2. Soil strength and fracture

2.1 Breaking modes

In the absence of a precise description of the behavior of a soil mass from its initial state to failure, soil mechanics has drawn inspiration from failure modes observed in nature to develop simplified behavior laws. Nature shows the existence of two main modes of failure:

• failure by sliding on a surface ;

• Fractures caused by plasticization and flow of a soil mass.

The representation of soil shear strength by a relationship between tangential stress τ and normal stress σ corresponds to the first mode of failure, which is the one that was the earliest to be identified and analyzed. Direct shear tests at the