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[Display omitted] • Coupled CFD-FEM thermo-mechanical analysis method was proposed. • Elaborate numerical model was established based on a large shield tunnel. • Structural fire safety of composite lining shield tunnel was investigated. • Effects of the timing of secondary lining and the tunnel depth were discussed. The composite lining has been extensively used in shield tunnels. In this study, the thermos-mechanical behaviors of composite lining shield tunnel structures under fire conditions were investigated. A method combined by computational fluid dynamics (CFD) and finite element method (FEM) is proposed to simulate the temperature distribution within the tunnel, the initial stress state of the structure, and the thermo-mechanical coupling process. The elaborate numerical model was established based on a large composite lining shield tunnel. The influence of the timing of secondary lining application and tunnel burial depth on the deformation, internal forces, and structural damage of the composite tunnel structure under fire conditions was investigated. According to the results, the composite lining experiences a significant increase in temperature on the fire-exposed side, which results in significant changes in bending moments and axial forces of the lining during fire. The timing of secondary lining application and burial depth have a significant impact on tunnel deformation during the fire. In comparison to other locations in the tunnel, the application of the secondary lining has the most significant impact on the internal forces at the tunnel arch position. And the burial depth has a considerable influence on the internal forces at both the tunnel top and arch positions. The post-fire damage to the secondary lining is more severe with the post casting and shallow depth. These results provide structural fire safety support for large shield tunnels with composite linings. [ABSTRACT FROM AUTHOR]

• A horizontal displacement formula for stratum due to shield tunnelling with a large longitudinal slope is presented. • A horizontal response analytical calculation model of the pile induced by shield tunnelling considering a large longitudinal slope is established. • The time-varying law of the horizontal response of the pile is analysed. • A parametric analysis evaluates the longitudinal slope angle, shield tunnelling parameters, and soil modulus. The horizontal displacement and bending moment of piles due to adjacent shield tunnelling with a large longitudinal slope were investigated. An analytical calculation method for the horizontal response of shield tunnelling to piles was presented, considering the included angle, the tunnel-soil-pile interaction, and the shielding effect of the pile group. A new formula for soil horizontal deformation was derived by combining the cutterhead thrust force, the frictional force between the shield and soil, the grouting pressure and the soil loss, and the pile response was calculated as it was subjected to the generated ground displacement. The reliability of the analytical method was verified by the existing analytical solution, field monitoring data, and numerical simulation results. A parametric analysis of the longitudinal slope angle, shield tunnelling parameters, and soil modulus was conducted to assess their influences on tunnelling-induced pile deflection and bending moment. Finding that the increase in the longitudinal horizontal response of the pile is more prominent before the shield arrives at its side and the instant peak values of the horizontal response of the pile increase with increasing longitudinal slope angle. As the shield arrives at the pile side, the increase in the longitudinal horizontal response of the pile reaches the maximum value, and the influence of changes in the longitudinal slope angle on the horizontal response of the pile is not significant. The transverse horizontal response of the pile changes more after the shield passes through it and shows an increasing trend, the peak value of the transverse horizontal response of the pile is nearly 1 time that of the shield arriving at its side with passing through it of 20 rings. The changing trend of the horizontal response of the pile with increasing longitudinal slope angle after the shield passes the pile is opposite to that before arriving at its side. The cutterhead thrust force and the shield shell friction force are suggested to be positively correlated with the grouting pressure to reduce pile deformation. The horizontal response of piles caused by the shield tunnelling load decreases with increasing longitudinal slope angle after the shield passes through. The horizontal response of the pile is more significantly affected by the change in the longitudinal slope angle of shield tunnelling in the stratum with a smaller soil elastic modulus. [ABSTRACT FROM AUTHOR]

• Existing tunnel exceeds M−N failure envelope in layered soil after new tunnelling. • Greater settlement and structure response occur owing to the larger soil arch formed. • Assuming a uniform ground can lead to unsafe design on twin-tunnel interaction. Tunnelling induces stress change and displacement in the ground. The excavation of a new tunnel in stratified soil can trigger different patterns of stress redistribution, which may adversely influence nearby tunnels. Research on multi-tunnel interaction has mainly been performed on the assumption of a uniform ground. The effects of different soil stratifications on tunnelling interaction remain poorly understood. In this paper, three-dimensional numerical parametric studies verified by previous centrifuge tests were carried out to analyse the twin tunnelling effects in two-layered soil. An advanced hypoplastic constitutive model that can capture stress-, path-, and strain-dependency of soil behaviour is adopted. Numerical cases investigated include perpendicular twin tunnelling in two sand layers with different relative densities and the location of the interface between the two sand layers. It is revealed that larger settlements and a wider surface settlement trough occur when tunnelling in two-layered soil strata than in a uniform ground. This is because of the wider and larger soil arch induced in two-layered soil strata. The structural response including tunnel deformation, induced bending moment, and induced hoop stress of the existing tunnel can be greater when tunnelling in layered soil strata than in a uniform ground owing to larger stress relief. Moreover, the combination of bending moment and hoop stress can exceed the M−N failure envelope of the structure in layered soil. A conventional simplified assumption of a uniform ground can underestimate of the influence of new tunnel excavation on existing tunnels, resulting in unsafe designs. [ABSTRACT FROM AUTHOR]

• Shaking table tests are conducted for near-valley station with diaphragm walls. • Impact of diaphragm walls on seismic responses of station is fully explored. • Diaphragm walls reduce lateral racking and shear effect of near-valley station. • Adverse effects of diaphragm walls on near-valley station are presented. • The influence of the valley depth-to-width ratio is discussed. Diaphragm walls are commonly employed as a permanent support for the building of metro stations near urban valley, and in conjunction with the interior sidewalls of the station structure to withstand the pressure from surrounding soils. Despite their prevalent use, the effect of underground diaphragm walls on the seismic response of stations is not yet fully understood. In this paper, a series of 1-g shaking table tests is designed to investigate the seismic response of a near-valley station with underground diaphragm walls within the elastic range. Modeling the stratum-structure-diaphragm walls system is accomplished by employing granular concrete reinforced with galvanized steel wires and synthetic model soils, and a station without diaphragm walls is included, serving as a benchmark for comparative analysis to understand the influence of diaphragm walls on the seismic behavior of the station. The experiment was designed for three depth-to-width ratios (DWRs), i.e. 1/3, 1/4, and 1/8, of arc-shaped valley topography, as well as the seismic excitations for the test include actual seismic records with the amplitude of 0.2 g, 0.4 g, and 0.8 g, respectively. Results show that the underground diaphragm walls enhance the lateral stiffness of the near-valley station compared to structures without diaphragm walls, and thus significantly reducing the racking deformation of structure during earthquakes. The presence of diaphragm wall would decrease the amplification of dynamic earth pressure caused by valley effect at the structural sidewalls, and significantly reduce the lateral vibration and shear effect of the station near a valley with a larger DWR. Notably, bending moment response at the connection between the diaphragm walls and structural sidewalls are dramatically amplified under strong seismic loading, and such adverse effects gradually increase with the DWR of the valley. [ABSTRACT FROM AUTHOR]

• Analysis on the geometric nonlinearity of longitudinal joint. • A new analytical model of joint with considering geometric nonlinearity. • Mechanical behavior of joint under complex loading condition. The previous mechanical analysis on the longitudinal joint of segmental linings commonly used the plane section assumption, in which the strains on the normal section are linearly and continuously distributed. This was proven to be overidealized according to a series of full-scale tests in existing literatures. In this paper, the nonlinear characteristics of deformation behavior for longitudinal joint were investigated with emphasis on the geometric nonlinearity of joint. A new analytical model of longitudinal joint was proposed with considering the geometric nonlinearity via employing the local plane section assumption to describe the strain distribution of joint section. The effectiveness of the proposed model was validated and parametric study (e.g. loading cycles and peak bending moment) was conducted. The results indicated the geometric nonlinearity of longitudinal joint refers to the increase area of free surface for the joint section. This nonlinearity will cause the discontinuous change of contact state which can be intuitively charactered by the height of concrete compression zone. Without considering the geometric nonlinearity, the area of concrete compression zone and bending capacity of the joint are overestimated by 45% to 100% based on the calculation result of the verification case. The increasing load cycles and peak load value deteriorate the deformation recoverability and ultimate capacity of joints. The degree of geometric nonlinearity is intensified with the increase of load cycles and peak load value but it can be alleviated if the concrete in external edge contact during overloading process. [ABSTRACT FROM AUTHOR]

This study formulates a tensionless Winkler solution to address the nonlinear behaviours in soil-pipe interactions induced by tunnelling. The solution accounts for critical aspects including joint stiffness degradation, gap formation, local soil yielding, and variations in soil bearing characteristics. Its reliability is validated through centrifuge model tests, proving its superiority over traditional linear approaches that often overestimate pipeline deflections and bending moments. Parametric studies further explore the impacts of greenfield settlements, joint position and stiffness, relative pipe-soil stiffness, and soil ultimate bearing capacity on jointed pipeline responses. [ABSTRACT FROM AUTHOR]

• A novel longitudinal beam-spring model of shield tunnel containing torsion. • The nonlinear torsional effect of joints due to bending moment is considered. • The iterative algorithm to obtain tunnel cross-section torsion is established. The longitudinal bending and shear deformations of existing shield tunnel induced by asymmetrical disturbances are normally accompanied by the cross-sectional torsion, and the longitudinal bending moment can result in the nonlinear torsional effect of circumferential joints. However, most existing soil-tunnel analysis methods simplify the shield tunnel as a one-dimensional continuous beam with constant stiffness lying on the elastic foundations, failing to consider its twisting motion. A novel analytical approach is therefore proposed in this work to better investigate the longitudinal bending-shear-torsional behaviors of in-service shield tunnel due to asymmetrical external loads. In this method, the existing shield tunnel is extended as a sequence of torsional Timoshenko short beams joined by linear rotational springs, linear shear springs and nonlinear torsional springs along its longitudinal direction. Simultaneously, the tangential and normal Winkler soil springs are evenly distributed on tunnel profile to emulate the soil responses to existing shield tunnel. The longitudinal displacement discontinuities at circumferential joints of existing shield tunnel are then solved using the state space method and the finite difference solution. In addition, the formulas of joint spring stiffness as the decisive parameters adopted in proposed approach are deduced. The effectiveness of the present solution is ultimately examined through two well-documented case histories respectively associated with up-crossing tunnelling and adjacent excavation, as well as comparison with previous analytical solutions. The impacts of several dominant factors on surface surcharge induced-tunnel longitudinal deformations are also further discussed, encompassing the joint rotational stiffness, longitudinal reinforced length of tunnel and load-tunnel relative location. The research implied that the proposed solution can satisfactorily evaluate the longitudinal discontinuous distributions of vertical displacement and cross-section torsion of shield tunnel induced by asymmetrical loads, in which the deformations of tunnel lining rings are mainly represented by rigid motion. The joint torsional property of shield tunnel is highly sensitive to its longitudinal bending moment, which makes a significant positive impact on the reduction of tunnel cross-section torsion. [ABSTRACT FROM AUTHOR]

• A novel Bayesian inference method was proposed to study deep excavations. • Machine learning with Bayesian Network could predict soil-structure interactions. • The Bayesian Network model could predict ground/wall responses accurately. • The sequential updating process could improve model performance effectively. • The Bayesian framework could be applied to various geological settings reasonably. The demand for deep excavations to construct underground infrastructure has been growing in metropolitan cities due to rapid urban development and limited land space. In this original research, a machine learning technique based on Bayesian Network is proposed to evaluate and predict soil-structure interactions caused by braced excavations. A novel framework based on Bayesian Network incorporating the Bayesian updating process was proposed to study the ground and wall responses induced by deep excavations. Three established case studies collected from the literature were used to implement and showcase the novel concept. Besides, original two-dimensional finite element models were developed to simulate the first two case studies involving deep excavations in clay in order to generate sufficient ground and wall response data for the training and updating phases of the proposed Bayesian model. The third deep excavation case study carried out in a totally different geology (predominantly sand) was then used to assess and verify the robustness of the Bayesian framework in predicting the soil-structure responses. It was eventually established that the developed novel Bayesian Network model managed to successfully predict the wall deflections and bending moments as well as the ground surface settlements of the third case study with good accuracy through using only two case studies as priors via the versatile sequential updating processes, albeit having totally different geological settings. [ABSTRACT FROM AUTHOR]

Progressive collapses of tied-back excavations caused by the failure of some anchors often occur. However, the mechanism through which partial failure evolves to global failure and how the local failure rate influences progressive collapse has not been investigated. In this study, anchor failure experiments involving tied-back excavation were designed to explore the load transfer path and rule in the case of partial anchor failure. The results showed that, anchor failure leads to an increase in the axial force on adjacent anchors, an increase in the maximum bending moment of capping beams, and easy damage in capping beams due to structural reinforcement. The lower the anchor elevation is, the greater the excavation depth, and the greater the load transfer coefficient (axial force). The failure rate of anchors has an effect on the stress redistribution of the structure and the soil arching effect. In multi-row anchored pile retaining excavations, more rows of anchor failure could trigger the remaining anchors failure because the load transfer coefficient (axial force) is greater for these anchors, which leads to large-scale excavation collapse. [ABSTRACT FROM AUTHOR]

Grouting lifting is the effective approach for treating differential settlement of existing tunnels, but it easily deteriorates the tunnel structure again and seriously threatens the operational safety of urban rail transit. Aiming to solve the above problem, this paper considers the interaction between the tunnel and the foundation, and establishes a series of analytical solutions of the stress, strain, axial force, and bending moment of the tunnel under single point grouting. Based on the characteristics of shield tunnel correction engineering and the analytical solution under single point grouting, the equation of the internal force and deformation of tunnel structure under multi-point grouting considering different grouting angles and different strata is obtained. The grouting correction engineering case for Shenzhen Metro Line 1 shows that the analytical solution to the displacement of the tunnel structure is well matched with the measured displacement of the tunnel structure is well matched with the measured data of the case and the difference is less than 15 %. The parameter analysis results indicate that the main loaded area of the tunnel under single point grouting is related to the stratum, and the main loaded area of the soft soil stratum is - 42. 4 ° + 42. 4 ° around the grouting point. Convergence deformation of the tunnel under multi-point grouting reaches the maximum value at the diameter passing through the center of the grouting zone and the minimum value at the diameter which is perpendicular to the former. Under the condition of limiting horizontal convergence deformation, the reasonable grouting angle range of the multi-point grouting zone is about 120 ° 150 °. This paper provides a theoretical basis that can immediately evaluate the mechanical structure performance and to determine whether to impose greater pressure when the tunnel is continuously lifted. [ABSTRACT FROM AUTHOR]

Considering the micro-arching effect between pipe umbrella, the paper investigates the mechanical response of pipe umbrellas during tunnel excavation by simulating pipe umbrellas as Timoshenko beams on a Pasternak foundation. The finite difference method is employed to solve the deflection, bending moment, and shear force of pipe umbrellas. By analyzing and contrasting with real-world engineering monitoring data, finite element computation results, and existing theories of other researchers, the reliability of the proposed method is verified, and good agreements are observed. A new parameter δ p is introduced in our method to represent the proportion of overlying load transmitted through the pipe umbrella, and influences of stiffness of the surrounding rock in front of the tunnel face, initial support stiffness, pipe umbrella design schemes, and excavation depth on the load transfer capacity of the pipe umbrella are subsequently investigated. The conclusions reveal that increases of surrounding rock stiffness, initial support stiffness, and diameter of pipe umbrellas would all enhance the stability of both the tunnel and pipe umbrellas. However, such enhancements have certain limitations. Particularly, when the excavation footage c <1 m, the pipe umbrella diameter should be specifically selected based on cases. [ABSTRACT FROM AUTHOR]

[Display omitted] • An innovative corrugated tube-rubber-concrete flexible joint (CTRCFJ) was proposed. • CTRCFJ without adding complexity to the structure and can bend and stretch/compress. • Buckling is reduced by 85.1% compared to the straight steel tube under reverse fault. • CTRCFJ is an effective mitigation method for permanent ground displacement. When water conveyance tunnels cross seismic fault fracture zones, significant deformation may occur, threatening the structural integrity of the tunnel. To resolve this problem, a novel corrugated tube–rubber–concrete flexible joint (CTRCFJ) for improving fault resistance is proposed for the first time. The effectiveness of the CTRCFJ was evaluated using advanced three-dimensional non-linear finite element analysis. The design parameters of the CTRCFJ, including rubber width, corrugated tube interval, and corrugated tube width, were selected based on the distribution of the axial bending moment of the lining structure. An orthogonal test was performed to determine the optimal combination of parameters. The effects of the number of crests, different steel grades, and wall thickness of the corrugated tube on the fracture resistance of the CTRCFJ are investigated. Results show that the CTRCFJ can effectively improve the dislocation resistance performance of composite lining water conveyance tunnels crossing reverse faults by reducing axial compression and bending strain. This study provides a reference for the structural design of composite lining structures that can cross reverse faults. [ABSTRACT FROM AUTHOR]

In order to study the mechanism of the influence of crack depth on the bearing capacity and stability of tunnel lining structure, the calculation model of internal force of lining with cracks is established based on the theory of fracture mechanics, and the variation of stability coefficient of lining structure with Ⅰ-Ⅱ composite cracks is analyzed. With the increase of the crack depth of the vault, the bending moment of the vault gradually decreases, the stress intensity factor increases, and the stability factor f gradually decreases. The critical depth of tensile and compressive crack instability propagation is 54.38% and 74.01% of the lining thickness, respectively. The relationship between load and displacement, internal force and contact pressure, crack propagation depth and time, and strain variation of the secondary lining vault during the loading process of cracks at different depths are analyzed. It is found that cracks have a significant impact on the bearing capacity of the secondary lining, and the deeper the cracks, the lower the bearing capacity and stability. When the crack depth is large, the tensile stress peaks in a short time, and applying smaller loads can also cause the crack to expand. When the crack depth is 0.9 d (d is the thickness of the secondary lining), the bearing capacity of the structure decreases by 72.57%. The research results can provide certain theoretical guidance for the prediction and prevention of lining crack propagation during tunnel operation. [ABSTRACT FROM AUTHOR]

• Refined FE analyses were conducted to study the seismic responses of shield tunnels. • The peak value of the dynamic bending moment of each segment was consistent. • Seismic analysis of shield tunnel by the stiffness reduction method may result in errors. • Lining segment exhibits considerable rotation under earthquake loading. The segment joints of a shield tunnel are generally the weak components of the structure, and they can be considerably damaged under strong seismic loading, thus influencing the overall seismic behavior. In this study, Finite Element simulation analyses of centrifuge shaking table tests and a shield tunnel project are carried out to investigate the influences of the segment joints on the seismic responses and the associated mechanisms. First, the reliability of the numerical procedure is validated by using the centrifuge shaking table test results. The numerical procedure is then used, together with the refined simulation approach for segment joints, to carry out extensive numerical analyses of the seismic responses of a shield tunnel project under different horizontal earthquake loadings. The research results show that due to the influence of the segment joints, the peak value of the dynamic bending moment of each segment tends to be more uniform. When the stiffness reduction method is used to consider the influence of the segment joints, the peak values of the dynamic bending moments can be too large, while the peak values of the dynamic axial forces may be too small. In addition, the tunnel segment rotates during an earthquake, resulting in relatively large relative displacements of the segment joints. [ABSTRACT FROM AUTHOR]

• An analytical model of the segment longitudinal joints considering different arrangements of double sealing gaskets. • The effect of the arrangement of sealing gaskets on the mechanical behavior of segment longitudinal joints. • The arrangement of double sealing gaskets and the location of bolts in the large-diameter shield tunnel. The sealing gasket of the traditional double waterproof system of the large diameter shield tunnel consists of two sealing gaskets which are arranged on the outside of the bolt hole(SGOH) or arranged on both sides of the bolt hole(SGBH). The different arrangement of the double sealing gaskets has significant effects on the mechanical behaviors of the longitudinal joints of the segment. In this paper, based on the energy equivalence principle, the actual distribution stress of the compression zone of the segment longitudinal joints was simplified equivalently as rectangular distribution stress. Then, a progressive mechanical model was established for the longitudinal joint, taking into account the effect of double sealing gaskets arrangements. Furthermore, the proposed mechanical model was validated through former experimental results and other analytical models. Finally, the effect of different arrangements of the double sealing gaskets on the mechanical behavior of the longitudinal joints of the segment was compared and analyzed. The results showed that the difference in the arrangement of the double sealing gaskets results in varying effects of the additional bending moment on the joint bending deformation under sagging and hogging moments, leading to the unequal bending stiffness of the joint. The difference in bending stiffness caused by the double sealing gasket arrangements is approximately 100∼150 MN·m/rad. To optimize the mechanical behaviors of the joints, the double sealing gaskets should be arranged on both sides of the bolt hole(SGBH), with the bolt located at the center of both the longitudinal joints and the core compressive zone of the longitudinal joints. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel precast segmental lining with a specific steel space truss was proposed. • The behavior of a novel lining was investigated experimentally and numerically. • The new lining's segmentation principle was determined. • The design parameters of lining joints were optimized. In recent years, precast segmental lining (PSL) has attracted much attention in tunnel engineering. In practice, the segmentation method is an essential consideration for PSL. This study proposes a novel PSL with a specific steel space truss. It uses physical and numerical modeling and considers three different segmentation principles in the experiments. In addition, this research develops an accurate three-dimensional finite element model, enabling a detailed analysis of the novel PSL. It also investigates the influence of joint thickness and stiffener on the novel PSL. The experimental results indicated changes in the lining's displacement, internal force, and failure mode. They revealed that cracks initially appear at the block joints. Considering the force and deformation of the lining, the method based on the maximum moment position principle is optimal because it can achieve the lowest deformation and bending moments. The numerical results showed that increasing the joint thickness or welding stiffeners raises the bending stiffness, reduces the joint opening, and decreases the plastic zone and strain value, thereby improving the joints' reliability. The findings are advantageous for the tunnel PSL design and can serve as a reference for joint optimization. [ABSTRACT FROM AUTHOR]

• Developed a hydraulic loading system to carry out a full-scale loading test. • The asymmetric unloading load of the three-ring segment is realized. • The influence of initial confining pressure loading method is considered. • The influence of foundation pit excavation on tunnel structure is explored. To explore the internal force and deformation response law of the Hangzhou subway shield segment under the influence of the excavation and unloading of the side foundation pit, the self-developed "shield segment hydraulic loading system" was used to carry out full-scale loading tests on the three-ring staggered assembled segments. A preloading loading test were performed before the formal experiment to determine the loading method of the initial confining pressure. Analyzed the change mechanism of the internal force and deformation of the segment structure are influenced by factors such as the foundation pit and the tunnel clear distance. A numerical model is established based on the experimental results, and the influencing parameters that are difficult to complete in the experiment, such as the depth of the tunnel, are analyzed in detail. The research results show that the equal load maintains a balanced state of the segment to the greatest extent, and its internal force and deformation results are large, which is suitable for this experiment. After the excavation of the foundation pit, the maximum positive and negative bending moment increases, while the maximum axial force decreases. The average maximum recovery rate of tunnel section is 15.6%. Moreover, the additional internal force and deformation of the segment will increase with increasing deformation parameters of the envelope structure and decrease with increasing clear distance between the foundation pit and the tunnel. The bending moment of the segment is more sensitive to changes in the confining pressure, the convergence change can be controlled within a limited range, and the trend of the change in the internal force of the bolt shows differences. The numerical model established in this paper can better analyze the stress state of the tunnel. When the tunnel is buried deep in the slightly upper position of the excavation surface of the foundation pit, the unloading effect that is received is largest. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of the influence of cross-sectional shape on the dynamic response of a tunnel and the surrounding soil. 1/20 scaled tunnel models with three different cross-sectional shapes (circle, rectangle and horseshoe) were tested. Two types of dynamic loads, sweep loads and train loads, were applied inside the model tunnels. The dynamic response of the tunnels and the soil were measured and both time and frequency domain analyses were performed. The experimental results show that the cross-sectional shape of tunnels can have a significant influence on the dynamic behavior of the tunnel. A clear amplification of the dynamic response of the rectangular tunnel was found, especially at tunnel invert. The distribution of the bending moment can also be changed due to variation of the tunnel cross-sectional shape. However, the test results demonstrate that the cross-sectional shape effects on the tunnel response are mainly in the near field and the influence decreases with increased distance from the excitation source. A limited effect of the cross-sectional shapes on soil response was also found. [ABSTRACT FROM AUTHOR]

The wetting-induced collapse of loess strata affects tunnel structures and is a key challenge in geotechnical engineering. In this study, we investigated the mechanisms that influence wetting-induced collapse/deformation of loess strata on the lining structure of a subway tunnel. First, a centrifugal field immersion unit and a monitoring system were developed based on the centrifuge working platform. The immersion device was composed of two sub-systems including a water supply system and a diffusion system. Subsequently, centrifugal model tests were performed under different wetting conditions to explore the impact of wetting-induced collapse of loess strata on the structure of the subway tunnel. The testing results showed that the non-uniform subsidence of the tunnel base induced by non-uniform wetting was the major cause of the collapse and deformation of the tunnel structure. The axial bending moment of the tunnel was found to be largest in the middle of the tunnel and decreased toward both ends. In addition, a layer of non-collapsible soil or properly treated collapsible soil with a certain thickness could effectively resist deformation induced by the subsidence of wetted loess. With an improved immersion simulation unit design and measurement device configuration, after being wetted, the pressure arch in the soil strata became weaker and gradually disappeared, resulting in a reduced range of the pressure arch on the side wall of the tunnel and complete elimination of the pressure arch effect on the top of the tunnel. [ABSTRACT FROM AUTHOR]

For the shield tunnel with double-layer lining, the type the joint surface between the segment and secondary lining has significant influence. Depending on the smoothness of the border and the transmit patterns of the forces between the segment and the secondary lining, the lining structure is divided into double-shell and composite structures. For the double-shell structure, only the axial force is transmitted through the border, while the axial and shear forces are transmitted through the border for the composite structure. In this paper, based on the similarity model test and the engineering background of the Shiziyang Tunnel, the influence of the joint surface type on the mechanical behaviors and failure characteristics of the lining structure is studied under the condition that the segment and secondary lining of the shield tunnel double-lining structure share the overload together. The result shows that (1) the differences in the bending moment values between the double-shell and composite structures are small, but the axial force values and the changing trends are obviously different; the load bore by the secondary lining and its ratio to the external load increases with the external load; further, the segment is still the main bearing structure; (2) after the macroscopic failure of the segmental lining structure, the double-shell lining loses stability rapidly, the bearing capacity loss is abrupt, and the damage of lining structure is more serious; when the composite structure loses stability, the failure process of the composite structure is longer, and the structural failure is ductile; (3) with the same load, the ratio of maximum displacement to the tunnel radius of the double-shell structure is larger, and the load level of the lining structure when failure occurs is lower. Generally speaking, the composite structure improves the overall stiffness of the double-lining structure, which is in favor of the joint load carrying of the segment and secondary lining to control the structural deformation; additionally, the bearing capacity of the composite structure is obviously better than that of the double-shell structure; therefore, it is recommended in the double-lining design of the shield tunnel. [ABSTRACT FROM AUTHOR]

Abstract This paper investigated the three-dimensional effect through full-scale loading tests on both a single (isolated) full-ring structure and a staggered-fabricated three-ring structure for a water storage and sewage tunnel. Some observations common for both the single-ring and three-ring tests were identified: both the internal forces and deformation of the testing specimen increase as the burial depth increases; whereas, the axial force decreases but the deformation of the testing specimen increases with increasing inner water head. Comparing with the single-ring tests, the deformation of longitudinal joints and the convergence deformation of the testing specimen are significantly reduced in the three-ring tests. The reasons may be two-fold: firstly, the staggered fabrication causes the redistribution of bending moment in the longitudinal direction, which transfers a certain amount of bending moment from the longitudinal joints to the segment body of adjacent rings; secondly, the presence of thrust force leads to the closure of circumferential joints, which co-acting with the longitudinal bolts, enforces the joints to deform together with the segment body of adjacent rings and thus restricts the deformation of the longitudinal joints. It is also shown that the mechanism of bending moment transmission is not unique but depends on the loading conditions and joint locations. [ABSTRACT FROM AUTHOR]

Abstract The seismic response of a tunnel lining structure is essential for its safety design and construction. The objective of this study is to investigate the seismic response of a tunnel lining structure embedded in a thick expansive soil stratum. First, the deformation and internal force characteristics of tunnel lining structure under seismic action are evaluated from a theoretical perspective. Second, a finite element (FE) analysis is carried out of the seismic responses of the tunnel lining structure and ground stratum. In the FE analysis, the bolted connections in the tunnel lining structure are simulated using the local stiffness reduction method (LSRM). The LSRM reduces the stiffness of the lining segments affected directly by the bolted connections using a stiffness reduction coefficient (SRC), while leaves the stiffness of the rest of the lining segments to remain to be unchanged. The results indicate that under the effect of transversal waves propagating in the direction perpendicular to tunnel axis, the transversal displacement characteristics of the tunnel lining structure are almost identical to that of the surrounding soil stratum. They both decrease with an increase in buried depth. The circumferential distributions of shear force, bending moment, and axial force in lining segments are varied, but the magnitudes of them increase with an increase in SRC. [ABSTRACT FROM AUTHOR]

Abstract This study derived the analytical solutions for the deflection, bending moment and shear force of a hard roof under an arbitrary front abutment pressure for both the initial and periodic fracture problems using the elastic foundation beam model. With the assumption that the overburden pressure is constant in a small section, the roof subjected to arbitrarily distributed pressures is divided into a number of sufficiently small sections, whose closed-form solutions can be obtained easily. The proposed analytical solutions are validated by comparisons with those calculated using a model reported in the literature, in which the overburden pressure is assumed to follow a normal distribution. The parametric studies on the foundation stiffness, front abutment pressure and support resistance show that: (1) with the decrease of elastic foundation stiffness, the deflection and the maximum bending moment significantly increase, and the shear force significantly changes in the unmined region; (2) as the peak point position moves inside the coal wall, the deflection of the goaf roof, as well as the maximum bending moment and shear force, obviously decrease; (3) with the increase of support resistance, the maximum bending moment and deflection significantly decrease, and the shear force significant decreases in the hydraulic support region; (4) when the coal seam is soft and peak position of the front abutment pressure is near the coal wall, initial fracture occurs at the midspan; otherwise, it occurs inside the coal wall. For the periodic fracture condition, the fracture position is closer to the coal wall when the coal seam is harder, and the fracture length increases when the peak position of front abutment pressure moves inside the coal wall. [ABSTRACT FROM AUTHOR]

Abstract When constructing an excavation, the segmental lining installed in an anisotropic rock mass is susceptible to asymmetrical pressure and substantial local instabilities. To promote liner stability in such cases, a support system that combines a segmental lining with a highly deformable expanded clay and rock bolts is proposed. A series of physical model tests were performed to study the effect of this support system. In the tests, different dip angles of the primary rock mass jointing, different thicknesses of the expanded clay and different lengths and spacings of rock bolts were considered. Variations in the internal forces and deformation of the segmental lining were recorded in each case. The distribution of the internal forces and deformation of the segmental lining were anisotropic due to the rock mass anisotropy; the maximum positive bending moment and positive deformation on the liner mainly developed in the direction normal to the stratification. By using a sufficiently thick deformable clay layer, the anisotropy of the load transferred to the segmental liner can be minimized. By adding rock bolts, the combined support system can take full advantage of the reinforcement effect of the rock bolts and the yielding effect of the deformable clay layer, and the effect of the rock bolts plus the yielding layer is greater than the sum of the two individual effects. Such a system can change the internal force and deformation distribution of the segmental lining to improve liner stability. [ABSTRACT FROM AUTHOR]

• Influence of 3D spatial effects of underground structure on its seismic response. • 2D model cannot correctly reflect deformation characteristics at 1/2 length of 3D model. • Results of 2D model roughly equivalent to that at 1/4 length of 3D model. • In-plane bending deflection of slabs well reflect structural 3D cooperative work degree. • Conversion coefficients from 2D model to 3D model with 95% probability guarantee. This paper takes an actual station of Nanjing Metro Line 2 as the research object and establishes the corresponding 2D and 3D overall time-domain nonlinear finite element models, respectively. By analyzing the results of the two models, the influence of the 3D spatial effects of the underground structure on the seismic response of the structure is studied. The results show that the 2D model cannot correctly reflect the deformation characteristics of the 3D station structure at the middle of the longitudinal length. The results obtained from the 2D model are roughly equivalent to the results at 1/4 of the longitudinal length of the 3D station structure. The overall longitudinal bending deflection of the station's top and middle slabs in their own plane can well reflect the 3D cooperative work degree of the station. Based on this, the conversion coefficient of the maximum inter-story displacement angle proposed in this paper can better convert the 2D model results. The minimum error between the converted 2D model results and the 3D model results is only 0.78%, and the maximum error is only about 10%. The maximum inter-story displacement angle conversion coefficient with 95% probability guarantee finally given is 1.447. Through the statistical analysis of the internal force ratio of the 3D model and the 2D model at the key cross-sections, this paper also gives the conversion coefficients with 95% probability guarantee for the bending moment, the axial force and the shear force of the key cross-sections, which are 0.696, 0.773, and 0.785, respectively. [ABSTRACT FROM AUTHOR]

• Reveal soil settlement with quartz sand. • The maximum allowable buried depth of the lining before damage is 150 m. • Tunnel end has the most significant convergence. • Severe damage occurs on the left part of the lining. Lining cracking is a typical damage phenomenon during the construction of metro tunnels, especially in the mixed strata with soft upper and lower hard rock, which severely affects the safety of the tunnel. In this paper, the physical model test was conducted to investigate the failure mechanism of metro tunnels. The full-face excavation apparatus was used to simulate tunnel excavation. The pressure cells, displacement transducers, strain gauges, and quartz sand marking method were adopted to monitor the mechanical response of the tunnel. Moreover, the failure evolution process of tunnel lining was recorded. The effectiveness of the model test was verified by numerical simulation. Meanwhile, the influences of the soil mechanical parameters, tunnel geometry, and lining stiffness on the failure behaviors of the tunnel were systematically discussed. The test results indicate that the tunnel excavation induced a slight deep soil disturbance. At buried depths ranging from 50 to 500 m, the soil settlement near the tunnel entrance was significant due to the uneven longitudinal compression, and the settlement decreased with increasing depth. The attenuation magnitude of the ground pressure in the vertical direction was greater than that in the horizontal direction. For the spring line, it was most affected by the coupling of axial compression and bending moment. The bearing capacity of the lining reached the limit when the buried depth was 150 m. The damage of the tunnel invert was the most severe, followed by the crown and the spring lines. In addition, the damage of the left part of the lining was more severe than that on the right part due to asymmetrical pressure. The entrance of the tunnel had little convergence, while the convergence at the end of the tunnel was the most significant. [ABSTRACT FROM AUTHOR]

• Polymer layer coating tunnel model had isolation effect in dynamic centrifuge tests. • Polymer layer reduced axial opening strain and bending moments of tunnel model. • Bending moments of tunnel models were calibrated before dynamic centrifuge test. • Polymer buffer layer did not absorb or filer the input motion. • Polymer layer coating tunnel model reduced seismic response of tunnel model. A buffer layer is one of the countermeasures to promote the anti-seismic performance of tunnels. A newly invented non-water reacting two-component polymer (NRTCP) is superior in impermeability and durability. The construction process of the NRTCP is time-saving and labor-saving. In this study, in order to verify the seismic isolation efficiency of the NRTCP around a shield tunnel model, a dynamic centrifuge test with several dynamic input signals was conducted. The seismic responses of shield tunnel models with and without the NRTCP layer were compared. The results show that the bending moments of the tunnel model with an NRTCP layer were lower than those without an NRTCP layer. Thus, this indicates that the NRTCP layer positively reduced the bending moment in the tunnel model. The axial strain in the tunnel model with the NRTCP layer was lower than that without the NRTCP layer. In addition, the frequency contents of the acceleration response of the tunnel models were rarely influenced by the NRTCP layer. These findings demonstrated the positive effects of the NRTCP buffer layer on seismic isolation in shield tunnels. [ABSTRACT FROM AUTHOR]

• Refined numerical model of segment joints considering complex geometric structure. • Clear insight into damage of concrete, stress of reinforcement and bolts. • Bearing capacity and deformation of joints under different load conditions. The lining of a shield tunnel consists of thousands of precast segments that are connected by bolts, resulting in many joints in the shield tunnel structure. The mechanical behaviour of segment joints, which are the most vulnerable part in structures, determine the bearing capacity and deformation characteristic of the overall tunnel structure. Therefore, it is necessary to study the stiffness, bearing capacity and deformation behaviour of joints under different load conditions. However, previous studies have mainly focused on the capacities of longitudinal joints (LJs) against bending moments, and circumferential joints (CJs) against shear forces. Actually, the interaction between segments can be much more complicated, and the joints may suffer other loads, but there have been few relevant studies on this topic. In this study, a series of three-dimensional numerical models were established to investigate the bearing capacity and deformation behaviour of the joints under different kinds of loads. The results showed that when LJs and CJs were subjected to tension or shear forces, the failure was caused by the damage of segment concrete, in other words, the mechanical properties of the concrete determined the ultimate bearing capacity of the joints. [ABSTRACT FROM AUTHOR]

• Whole-process monitoring of prefabricated station was conducted for the first time. • Mechanical behavior and deformation of the joint were discussed. • Performance of the grouted mortise-tenon joints was analysed in different stages. • The monitoring and numerical calculation results have been compared. • Offer available reference for prefabricated station design in actual application. Changchun Metro Line 2 implemented the whole-process monitoring and measurement of prefabricated metro station for the first time. This paper analyzes the mechanical behavior and deformation of the joints of vault and arch springing, which bear high bending moments, in order to study the mechanical properties of these joints, which play a key role in the bearing capacity of prefabricated structure. In addition, it also discusses the trend of the joint stress, relative deformation and relative rotation angle as it changes with the construction process and after opening for traffic. Furthermore, it analyzes the performance of the joints in different construction stages, and compares it with the numerical calculation results based on the long-term in-situ monitoring and survey data carried out at the prefabricated station of Changchun Metro during the whole construction period. As indicated by the result, at the beginning of the assembly and arching into a ring of the components at the top, the structure undergoes a self-adaptation period, where the joints are subject to heavy stress and deformation. After backfilling and water level recovery, the joints are subject to increased axial force and reduced stress and deformation, resulting in a positive effect on the bearing capacity of the joints. The overall change range in joint stress is rather small. During the construction period, the two tenons of double-tenon joint of vault bear external force by mutual engagement, which are subject to two types of stresses, namely, tensile stress and compressive stress. The concave-convex tenon of single-tenon joint at the arch springing is markedly engaged with each other, with the tenon subject to a bending effect. The test results are relatively stable, and the overall deformation at the joint of vault and arch springing is relatively small, with sufficient safety margin from the end of construction disturbance through the operation stage till today. The research results demonstrate great practical guiding significance for the joint and structure design of prefabricated metro station. [ABSTRACT FROM AUTHOR]

• A semi–analytical force–method solution for a straight–jointed tunnel is proposed. • Nonlinear joint stiffness is considered and expressed as a function of M, N and θ. • An incremental iterative algorithm is established for the strong nonlinear system. • The tunnel ultimate bearing capacity under different load distributions is analyzed. The ultimate bearing capacity of a segmental tunnel lining significantly influences its service performance, particularly when the surrounding soil pressure changes. In this study, a semi–analytical force–method solution for a straight–jointed segmental tunnel lining was derived. The nonlinear joint stiffness was considered and expressed as a function of the bending moment, axial force, and rotational angle of the joint. An incremental iterative algorithm and the corresponding convergence criterion of the proposed model were established for the strong nonlinear system. The proposed model was then verified using the full–scale test results for the loading behavior of the joint and the segmental lining. The structural response of the tunnel under different external load distributions was analyzed using the proposed model, and the main conclusions are as follows. As the lateral pressure coefficient increases, the ultimate bearing capacity of the tunnel lining increases and the length of the first yield platform of the convergence deformation decreases. When P 3 /P 1 increases from 0.71 to 0.83, the ultimate bearing capacity of the tunnel lining increases by 20%, but it has no significant effect on the ultimate convergence deformation. For the lining of the shield tunnel of the Shanghai Metro, the recommended convergent deformation limit is 110 mm. [ABSTRACT FROM AUTHOR]

• Shaking table model tests were conducted to examine seismic behavior of THMA culvert. • The embankment sequences significantly affect the deformation mode of THMA culvert. • The culvert exhibits alternative left and right bias deformation in dry sand case. • Simultaneous backfilling on all sides of the THMA culvert is the safest sequence. A three-hinged multi-arch (THMA) culvert is a new type of underground structure that is gradually being adopted in practical engineering in Japan as an alternative to elevated bridges. However, there exist reports that THMA culverts were seriously damaged by earthquakes in the past, and research on the behavior of THMA culverts during the embankment process and exposure to post-construction earthquakes is insufficient. Therefore, to carefully investigate the static/seismic mechanical behavior of the THMA culvert, systematic 1 g shaking table model tests were carried out in the current study. THMA culverts were constructed using different embankment procedures in dry/saturated sand ground, and then subjected to seismic loading. The results showed that the different embankment procedures significantly affect the deformation mode of the multi-arch culvert on both dry and wet sand grounds, and the construction sequence involving simultaneous backfilling on both sides of the culvert can reduce the generated bending moments during seismic vibration. Furthermore, in all cases, the bending moments generated during the earthquake were significant on both sidewalls of the culvert in the range of 30°–60° from the ground, and the bending moments in the region near the hinges were smaller, which provided basic data for the optimized design of the culvert. [ABSTRACT FROM AUTHOR]

• The influences of groundwater fluctuation on tunnels were investigated. • Tunnel behaviors in terms of shapes and site conditions were revealed. • Self-designed experimental equipment and innovative methods were proposed. • Buoyancy reduction and time lag effect in both sand and clay were analyzed. Groundwater is one of the important factors affecting the stability of underground structures. However, the researches on the tunnel responses under the change of groundwater level based on the experiments are still insufficient. In this study, the interaction between underground tunnels and soil during the change of groundwater level in sand and clay stratums was investigated through the model experiments. Adopting the self-designed experimental equipment and innovative experimental methods, the contact pressure, bending moment, pore water pressure, buoyancy, and vertical displacement of two types of tunnels throughout the testing process were monitored. The impact of time lag effect on tunnels was also discussed. Force analysis indicates possible critical locations of both tunnels along the whole groundwater change process. The groundwater buoyancy in clay was reduced by time lag effect. The structures showed periodic sinking and floating behavior, and substantial settlement occurred with the decline of water level in the final stage. [ABSTRACT FROM AUTHOR]

Sharp transitions in structure stiffness and/or ground properties have a significant influence on the seismic response of tunnels. These issues are not well understood yet, or at least not well considered during design. An analytical solution is derived to investigate the seismic response of long tunnels, built in non-homogeneous ground, subjected to sinusoidal shear motions. It is assumed that the tunnel is excavated in two different soil deposits that have a sharp contact, and there is a transition zone through the contact. It is also assumed that the tunnels can be represented as beams on an elastic medium. Continuity at the contact between the different contact sections of the tunnel is imposed to solve the governing equations of equilibrium. In addition, wave passage effects along the tunnel are considered by including a phase angle in the far-field displacements. Explicit formulations are obtained for tunnel deflection, bending moments and shear forces. The solution is verified by providing comparisons between its results and those from the Finite Element program ABAQUS. A parametric analysis is presented where the effects of the stiffness of the structure, the shear velocity of the soil and the length of the transition zone are investigated. [ABSTRACT FROM AUTHOR]

[Display omitted] • The feature of large and complex underground pipe structures (LCUP) is reviewed. • The seismic response law of complex spatial cross-node pipes of LCUP is revealed. • The usability of seismic analysis methods for LCUP in hard rock sites is discussed. • The technical route of seismic design for LCUP in hard rock sites is constructed. • A balance of safety, accuracy and efficiency for LCUP seismic analysis is realized. In order to study the seismic design methods of large and complex underground pipe structures (LCUP) in hard rock sites, firstly, the structural characteristics of LCUP and the calculation principles of four typical seismic analysis methods were carefully sorted out. Secondly, based on different seismic analysis methods, the deformation and internal force response of three representative pipes were studied in detail, and the calculation accuracy and error causes of each seismic analysis method were analyzed. Finally, the advantages, disadvantages and applicability of the seismic analysis methods on typical pipes were systematically explained, and the technical route for seismic design of LCUP was proposed. The results showed that both the reaction displacement method and the reaction acceleration method can effectively analyze the seismic performance of standard section structures. The three-dimensional time-history analysis method was comprehensive and highly accurate for seismic analysis of complex spatial cross-node structures. The rigid connections of different pipes leaded to serious discontinuities in stiffness at the intersection, and the deformation and bending moment distributions were highly concentrated at the height away from the intersection. The influence range of the intersection was about three times the width of the intersection, and LCUP was classified according to this influence range. For standard section structures, seismic analysis should be carried out using the reaction acceleration method or reaction displacement method, while for spatial cross-node structures, three-dimensional time-history analysis should be used for seismic analysis. This scientific seismic design method achieved multiple optimal balances of seismic safety, analysis accuracy, efficiency and economy in the seismic analysis of large and complex underground structures. A theoretical basis and useful reference for the scientific design, accurate prediction and seismic safety evaluation of seismic response of LCUP were provided in this paper. [ABSTRACT FROM AUTHOR]

This paper presents an approach to determine the nonlinear bending moment – rotation relations for longitudinal joints of segmental concrete tunnel linings with flat concrete contact areas based on the rules for confined concrete and partially loaded areas according to Eurocode 2. It is demonstrated that the resulting bending moment – rotation relations show better agreement with experimental data than other approaches from the literature. The proposed approach allows to establish the bending moment – rotation relations for both serviceability limit state (SLS) and ultimate limit state (ULS) and thereby a tunnel lining design consistent with Eurocode 2. The practical implications for the design concept of tunnel linings are discussed. [ABSTRACT FROM AUTHOR]

Understanding the behaviour of existing tunnels subjected to in-service deformations, as a result of the construction of underground works (e.g. new tunnels) in their proximity, is of importance in order to safeguard infrastructure within the urban environment. The associated deformations that take place during tunnelling have to be carefully assessed and their impact on the existing tunnels needs to be considered. A half-scale segmental grey cast iron (GCI) tunnel lining ring was tested as part of an extensive research project investigating the impact of new tunnel excavations on existing tunnels conducted at Imperial College London. A sophisticated experimental arrangement was developed to deform the ring in a variety of modes under combined displacement and load control. This paper reports on experiments carried out to assess its structural response when subjected to large deformations. The tests reported are the first to be conducted on a realistic scale model under carefully controlled conditions, and provide valuable insight into the behaviour of a GCI segmental ring during distortions commonly observed in reality. Details of the experiments, including the adopted test set-up and the instrumentation employed, are presented. The measured bending moments around the ring, as a result of the applied deformations, are determined and compared with those predicted using the well-known equations given by Morgan (1961) and Muir Wood (1975), often used in industry, as well as those obtained assuming an elastic continuous ring. [ABSTRACT FROM AUTHOR]

[Display omitted] • A mechanical model JICM was established for assessing assembled joints. • JICM reflected the multi-parameter effects of multi-type assembled joints. • JICM revealed the dominant reason for brittle damage of assembled joints. • JICM predicted the failure process of assembled joints accurately. • JICM predicted the deformation of assembled components accurately. Modern prefabricated underground structures are the key advancement in the low-carbon urban construction. The rapid development of prefabricated structures has increased the diversity of assembled joints. It becomes more difficult to evaluate the joint performance considering multiple parameters, such as joint size, morphology, interface properties, and axial forces. Therefore, this study developed an assembled joints mechanical assessment model to solve the multi-parameter problems. First, the interface contact formulas were established for typical interfaces. Second, a joint interfaces contact model (JICM) was established based on the above formulas and multiple parameters. Finally, model validation and case study were performed for JICM. The results show that: (1) JICM fitted well with the experimental data, and reflected the multi-parameter effects of multi-type assembled joints. (2) JICM calculated the contact stress at the outer edge of the interfaces can be twice larger than the bending stress. Accordingly, JICM revealed the key reason for brittle damage of assembled joints. (3) JICM demonstrated promising joint design capabilities: it calculated that the bending moment contribution of one interfaces group was dominant; it further calculated the deformation and bearing capacity of the assembled joints at the yield and ultimate moments. (4) JICM demonstrated promising joint deformation control: it calculated the rotation developments of assembled joint and continuous section were not consistent; it further presented the deformation pattern of the assembled components was a folded line and can be 20% larger than that of a continuous component. JICM provides a tool for parametric design of joints and quantitatively evaluates multi-parameter effects, demonstrating practical value for urban green low-carbon construction. [ABSTRACT FROM AUTHOR]

• Effect of strength of improved ground differs in Level 1 and Level 2 earthquakes. • Expansion of plasticized improved ground affects seismic tunnel behavior in Level 2 earthquake. • Optimum improvement in strength differs depending on ground-improvement pattern. • Weight of improved ground has only small effect on seismic tunnel behavior. Pre-ground improvement is an auxiliary method for shallow overburden tunnels in which the tunnel excavation is performed after improving the ground on top of the tunnel with/without sides by the replacement method and/or the cement mixing method. Although it has been confirmed in previous studies that pre-ground improvement enhances the tunnel face stability and reduces the settlement of the ground surface during the tunnel excavation, the seismic behavior of tunnels with pre-ground improvement has not been fully investigated. Therefore, the authors conducted a shaking table experiment under the centrifugal acceleration of 50 g in order to understand the effect of the ground-improvement pattern on the seismic behavior of a shallow overburden tunnel. Furthermore, the authors performed a two-dimensional elasto-plastic finite element analysis of the centrifuge model experiment to verify the applicability of the numerical method and to examine the influence of the stress release associated with the tunnel excavation, which cannot be considered in the centrifuge model experiment. In this study, the effects of the strength and weight of the improved ground on the seismic behavior of a shallow overburden tunnel are investigated using the same numerical method. The results confirm that, for a Level 1 earthquake, the increments in bending moment are almost the same regardless of the strength of the improved ground in both ground-improvement patterns. On the other hand, for a Level 2 earthquake, the seismic behavior of a tunnel has different tendencies depending on the ground-improvement pattern because of the expansion of the plastic area in the improved ground and the difference in stiffness between the improved ground and the surrounding ground. Therefore, the optimum strength of the improved ground differs depending on the ground-improvement pattern. It is also found that, although the translational motion of the tunnel with the improved ground is slightly greater when the weight of the improved ground is larger, the increase in weight due to the ground improvement has only a small effect on the seismic behavior of the tunnel because the behavior of the tunnel during an earthquake is dominated by the surrounding ground. [ABSTRACT FROM AUTHOR]

• Full-scale compression-bending tests that verifying the numerical model are carried out. • The moment–rotation curves of six different segmental joints are analyzed. • The characteristic equations of moment-rotation relationship of segmental joint are proposed. The compression–bending stiffness of segmental joints plays a crucial role in the analysis of segment lining structures, and setting reasonable values for joint compression–bending stiffness is a key determinant of the accuracy of mechanical calculations of lining structures. In this study, a three-dimensional refined numerical model of segmental joints is established, and solid elements are used to simulate important components, such as segments and bolts, which essentially reproduce the structural characteristics of the joints and accurately reflect the contact relationship between the segments. Full-scale compression–bending tests are conducted to verify the numerical model, and the compression–bending calculation and analysis of six different segmental joints are performed. The characteristic equations for calculating the compression–bending stiffness of segmental joints with different thicknesses are determined. The results show that the refined three-dimensional model can accurately reflect the compression–bending stiffness of the segmental joints, and the deviations between the numerical and the experimental results appear when damage phenomenon occur on the joint surface. Axial force can improve the compression–bending stiffness of segmental joints, and its influence increases with an increase in the bending moment and decreases with an increase in the thickness. The moment–rotation curves of the joints with different thicknesses are similar. Moreover, increasing the thickness effectively improves the compression–bending stiffness of segmental joints, and the influence increases with increasing bending moment or decreasing axial force. The moment–rotation relationship of a segmental joint has two characteristic curves, I and II, and the type of characteristic curve primarily depends on the structural characteristics of the joint surface. The characteristic equations based on the data-fitting method and the moment–rotation data of segmental joints with different thicknesses can be used to simply and accurately calculate the compression–bending stiffness of segmental joints. [ABSTRACT FROM AUTHOR]

• Centrifuge tests of impacts of reinforced wall on existing tunnel subjected to lateral excavation is performed. • Reinforced wall decreases the tunnel heave but increases the horizontal displacement. • Function mechanism behind the reinforced wall is extensively studied. Reinforced wall is widely adopted to protect existing structures subjected to nearby underground constructions. This study carried out two three-dimensional centrifuge tests (T -W with reinforced wall and T-0 without reinforced wall) and numerical back-analyses to investigate the ground and tunnel responses due to a nearby excavation in dry sand. In terms of the changes of earth pressures on the tunnel, the reinforced wall shows slight impacts at most positions, but areas between right heel and invert, and between left shoulder and crown. Tunnel heaves and longitudinal bending moments in vertical direction are slightly decreased due to the reinforced wall, while horizontal displacements and longitudinal bending moments in horizontal direction are appreciably increased. In terms of the effectiveness of the reinforced wall on mitigating tunnel horizontal displacements, four zones can be divided from shallow to deep: positive, negligible, negative, and negligible. Arching effects induced by the diaphragm wall deformations and continuous reinforced wall displacements are found to be the mechanism behind the reinforced wall-induced increases in tunnel horizontal displacements. [ABSTRACT FROM AUTHOR]

• The proposed design model guides the selection of required liner thicknesses. • Bending moments induced by earth loading govern liner design. • Required thicknesses are subject to the level of deterioration within the host pipe. • Perfect pipe rigidity assumed in the design model results in safe liner thicknesses. Reinforced concrete pipes experiencing unsafe levels of deterioration, and which require repair or replacement may be rehabilitated using trenchless technologies that avoid traffic impediments and costly excavations. One such trenchless solution involves the application of cementitious liners which are centrifugally sprayed out onto the interior of the host pipe to provide additional structural capacity and/or hydraulic repairs to the buried pipe. This paper presents a method for selecting the required liner thickness to prevent cracking and further deterioration in damaged reinforced concrete pipes. The design method is dependent on the burial conditions and extent of deterioration present in the host pipe: cracking, loss of concrete cover, loss of tensile reinforcement or the total breakdown of the host pipe. The bending moments resulting from earth loads are considered the controlling load source governing design. Design equations are developed to permit selection of minimum liner thickness needed for each of the deterioration types, considering composite interactions between the sprayed liner and the remaining components (concrete and steel) of the reinforced concrete pipe. Sample calculations are presented for the liner thicknesses required to rehabilitate Class 3, Class 4 and Class 5 pipes experiencing varying extents of deterioration. The results from the design models are evaluated using finite element simulations of the repaired pipe system. Comparisons between the strain distributions calculated using the design model and the finite element analyses demonstrate that the design model produces safe estimates of required liner thicknesses. [ABSTRACT FROM AUTHOR]

[Display omitted] • Four UHPC joint types (unbolted, straight, inclined, and curved bolted) are designed. • Axial force increases the bending stiffness of the joint with better elastic performance. • Stiffness of the straight bolt joint is the highest under positive bending moment. • Joints reinforced by curved bolt bear secondary loading better. This study used ultra-high performance concrete (UHPC) to design joints reinforced by various bolt types: straight, inclined, and curved bolts. Full-scale samples were prepared in the laboratory. Loading tests on these joints and the unbolted one were carried out based the actual stress state of the tunnel, and various axial force magnitudes and bending moments were combined to obtain the deformation characteristics. The fitting law of the bending stiffness of joints was presented. It is found that axial force increases the bending stiffness of the joint with better elastic performance. The bending stiffness of the straight bolt joint is the highest under positive bending moment during the first loading process, while that of the curved bolt joint is the greatest under negative bending moment during the second loading process. The test results indicate that joints made using UHPC (especially the curved bolt joint) can be used to build a more resilient prefabricated tunneling system. [ABSTRACT FROM AUTHOR]

[Display omitted] • The disaster testing system was innovatively used to conduct tests on concrete pipes. • The intact, corroded without void, and corroded with void pipes were investigated. • A multiparameter formula for the pipe stress with void and corrosion was developed. The disaster testing system of buried pipes developed in this work is innovatively used to examine intact pipes, internally corroded pipes without erosion voids, and internally corroded pipes with erosion voids. The measured data are verified by the finite element models. Parametric analyses are conducted to investigate the statistical relationship between the maximum principal stress on the pipe and the defect parameters, and a multiparameter formula used to predict the maximum stress on the pipes is finally derived. The results show that the corroded pipes without erosion voids have the largest circumferential bending moment. The circumferential bending moments of the bell and spigot of the pipes with erosion voids and internal corrosion decline with an increase in the width and length of the void. The parametric sensitivity analysis demonstrates that the stress on the pipes with erosion voids and internal corrosion has a positive linear relationship with the corrosion depth, a quadratic polynomial relationship with the width and length of corrosion, and a negative linear correlation with the width and length of the void. The proposed multiparameter formula can accurately predict the maximum principal stress on the pipes with erosion voids and internal corrosion. The results fully reveal the mechanism of the combined effect of corrosion and voids on the mechanical properties of concrete pipes. [ABSTRACT FROM AUTHOR]

• A relatively comprehensive investigation of twin stacked tunnel is performed. • Pillar depth between stacked tunnels should not be too small. • Lagging distance between stacked tunnels should be larger than 4 to 5 tunnel diameters. Twin stacked tunnels become more and more common due to the limited surface area and underground space. Most researchers have already paid attention separately to the influence of the twin stacked tunnels' position and construction sequence. However, the influence of the lagging distance and pillar depth of twin stacked tunnels has not yet been fully investigated when considering the tunnels construction sequence. The present study aims at analysing the effect of the construction sequence, pillar depth between tunnels, and lagging distance between stacked tunnels' faces on the structural forces induced in the tunnel linings and on the surface settlements. For this purpose a three-dimensional (3D) numerical model was developed using the finite difference element code FLAC3D. Most of the parameters of the slurry-type shield machine were simulated. The analyses included the influence of the face support pressure, shield conicity, shield weight, grouting pressure, harden grout, jacking forces, the weight of the backup train and jointed segmental lining. The results indicated that the most critical scenario, both in terms of ground settlement and internal forces in the tunnel lining is when the upper tunnel (UT) is excavated first and followed by the lower tunnel (LT) excavation. On the other hand, the following upper tunnel should be excavated at a lagging distance, which is larger by about 4 to 5 times the tunnel diameter behind the preceding lower tunnel. The research results reviled that designing a too small pillar depth between stacked tunnels should be avoided. This concerns pillar depths of about 0.25 to 0.5 times the tunnel diameter, as indicated in this study. It could cause a great increment in the normal displacements, longitudinal forces, and bending moments in the lower tunnel lining. [ABSTRACT FROM AUTHOR]

• A sophisticated numerical model of segmental ring considering rebars and bolts. • Failure mechanism of segmental ring was studied base on the plastic hinge theory. • Comparison of structural response between unloading and loading condition. A shield tunnel lining structure is a type of complex structure system, and few systematic studies have been performed on its transverse deformation response under upper unloading conditions. In this paper, a sophisticated numerical model of segmental rings considering rebars, bolts, sealing grooves and caulking grooves was built to analyze the transverse deformation response of shield tunnels under upper unloading conditions. The failure mechanism of the segmental ring was studied based on the plastic hinge theory, and the controlling index of convergence deformation was proposed for engineering applications. Meanwhile, the structural response and failure process of the segmental ring under unloading were compared with those under continuous loading conditions. The results show that with the decrease in load, the tunnel deformation mode changes from the horizontal ellipse to the vertical ellipse, and the maximum vertical convergence deformation of the segmental ring is 46.3 mm when the unloading ratio is 0.68. The bending moment and axial force of the segmental ring decreases, subsequently reverses and keeps increasing until the structure failure. Concrete cracks mainly appear on the waist, crown and invert of the shield tunnel and penetrate along the thickness and longitudinal directions. The plastic hinge successively forms at 73°, 287°, 180°, 8°, and 352° locations during unloading. The variation in the segmental ring stiffness under the overlying unloading condition first recovers and subsequently continues decreasing until structural failure. [ABSTRACT FROM AUTHOR]

[Display omitted] • Behaviour of shotcrete with steel sets in tunnels is modelled by analytical method; • The stiffness of hybrid support system is obtained in any locations along tunnel; • The non-uniform deformation of shotcrete between consecutive steel sets is obtained; • Developed bending moment on shotcrete between steel sets can be crucial for design; • A greater stiffness of steel set, a greater bending moment on shotcrete layer. The appropriate support system for tunnels in squeezing conditions is the hybrid supporting system consisting of steel sets and a shotcrete layer. The stiffness of this hybrid supporting system is variable between two consecutive steel sets, i.e., a greater value in the vicinity of the steel sets than in other locations. This is because, the contribution of steel sets to confine tunnel convergence is not uniform, but depends upon the distance to each steel frame. Accordingly, the tunnel convergence becomes non-uniform between two consecutive steel sets, and a bending moment is developed on the shotcrete layer, which may influence its design. In this paper, an analytical model is presented to simulate the shotcrete performance between two consecutive steel sets. For this purpose, the shotcrete layer is considered a shell fastened by consecutive steel rings. Using the principles of the theory of shells in the axisymmetric condition, it is assumed that the shotcrete is composed of longitudinal elements glued together from their sides. Therefore, the problem is simplified to analyzing continuous beams leaned on consecutive supports but subjected to loading from the radial and tangential directions. By employing the convergence confinement method, the non-uniform displacements are found. The results obtained with this theoretical simulation well agree with those of numerical modeling by FLAC3D. Therefore, the presented theoretical solution can provide a tool for the preliminary design of the shotcrete layer and an initial estimate of its performance to be used in more complicated numerical models. The results show that the non-uniform radial displacement and bending moment enhance by increasing the tunnel depth and decreasing the rock mass quality. As well, as the thickness of the shotcrete increases, the bending moment reduces. [ABSTRACT FROM AUTHOR]

• Mechanical behavior of the concrete pipes with a void at the invert are revealed. • Voids at the invert have a great influence on the pipe segments on its two sides. • The effectiveness of PU grouting in pipes with a void at the invert is evaluated. Research on pipes ringed by erosion voids is mainly concerned with circumferential mechanical behavior but ignores longitudinal bending responses induced by the relative motion of the pipe joints at voids. There has been limited innovative experimental investigation into pipe behavior that assesses the effectiveness of polyurethane (PU) grouting in rehabilitating erosion voids. Mechanical behavior tests of concrete pipes of intact, with an erosion void at the invert, and PU grouted were performed using a self-developed large-scale disaster testing system, and the measured results were numerically verified. Results show that the peaks of the positive and negative longitudinal bending moments of the intact pipes and pipes with a void at the invert appear at SgL2 and SgR2, SgL3 and SgR3, respectively, and the positive longitudinal bending moment is about 1.6–3.0 times higher than the negative ones. Voids at the invert have a significant effect on the longitudinal bending moments of the pipe segments located on the adjacent sides of the void. The longitudinal bending moments of the pipes with different void widths decrease with the increase of void width at SgL1 and SgR1 and increase with the increase of void width at SgL2 and SgR2, SgL3 and SgR3. The longitudinal bending moments of the pipes with different void lengths increase with the increase of void length in the whole longitudinal length. The circumferential bending moments at the bell-and-spigot decrease as the void width and void length increase. The maximum increases in the vertical displacements and maximum shear displacements of the pipes with different void widths and void lengths are 63–150% and 150–221%, 225–325%, and 325–550%, respectively, compared with the intact pipes. The rehabilitation efficiencies of the longitudinal and circumferential bending moments and the vertical displacement ranged from 85–93% and 85–94%. [ABSTRACT FROM AUTHOR]

• The internal force of a horseshoe tunnel is deduced by the force method principle. • Influence of the upper pilot tunnel excavation on the existing tunnel is studied. • Influence of the lower pilot tunnel excavation is studied by step-by-step unloading. • The accuracy of the proposed model is validated by comparing with other approaches. This paper proposed a calculation model for the internal force around a horseshoe tunnel based on the principle of force method. A method is proposed for computing the lateral elastic resistance of the non-circular closed ring structure. The influence of the newly constructed station of Beijing Subway Line 12 approach crossing the existing Subway Line 5 was studied with the upper and the lower pilot tunnel excavation considered. The study denotes that the maximum positive and negative bending moments of the existing tunnel increased are due to the upper pilot tunnel construction, but the influence is limited. The influence on the existing tunnel is studied by a step-by-step excavation unloading method, with each unloading width assumed to be the same as the spacing of one steel arch frame in the lower pilot tunnel excavation stage. Lastly, the construction parameters affecting the internal forces of the existing tunnel are analyzed, including the distance between the unloading surface and the existing tunnel, the unloading area, and the total excavation steps of the lower pilot tunnel. [ABSTRACT FROM AUTHOR]

• The full-scale test of concrete pipes after CIPP rehabilitation was carried out. • A numerical model of concrete pipes after CIPP rehabilitation was established. • The mechanical properties of the pipe-liner composite structure were studied. • Various cover depths, traffic loads and fluid loads were considered. The pipeline is mainly subjected to soil load, fluid load and traffic load, the mechanical properties of corroded concrete pipes after cured-in-place-pipe (CIPP) rehabilitation under multi-field coupling have not been revealed. In this paper, a 3D numerical model of buried concrete pipes after CIPP rehabilitation under the coupled effects of the soil load, traffic load and fluid load were established based on the results of the full-scale test. The influence of cover depth, traffic load and fluid load on the maximum principal stress of the pipeline, vertical displacement of the pipeline, the von Mises stress of the CIPP liner and the circumferential bending moment of pipe-liner composite structure were analyzed. Results showed that the cover depth and traffic load have a greater influence on the crown, invert, and springline of the pipe and has a lesser influence on the shoulder and haunch of the pipe; The stress and the circumferential bending moment of the pipeline is mainly caused by the soil load, the vertical displacement of the pipeline and the von Mises stress of CIPP liner is mainly caused by the traffic load, and the fluid load has little effect on the pipe-liner composite structure. [ABSTRACT FROM AUTHOR]

The inevitable influence of large excavation in soft soils on nearby tunnels is of great concern in practice. In this paper, the influence of a nearby large excavation on existing metro tunnels of the Ningbo Metro Line 1 in sensitive soft soils is investigated and presented. Considerable displacement in the left tunnel closer to the excavation induced by the nearby excavation was revealed by field monitoring. Visible cracks and leakages were observed in left tunnel linings. Three dimensional numerical simulations are conducted to investigate the responses of the ground and left tunnel due to the adjacent excavation. The development of bending moment and displacement of the left tunnel during different construction stages of the nearby excavation is obtained. Then the interaction mechanism between the nearby excavation, surrounding soils and existing twin tunnels is investigated, which is of significance to the interpretation of the influence of the nearby excavation on the existing twin tunnels. Several protective measures for alleviating the influence of adjacent excavation on left tunnel are studied, including divided excavation, soil improvement and a cut-off wall. It is found that the left tunnel is influenced to varying degrees during different construction stages and the time effect is distinct for this large excavation in soft soils, which would be suggestive to engineers to pay more attention to the protection of adjacent tunnel during the crucial construction stages. The bending moment and displacement of the left tunnel is strongly related to the unloading effects and displacement of surrounding soils, which can be alleviated by means of proper improvement of excavation sequence. Comparatively, longitudinally divided excavation is more effective in protecting the left tunnel than soil improvement or a cut-off wall. This study is of certain reference value for protecting metro tunnels adjacent excavation in soft soils. [ABSTRACT FROM AUTHOR]