Text 8 footings and foundations

The more common types of foundation structure are: isolated spread footings (one to each column); combined footings (one to two columns); and mat, or raft, foundations (all columns resting upon a heavy slab).

These forms may sometimes rest directly on the bearing soil or they may rest on the heads of piling. Sometimes the columns are set on large cylinders of plain concrete, from 4or 8 ft. in diameter and extend to rock or other hard stratum at depth. This form is called “pier” or “caisson” foundation.

Spread footings may be used under either walls or columns and may be of timber, steel, plain masonry, or reinforced concrete.

The problem is to enlarge the base sufficiently to secure bearing area to carry loads.

Wall Footings – the footings in this case act as a cantilever beam with the load acting upward and the support at the middle. It is commonly assumed that the bearing is uniformly distributed over the bearing area, although the bearing may be somewhat greater at the middle that at the edges.

Continuous footings under columns – when footing are built continuously under columns, the footing takes a beam carrying concentrated loads at column points acting downwards and distributed load acting upward. The distribution of the pressure cannot be calculated directly, but the problem is to proportion the width of the footing stab so that the slab as a continuous beam under a distributed load upward will have upward reactions at column points not less than the column loadings.

Floating foundations is a term applied to practically monolithic footings built under an entire structure which have been used widely for construction on soils of more or less plastic nature. Floating foundations have been constructed as slabs or mats, a combination of beams and slabs, inverted barrel arches and slabs, and inverted groined arches.

The obvious advantage of floating foundations is that if uneven settlement occurs, serious injury to the structure does not result.

Pile foundations are used in the areas where the other kinds of foundation cannot be constructed. By driving bearing piles, the support of structure is transferred in an irregular manner to the earth stratum. The proportions of the load carried down by the piles and by the intervening soil depend on the nature of the soil – the stiffer the soil, the greater proportion is carried thereby.

Timber piles. - Timber piles have been used almost exclusively in the past in foundation work; but with the growing scarcity of timber, and the development of concrete, reinforced concrete piles are coming to be extensively used.

Pile driving. – Pile may be driven by means of a pile driver or a water jet. Pile drivers may be classed as drop hammers and steam hammers.

The steam pile driver has many advantages over the drop hammer. Drop hammers operate at about 5 or 6 blows per minute, while steam hammers strike about 60blows per minute for a single-acting and 120 per minute for a double-acting. After piles are driven, they are sawed off at a given elevation, and the foundation footings are placed directly thereon, the heads of the piles projecting into the concrete from 6 to 12 in.

The engineer in charge of pile driving should carefully inspect the piles previous to driving so that they meet the specifications and, after they are driven, to discover injured piles.

Some other kinds of foundation work. – Pier foundations are used where a stratum of high supporting capacity is under soil strata.

Cofferdams are enclosures constructed to exclude water from the area of operation during construction.

Caissons – the word “caisson” is derived from the French word meaning “case”.

As applied to engineering construction, a caisson may be defined as a large watertight box used to exclude water or other fluid and semi fluid materials during excavating of foundations and the construction of substructures and ultimately becoming an integral part of the structure. This box or caisson, may be “open”, without a bottom, the water being allowed to rise naturally inside; or may have a bottom and be carried down by building the masonry pier inside. If the box is inverted, the water may be excluded by air pressure, and the box becomes a pneumatic caisson consists of a box with cross walls. Sometimes the caisson is constructed of reinforced concrete or a steel cylinder.