The geotechnical earthquake engineering profession has struggled with the inherent complexity of the multiphase soil response to cyclic loading owing to the progressive nature of the generation of excess pore pressure (EPP) and degradation of soil stiffness and strength. One approach to improve understanding of the cyclic response and correlate the transition from a two-phase saturated soil to a single, fluid-phase liquefied soil is to treat the soil as a non-Newtonian viscous liquid. However, the work to date suggests that the viscous fluid model approach can only be implemented following the onset of sustained soil liquefaction. This paper presents a unified thixotropic fluid model and framework that effectively links the pre-shear soil fabric and its progressive cyclic response to the onset and maintained state of soil liquefaction. The framework treats the soil fabric as a fluid net-type structure proposed for use with thixotropic fluids, and presents the constitutive state and rate equations describing the deconstruction of the liquefiable soil fabric in response to cyclic loading. The unified framework uses physically meaningful soil parameters that can be obtained from common cyclic laboratory tests to seamlessly link the state-dependent and shear-strain-rate-dependent nature of soils to the generation of EPP, the latter of which is shown to increase in significance as EPP accumulates. The proposed thixotropic-induced excess pore pressure model should prove advantageous for use in forward modelling of the stress–strain rate response of liquefiable soil and generation of EPP.

The influence of pile geometry on internal sand movement around a laterally loaded pile was visualized in this paper by utilizing transparent soil and digital image correlation (DIC) methods. Transparent sand was made of amorphous silica gel and a pore fluid with a matching refractive index. An optical system consisting of a laser light, a camera, a loading frame, and a computer was developed to illuminate a targeted section inside a transparent soil model. A distinctive laser speckle pattern was generated by the interaction between the laser light and transparent soil. Two laser speckle images before and after deformation were used to calculate the displacement field using DIC. Two different cross-sectioned piles were used in the tests: a square-sectioned pile and a circular pile. Both piles were made of acrylic and had a diameter or width of 12.7 mm. Two different failure planes were observed in the tests, where a larger zone was mobilized by the circular pile compared to the one in the square-sectioned pile.