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In this study, a three-dimensional vehicle-track-tunnel-soil model for evaluating the dynamic response of a double-line metro tunnel in a poroelastic half-space is developed. The model development consists of two steps. In the first step, the vehicle-track-tunnel-soil model in a full-space is developed. In the second step, based on the full-space model, the dynamic response of the poroelastic half-space is solved by using the 2.5-dimensional boundary integral equation and the Green’s function. The developed vehicle-track-tunnel-soil model is validated against existing tunnel models and is demonstrated through a case study of the dynamic response induced by one-way and two-way train loads. [ABSTRACT FROM AUTHOR]

An analytical method is presented to calculate ground vibrations from a tunnel in a multi-layered half-space. Using the transfer matrix method, the dynamic system matrix for the multi-layered soil overlying a half-space or bedrock is obtained. The tunnel is coupled with the multi-layered soil via transformations between the cylindrical waves and plane waves. The proposed method provides a high-efficient tool to predict vibrations induced by underground railways. Numerical simulation suggests that the critical velocity under the moving harmonic load mainly depends on the properties of soils below the tunnel. There is no obvious critical velocity under the moving harmonic load. [ABSTRACT FROM AUTHOR]

A two-and-a-half-dimensional (2.5-D) coupled finite element–boundary element (FE–BE) model is presented to simulate the three-dimensional dynamic interaction between saturated soils and structures with longitudinally invariant geometries. A regularized 2.5-D boundary integral equation for saturated porous media is derived that avoids the evaluation of singular traction integrals. The 2.5-D coupled FE–BE model is established by using the continuity conditions on the soil–structure interface. The developed model is verified through comparison with an existing semi-analytical method. Two case studies of a tunnel embedded in a poroelastic half-space and the efficiency of a vibration isolating screen are presented. [ABSTRACT FROM AUTHOR]

Recently, the pit-in-pit braced excavations are frequently adopted in the geotechnical engineering projects in China due to the large demand of urban underground space utilization. In this study, a simplified approach for basal-heave stability analysis of pit-in-pit braced excavations in soft clays is developed based on the limit equilibrium method. The configuration of failure surfaces is obtained by using finite element method with the shear strength reduction technique. The factors of safety computed with the simplified approach are comparable to those from the finite element simulation. This simplified approach is demonstrated through a case study of pit-in-pit excavations in Shanghai. [ABSTRACT FROM AUTHOR]

This study proposes an improved tunnel model for evaluating train-induced dynamic stress in saturated soils, which can consider multiple moving loads, grouting layer and pore-water pressure. Using Shanghai Metro’s actual parameters for train speed, tunnel, grouting layer and soils, the analysis of the spatial distribution of dynamic stress for soils and stress state of various locations under moving train loads shows that neglecting effects such as pore-water pressure can lead to underestimating dynamic normal stress and overestimating dynamic shear stress in the soils below tunnel. This model can be further extended to investigate principal stress axes rotations and tunnel settlement. [ABSTRACT FROM AUTHOR]

This paper proposes a thin layer method (TLM) for computing 3D dynamic responses in a layered transversely isotropic (TI) half-space with irregular interfaces. Utilizing Fourier transforms and modal superposition, stiffness matrices for semi-infinite and finite-length thin layer elements are derived, enabling the simulation of wave propagation in the finite-depth domain with irregular interfaces. The complex frequency shifted perfectly matched layer (CFSPML) within the TLM framework is developed to simulate wave attenuation in the underlying TI half-space, addressing the instability issues of the classical PML in the TI medium. The finite-length thin layer element can be of arbitrary length and is independent of the frequency, thus enhancing computational efficiency compared to discrete numerical methods like the FEM. The accuracy of the proposed method is verified with existing methodologies. The dynamic responses of a two-layered TI half-space with a Gaussian-shaped interface induced by either surface or buried dynamic point loads are investigated. The influences of the position and size of the irregular interface, as well as the transversely isotropy, are analyzed. Numerical results demonstrate that the presence of the irregular interface significantly changes the distribution of displacement fields. The influence of the irregular interface is highly dependent on the loading frequency and observation position. [ABSTRACT FROM AUTHOR]

Hydroacoustic noise generated by trains running in underwater tunnels significantly impacts aquatic ecosystems, particularly fish breeding grounds and the habitats of endangered species. This underscores the necessity of investigating the characteristics of wave propagation and noise radiation emanating from underwater tunnels. This study proposes a novel analytical method for calculating 3D responses due to a dynamic point load acting on an underwater tunnel in stratified soils that considers dynamic tunnel–soil–fluid interactions. Stratified soils consist of multiple sediment layers, which are simulated as saturated porous media, overlying substrate layers, which are modeled as phase elastic media. The transfer matrix method is applied to simulate wave propagation in a layered fluid–solid system. The functions describing various types of waves scattered at the layer interfaces and the tunnel–soil interface are unified by wave transformation, thereby deriving an analytical solution for dynamic tunnel–soil–fluid interactions. A comparison with existing methods via numerical cases confirms the accuracy of the proposed method. Vibrations and hydroacoustic noise owing to dynamic loads acting on an underwater tunnel buried either in a saturated sediment layer or a single-phase substrate layer are investigated. The influences of tunnel burial depth, layering, as well as the porosity and permeability of saturated sediment are systematically analyzed. This research enhances the understanding of wave propagation and noise radiation in water emanating from underwater tunnels, facilitating the development of vibration isolation measures and protecting aquatic ecosystems. [ABSTRACT FROM AUTHOR]

In this paper, a probabilistic assessment of supported excavations in spatially varied sands is presented. Random finite element modelling (RFEM) is performed to simulate excavation-induced responses. A procedure for automating the Monte Carlo simulation is developed to facilitate the RFEM. The effects of soil vertical spatial variability on several major geotechnical and structural failure modes, including geotechnical ultimate failure, geotechnical serviceability failure, wall bending failure, wall shear failure, and strut buckling failure, are explicitly investigated. This study demonstrates the importance of addressing the spatial variation of soil properties by considering multiple failure modes for complicated soil-structural interaction problems. [ABSTRACT FROM AUTHOR]

Bridge-embankment transition zones, in which unexpected amplifications of vehicle-track interactions and infrastructure degradation are concentrated, are critical areas of high-speed railways. To mitigate the dynamic issues induced by uncertain filler stiffness, an available stiffness combination of transition zone components is required. Combining with robust designs, a multi-objective optimisation methodology is proposed to determine the compromise design of transition zones, which considers trade-offs among dynamic stress, dynamic displacement and system robustness. An optimal stiffness combination of subgrade bed surface layer (E 1) and graded broken stone area (E 2) in a typical bridge-embankment transition zone is presented by employing this methodology. The results indicate that the Pareto fronts visually show the dynamic response contradiction. Based on the conflicting evaluation criteria, i.e., minimizing the displacement and stress of the subgrade and maximizing the system robustness, the knee points objectively render the best compromise design with E 1 and E 2 values of 1200 and 1000 MPa, respectively. Not only the dynamic response of the vehicle-track-subgrade system but also the negative effect brought by filler uncertainties are well controlled. Such a methodology is available for configuration design of projects in which uncertainties exist and design criteria are hard to weigh. [ABSTRACT FROM AUTHOR]

The perfectly matched layer (PML) has proven to be efficient for absorbing elastic waves in the unbounded domain. This strategy is frequently coupled with the semi-analytical approach, such as the thin layer method (TLM), to simulate wave propagation in a layered half-space subjected to dynamic sources. However, beyond spurious reflections due to discretization and zero-frequency singularity, traditional PML may cause exponentially increasing backpropagation waves attributed to material anisotropy when simulating transversely isotropic (TI) media, resulting in significant computational errors. This paper explores the coupling of the complex frequency shifted perfectly matched layer (CFSPML) with the TLM to provide a highly efficient and accurate tool for modelling wave propagation in a layered TI medium. The 3D fundamental solution for a dynamic point source acting on a multi-layered TI half-space is therefore derived. Numerical cases are conducted to evaluate the performance of the TLM-CFSPML in comparison to the TLM-PML. Various factors such as the material anisotropy, stiffness, load and observation positions, and loading frequency are systematically considered. The numerical results demonstrate that the CFSPML can address the computational instability issues related to the anisotropy of the medium. The TLM-CFSPML exhibits computational stability with all the considered scenarios while maintaining high computational efficiency. [ABSTRACT FROM AUTHOR]

This paper investigates the three-dimensional (3D) axisymmetric dynamic response of an unsaturated poroelastic transversely isotropic half-space subjected to different vertical transient loading types. Taking into account the compression deformation of soil particles, pore fluid, the viscous and inertial coupling between different phases, as well as the material anisotropy of soil skeleton, the dynamic governing equations of unsaturated poroelastic transversely isotropic half-space medium in cylindrical coordinate system are established. Then, the general solutions for the soil skeleton's displacement and stress, as well as the relative displacements and pressures of pore-water and pore-air, are obtained by using Laplace-Hankel transform. Finally, the Durbin numerical inversion method and Gauss-Kronod algorithm are applied to solve the transient Lamb's problem in time domain. After validating the correctness of the proposed solution, the numerical examples focused on the effect of material anisotropy, transient loading type, the degree of saturation and distance from the loading source on the vertical and radial surface displacements of an unsaturated transversely isotropic poroelastic half-space are investigated. [ABSTRACT FROM AUTHOR]

This paper proposed an analytical solution for investigating the dynamic response of a pipe pile subjected to a torsional harmonic excitation torque in unsaturated soils. Based on the wave equations of three-phase porous media, the capillary pressure and inertial coupling effect between water and gas phase are taken into account, as well as the influence of saturation degree on soil's dynamic shear modulus. The pile is modeled using one-dimensional elastic theory. The dynamic governing equations of outer and inner unsaturated soils are derived by virtue of the separation of variables method. Accounting for the boundary and continuity conditions of pile-soil interface, a closed-form solution for the torsional dynamic response of a pipe pile is derived in frequency domain. The proposed analytical solution is verified by comparing with the two existing solutions available in literature. Finally, a parameter study is undertaken to portray the influence on the dynamic response of a pipe pile. [ABSTRACT FROM AUTHOR]

A double-line metro tunnel was constructed beneath an existing high-speed railway bridge in a city of east China. To avoid the unexpected displacement of the bridge resulting from the shield tunneling, the displacement isolation piles were employed between the shield tunnels and the pile foundation of the bridge. An accompanied optimization design issue of the isolation piles is analyzed in this work. The optimization target is not only that the displacement criteria of the bridge pile foundation during tunneling process must be met, but also an outstanding vibration mitigation effect in operation process should be achieved. In this work, a steady-state finite element analysis (FEA) is employed to evaluate the limit of the pile spacing according to the displacement criteria of the high-speed bridge. After that an analytical model and a three-dimensional (3D) finite-infinite element model of the bridge-foundation-tunnel system are established, respectively, for further optimization of the pile parameters from the view point of vibration isolation effect. The optimized pile spacing of the isolation piles for the real project case is finally recommended based on in-situ measurement data and the evaluation results. [ABSTRACT FROM AUTHOR]

When using an implicit numerical integration algorithm to implement a highly nonlinear elasto-plastic constitutive models in FEM simulations, some difficulties would encounter including Jacobian matrix singularity and non-convergence. Based on the composition of the elastoplastic constitutive model and the characteristics of the traditional implicit algorithms, this paper proposes three improved implicit algorithms, namely the Homotopy-Newton-CPPM algorithm, the CG-Newton-CPPM algorithm and the two-stage algorithm, which would contribute to improve the convergence and deal with singularity of the Jacobian matrix. Besides, the convergence and the computational efficiency as well as the accuracy of five algorithms, including a conventional explicit algorithm, a conventional implicit algorithm and three improved implicit algorithms, are compared with reference to numerical simulations of single element tests and multi-element analysis. [ABSTRACT FROM AUTHOR]