A unified thixotropic fluid model considering stage characteristics for soil liquefaction

The rational evaluation on the performance evolution of liquefiable soil subjected to the earthquake is essential to solve the problem of large deformation. The undrained cyclic triaxial tests are conducted on the saturated Nanjing fine sand to study the relationship of shear stress–strain, the development of excess pore water pressure, and the evolution of the fluidity including apparent viscosity and average flowing coefficient during the liquefaction process. An obvious stage characteristic is observed in the process of soil liquefaction, which can be divided into four stages including the solid stage, solid–fluid transition stage, thixotropic fluid stage, and stable fluid stage depending on the growth rate of excess pore water pressure ratio, which is related to the residual shear strain of soil. A higher cyclic stress ratio and lower confining pressure lead to a lower number of cycles and excess pore water pressure ratio for triggering the stage transformation. Moreover, a linear relationship between the required number of cycles and the corresponding pore pressure ratio for stage separation is demonstrated. Furtherly, the modified state equation describing the connection between shear stress and shear strain rate is obtained by introducing Gompertz growth function to express the relationship between the apparent viscosity and excess pore water pressure ratio. Consequently, a modified thixotropic-induced excess pore pressure (MTEPP) model is established by considering the stage characteristics of soil liquefaction by adapting varying destruction parameters c . At last, the flow chart using MTEPP model to calculate the excess pore pressure ratio and shear strain of liquefiable soil is proposed and implemented. Compared with the experimental results, the proposed MTEPP model can well describe the behaviors of soil liquefaction during the stage transition process.