Geotechnical Engineering

DHARM

564 GEOTECHNICAL ENGINEERING

For a continuous footing of width b, it is already seen that,
qult = cNc + γDf Nq + 0.5 γ b Nγ
Thus, the bearing capacity of a circular footing of diameter equal to the width of a con-
tinuous footing is 1.3 times that of the continuous footing, or at least nearly so, if the footings
are founded in a purely cohesive soil (φ = 0); the bearing capacity of a square footing of side
equal to the width of a continuous footing also bears a similar relation to that of the continuous
footing under similar conditions just cited.
Further, the corresponding ratios are 0.6 and 0.8 in the case of circular footing and
square footing, respectively, when the footings are founded in a purely cohesionless soil (c= 0).
The ‘‘benefit’’ of surcharge or depth of foundation, as it is called, is only marginal in the
case of footings on purely cohesive soils, since Nq is just equal to 1; in fact, the increase in
bearing capacity due to depth is just equal to the surcharge γDf and it is only the difference
between the gross and net values of bearing capacities. However, this benefit or increase in
bearing capacity is significant in the case of cohesionless soils or c – φ soils (φ > 0), especially
when the angle of shearing resistance and hence Nq-value are very high as for dense sands.
While the bearing capacity of a footing in pure sand may be increased either by increas-
ing the width or depth below ground at a given density index, the value may be increased by
densification. However, these avenues are not useful in the case of footings in pure clays.

The differences in the bearing capacity values arising out of differences in the size of the
footing and in the shape of the footing are termed ‘size effects’ and ‘shape-effects’, respectively.

14.5.6Meyerhof’s Method

The important difference between Terzaghi’s and Meyerhof’s approaches is that the latter

considers the shearing resistance of the soil above the base of the foundation, while the former

ignores it. Thus, Meyerhof allows the failure zones to extend up to the ground surface (Meyerhof,

1951). The typical failure surface assumed by Meyerhof is shown in Fig. 14.11.

The significant zones are: Zone I ABC ... elastic

Zone II BCD ... radial shear

Zone III BDEF ... mixed shear

Logarithmic spiral

b

Df

AB

FE

I

Elastic zone

II

D

(90° – )f

C Radial shear zone

III

Mixed shear zone

Fig. 14.11 Meyerhof’s method for the bearing capacity of shallow foundation