Mohr Circles

Описание: Mohr Circles

I want to share a passage from a book I really like and have already used in previous posts

In both field and laboratory tests, it is generally assumed that the Mohr´s failure envelope is a straight line with a slope equal to the angle of internal friction and an intercept at the origin equal to the cohesion. For granular materials with no cohesion, the Mohr´s envelope should pass through the origin. However, when two triaxial compression tests under two different confining pressures are run, the failure envelope, which is a line tangent to both circles, does not pass through the origin, because the Mohr´s failure envelope is actually a curve with the angle of internal friction decreasing with the increase in confining pressure owing to the increasing breakdown of particles under higher stresses. Consequently, a line tangent to both circles may result in a cohesion intercept, which does not exist in reality. The current practice is to tilt the envelope slightly and force it to pass through the origin. When a cohesionless material is placed in an embankment, the most critical failure surface obtained by LEAME is a very shallow circle, which barely scratches the surface of the slope. This type of failure surface is impossible, because the higher friction angle near to the surface will prevent the formation of very shallow failure surfaces. As the depth increases, the friction angle decreases, but the normal stress increases, and there is a critical depth at which the factor of safety is at minimum. A theoretical method for analyzing a curved envelope is presented (See Chapter 3 – Shear Strength- Huang (2014) Slope Stability Analysis by LEM), in which the conventional soil parameters, c and ϕ , are replaced by a new pair of parameters, ϕ0 and Δϕ.
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