Failure plane is one at which angle of obliquity is maximum. Failure shear stress is one at which difference of shear stress and shear strength is minimum. θc = Angle of the critical plane to the major principal plane From triangle property, External angle = Sum of internal angle 2θc=
π
2
+βmaximum θc=
π
4
+
βmaximum
2
where, βmax= Angle between resultant stress and normal stress on critical plane = Friction angle of soil =ϕ Calculation: the angle of failure plane relative to the major principle plane in a triaxial test, θ=