AbstractA thermodynamics-based constitutive model is developed for calcareous sand treated by microbially induced calcite precipitation (MICP) to describe the effects of biocementation and its degradation by cyclic shearing within the framework of nonequilibrium thermodynamics. The elastic potential function implemented within the constitutive model leads to a hyperelastic representation of stress-strain-strength with considerations of true cohesion and stress-density state dependency, yielding a theoretical stress state boundary surface for sands treated with different levels of MICP. In addition, the concepts of configuration entropy and locked energy are defined to describe energy dissipation and corresponding irreversible deformation accumulated during cyclic loading. The effects of MICP treatment on the cyclic behavior of sands can be well predicted through the definition of the MICP-induced increase in soil density and a bonding parameter that varies as a function of the reaction index, representing the concentration and volume of microbial reactant. Predictions of a series of undrained cyclic triaxial tests of sands with and without MICP are made to validate the model. It is shown that a two-stage degradation of the bonding between sand particles should be considered for better predictions of the cyclic behavior. In general, the model sufficiently captures the cyclic stress-strain hysteresis and excess pore pressure generation in MICP-treated sand and gives insights into the underlying mechanisms.