Twin-arch tunnels are widely employed in mountainous regions worldwide due to terrain constraints and alignment difficulties. However, when crossing near-fault seismic zones, these structures face significant seismic challenges. Compared with far-field earthquakes, near-fault earthquakes are characterized by long- period, highvelocity pulse motions, which impose more severe impacts on tunnel structures. Compared with conventional single-hole tunnels, twin-arch tunnels exhibit larger spans, weaker connections linking the main lining to the central wall, and unfavourable width-to-height ratios of the central wall, making their seismic resistance more vulnerable. Therefore, this study focuses on a near-fault twin-arch tunnel project and performs a shaking table model test to examine the dynamic response of the twin-arch tunnel and the performance of steel strip reinforcement under pulse-like ground motions. The results show that the acceleration amplification factor increases with elevation but decreases above the twin-arch tunnel with higher peak ground acceleration due to plastic flow damping. Conversely, it increases below the tunnel as the soil becomes denser and dissipates less energy. Under pulse-like ground motion, the acceleration amplification factor beneath the tunnel is lower than that under ordinary ground motion. The central wall mainly transmits seismic inertial forces, with its acceleration influenced by structural connections, whereas dynamic amplification in the inverted arch results from energy concentration because of its geometry. The arch shoulder adjacent to the central wall experiences the highest peak soil pressure. Pulse-like ground motion has a stronger impact on the soil above the tunnel and induces higher peak strains and internal forces than ordinary ground motion does. Steel strip reinforcement significantly reduces the overall response force of the twin-arch tunnel to pulse-like ground motion but increases the strain concentration at the reinforced-unreinforced junctions.

With the development of large-scale mechanized construction techniques, tunnel excavation is predominantly executed using either full-face or large-face methods, often supplemented with anchor-bolt reinforcement. However, the reinforcement mechanism of prestressed anchor bolts and the impact of excavation methods on the anchorage layer are yet to be comprehensively clarified through an integrated lens that bridges the macroscopic bearing capacity with mesoscopic mechanical properties. In this study, diverse support types and excavation methods were considered to perform a comprehensive series of loading and failure tests on tunnel anchorage layers. Through the incorporation of stress monitoring, P-wave velocity analysis, and particle image velocimetry (PIV), this study revealed the reinforcement mechanisms of prestressed anchor bolts. In parallel, it delineates the influence of excavation methods on both the macroscopic bearing capacity and mesoscopic mechanical properties of the anchorage layer. The experimental findings revealed that prestressed anchor-bolt reinforcement induced a progressive evolution in the surrounding rock, characterized by sequential modifications in stress, integrity, mechanical properties, ductility, and bearing capacity. Relative to the unsupported conditions, the prestressed anchor-bolt reinforcement yielded substantial enhancements: stress improved by approximately 245.5%, integrity by 14.3%, mechanical properties by 9.8%, ductility by 147.7%, and bearing capacity by up to 500%. In unsupported conditions or with anchor bolts, large-face excavation demonstrated superior performance relative to full-face excavation, enhancing both the mesoscopic mechanical properties and macroscopic bearing capacity by approximately 2.8%–6.9% and 50%–100%, respectively. The findings indicate that large-face excavation is the preferred method under these support conditions. However, when prestressed anchor-bolt reinforcement is used, the differences between the two construction methods become negligible, rendering full-face excavation the more practical construction option.

When a train catches fire inside an extra-long tunnel, fixed-point rescue is widely used. The distance between fixed points determines whether the train can stop safely. However, in many countries, regulations dictate a maximum of 20 km, and each tunnel and train have different characteristics. Fixed points are mainly designed based on experience, which may be detrimental to precise risk control. In this paper, a risk probability calculation model for fire train fixed-point stopping in railway tunnels is developed, which takes into account train operation characteristics and tunnel engineering parameters. Based on quantitative calculations, the role of major factors is discussed, and an empirical formula for safe deceleration at different station distances is established. The study reveals that: 1) the reliability of the 20-km distance standard is contingent on train equilibrium speeds exceeding 80 km/h; 2) at a distance of 20 km, the risk probability of fixed-point stopping varies from 0.011% to 10.06%. Better train braking capacity and greater uphill slopes can reduce the risk, with the latter playing a more significant role; and 3) there is a safe deceleration that maintains the risk probability below 0.03%. This is determined primarily by longitudinal grades and train braking capacity. As the fixed-point distance increases, the required safe deceleration grows exponentially.

The prestress value of the anchor bolt significantly influences the support effect of weak surrounding rock. An insufficient prestress value may lead to inadequate support, while an excessively high prestress value can cause damage to the surrounding rock. Previous studies have predominantly focused on failure at the anchorage segment, while neglecting failure at the loading end. However, in weak surrounding rock, excessively high prestress may result in crushing failure at the loading end, which in turn adversely affects the overall support effectiveness. To address this issue, a theoretical model is developed that accounts for both shear-slip failure of the anchorage section and crushing failure of the surrounding rock at the loading end. The model is used to predict the ultimate anchorage force (Pmax), the ultimate prestress at the loading end (Fbmax), as well as key design parameters of prestressed anchor bolts, including the bearing plate size (B) and the anchorage length (lg). Its accuracy is validated against pull-out test data from four anchorage systems, with deviations below 20%. Taking Grade IV and Grade V surrounding rock as examples, the model is applied to determine appropriate prestress levels and prestressed anchor bolt parameters. Based on the shear-slip failure model, the variation of Pmax with different anchorage lengths and rock mass integrity is analyzed. Based on the loading end crushing failure model, the stress distribution characteristics of the surrounding rock under single-bolt support and systematic bolting are comparatively investigated, and the failure depth corresponding to loading end crushing is determined. When the anchor bolt prestress is 60 kN, reasonable design values for the lg and B are obtained. This research can provide theoretical support for tunnel designers to dynamically evaluate the rationality of anchor bolt parameters according to on-site geological conditions.

Seismic fault zones are widely distributed worldwide and tectonic compression often gives rise to geologically complex mountain ranges. In such mountainous areas, where rugged terrain and weak geological conditions prevail, twin-arch tunnels are often the only viable solution for dual-line highways or railways due to limited construction space and alignment constraints. When these tunnels cross near-fault seismic zones, they encounter substantial earthquake-engineering difficulties, especially from the long-period pulse effects and intense vertical ground motions characteristic of near-fault earthquakes. However, existing studies lack a theoretical method specifically suited to the assessment of seismic performance concerning near-fault twin-arch tunnels. To address this gap, this study first developed a seismic mechanical model specifically designed for twin-arch tunnels. Next, numerical simulation was combined to obtain methods for determining the relevant computational parameters of the theoretical model. Subsequently, the validity of the seismic calculation method for twin-arch tunnels was confirmed through numerical simulations and physical model tests. Lastly, a parametric analysis was performed using the proposed methodology. The analysis reveals a nuanced relationship between the structural geometry and stress response of the twin-arch tunnel. Moderately increasing the centre wall’s width-to-height ratio is shown to alleviate stress concentrations therein. Conversely, an excessive ratio exacerbates both tensile and compressive stresses within the primary tunnel liner. Regarding geotechnical and seismic parameters, a greater burial depth improves structural stability by inducing higher compressive stress, thereby suppressing tensile failure. Finally, the horizontal PGV/PGA ratio exerts a limited influence on the inner liner but has a more pronounced impact on the outer liner and the centre wall.

In groundwater environments, tunnel shotcrete remains in long-term contact with seepage water and is therefore susceptible to calcium leaching deterioration, which may threaten the long-term structural safety and service performance of tunnels. In this study, laboratory experiments were conducted to simulate the seepage-induced leaching process of shotcrete under tunnel groundwater conditions. Combined with multiple microstructural characterization techniques, the leaching zoning characteristics of shotcrete were systematically investigated. Based on the Ca/Si ratio, quantitative classification criteria for different leaching zones were established, including the un-leached zone (Ca/Si > 3.2), leaching transition Zone I (2.1 < Ca/Si <= 3.1), leaching transition Zone II (1.25 < Ca/Si <= 2.1), and the fully leached zone (1.25 < Ca/Si <= 2.1). The leaching depths were measured using both the phenolphthalein method and the Ca/Si method, and the effects of groundwater flow velocity, HCO3- concentration, accelerator dosage, and water-to-binder ratio on the development of different leaching zones were systematically analyzed. On this basis, a combined phenolphthalein-Ca/Si testing method was proposed, and a predictive model for shotcrete leaching depth was established, with the prediction error of less than 6%. Based on the proposed model, the long-term service performance of shotcrete in operating tunnels was further evaluated. The results indicate that the leached zone ratio of shotcrete may reach up to 40.4% after 100 years of service. This study provides a theoretical basis and technical support for the durability assessment and service-life prediction of shotcrete in water-rich tunnels.

Excessive surrounding rock pressure induced by large deformation often results in the failure of rigid support systems. Existing research on large deformation control has primarily focused on soft rock tunnels. To address the challenge of support failure in high-geostress hard-rock tunnels, an energy-dissipating device was developed to enable the controlled release of rock mass energy. Laboratory tests were conducted to establish the relationship between key design parameters and mechanical performance indicators of the device. Based on laboratory testing and field monitoring data from sections of large deformation, the structural design parameters for the energy-dissipating device were established. The designed energy-dissipating yielding support was subsequently implemented in a high in-situ geostress hard rock tunnel undergoing large deformation. Field monitoring results indicate that the device significantly increased the allowable deformation capacity and effectively reduced stress levels in the support structure. Consequently, common failure issues such as shotcrete cracking, spalling, and steel frame distortion were effectively mitigated. The proposed energy-dissipating device demonstrates distinct advantages in terms of structural integrity, material accessibility, manufacturability, economic efficiency, and ease of installation.

Most of the existing deformation control criteria for shield tunnel linings under adjacent construction disturbance are empirical and specific case-based, which is lack of mechanism-driven analysis and generalizability. In this paper, a systematic comparison of mechanical behavior and failure mechanism for shield tunnel linings under upper loading, lateral unloading and upper unloading condition was conducted by employing elaborate numerical simulation method. A performance-based deformation control criteria and safety level classification standards of tunnel lining structure were proposed based on the analysis of structural performance evolution. The results show that the deformation outline of the linings under upper loading, lateral unloading and upper unloading conditions are symmetric horizontal oval, asymmetrical horizontal oval and symmetric vertical oval, respectively. Deformation mode has significant impact on crack distribution and development. Crack distribution for upper loading case is basically symmetric along the 0 degrees-180 degrees axis while the crack distribution for lateral unloading case is asymmetric with expanding towards the excavation side. The variation of convergence deformation curve for segmental linings presents a multi-stage evolution combining linear and nonlinear characteristics, which can be divided into four stages: linear growth stage, nonlinear growth stage, rapid growth stage and failure stage. Based on the performance-based deformation control criteria, the specific range of convergence deformation for construction (CP1) and reinforcement controlling points (CP2) in upper loading/lateral unloading cases were given as 4.8 parts per thousand similar to 6.3 parts per thousand D /6.5 parts per thousand similar to 6.9 parts per thousand D and 12 parts per thousand similar to 13.3 parts per thousand D /11.4 parts per thousand similar to 12 parts per thousand D, respectively. The findings will provide guidance for the tunnel structural protection and maintenance.

In a mixed traffic flow environment, the complex vehicle following behavior and the illuminance changes in the tunnel entrance/exit section can exacerbate the instability of traffic flow characteristics. In order to investigate the characteristics of the mixed traffic flow in the tunnel entrance and exit section affected by illuminance, a model of human-driven vehicles (HDVs) is constructed based on experiments. Then, considering the degradation phenomenon of Intelligent Connected Vehicles (ICVs), we construct the ICVs following the model by considering the delay of vehicle recognition time caused by illuminance change. Finally, the mixed traffic flow model is constructed, and the influence of illuminance on the flow in the entrance/exit section under different ICVs penetration rates is analyzed by simulation and evaluated in terms of access efficiency and stability. The results show that when the penetration rate of ICVs in the mixed traffic flow reaches $100 \%$, the maximum flow rate of the traffic flow is enhanced by 1.4 times. The standard deviation of vehicle speed is negatively correlated with the penetration rate of ICVs, which is decreased by $85.45 \%$, and the ICVs reduce the influence of illuminance on the traffic flow and effectively reduce the speed fluctuation of the vehicles in the entrance and exit sections.

This study introduces a cloud-native framework that alleviates the intricate workflows, limited real-time performance, and heavy software dependency inherent in conventional tunnel-support analyses. A purely data-driven surrogate, trained on 4,933 heterogeneous tunnel sections, replaces time-consuming finite-difference simulations. Latin-hypercube sampling was used to populate the design space and to train a single full-section model that jointly predicts all stability indices. Displacement errors typically fall between 1.3 mm and 2.1 mm, with safety-factor errors around 0.11, while inference is approximately 100 times faster than finite-difference solutions. A lightweight blocking multi-thread HTTP service encapsulates the model, delivering real-time support-design feedback via standard web browsers. The proposed approach lowers the technical threshold of tunnel-support analysis and provides an efficient, real-time numerical computing solution for tunnelling in complex geological conditions.

Spill fires during fuel transportation can be extremely catastrophic when they occur in tunnels due to the confined space. Understanding flame behavior is essential for managing the associated fuel management and thermal processes. This article investigates the flame base drag length and flame tilt angle of diffusion flames resulting from spill fires in tunnels, with an aim to shed light on how the confinement effect in the tunnel space affects the diffusion fire flame morphology. The results show that tunnel walls and ceiling influence flame geometry by increasing the flame tilt angle and extending the flame base along the tunnel floor. In addition, the tunnel cross-sectional aspect ratio, i.e., the height to width ratio, has been determined as an additional influencing factor, particularly affecting the flame tilt angle. Notably, the flame tilt angle tends to increase as the tunnel height increases, while maintaining a constant cross-sectional area. The proposed correlations also suggest that the equivalent ventilation rate, used to correlate flame geometry in open spaces, is approximately 0.91 times the reference wind speed measured at a height of 2 m above the ground in open fields. The findings provide an accurate method for depicting flame morphology in tunnels and for harmonizing flame geometry correlations across diverse indoor and outdoor environments.

During strike-slip fault dislocation, multiple fault planes are commonly observed. The resulting permanent ground deformation can lead to profound structural damage to tunnels. However, existing analytical models do not consider multiple fault planes. Instead, they concentrate the entire fault displacement onto a single fault plane for analysis, thereby giving rise to notable errors in the calculated results. To address this issue, a refined nonlinear theoretical model was established to analyze the mechanical responses of the tunnels subjected to multiple strike-slip fault dislocations. The analytical model considers the number of fault planes, nonlinear soil‒tunnel interactions, geometric nonlinearity, and fault zone width, leading to a significant improvement in its range of applicability and calculation accuracy. The results of the analytical model are in agreement, both qualitatively and quantitatively, with the model test and numerical results. Then, based on the proposed theoretical model, a sensitivity analysis of parameters was conducted, focusing on the variables such as the number of fault planes, fault plane distance (d), fault displacement ratio (η), burial depth (C), crossing angle (β), tunnel diameter (D), fault zone width (Wf), and strike-slip fault displacement (Δfs). The results show that the peak shear force (Vmax), bending moment (Mmax), and axial force (Nmax) decrease with increasing d. The Vmax of the tunnel is found at the fault plane with the largest fault displacement. C, D, and Δfs contribute to the increases in Vmax, Mmax, and Nmax. Additionally, increasing the number of fault planes reduces Vmax and Mmax, whereas the variation in Nmax remains minimal.

This study investigates the effect of cross braces on the lateral stability and curve passability of heavy haul freight wagons. A vehicle kinetic model of a heavy haul freight wagon was established based on the vehicle-track coupling kinetic theory. The dynamic performance of empty and heavy freight wagons in a straight line section and a curve section with or without cross braces was simulated and analysed. The running stability, acceleration, safety, and wear indexes were analysed. The results demonstrate that the utilisation of cross braces has a considerable impact on enhancing the bogie's stability and the vehicle's critical speed. With cross braces, the vehicle can meet operational requirements at speeds of up to 120 km/h, compared to 70 km/h without them. Nonetheless, it is recommended to install cross braces to minimize wear and tear. The general trend in straight and curved section operation is that the presence or absence of cross braces has a minimal effect on acceleration and stability. Without cross braces, the wheelset lateral force increases, the attack angle of the wheelset decreases, and the wear index increases. Conversely, the use of cross braces can reduce wheel wear.

A review concerning the development and performance of water-based fixed fire fighting systems (WFFFS) in road tunnels is presented. Conventional WFFFS types, including automatic sprinkler system, deluge system, and water mist system, have gained recognition as valuable supplements to ventilation systems in combating tunnel fires. Full-scale projects through international collaborations have been instrumental in formulating guidelines for facilitating the installation and optimal functioning of WFFFS in road tunnels. Extensive small-scale experiments, along with numerical investigations documented in literature, greatly contribute to complete exploring the performance of WFFFS. Owing to these efforts, advantages of WFFFS, such as reducing fire size, cooling hot smoke, and limiting smoke spread are well affirmed, and the shed mechanisms behind these fire fighting effects now have been well understood. However, WFFFS also present challenges from aspects such as disrupting smoke stratification and augmenting the production of toxic combustion products. From the review, further development of WFFFS in road tunnels relies on establishing methods of specifying design fires that accounts for suppression effects of WFFFS, as well as gaining deeper insights into the cooperative and interactive mechanisms between WFFFS and mechanical ventilation systems.

Clay-sulfate rock, due to its significant expansiveness, poses a major challenge to tunnel engineering design. Understanding its swelling potential, which includes both swelling strain and swelling pressure, is a key prerequisite for ensuring safe and stable tunnel design. Water significantly influences the swelling potential and constitutive model of clay-sulfate rock. Four types of swelling tests (FST, SPT, WSPT, and CLST) were carried out on clay-sulfate rock. The results show that with initial water contents of 0

Previous analytical models of fault-crossing tunnels have predominantly been based on the assumption of the tunnel being continuous, which is barely present in practical engineering projects. In this study, a theoretical model for the mechanical analysis of segmental tunnels subjected to reverse fault dislocation is proposed, wherein the interactions between the segmental lining and joints are incorporated, significantly improving the application range. A numerical concrete damaged plasticity (CDP) model is subsequently developed, which incorporates both the segmental lining and joints, to investigate the damage characteristics of the segmental tunnel under reverse faulting. Compared with the experimental and numerical results, the proposed theoretical model not only precisely captures the segmental character of the mechanical response of the segmental tunnel under reverse fault dislocation but also has a high degree of computational accuracy, with a maximum error of approximately 7.2 %. Drawing upon the theoretical and numerical models, a parameter analysis is conducted on the segment length (Lseg), distribution range of the segmental lining (Wseg), and fault displacement (Rfd). The results show that the Lseg value should not exceed 20 m and that the Wseg value should be no less than the fault zone width.

To gain an in-depth understanding of the failure characteristics and reinforcement mechanisms of the anchorage layer in tunnel, a series of loading failure experiments were conducted. By isolating the anchorage layer, its bearing capacity was effectively quantified. This study systematically examined the stress, deformation, and failure characteristics of the surrounding rock under unsupported conditions (surrounding rock layer, SRL), anchor bolt support (anchorage layer, AL), and pre-tensioned anchor bolt support (pre-tensioned anchorage layer, PAL). Furthermore, the incorporation of P-wave velocity (Vp) and PIV testing provided a robust framework for elucidating the reinforcement mechanisms of the anchorage layer. The failure of SRL initiated at the tunnel shoulders, whereas for AL and PAL, failure originated at the tunnel crown. The Vp at the tunnel crown of PAL and AL exhibited increases of approximately 11.0

The reinforcement of tunnels in weak surrounding rock using anchor bolts establishes an anchorage layer that collaborates in bearing loads, constituting a widely employed reinforcement technique. However, the bearing capacity of the anchorage layer remains unquantified, its failure response inadequately investigated, and the reinforcement mechanism of anchor bolts has yet to be comprehensively elucidated through a synthesis of mesoscopic and macroscopic perspectives. This study undertakes a series of anchorage layer loading and failure experiments under varying prestress values, aiming to quantify the bearing capacity by isolating the anchorage layer and investigating its failure response. To elucidate the reinforcement mechanism of the anchorage layer, the experiments integrate stress testing, P-wave velocity analysis, and PIV measurement. These methods provide a comprehensive perspective, elucidating stress distribution, integrity, and mechanical parameters at the mesoscopic perspective, alongside deformation and bearing capacity at the macroscopic perspective. The experimental results indicate that the failure of the surrounding rock layer (SRL) begins at the tunnel shoulder, while the failure of the anchorage layer (AL) and prestressed anchorage layer (PAL) begins at the tunnel crown, and the failure modes of AL and PAL are consistent. In comparison with SRL, both AL and PAL demonstrate notable enhancements in stress distribution, integrity, mechanical parameters, ductility, and bearing capacity. Notably, increasing the value of prestress further amplifies these improvements. For instance, relative to SRL, PAL (6P) exhibits a remarkable 244% increase in radial stress, a 23.26% rise in integrity, improvements of 23.21% in elastic modulus and 15.46% in cohesion, an approximate 217.75% enhancement in failure displacement, and striking increments of around 400% and 325% in failure load and ultimate load, respectively. From the mesoscopic perspective, the installation of anchor bolts modulates the stress distribution within the surrounding rock, enhances its integrity, and strengthens its mechanical parameters. This is subsequently manifested from the macroscopic perspective, where the ductility and bearing capacity of the surrounding rock are significantly enhanced.

Bolts reinforcing surrounding rock is a common reinforcement method. In this paper, through finite-discrete element coupling numerical simulations and model tests, the beam effect within the bolt reinforcement layer (BRL) is visualized, and the failure response of the overall BRL is investigated. First, a finite-discrete element (FDM-DEM) coupled numerical model is developed to conduct loading failure tests on BRL. Second, model tests are conducted to validate the FDM-DEM numerical model, which exhibits concordance with the failure location, evolution process, failure load (6F), and ultimate load (6U) of experimental results observed in BRL. While BRL without bolt reinforcement (BRLW) presents a "fragmentation" failure, BRL exhibits a comprehensive deformation failure owing to the beam effect, characterized by minor rockfalls of the tunnel crown. Finally, a parameter analysis was conducted to investigate the impact of bolt length (Lb), bolt spacing (Bs), and bolting range (Br) on the failure response and beam effect of BRL. The results suggest that the increase of Lb and Br and the decrease of Bs will each strengthen the beam effect of BRL, consequently enhancing the 6F and 6U.

China’s loess distribution area covers 640,000 km2, accounting for about 6.6% of its land area and about 4.9% of the world’s total loess distribution area. By the end of 2024, 838 highway loess tunnels with a length of 917 km and 470 railway loess tunnels with a length of 520 km had been built in China. With accumulated construction experience, China’s loess tunnel construction technology has been significantly developed. There is an urgent need to systematically summarize and sort out the latest achievements and research results in the field of loess tunnels, and to form a set of strong theoretical system and effective technical support, so as to guide the design and construction of loess tunnels for high-quality projects, serve the strategy of strengthening China’s transportation, expand the field of overseas engineering, and enhance the international competitiveness. The Journal of Traffic and Transportation Engineering (English Edition) invites more than 20 experts and scholars in the field of tunnel engineering to analyze the current situation and trend based on the engineering characteristics of loess, loess tunnel design theories and methods, loess tunnel construction methods and technologies, disease perception and prevention measures in loess tunnel operation and maintenance, and typical loess tunnel engineering cases and other topics. Overall, this review paper is able to provide references and insights for researchers and engineers in the field of loess tunnel engineering.