Undrained Strength of Compacted Clay under Principal Stress Reorientation

Abstract

Consolidated-undrained tests on saturated compacted clays are used to provide strengths for design scenarios such as rapid drawdown, rapid flood, and earthquake loading of dams and levees. Geotechnical engineers have long recognized the influence of stress path, particularly principal stress reorientation from consolidation to failure, on the undrained strength of clay. While stress reorientation effects have been explored for undisturbed soils, their influence on the strength of compacted clay has not been examined in detail. The results of triaxial tests with stress reorientation and direct simple shear tests on a compacted clay are presented and compared to results from isotropically consolidated triaxial compression (ICU-TC) tests. Triaxial extension (TE) tests are shown to impose a stress system very dissimilar to the plane strain field conditions. TE tests also have problems with necking. The undrained strengths from TE tests were 25–55% lower than those measured in ICU-TC. In contrast, triaxial compression tests that incorporate a 90° stress reorientation resulted in only a slight reduction in strength from ICU-TC. Compacted clays were found to be difficult to test in conventional direct simple shear equipment because of their stiffness and initially unsaturated condition. Further improvement of DSS testing techniques is needed before these tests can be used for compacted clays.