Bearing Capacity of Soil

The purpose of the foundation being to transmit the load of the structure to the ground, it is necessary to know the capacity of the soil (bearing capacity) to withstand this pressure. The definitions of various terms used in connection with bearing capacity are given below : 

Ultimate Bearing Capacity 

The intensity of loading at the base of the foundation which would cause shear failure of the soil support. 

Safe Bearing Capacity 

Maximum intensity of loading the foundation will safely carry without the risk of shear failure. 

Allowable Bearing Capacity 

The net intensity of loading that the foundation will carry without undergoing settlement in excess of the permissible value for  the structure under consideration but not exceeding the safe hearing capacity. 

The ultimate bearing capacity can be determined 

1. by plate load test, 
2. theoretically, based on the characteristics of various types of soils, and 
3. by making use of penetration ad other test results. 

The bearing capacity that can be adopted for the &sign of a foundation depends on the characteristics of the soil, the depth and dimensions of the foundation and the degree of settlement that can be allowed for the structure. There are two approaches to the determination of the bearing capacity. 

Based on the shear strength characteristics of the soil, the ultimate bearing capacity of the soil at a given depth for specific dimensions of the foundation can be calculated. A factor of safety can be applied to the ultimate bearing capacity to arrive at the safe bearing capacity. If calculation of settlements indicates that it is within the permissible limit then the safe bearing capacity would be the allowable bearing capacity.  However, if the calculation of the settlements shows 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 determine the bearing capacity on the basis of in-situ tests. 

In the case of cohesion-less soils, the main problem is to obtain satisfactory undisturbed soil samples for determination of strength. However, generally in such soils the allowable bearing pressures are governed by settlement consideration rather than ultimate bearing capacity due to shear failure. In cohesion-less soils, therefore, bearing capacity can be calculated on the basis of penetration tests and empirical relating allowable bearing pressure to permissible settlements for foundations of given dimensions. 

The procedure of calculation of ultimate bearing capacity  basis of shear strength characteristics is widely used in the of silts and clays. 

For a foundation on rock, allowable bearing pressures more by the stresses on the foundation elements.