Bearing Capacity of Soil

Bearing Capacity of Soil

1. Ultimate bearing capacity It is the maximum gross pressure on soil at the base of footing at which

soil fails in shear.

2. Net bearing capacity It is the maximum net pressure at which soil fails in shear and is equal to

ultimate bearing capacity minus overburden pressure

where D is the depth of the foundation.

3. Net safe bearing capacity

=

4. Gross safe bearing capacity

=

5. Safe settlement pressure It is the net pressure which the soil can carry safely without exceeding

the allowable settlement.

6. Allowable bearing pressure It is the maximum allowable net bearing pressure from the

consideration of shear failure and settlement failure. It is also known as allowable bearing

capacity of soil.

* Minimum depth of foundation: Rankine’s theory is used to determine minimum depth of foundation.

According to it,

Dmin 

= , where q = Intensity of loading

According to I.S. specifications in any case it should not be less than 800 mm and 900 mm in sand

and clay respectively.

* Terzaghi developed the following equation of ultimate shearing capacity of soil for strip footing by

analysing the forces acting on the wedge of the soil mass.

qult = 

where C = Unit cohesion

g = Effective unit weight of soil

Df = Depth of foundation

Nc

, Nq and Ng are the bearing capacity factors, which depend upon the angle of friction, f as given in

Table 12.5.

Table 12.5 Bearing capacity factors of Terzaghi

Ef ect of water Table: If foundation is in contact with the water table, bearing capacity is reduced

due to:

1. Reduction in cohesion

2. Reduction in effective unit weight of soil

Note that reduction in the effective angle of shearing resistance is not much in the above case.

To account for effect of water table Terzaghi’s equation for ultimate bearing capacity may be

modified as

qult = CNc + gDf Nq Rw1 + g B Ng Rw2

where Rw1 = reduction factor for water given in Table 12.5 above the base level of foundation

Rw2 = reduction factor for water table below the base level of the foundation

The reduction factors can be as high as 50% in granular soil.

When the water table is under the base of the footing at a depth equal to the width of footing the

bearing capacity is reduced to three-fourths.

* The bearing capacity equations given above are derived for a strip footing of infinite length, which

is a plain strain case. For a square or circular footing which are three-dimensional case Terzaghi

gave the following approximate equations for bearing capacity per unit area:

1. For square footing

qult = 1.3 CNc + g Df Nq + 0.4 g B Ng

2. For circular footing

qult = 1.3 CNc +g DNq + 0.3 g BNg