The flexible joints and segmental lining serve as effective seismic measures for tunnel in high-intensity seismic area. However, the tunnel axial deformation at flexible joints has not been fully incorporated into analytical models. This study presents a novel mechanical model for flexible joints that considers tension (compression)shear-rotation deformations, replacing the traditional shear-rotation springs model. An improved semi-analytical solution has been developed for the longitudinal response of a tunnel featuring a three-way flexible joint mechanical model subjected to fault movement. The nonlinear elastic-plastic foundation spring, the soil-lining tangential interaction, and the axial force of tunnel lining have been considered to improve the applicability and precision of proposed method. The proposed solution is compared with existing models, such as short beams connected by shear and rotation springs, by examining the predictions against numerical simulations. The results indicate that the predictions of the proposed model align much more closely with the outcomes of the numerical simulations than those of the existing models. For the working conditions selected in 4, neglecting the tension-compression deformation at flexible joints an 81.8% error in the peak axial force of the tunnel and a 20.2% error in the peak bending moment. The reason is that ignoring the axial deformation of these joints results in a larger calculated axial force on the lining, which subsequently leads to increased bending moment and shear force. Finally, a parameter sensitivity analysis is conducted to investigate the effect of various factors, including flexible joint stiffness, segmental lining length, and the length of the tunnel fortification zone.

PurposeShield tunnel is usually used as permanent underground facilities with a design service life of 100 years, and operational safety is very important. The objective of this paper is to investigate the failure mechanism and resilience evolution of double-layer lining structures of shield tunnels and to maintain the safety of structural operation.Design/methodology/approachA macro-micro model is established based on the refinement concept, considering the influences of hand-hole weakening, multi-contact interactions and reinforcement bars. The macro model describes the stress and deformation of the soil-reinforced structure using the stratum-structure method. The micro model introduces the total strain crack model, which accurately characterizes the tensile, compressive and shear behavior of concrete, calculating the millimeter-scale crack characteristics at the interface between the double-layer lining and the concrete. The mechanical response and resilience evolution of the reinforced structure are studied.FindingsThe results show that the segmental lining joint is the weakest part of the reinforced structure. The primary failure modes include the destruction of the arch vault and left-right spandrel joints, fractures in the tension zone and crack propagation and penetration at the interface. The segmental lining and secondary lining are not perfectly connected, resulting in different internal force distribution patterns, and the secondary lining exhibits a deformation mode different from the typical elliptical type. There is a significant difference between the normal and tangential displacement distributions at the interface of the double-layer lining structure, with interface failure mainly characterized by shear slip. Reinforcement of the secondary lining can significantly enhance the resilience of the segmental lining, and the resilience recovery of the structure is more pronounced with earlier reinforcement intervention.Originality/valueThis study demonstrates notable originality and value. It develops a refined model to simulate the failure and damage of a double-layer lining structure, with millimeter-scale simulations of crack propagation at the interface of the interlayer area. A framework for evaluating the structural resilience of shield tunnels reinforced with double-layer linings is established, and the evolution of performance and structural resilience throughout the loading process and subsequent lining reinforcement was thoroughly analyzed. The findings provide valuable recommendations for the reinforcement of double-layer linings in shield tunnel projects.

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.

With the expansion of international terrorism and the potential threat of attacks against civil infrastructure, the dynamic response and failure modes of underground tunnels under explosive loads have become a prominent research topic. The high cost and inherent danger associated with explosion experiments have limited current research on tunnel internal explosions, particularly in the context of scaled model tests of shield tunnels. This study presents a series of scaled model tests under 1g-condition simulating internal blast events within a shield tunnel based on the prototype of the Shantou Bay Tunnel, considering the influences of surrounding stratum and equivalent explosive yield. Three different TNT explosive yields are considered in the model tests, namely 0.2, 0.4, and 1.0 kg. The model tests focus on the damage behavior and collapse modes of the shield tunnel lining under internal explosive loads. The model tests reveal that the shield tunnel is prone to damage at the joints of the tunnel crown and shoulder when subjected to internal explosive loads, with the upper half of the tunnel lining experiencing segment collapse, while the lower half remains largely undamaged. As the TNT equivalent increases, the damage area at the tunnel joints expands, and the number of segment failures in the upper half of the tunnel rises, transitioning from a damaged state to a collapsed state. The influence of stratum-structure interaction is investigated by comparing two models, one with overburden soil and the other positioned at the ground surface. The model tests reveal that the presence of soil pressure and confinement can significantly enhance the tunnel resistance to internal blast loads. Based on the observation of the model tests, five different damage modes of segment joints under internal explosion are proposed in this study.

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.

Lining-soil systems are always designed to apply in the severe environment, including high temperature and impact loads. In this work, the thermo-hydro-mechanical responses of a lining-soil system are implemented under a sudden heating. To distinguish from previous works, the thermal contact resistance and the partition coefficient at the interface are considered. And the lining material is regarded as a continuous medium and the surrounding soil as a saturated porous medium. In the numerical part, the closed-form solutions pertaining to temperature, displacement, radial stress and pore water pressure are derived utilizing the Laplace transformation technique and its numerical inversion method. The rationality of the method is verified by comparison, and the effects of the thermal contact resistance, the partition coefficient, interfacial contact models and lining materials on each physical field are discussed. The results indicate that interfacial conditions have a significant impact on the heat transfer process of the lining-soil system.

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.

Lined canals in cold regions often experience severe frost damage, posing a significant threat to the water supply safety. This paper proposes a modified analytical solution for the response of canal lining under soil frost heave, which is based on a Timoshenko beam on a Pasternak foundation considering tangential contact between the lining and frozen soil. This analytical solution can provide any solution using Timoshenko or Euler-Bernoulli beams on Pasternak or Winkler foundations with or without tangential contact. Then, the modified analytical solution, along with the traditional analytical solutions using Euler-Bernoulli beams on Winkle foundations, is compared against both model test and numerical simulation results. The modified analytical solution performs better than the traditional solution. Finally, the effects of foundation model, beam model, and tangential contact in simulating canal frost heave were discussed, and some measures to mitigate canal frost heave are proposed. The results show that solutions based on Winkler foundation overestimate frost heaves and tensile stresses of canal linings, and solutions without considering tangential contact only obtains overestimated frost heave. In addition, solutions based on Euler-Bernoulli beams underestimate frost heaves slightly and overestimate tensile stresses slightly. Therefore, the Pasternak foundation, Euler-Bernoulli beam, and tangential contact model can be used to simulate canal frost heave. The modified analytical solution directly uses field measurements of soil free frost heave to calculate canal frost heave, thereby enhancing result reliability. This analytical solution provides a simple method for canal frost heave design, and can be applied to frost heave analyses of flat and inclined structures.