where τ is the shear strength; σ the normal stress component across the slip surface; c and φ are cohesion and angle of internal friction that are the mathematical constants defining a linear relationship between τ and σ . However, Terzaghi's perception of the soil as a two-phase system in 1920 has brought the effective stress principle which is described as , where u_w is the neutral stress or positive pore water pressure.

Based on his effective stress principle, it was demonstrated that shear stresses in the soil can only be resisted by the solid particles. Thus, the shear strength should be expressed as a function of effective normal stress at failure as

.......................................................................................................(6.2)

in which the effective stresses are used in place of total normal stresses. The dash over τ is not used as water does not contribute to shear. Equation (6.2) describes that the failure occurs at any given point in the soil mass under the application of a critical combination of shear and effective normal stresses. The stress state in soil is conveniently represented by Mohr's circle defined by the effective principal stresses as shown in Fig. 6.1a. The line drawn tangent to all the Mohr's circles at failure is the failure envelope given in Fig. 6.1. A state of stress represented by stress points above the failure envelope do not exist. On the other hand, if the state of stress of a soil locatesd entirely below the failure envelope, the shear strength of the soil is not exceeded and the soil mass is stable.The shear stresses along the failure envelope describe the shear strength of the soil under the respective effective normal stresses. The envelope passes through origin when the cohesion intercept is zero, which is the case of sands.