Anisotropy of Undrained Shear Strength of an Over-Consolidated Soil by Triaxial and Plane Strain Tests

Abstract

In order to find out the real cause leading to the anisotropy of undrained shear strength of clayey soils, two types of undrained shear tests were carried out; 1) undrained plane strain tests on lightly over-consolidated kaolin clay, and 2) undrained triaxial tests on heavily over-consolidated kaolin clay. Analysis of the data has shown that the anisotropy of undrained shear strength of over-consolidated kaolin clay is primarily caused by the direction-dependent (anisotropic) shear strength parameters c' and ϕ' in terms of effective stresses. The anisotropy of pore water pressure parameter Ay is artificially generated as a result of the anisotropic change of the undrained shear strength itself, rather than of the anisotropic excess pore-pressure development during shearing, and is of secondary, if any, importance to interpret why the undrained shear strength is anisotropic. Intensity of the anisotropy in the undrained shear strength is not altered much by either changing the boundary conditions, such as triaxial and plane strain or increasing the over-consolidation ratio up to 32. The minimum values of cohesion c' and friction angle ϕ' appear when a specimen is compressed with the major principal stress inclined at about 30° to the consolidation plane. When a specimen is compressed either normal or parallel to the consolidation plane, values of cohesion c' and friction angle ϕ' are high with no significant difference between them. The difference between the maximum and the minimum friction angle ϕ' is almost the same as that observed in the triaxial tests of sands. In the case of sands, however, a specimen compressed normal to the bedding plane yields the maximum friction angle ϕ', while a specimen compressed parallel to the bedding plane yields the minimum value in the triaxial tests and a value close to the minimum in the plane strain tests. These observations suggest that the micro-deformation mechanism leading to the strength anisotropy is quite similar in both clay and sand, but not exactly the same.