Constructing a shield tunnel that crosses under a river poses considerable safety risks, and risk assessment is essential for guaranteeing the safety of tunnel construction. This paper studies a risk assessment system for a shield tunnel crossing under a river. Risk identification is performed for the shield tunnel, and the risk factors and indicators are determined. The relationship between the two is determined preliminarily by numerical simulation, the numerical simulation results are verified by field measurements, and a sample set is established based on the numerical simulation results. Fuzzy comprehensive evaluation and a backpropagation neural network are then used to evaluate and analyze the risk level. Finally, the risk assessment system is used to evaluate the risk for Line 5 of the Hangzhou Metro in China. Based on the evaluation results, adjustments to the slurry strength, grouting pressure, and soil chamber pressure are proposed, and the risk is mitigated effectively.

Earth Pressure Balance (EPB) shield machines are considered to be the most efficient tunneling method for Metro tunnels due to their adaptability to a great variety of ground conditions, higher construction efficiency, and providing a safer working environment. There are many guidelines available for EPB shield machine selection. However, these guidelines are very general and cannot be used directly for an upcoming project. This paper takes Chengdu Metro Line 6 in China as the engineering background; the studied area is typical of a water‐rich sandy and cobble stratum with high content of cobble. Three types of EPBs in the two continuous intervals exhibit significant differences in performance and encounter many difficulties such as wear of the cutter disc and tools, clogging, and severe surface settlement during the operation. These difficulties prevent the construction efficiency, increase the cost of the project, and cause delays in construction period. The causes of these difficulties are summarized by recording and comparing the operational parameters of the three types of EPBs. These parameters that are summarized include the advance rate, total thrust, torque, and the rate of rotation of the cutter‐head. In addition, the surface settlements are also compared. The results indicate that the opening rate, maximum opening size, and the opening position of the cutter‐head are key factors that affected the geological adaptability of the shield machine in water‐rich sandy and cobble strata. Among the three factors, the maximum opening size and opening position are the most important factors influencing the strata adaptability of the cutter‐head. To avoid frequent jams of the cutter‐head, the maximum torque should be not less than 6,500 kNm. The maximum opening size should not be less than 420 mm × 420 mm. The effect of increasing the central opening of the cutter‐head is that large cobbles and boulders can be discharged through the central opening when they cannot be discharged through the opening near the original position of the cobbles and boulders. This paper provides specific guidance on the selection of cutter‐head for shield machines in water‐rich sandy and cobble strata.

Water leaking into metro segments is related directly to the presence of underground water. However, to date, the underground confined water channels formed by metro underpass reinforcement structures have not been considered as causes of the problem. In the present study, based on field investigations and geological data, confined water is identified as the source of segment leakage on Line 5 of Hangzhou Metro in China. It is then hypothesized that the problem stems from an underground confined water channel formed under the reinforced structure of the subway. Numerical simulations predict a settlement difference of 8–10 mm between the reinforcement area and the weak strata, and field monitoring data show this difference to be 8.62 mm. This consistency between the numerical simulations and the field measurements verifies the hypothesis. Finally, given the causes of underground confined water channels, corresponding technical measures such as piecewise secondary grouting are proposed to cut them off and avoid metro‐segment leakage. The present research results contribute to the causes of and the laws governing the metro‐segment leakage that occurs at subway stations and other reinforced structures.

With the acceleration of urbanization in China, more underpasses will be constructed in big cities to alleviate the great traffic pressure. The prefabricated and assembly construction method has been introduced to replace the traditional cast‐in‐place method to achieve quick construction. However, for a fully prefabricated and assembled underground structure (PAUS) with large cross section, the structure must be cut into segments in transverse direction to reduce the size and weight for easy transportation and assembly. Therefore, how to develop an optimal partition scheme is a new problem to be studied. Firstly, three preliminary partition schemes were proposed based on the internal force distribution and completed engineering practices. Then, the three schemes were compared in terms of bending moment, shear force, and axial force. The construction efficiencies were also compared with special emphasis on difference of the build period. Finally, an optimal partition scheme was determined and successfully applied in the real project. Furthermore, the construction period of this partition scheme was 1/3 of the traditional cast‐in‐place method. The results of the current paper can provide some design guidance to large cross‐sectional underpasses and other underground structures in the partition stage.

With the rapid development of the urbanization, many underpasses are designed and constructed in big cities to alleviate the huge traffic pressure. The construction method has been changed from traditional on-site concrete pouring technology to prefabricated assembly technology. However, this change will inevitably bring out some new problems to be studied such as the behaviour of the radial joints. In this study, the numerical simulation model of Moziqiao precast and assemble underpass with large asymmetric cross section was constructed by using the ABAQUS software to study the transient response of the underpass induced by ground surface dynamic load. Based on the similarity theory a 1/10 scaled model test was carried out to study the long-term radial joint behaviour of the underpass considering the prestress loss during the 2000 000 loading cycles. The results transient dynamic response from computed and tested was compared in terms of acceleration. The comparison showed that the transient response accelerations have good consistency. The results of the physical model test were analysed in terms of joint opening, closure, and slipping. The accumulative joint opening was closely correlated to the prestress level, and the joint opening at different prestress levels increased with the loss of the prestress. The joints closure decreased with the increase of the previous accumulative color value. The joint slipping mainly attributed to the slipping of the top segment. Both the opening and slipping of the joints at RJ 1 were larger than that of RJ3 due to the wider span of RJ1, which reflected an asymmetric effect. This study revealed the long-term accumulative behaviour of the radial joints, which convinced us that the long-term accumulative deformation of the joints should be taken into consideration during the design stage for similar projects.

Prefabricated cut-and-cover construction offers rapid assembly, tighter quality control, and lower environmental impact than conventional cast-in-place methods, yet its seismic performance in shallow, asymmetric, large-span urban underpasses remains insufficiently studied. This study quantifies the response of an asymmetric prefabricated underpass using a 3D finite-element model with viscoelastic boundaries and benchmarks it against a cast-in-place counterpart under three representative ground motions (Wolong, Kobe, and Northridge). Prefabrication reduces ground-surface acceleration amplification by up to 32%, evidencing superior damping capacity; however, deformation demand increases, with a maximum inter-storey drift ratio of 1/489 at the three-lane sidewall under 0.4 g, 36% higher than the cast-in-place case yet below the plastic limit of 1/250. Joint actions satisfy waterproofing and safety requirements (peak opening 1.582 mm, slip 0.403 mm; allowance 2 mm). Taken together, these results clarify a performance that prefabrication mitigates seismic accelerations but heightens inter-storey drift ratios and offer practical guidance to designers and practitioners on scheme selection and admissible drift ratio targets to enhance seismic resilience.