CONSTRUCTION OF TEE OBIB. The crib is a timber structure below the caisson, which transmits the pressure to the bed of the foundation. A crib is essentially a grillage (see § 705 and § 793) which, instead of being built in place, is first constructed and then sunk to its final resting place in a single mass. A crib is usually thicker, i.e., deeper, than the grillage. If the pressure is great, the crib is built of successive courses of squared timbers in contact; but if the pressure is small, it is built more or less open. In either case, if the crib is to rest upon a soft bottom, a few of the lower courses are built open so that the higher portions of the bed may be squeezed into these cells, and thus allow the crib to come to an even bearing. If the crib is to rest upon an uneven rock bottom, the site is first leveled up by throwing in broken stone, although this is a poor method. If the bottom is rough or sloping, the lower courses of the crib are sometimes made to conform to the bottom as nearly as pos sible, as determined from soundings; but this method requires care and judgment to prevent the crib from sliding off from the inclined bed, and should be used with great caution, if at all.
The crib is usually built afloat. Owing to the buoyancy of the water, about one third of a crib made wholly of timber would pro ject above the water, and would require an inconveniently large weight to sink it; therefore, it is best to incorporate considerable stone in the crib-work. If the crib is more or less open, this is done by putting a floor into some of the open spaces or pockets, which are then filled with stone. If the crib is to be solid, about every third timber is omitted and the space filled with broken stone.
The timbers of each course should be securely drift-bolted (§ 795) to those of the course below to prevent the buoyancy of the upper portion from pulling the crib apart, and also to prevent any possi bility of the upper part's sliding on the lower.