A theoretical framework to describe undrained yielding and potential fluidisation of soft subgrade soil under cyclic loading

Soft soils subjected to cyclic loading under undrained conditions can experience either undrained yielding (i.e. plastic yielding that has reached the critical state) or the transition to a fluid-like state caused by the upward migration of fine particles and moisture. The latter changes the internal soil fabric and pore structure, resulting in a non-uniform change in the soil state. This paper presents a theoretical framework that incorporates both empirical and critical state-based constitutive approaches to determine the evolution of excess pore water pressure, localised void ratio changes and spatial state redistribution within soil subjected to undrained cyclic loading. The proposed framework is verified against two sets of experimental data obtained from a large-scale one-dimensional cylindrical apparatus and the cyclic triaxial apparatus. A liquidity index (IL) criterion is introduced to identify any potential for the occurrence of localised fluidisation as a function of the number of loading cycles and depth, while critical state principles are also used to assess undrained yielding in triaxial tests. The proposed framework can identify the onset of undrained yielding, early softening-induced potential fluidisation and the attainment of stable states with increasing number of loading cycles. The model output for various cyclic loading conditions provides a valuable insight to improve the future design of rail infrastructure built over a soft subgrade.