Subsea tunnels cross complex geological structures, such as weathered troughs with fractured rock masses and high permeability, and are prone to water and mud inrush. To minimize the risk of subsea tunnelling, a novel method consisting of a multi-index evaluation system and a computational model using the interval number TOPSIS method was established. The multi-index evaluation system was formed by eight evaluation indices that can potentially affect water and mud inrush: sea depth, subsea tunnel burial depth, scale of weathering trough, interface angle, strength of surrounding rock, permeability of weathering trough, cyclical footage, and grouting reinforced region. The risk levels of water and mud inrush were divided into four grades. Considering the uncertainty of the evaluation indices, an evaluation vector of interval numbers was adopted. The triangular fuzzy number membership function was used to determine the membership degree, and the 1–9 scales method was used to construct the judgment matrices, which can obtain the weight of evaluation indices. Furthermore, the weight values of the evaluation indices combined with the membership degree were used to obtain the result vector, which can be analyzed using the interval number TOPSIS method. This novel assessment method was applied to the FWK15+350 of the Haicang tunnel successfully. The risk level fell into IV, which represents a high-risk section. The results showed a high degree of congruence with the prevailing circumstances, thereby validating the credibility of the proposed methodology.

The crossing of weathered troughs by subsea tunnels poses significant water inrush hazards, primarily owing to the highly weathered, low-strength, and high-permeability nature of the rock masses. To accurately simulate the full-process evolution of these hazards, this study establishes a three-dimensional fluid-solid coupling numerical simulation model based on the Xiamen Second West Passage Subsea Tunnel project using numerical simulation software. Simulations were conducted under three distinct working conditions, including unsupported, initially supported, and advance grouting, with systematic monitoring performed at specified excavation distances. Results indicate that, in the unsupported scenario, vault displacement remains relatively limited in the early stages but increases sharply after the tunnel face enters the weathered trough. The concentration of the first principal stress shifts from the arch shoulders and feet toward the vault, while fracture expansion alters seepage pathways and reduces pore water pressure in the surrounding rock. The main influence range of excavation-induced disturbance is approximately one tunnel diameter. Notably, both initial support and advance grouting effectively reduce pore water pressure, enhance rock mass stability, and decrease water inrush risk. This study elucidates the evolutionary mechanisms of water inrush hazards and provides support for safety control in similar subsea tunnel engineering projects.

Surrounding rock deformation in soft rock tunnels is controlled by complex nonlinear interactions among geological conditions, construction parameters, and support measures, making accurate prediction challenging. In this study, a project-scale deformation prediction dataset was constructed using field monitoring data from a sandy shale tunnel project. Eight engineering factors were selected as input variables, including excavation method, initial support strength, closure time, tunnel burial depth, lithology, rock integrity, groundwater condition, and the relative orientation between major structural planes and the tunnel. A hybrid prediction framework integrating a novel whale optimization algorithm (NWOA) and a long short-term memory (LSTM) network was developed. The proposed NWOA improves the standard whale optimization algorithm by introducing a nonlinear convergence strategy, an adaptive weight coefficient, and a dynamic spiral position updating mechanism to enhance the hyperparameter search process of the LSTM model. Model performance and stability were further assessed using repeated and nested cross-validation. The corresponding RMSEs were 0.2294 ± 0.0734 and 0.2219 ± 0.0751 percentage points, the MAEs were 0.1427 ± 0.0350 and 0.1505 ± 0.0585 percentage points, and the R² values were 0.8660 ± 0.0548 and 0.8732 ± 0.0599, respectively. These comparable results support project-specific predictive performance for the investigated tunnel sections.

Water and mud inrush is a major hazard impeding the construction sustainability of tunnel crossings of water-rich active faults, where seismic-induced fracturing may compromise the integrity of water-resisting rock masses. Natural rock masses commonly contain pre-existing cracks and mineral veins, but their influences on dynamic failure under coupled geostress, hydraulic pressure, and seismic loading remain insufficiently understood. Taking the Kangding No. 2 Tunnel of the Sichuan–Tibet Railway crossing the Yunongxi active fault as the engineering background, a coupled hydro-mechanical discrete element model was established to investigate representative pre-existing crack and quartz-vein configurations corresponding to inclinations of 0°, 30°, 60°, and 90°. The model was subjected to principal stresses of 23.7 and 12.7 MPa, a groundwater pressure of 3.33 MPa, and combined horizontal and vertical seismic loading, with the horizontal component prescribed at 0.8 g and 5 Hz. The results show that pre-existing crack tips acted as preferential sites for fracture initiation, whereas quartz veins generally impeded fracture propagation before vein rupture. Shear fractures dominated the dynamic failure process in all investigated cases. The tensile-fracture ratios ranged from 7.90% to 13.15% for the crack-containing models, with the maximum value occurring in the 30° configuration, compared with only 7.01–7.84% for the vein-containing models. The AE b values of the crack-containing models ranged from 2.14 to 2.94 and exhibited a trapezoid-shaped variation among the investigated configurations, whereas the vein-containing models showed an oblique N-shaped variation, with a maximum b value of 2.23 in the 90° configuration. Abrupt increases and pronounced fluctuations in AE counts and energy accompanied intensified fracture propagation and model failure. These results provide numerical indications of potentially fracture-prone structural configurations, which can guide precise directional grouting, early warning and ecological protection in the migration of water and mud inrush, eventually supporting the construction sustainability of the tunnel.