If the same triaxial test results of UU, CU and CD tests are plotted in terms of effective stresses taking into consideration the measured pore water pressures, it is observed that all the Mohr circles at failure are tangent to the same failure envelope, indicating that shear strength is a unique function of the effective stress on the failure plane.

This failure envelope is the shear strength envelope which may then be written as

where c' = cohesion intercept in terms of effective stress
f' = angle of shearing resistance in terms of effective stress

If is the effective stress acting on the rupture plane at failure,is the shear stress on the same plane and is therefore the shear strength.

The relationship between the effective stresses on the failure plane is