Geotechnical Engineering

DHARM

630 GEOTECHNICAL ENGINEERING

Footings must be constructed only on slopes which are stable. The stability of the slope
itself may be endangered by the construction of footings.

15.4.9Construction of Spread Footings

Footings are relatively simple to construct. The inspection of subsoil conditions, the realtive
depth of footing and dewatering of excavation when necessary require special attention. De-
pending on the nature of soil, bracings may be required for this sides.

The average soil condition based on the soil boring results must be ascertained. As the
foundation is constructed, the actual soil conditions encountered must be checked with respect
to the boring analysis.

Adjacent footings should be constructed such that their difference of levels, if any, does
not introduce undue additional stress at the lower footings and also that the lower footing does
not affect the stability of the upper one. This difficulty is generally avoided by keeping the
difference in the elevations of footings not greater than one-half the clear distance between
the footings. It is always a good practice to construct the lower footings first, so that the eleva-
tion of the upper footing may be adjusted if necessary.

The excavation should be kept dry during the construction period because free water
gives rise to many difficulties. The soil conditions under water cannot be easily inspected. In
clay soils, free water tends to soften the upper portion of the soil and cause settlements. Plac-
ing concrete under water also poses problems. For these reasons, it is considered necessary to
dewater the excavations where necessary.

For certain recommendation in this regard, the reader is referred to ‘‘IS: 1080-1980
Code of Practice for design and construction of simple spread foundations (First Revision)’’.

15 .5 Strap Footings

A relatively common type of combined construction is the ‘strap footing’ or ‘cantilever footing’,
as has already been seen in Sec. 15.2. This is usually employed when the footing of an exterior
column cannot be allowed to extend into adjoining private property. Straps may be arranged
in a variety of ways (Fig. 15.2), and the choice depends upon the specific conditions of each
case.

15.5.1 The Cantilever Principle

The cantilever principle is largely concealed in actual footings of this type. This principle is
illustrated in Fig. 15.24.

Pe Pi

b bi

Fig. 15.24 Cantilever principle of strap footing