Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 609

An isolated footing may be square, circular, or rectangular in shape in plan, depending upon
factors such as the plan shape of the column and constraints of space.

If the footing supports more than one column or wall, it will be a strap footing, combined
footing or a raft foundation.

The common types of spread footings referred to above are shown in Fig. 15.2. Two
miscellaneous types—the monolithic footing, used for watertight basement (also for resisting
uplift), and the grillage foundation, used for heavy loads are also shown.

Footing Footing

Wall

Section

Plan

Section

Plan

Column

Footing

Column

Pedestal

Section

Wall Column

Concrete pad

Joist

(a) Continuous

footing

(b) Isolated

footing

(c) Isolated footing

with pedestal

(d) Monolithic

footing

(e) Grillage

Fig. 15.2 Common types of spread footings

Strap footings

A ‘strap footing’ comprises two or more footings connected by a beam called ‘strap’. This is also
called a ‘cantilever footing’ or ‘pump-handle foundation’. This may be required when the foot-
ing of an exterior column cannot extend into an adjoining private property. Common types of
strap beam arrangements are shown in Fig. 15.3.

Combined footings

A combined footing supports two or more columns in a row when the areas required for individual
footings are such that they come very near each other. They are also preferred in situations of
limited space on one side owing to the existence of the boundary line of private property.

The plan shape of the footing may be rectangular or trapezoidal; the footing will then be
called ‘rectangular combined footing’ or ‘trapezoidal combined footing’, as the case may be.
These are shown in Fig. 15.4.

Raft foundations (Mats)

A raft or mat foundation is a large footing, usually supporting walls as well as several columns
in two or more rows. This is adopted when individual column footings would tend to be too
close or tend to overlap; further, this is considered suitable when differential settlements aris-
ing out of footings on weak soils are to be minimised. A typical mat or raft is shown in Fig. 15.5.