Gang Wei

Shield tunnels enter the operation and maintenance stage, ensuring the long-term safety of segmental linings has become a critical challenge. To overcome the limitations of traditional passive repair, this study introduces novel steel–concrete composite reticulated shell segments (SCRSs) based on the secondary service life concept. A scaled-model surcharge test was conducted to evaluate the baseline performance and failure mechanisms of unstrengthened segments (S-CN). Subsequently, a secondary concrete layer was cast inside the deformed segments (S-CR) to assess post-damage structural recovery, supported by theoretical ring mechanics analyses. Experimental results indicate that the unstrengthened SCRS exhibits pronounced load-bearing capacity and ductility, characterized by a progressive three-stage response encompassing the elastic stage, yield plateau, and post-yield stage; the test was terminated at a load of 141.95 kN with the curve still ascending, so this value represents a terminal load and a lower bound on the ultimate capacity. This behavior stems from the sequential engagement of outer reinforcement and the inner steel framework, providing clear deformation warnings prior to structural failure. Furthermore, casting the secondary concrete layer enhanced both the stiffness and the load-bearing capacity of the deformed specimen, which sustained a maximum total load of 203.14 kN; this value is a local failure threshold governed by concrete crushing beneath the concentrated loading heads rather than a structural peak load. The as-built specimens were fabricated from commercially processed steel members and hand-mixed concrete, and their measured material properties (a measured ultimate tensile strength of about 240–400 MPa for the steel, varying between two test batches; a cubic compressive strength of 27 MPa for the concrete) were used throughout the analysis. The experimental data support the feasibility of the secondary service life concept as a proof-of-concept study of the segment body, within the limitations of a single scaled specimen and short-term monotonic loading. This composite design offers a potential pathway for shifting shield tunnel linings from reactive repair to proactive, whole-life management.

To study the changes in circumferential internal forces of an existing shield tunnel when an overlapping tunnel passes through, a surrounding pressure redistribution model for overlapping tunnels was established to calculate the circumferential surrounding pressure of the existing tunnel. Combined with an engineering case, a detailed three-segment tunnel model was created using Abaqus 2023 finite element software. The circumferential forces of the tunnel were applied to the model, and the circumferential internal forces of the segments were extracted to study the variation law of segment circumferential internal forces. The research results indicate the following: (1) During the excavation of overlapping tunnels, the circumferential surrounding pressure of the existing tunnel decreases, and the circumferential surrounding pressure at any position exhibits a symmetric “8”-shaped distribution. (2) During the excavation of the new tunnel, the circumferential internal forces of the existing shield tunnel increase. In front of the excavation face, the additional internal forces of the tunnel decrease with distance, while within approximately 6 m behind the excavation face, the additional internal forces of the tunnel increase sharply. (3) As the angle between the line connecting the axes of the two tunnels and the z-axis increases, the additional internal forces of the existing tunnel decrease; as the clear distance between tunnels increases, the circumferential internal forces of the existing tunnel decrease, and the additional bending moments and additional shear forces approximately show linear variation.

Tunnelling-induced safety risks from adjacent piles have become increasingly severe with the rapid development of urban underground space. Model tests have become essential for revealing the complex pile-tunnel interaction mechanism. This paper reviews the research progress of model tests on the influence of single-line tunnelling on adjacent piles, focusing on test soil materials, tunnel simulation methodologies, analysis of test results, and research prospects. However, current model test studies are constrained by several critical limitations, including insufficient similarity between soil materials and prototype conditions, and overly idealized simulation of tunnel excavation. This paper identifies a significant research gap: the inability of current volume-loss techniques to capture 3D dynamic factors (e.g., face pressure and grouting timing) and the lack of meso-scale observation at the pile-soil interface. This review provides a systematic synthesis of these methodological challenges and proposes future research prospects to provide a more scientific basis for engineering design and risk control.

With the growing scarcity of surface space, underground development has become essential for expanding human living space. Among various tunneling methods, pipe jacking stands out due to its economic advantages and minimal environmental impact. Recently, vertical pipe jacking has been explored as an innovative technique for constructing shafts that connect horizontal tunnels to the ground surface. However, the evolution of jacking force during vertical pipe jacking with increasing jacking distance remains poorly understood. Understanding this evolution is critical for selecting jacking equipment, designing the horizontal tunnel lining against reaction forces, and preventing construction failures. Unlike horizontal pipe jacking where self-weight is negligible, the proposed model reveals that in vertical pipe jacking the self-weight of the pipe and machine above the excavation face increases with jacking distance while the overburden pressure decreases, resulting in a parabolic-like jacking force trend—a novel finding not reported in previous pipe jacking literature. This paper proposes theoretical formulas to quantify the three components constituting the jacking force: face resistance at the cutting head, frictional resistance along the pipe surface, and the dead weight of the machine and pipe above. The influence of jacking distance on each component is systematically analyzed. Parametric studies under standard and varied conditions reveal that under standard conditions, jacking force follows a parabolic trend—rapid initial increase, followed by slower growth, and eventually a slight decrease. The maximum jacking force consistently occurs at L = L₀ − 1 m, identifying the most unfavorable construction stage where special attention to tunnel lining deformation is required. Increasing outer diameter transitions the force curve from quasi-parabolic to “half diamond” shape, while doubling the friction coefficient approximately doubles the jacking force. These findings provide practical guidelines for vertical pipe jacking design and construction, including equipment capacity selection, friction reduction strategies, and monitoring priorities.

To evaluate the coupled deformation of existing shield tunnels induced by multi-segment excavations with isolation piles, this study develops an integrated analytical framework combining a Kerr three-parameter foundation-plate model with a three-dimensional image-source solution. A closed-form expression for the soil displacement field is first derived by incorporating layered soil conditions, staged excavation, and associated spatial effects. The soil–pile interaction of isolation piles is then modeled using the Kerr foundation, and the flexural response is obtained through variational formulation and finite-difference discretization. These responses are sequentially propagated through the excavation stages, enabling the superposition of multi-pit effects on the final retaining-wall deformation. The image-source method and a volume-equivalent transformation are further used to convert wall deformation into an additional stress field acting on the tunnel, which is ultimately coupled with a tunnel–soil deformation–coordination model to compute horizontal tunnel displacements. This unified workflow establishes a continuous mechanical transfer chain—from excavation-induced soil loss to isolation-pile bending and finally tunnel deformation. Parametric analyses show that lateral displacement of the retaining structure is jointly governed by wall bending and pit-bottom uplift, producing a right-skewed “S-shaped” profile. The bending-moment peak shifts toward earlier-excavated zones, indicating a memory effect of excavation sequencing. Two engineering cases verify that the proposed method accurately reproduces the magnitude and depth of measured wall deflections, while predicted tunnel displacements show a near-Gaussian pattern with high accuracy near the peak. The analytical framework provides a robust theoretical basis for optimizing pit segmentation and excavation sequencing adjacent to shield tunnels.

The deep foundation pit excavation of subway will cause horizontal displacement, uneven settlement and other adverse effects on the adjacent shield. The use of servo steel strut has a certain effect on deflection correction, but the current understanding of the influencing factors of deflection correction is not comprehensive. Based on structural and spatial symmetry, the influence of tunnel depth, tunnel and foundation pit clear distance and deformation control quantity of enclosure structure on deflection correction quantity was studied by symmetrically designed model test and numerical simulation, and the prediction formula of deflection correction quantity considering tunnel and foundation pit clear distance and deformation control quantity of enclosure structure was proposed. The results show that with an increase in the tunnel’s burial depth, deflection correction decreases significantly. When the tunnel is near the foundation pit bottom, there is no significant correction effect, and the control law of the tunnel ground pressure under the servo steel strut loading is consistent with the correction law. Deflection correction is negatively correlated with the tunnel and foundation pit clear distance, and positively correlated with the deformation control of the diaphragm wall. The curve of the deformation control of the enclosure structure and the deflection correction is parabolic. The deflection correction is an exponential function of the ratio of the deformation control of the enclosure structure to the clear distance between the tunnel and the foundation pit, and the servo deflection correction follows a normal distribution along the longitudinal axis of the tunnel, showing obvious symmetry characteristics in the foundation pit influence zone.

Granular materials exhibit complex mechanical behaviors during unloading, yet the underlying micro- and meso-scale mechanisms remain unclear. This study employs a discrete element method to simulate a series of triaxial tests on sand and pebble specimens with varying initial densities under different unloading stress paths. While dense specimens demonstrate strain softening and dilatancy, loose samples exhibit shear contraction. To quantify the underlying fabric evolution, persistent homology (PH) theory is adopted to analyze the particle contact force networks. The results reveal that the average strength of this network correlates strongly with the macroscopic stress–strain response. For dense samples, network strength rapidly increases to a peak coinciding with the deviatoric stress maximum, then gradually decreases with further shear. Crucially, this evolution is path-dependent: the average contact force network strength increases approximately 20% more during unloading in the minor principal stress direction compared to unloading in the major principal stress direction. This quantitative analysis of force chain degradation provides a mechanistic explanation for the observed strain softening, highlighting the dominant role of the unloading stress path. In contrast, loose specimens, which initially lack an obvious force chain network, show negligible microstructural evolution during unloading.

Aiming at the deformation problem of sand-cobble composite strata induced by tunnelling, the stress path of soil during tunnel excavation was analyzed by numerical simulation. Two stress path schemes (σ₁ reduction-σ₃ reduction, σ₁ reduction-σ₃ stability) were proposed. The triaxial test model of composite stratum is constructed. The DEM simulation results demonstrated significant differences in the macroscopic mechanical response of soil under different stress paths. The simultaneous unloading of axial pressure and confining pressure will accelerate soil softening, while the decrease of confining pressure unloading rate can improve the toughness of soil. With the increase of the thickness of cobble stratum, the bearing capacity and toughness of the soil show an increasing trend, and the increase of cobble particles enhances the overall stability of the soil. In addition, the increase of stress ratio also leads to the improvement of soil strength and stability, so that the soil can better resist the softening effect caused by stress release during unloading.

Maintaining stable tunnel face pressure in slurry shield tunneling is critically dependent on the formation of a low-permeability filter cake. However, the knowledge of the filter cake and mud spurt is not specifically understood. Using a modified fluid loss test, this study investigates the formation and hydraulic properties of filter cakes from various slurry mixtures under different pressures. The key findings reveal that CMC-Na (sodium carboxymethyl cellulose) serves as the most effective additive for enhancing slurry performance. A comprehensive database of constitutive model parameters for 15 slurry compositions was established, enabling precise prediction of the filter cake’s hydraulic conductivity and void ratio under any pressure. Analysis of the cyclic formation process revealed that the dynamic filter cake averages two-thirds of the maximum thickness, offering a key parameter for stability control. Furthermore, a practical mud spurt model was proposed that predicts slurry penetration by avoiding the need for site-specific empirical constants or complex column tests, relying instead on standard geotechnical and slurry parameters. The results provide practical criteria for filter cake formation and directly applicable models to optimize slurry design, thereby enhancing the control and safety of shield tunneling.