Experimental and Theoretical Study on Static and Cyclic Liquefaction Behaviours of Sands


Student thesis: Doctoral Thesis

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Awarding Institution
Award date29 Dec 2021


Sand liquefaction refers to a phenomenon that soil transfers into a state of flow during a static or cyclic loading. This study intends to combine conventional experiments, three-dimensional X-ray tomography to investigate soil liquefaction under static or cyclic loads.

This study investigated the dependence of three-dimensional grain shape characteristics on particle size by applying X-ray µCT. A series of packing and triaxial shearing experiments are performed, which suggest that the macroscale behaviour of sands at different scales of stress states progressively correspond to the characteristic scales of the microscale morphological descriptors of particles. That is, the packing behaviour are more reliant on grain aspect ratio (AR), whereas the critical state mechanical behaviours are more reliant on grain sphericity. As the AR of particulates increases, the extreme void ratios and packing index decrease. The shearing friction angles at the critical state decrease as the sphericity of grains increases and the intercept void ratios become smaller.

Despite a substantial amount of research on the static instability and liquefaction of sands, the crucial role of the excess pore water pressure in the undrained shearing behaviour of a saturated sand has not been fully understood. In this paper, a fundamental investigation into the role of the excess pore water pressure in the static liquefaction and critical state behaviours of saturated sands was carried out through the conduction of a comprehensive triaxial shearing testing program on Zhujiang River sand. Particularly, two parallel series of isotropically consolidated drained and undrained tests were performed on two groups of sand specimens with the same initial void ratio to explore the relationships between drained and undrained shearing responses, which facilitates a deep understanding of the effects of the excess pore water pressure. Experimental results show that there is a unique linear correlation between the normalized excess pore water pressure and the normalized mean effective stress with respect to the initial effective confining pressure at the critical state, which essentially suggests the existence of a unique line of excess pore water pressure at the critical state for the undrained shearing of saturated sands. The experimental findings form a vital basis for the development of an enhanced critical state soil mechanics framework that unifies the drained and undrained behaviour of saturated sands.

This study aims to explore the theoretical mechanism of the new unique line of excess pore water pressure at the critical state for saturated sands under monotonic undrained shearing based on the analytical method of stress path during triaxial shearing test. A unified model of critical state soil mechanics incorporating excess pore water pressure is built with its unique lines in different forms. The connections between different constants of the traditional and new critical state lines in the unified model are established. Meanwhile, a theoretical predictive model for deviatoric stress and excess pore water pressure at the critical state based on the initial condition of saturated soil specimen is proposed. For model verification, a series of drained and undrained triaxial shearing tests were performed on four kinds of sands with different particle shape characteristics. Experimental results show that the theoretical equations of the unique lines in different forms are well verified and the predictive model for deviatoric stress and excess pore water pressure at the critical state works well for all tested sands. It suggests that the gradients or constants of the traditional and the new critical state lines are significantly affected by the sand grain morphology characteristics. It is meritorious for the contribution of the rounded sand grains on the developing of excess pore water pressure during shearing at the critical state. The unified critical state soil mechanics with the new critical state lines gives a chance to open a new window and provides a new approach to understand the static liquefaction susceptibility and the constitutive model of soils or granular matter.

Microscale particle shape characteristics play a crucial role in macroscale physical and mechanical behaviours of granular soils. This study focuses on providing a novel approach to involve influences of grain shape characteristics into the traditional critical state line (CSL) for sands. A unified CSL is proposed to unify the locations of CSLs of the void ratio and effective mean stress for different sands with different particle shape characteristics from angular to rounded. Data from four series of monotonic triaxial shearing testing programmes on four kinds of silica sands with different particle shape characteristics is presented and the critical state loci for specimens of all sands are well located on one unified curve in the form of the unified CSL. Based on the unified CSL, one predictive model for the location of CSL for a specific sand with the known maximum and minimum void ratios measured by physical packing tests is proposed. With the revealed correlations between packing behaviours and grain shape characteristics, another predictive model with the known quantitative grain shape characteristics measured by X-ray uCT tests is further proposed. The prediction values for constants of the CSL calculated by both models with the properties measured by simple physical material tests are consistent with the experimental results through standard triaxial shearing tests. It is known that the state parameter is an important concept to describe sand behaviours. In this study, a normalized state parameter (NSP) is suggested as an alternative single parameter measure of sand behaviours based on the unified CSL. It is a physical parameter in the plane of unified CSL with reference to the corresponding critical state to predict and quantify sand behaviours through combining the effect of normalized void ratio and mean effective stress level. The dependents of mechanical behaviours under both drained and undrained conditions on the NSP are presented based on the experimental results of four tested sands. It is suggested that the unified CSL and NSP are fundamental physical concepts and will be helpful to enhance the framework of critical state soil mechanics (CSSM) and the constitutive modelling from divergence to convergence by involving microscale particle physical properties.

A novel cyclic triaxial shearing apparatus is designed based on the synchrotron X-ray CT experimental platform of the BL13W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) for soil liquefaction study in the grain-scale. It is developed for observing the internal structure evolution under compression and tension loading and identifying both qualitative and quantitative conditions for sand liquefaction. The grain contacts for cyclic shearing and flow liquefaction are investigated by experimental method with the CT-based cyclic triaxial shearing apparatus. For analyzing a large-scale digital X-ray image, a novel approach for analyzing huge images of granular assembly is set up.

    Research areas

  • soil liquefaction, excess pore water pressure, critical state, X-ray micro-computerized tomography, apparatus designing, triaxial shearing test