Shear behaviour in numerical triaxial compression tests by 3D fluid-coupled DEM: a fundamental study on mechanisms of landslide initiation

Shear behaviour in triaxial compression tests was numerically examined by 3D fluid-coupled DEM in a fundamental study on the mechanisms of landslide initiation. Changes in pore-water pressure because of undrained compression were calculated by introducing a measurement sphere surrounding a ball which represented the soil particles. The pore-water pressure was assigned to the measurement sphere as the product of the changes in volumetric strain and modulus of compressibility of water, and resulting flow due to the pressure differences among neighbouring measurement spheres was given on the basis of Darcy’s law. Specimens for the numerical triaxial compression tests were prepared with wide ranges of initial void ratio by use of samples with three different grain-size distributions. In the loose specimens, positive pore-water pressure build-up and subsequent liquefaction were numerically reproduced. In contrast, in the dense specimens, negative pore-water pressure was observed, showing upgrade of soil structure. For the medium specimens, phase transformation, which is a typical characteristic of granular soils, was successfully reproduced. Although steady-state lines were found for each sample on a p_\textss^￠ - e₀ p_{\text{ss}}^{\prime } - e_{0} diagram, there was a gap at a specific initial void ratio. Although the angles of internal friction were inversely proportional to the initial void ratio in both the undrained and drained tests in the medium to dense specimens, the observed angles of internal friction were smaller than the particle-to-particle friction angle. This result may be attributable to the fact that the balls could not form the complicated soil structures created in reality by particles with complex shapes.