FOUNDATIONS UNDER WATER The class of foundations to be discussed in this chapter could appropriately be called Foundations for Bridge Piers and Abut ments, since these are the principal ones that are laid under water. The principles to be considered apply to foundations in water-bearing soils. In this class of work the chief difficulty is in excluding the water preliminary to the preparation of the bed of the foundation and the construction of the artificial structure. This usually requires great resources and care on'the part of the engineer. Sometimes the preservation of the foundation from the scouring action of the current is an important matter.
Preventing the undermining of the foundation is generally not a matter of much difficulty. In quiet water or in a sluggish stream but little protection is required, in which case it is sufficient to de posit a mass of loose stone, or riprap, around the base of the pier. If there is danger of the riprap's being undermined, the layer must be extended farther from the base, or be made so thick that, if under mined, the stone will fall into the cavity and prevent further damage. A willow mattress sunk by placing stones upon it is an economical and efficient means of protecting a structure against scour. A pier may be protected also by inclosing it with a row of piles and depositing loose rock between the pier and the piles. In minor structures the foundation may be protected by driving sheet piles around it.
If a large quantity of stone be deposited around the base of the pier, the velocity of the current, and consequently its scouring action, will be increased. Such a deposit is, however, an obstruction to navigation, and therefore is seldom permitted. In many cases the only absolute security is in sinking the foundation below the scouring action of the water. The depth necessary to secure this adds to the difficulty of preparing the bed of the foundation.
The principal difficulty in laying a foundation under water consists in excluding the water. If necessary, masonry can be laid under water by divers; but this is very expensive and is rarely resorted to: There are five methods in use for laying foundations under water: (1) the method of excluding the water from the bed of the foundation by the use of a coffer-dam; (2) the method of founding the pier, without excluding the water, by means of a timber crib surmounted by a water-tight box in which the masonry is laid; (3) the method of sinking iron tubes, timber cribs, or masonry wells to a solid sub stratum by excavating inside of them; (4) the method in which the water is excluded by the presence of compressed air; and (5) the method of freezing a wall of earth around the site, inside of which the excavation can be made and the masonry laid. These several methods will be discussed separately in the order named.