Geotechnical Engineering

DHARM

SHALLOW FOUNDATIONS 611

Strong Stratum

Weak

Strata

Hard stratum

Wall Column Wall

Section

Plan

Fig. 15.5 Raft (Mat) foundation (Flat slab type)

Deep footings

According to Terzaghi, if the depth of a footing is less than or equal
to the width, it may be considered a shallow foundation. Theories of
bearing capacity have been considered for these in Chapter 14. How-
ever, if the depth is more, the footings are considered as deep footings
(Fig.15.6); Meyerhof (1951) developed the theory of bearing capacity
for such footings.

Pile foundations

Pile foundations are intended to transmit structural loads
through zones of poor soil to a depth where the soil has the
desired capacity to transmit the loads. They are somewhat
similar to columns in that loads developed at one level are
transmitted to a lower level; but piles obtain lateral support
from the soil in which they are embedded so that there is no
concern with regard to buckling and, it is in this respect
that they differ from columns. Piles are slender foundation
units which are usually driven into place. They may also be
cast-in-place (Fig. 15.7).

A pile foundation usually consists of a number of piles, which together support a struc-
ture. The piles may be driven or placed vertically or with a batter. More detailed treatment of
this type of foundation is given in Chapter 16.

Df D>bf

b

Fig. 15.6 Deep footing

Fig. 15.7 Pile foundation