Soil is a natural material that we work with. Human has less control over its properties as compared to other civil engineering construction materials, such as concrete and steel.

The existence of pore fluid in the soil mass complicates further the situation.

Its behavior is so complex that a "reasonable" rule does not apply.

Understanding of soil basic behavior, modeling of this behavior, and incorporating it into an analytical procedure through mathematical models are becoming increasingly necessary.

On the other hand, in design, we have to replace this sophisticated process with a much simplified procedure.

The success of analysis, however, does not come if a reasonable prediction of performance cannot be made.

So, soil is simple but the understanding of soil involves a sophisticated process which encompasses soil testing, modeling, numerical and simplified analyses, and monitoring of field performance. 
by Hoe Ling in 1996