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The research on the concrete structure built with self-healing materials brings inspiration to increase the safety and sustainability of underground structures in the whole life cycle. The utilization of microencapsulated healing agents in self-healing concrete has demonstrated efficacy in the repair of microcracks within concrete structures. Nevertheless, there exists a dearth of effective methodologies for assessing the impact of microcapsule parameters on the mechanical properties of self-healing concrete. This study introduces an innovative three-dimensional micromechanical model that can be utilized to analyze the micromechanical response of microencapsulated self-healing concrete under tensile loading conditions. The 3D micromechanical model is accomplished through the utilization of the elastic secant compliance tensor. Subsequently, a comprehensive examination is undertaken to analyze the progression of damage-healing in self-healing concrete incorporating microcapsules. Finally, a parametric investigation is conducted to elucidate the impact of the micro-parameters on the mechanical behavior of self-healing concrete. The present discovery holds significant implications for the development of microencapsulated self-healing concrete for underground structures, particularly in terms of establishing appropriate parameters. [ABSTRACT FROM AUTHOR]

In this paper, a resilience assessment framework for microencapsulated self-healing cementitious composites is proposed based on a micromechanical damage-healing model. A 3D micromechanical analytical model is constructed to analyze the performance evolution during the damage-healing process of self-healing concrete. The resilience assessment of microencapsulated self-healing concrete is defined by virtue of the residual stiffness, self-healing effect on stiffness and damage cumulative on stiffness, which corresponds to three main features of resilience; namely, the robustness, recoverability and adaptability. The assessment results indicate that the release of healing agents within microcapsules and healing process of extended microcracks allows the microencapsulated self-healing concrete to have higher resilience than conventional concrete. Moreover, a parameter sensitivity analysis is conducted to investigate the influence of the healing efficiency, the applied initial damage and the fracture toughness of the repaired microcrack on resilience of microencapsulated self-healing concrete. The results indicate that higher healing efficiency and applied initial damage leads to high resilience, and fracture toughness of the repaired microcrack makes less difference to the results. The findings of this paper lay a theoretical foundation for the resilience design of self-healing material layer of underground structures. [ABSTRACT FROM AUTHOR]

Various adjacent construction projects are inevitably encountered during the large-scale construction of urban underground transportation. The case of a twin shield tunnel excavated below an existing multi-arch culvert bridge in close vicinity has not yet been thoroughly studied. A case of a shield tunnel excavation below an existing multi-arch culvert bridge in Foshan is presented. First, a fully coupled three-dimensional model incorporating fluid and solid interactions was established to simulate the twin shield tunnel excavation below the existing multi-arch culvert bridge in close vicinity. Subsequently, the numerical model was validated using the modified Peck empirical formula. Finally, the influence of different foundation reinforcement deformation moduli, tunnel excavation face support pressures and grouting pressures on ground deformation was discussed. The results indicate that the area most significantly affected by the excavation of the tunnel passing underneath is not within the section of the multi-arch culvert bridge after applying the foundation reinforcement design. The area most significantly impacted is concentrated as the shield tunnel commences excavation and advances beneath the location corresponding to the 1-hole of the steel corrugated pipe. The range of influence of the ground disturbance extends approximately 7 m in front of the excavation face before the tunnel passed through. The range of ground disturbance decreases to 4–6 m in front of the excavation face after the tunnel passed through. The excavation face support pressure and grouting pressure have a minor impact on the settlement of the multi-arch culvert bridge under this reinforcement design. The results provide useful references for the reinforcement design for tunnel excavation adjacent to existing structures. [ABSTRACT FROM AUTHOR]

The large underground structure in saturated stratum is vulnerable to float during an earthquake. This results in a plethora of problems, such as uneven settlement, large internal force and structure damage. In this study, the seismic behavior of a three-story, two-span subway station in complex saturated stratum was investigated through water-soil fully coupled 3D finite element method. In the analysis, the nonlinear soil behavior was described by a cyclic mobility model and the structure was calculated using a nonlinear constitutive model which can consider the influence of axial force on its bending moment. Joints elements were used to model the contact surface between soil and structure. The development of the soil displacement and internal force of the station structure at different times during a huge earthquake was examined. The bearing capacity of the station structure members was analyzed. The results indicated that the vertical displacement was spatially distributed. The larger additional seismic stresses appeared in the bottom slab and side wall. Failed elements were observed in both the walls and slabs, and the walls were more easily damaged than the slab. The results provide useful reference for the large underground structure seismic design in the construction of urban underground space. [ABSTRACT FROM AUTHOR]

[Display omitted] • Prefabricated Machinery-excavation Structure necessitates compatibility with machine. • PMS decision problem presents a multi-sour information cross-fertilization. • Machinery-structure Automatic-adaptive Design Model was built for PMS planning. • MADM reveals the safety performance patterns of combined excavation. • MADM exhibits remarkable decision-making and optimization capabilities. The Prefabricated Machinery-excavation Structure (PMS) represents a pioneering solution that addresses the technical challenges for the construction of underground spaces in bustling urban environments. Unlike traditional engineering designs, the decision-making process for PMS involves the integration of multi-source information from both machinery and structure. Currently, the intricate mechanisms underlying this process remain elusive, presenting formidable technological hurdles that impede construction safety and industrial chain efficiency. In response to this issue, the present study has developed a Multi-source Automatic-adaptive Design Model (MADM) that integrates automatic modeling, automatic design, and simulation techniques. Through rigorous case studies, it has been observed that the initial excavation structure's capacity to withstand soil pressure is directly proportional to the optimization achieved in subsequent substructures. Notably, the optimization of steel rebar in temporary components is particularly evident, achieving a 20% increase compared to other schemes. Moreover, MADM favors a combined left–right excavation approach, which advocates for a 20–83% enlargement of the roof size for the up-down combination. Its robust decision-making and optimization capabilities are further evidenced by its preference for smaller excavation faces and substructure sizes, ultimately resulting in a substantial 30% reduction in concrete and steel usage. This study provides a scientific decision-making framework for PMS machinery-structure multi-source information planning, enhancing both safety and industrial chain efficiency. [ABSTRACT FROM AUTHOR]

The development of cities often involves the construction of new tunnels buried underneath densely distributed existing structures. When tunnels experience complicated and difficult conditions, coupling failure mechanisms often develop, in which the failure of one structure results in the failures of adjacent structures caused by soil failure initiated from the excavation of the new tunnel. Four centrifuge tests were performed in this study to reveal three major mechanisms, i.e., rapid sand flow, partial failure and overall collapse induced by the instability of a tunnel face and the effects of soil types and buried existing structures. Data are presented about the deformation and the failure mechanisms. Effects of soil properties and groundwater are discussed. The tests indicate that rapid sand flow can be easily triggered by tunnel face instability, a chimney-like mechanism creating gaps underneath existing structures. In cohesive soil, failure may be limited in front of the tunnel face due to the formation of arching, rendering a partial collapse. An overall collapse may occur in less cohesive soil when involving changes in underground water, which is a failure mode of a ground block bounded by two single shear planes extending from the tunnel face to the surface. It was observed that the bending deformation of the existing tunnel is well correlated with the failure mode, and a limited partial collapse had the smallest impact on the tunnel. The magnitude of the deformation of the structures depended not only on the failure mode but also on the scope and orientation of the failure. [ABSTRACT FROM AUTHOR]

Constructing large-section underground structures using the pipe jacking technique presents challenges, including the insufficient size of the rectangular pipe-jacking machine and the transportation and hoisting of large prefabricated ring structures. This paper introduces and investigates the construction, challenges, and key techniques involved in jacking a super-large rectangular prefabricated tunnel underpass a rainwater box culvert. The adoption of an assembly concept for manufacturing both the super-large rectangular pipe-jacking machine and ring structures, along with innovative in-shaft assembly and jacking techniques, effectively addresses issues in super-large section pipe-jacking engineering. The on-site monitoring data analysis reveals that the jacking force for the super-large rectangular tunnel significantly exceeds the estimated value. This is primarily attributed to excessive resistance during jacking in the reinforcement areas of the launching and receiving shafts, constituting 55 % of the total jacking force. Additionally, the jacking resistance from the soil chamber pressure accounts for approximately 22 % of the total force. The frictional resistance of the ring structures represents only 23 % of the total jacking force, with the average surface friction force approximately at 12.4 kPa. Vertical and horizontal relative deviations of the tunnel were controlled at 32 mm and 22 mm, respectively. The settlements of the ground surface and box culvert induced by pipe-jacking were vulnerable to the grouting pressure of lubricants, with maximum values of 35.9 mm and 39.1 mm, respectively. The experience learnt from this project can provide valuable insights for the application of large-section rectangular pipe-jacking technology in underground space development. [ABSTRACT FROM AUTHOR]

• A novel method that integrates multiple damage indices to evaluate the seismic resilience of tunnels was introduced. • A combination of FEM and earthquake attenuation models is utilised for multiple damage indices. • The robustness grade is determined by Cloud Model method. • The repair time is quantified through a repair function based on robustness grades. The subterranean environment of tunnels poses considerable uncertainty as tunnel structures are ensconced in soil, unlike their above-ground counterparts. This significantly complicates tunnel risk assessment during earthquakes. This study introduces a novel method that integrates multiple damage indices to evaluate the seismic resilience of tunnels. Initially, seismic attenuation is introduced to calculate earthquake exceedance probabilities for various tunnel damage indicators, employing finite element methods (FEM). A robustness evaluation criterion scale value is established based on the amalgamation of multiple tunnel damage indices. Standard Cloud Models are then generated utilising the robustness evaluation criteria. Subsequently, the independent and correlated weights of the robustness evaluation indices are determined using the CRITIC-G1 and decision-making trial and evaluation laboratory (DEMATEL) methods, respectively. A game theory (GT) method is then utilised to amalgamate and allocate weights to these robustness evaluation indices. The evaluation Cloud Models are subsequently generated using a backward cloud generator, based on the division of damage grades for the evaluation criteria and combination weights. Finally, the robustness grade is determined by comparing the similarities between the standard and evaluation Cloud Models. The repair time of the tunnel is quantified using a repair function based on robustness grades. The efficacy of the seismic resilience assessment method is discussed based on three hypothetical cases, providing valuable guidance for assessing the seismic resilience of underground structures. [ABSTRACT FROM AUTHOR]

The necessity for low-carbon underground structure construction in urban sustainable development is clear. Despite widespread green construction techniques, cast-in-situ (CIS) construction is still common in underground projects, presenting significant environmental challenges. Consequently, this study presents the Prefabricated Two-wall-in-one Diaphragm Wall (PTDW), a single-walled scheme, and investigates its GHG reduction efficiency. First, the technical advantages of PTDW were introduced in detail. Second, the Carbon-mechanics Dual-control Assessment Model (CDAM) was constructed to evaluate the GHG reduction efficiency of underground construction techniques. Finally, CDAM was used for a comparative analysis of single-walled and traditional double-walled schemes. The research findings are as follows: (1) The vulnerability of the prefabricated single-walled scheme is not inferior to the CIS double-walled scheme. (2) PTDW technology reduces GHG emissions intensity by 24.96% compared to traditional CIS construction. This reduction mainly results from avoiding traditional CIS temporary measures. (3) Across all restoration levels and areas, the single-walled scheme's GHG reduction efficiency consistently exceeds that of the double-walled scheme. PTDW technology offers significant advantages in structural resilience and GHG reduction efficiency. It also reveals that PTDW technology not only reduces GHG emissions intensity but also enhances the GHG reduction efficiency. These findings provide both a theoretical foundation and a practical tool for evaluating the GHG reduction efficiency of underground structural construction technologies. [ABSTRACT FROM AUTHOR]

• A resilience assessment framework for underground structures is proposed. • Performance-Time-Cost three-dimensional space is constructed. • The composite performance of the evaluated system is continuously evaluated. • The formulas for comprehensive resilience assessment are derived. Scientifically and reasonably evaluating the safety of existing underground structures under adjacent construction disturbances is of great importance. In this paper, a resilience assessment framework including five steps is proposed to investigate the composite performance and comprehensive resilience of the evaluated structure-stratum system under adjacent construction disturbance. A Performance-Time-Cost three-dimensional space is constructed to interpret the evolution laws of composite performance and recovery costs over time. Using the cross-matrix method from the concept of resilience, the resilience indicator system is constructed. Moreover, the composite performance with multiple indicators of the evaluated system is continuously evaluated using the set pair analysis theory and the combined weights in terms of game theory. Furthermore, the formulas for comprehensive resilience assessment are derived from the perspectives of maximum and cumulative damage of composite performance. Finally, the correctness and rationality of the resilience assessment are verified by engineering applications. The resilience framework proposed in this paper provides theoretical and technical support for the safety state evaluation and control technology of existing underground structures under adjacent construction disturbances. [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]

Accurately predicting the overlying earth pressure of underground structures is a key point in the design and construction of underground structures. On the basis of the existing composite function model describing the load–displacement curve (LDC) and experimental data, a intergrating knowledge-data-driven method to predict LDC is proposed in this paper. The buried depth (H), trapdoor width (B), soil weight (γ) and internal friction angle (φ) are selected as multi-characteristic input variables. The initial modulus of arching a , the minimum soil arching ratio ρ min , the minimum soil arching ratio displacement ξ min and the ultimate soil arching ratio ρ ult are output variables. Two swarm intelligence optimization algorithms (i.e., sparrow search algorithm (SSA) and particle swarm optimization (PSO)) are used to optimize the parameters of the established generalized regression neural network (GRNN), and the knowledge-data cooperatively driven prediction of the LDC is realized. The results show that the GRNN model optimized by swarm intelligence algorithm has better prediction performance than the GRNN model. The LDC obtained from the output parameters of three GRNN models are compared with the results of the trapdoor experiments. The comparison results show that the LDC obtained by the GRNN model optimized by swarm intelligence algorithm are more consistent with the experimental results than that those obtained by GRNN model, and the prediction performance of the SSA-GRNN is slightly better than that of the PSO-GRNN. [ABSTRACT FROM AUTHOR]

The formation of discontinuities within soil masses due to arching phenomena is a critical issue in geotechnical engineering, profoundly influencing the stability and integrity of underground structures. This study investigated the microscopic mechanisms and associated stress redistribution in the soil mass above an actively moving trapdoor. Utilizing the Discrete Element Method (DEM) model, the influence of varying initial soil packing densities on the induced arching phenomena is examined concerning the soil displacement mechanism and shearing configuration: transfer from inner triangular to vertical bands is observed in dense soil while only vertical shearing bands are observed in initially loose soil. To account for principal stress trajectories under differing initial density conditions, an analytical model was developed to quantify the variations in soil stress. The findings reveal a noteworthy impact of the initial soil void ratio e , on the pattern evolution and stress redistribution of soil arching, manifesting a passive stress limit coefficient, K p , in denser soils along the central line above the trapdoor. The analytical model demonstrates robust agreement with both numerical and experimental data in soil loosening stress distribution and the load–displacement response of the trapdoor. [ABSTRACT FROM AUTHOR]

[Display omitted] • Clay replacement can reduce the structure internal force under earthquake loads. • Unilateral drainage diaphragm wall utilizes cushioning performance of liquefied soil. • The combined treatment exhibits excellent anti floating effects on the structure. After the concept of 'resilient city' was introduced, the anti-seismic and resilience enhancement of underground structures gradually became an important issue in engineering. In this study, a method for enhancing the resilience of a soil–structure system subjected to earthquakes is proposed. First, a water-soil fully coupled numerical model was established to analyse the seismic behaviour and enhancement effect of a station structure subjected to an earthquake in liquefiable ground. In the analysis, an elastoplastic cyclic mobility model was used to describe the soil behaviour, whereas an interface joint element model was used to consider the soil-structure interaction. Subsequently, a non-liquefied clay replacement method was designed, and the effects of the replacement position and thickness on the floating displacement and internal force of the structure were analysed. To fully utilise the cushioning effect of the liquefiable sand and the anti-floating drainage near the station structure, a unilateral drainage diaphragm wall that does not drain in the far-field soil and drains in the near-field soil is proposed around the station structure. Based on the analysis results of different cases considering the soil replacement position, replacement thickness, distance between the unilateral drainage diaphragm wall and station structure, and buried depth of the wall, combined measures of clay replacement with unilateral drainage diaphragm wall were examined. The results show that the combination of the two measures significantly reduced the floating displacement, restrained the additional internal forces of the structure, reduced the time for the structure to reach stability after an earthquake, which is helpful to enhance the ability of self-recovery and improve the seismic resilience of the underground structure. [ABSTRACT FROM AUTHOR]

Earthquake-induced liquefaction can lead to uplift displacements of underground structures. While the behaviour of a single underground structure has been extensively studied, the coupled response of a shallow buried circular tunnel adjacent to an underpass with a rectangular section in liquefiable ground remains unclear. In this study, the finite difference method based on the fully coupled effective stress is employed to analyse the seismic response of two adjacent underground structures. Firstly, comprehensive parameters of Hostun sand for P2Psand constitutive model was calibrated. The accuracy of the numerical method is verified through a centrifuge test. Subsequently, comparisons are made between the seismic response of the tunnel-soil-underpass interaction and other cases: free field, tunnel, and underpass. The tunnel-soil-underpass interaction in liquefiable ground is analysed, with a special emphasis on the development of excess pore pressure, acceleration response of the soil, displacement and deformation of the structures. A section deformation coefficient was proposed to depict the tunnel deformation. Finally, horizontal spacing, buried depth of tunnel and underpass are discussed to clarify the interaction of two structures. The optimal relative position of two structures is suggested, providing valuable insights for the spatial design of underground structures in liquefiable ground. [ABSTRACT FROM AUTHOR]

• PUS-based DFOS monitoring system is adaptable to various PUS components. • PUS-based DFOS monitoring system accurately reflects the component performance. • A hierarchical performance assessment framework is proposed for PUS. • PUS-CIMAM highlights the key process and regions for the large-span components. • PUS-CIMAM reveals the impact of adjacent ring construction disturbances. Mechanical performance of prefabricated structural components during construction is critical for ensuring long-term safety and durability of prefabricated underground stations (PUS). This study aims to address the issues of structure mechanical performance of large-scale prefabricated components during assembling process. First, a strain transfer model for five-layered armored fiber optic is proposed and validated experimentally through test. Second a PUS-based distributed fiber optic sensors (DFOS) monitoring system was established. Third, a PUS based hierarchical performance assessment framework was constructed. Finally, a PUS based theoretical Construction Intelligent Monitoring Assessment Model (CIMAM) was established to quantitively assess structure safety level. A case study of Shenzhen Metro PUS was performed, and the results showed that: (a) PUS-based DFOS monitoring system presents high accuracy and high resolution, (b) The monitoring system reflects the PUS reactions during the whole construction process with good adaptability and intelligence, (c) PUS-CIMAM results show that the assembly process of components is a significant control link, particularly the performance of large-span roof during assembling process is a key controlling component, and (d) PUS-CIMAM reveals the adjacent ring construction has a disturbing effect. [ABSTRACT FROM AUTHOR]

This paper investigated the stress-transfer mechanisms in soil around a circular tunnel (CT) and a rectangular tunnel (RT) using the finite element method (FEM). Compared with the CT, there was a self-weight stress zone in the local soil above the RT, and the shear stress was concentrated only near the sliding surface, which caused the trajectory of the minor principal stress in this area to take an inverted arch shape. Then, a multi-arch model consisting of the end-bearing arch and the friction arch was proposed for estimating the distribution of the vertical earth pressure on a deep RT in dry sand. Unlike the CT, the friction arch in the RT was based on the assumption that the minor principal stress was a circular arc. The distribution factor of the vertical earth pressure on the RT was obtained. Note that a modified coefficient in the distribution factor was introduced, which was obtained by numerical simulation at different cover depth ratios and internal friction angles. Finally, the theoretical, experimental and numerical results were compared to evaluate the validity of the proposed model. The theoretical results in this paper were in good agreement with the experimental and numerical results. [ABSTRACT FROM AUTHOR]