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.

It is an effective approach to use Steel Tube Slab (STS) structure combined with the Pile-Beam-Arch (PBA) method to construct a large-space underground station. Traditional construction methods cannot meet the requirement of construction because of the complicated soil layers and high building densities in urban areas. The STS method can effectively increase the rigidity of the supporting system by using steel tubes. Firstly, the stress of bolts and steel tubes are investigated in the construction process based on the field monitoring data. Subsequently, FLAC3D is used to establish a three-dimensional model, which is verified based on the in-situ monitoring data; the effect of excavation process on ground settlement, deformation of STS structure and bridge pile are studied by numerical results. Moreover, the key parameters such as welding of flanges and the step length are studied. The results show that the stress of the steel tubes and flanges does not exceed the designed strength during the construction process. Based on the numerical simulation data, it is indicated that the STS structure can be a very effective and dependable measure in controlling and reducing the surface settlement and the existing adjacent buildings. The numerical results can be used to guide the later construction.

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.