Soil-particle and pore orientations during drained and undrained shear of a cohesive sandy silt-clay soil

The orientations of particles, pores, and other constituents of an artificially made cohesive sandy silt-clay soil were studied to investigate how they change during drained and undrained shear. The results show that the orientation pattern before shearing is nearly random, although there may be some degree of preferred orientation caused by the overburden pressure. The degree of preferred orientation increases as the shearing increases until failure in both the drained and undrained tests and increases towards the failure plane. After failure, the degree of preferred orientation does not change considerably near the failure plane but does continue to increase away from it. The number of oriented particles, pores, and other constituents increases, but their averages stay about the same as the shearing continues after failure in the drained tests. The differences between the degrees of preferred orientations 5 and 10 mm away from the failure planes at different shear (horizontal) displacements are much less in the drained tests than in the undrained tests, indicating formation of a wider deformation zone in the drained tests. This is probably because particles in the drained tests have enough time to respond to the applied shear stresses and change their orientation. This may explain why deformations occur in wide zones along tectonically active creeping (aseismic) faults, whose mechanisms are analogous to those of drained shear tests, and in narrow zones along seismic faults, whose mechanisms are analogous to those of undrained shear tests.