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Objective As the sand layer located at the launching end of the slurry shield section is connected with seawater during the tunnel construction of Xiamen Metro Line 3, accidents like water and mud gushing, ground collapse may occur when the shield starts. Therefore, it is necessary to study the mud film stability of urban slurry shield tunnel to ensure the construction safety. Method A numerical model is developed to simulate the mud film formation and infiltration process under different permeability conditions by using the DEM-CFD (Discrete Element Method-Computational Fluid Dynamics) coupling method. The impermeability stability of the mud film is evaluated by analyzing the spatial distribution of slurry particles, the mud film hydraulic pressure changes, and the force chain network. Result & Conclusion The increase in sand particle size and slurry cohesion inhibits and enhances mud film formation, its long-term impermeability and mechanical stability under dynamic hydraulic loads respectively. The increase in cohesion enhances both the mud film forming density and the impermeability stability. However, the enhancement of impermeability stability is limited for the mud film on smaller grain size stratum. In sandy stratum with large pores and low cohesion, the mud film is prone to collapse, but increasing the cohesive force can effectively enhance its stability. Therefore, by selecting the appropriate particle size ratio and slurry cohesion, mud film impermeability stability can be improved to meet the engineering requirements.

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To guarantee the safety of tunnel construction and the continued use of nearby structures, it is crucial to accurately forecast the size and extent of the plastic region that may occur due to tunnel excavation, as well as examine the impact on resilience. In this paper, the influence mechanism of tunnel construction on adjacent pile foundation and resilience assessment is investigated. Firstly, the stratum deformation and stress induced by tunnel construction are derived based on the thin-walled theory considering the influence of tunnel structure stiffness. Moreover, the resilience assessment based on the characteristics of the stratum plastic region is proposed to describe the degree of disturbance caused by tunnel construction to the adjacent pile foundation. Then, a comparison with a numerical simulation is conducted to verify the correctness of the prediction method of the stratum plastic region proposed in this paper. Finally, parameter sensitivity analysis is carried out, which indicates that pile parameters, soil parameters, and different tunnel outline conditions have a great influence on the prediction results. In order to reasonably control the impact of tunnel construction on the surrounding environment, safety control techniques, including advance grouting reinforcement and grouting uplift, need to be carefully designed. [ABSTRACT FROM AUTHOR]

With the development of large-diameter shield tunnels, how to realize effective security and stability control of shield tunnel lining has become a significant research topic. This paper investigates the deformation and failure mechanism of lining large diameter shield tunnels in depth and discusses the deformation characteristics and influencing factors of the lining of the shield tunnel with various diameters through the software of finite element analysis ABACUS. A set of models with varying diameters is built under identical stress conditions in order to maintain control over the variable. The utilization of the elastic–plastic model is observed in the application of bolts and rebar. The utilization of the Concrete Damage Plasticity model has been taken into account for the concrete lining. For the sake of comparison, the crown displacement of the shield tunnel, strain in tension and compressive zones, bolt stress and strain, deformation and intemal force distribution around the shield tunnel, and cracks in the tension zone, are carefully studied. An in-depth analysis is conducted to elucidate the variations in damage evolution mechanisms across linings of different sizes, within the framework of plastic hinge theory. The results indicate that the convergence deformation of large-diameter tunnel lining increases significantly during loading compared with that of small-diameter tunnel. Moreover, the probability of brittle failure is higher in big-diameter shield tunnels compared to small-diameter tunnels, indicating that these larger tunnel structures are more prone to suffering geometric instability. [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]

The high-energy-level dynamic compaction method is widely used in various foundation treatment projects, but its reinforcement mechanism still lags behind the practice. In view of this, a three-dimensional fluid–solid coupling dynamic analysis model was established on the basis of the FDM–DEM coupling method. The variation trends of crater depth, soil void ratio, vertical additional dynamic stress, and pore water pressure during the process of dynamic compaction were analyzed. The results indicate that the curvature of the crater depth fitting curve gradually decreases with the increase in strike times, tending to a stable value. The initial particle structure is altered by the huge dynamic stress induced by dynamic compaction. As strike times increase, the soil void ratio decreases gradually. The vertical additional dynamic stress is the fundamental reason resulting in foundation compaction. Precipitation preloading before dynamic compaction can improve the reinforcement effect of dynamic compaction, making up for the deficiency that the vertical additional dynamic stress attenuates rapidly along the depth direction. The simulated CPT results illustrate that the modulus of foundation soil can be increased by 3–5 times after dynamic compaction. The research results can provide important reference for similar projects. [ABSTRACT FROM AUTHOR]