The effects of normal fault movement on the failure mechanism of water conveyance tunnels considering multi-field interaction

Tunnels of large water conveyance projects often cross active faults and are therefore exposed to permanent ground deformation induced by normal fault dislocation. In this study, a three-dimensional multi-field coupling model is developed to investigate the mechanical response and failure mechanism of a water conveyance tunnel crossing a normal fault. The model integrates surrounding rock, fault fracture zone, reinforced concrete lining, and internal pressurised water through an ABAQUS–FLUENT–MpCCI framework. The surrounding rock and fault zone are described by a Mohr–Coulomb elastoplastic model, the lining is simulated using the concrete damage plasticity model, and the internal water flow is governed by the Reynolds-averaged Navier–Stokes equations with the RNG k–ε turbulence model. The numerical model is qualitatively validated against a scaled physical model test of a tunnel crossing a normal fault, focusing on the localisation of deformation and damage in the fault zone. A systematic parametric analysis is then conducted to examine the influence of fault displacement, fault dip angle, fault zone width, fault cohesion and lining strength. The overall lining damage in tension (OLDT) index is adopted to quantify the global damage state of the lining. The results show that normal fault dislocation induces strongly localised deformation and damage within the fault fracture zone, whereas tunnel segments far away remain essentially elastic. The crown and invert near the fault are confirmed to be the most vulnerable regions due to combined bending and shear. Increasing fault displacement significantly amplifies the lining damage in the fault zone, while variations in fault dip angle and fault width mainly affect the axial extent of the highly deformed region. Higher fault cohesion and higher lining strength both reduce the OLDT value and shrink the severely damaged zone. The internal water slightly modifies the stress distribution in the lining through fluid–structure interaction but does not change the fundamental failure mode. These findings provide a basis for performance-based design and mitigation measures for water conveyance tunnels crossing normal faults.