Shear behaviour of loess under varied drainage conditions subjected to monotonic and cyclic loads

The role of drainage conditions in altering the shear behaviour of loess is a fundamental concern, yet remaining unclear in the current literature. This paper presents an experimental investigation into the problem on saturated intact loess through static and cyclic triaxial tests. Four types of drainage conditions were controlled during shearing, including drained, undrained, water-absorbing and partially drained conditions. Results from the static triaxial test revealed that the loess exhibited a strain-hardening behaviour in the drained condition. When the drainage of the loess sample was gradually restricted, it showed strain-softening behaviour, and the lowest shear strength occurred in the sample under the water-absorbing condition. A fairly good correlation has been established between the strain increment ratio and the stress ratio at the asymptotic state. When subjected to the cyclic loads, the loess sample demonstrated the cyclic mobility type of failure mode. A smaller number of cycles were found at failure for the loess samples under the water-absorbing condition than that under the conventional undrained condition. Of a particular interest was the partially drained condition. The mean effective stress of the loess sample firstly reduced, and it then regained as the cyclic load gradually proceeded. At the meanwhile, the excess pore water pressure of the sample reduced after reaching a peak value. In this study, the dynamic elastic modulus and the damping ratio of the loess samples were also examined. Compared with the undrained case, the dynamic elastic modulus is greater under the partially drained condition, and it is lower under the waterabsorbing condition. Accordingly, an opposite trend was found for the damping ratio. The above experimental results provide evidences on how the drainage conditions expedite or retard the occurrence of shear failure in loess, confirming the significant impact of the drainage conditions on the shear behaviour of loess.