in architecture, is that part of a building which is under ground. See Building.

The foundation is properly so much of the masonry as reaches as high as the surface of the ground, and ought always to be proportioned to the load or weight of the building that it is to bear. Sometimes the foundation is massive, and continued under the whole building, as in the antique arches and aqueducts, and some amphitheatres; but it is more usually in spaces or intervals, either to avoid expense, or because the vacuities are at too great a distance, in which latter case they make use of insulated pillars bound together by arches.

Palladio allows a sixth part of the height of the whole building for the hollowing or under-digging; unless there be cellars under ground, in which case he would have it somewhat lower; and as to thickness, double the width of the wall is no bad rule.

Foundation of Bridges, is laid after different manners. The first is by inclosing all round the space of ground you would build upon, by dams made with piles set deep in the ground in double rows, well strengthened and bound together with cross pieces and cords, and filling the vacant spaces between them with chalk or other earthy matter. This being done, the water must be emptied out, and the foundation dug according to the quality of the ground, driving down piles, if it be necessary, upon which the walls of the foundation must be laid. But this method is only practicable in building on such rivers, where the water is neither very rapid, nor very deep. The second is done by laying the foundation on grate-work, rafts of stout oak well bound together, and made fast at the surface of the water with cables or machines, and building upon them large quarters of stone, cramped together, and joined with good mortar, or cement, and afterwards letting them descend softly by these cables and machines perpendicularly to the bottom of the water. This was the method practised in laying the foundation of Westminster Bridge, the grating being made of the bottom of a frame called by the French Caisson, the sides of which were so contrived, that they might be taken off, after a pier was finished. The third is by drawing off all, or the greatest part of the water of the river into some other place.