent inflow of water through seams in the rock.
199. Crib Cofferdams.—Timber cribs built on land and floated into position are frequently used as cofferdams, and for shallow depths, these may he made of a framework of timber with a covering of planks upon the outside. Usually the crib is formed of two walls
made of squared timbers laid on top of each other, tied together, and braced with framework, and is sunk by loading with gravel or earth, and sometimes filled with puddle to increase its watertightness. The crib itself may be made practically water-tight, so that leakage is restricted to the space below the crib. In using this method there are no braces across the space in which the foundation is to be placed.
The bottom should be leveled before sinking the crib, or when on bed rock, the bottom of the crib may be made approximately to fit the surface of the rock. Sheet piling may be driven around the outside of the crib to prevent leakage under the crib when the crib does not lie upon the rock. Tarpaulins fastened to the crib near the bottom are frequently used to prevent leakage under the crib. A deposit of puddle around the base of the crib is generally sufficient to seal the bottom against excessive leakage in ordinary work, but a layer of concrete over the rock bottom is sometimes necessary.
Cribs are sometimes made so that they may be removed and re peatedly used. These have sometimes been used for bridge piers, being made in two parts joining together on the short sides so that they may be taken from around the foundation after it is constructed. Watertight compartments are provided which may be pumped out when it is desired to float the cribs. Sometimes sheet-piling is used around these cribs, which may be withdrawn before raising them.