THESIS
2015
xvii, 246 pages : illustrations (some color) ; 30 cm
Abstract
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...[
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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 tum 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.
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