Bearing Capacity Criteria

The design of foundation is primarily based on the concept of bearing capacity of the soil. Thus the bearing capacity is defined as the load or pressure developed under the foundation without introducing damaging movements in the foundation and in the super-structure supported on the foundation. Since damaging movements may result from foundation failure (collapse) as well as from excessive settlement, the following criteria should always be used in evaluating the bearing capacity:

1. Adequate factor of safety against failure by collapse.
2. Adequate margin against excessive settlement.

The bearing capacity after allowing a certain factor of safety over the ultimate bearing capacity against failure (collapse) is termed as safe bearing capacity.

The bearing pressure which is safe against failure and at the same time does not cause settlement more than the permissible settlement is called allowable bearing capacity or allowable soil pressure.

Safe bearing capacity or allowable soil pressure is not a fixed value for a given soil but depends on density, strength, depth of foundation, cohesion, etc. In general, for cohesionless soils it varies from 100 to 450 kN/m² and for cohesive soils from 150 to 450 kN/m². The problematic soils are loose dry fine sand and expansive clays (black cotton soils). However, a suitable foundation can be provided based on the field condition.