Construction of Thl D

The crib is sometimes made of squared timbers laid one on top of the other, and drift-bolted together. The timbers must be securely fastened together vertically, or the buoyancy of the water will lift off the upper courses. The joints between the timbers may be made water-tight by placing cement grout between them during the construction of the crib, or by driving oakum into the joints after the crib is built. A crib made in this way in combination with sheet piles can be used in water 10 or 12 feet deep. The principal advantage of this form of construction is that there are no braces across the dam to interfere with the excavation and the laying of the masonry.

Either of the above forms of crib coffer-dam can be used upon a moderately level rock bottom, by driving wood sheet piles outside of the crib until the point is bruised enough to make a fairly good fit against the rock and then depositing a bank of clay against the bottom of the piles.

Movable

A movable or floating crib coffer-lam has been used, and was considered to have been a success.* It consisted of a crib with double walls built of squared timbers; and had several pockets into which stone could be thrown to sink it, and also several water-tight compartments to facilitate its movement from place to place. It was made in halves to allow of its removal from around the finished pier. The halves were joined together by fitting timbers between the projecting courses of the crib, and then passing long bolts vertically through the several courses.

Double

Sometimes two coffer-dams are em ployed, one inside of the other, the outer one being used to keep out the water, and the inner one to keep the soft material from flowing into the excavation. The outer one may be constructed in any of the ways described above. The inner one is usually a frame-work sheeted with boards, or a crib of squared timbers built log-house fashion with tight joints. The inner crib is sunk (by weighting it with stone) as the excavation proceeds. The advantages of the use of the inner crib are: (1) the coffer-dam is smaller than if the saturated soil were allowed to take its natural slope from the inside of the dam to the bottom of the excavation; (2) the space between the crib and the dam can be kept full of impervious material in case of any trouble with the outside dam; (3) the feet of the sheet piling are always covered, which lessens the danger of undermining or of an inflow of water and mud under the dam; and (4) it also reduces to a minimum the material to be excavated. •

A serious objection to the use of coffer-dams is the difficulty of preventing leakage under or through the dam. One of the simplest devices to prevent leaks is to deposit a bank of gravel around the outside of the dam; then if a vein of water escapes below the sheet piling, the weight of the gravel will crush down and fill the hole before it can enlarge itself enough to do serious damage. If the coffer-dam is made of crib-work, short sheet piles may be driven around the bottom of it; or hay, willows, etc., may be laid around the bottom edge, upon which puddle and stones are deposited; or a broad flap of tarpaulin may be nailed to the lower edge of the crib and spread out loosely on the bottom, upon which stones and puddle are placed. A tarpaulin is frequently used when the bottom is very irregular,—in which case it would cost too much to level off the site of the dam; and it is particularly useful where the bottom is rocky and sheet piles can not be driven.

When the. bed of the river is rock, or rock covered with but a few feet of mud or loose soil, a coffer-dam only sufficiently tight to keep out the mud is constructed. The mud at the bottom of the inclosed area is then dredged out, and a bed of concrete deposited under the water (§ 347). Before the concrete has set, another cofferdam is constructed, inside of the first one, the latter being made water-tight at the bottom by settling it into the concrete or by driving sheet piles into the concrete. However, the better and more usual method is to sink the masonry upon the bed of concrete by the crib and open-caisson process—see Art. 2 of this chapter.

It is nearly impossible to prevent considerable leakage, unless the bottom of the crib rests upon an impervious stratum or the sheet piles are driven into such a stratum. Water will find its way through nearly any depth or distance of gravelly or sandy bottom. Trying to pump a river dry through the sand at the bottom of a coffer-dam is expensive. However, the object of a coffer-dam is not to prevent all infiltration, but only to so reduce it that a moderate amount of pumping will keep the water out of the way. Probably a coffer-dam was never built that did not require considerable pumping; and not infrequently the amount is very great,—so great, in fact, as to make it clear that some other method of constructing the foundation should have been chosen.