Determination of the bearing capacity of a foundation The bearing capacity of a foundation can be determined in terms of a general shear mode, which is based on the theory of plasticity. This was the basis of Terzaghi's theory, which was based on Prandt and Reissner's solution assuming a rough foundation surface. Lower and upper bound plasticity theorems can be applied to get exact solutions, however this is based on the assumption that the soil is a perfectly plastic material.

This assumption is only realistic for homogenous clays of low compressibility and subjected to the general shear mode of failure. However in most cases, it is settlement and servicability that are the limiting criteria.

Various equations have been established for calculating the ultimate bearing capacity of shallow foundations based on Terzaghi's equation. These take into account the shape, depth and the loading conditions as well as the variability of the soil strata. An exact stability analysis would involve solving simultaneously the conditions of equilibrium and compatibility throughout the failure surface. This is impractical since all the following conditions must be satisfied:

Each point within the soil mass must be in equilibrium
The stresses within the soil must be in equilibrium wit the applied stresses to the foundations
The strains occurring at a point must be compatible with the strains at all the surrounding points
The strains must be related to the stresses at every point by an appropriate stress-strain relationship for the soil
The failure criteria for the soil must not be exceeded at any point within the soil

A complete stress-strain assessment of the soil mass would be very complicated because of the difficulty in modelling the actual stress-strain behaviour of the soil. The most successful method are limit equilibrium methods - a collapse mechanism is assumed with blocks of soil sliding along their slip surfaces and the stability of the soil mass is examined with each component block in static equilibrium. The critical condition is found and the external loading on this is taken to be approximately equal to the collapse load.

This method combines some of the features of the upper and lower bound methods. Like the upper bound method, a collapse method is assumed but it is assumed that the soil can deform sufficiently for the mechanism to become compatible. The equilibrium is also examined as in the case of the lower bound method but no check is made on the stresses in the soil mass surrounding the slip surface. Therefore in general the method does not provide an upper or lower bound solution, nor is the critical failure mechanism necessarily the actual collapse mechanism, but it is one that has been observed to develop in laboratory models and full size foundations.

	Created by rhiannonsian over 9 years ago

Degree level Geotechnics (Bearing Capacity) Note on General Bearing Capacity Equation, created by rhiannonsian on 09/04/2015.