Concrete segments are commonly utilized as linings in shield tunnels to support the load from the surrounding ground, with their mechanical performance playing a crucial role in ensuring tunnel safety. During the construction of shield tunnels, these segments are assembled on-site, and grouting is performed concurrently to promptly fill the gap between the segment and the surrounding ground. However, inadequate grouting can lead to the formation of voids, which are hidden construction defects that compromise the mechanical stability of the tunnel segments. This study explores the impact of grouting voids on the mechanical performance of concrete segmental tunnels during construction using a 3D numerical simulation. A 3D finite-element model of a segmented shield tunnel with grouting voids was developed based on the load-structure method. The analysis focused on the effects of void characteristics, such as their angle, position, and length, on the tunnel's mechanical behavior. The results indicate that voids located at the tunnel crown reduce the vertical convergence of the tunnel cross-section, while voids at the waist exacerbate its horizontal convergence. Additionally, the presence of voids alters the bending moment distribution in the segments. Compared to the case without a void, there is a reversal of the bending moment when the void is located at the crown, and the bending moment increases from −13 kN·m to 24 kN·m, potentially causing tensile damage. Furthermore, voids also induce stress concentration within the segments, and the maximum stress concentration factor (SCF) occurs at the center of the voids as 2.44. However, when a circumferential joint intersects the void, joint opening causes stress redistribution, with the most significant stress concentration appearing at 45° on both sides of the void. These findings contribute to better damage recognition and enhance the safety assurance of concrete shield tunnels.

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Monitoring of cracks and crack growth rates is a crucial aspect of structural health monitoring for concrete infrastructure, and multiple manual and automatic monitoring techniques have emerged over the years. This study focuses on an in-depth review of concrete crack sensing using distributed fiber optic sensing (DFOS) technology. DFOS provides the option to sample distributed data points through dedicated optical fibers or cables, thereby effectively addressing the spatial limitations associated with conventional discrete point sensors such as foil strain gauges and transducers. The main findings include that (1) smart concrete crack sensing generally involves three objectives: detecting crack initiation, identifying the crack location and determining the crack width and its evolution; (2) for DFOS used for crack sensing, the three main sensing principles are to measure localized strain spikes in optical fibers or cables that span across cracks, to detect signal intensity losses caused by micro-bending of optical fibers in proximity to cracks and to measure precise local temperature variations within the crack areas; (3) strain-based crack sensing has become the predominant method due to its superior sensing performance and application versatility. This dominance is supported by extensive experimental demonstrations and successful implementations in field monitoring practices; (4) the sensitivity of optical fibers or cables to concrete cracks depends on the installation method, while quantitative crack width measurements require the precise determination of crack locations followed by a subsequent integration or exponential fitting of strain along the length at fiber-concrete interface. This study helps to advance the application of the smart DFOS for structural health monitoring and maintenance of concrete infrastructures.@en

The development of modern cities has often led to increased traffic congestion and limited usable space. One effective solution to these problems is to construct roadway tunnels, which can expand urban space and alleviate traffic congestion. However, building large underpass tunnels in urban areas, especially in soft ground, presents technical challenges owing to its extensive environmental disturbance. The pipe-umbrella box jacking method offers a promising solution to these challenges as this method does not require breaking the ground from the surface or disrupting surface traffic. This study reviews the key techniques of the pipe-umbrella box jacking method for constructing large rectangular tunnels with small burial depths in soft ground. The study focuses on three main technical issues: pipe umbrella installation, soil face excavation, and box jacking control. Furthermore, two tunnel projects in Shanghai, China, are presented as case studies to showcase the construction challenges and countermeasures during box jacking tunneling with pipe-umbrella. Practical construction experience has revealed that the installation of the pipe umbrella before box jacking serves as an effective method of ground pre-support. However, its performance can be influenced by factors such as pipe installation deviation and inter-pipe connection design. Both open-face steel grid extrusion (SGE) shields and closed-face earth pressure balanced (EPB) shields can be used for soil excavation during box jacking, with different work modes resulting in distinct ground deformation patterns. Additionally, lubrication grouting has been found to effectively reduce friction resistance and affect vertical ground deformation. The study also examines the lessons learned from case studies and proposes optimization measures. This study can serve as a valuable reference for the tunneling industry, providing insights into the pipe-umbrella box jacking method and its application in constructing large rectangular tunnels in soft ground.@en

With the rapid urbanization and development of metropolises, urban road tunnels have been constructed at an increasing rate, significantly alleviating urban traffic pressure, and improving urban resilience. Fire hazards have become a major threat to modern road tunnels due to the growing popularity of electric vehicles and high-density transportation of goods, particularly flammable materials. Asphalt pavements, as an essential component of road tunnels, may release harmful effluences and smoke under high temperatures, exacerbating the fire and adding risk to life safety. It is hence critical to investigate fire-retarding asphalt materials and their potential use in urban road tunnels pavements. This paper provides a comprehensive review of fire-retarding asphalt pavements for urban road tunnel pavements. The review covers tunnel fire generation mechanisms, evaluation methods, flame retardants for asphalt pavements, and recent developments in flame retardant technologies. By investigating these aspects, this paper aims to better understand the flammability of asphalt mixtures and asphalt pavements in urban road tunnels, promote the research of flame-retardant technology, and ultimately reduce the damage and loss caused by asphalt road tunnel fire accidents. Additionally, this study identifies the limitations of current research and provides an outlook for future research to contribute to the resilience of urban road tunnel structures and the longer service life of asphalt pavement in semi-closed road tunnels.

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This study demonstrates the design and field implementation of an innovative servo concrete bracing system in foundation pit excavation. The bracing system comprises concrete struts, revised purlins, and hydraulic jacks, and its field performance is evaluated in a deep foundation pit project in Shanghai, China. The field measurements demonstrate that the servo bracing system effectively reduces the maximum lateral displacement of the retaining wall by up to 31%. Moreover, the servo jacks modify the wall’s flexural behavior by introducing local inflection points at certain depths and driving the displacement peak upward. Furthermore, the system’s performance varies with strut configuration, and servo forces influence not only the corresponding acting strut but also the adjacent struts’ behavior, implying that the monitoring scope should be expanded when applying the servo bracing system in actual engineering. This study provides a meaningful technical reference for future servo concrete bracing system applications in foundation pit engineering. @en

Distributed fiber optic sensors (DFOSs) possess the capability to measure strain and temperature variations over long distances, demonstrating outstanding potential for monitoring underground infrastructure. This study presents a state-of-the-art review of the DFOS applications for monitoring and assessing the deformation behavior of typical tunnel infrastructure, including bored tunnels, conventional tunnels, as well as immersed and cut-and-cover tunnels. DFOS systems based on Brillouin and Rayleigh scattering principles are both considered. When implementing DFOS monitoring, the fiber optic cable can be primarily installed along transverse and longitudinal directions to (1) measure distributed strains by continuously adhering the fiber to the structure's surface or embedding it in the lining, or (2) measure point displacements by spot-anchoring it on the lining surface. There are four critical aspects of DFOS monitoring, including proper selection of the sensing fiber, selection of the measuring principle for the specific application, design of an effective sensor layout, and establishment of robust field sensor instrumentation. These four issues are comprehensively discussed, and practical suggestions are provided for the implementation of DFOS in tunnel infrastructure monitoring.@en

Structural health monitoring is essential for the lifecycle maintenance of tunnel infrastructure. Distributed fiber-optic sensor (DFOS) technology, which is capable of distributed strain measurement and long-range sensing, is an ideal nondestructive testing (NDT) approach for monitoring linear infrastructures. This research aims to develop a distributed sensing network utilizing DFOS for structural integrity assessment of concrete immersed tunnels. The primary innovations of this study lie in the development of a general flowchart for establishing a sensing network and obtaining reliable field data, as well as its subsequent validation through a detailed case study. Concentrated joint deformations in typical immersed tunnels, detectable by the DFOS, are key indicators of structural integrity. This study addresses crucial elements of field monitoring system design, including the selection of appropriate optical fibers or cables and the determination of vital interrogator system parameters. It also covers sensor parameter determination, installation techniques, field data collection, and postanalysis. Furthermore, this research is exemplified by a case study that illustrates the successful implementation of a distributed sensing network in an operational immersed tunnel, and monitoring data reveals cyclic structural deformations under impacts of daily tide and seasonal temperature variations. The data obtained from this network play a significant role in subsequent condition assessments of tunnel structures. The research findings contribute to the assessment of large-scale infrastructure health conditions through the application of DFOS monitoring.@en

Quasi-rectangular shield tunneling is a cutting-edge trenchless method for constructing metro tunnels with double tubes, owing to its advantages in saving underground space and reducing ground disturbance. However, the conventional quasi-rectangular shield tunneling method is not applicable when constructing a tunnel without a center pillar, such as a scissor crossover section of a metro line. Therefore, the 0−θ tunneling method, which combines the quasi-rectangular shield and pipe jacking methods, was investigated in this study to solve the aforementioned construction challenges. This study presents a case study of the Sijiqing Station of the Hangzhou Metro Line 9 in China, in which the 0−θ method was first proposed and applied. Key techniques such as switching between two types of tunneling modes and the tunneling process control in complex construction environments were investigated. The results demonstrated that the 0−θ method can address the technical challenges presented by the post-transition line with a high curvature and a scissors crossover line. In addition, the adoption of the 0−θ method ensured that the transformation between shield tunneling and pipe jacking was safe and efficient. The ground settlement monitoring results demonstrated that the disturbance to the surrounding environment can be limited to a safe level. This case study contributes to the construction technology for a metro tunnel containing both post-transition lines with a small turning radius and a scissors crossover line. A practical construction experience and theoretical guidance were provided in this study, which are of significance for both the industry and academia.

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This paper presents a discussion of “Study on Asphalt-Cement
Materials for Seismic Isolation Layer of Shield Tunnels” by Qi Yang,
Ping Geng, Liangjie Wang, Bingbing Zhao, and Pingliang Chen.
https://doi.org/10.1061/(ASCE)MT.1943-5533.0004466.@en

Seasonal joint deformations within an immersed tunnel are important indicators to assess structural behavior and therefore should be monitored in detail. In this study, distributed optical fiber sensors (DOFS) are applied to precisely measure the seasonal joint deformations in an immersed tunnel for the first time. Measurements over a one-year period specifically reveal the impact of seasonal temperature variations on the joint opening and uneven settlement deformation. Field monitoring shows that the variation in joint opening exhibits a cyclic behavior and is strongly correlated with temperature change. The immersion joints generally show a larger range of seasonal opening (with a maximum of about 6 mm) than dilation joints, but at several dilation joints significant opening also occurs. The uneven or differential settlement at most joints stays below 1 mm, except at a few joints where the range is above 1 mm, which are indications of underlying structural defects in the tunnel. The observed joint uneven settlement also shows a seasonal variation, but the correlation with temperature is weak. The impacts of seasonal deformation on the structural integrity and watertightness of the tunnel are assessed, and further suggestions on tunnel maintenance and inspection are made.

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Daily and seasonal deformation behavior of immersed tunnels potentially impacts the structural integrity. In this study, distributed optical fiber sensors (DOFS) are used to instrument both dilation and immersion joints of the Heinenoordtunnel, an immersed tunnel in the Netherlands. This DOFS system proves capable of measuring joint opening and uneven settlement at half-hour intervals. The field monitoring shows the Heinenoordtunnel behaves more like a rigid body and exhibits a cyclic vertical movement under daily tide impacts over a period of 12 hours. Moreover, the joints show a cyclic seasonal opening which is negatively correlation with temperature variations, i.e. the tunnel joints are compressed when the outside temperature rises and vice versa. These monitoring results@en

The short-term deformation behavior of immersed tunnels due to daily or monthly temperature changes and tidal variations is often not monitored but forms important input for a structural health assessment of the tunnel. In this study, distributed optical fiber sensors (DOFSs) are used to monitor the short-term (daily and monthly) deformation behavior of an immersed tunnel. Joint opening and the relative settlement differences between tunnel elements are monitored simultaneously at subhour intervals. Measurements show that the variation in the joint opening is strongly correlated with temperature change, and the joint gap has a tendency to open at low temperatures and to close at increasing temperatures. Simultaneously, the entire immersed section behaves more like a rigid body and moves upwards and downwards periodically due to tidal fluctuations in the river, with an observed vertical movement of slightly less than one millimeter. The tide also causes local tilting of tunnel segments, and this tilting behavior differs between winter and summer, which implies that the (seasonal) temperature-induced joint deformations affect the robustness of the tunnel to tidal loads. A soil-tunnel structure interaction analysis reveals that the cyclic vertical movement of the tunnel is driven by retardation of the tidal wave in deeper soil layers, which can be captured by a coupled flow model. This study provides new insights into the short-term deformation behavior of immersed tunnels.

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Monitoring the deformations of immersed tunnels is important during the entire tunnel service life to assess the structural integrity of the tunnel. Conventional joint deformation monitoring is based on manual levelling measurements and normally occurs only at multi-year intervals, which does not allow to capture short term deformation behavior. In this study a new joint monitoring system using distributed optical fiber sensors (DOFS) is developed. A special sensor layout is designed that allows simultaneous measurements of both horizontal joint opening and vertical uneven settlement of the immersion and dilation joints. For this sensor scheme the transfer relation from fiber strain to joint deformation is derived and verified by in-lab experiments. The sensor system proves to be able to detect sub-millimeter joint deformations, indicating a more than sufficient accuracy for structural monitoring of immersed tunnel joints. Subsequently, the First Heinenoordtunnel in the Netherlands is instrumented using this distributed optical fiber sensing system, in order to obtain additional data for both long-term and short-term assessment of its structural condition.

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Backfill grouting plays a vital role in shield tunneling. This paper aims to present a comprehensive review of the development and progress of backfill grouting materials specifically designed for shield tunneling. Initially, the various components of grouts, such as pozzolanic materials, filling fine aggregates, and chemical additives, are introduced and discussed in detail. Subsequently, this study investigates critical properties including workability, mechanical properties, and durability of the grouts. Additionally, the principal factors influencing the properties are summarized, along with recommended ranges for specific geological conditions. Furthermore, the paper elucidates the diffusion mechanism of grouting mortars by presenting the current grouting models employed in shield tunneling. Recent advancements in grouting materials are extensively studied and extended, offering new perspectives for future grouting technology in shield tunneling. This study provides valuable insights into overcoming the existing challenges associated with shield tunnel grouting and promoting the evolution of current grouting materials.

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Pipe jacking has been the dominant trenchless technology for constructing small (\2 m) to medium-diameter (\4 m) tunnels. Uncertainties and construction difficulties increase significantly when the diameter of the tunnel exceeds 4 m. This paper presents a case study of the largest concrete pipe-jacking tunnel project in the world, the sewerage tunnel along Jinshan Lake, Zhenjiang, China. In this project, an underwater tunnel with a diameter of 4.67 m was constructed by the earth pressure balance (EPB) pipe-jacking method. The case study reports project background, and geological and hydrogeology conditions. The key techniques such as the selection of pipe-jacking machine, jacking force estimation and control, design of intermediate jacking station, grouting process control, launching, and reception of the tunnel boring machine, trajectory control of pipe jacking, and ventilation and gas monitoring during the construction period were investigated and discussed. Furthermore, to overcome the technical difficulties associated with the oversized jacked tunnel, the corresponding countermeasures were adopted point by point, so that the safety of the whole project could be guaranteed. This study filled the knowledge gap of technical know-how for large-diameter (over 4.5 m) pipe-jacking tunnel and is expected to provide practical guide for future large-diameter pipe-jacking tunnels.

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Modelling slab track-bridge interaction subject to a moving train usually involves solving complex high-dimensional matrix equations which is time-consuming. This research works to optimize the auto-assembling process in the slab track-bridge coupling matrices formulation and improve the computational efficiency by truncating the dynamic matrices used in time integral scheme. To achieve the above goals, the key issue is to appropriately couple the systems’ dynamic matrices in conditions where the elemental sizes of the track slab and the bridge are inconsistent in 3-D space. Besides, by firstly clarifying the degrees of freedom vector of the rail, the track slab and the bridge girder participated in each time step, dynamic matrices characterizing the train-slab track-bridge interaction are truncated with time to reduce the matrix size. This present study has demonstrated the solutions for above problems. Apart from model validations, some numerical examples are presented to show applicability of the proposed methods.@en

Distributed optical fiber sensors (DOFS) allow for distributed strain sensing and can be installed to function as extensometers for measuring point-displacements. This paper discusses the metrics of optimal sensing fiber selection for point-displacement measuring. Key metrics include the physical structure, mechanical parameters and light transmission coefficients. Calibration tests for verification of the optical fiber properties are designed and results of four fiber types are presented. Finally, creep and relaxation behavior of optical fibers is discussed based on manual tension test results, and a quantification model is proposed to assess the induced measurement error for sensing fiber. The maximum (absolute) measurement error for two common fiber types used in point displacement measurements is determined to be below 8%, and the study shows that pretensioning of the fiber helps to reduce such measurement errors.

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In this paper, a probabilistic analysis is implemented to determine the settlement of immersed tunnel elements and the effects of subsoil stiffness variability. A soil-structure interaction model is used to study the effects of subsoil stiffness on the shear behavior of tunnel joints. Site investigation data is obtained from the Hongkong-Zhuhai-Macau Bridge (HZMB) Tunnel project in China. Two probabilistic methods, the Point Estimate Method and Monte Carlo simulation, are compared when determining the tunnel settlements. The first is computationally more efficient and has sufficient accuracy, while the latter is extremely accurate with higher computational costs. Based on the settlement results, spatial variability of the soil stiffness is quantitatively assessed. The soil-structure interaction analysis and derivation of shear forces in tunnel joints is performed by coupling FE analysis to a Monte-Carlo model. The results show that tunnel structure behavior is significantly influenced by the soil parameters uncertainty.

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The performance and deformation of ballast bed are significantly influenced by the particle morphology (size and shape), the rheology (translation and rotation), and the degradation (breakage and abrasion). Regarding the ballast particle morphology, the ballast particle size is generally measured by sieving and described with the Particle Size Distribution (PSD), while the particle shape is normally classified as three characteristics, the form, angularity, and surface texture. Quantifying particle morphology with current manual methods is difficult to obtain accurate results (often subjective). Concerning the ballast particle rheology, almost all the related studies are based on numerical simulations, e.g. the Discrete Element Method (DEM). A limited number of studies were performed to record the translation and rotation with the electronic devices embedded in ballast layer. However, the numerical simulations can only precisely reflect the ballast particle rheology in quasi-static tests (e.g. direct shear test), and the electronic devices can only record the ballast particle rheology in the limited areas, where they were placed. The ballast breakage could be evaluated by the change of the PSD, but the determination of PSD involves significant errors. Additionally, the manual methods could not fully quantify the ballast abrasion. As a result, more accurate evaluation methods need to be developed and utilised for the validation and confirmation of the degradation-related studies.Towards these limitations, the studies on two-dimensional (2D) and three-dimensional (3D) image analysis methods for granular materials are reviewed, discussing their existing and potential utilisation in railway ballast applications. This paper can be of interest to the researchers, who are dealing with the performance and deformation of ballast bed. Additionally, a special attention can be paid to utilising the image analysis for accurate particle morphology quantification, particle rheology investigation and ballast degradation evaluation.@en