Stress relief disturbance and residual pore pressure in cohesive soils

Sampling induces structural modifications in clay which depend on the stress and strain paths followed during sampling history. A complete description of disturbance is very difficult, because it originates in situ during drilling and is completed in the laboratory with reloading at the pre-existing stress conditions. In this paper, only the effects of undrained relief of confining effective stresses are investigated for reconstituted natural clays of varying plasticity. During undrained stress relief, negative pore pressure (suction) occurs: this residual stress was selected as a reference parameter for disturbance appraisal and compared with initial conditions before unloading. For the same type of soil, consolidation was carried out in various stress conditions (isotropic and anisotropic), and both normally consolidated and overconsolidated specimens were made. After undrained unloading, residual stress was evaluated by means of a controlled swelling technique using a modified oedometer and a triaxial device. This method was validated by comparisons with suction plate measurements performed on reconstituted kaolin clay specimens. Experiments showed various causes of loss of suction in samples, related to plasticity index, stress history, stress conditions and released stress level. For normally consolidated non-structured clays, undrained stress relief gives rise to some important disturbance processes, more evident for anisotropically consolidated samples. Residual stress may be considered meaningful for stress relief disturbance appraisal.