76. This term is generally applied to those walls which are below the surface of the ground, and which support the superstructure. Walls whose chief office is to withhold a bank of earth, such as around areas, are called retaining walls.
Foundation walls are built either of stone or brick, the former being the more common. Brick walls for foundations are only suitable in very dry soils, or in the case of party walls, where there is a cellar or basement each side of them.
As the method of building brick foundations is the same as for any brick wall, it will not be described here, but taken up in the chapter on Brickwork.
The best stones for foundations are granites, compact sandstones, slates and blue shale. The less porous the stone the better it will stand the dampness to which it must be subjected. As a rule laminated stones make the best wall, as they split easily and give flat and parallel beds. If the only stone to be had is boulders or field stone, they should be split so as to form good bed joints. Corbel or round stones should never be used for building foundation walls, and for all buildings exceeding three stories in height, block stone or the best qualities of laminated stone should be used.
The usual practice in building foundations is to use the stone just as it is blasted from the quarry, or, if the building is built on a ledge, from the foundation itself, the stone receiving no preparation other than breaking it up with a sledge hammer, and squaring one edge for the face. Too great irregularity and unevenness is overcome by a sparing use of the stone hammer and by varying the thickness of the mortar joint in which the stones are bedded. The strength of the wall, therefore, depends largely upon the quality of the mortar used.
The wall should be leveled off about every 2 feet, so as to form irregular courses, and the horizontal joints should be kept as nearly level as possible.
When block stone is used the stones are generally from 18 inches to 2 feet thick and the full width of the wall. They are commonly roughly squared with the hammer, and but little mortar is used in the wall. Only in a few localities, however, are such stones obtainable at a price that will permit of their use, so that as a rule stone split from a ledge and called "rubble" is the material with which the architect will have to deal.
Aside from the quality of the stone and mortar, the strength of a rubble wall depends upon the manner in which it is bonded or tied together by lapping the stones over each other. About every 4 or 5 feet in each course a bond stone should be used; that is, a stone that will go entirely through the wall, and, by its friction on the stones below, hold them in place. A stone that goes three-fourths of the way through the wall is called a three-quarter bond. It is usually customary to specify that there shall be at least one through stone in every 5 or 10 square feet of the wall, depending upon the character of the stone and nature of the building. Fig. 38 shows a portion of wall built of square or laminated stone, with through bond stone, B B, and three-quarter bond stones at A A. A good three-quarter bond is nearly equal in strength to a through bond, and when the character of the stone will permit of the wall being built largely of flat stone extending two-thirds of the way through the wall, it will not be necessary to use more than one through stone to every 10 square feet of wall. No stone should be built into the face of a wall with a less depth than 6 inches, although stone masons will often set a stone on edge, so as to make a good face and give the appearance of a large stone, when it may be only 3 inches thick. All kinds of stones should always be laid so that their natural bed, or splitting surface, will be horizontal. It is also important that the stones shall break joint longitudinally, as in Fig. 38, and not have several vertical joints over each other, as at A A, Fig. 39. The angles of the foundation should be built up of long stone, laid alternately header and stretcher, as shown in Fig. 40. The largest and best stone should always be put in the corners, as these are usually the weakest part of the wall.