Safe Bearing Capacity

It is the maximum intensity of loading a foundation will safely carry without the risk of shear failure.

Allowable Bearing Capacity

It is the maximum net intensity of loading which could be imposed on the soil along with no possibility of excessive settlement.

The ultimate bearing capacity could be determined

(1)        Through plate load test,

(2)        Theoretically, based on the characteristics of several types of soils, and

(3)        Through making use of penetration and other test results.

The bearing capacity which could be adopted for the design of a foundation depends on the features of the soil, the depth and dimensions of the foundation and the degree of settlement which could be permitted for the structure. There are two approaches to the determination of the bearing capacity.

Based on the shear strength features of the soil, the ultimate bearing capacity of the soil at a given depth for specific dimensions of the foundation could be compute. A factor of safety can be applied to the ultimate bearing capacity to arrive at the safe bearing capacity. If calculation of settlements denotes that it is inside the permissible limit then the safe bearing capacity would be the permitted bearing capacity. Therefore, if the calculation of the settlements display that it is beyond acceptable limit then a lower value has to be adopted for the allowable bearing capacity based on the permissible settlement. The second approach is to denote the bearing capacity on the basis of in-situ tests.

In that case of cohesionless soils, the major problem is to acquire satisfactory undisturbed soil samples for determination of shear strength. Thus, commonly in such soils the allowable bearing pressures are governed through settlement consideration rather than ultimate bearing capacity because of shear failure. In cohesionless soils, thus, the bearing capacity could be computed on the basis of penetration tests and empirical methods relating allowable bearing pressure to permissible settlements for foundations of provide dimensions.

The process of calculation of ultimate bearing capacity on the basis of shear strength characteristics is broadly used in the case of silts and clays. For a foundation on rock, allowable bearing pressures are governed more through the stresses on the foundation elements.