Rainfall-induced landslide hazards are a common, serious and continual problem to many places throughout the world under tropical and subtropical climates. Both surface infiltration and the springs from the bedrock are the sources to increase the pore water pressure. It is believed that pore-water-pressure built-up, together with shearing-induced one, is the key factor dominating the initiation of flow landslides. However, several critical issues related to the mechanisms that govern how the sliding of different types slopes, e.g., loose or dense, is triggered and then gradually or suddenly transformed into a liquefied viscous flow still remain unclear, hypothesized, or even unknown. Therefore, in this study, coupled finite element numerical simulations on the previously published large-scale experimental landslide are carried out to further understand these unanswered issues.

In order to better simulate the process of flow landslide, a coupled analysis, which clearly considers not only the soil deformation under either drained or undrained conditions during the rise of groundwater table and subsequent seepage processes but also the shearing-induced pore pressures in response to landslide movement, has to be implemented in the FEM simulations. In this context, a state-dependent dilatancy sand model, which can well capture the responses of dense and loose soil under either drained and undrained conditions, the phase transformation, and any stress-state evolution under a unified model parameter set, are therefore adopted in the FEM simulations herein.

The FEM simulations on flow landside originated from a loose slope have revealed the following insights relevant to the failure process and initiation mechanisms. The shear zones are found to link together as a whole and penetrate through the whole slope within ~1 second in the end. This gives rise to a sudden overall failure in a flow form. The local soil responses explain how such a failure process can be formed. As expected, the increase of pore water pressure is the key factor to induce failure. Inside the shear zone, the soil experiences two stages: shearing stage and flow failure stage. During the shearing process, deviatoric stress q continues to increase whereas mean effective stress p' keeps decreasing and so does the associated shear resistance. Such q and p' responses during the shearing stage ultimately promote soil to enter the flow failure stage where q and p' suddenly decrease and finally reaches the critical state line. In addition, there is a sudden increase in the rate of shear strain intensity and the strength of the soil is no longer sufficient to withstand the static stress that is acting on the soil before flow failure.

Different from the failure process in the loose slope, the failure of dense slope occurs intermittently. The soil in the dense slope initially is also subjected to a continuous shearing where q increases and p' decreases and then enters the failure stage where both q and p' start to decrease and move towards to the critical state line. Afterwards, the accelerated shearing process gives rise to the first slip episode that near the slope surface. The volumetric dilation is produced and induces a negative excess pore water pressure. This in turn increases p' to impede the further movement of the soil mass from the first slip episode and to give rise to a phase transformation on the stress path. The induced negative excess pore water pressure then gradually dissipates, which in turn makes the pore water pressure gradually recover and p' decrease. This not only leads to another phase transformation on the stress path but also produce another slip episode. Such a slip-stop process repeats until the dilation-induced negative excess pore water pressure cannot temporarily hold the sliding mass. Then, the small scale, flow failure occurs locally.

It has also been found that the failure pattern of landslides originated from a loose slope and the associated local soil responses can be significantly influenced by the soil permeability. The simulations of slopes with the medium and the lowest soil permeability exhibit similar failure patterns that a deep and global failure mode is observed. However, the failure mode of the slope with the highest permeability is quite different with the other two cases. The failure initiates from the weakest points that are around the toe of the slope and finally a local slope failure occurs along the slope surface around that region.