A crib is essentially a framework (called a birdcage by the English) which is made of timber, and which is filled with stone to weight it down. Such a construction is used only when the entire timber work will be perpetually under water. The timber framework must, of course, be designed so that it will safely support the entire weight of the structure placed upon it. The use of such a crib necessarily implies that the subsoil on which the crib is to rest is sufficiently dense and firm so that it will withstand the pressure of the crib and its load without perceptible yielding. It is also necessary for the subsoil to be leveled off so that the crib itself shall not only be level but shall also be so uniformly supported that it is not subjected to transverse stresses which might cripple it. This is sometimes done by dredging the site until the subsoil is level and sufficiently firm. Some of this dredging may be avoided through leveling up low spots by depositing loose stones which will imbed themselves in the soil and furnish a fairly firm subsoil. Although such methods may be tolerated when the maximum unit-loading is not great (as for a breakwater or a wharf), it is seldom that a satisfactory foundation can be thus obtained for heavy bridge piers and similar structures.