Types and Suitability of Shallow Foundations

Shallow foundations are all suitable for building and are sub-divided into a number of types according to their size, shape and general configuration. They are discussed below.

1. Spread Footings

These footings are the most common of all types of footings with minimum cost and complexity of construction (Fig. 15.1(a)). It necessarily provides the function of distributing the column load over a wide area taking care of the strength and deformation characteristics of the soil. These types of footings are also known as pad footings, isolated footings and square or rectangular footings (for length of footing, L, and width of footing, B, ratio less than 5).

Figure 15.1 Types of shallow foundations

2. Combined Footing

These footings are formed by combing two or more equally or unequally loaded columns into one footing. This arrangement averages out and provides a more or less uniform load distribution in the supporting soil. Further distribution prevents variation of settlement along the footing. These footings are usually rectangular in shape. It may be modified to a trapezoidal shape so as to accommodate unequal column loadings or column close to property line. It may be provided with a strap to accommodate wide column spacing or columns close to property line (Fig. 15.1(b)).

3. Continuous Footing

These footings carry closely spaced columns or a continuous wall such that the load distribution is uniform and load intensity is low on the supporting soil (Fig. 15.1(c)). These footings are also named as strip footings or wall footings (for L/B ratio greater than 5).

4. Mat or Raft Foundation

These are characterised by the feature that columns frame into the footing in two directions. Any number of columns can be accommodated with as low as four columns (Fig. 15.1(d)). In the majority of the cases, mat foundations are used where the soil has low bearing capacity. By combining all individual footings into one large mat, the unit pressure in the sub-soil is reduced.

Since the bearing capacity increases with increasing depth and width of the foundation and the settlement decreases with the increasing depth of foundation, the advantage of mat foundation is two-fold. Mat foundation is also preferred when the total area of the footings exceeds 50% of the total plinth area.