Effects of Stiff and Flexible Boundary Conditions in Triaxial Extension Tests on Cross-anisotropic Sand Behavior

Drained extension tests on cylindrical specimens indicate that strain localization is consistently encountered in the form of specimen necking. The cause of the strain localization, which is initiated early in the test, is the inherent instability in the axisymtnetric extension test, which allows stresses, and therefore deformations, to concentrate at the weakest part of the specimen. This instability is the result of the inward radial strains experienced in these tests. The conventional extension test is therefore unreliable for determining soil strength in extension. A method is employed to enforce uniform strains in extension tests on cylindrical specimens by the use of curved steel plates separated by lubricated latex membranes. Using such harnesses, two series of tests were performed on cross-anisotropic sand, one with short plates and one with long plates. The results of these tests are compared with results from conventional extension tests and with results from true triaxial tests at the corresponding stress states. The extension tests with long plate harness are the most successful in maintaining uniform strains, and they result in higher rates of dilation and higher strengths than those obtained in the conventional extension tests. These results also compare well with the results from true triaxial tests with stiff boundaries. Detailed comparisons are presented.