Geotechnical Engineering

DHARM

572 GEOTECHNICAL ENGINEERING

That is to say,

qnet ult = qult – Df ...(Eq. 14.104)

qns =

qnet ult

η

...(Eq. 14.105)

qs = qns + γDf =

qnet ult

η

+ γDf

or qs =

()qDult−γf

η

+ γDf ...(Eq. 14.106)

or qs =

qult

η

+

γη

η

Df()− 1

...(Eq. 14.107)

An expression for qnet ult is sometimes written by combining γDf terms, somewhat as

follows:

qnet ult = qult – γDf = cNc + γDf(Nq – 1) +

1

2

γb Nγ ...(Eq. 14.108)

However, the procedure involved in Eqs. 14.104 to 14.106 is advocated to avoid possible

confusion.

In the case of a surface footing founded on a purely cohesive soil, (Df = 0, φ = 0°, Nq = 1,

and Nγ = 0), the safe bearing capacity is obviously cN. c q

ηη

=

F

HG

I

KJ

ult itself; it is qult

η

for a surface

footing founded on any soil, for that matter.
The factor of safety generally chosen is 3. However, it may sometimes be as high as 5.
Apparently this may appear to be high, but is not so high when one remembers the innumer-
able factors and imponderables involved in the determination of the bearing capacity. It can be
easily demonstrated that, even with a small variation of φ, the value of qult may change consid-
erably. This justifies the statement made above.

The allowable bearing pressure (Sec. 14.1) is the smaller of the two values—the safe
bearing capacity to avoid the risk of shear failure and the maximum net allowable pressure
that will produce settlements of the structure within tolerable limits.

More of this will be seen in Sec. 14.13 since the bearing capacity of sands is governed
invariably by settlement rather than shear failure.

14.8 Foundation Settlements

Settlement—total settlement and differential settlement of foundations and consequently of

the structures above the foundations—and its determination has been presented in detail in

Chapter 11 on ‘‘Settlement Analysis’’.

Some of the important aspects related to the bearing capacity of footings will be summa-

rised in the subsections to follow.

14.8.1Source of Settlement

Foundation settlements may be caused due to some or a combination of the following factors:

(i) Elastic compression of the foundation and the underlying soil, giving rise to what is

known as ‘immediate’, ‘contact’, ‘initial’, or ‘distortion’ settlement,