Geotechnical Engineering

DHARM

BEARING CAPACITY 569

The steps involved are as follows :

(i) The ratios Df /b and c

γb

are determined.

(ii) The parameter ρ is found from Fig. 14.12, the appropriate chart for the particular

value of Df /b being picked, since the value of φ is known and

c

γb

is also known.

Fρ=

HG

I

KJ

R

b

, whereRis the radius of the rupture surface.

(iii) The bearing capacity factors of Balla Nc, Nq, and Nγ are determined from the charts
of Fig. 14.13, for the particular value of ρ determined in (ii) above and for the known value ofφ.
(iv) The ultimate bearing capacity is determined by using these factors and other relevant
quantities in Balla’s formula.

The limitations are that it should be used when

D

b

f ≤ 1.5 and that it is applicable to

continuous footings only.

14.6 EFFECT OF WATER TABLE ON BEARING CAPACITY

The Terzaghi equation for bearing capacity,

qult = cNc + γDf Nq +

1
2 γ^ b Nγ,
contains the unit weight, γ, and the cohesion, c, of the soil directly, and its angle of shearing
resistance, φ, indirectly, since the bearing capacity factors, Nc, Nq, and Nγ depend upon the
value of φ.

Water in soil is known to affect its unit weight and also the shear parameters c and φ.
When the soil is submerged under water, the effective unit weight γ′ is to be used in the
computation of bearing capacity. Similarly, the effective stress parameters, c′ and φ′, obtained
from an appropriate test in the laboratory, on saturated sample of the soil, are to be used.

However, the effect of water table on the shear parameters of the foundation soil is
usually considered small and hence, ignored. But the effective unit weight γ′ is roughly half
the saturated unit weight; consequently there will be about 50% reduction in the value of the
corresponding term in the bearing capacity formula.
It should be now obvious that the location of the ground water table and its seasonal
fluctuations have a bearing on the capacity of a foundation. There will be no effect or reduction
in the bearing capacity if the water table is located at a sufficient depth below the base of the
footing. In fact, this minimum depth below the base of the footing is set at a value equal to the
width of the footing since the maximum depth of the zone of shear failure below the base is not
expected to exceed this value ordinarily. If the water table is above this level, there will be a
reduction in the bearing capacity. If the water table is at the level of the base of the footing, γ′
is to be used for γ in the third term, which indicates the contribution of the weight of the soil in
the elastic wedge beneath the base of the footing, since the entire wedge is submerged; that is
to say, a reduction factor of 0.5 is to be applied to the third term. For any location of the water
table intermediate between the base of the footing and a depth equal to the width of the foot-
ing below its base, a suitable linear interpolation of the necessary reduction is suggested.