The spalling of concrete poses a serious threat to its normal serviceability under elevated temperatures, particularly for ultra-high-strength levels. This study presents a comparative investigation into the spalling behaviors of alkali-activated ultra-high performance concrete (AAUHPC) with different precursors of ground granulated blast furnace slag (GGBFS), metakaolin (MK), silica fume (SF), and ordinary Portland cement-based UHPC (OPC-UHPC) during heating to 800 degrees C. The compressive strength, flexural strength, spalling performance, thermogravimetric analysis, microstructure, pore structure, and temperature gradients of AAUHPC are investigated to reveal its spalling mechanism. Results indicate that incorporating MK and SF facilitates the geopolymerization process, leading to an increase in C-(N)-A-S-H gels, thereby enhancing compressive and flexural strength. Crucially, AAUHPC demonstrates significantly superior spalling resistance compared to OPC-UHPC, exhibiting no explosive spalling even under high heating rates or saturated conditions, which is attributed to its interconnected porous structure that facilitates vapor release. Furthermore, the incorporation of MK and SF increases the proportion of gel pores, leading to more thermally induced cracks at higher heating rates and moisture contents. In addition, environmental assessments indicate that compared to OPC-UHPC, AAUHPC reduces its total carbon emissions by up to 49.2%. These findings collectively clarify the fire resistance mechanisms of AAUHPC and support its application in high-temperature environments.

In response to the growing low-carbon demand for urban shield tunneling, this study presents a mini review of low-carbon backfill grouting materials, focusing on their material taxonomy, performance mechanisms, and engineering adaptation pathways. The review systematically summarizes recent advances in three major categories of sustainable grouts: alkali-activated grouts (AAG), asphalt-related composite grouts, and shield tunnel spoil (STS)-based recycled materials. Despite significant progress, persistent challenges remain in long-term performance evaluation, mix design standardization, and adaptation to complex hydrogeological conditions. Future research should integrate material innovation, multi-source solid waste synergy, and digital construction technologies to establish a performance-driven, low-carbon-oriented framework for grouting materials. This review provides holistic insights into the sustainable development of shield tunneling materials and promotes the transition toward more environmentally friendly, low-carbon, and resilient underground infrastructure systems.

To overcome the limitations of insufficient performance enhancement, poor construction quality control and low efficiency associated with existing reinforcement methods for shield tunnels, a prefabricated UHPC panel (PUP) reinforcement method is proposed, along with a novel joint for panel connection, referred to as the assembled bolt-plate joint (ABPJ). A series of flexural tests are conducted to systematically reveal the effects of load type and key design parameters on the mechanical behaviour of the ABPJ, and corresponding optimization strategies are proposed. A refined finite element (FE) model for the interlayer composite damage behaviour of bonded reinforced segmental structure was developed based on coupled CZM (Cohesive Zone Model)-XFEM (eXtended Finite Element Method) method, clarifying the influence of the ABPJ on the load-bearing performance and damage mechanisms of the reinforced structure. The results show that the technical framework of the prefabricated UHPC panel reinforcement method encompasses three key aspects: standardized design, industrialized production, and prefabricated construction. The flexural failure of the ABPJ is governed by the formation of plastic hinges, which may develop either at the groove wall or at the joint interface. The former is caused by the combined effects of flexural tension and the weakening of the cross-section due to grooving, while the latter results from insufficient stiffness of the connecting plate. To ensure adequate bidirectional flexural stiffness, the ABPJ connecting plate thickness should not be less than 20 mm. Under axial compression combined with positive bending, the development of gaps at the ABPJ compromises the structural integrity, leading to interlayer composite failure characterized by adhesive layer cracking and interfacial debonding, and a reduction in load-bearing capacity of approximately 25 % for the reinforced structure. Under axial compression combined with negative bending, the high level of axial force helps maintain compression at the joint interface, making the presence of the ABPJ have a negligible effect on the reinforcement effectiveness.

Ground-penetrating radar (GPR) has been widely used to detect concealed defects behind tunnel linings. However, the limited penetration capability of traditional GPR in complex media such as concrete constrains tunnel-lining inspection. This limitation makes it difficult to obtain clear images of deeper structures. Furthermore, achieving rapid, multi-directional, and multi-angle inspections with traditional methods is challenging, especially in operational tunnels. In this paper, a vehicle-mounted multi-antenna synthesized ultra-wideband (UWB) GPR system is designed for tunnel inspection. First, we introduce a synthesized UWB signal design method that uses a field-programmable gate array (FPGA) to dynamically generate transmit signals from 0.3 to 1.0 GHz. This approach balances inspection depth and resolution. Second, we propose a delayed phase-locked synchronization technique to achieve high-precision time synchronization between the transmitter and receiver. Additionally, we design a semi-elliptical antipodal Vivaldi antenna array that outperforms conventional designs and obtains higher-quality inspection data in real tunnel environments. We systematically verified the feasibility and effectiveness of the proposed system through four experiments, including reflection analysis of multi-material targets, non-contact detection of buried objects in sandy media, dynamic scanning-based imaging in tunnel structures, and identification and imaging of typical defects in complex tunnel environments. The experimental results demonstrate that the system can effectively detect various types of subsurface targets under non-contact conditions. In the future, the system is expected to provide an efficient solution for multi-directional, multi-angle, deep, and rapid tunnel defect detection, particularly in complex engineering scenarios such as operational tunnels.

To address the bottlenecks of low construction efficiency and poor-quality control in existing strengthening technologies, a prefabricated UHPC panel (PUP) strengthening method tailored for metro shield tunnels is proposed. A 1/5-scaled model test was conducted to systematically investigate the load-bearing capacity and failure mechanism of the novel strengthened structure, thereby validating its performance enhancement and engineering applicability. The results indicate that the developed fiber micro-particle concrete (FMPC) can reasonably replicate the tensile and compressive properties of prototype UHPC materials, offering a reliable material basis for scaled model test. A multi-index evaluation system was established to comprehensively quantify the strengthening effectiveness, incorporating stiffness retention, interface compatibility, and internal force distribution. The performance evolution of the strengthened structure is a multi-mechanism coupled process, which can be categorized into the quasi-elastic stage, nonlinear degradation stage, and failure stage.Triggered by the degradation of interface compatibility and further aggravated by structural cracking, the load-sharing capacity of the strengthening layer declined significantly, ultimately leading to structural failure. The PUP method achieved a load-bearing capacity enhancement approximately 1.6 times that of the conventional bonded steel-plate strengthening method. This improvement is primarily attributed to the superior flexural stiffness of the PUP system and the enhanced mechanical compatibility of cementitious materials across the bonded interface. The joints of the PUP system exhibited no mechanical weakness, confirming the feasibility of a circumferentially non-uniform stiffness design, which is essential for achieving assembly-based strengthening.

With advances in computer vision and modern surveying technologies, intelligent inspection systems and automatic recognition methods are increasingly used in highway tunnel maintenance. However, existing mobile inspection methods still struggle to balance high-speed operation, fine-crack recognition, and comprehensive assessment of multiple defects. This study proposes an automatic recognition and quantitative assessment method for multiple visible defects in highway tunnels based on a vehicle-mounted multisensor inspection system. The system integrates high-resolution imaging, infrared illumination, 3D laser scanning, mileage positioning, and high-speed data storage, enabling continuous full-section data acquisition at speeds up to 80 km/h. A structural-feature-constrained mileage correction strategy is developed to reduce accumulated localization errors. For crack analysis, a multilevel framework combining two-stage CNN screening, cascaded segmentation, crack trajectory tracking, and subpixel edge extraction is established for crack recognition and 0.1 mm-level width measurement. Water leakage and spalling are extracted through visible–infrared image fusion and adaptive boundary refinement, while cross-sectional deformation is calculated using 3D tunnel axis reconstruction, point-cloud filtering, and cross-section fitting. Field tests and controlled experiments demonstrate that the system can rapidly identify, locate, and quantify multiple tunnel defects, providing a practical reference for intelligent tunnel inspection and maintenance.

From a spectroscopic perspective, this paper systematically elucidates the mechanism of rock strength softening. By integrating uniaxial compressive strength tests, near-infrared spectroscopy, as well as one-dimensional and two-dimensional correlation spectroscopy analyses, it thoroughly reveals the intrinsic mechanism and spectral response characteristics of sandstone strength softening during water absorption. The results indicate that: (1) during the water-induced softening process of sandstone, four characteristic absorption peaks are observed in its one-dimensional near-infrared spectrum. Specifically, the peak intensities at approximately 1400 nm and 1900 nm increase significantly, while those at around 2200 nm and 2320 nm show only minor changes; (2) during the rapid softening phase (AB stage), the sensitivity of characteristic spectral bands to moisture content follows the order 1935 nm > 1440 nm; in the slow softening phase (BC stage), the sensitivity order remains 1935 nm > 1440 nm.(3) During the water absorption and strength softening process of sandstone, changes in the population of water molecules near 1440 nm and 1935 nm are the primary factors influencing water-induced strength softening. Notably, the variation in adsorbed water around 1935 nm exhibits the highest sensitivity and is the dominant mechanism responsible for the strength softening of sandstone. (4) The strength-saturation relationship follows an exponential decay, while the spectral absorbance increases monotonically with saturation. When both are examined as functions of saturation, the spectral characteristics and strength softening exhibit consistent parallel trends. The spectral characteristics provide an effective means to qualitatively interpreting and monitoring rock strength softening in water.

ObjectiveThe excavation of a newly built tunnel causes the stress release of surrounding soil，subsequently altering the attitude and inducing displacement of the existing tunnel，which threatens its structural health. Therefore，it is imperative to study the attitude response of the existing tunnel and the influence range of the newly built tunnel. MethodTaking a shield tunnel section between Yushan Square Station and Zhujiang Road Station on Suzhou Metro Line S1 as an example of an existing tunnel structure，the displacement control method is employed to control the ground loss during new tunnel construction. Analysis and discussion are conducted according to different crossing modes and convergence modes of the newly built tunnel，as well as varying clearances between the new and existing tunnels. The response characteristics of the existing tunnel structure are analyzed using finite element numerical simulation technology. Result & Conclusion In under-crossing scenarios，the displacement and deformation of the existing tunnel exhibit a monotonic relationship with the clearance. The vertical displacement of the existing tunnel increases as the clearance decreases，while the range of the deformation section decreases as the clearance decreases. In over-crossing scenarios，the vertical displacement of the existing tunnel shows a non-monotonic relationship with the clearance. Additionally，the horizontal convergence of the new tunnel excavation face exerts a certain influence on the vertical displacement and the minimum longitudinal curvature radius of the existing tunnel. In both over-crossing and under-crossing scenarios，the minimum longitudinal curvature radius of the existing tunnel should be taken as a structural safety evaluation index. For under-crossing scenarios，the clearance should be controlled more than 0.9 times the tunnel diameter，while for over-crossing scenarios，it should be controlled more than 0.5 times the tunnel diameter.

ObjectiveWith the growth of metro operation years and the continuous expansion of mileage, the number of tunnel defects keeps on rising, accompanied by coupled effects. Existing evaluation methods can hardly reflect the overall structural health conditions comprehensively. To enhance the scientific basis of operation and maintenance decision-making, it is necessary to establish a standardized management system to achieve standardized management and comprehensive evaluation of defect information, thereby ensuring the long-term operational safety of metro tunnels. MethodFirst, based on statistical results of shield tunnel defects from various provinces and cities, the common defect types are summarized. From the perspective of the coexistence of defects, the necessity of comprehensive structural evaluation is discussed; from existing defect evaluation methods, the shortcomings of monotonous evaluation objects and fragmented evaluation systems are identified. Then, the database concept is introduced to organize, standardize, record, and manage all inspection information related to each defect type. Finally, a comprehensive structural health evaluation method for shield tunnel rings is designed by combining monotonous defect structural health evaluation and mathematical model approach, and an actual case is evaluated. Result & Conclusion Since the defect types serving as the management target are representative, and the defect information records are complete and mutually independent, the structural health evaluation of monotonous defects can be efficiently completed. By integrating the database with the mathematical model method, problems of difficult data acquisitions and nonstandard data formats are solved, an efficient comprehensive evaluation is achieved, allowing a process-oriented comprehensive evaluation of the structural health conditions of all shield tunnel rings within a specified section.

ObjectiveStatic pressure pile construction exerts squeezing effects on the surrounding soil layers and causes changes in pore water pressure over a period of time, thereby affecting the displacement and deformation of existing metro tunnel structures. Analysis of this process is helpful for standardizing control methods for projects adjacent to existing metro tunnel structures. MethodTaking the Yushan Square Station - Zhujiang Rd. Station shield tunnel interval on Suzhou Rail Transit Line 11 as a case study, the project overview is introduced. A single row of static pressure piles on one side is arranged. Under the effects of soil squeezing and excess pore water pressure, multiple working conditions are established based on different horizontal and vertical distances between the piles and the tunnel. A finite element numerical analysis is conducted to obtain the response results of tunnel displacement and deformation under each working condition. Finally, by superimposing the effects of soil squeezing and excess pore water pressure, the displacement and deformation of the metro tunnel under the identical working conditions and at the same construction stage are analyzed, and the influence patterns of static pressure pile construction on the displacement and deformation of existing metro tunnels are summarized. Result & Conclusion The horizontal displacement of existing tunnels is mainly dominated by soil squeezing effect, while the vertical displacement is mainly caused by excess pore water pressure. The horizontal influence range is approximately 40 m, and the vertical influence depth is about two-thirds of the pile length. The horizontal deformation of tunnels in response to construction is greater than the vertical deformation. The maximum horizontal displacement occurs during the dissipation stage of excess pore water pressure, and its occurrence time is delayed with increasing pile-tunnel clear distance and advanced with increasing tunnel burial depth.

The application of novel cementitious materials in shield tunnel strengthening has become an important development trend. However, insufficient integration between material development and structural design limits mechanical compatibility among the strengthening materials, the bonding interfaces, and the structural system. To address this limitation, a multiscale computational framework is developed for segmental linings strengthened with prefabricated UHPC panels. It elucidates damage evolution across scales and the resulting global performance degradation, providing a basis for material-interface-structure integrated design. Under combined bending and compression, interfacial failure preferentially initiates in bending-shear-dominated sections, driving internal force redistribution toward bending-dominated sections, and ultimately governing global structural failure. Increasing the strengthening layer thickness from 60 mm to 100 mm raises the bending moments at initial cracking and structural failure by 46.8% and 40.5%, respectively. Increasing the reinforcement ratio from 1.4% to 2.5% shifts the governing failure mode from mid-span cracking to interfacial failure. At a fiber volume fraction of 1.5%, early cracking occurs in the strengthening layer due to insufficient spatial fiber coverage. Anchor bolt density has a limited influence on overall strengthening performance compared with interfacial effectiveness.

ObjectiveTo eliminate the impact of assembly errors on the safety stress evaluation of a shield tunnel target ring structure, it is necessary to study a new method for the health and safety monitoring and evaluation of underground structures. MethodA method for the structural safety evaluation of a single ring is proposed, based on construction records, onsite defect inspection results, and numerical analysis results from a specific subway shield tunnel project. First, finite element numerical analysis is used to calculate the convergence deformation of the target ring under design loads. By comparing the initial detection data, the potential assembly errors are calculated. Then using the onsite cross-section convergence deformation detection results, the actual load-induced convergence deformation is calculated, which is subsequently used as a boundary constraint to back-calculate the true stress conditions of each structure in the tunnel section. Finally, the health status of the structure is judged by comparing the material strength standards of each structure. The calculation accuracy is then verified by correlating the actual onsite condition of each structure. After confirming that the calculated results are generally consistent with the actual condition of each structure, the remaining bearing capacity ratio of the target ring is calculated. Thus, a quantitative evaluation of the target ring structural health status is achieved. Result & Conclusion The evaluation results show a high degree of consistency with the actual on-site structural performance. The proposed evaluation method features a clear procedure and strong operability, reflecting the characteristics of the structural condition effectively, particularly in sections with abnormal records during the assembly period.

Middle rock pillars (MRPs) play a crucial role in the stability of bifurcated small clear-distance tunnels. Assessing the stability of the MRP is a key challenge in design and construction. This study focuses on the bifurcated small clear-distance section of the Xiamen Haicang Shugang evacuation channel underground interchange tunnels. The stability criteria for the MRP during both the early design and later construction stages were analyzed by using the strength reduction method (SRM) via numerical simulations. In the design stage, the SRM was applied to determine the stability limit state of the MRP. Relationships between rock mass density, cohesion, and elastic modulus were identified, and these parameters were combined with basic cohesion values for an initial stability assessment. During the construction stage, the full excavation process was analyzed by examining the distribution and changes in the plastic zone of the rock mass. Two key construction stages, a 10 m excavation on the main line upper step and a 10 m excavation on the ramp upper step, were identified as points where the plastic zone of the MRP began to form on the sidewall and the center, respectively. Multiple linear regression was used to determine the displacement, stress, and plasticity criteria for MRP stability. A comprehensive criteria formula incorporating the width–span ratio, tunnel vault settlement, and horizontal clearance convergence was developed, providing technical guidance and a scientific basis for similar projects.

To clarify the influence of reinforcement corrosion on the mechanical performance of road tunnel linings, localized tests on reinforcement-induced concrete expansion are conducted to identify cracking patterns and their effects on load-bearing behavior. Refined three-dimensional finite element models of localized concrete and the entire tunnel are developed using the concrete damaged plasticity model and the extended finite element method and validated against experimental results. The mechanical response and crack evolution of the lining under corrosion are analyzed. Results show that in single-reinforcement specimens, cracks propagate perpendicular to the reinforcement axis, whereas in multiple-reinforcement specimens, interacting cracks coalesce to form a π-shaped pattern. The cover-layer crack width exhibits a linear relationship with the corrosion rate. Corrosion leads to a reduction in the stiffness and load-bearing capacity of the local concrete. At the tunnel scale, however, its influence remains highly localized, and the additional deflection exhibits little correlation with the initial deflection. Local corrosion causes a decrease in bending moment and an increase in axial force in adjacent linings; when the corrosion rate exceeds about 15%, stiffness damage and internal force distribution tend to stabilize. Damage and cracks initiate around corroded reinforcement holes, extend toward the cover layer, and connect longitudinally, forming potential spalling zones.

The issue of geotechnical hazards induced by excavation in soft soil areas has become increasingly prominent. However, the retaining structure and surface settlement deformation induced by the creep of soft soil and spatial effect of the excavation sequence are not fully considered where only elastic–plastic deformation is used in design. To understand the spatiotemporal effects of excavation-induced deformation in soft soil pits, a case study was performed with the Huaxi Park Station of the Suzhou Metro Line S1, Jiangsu Province, China, as an example. Field monitoring was conducted, and a three-dimensional numerical model was developed, taking into account the creep characteristics of mucky clay and spatiotemporal response of retaining structures induced by excavations. The spatiotemporal effects in retaining structures and ground settlement during excavation processes were analyzed. The results show that as the excavation depth increased, the horizontal displacement of the diaphragm walls increased linearly and tended to exhibit abrupt changes when approaching the bottom of the pit. The maximum horizontal displacement of the wall at the west end well was close to 70 mm, and the maximum displacement of the wall at the standard section reached approximately 80 mm. The ground settlement on both pit sides showed a “trough” distribution pattern, peaking at about 12 m from the pit edge, with a settlement rate of −1.9 mm/m per meter of excavation depth. The excavation process directly led to the lateral deformation of the diaphragm walls, resulting in ground settlement, which prominently reflected the time-dependent deformation characteristics of mucky soft soil during the excavation process. These findings provide critical insights for similar deep excavation projects in mucky soft soil, particularly regarding excavation-induced deformations, by providing guidance on design standards and monitoring strategies for similar geological conditions.

The mechanical properties of the steel-plate-reinforced segmental lining are generally determined by the load-bearing capacity of reinforced joints. However, there is a lack of valid calculation methods for compression-bending bearing capacity, and researchers mainly rely on experience and analogy for the design of reinforced joints. This paper proposes an analytical model based on the deformation and stress characteristics of the joint surface to calculate the compression-bending capacity of the steel-plate-reinforced joint. After verifying the applicability of this analytical model through finite element simulations, the evalution rules of the load-bearing capacity of the reinforced joint were attained, followed by a quantitative investigation into the influence of joint parameters on it. The results show that: (1) the bearing capacity curve of the reinforced joint under different axial forces can be separated into two parts, with the maximum ultimate bending moment found at the demarcation point, where the steel plate yielding and joint failure occur simultaneously; (2) the steel plate strength and cross-sectional area have a strong influence on the bearing capacity of the reinforced joint when the axial force is under 0.15RFF, where RFF is the axial force at pure-compression failure); (3) the concrete strength and segment width have a prominent influence on the curve when the axial force is over 0.30RFF; (4) the impact of the fictitious strain, bolt strength, bolt diameter, and bolt location on the bearing capacity is minimal in range and amplitude.

[Objective]Earth chamber pressure is a key pa-rameter for EPB(earth pressure balance)shield construction assessment.Accurate prediction of earth chamber pressure helps construction technicians take timely control measures to ensure subway tunnel construction safety.Therefore,it is nec-essary to study the earth chamber pressure prediction method of EPB shield.[Method]A multi-branch LSTM(long and short term memory)-DNN(deep neural network)fusion model is proposed.LSTM branch extracts its time series evolution char-acteristics by backtracking historical data,while DNN branch extracts excavation state characteristics.The two branches are combined and then integrated through a fully connected layer to realize the prediction of earth chamber pressure.This multi-branch model is verified based on the actual shield tunnel data of Jinan Rail Transit Line 1,and compared with LSTM and DNN models respectively.[Result & Conclusion]The pre-diction model of earth chamber pressure based on LSTM-DNN fusion algorithm can converge efficiently,and has good predic-tion effects on the training set and the verification set.In the subsequent 100-step test,the predicted value of earth chamber pressure obtained by the LSTM-DNN fusion model better re-flects the change trend of the actual value,with an average de-viation of 7.65 kPa and a relative error of 6.09％,indicating a higher prediction accuracy.

Investigating the mechanical responses and strengthening effects of surcharge-disturbed shield tunnels with varying cross-sections is crucial for ensuring the effectiveness of structural safety evaluations and treatments. This study introduces an elaborate modeling approach for segmental rings of three different sizes. Using calibrated finite element (FE) models and a proposed numerical simulation method for strengthening deformed segmental rings, the variations in surcharge-induced mechanical behaviors and strengthening effects in shield tunnels with typical cross-sections were analyzed. Based on these findings, safety classification standards and recommendations for selecting strengthening measures for shield tunnels with typical cross-sections are established. The results indicate that surcharge-disturbed shield tunnels with typical cross-sections exhibit certain variations in structural internal force levels and transmission characteristics, as well as in the timing of plastic hinge formation and joint mechanical response. Additionally, there are also commonalities in the overall deformation and damage mechanisms of the structures. For instance, the stable and accelerated development stages of segmental deformation are primarily driven by segment cracking, while the rapid development stage is triggered by both the surrounding stratum reaching passive earth pressure and the formation of structural plastic hinges. Accordingly, a safety classification standard for single-line shield tunnels with typical cross-sections is established. For a 6.2 m diameter tunnel, a steel plate-UHPC composite structure (SUCS) is recommended if spatial conditions permit, owing to its excellent interface properties. For a 6.7 m diameter tunnel, both SUCS and prefabricated UHPC slab (PUS) are preferred, provided that the layer thickness and interface measures are appropriately designed. For an 8.8 m diameter tunnel, EBSP, SUCS, and PUS are all viable options, although modifications to the conventional strengthening measures are necessary to enhance mechanical performance.