What is the practical difficulty in defining the values and effects of factors in the general bearing capacity equation.

Bearing capacity is the most important parameter in soil engineering which governs the design of foundation. The bearing capacity of soil is defined as the capacity of the soil to bear the loads coming from the foundation and super-structure. The pressure which the soil can easily withstand against load is called allowable bearing pressure. 

There are various methods available now to estimate the bearing capacity of soil, put forth by scientists like Prandtl, Terzaghi, Meyerhoff, Hansen, Vesic and others. Among all the methods discussed above, Terzaghi's method of estimating bearing capacity is majorly used.

There are quite practical difficulties in defining the values in the equation of bearing capacity due to complex nature of soil. Those difficulties were:

• As already discussed, the soil is a complex material. So, unlike other materials, it is neither homogeneous nor isotropic. This was the main practical problem and to govern the equation for bearing capacity, it was assumed by Terzaghi that the soil is homogeneous and isotropic but practically, its not possible.
• Load applied on the footing is practically non-vertical and eccentric also. But during analysis of Bearing capacity, it was very difficult to generate equation with such loading condition. So, it was also assumed that the loading applied is vertical and non-eccentric to simplify the equation.
• Shear strength applied by soil above the base of the footing. If it was to be considered, then the situation would have been more complicated. So, it was assumed that the soil above base of the footing has no shear strength and it acts as only surcharge load.
• There is no same shearing pattern in the soil because of its homogeneous nature. It was a bigger practical problem in governing the bearing capacity equation. So, later on, it was assumed in Reissner's theory that the shear pattern is unaltered.
• Non uniform contact pressure between the foundation and underlying soil. But it was also assumed uniform in all bearing capacity equations.

So, these were the main practical difficulties which were solved by assuming certain conditions which were practically not true, but without these assumptions, it was impossible to govern the various bearing capacity equations.