Geotechnical Engineering

DHARM

BEARING CAPACITY 563

If the stress-strain relations are intermediate between C 1 and C 2 in Fig. 14.9, the bear-
ing capacity is intermediate between qult and qult′.

Terzaghi’s bearing capacity factors are plotted in Fig. 14.10 for general shear as well as
for local shear. As a general guideline, if failure occurs at less than 5% strain and if density
index is greater than 70%, general shear failure may be assumed, if the strain at failure is 10%
to 20% and if the density index is less than 20%, local shear failure may be assumed, and, for
intermediate situations, linear interpolation of the factors may be employed.

The bearing capacity factors of Terzaghi are tabulated in Table 14.3 for certain values
ofφ:

Table 14.3 Terzaghi’s bearing capacity factors

Terzaghi’s bearing capacity factors

Angle of shearing Nc Nq Nγ

resistance φ°

0 5.7 1.0 0.0

5 7.3 1.6 1.5

10 9.6 2.7 1.2

15 12.9 4.4 2.5

20 17.7 7.4 5.0

25 25.1 12.7 9.7

30 37.2 22.5 19.7

35 57.8 41.4 42.4

40 95.7 81.3 100.4

45 172.3 173.3 297.5

50 347.5 415.1 1153.0

Bearing capacity of shallow circular and square footings

By repeating the reasoning which led to Eq. 14.67, the bearing capacity of circular footings
has been proposed by Terzaghi as follows, from the analysis of experimental data available.

qultc = 1.3 cNc + γDf Nq + 0.3 γ d Nγ ...(Eq. 14.81)

where d = diameter of the circular footing.

The critical load for the footing is given by

Qultc =

πd^2

4

F

HG

I

KJ

. qultc ...(Eq. 14.82)

Similarly, the bearing capacity of a square footing of side b is:

qults= 1.3 cNc + γDfNq + 0.4 γb Nγ ...(Eq. 14.83)

The critical load for the footing is given by

Qults = (b^2 ). qults ...(Eq. 14.84)