Severe scaling and spalling are commonly observed on tunnel lining surfaces in sulfate-rich environments. Due to humidity gradients, sulfate solution in rock fissures migrates through capillary action to the concrete exposed face, leading to physical crystallization precipitation at free-face zone and chemical sulfate attack at soil-facing zone, resulting in concrete expansion and crack. Existing models focus on full immersion or wet-dry cycles, which have obvious errors in predicting concrete damage under similar partial immersion. Considering the time- varying characteristics of saturation, porosity, calcium leaching and crack, a transport-reaction-expansion model for lining concrete under dual sulfate attacks and water evaporation was established. The spatiotemporal distribution of phase composition and the influence of modeling parameters on concrete expansion were revealed. The expansion strain caused by dual sulfate attacks and changes in the water evaporation zone was discussed. These findings provide a theoretical foundation for the durability design of lining concrete in sulfate- rich environment.

This study investigated the bearing capacity and failure characteristics of a shield tunnel lining structure subjected to top overload and simultaneous unloading on both sides of a tunnel, considering the presence of internal water pressure. The results show that the structural response of the shield tunnel lining is most unfavourable under the condition of a fully filled pipe, where the internal water pressure reduces the axial force of the lining ring section, compared with the conditions of an empty pipe and a partially filled pipe. When the internal water pressure increases from 0 MPa to 0.6 MPa, the convergence deformation of the lining ring under a top overload of 400 kPa increases by 23.6%, resulting from a reduction of 27.2% in the maximum axial force at the lining section. Similarly, the convergence deformation of the lining ring under simultaneous unloading of 400 kPa on both sides of the tunnel increases by 21.6% because of a reduction of 56.4% in the maximum axial force at the lining section. The shield tunnel lining rings under the action of internal water pressure when subjected to top overload or simultaneous unloading on both sides of the tunnel exhibit the same failure characteristics. As the overload or unloading value increases, the lining ring deformation gradually increases, the joint opening exceeds the waterproof design limit, and the bolt enters a plastic yield state as its stress exceeds the yield strength. Cracks occur in the concrete at the positions of the lining segments, segmental joints, and handholes because of the large strain values. Moreover, the stress of the steel bars, joint panels, and anchor bars inside the lining segments may exceed their yield strength. During the top overload, the bending moment and axial force of the lining ring increase, whereas when unloading on both sides of the tunnel, the bending moment increases and the axial force decreases. Compared with the case with an overload value of 400 kPa, the maximum positive and negative bending moments of the lining ring under a lateral unloading value of 400 kPa decrease by 11.5% and 14.4%, respectively, whereas the maximum axial force decreases by 73.1%. This considerable decrease in axial force during lateral unloading leads to greater eccentricity and a more adverse structural response of the lining structure than does top overload. Therefore, during the operation of shield tunnels with internal water pressure, the influence of unloading on both sides of the lining structure caused by soil stress relaxation should be taken seriously.

In deep-buried long tunnels, train derailment accidents pose a serious threat to the stability of the tunnel lining structures and the safety of personnel along the line. To address the impact damage to the secondary lining caused by high-speed train derailments, a three-dimensional nonlinear dynamic analysis model of the Electric Multiple Unit (EMU) - lining - soil system was established. The advantages of this model include: it fully considers the complex streamlined design of the EMU front end, the nonlinearity of lining materials, and the M-C elastic structural model of the soil, allowing for accurate simulation of the contact and deformation between the EMU and the lining. The results indicate that the first 30 ms of the collision process are extremely intense, primarily involving the first three train vehicles. Among these, the head vehicle experiences the greatest reduction in kinetic energy and plastic dissipated energy, resulting in the most severe plastic deformation of the vehicle body. The impact load exhibits a distinct multi-peak characteristic, mainly composed of lateral impact force components. The area of displacement change in the lining expands continuously along the direction of the train, with peak displacements stabilizing after 30 ms. The lining primarily suffers from tensile failure, with multiple tensile cracks appearing in areas distant from the collision, while compressive damage is mainly concentrated at the point of direct impact. As the collision angle increases, the range of compressive damage along the longitudinal direction becomes narrower. The ratio of tensile damage area to compressive damage area is mainly influenced by the collision angle. In the design of tunnel structures for impact resistance, special attention should be paid to the lateral impact resistance and tensile failure capacity of the tunnel structure.

Taking the tunnels crossing active faults in China's Sichuan-Tibet Railway as the research background, experimental studies were conducted using a custom-developed split model box. The research focused on the cracking characteristics of the surrounding rock surface under the action of strike-slip faults, the progressive failure process of the tunnel model, and the mechanical response of the tunnel lining. In-depth analyses were performed on the tunnel damage mechanism under strike-slip fault action and the mitigation effects of combined anti-dislocation measures. The results indicate the following: Damage to the upper surface of the surrounding rock primarily occurs within the fault fracture zone. The split model box enables the graded transfer of fault displacement within this zone, improving the boundary conditions for the model test. Under a 50 mm fault displacement, the continuous tunnel experiences severe damage, leading to a complete loss of function. The damage is mainly characterized by circumferential shear and is concentrated within the fault fracture zone. The zone 20 cm to 30 cm on both sides of the fault plane is the primary area influenced by tunnel forces. The force distribution on the left and right sidewalls of the lining exhibits an anti-symmetric pattern across the fault plane. The left side wall is extruded by surrounding rock in the moving block, while the right side wall experiences extrusion from the surrounding rock in the fracture zone, and there is a phenomenon of dehollowing and loosening of the surrounding rock on both sides of the fault plane; the combination of anti-dislocation measures significantly enhances the tunnel's stress state, reducing peak axial strain by 93% compared to a continuous tunnel. Furthermore, the extent and severity of tunnel damage are greatly diminished. The primary cause of lining segment damage is circumferential stress, with the main damage characterized by tensile cracking on both the inner and outer surfaces of the lining along the tunnel's axial direction.

In designing earthquake-resistant structures, we traditionally select dynamic loads based on the recurrence period of earthquakes, using individual seismic records or aligning them with the design spectrum. However, these records often represent isolated waveforms lacking continuity, underscoring the need for a deeper understanding of natural seismic phenomena. The Earth's crustal movement, both before and after a significant earthquake, can trigger a series of both minor and major seismic events. These minor earthquakes, which often occur in short time before or after the major seismic events, prompt a critical reassessment of their potential impact on structural design. In this study, we conducted a detailed tunnel response analysis to assess the impact of both single mainshock and multiple earthquake scenarios (including foreshock-mainshock and mainshock- aftershock sequences). Utilizing numerical analysis, we explored how multiple earthquakes affect tunnel deformation. Our findings reveal that sequential seismic events, even those of moderate magnitude, can exert considerable stress on tunnel lining, resulting in heightened bending stress and permanent displacement. This research highlights a significant insight: current seismic design methodologies, which predominantly focus on the largest seismic intensity, may fail to account for the cumulative impact of smaller, yet frequent, seismic events like foreshocks and aftershocks. Our results demonstrate that dynamic analyses considering only a single mainshock are likely to underestimate the potential damage (ie., ovaling deformation, failure lining, permanent displacement etc.) when compared to analyses that incorporate multiple earthquake scenarios.

In shield tunneling, the joint is one of the most vulnerable parts of the segmental lining. Opening of the joint reduces the overall stiffness of the ring, leading to structural damage and issues such as water leakage. Currently, the Winkler method is commonly used to calculate structural deformation, simplifying the interaction between segments and soil as radial and tangential Winkler springs. However, when introducing connection springs or reduction factors to simulate the joint stiffness of segments, the challenge lies in determining the reduction coefficient and the stiffness of the springs. Currently, the hyperstatic reflection method cannot simulate the discontinuity effect at the connection of the tunnel segments, while the state space method overlooks the nonlinear interaction between the tunnel and the soil. Therefore, this paper proposes a numerical simulation method considering the interaction between the tunnel and the soil, which is subjected to compression rather than tension, and the discontinuity of the joints between the segments. The model structure and external load are symmetrical, resulting in symmetrical calculation results. This method is based on the soft soil layers and shield tunnel structures of the Shanghai Metro, and the applicability of the model is verified through deformation calculations using three-dimensional laser scanning point clouds of sections from the Shanghai Metro Line 5. When the subgrade reaction coefficient is 5000 kN/m3, the model can effectively simulate the deformation of operational tunnels. By adjusting the bending stiffness of individual connection springs, we investigate the influence of bending stiffness reduction on the bending moment, radial displacement, and rotational displacement of the ring. The results indicate that a decrease in joint bending stiffness significantly affects the mechanical response of the ring, and the extent and degree of this influence are correlated with the joint position and the magnitude of joint bending stiffness.

Shaoguan was hit by a extremely heavy rainstorm, and the mountain watercatchment of Dabaoshan Tunnel in the southern of Beijing HongKong Macao Expressway in Guangdong increased sharply. Due to therapid rise of groundwater level, water and mud gushed at ZK141+227 ofDabaoshan, and serious water seepage occurred in other areas, bringing soilinto the tunnel, which seriously hindered the safe passage of the tunnel.According to the on-site investigation of water and mud gushing, it wasfound that there were branches sandwiched in the mud gushing out, andat the same time, it was found that there was water leakage at the foot ofsome walls where drainage holes were added. Based on the fluid structurecoupling mechanism, the seepage mechanism of highway tunnels was deeplyexplored, and the mechanical properties of tunnels under seepage were ana-lyzed through experimental data and numerical simulation. The experimentalresults show that under the action of seepage, the stress distribution of the tunnel lining changes, and the phenomenon of local stress concentration isobvious. When the seepage pressure reaches 3.5 MPa, cracks appear in thetunnel lining, with a total of 5 cracks. The distribution of cracks is closelyrelated to the seepage field. The numerical simulation further reveals theinteraction mechanism between the seepage field and the tunnel structure,confirming the influence of the seepage field on the stability of the tunnellining. When the seepage pressure increases to 4.0 MPa, the displacementchange rate of the tunnel lining reaches 0.3 mm/m, and the maximum liningstress is 15.7 MPa. The purpose of this study is to propose a maintenance planfor highway tunnels to improve their safety. Consider the impact of seepageon tunnel structure and adopt effective waterproofing and drainage design.Further research on the seepage mechanism and tunnel mechanical propertiesis recommended to provide more reliable theoretical support for engineeringapplications.