Internal erosion has been widely detected in both natural deposits and filled structures, which is
one of the leading causes of failures or incidents of large and small dams, dikes and levees. It
poses great risk on earthen structures as the process is not transparent until it has progressed
enough to be visible or detected by measurements. With the movement and migration of soil
particles under seepage flow, the contacts between soil particles will decrease and soil force
chains may buckle, leading to significant settlement, a coarser and more permeable structure,
and a higher possibility of downstream instability or even failure of the earthen structures.
Since internal erosion is almost unavoidable in real engineering projects, it is necessary and
important to conduct investi...[
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Internal erosion has been widely detected in both natural deposits and filled structures, which is
one of the leading causes of failures or incidents of large and small dams, dikes and levees. It
poses great risk on earthen structures as the process is not transparent until it has progressed
enough to be visible or detected by measurements. With the movement and migration of soil
particles under seepage flow, the contacts between soil particles will decrease and soil force
chains may buckle, leading to significant settlement, a coarser and more permeable structure,
and a higher possibility of downstream instability or even failure of the earthen structures.
Since internal erosion is almost unavoidable in real engineering projects, it is necessary and
important to conduct investigations on soil behaviours after subject to particle loss during the
internal erosion process. Therefore, the main objectives in this thesis are to study soil
deformation characteristics and evolution of stress-strain behaviour induced by internal erosion
under complex stress states.
In this thesis, a photographic method was developed. By combining this photographic
technique with a modified erosion-triaxial apparatus, a seamless study of soil deformations
during internal erosion and changes in the subsequent post-erosion soil mechanical behaviours
was performed. A high-resolution digital camera was installed to take images of the soil
specimen which was wrapped with a marked membrane at a certain time interval. The locations
of reference points on the sample were traced at different times, and the captured deformations
(pixels) on an image can be converted to the deformations of the soil sample using horizontal
and vertical scale factors. Based on the movements at the grid points, not only can a qualitative
description of soil profile changes with increasing particle losses be obtained, a systematic
analysis of how the soil axial, radial and volumetric strains develop can also be achieved. Very
significant soil deformations during the internal erosion process can be captured. With the
increasing loss of soil particles, both the axial and radial strains and the void ratio increased.
Larger soil deformations developed under a larger shear stress ratio or a higher mean effective
stress. Heterogeneous soil deformations also developed across the specimen because of the
nonuniformity of the soil pores and the nonuniform migration of fine particles. The maximum
lateral deformation occurred at the middle part of the soil specimen, whereas the soil
deformations along the vertical direction were relatively uniform
During the entire erosion process, the soil experienced three stages of erosion and deformation
but reached a new equilibrium in each stage. The soil deformation and erosion amount were
closely related. In the initial stage, only small deformations occurred due to a small amount of
particle loss. In the second stage, significant soil deformations developed after a large amount
of particles had been lost. The strong force chains buckled due to the lack of supports from fine
particles, leading to obvious deformations. In the third stage, the coarse grains already formed a
new stable soil structure, and the loss of the remaining small amount of particles had little
effect on the soil skeleton. Hence only limited additional deformations developed.
Soil fabric and soil microstructure change when subjected to particle loss during the internal
erosion process. The post-erosion soil changed its initial strain softening behaviour to a more
strain hardening response and exhibited a contractive tendency in the soil volume. The
contractive degree and reduction in shear strength increase with increasing amount of eroded
fine particles. Because of the changing grading induced by the loss of fine particles, the critical
state of the post-erosion soil also changed. There is a clear trend of rising critical states,
reduced dilative tendency and decreased soil stiffness during shearing with increasing amount
of erosion.
The mechanism of internal erosion under cyclic hydraulic conditions has not been studied in
the literature. The hydraulic gradient in the soil may vary significantly with time in many
practical engineering projects, such as in river banks or soil beneath a vibrating spillway slab.
Therefore, in addition to internal erosion in the one-directional hydraulic conditions, the
internal erosion process under cyclic hydraulic conditions was also investigated in this thesis.
By conducting a series of laboratory tests, it was found that when subjected to almost the same
hydraulic gradient, a much larger amount of fine particles eroded under cyclic loading
conditions than under the unidirectional hydraulic gradient situation. The potential for particle
bridging, arching and clogging is lower under cyclic seepage; thus additional particle loss can
be induced. However, the influence of cyclic loading diminishes as the cyclic loading sustains,
exhibiting a ratcheting behaviour. The maximum loss of fine particles and the largest hydraulic
conductivity occurred in the first cyclic stage, then the erosion rate decreased and finally
reached a new stable state.
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