This study evaluates the dynamic behavior of a subsea railway tunnel during an earthquake, considering ground conditions and seismic wave characteristics using the finite difference modeling method. A comprehensive ground-tunnel structure system model was constructed to analyze the structure's response during earthquakes, yielding significant results. Analysis of lining stress values in the subsea tunnel revealed that the maximum compressive stress in the soil part is significantly larger than in the rock part in composite ground conditions, and the maximum compressive stress in the fractured zone is increased by up to 10 times compared to the rock zone. In addition, a seismic fragility curve for subsea tunnels was derived from a series of analytical results. The analysis indicates that the probability of minor damage exceeds 50 % for earthquakes of about 0.32 g and above, while the probability of moderate damage exceeds 50 % for earthquakes of 0.39 g and above for subsea railway tunnels passing through various ground conditions.
Fibre reinforcement technology has been widely adopted in soil improvement due to its cost-effectiveness, simplicity, and environmental benefits. In many fibre reinforcement projects, the soil is often in an unsaturated state. However, the numerical simulation mechanisms of fibre-reinforced unsaturated soils remain poorly understood. In this study, a Vangenuchten (VG) model considering fibre incorporating fibres was proposed based on the original VG model. This model considering fibre accurately describes the soil water characteristic curve (SWCC) of fibre-reinforced sand (FRS), as verified by water-holding characteristics tests. Then, unsaturated triaxial tests confirmed the applicability of an unsaturated soil elastoplastic constitutive model and a fully coupled soil-water-air finite element-finite difference (FE-FD) method for simulating the mechanical behaviour of unsaturated FRS. Finally, using the SWCC parameters derived from the VG model considering fibres and mechanical parameters from saturated triaxial tests, slope models were established to analyse the stability of both unreinforced and fibre-reinforced slopes. The results show that the interweaving action of fibres within the soil enhances its strength, reduce permeability, and decreases both saturation and pore water pressure, ultimately increasing slope stability. This study provides valuable insights into the SWCC characteristics and the numerical calculation of FRS under unsaturated conditions.
Geosynthetic-encased stone columns (GESCs) represent an efficient and cost-effective solution for enhancing weak soil foundations. The deformation and load-bearing mechanisms of GESC-improved foundations under traffic flow are complicated due to substantial particle movements and soil disruption. A three-dimensional discrete-continuum coupled numerical model was proposed in this study to investigate the cyclic behavior of GESC-improved soft soil. The reliability and accuracy of proposed model was validated through experimental data. The effect of cyclic loads, bearing stratum, and geogrid encasement was investigated. Microscopic investigation of particle movement, contact force distribution, and stress transfer mechanism was performed. The vertical loads transferred from the column to the surrounding soil with the interaction effect between the aggregates and the soil. The stress concentration ratio decreased with the increase in depth. The geogrid encasement facilitated the load transfer process by effectively confining the particles and enhancing the column stiffness. The particles in the low segment of floating column exhibited large downward displacements and punching deformation. The geogrid encasement and cyclic loads contributed to enhanced compaction and coordination number of the aggregates.
To investigate the one-dimensional nonlinear consolidation characteristics of a double-layer foundation under multi-stage loading, a one-dimensional nonlinear consolidation equation for the double-layer foundation was established, and numerical solutions were obtained through the finite difference method. The accuracy of the proposed solution was validated by comparing it with existing analytical solutions and finite element analysis results. Based on these comparisons, the influence of nonlinear parameters, double-layer soil properties, and loading conditions on the consolidation behavior of the double-layer foundation was further examined. The results indicated that, under multi-stage linear loading conditions, an increase in the initial permeability coefficient ratio of the double-layer foundation resulted in a significant reduction in excess pore water pressure and an acceleration of consolidation. The compression index was found to predominantly affect the later stages of consolidation, with minimal impact on the early stages. The consolidation rate was observed to increase as the permeability coefficient ratio decreased. Despite notable differences in early consolidation behavior under varying loading conditions, the findings reveal that these discrepancies are alleviated in the later stages, ultimately resulting in no significant overall difference in the time required for the foundation to achieve complete consolidation.
Seepage plays a crucial role in the mechanical behavior and damage modes of geotechnical materials. In this work, based on the unsteady seepage equation, a hydraulic coupling numerical simulation algorithm combining interpolation finite difference method (FDM) and discrete element method (DEM) is proposed to explore the intrinsic mechanism of the interaction between geotechnical materials and the seepage process. The method involves constructing an irregular fluid calculation grid around each particle and deriving the two-dimensional unsteady seepage governing equation and its stability conditions using interpolation and the FDM. The efficiency of the seepage calculation was investigated by numerically varying the parameters of the difference format. The method was applied to simulate the generation of gushing soil in a sinking area of a sunk shaft under hydraulic drive conditions. The results indicate that the improved FDM can effectively simulate the two-dimensional seepage of soil with high calculation efficiency. The hydraulic conductivity and time step positively correlate with the calculation efficiency of the difference format, whereas the spatial step has a negative correlation. The proposed method also accurately reflects the process of gushing soil damage. These results provide a solid theoretical basis to study the geotechnical seepage field and its associated damage mechanisms.
In this paper, a recently developed unified critical state model (CASM-S), which is applicable to predict the mechanical behaviours of sand and overconsolidated clay, is numerically implemented into the Fast Lagrangian Analysis of Continua, i.e. the FLAC(3D), for engineering applications. The implicit integration algorithm incorporated with the line search method is employed to implement CASM-S model. The additional parameter u is calibrated by genetic algorithm, whilst the rest of the material parameters are determined following the literature upon applicable. Validation of CASM-S model and its numerical implementation has been well demonstrated by a series of drained and undrained triaxial compression tests conducted on clay and sand. In terms of stress-strain relations, volumetric versus axial strain, and negative pore pressure versus axial strain, the model predictions agree well with the experimental results. Then, a case study is performed to demonstrate the applicability of the CASM-S model to analyse geotechnical problems, e.g. the foundation pits excavated from Berlin sand within the FLAC(3D), where lateral deflections of the diaphragm wall and vertical displacements in a designated are evaluated. Conclusions can be drawn that the predictions of CASM-S model are almost identical to the field data, demonstrating a good performance in engineering applications.
The paper presents a refined implicit two-phase coupled Material Point Method (MPM) designed to model poromechanics problems under static and dynamic conditions with stability and robustness. The key variables considered are the displacement and pore water pressure. To improve computational efficiency, we incorporate the Finite Difference Method (FDM) to solve pore pressure, stored at the center of the background grid where the material points reside. The proposed hydromechanical MPM cannot only effectively addresses pore pressure oscillation, particularly evident in nearly incompressible fluids-a common challenge with Galerkin interpolation, but also decreases the degrees of freedom of the system equations during the iteration process. Validation against analytical solutions and various numerical methods, encompassing 1D and 2D plane-strain poromechanical problems involving elastic and elastoplastic mechanical behavior, underscores the method's resilience and precision. The proposed MPM approach proves adept at simulating both quasi-static and dynamic saturated porous media with significant deformation.
This study investigates the influence of the soil-structure interaction (SSI) on the seismic performance of structures, focusing on the effects of foundation size, soil type, and superstructure height. While the importance of SSI is well recognized, its impact on structural behavior under seismic loads remains uncertain, particularly in terms of whether it reduces or amplifies structural demands. A simplified dynamic model, incorporating both the mechanical behavior of the soil and structural responses, is developed and validated to analyze these effects. Using a discrete element approach and the 1940 El Centro earthquake for validation, the study quantitatively compares the response of soil-interacting structures to those with fixed bases. The numerical results show that larger foundation blocks (20 m x 20 m and 30 m x 30 m) increase the seismic response values across all soil types, causing the structure to behave more like a fixed-base system. In contrast, reducing the foundation size to 10 m x 10 m increases the flexibility of structures, particularly buildings built on soft soils, which affects the displacement and acceleration response spectra. Softer soils also increase natural vibration periods and extend the plateau region in regard to spectral acceleration. This study further finds that foundation thickness has a minimal impact on spectral displacement, but structures on soft soils show more than a 15% reduction in spectral displacement (SD) compared to those on hard soils, indicating a dampening effect. Additionally, increasing the building height from 7 to 21 m results in a more than 20% decrease in SD for superstructures with natural vibration periods exceeding 2.4 s, while taller buildings with longer natural vibration periods exhibit opposite trends. Structures built on soft soils experience larger foundation-level displacements, absorbing more seismic energy and reducing earthquake accelerations, which mitigates structural damage. These results highlight the importance of considering SSI effects in seismic design scenarios to achieve more accurate performance predictions.
The interaction of closely-spaced footings on soils is of concern for recent decades. The inherent variability of soil makes the topic more challenging. This study investigates the behaviour of twin strip foundations on both unreinforced and geogrid-reinforced spatially random sands. The Random Finite Difference is performed by coupling Matlab and FLAC2D in each Monte Carlo simulation. Two types of sands, dense and loose, are assumed as the spatially correlated log-normal random fields and the friction angle is considered a random parameter. This study discusses how much the bearing capacity of twin footings is impressed by the heterogeneity of random sands. The unreinforced soil results show that when the uncertainty of phi is high, the homogeneous soil assumption could overestimate the interference effect by about 18% and 9% in the dense and loose sands, respectively. While soil reinforcement reduces the difference between results obtained from probabilistic analysis with those calculated with deterministic analysis. Moreover, the isotropic random fields with rapid fluctuation of phi yield the greatest interference influence in the unreinforced and reinforced dense sands. The combined impact of interference and reinforcement is greater in loose sand than in dense sand, regardless of whether the soil is heterogeneous or homogeneous.HIGHLIGHTRandom heterogeneous variability of phi on the interaction of the twin foundations on both unreinforced and geogrid-reinforced sand was investigated.For twin footings on unreinforced dense sand, Scr/B changes from 1.25 to somewhere between 1.25 and 1.5 by increasing COV phi, while on the unreinforced loose sand, Scr/B = 1 remains constant, irrespective of COV phi values.For high variability of phi, the assumption of soil without variability results in overestimation of twin footings interference effect by about 18% and 9%, which means the risk acceptance yields irrevocable damages.Increasing the number of geogrid layers, N, declines the risk of estimating the analysis with homogeneous soil, especially for high variability of phi. By all means, the footing interference and geogrid effects become lower when N is added.The interference effect is usually higher for twin footings on unreinforced sand than on the same reinforced sand. Overall, the integrated influence of interference and reinforcement in loose sand is more than that in dense sandThe combined influence of interference and the first geogrid layer is the most in the isotropic random field (theta x = theta y = 1 m) for both sand since the rapid variation of phi in both directions increases the influence and reinforcement effects.The deterministic analysis underestimates the interaction of twin foundations on the reinforced loose sand compared to the anisotropic random fields.
Particle morphology plays a crucial role in determining the mechanical behavior of granular materials. This paper focused on investigating the effects of boundary conditions on the triaxial mechanical properties of soil samples, with particular consideration given to the influence of particle shape. To achieve this, a numerical model was proposed, which couples the finite difference method (FDM) and the discrete element method (DEM) to simulate the behavior of a rubber membrane and soil particles, respectively. The particle morphology was accurately reconstructed using spherical harmonics (SH) analysis, and the shell cells in the FDM were utilized to construct the boundary modeling. Through a series of simulations, the macroscopic and microscopic mechanical responses of soil particles, both within and outside the shear band, were investigated. The obtained simulation results were then compared with those derived from the DEM simulation using a particle-based membrane. The research findings pertaining to the influence of boundary conditions and particle shape provide significant contributions to our understanding of granular material behavior. These findings offer valuable insights that can be applied in the design and analysis of geotechnical structures.