Discrete element analysis of fabric evolution in cyclic liquefaction of granular soils

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 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.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology