Shear Strength Properties of a Sodium Illite

Osmotic repulsive forces between clay particles appear as a component of the effective normal stress, but do not withstand shearing stresses; hence, the osmotic forces reduce the effective angle of internal friction. This was determined from a series of effective-stress triaxial compression tests that were performed on sedimented samples of 0.1N sodium illite and the results were compared with a similar series of tests on 0.01N calcium illite. The tests also suggested that the size of the hysteresis loop in the effective-stress Mohr-Coulomb failure envelope is a function of the strain at failure rather than the void ratio and that the failure strain depends on the initial soil structure and the type and relative magnitude of interparticle physico-chemical forces. The shear-induced pore-water pressures for normally consolidated samples at 30 psi were functions only of strain, not being effected by a wide range in strain rate.