Thermodynamic basis of granular STZ model and its application in revealing shear resistance reduction mechanisms of granular soils under vibration

The granular shear-transformation-zone (STZ) model is an energy-based constitutive model for studying granular materials subjected to vibration. This paper provides a detailed analysis of the thermodynamic foundation of the granular STZ model and applies it to investigate the mechanism of vibration-induced shear resistance reduction (ViSRR) in granular soils. Firstly, using the principles of thermodynamics, an energy conversion equation for a mechanical-energy-governed granular soil system is derived, accounting for vibration energy, configurational energy, strain energy, and contact energy dissipation. Then, by incorporating the critical state concept from soil mechanics, the energy conversion function is used to develop the evolution law of configurational temperature, one of the three governing functions of the granular STZ model. The analysis shows that ViSRR is influenced by the rate of strain energy development and the input rate of vibration energy, and that a limitation in soil deformation during vibration is necessary for ViSRR to occur. Furthermore, the contact energy dissipation arising from inelastic contact deformation and particle collisions contributes to enhancing a granular soil’s shear resistance by absorbing part of the external energy applied to the soil.