Numerical study on the hydro-mechanical behaviour of swelling rocks in long-term floor stability

Rock swelling induced by clay minerals causes volumetric expansion and triggers excessive floor heave, thereby compromising tunnel stability. Predicting long-term floor stability is challenging due to limited understanding of the time-dependent characteristics of rocks. Interactions between clay minerals and water further complicate floor heave mechanisms. This study develops a field-scale numerical model to investigate the time-dependent behaviour of swelling rocks. The model couples water diffusion with the mechanical deformation of the rock matrix and incorporates the viscoelastic properties of the rock. A material diffusivity constant is introduced to account for the influence of internal stress on the effective diffusion coefficient during water transport. Viscoelastic properties, including instantaneous elastic and creep responses, are captured using the Burgers model. The model is calibrated against field monitoring data and experimental observations. Sensitivity analyses are conducted to identify factors influencing the displacements of the roof, sidewall, and floor. Results reveal that the stiffness and the maximum swelling stress of the claystone floor have limited impacts on model displacements. The deformation behaviour is primarily governed by the viscosity properties defined in the creep model. Viscosity parameters related to the Maxwell dashpot dominate the long-term behaviour of the swelling rock. Model predictions indicate that higher viscosities delay strata deformation and prolong the time to stabilisation, potentially persisting beyond the service life of the tunnel. These findings are expected to provide insights into long-term stability analysis and to contribute to floor heave management.