Tunnel excavation and swelling analysis of expansive bedrock with multiphysics elasto-plastic model capable of describing different swelling behavior due to exchangeable cation species

Swelling of smectite-bearing bedrock can cause severe tunnel deformation, depending on the type of exchangeable cation present in the interlayer structure. This study proposes an extended expansive bedrock model capable of capturing distinct swelling behaviors induced by different cation species. The model incorporates a double-layer repulsive force, formulated based on Stern theory, into a previously developed finite elastoplastic framework. Finite element analyses of tunnel excavation and subsequent swelling were performed using the proposed model. The results indicate that yielding of the bedrock skeleton acts as a trigger for accelerated swelling deformation, and that the swelling behavior is strongly influenced by the type of exchangeable cation: in sodium-type smectite, pronounced swelling occurred primarily at the tunnel invert, whereas calcium- and potassium-type smectites exhibited only minor expansion. The analysis also investigated the mechanical interaction between the expansive bedrock and an invert concrete layer. Under the assumed conditions, compressive axial stresses exceeding 20 MPa developed in the invert, suggesting that the swelling pressure can surpass the compressive strength of ordinary unreinforced concrete. These findings elucidate the fundamental mechanism of tunnel invert deformation, highlighting the distinct swelling behaviors associated with various exchangeable cation species, clarifying the multiscale and multiphysics interactions between electrochemical processes in the interlaminar region and the elastoplastic response of the surrounding rock mass, and quantitatively demonstrating the mitigating effect of the invert on swelling-induced tunnel deformation.