Deep excavation projects in urban areas may adversely affect underlying tunnels. Differential settlement and additional stresses in existing tunnels occur frequently due to excavation construction, which leads to the opening of circumferential joints or even to water leakage. To ensure the security of existing tunnels under deep excavation during construction, this paper proposes a risk assessment methodology that aims to evaluate the risks of circumferential cracks of preexisting shield tunnels. Existing tunnels are assumed to be a beam on a Winkler elastic foundation. The longitudinal deformations of the tunnel and internal forces in tunnel structures are obtained from the singular function method. Based on the equivalent axial stiffness theory, the relationship between the opening of circumferential joints and the additional bending moment is established. The Monte Carlo simulation technique is used to quantitatively estimate the potential risk levels of circumferential joints. The calculated results are verified by in situ observations.

Double-O-tube shield tunneling has attracted increasing attention because it offers cost-efficiency in underground construction. Prediction of ground surface settlement and the variety of additional stresses induced by shield construction is crucial to underground construction in metropolises since excessive settlement could trigger potential damage to the surrounding environment. The additional stresses induced by the propulsion of double-O-tube shields are calculated by means of the Mindlin’s equations of elasticity. The characteristics of additional stresses are analyzed with compound Gauss-Legendre integral arithmetic, and the frontal additional thrust, the lateral friction, and the ground loss are taken into account. Subsequently, based on field measurements, the maximum settlement coefficient and width of the settlement trough coefficient of the typical Peck formula are modified. The predictive curve of the Peck formula is closer to the engineering measured data than that of the typical formula. The cut-off functions of ground surface settlement caused by double-O-tube tunnel shield construction are proposed and can predict the shape of ground surface settlement, such as single peak or double peak. The correctness of the proposed functions is verified based on an engineering project.

Numerical analyses have been widely used to investigate the stability of deep excavations. The results of this type of analyses rely on the constitutive model that is adopted for the analyses. Although many advanced constitutive models have been developed for the simulation of soil behavior, they are often complex, expensive to use, and not well understood by practicing engineers. Hence, simple constitutive models, such as the Mohr–Coulomb model, are widely used in geotechnical engineering practice. However, these simple models, fail to represent some key features of soil behavior. Of particular importance in granular soils is the effect of changes in soil density on the soil behavior, an aspect that is often overlooked when simple constitutive models are used. In this study, a series of triaxial tests will be conducted on reconstructed gravelly sand to investigate the effect of the relative density and confining pressure on strength and stiffness characteristics of the soil. According to the test results, variations of the soil stiffness and peak deviatoric stress with relative density and confining pressure will be established. A novel disturbance function will then be proposed based on the disturbance state concept to capture the effect of soil density on its behavior. Subsequently, a constitutive model that is based on the original Duncan and Chang model will be developed that considers the effects of the confining pressure and relative density of the soil using the proposed disturbance function. The model is implemented in ABAQUS software and is verified by comparing the numerical results with experimental data. The application of the model is presented through the simulation of a deep supported excavation. The numerical results show that the model successfully simulates horizontal displacements of the support piles, with errors of <5% for the monitoring points. In particular, it is highlighted that the model is superior to the widely used Mohr–Coulomb model in the simulation of excavation problems, and yielded a safer design than that obtained using the Mohr–Coulomb model.

This study presents a novel construction pre-supporting system for large underground space excavation with shallow depth, Steel Tube Slab system (STS), in which adjacent steel pipes are connected by a couple of flanges, bolts and concrete for flexural capacity and lateral stiffness of the whole structure. The STS method is employed for the first time for the construction of the ultra-shallow buried and large span subway station in China, during which ground settlement and structural deformation are monitored. A numerical model for the subway station is established with reliability verified by monitored data comparison from numerical results and investigation on the effect of large span underground excavation on surrounding soil surrounding soil and existing buildings in soft soils. Unlike traditional methods, the STS method can effectively control and reduce the ground settlement during construction, thereby rendering it ideally suited for application in soft soils.

This paper developed a new ultrasonic computerized tomography (CT) method for damage inspections of a steel tube slab (STS) structure based on compressive sampling (CS). CS is a mathematic theory providing an approximate recovery for a sparse signal with minimal reconstruction error from under-sampled measurements. Considering the natural sparsity of the damage, CS algorithm is employed to image the defect in the concrete-filled steel tube of Shenyang Metro line 9 for reducing the work time. Thus, in the measurement stage, far fewer ultrasonic measurement paths were selected from the dense net of conventional ultrasonic CT techniques to capture the underlying damage information. Then, in the imaging stage, ℓ₁-norm minimization algorithm of CS theory is selected to recover the internal damage via fusing measurement data and solving optimization problem. The functionality of the proposed method is validated by three numerical concrete tube models with various conditions. Additionally, both the conventional ultrasonic CT technique and the proposed one are employed for ultrasonic inspection of the STS structure in Shenyang Metro line 9. Both the numerical and experimental results indicate that the proposed ultrasonic CT improved by CS has a great potential for damage detection, which provides an alternative accurate and effective way for non-destructive testing/evaluation (NDT/E).

Recently, the Steel Tube Slab (STS) curtain method has been successfully used in the construction of ultra-shallow metro stations in modern urban areas. By using the STS curtain method, the external surfaces of pipes are welded with flange slabs, and then the pipes are jacked into soils. However, the nature and extent of the influence of flange slabs on the calculation of jacking force is unclear. This paper presents the first combined theoretical and case study to calculate the increase of jacking forces for circular steel pipes with welded flange slabs during the entire jacking process. The increase of jacking forces was assumed to balance the friction at the soil-pipeline interface. Theoretical formulae were firstly developed to calculate earth pressure based on pressure arch theory and elastic foundation beam theory. A case study was then performed to verify the theoretical results with the field data from Olympic Metro Station on the Shenyang Metro Line 9 in China. Subsequently, factors influencing the calculation of jacking force were analyzed and discussed. Results show that the theoretical formulae produced good results with minor errors. The implications of these findings regarding the prediction of jacking force and selection of hydraulic jack are discussed.