THESIS
2017
xvi, 194 pages : illustrations (some color) ; 30 cm
Abstract
Liquefaction of granular soils has caused significant damage to infrastructure during past
earthquakes, including loss of bearing capacity of soil foundation, excessive deformation and
large lateral spreading of the ground. Understanding the evolution of microstructure in granular
soils can provide significant insights into constitutive modeling of cyclic liquefaction. This
study aims to explore the evolution of micromechanical structure of granular soils in cyclic
liquefaction. A variety of undrained cyclic simple shear tests are conducted using Discrete
Element method (DEM), from which particle-scale information can be obtained. The major
findings of the thesis include:
(a) A new index D
c, termed as “centroid distance”, is proposed to quantify the particle-void
fabric of gran...[
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Liquefaction of granular soils has caused significant damage to infrastructure during past
earthquakes, including loss of bearing capacity of soil foundation, excessive deformation and
large lateral spreading of the ground. Understanding the evolution of microstructure in granular
soils can provide significant insights into constitutive modeling of cyclic liquefaction. This
study aims to explore the evolution of micromechanical structure of granular soils in cyclic
liquefaction. A variety of undrained cyclic simple shear tests are conducted using Discrete
Element method (DEM), from which particle-scale information can be obtained. The major
findings of the thesis include:
(a) A new index D
c, termed as “centroid distance”, is proposed to quantify the particle-void
fabric of granular packing. The irreversible change of D
c indicates the collapse of large
pores during cyclic liquefaction. It can be used as an effective indicator to describe the
change in the internal structure, which influences the cyclic mobility and post-liquefaction
deformation.
(b) Two fabric descriptors (E
d and A
d) are proposed to quantify the shape and orientation of
particle-void distribution in the packing. These descriptors are applicable in both solid-like
state and fluid-like state during cyclic loading of the granular soils. A hardening state line
(HSL) is further defined in the E
d - A
d space to for transition between the flow state and the
hardening state. Existence of HSL indicates that the load-bearing structure in the
post-liquefaction stage can be formed only if either E
d or A
d becomes sufficiently large.
(c) Samples with different intial fabrics are prepared with a pre-shearing method to explore the
influence of initial fabric to cyclic liquefaction. The DEM simulation demonstrates that
samples with higher degree of fabric anisotropy have much lower liquefaction resistance
compared with an isotropic sample. A relationship is further proposed to parameterize the
liquefaction resistance with coordination numbers. Evolution of fabric before initial
liquefaction is profoundly influenced by intial fabric of samples. However, in
post-liquefaction cyclic loading, all samples with different initial fabrics evolve towards
the same highly anisotropic fabric, resulting in similar post-liquefaction behaviors.
(d) Multi-directional cyclic loading involves variation in both shear stress magnitude and
orientation. Three types of loading paths (circular, oval and figure-8) and the
uni-directional loading are applied on a medium-dense granular packing to explore the
signature of micromechanical structure in multi-directional loading conditions. Under
circular/oval loading tests, the packing will not experience a flow state even though large
shear deformation can be developed, which is different from the uni-directional loading
case. Evolution of contact-based fabric trajectory starts from a shape similar to the shear
stress path to a steady-state circle, which is almost identical for all oval and circular loading
tests. The same steady state of coordination number and particle-void fabric (D
c) will be
reached in oval/circular loading, which corresponds to that of the hardening state in
uni-directional loading.
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