Bearing Capacity of Soil in Foundation

Bearing Capacity of Soil in Foundation

Foundations are the sub-structural elements which are in direct contact with the ground and transmits the load of the superstructure to the earth in such a way that neither supporting soil failed in shear nor in excessive settlement.

Broadly, foundations may be grouped as shallow or deep foundation depending on the depth of installation of foundation.

Modes of Failure of a Structure

A structure when loaded may fail in following two ways :

1. Failure due to shear : When the load supporting power of soil is lesser than structural load at the foundation level.

2. Failure due to excessive settlement : When the settlement in the soil exceed the tolerance limit. In particular, differential settlements should not cause any unacceptable damage which may interfere with the function of the structure.

Bearing Capacity

The load carrying capacity of foundation soil or rock which enables it to bear and transmit loads from a structure is known as bearing capacity.

Factors Affecting Bearing Capacity

1. Nature of soil and its physical and Engineering properties.

2. Nature of foundation and other details such as the size, shape, depth and rigidity of the structure.

3. Location of the ground water table relative to the foundation level.

4. The total and differential settlements that the structure can with stand without fractional failure.

5. Initial stresses condition of the soil due to prehistory or due to the existing structure near proposed foundation.

6. Nature and type of loading i.e. centric or eccentric.

7. Mode of shear failure i.e. General, Local or Punching shear failure.

Allowable Bearing Pressure

A Foundation is said to be safe when it satisfied two conditions :

1. There should be adequate factor of safety against shear failure.

2. The settlement should be within permissible limit i.e. it should not be great enough to damage the structure.

Out of above two conditions, which ever gives a lower value of load intensity is referred to as the allowable bearing pressure.

i) For Granular Soils : For granular soils, generally governing criteria is safe settlement pressure except in narrow footings on loose sand.

ii) For Cohesive Soils : Allowable bearing pressure for cohesive soils is generally governed by shear criteria i.e. net safe bearing capacity except soil, normally consolidated clays. 

[ NOTE : Generally a factor of safety of 3 against shear also ensure safety against settlement. ]

Few Important Definitions

1. Gross pressure intensity : It is the total pressure at the base of foundation due to the weight of superstructure, self weight of footing and weight of the earth fill.

[NOTE : The gross pressure intensity at the time of failure is nothing but the ultimate bearing capacity of the soil. In general, gross pressure intensity may be greater, equal or smaller than bearing capacity. ]

2. Net pressure intensity : It is that part of gross pressure at the base of footing which is in excess to initial effective overburden pressure.

3. Ultimate bearing capacity : Ultimate bearing capacity is defined as the minimum gross pressure at the base of the foundation at which the soil just fails in shear.

OR

The maximum gross pressure, which a foundation can with stand without the occurrence of shear failure.

4. Net ultimate bearing capacity : It is that maximum net pressure which can be applied at the base of the foundation without shear failure.

OR

It is the minimum net pressure at which the soil just fails in shear.

5. Net safe bearing capacity : It is that net pressure which can be applied safely at the base of footing without the risk of shear failure.

6. Safe bearing capacity : It is that gross pressure at the base of footing which can be applied safely without risk of shear failure.

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