Purpose of Footings:
If a man stands on soft mud, marshy ground, or quicksand, he sinks into a greater or less depth, proportional to his weight. If, however, he stands on a plank or a wooden platform, or on a post or posts driven through the mud or marsh to firmer ground, his weight is distributed over a larger area in the first case and carried down to a better foundation in the second.

The same thing is true of the footings of buildings. By spreading the load, or weight of the structure, over a larger area or bearing surface, the weight of the building is more evenly distributed, and the likelihood of a settlement, due to compression of the ground, is greatly diminished. For this reason, the higher and heavier the building is to be, the wider and deeper the supports of footings for the foundation must be and if extremely soft or yielding ground is encountered, piling should be resorted to in order to carry the weight of the building to a more solid base.

Footings may be of iron, timber or large, flat building stones laid directly on the ground or on a bed of concrete, or they may be concrete alone or with reinforcement, or of concrete and stepped-up brickwork. Where piling is used, heavy capping timbers are often placed on the heads of the piles, with either stone or concrete footings resting on them; or large footing stones may be laid directly on the piles
Special Footings:
Footings on Rock and Gravel. In placing foundation footings on a rock, it is sometimes found that some portions of the footings will rest on the rock, and others, owing to the diversified character of the surface, will rest on clay, sand, or gravel. The settlement of the foundation walls - and as a necessary consequence, that of the whole building - will then be uneven, as the walls resting on the rock will not settle, while those resting on the sand, gravel, or clay, by compressing the material on which they are carried will settle.

Fig. 58 illustrates the method employed to obtain equal settlement. In a are shown the rock and gravel before leveling or excavating, the clay or sand being shown at a and the rock at b. It is customary to remove the rock to a certain level, as shown in b b. The softer solid a is the removed and leveled off, as at c c, and a bed of concrete about 3 feet thick, as shown at d, is then put down. This concrete is brought to the level of the rock, as at b b, and on this base, the brick or stone foundation wall e is built.

In erecting footings on solid rock, it is not considered necessary to cut the footing bed level over the entire surface of the rock, nor even to cut a series of horizontal surfaces resembling steps, as is frequently done in softer soils, but it is necessary to roughen the surface of the rock so as to prevent the footing from the slipping on its foundation. After this is done, concrete may be put in to bring the foundation to its proper level. If the structure is to be only three of four stories in height, stone or brick may be used instead of concrete, but a concrete base is usually preferable.

Footing on Sloping Ground:

Footing courses built on slopes - especially of clay - are always liable to slide. This tendency to slide, however, may be overcome by cutting horizontal steps in the slope, as shown in Fig 59, where the slope e f is stepped off, as shown at a, in order that the footings b may have a horizontal bearing. These footings may be either of stone or of concrete, but if the former material is used, great care must be exercised to secure a perfect bond at the stepping places, and the foundations should be laid in as long sections as possible.