Question

Mohr-Coulomb failure criterion: ?? = ? ′ + ???′ tan?′

Direct shear tests were conducted on dry soil originally. Since water ingress in the soil would reduce the soil strength. If I repeat the direct shear tests using saturated soil, will the values of ??, c’ and ?′ be different from those obtained from the dry test? If yes, how and why?

Answer 1

Shear strength (Ʈ'f) of soil is characterized by cohesion (c'), friction angle (?′) and dilatation (increase in soil volume when its being sheared). The two parameters cohesion and friction angle define the soil maximum ability to resist shear stress under defined load.

Cohesion mobilises at the beginning of stress conditions and reaches maximum values around the plastic limit that is at the beginning of structural collapse. Cohesion decreases at water content heading towards the liquid limit (wL) and increases towards the shrinkage limit (ws).

Internal friction is generally defined as resistance of two planes moving against each other, determined by their grading. Friction increases with increase in normal load, provided that the soil specimens are allowed to consolidate. It is expected for the shear strength to grow with the decrease in water content. This is because that the clayey materials are compacted drier than optimum moisture content.

Thus, the cohesion of soil (c) decreases in saturated conditions. Friction angle (?′) doesn't vary significantly with water content. Hence, the Shear strength (Ʈ'f) is expected to decrease with the increase in water content.