Foundation, the substructure upon which all erections are built. Its nature will depend on the soil underneath, being as a rule much simpler in construction when the soil is good firm ground. Rock should be tested in solidity and thickness, and foundations built square thereon. Gravel or hard clay is good if of sufficient thickness and if compressed with rammers. Loose ground must be strengthened artificially. Concrete is much used as a basis, especially over compressible soils, upon which the footings are laid and the structure raised. Piles may also be driven in to support the foundations; they are specially desirable when the structure adjoins the sea, river, or marsh, and are then planted so as to prevent lateral yielding of the subsoil. Submarine foundations may be prepared by simply dropping large boulders in the sea in suitable positions, but the more systematic method of using caissons or coffer-dams (q.vi) is now much employed for the construction of foundations under water.