Stress–dilatancy and micromechanics of sand under partially drained conditions

The mechanical behaviour of soil subject to shear loading or deformation is typically considered either completely drained or undrained. Under certain conditions, these drained and undrained scenarios can represent boundaries on the allowed volumetric strain. There is growing interest in exploring the response under intermediate conditions where partial drainage is allowed, particularly in the development of new approaches to mitigate the risk of liquefaction induced failure and the design of off-shore structures. This study uses the discrete element method (DEM) to investigate the effect of partial drainage conditions on the mechanical behaviour of spherical assemblies. Samples with different interparticle friction values are isotropically compressed and then subjected to undrained, drained, and partially drained triaxial shearing. The partially drained conditions are simulated in the DEM samples by applying a controlled volumetric strain that is a fraction of the drained volumetric strain. Results on loose samples indicate that allowing drainage enhances peak shear resistance and can also prevent liquefaction. Moreover, dense samples show a substantial increase in shear resistance when small changes in drainage and volumetric strain take place. The peak stress ratio and the stress ratio at the phase transformation point are insensitive to the drainage level. There is a linear correlation between the state parameter and the drainage level at the peak stress ratio and the phase transformation point. This observation could be used to trace partially drained stress-paths and could also aid the development of uncoupled constitutive models that account for drainage effects.