Long-term stability of a tunnel excavated in expansive stratum: response to water infiltration and influence of surrounding ground stiffness

Expansive soils are widely distributed globally and frequently intersect tunnel construction projects. Although various techniques are available to reinforce and stabilize tunnels during construction, the swelling behavior of expansive soils remains challenging to predict in the operational phase. This study investigates the swelling characteristics of expansive soils induced by water infiltration and their mechanical impact on tunnels. The results indicate that the swelling rock model proposed by Wittke-Gattermann and Wittke (2004) effectively simulates the interaction between expansive soils and tunnels, successfully reproducing a pressure evolution process characterized by three stages: rapid increase, plateau, and renaissance. During the plateau phase, expansive soils reach their maximum swelling under the loading of the surroundings, temporarily ceasing pressure growth while inducing significant shear deformation in the surrounding ground. As inner saturation progresses, swelling pressure spreads outward, triggering a renaissance phase until the system reaches equilibrium. The study further examines the influence of surrounding ground stiffness, showing that low-stiffness ground undergoes large deformation with limited pressure on the tunnel, while high-stiffness ground restricts deformation but transfers greater pressure to the tunnel lining, especially at the invert. Furthermore, real-scale simulations with varying locations of expansive soil indicate that as the expansive soil layer shifts vertically from the model bottom to the top, the surrounding ground stress state transitions from vertical to horizontal compression, thereby altering tunnel deformation patterns and influencing potential failure modes. These findings provide valuable insights into the long-term performance of tunnels in expansive soil environments, highlighting the importance of considering both stiffness contrasts and soil layer positioning in tunnel design.