Geotechnical Engineering

DHARM

BEARING CAPACITY 571

Df

zq

zg

b

1.0

9.0

8.0

0.7

0.6

0.5

0 0.2 0.4 0.6 0.8 1.0

z/D orz/bqf g

RorRq

g

b

(a) Location of water table (b) Reduction factors R and Rq g

Fig. 14.14 Effect of water table on bearing capacity
Note. For zq > Df (the water table is below the base of the footing), Rq is limited to 1.0. For 0 ≤ zq
≤ Df (the water table is above the base of the footing), Rγ is limited to 0.5. for zq > (Df + b) or zγ > b, Rq as
well as Rγ are limited to 1.0. For zq = 0, Rq as well as Rγ are limited to 0.5.

14.7 Safe Bearing Capacity

Safe bearing capacity, as already defined in Sec. 14.1, is the maximum pressure intensity that
the soil will safely transmit without the risk of shear failure irrespective of settlement that
may occur.

The value of the safe bearing capacity is determined by applying a suitable factor of
safety to the ultimate bearing capacity, determined by any one of the methods available. The
ultimate value is composed of three terms–one due to the cohesion of the soil, another due to
the weight of the soil in the elastic zone, and a third due to the depth of foundation or sur-
charge. The contribution due to this third term is a factor Nq times the surcharge or original
overburden pressure, γDf.

Since the soil has already been subjected to this original overburden pressure, there is
no need to apply a factor of safety greater than unity to this component of the ultimate bearing
capacity.

It has also been seen that the gross bearing capacity minus the original overburden
pressure or surcharge pressure at the level of the base of the foundation is called the net
bearing capacity (Sec. 14.1).

Thus, the following procedure may be specified for arriving at the safe bearing capacity:

(i) The surcharge pressure, γDf, is deducted from the gross ultimate bearing capacity

qult, to give the net ultimate bearing capacity, qnet ult.

(ii) The net ultimate bearing capacity is divided by the chosen factor of safety η, to give

the net safe bearing capacity, qns.

(iii) Finally, the surcharge pressure is added to the net safe bearing capacity, to give the

safe bearing capacity, qs, against shear failure.