Despite over six decades of field and laboratory investigations, theoretical studies, and advances in constitutive modeling, questions remain on the fundamental issues concerning liquefaction mechanisms, the collective influence of multiple factors on excess pore water pressure (EPWP) generation, and liquefaction triggering criteria. This paper presents the general apparent viscosity-and average flow coefficient-based methodology for quantifying the solid-liquid phase-change process of liquefiable soil under undrained cyclic loading. The analysis reveals that the evolution of the soil particle-fabric system is the fundamental physico-mechanical mechanism behind EPWP generation in a liquefiable soil, with the accompanying change in soil physical state serving as the intrinsic mechanism driving EPWP generation. The study further identifies the physico-mechanical foundations of EPWP generation, as well as the inherent causes and a unified quantitative characterization of the coupled influences of multiple factors on EPWP generation. This work presents the novel observation that the marginal peak excess pore pressure ratio (ru,pm) between the solid-liquid mixed phase and the liquid phase of liquefiable soil can be identified accurately and that ru,pm is characterized by its inherent robustness. A ru,pm value of 0.90 can be used as a liquefaction triggering criterion for soils both in laboratory element tests and in the field. Another original finding is that the liquefaction triggering resistance curve is the threshold state curve between solid-liquid mixed phase and transiently liquid phase of a liquefiable soil and is unique for a specific initial physical state. The definitions of liquefaction triggering and corresponding liquefaction triggering resistance are clear and unambiguous and have the same physico-mechanical basis. The insights obtained in this paper will potentially enable the scientific and engineering communities to reinterpret the liquefaction mechanism, its evaluation, and liquefaction mitigation strategies.

Evaluating cyclic liquefaction of soil from the perspective of energy dissipation provides a more comprehensive insight into its liquefaction mechanism. This study conducted a series of undrained cyclic triaxial tests using discrete element method to investigate the influence of plastic fines content (FC) on the dynamic characteristics of sand-clay mixtures. A new evaluation index, the Viscous Energy Dissipation Ratio (VEDR), is introduced to assess the energy dissipation performance of sand-clay mixtures. Macroscopically, it is shown that when FC 30 %, the trend reverses. In terms of energy dissipation, as the fines content increases, VEDR gradually transitions from the sand-like to the clay-like mode, exhibiting a unique transitional mode when FC = 50 %. Microscopically, the development of bond breakage is highly similar to that of VEDR. The bond breakage facilitates particle sliding and rolling, which is the fundamental factor causing the differences of energy dissipation between pure sand and sand-clay mixtures. This paper contributes to the mechanistic study of liquefaction criteria based on energy theory by establishing the connection between microscopic particle behavior and macroscopic energy dissipation during the cyclic liquefaction process.

Loess has a unique structure that makes it susceptible to liquefaction during intense seismic activity. Liquefaction is closely linked to microstructural changes due to hydraulic coupling. This study examined the threedimensional microstructure evolution of loess in various liquefaction states using dynamic triaxial tests and high-precision micrometer CT scanning. As the ratio of pore water pressure (Rwp) increases, the size of loess particles tends to decrease while the roundness is inclined to increase. Moreover, the morphology and orientation of particles remain relatively stable under such circumstances. In addition, increasing Rwp will decrease the number of macropores, increase the number of mesopores and fine-pores, and decrease the size of throats and channel length, with which petite throats and pores become more prominent. Consequently, liquefaction gradually opens closed pores, enhances soil connectivity, and divides large pores to increase small to mediumsized pores, improving pore distribution uniformity. Liquefaction induces the pore shape coefficient to decrease, the number of slim pores to increase, and irregular and circular pores to decrease. These findings provide a scientific foundation for preventing and evaluating loess liquefaction disasters and shed light on the microscopic mechanisms of loess liquefaction.