Last edited 15 May 2019

Bearing capacity

The term 'bearing capacity' refers to the capacity of soil to support applied loads that are acting on it. In the construction industry, this typically relates to the capacity of soil to support building foundations, in which case, the bearing capacity can be calculated from the maximum average contact pressure between the foundation and the soil that would not produce shear failure.

Three modes of failure limit bearing capacity:

General shear failure.
Local shear failure.
Punching shear failure.

The ultimate bearing capacity of soil (qu) is the maximum pressure which can be supported without failure occurring.

The net ultimate bearing capacity (qnu) does not take into consideration the over-burden pressure and can be calculated as:

qnu = qu - Ydf

Where:

Y = unit weight of soil
Df = foundation depth

The net safe bearing capacity (qns) considers only shear failure, and can be calculated as:

qns = qnu / F

Where

F = factor of safety.

The allowable bearing capacity (qs) is the ultimate bearing capacity divided by a factor of safety, and can be written as:

qs = qu / F

On particularly soft soil, significant settlement can occur without shear failure. In such instances, the maximum allowable settlement is used as the allowable capacity.

Karl von Terzaghi developed a theory for evaluating the ultimate bearing capacity of shallow foundations. His theory states that a shallow foundation is one where the depth is less than or equal to its width.