Undrained shear strength

[MilanoGeo]

How can the undrained shear strength parameter (Cu) be defined from a theoretical point of view? What is the difference between using a Tresca approach to failure or a Mohr-Coulomb one in soil mechanics? Why is Cu higher for unsaturated soils than for saturated soils?

 

[KAM_imported]

Undrained shear strength refers to the shear strength of a saturated clay where no change in water content occurs during loading. When plotted on a Mohr's circle, the failure circles are the same diameter and result in a horizontal rupture line--the so called phi=0 concept.

The shear strength of partially saturated soils is related to effective stress, and the above-mentioned phi=0 concept does not apply. It is difficult to measure the strength of these soils because the void space includes both water and gas (air). Lab tests to estimate strength must therefore duplicate field conditions (Sr, same total stress, same pore water and pore air pressures ) as closely as possible. In-situ testing (Marchetti dilatometer, for example) can also be used to estimate the undrained shear strength. Although the shear strength of a partially saturated soil is normally greater than that of a saturated soil, most textbooks do not recommend using this increased strength in design because it will be lost if the soil subsequently becomes saturated.