An experiment study on stress relaxation of unsaturated lime-treated expansive clay

Stress relaxation, essentially time-dependent behavior of soil, is crucial to the long-term safety of earth structures. Herein, a series of triaxial relaxation experiments were carried out to advance the understanding of stress relaxation behaviors in the unsaturated lime-treated expansive clay. The roles of the relaxation time, the magnitude of the strain imposed prior to the start of the relaxation process, and the loading mode and rate on the unsaturated relaxation behaviors were thoroughly discussed. The results revealed that the stress relaxation process of tested soils consists of three phases of instantaneous relaxation, decelerated relaxation, and nearly non-relaxation. The initial and residual amounts of the deviatoric stress increased with higher pre-strain and loading rate, while the initial relaxation velocity increased with the increased strain loading rate. It may suggest that the loading rate of the pre-strain contributed to the pattern of the relaxation curves and the duration of time before reaching the nearly non-relaxation phase, while the magnitude of the pre-strain affected significantly the residual amount of the deviatoric stress. The specimen with a small pre-strain produced a large volumetric deformation, while the specimen imposed with a fixed pre-strain imposed at a higher loading rate showed a smaller volumetric strain. In combined multiple relaxation experiments, the specimens exhibited the cumulative damage and the different variations of the volumetric strain in each relaxation, and the amount of relaxed stress before the shear failure of the specimen was larger than that after the failure of the specimen with the same pre-strain. This will provide more comprehensive knowledge for the development of an appropriate constitutional model, incorporated with time effects on the unsaturated lime-treated expansive clay, and for the improvement of the testing method.