Showing papers in "Computers and Geotechnics in 2020"
TL;DR: In this paper, a new numerical approach for modeling the localised leakage of shield tunnel lining is proposed, in which the leakage is simulated using a one-dimensional leakage element, which overcomes the limitations on the solid meshing of the leakage element and also reduces computational error due to the size difference between the solid leakage elements and lining element.
Abstract: A new numerical approach for modelling the localised leakage of shield tunnel lining is proposed, in which the localised leakage is simulated using a one-dimensional leakage element. The proposed method overcomes the limitations on the solid meshing of the leakage element and also reduces computational error due to the size difference between the solid leakage element and lining element. The simulated results show that groundwater leakage causes a decrease in the pore pressure in the surrounding ground, which is difficult to capture with existing modelling methods. Localized leakage at the joints leads to tunnel deformation in the shape of an inclined oval in the transverse direction. The leak location in the transverse direction has a significant impact on the tunnel deformation in the longitudinal direction. Leakage at the roof has less impact on tunnel settlement, but its impact on the ground surface settlement should not be overlooked.
123 citations
TL;DR: In this article, a series of techniques are incorporated into the original NMM (numerical manifold method) to simulate the entire process of the progressive failure of rock slopes, including strength-based LT criterion, which can account for tensile cracks, tensile shear cracks and compressive-shear cracks.
Abstract: To simulate the entire process of the progressive failure of rock slopes, a series of techniques are incorporated into the original NMM (numerical manifold method). To reflect the stress concentration near the crack tip, the Williams' displacement series is adopted to enrich the global displacement function of the NMM. Furthermore, the most recently proposed strength-based LT criterion, which can account for tensile cracks, tensile-shear cracks and compressive-shear cracks, is adopted to determine the crack propagation direction and length. Three typical numerical examples, including a Mode-I crack problem, a Mode-II crack problem and a Brazilian disc problem are adopted to verify the numerical model. The numerical results indicate that the numerical model is capable of accurately simulating the Mode-I crack propagation problem, the mode-II crack propagation problem and the failure process of Brazilian disc. Furthermore, the numerical model is adopted to investigate the entire progressive failure process of two rock slopes. The corresponding results indicate that the numerical model can not only simulate the propagation and coalescence of multiple cracks in rock masses but also the opening/sliding of rock blocks along discontinuities. The proposed numerical model warrants further investigation.
107 citations
TL;DR: In this article, a novel approach is presented to describe the dynamic interaction system of a large-diameter floating pipe pile and surrounding soils, taking the three-dimensional wave effects into account.
Abstract: A novel approach is presented to describe the dynamic interaction system of a large-diameter floating pipe pile and surrounding soils, taking the three-dimensional wave effects into account. The corresponding analytical solutions for longitudinal complex impedance are obtained and subsequently validated via comparisons with existing solutions. Comparative analyses are also performed to illustrate the difference between the present and previous solutions, concerning the wave propagation effect in the radial direction on the longitudinal dynamic vibration of pile shaft. Furthermore, the effects of Poisson’s ratio and visco-elastic support beneath the pile toe, on the longitudinal dynamic vibration of pile shaft, are investigated. It is indicated that the presented approach and corresponding solutions provide a more wide-ranging application for longitudinal vibration analysis of a large-diameter floating pipe pile, which can also be reduced to analyze the longitudinal vibration problems of large-diameter floating solid pile and fixed- end pipe pile.
92 citations
TL;DR: In this paper, the Johnson-Holmquist damage constitutive model was adopted, based on the LS-DYNA secondary development technology, and smooth particle hydrodynamics (SPH) was introduced to simulate the fluid.
Abstract: During drilling and blasting construction, a water inrush can be caused by damage to a water-resisting rock mass. Tunnel water inrush is a common anthropogenic geological hazard in engineering. The traditional finite element method (FEM) is less accurate in dealing with large deformation problems, and the Discrete element method (DEM) cannot realize engineering scale operation. In this study, the Johnson-Holmquist damage constitutive model was adopted, based on the LS-DYNA secondary development technology. A fluid grid was used in the blast damage area, and smooth particle hydrodynamics (SPH) were introduced to simulate the fluid. The far area of the tunnel is calculated by solid grid, forming a coupled algorithm of FEM and DEM. Based on a case study of the Yonglian Tunnel, this study simulated the evolution of a tunnel water inrush under the action of blasting. Through contrastive analysis, the numerical simulation results for the water inrush owing to damage to the water-resisting rock were found to correspond to those found using field tests. Therefore, this method can provide reference for underground engineering construction.
73 citations
TL;DR: In this paper, a new analytical model for calculating the horizontal dynamic impedance of pile groups with arbitrary members connected with a rigid pile cap is presented, which allows robustly the effect of pile-to-pile interaction on the impedance of the group, by accounting for the secondary waves generated by the vibration of the receiver pile, and how these alter the response of the source pile.
Abstract: This paper presents a new analytical model for calculating the horizontal dynamic impedance of pile groups with arbitrary members connected with a rigid pile cap. The derived solution allows considering robustly the effect of pile-to-pile interaction on the impedance of the group, by accounting for the secondary waves generated by the vibration of the receiver pile, and how these alter the response of the source pile. This is achieved by introducing a modified expression for the pile-to-pile interaction factor that accounts for coupling of source and receiver pile displacements. It is shown that this effect can be important for closely-spaced pile groups subjected to high frequency excitations. The proposed solution can be straightforwardly used to determine the frequency-depended impedance of large pile groups comprising an arbitrary number of piles, which numerical modelling can be cumbersome.
66 citations
TL;DR: In this paper, a nonlinear strength reduction method based on the Generalized Hoek-Brown (GHB) criterion has been proposed to find an optimal set of parameters that trigger rock slope failure.
Abstract: The strength reduction method (SRM) combining with the Generalized Hoek-Brown (GHB) criterion has become a widely used means to assess the stability of rock slopes. Therefore, various efforts have been made to establish a standard reduction strategy for strength parameters. This paper presents a new nonlinear SRM based on the GHB criterion, and the core novelty of the proposed method is to provide a reduction strategy with precise physical meaning to find an optimal set of parameters that trigger rock slope failure. To verify the validity of the proposed method, two illustrative examples are analyzed. The results show that the proposed method could lead to a reasonable safety factor, and the critical sliding surface obtained by the proposed method can reflect the tensile crack in the steep slope. Finally, to distinguish the computational results and efficiency among the proposed method and other preexisting reduction methods, other more 8 slopes selected from the literature are used for research objects. The results show that the safety factors resulted from the proposed method are very close to those acquired by the most accurate method available at present (Hammah’s method), however, its computational efficiency is comparatively higher than that of Hammah’s method.
62 citations
TL;DR: In this paper, a new failure mechanism is constructed on the basis of the deformation characteristics of the rock mass around an existing tunnel induced by an adjacent excavation, which is derived in the framework of the upper bound theorem in conjunction with a variational approach.
Abstract: Foundation pit excavation may disturb the initial stress field and induce the deformation of the surrounding rock mass. Due to the excavation, the originally compact rock mass and lining structure may become loose with adverse impacts on the normal operation of the existing tunnel. A new failure mechanism is constructed on the basis of the deformation characteristics of the rock mass around an existing tunnel induced by an adjacent excavation. Using this failure mechanism, the upper bound solution of the slip surface equation for the rock mass around an existing tunnel is derived in the framework of the upper bound theorem in conjunction with a variational approach. The shape and range of the slip surface are plotted for different parameters. A three-dimensional numerical model is constructed to simulate the deformation of the rock mass around the existing tunnel induced by adjacent excavation. By comparing the analytical results for the slip surface with the failure surface provided by numerical simulation, it is found that the differences between the analytical results and the numerical results are small. This comparison shows that the proposed method is valid for investigating the deformation of the rock mass around the existing tunnel induced by adjacent excavation.
62 citations
TL;DR: In this article, a new failure mechanism that combined with the Protodyakonov's theory and the multi-block failure mechanisms is proposed to deal with the face stability of deep tunnels in sandy layer.
Abstract: This paper aims at proposing a new failure mechanism to deal with the face stability of deep tunnels in sandy layer. Numerical simulations were conducted to analyze the influence of different variables on the limit support pressure and the failure zone. Based on the results of the numerical simulation, a new failure mechanism that combined with the Protodyakonov's theory and the multi-block failure mechanisms is proposed. The limit analysis method is adopted to obtain the limit support pressure and failure zone. Finally, the limit support pressure and failure zone derived from the new mechanism are compared with those given by numerical simulation, previous researches of limit analysis and centrifugal test. The results show that the new failure mechanism based on Protodyakonov's theory is more reasonable and accurate to analyze face stability of deep tunnels.
59 citations
TL;DR: In this article, a modified one-dimensional conditional Markov chain model is proposed to model the stratigraphic boundary uncertainty (SBU) arising from limited site investigation data on slope stability analysis with the consideration of soil spatial variability.
Abstract: Slope stability analysis based on different stratigraphic boundary conditions may have significant differences in terms of factor of safety (FS) and the location of the critical slip surface. Therefore, traditional reliability analysis of layered slopes that assumes the stratigraphic boundary between two soil layers being a deterministic line or surface can be misleading. This study aims to investigate the influence of the stratigraphic boundary uncertainty (SBU) arising from limited site investigation data on slope stability analysis with the consideration of soil spatial variability. A modified one-dimensional conditional Markov chain model is proposed to model the SBU. Cholesky decomposition technique is subsequently utilized to simulate the inherent spatial variability of soil properties based on the simulated stratigraphic boundary within the framework of Monte Carlo simulation. A sample example on a two-layered soil slope with limited number of boreholes shows that the proposed method can well simulate the stratigraphic boundary based on limited borehole information. The statistics of FS and probability of failure are found not to increase or decrease monotonically with the number of boreholes, but can converge to the correct results if the number of boreholes increases. Moreover, the conventional reliability analysis with an implicit assumption of deterministic stratigraphic boundary condition can significantly overestimate the reliability of the slope, but this overestimation decreases with the increase in the scale of fluctuation.
55 citations
TL;DR: In this paper, the authors proposed a theoretical approach to evaluate the variation in excess pore water pressure caused by the installation of a jet grouting column in clay, accounting for the chronological sequence of construction.
Abstract: The injection of large volumes of pressurized water and grout into the subsoil during jet grouting generates a sudden increase in excess pore water pressure. This study proposes a theoretical approach to evaluate the variation in excess pore water pressure caused by the installation of a jet grouting column in clay, accounting for the chronological sequence of construction. The jet grouting column installation is simulated through the undrained expansion of a series of spherical cavities. Partial dissipation during the construction process is considered due to the gradual installation of the grouting columns. The relationship between the ultimate cavity radius (au) and the radius of the jet grouting column (rc) is established to represent the influences of both jetting parameters and soil properties on the generated excess pore water pressure. The proposed model is validated using two case studies, one conducted in Singapore marine clay and the other in Shanghai soft clay.
52 citations
TL;DR: In this paper, a homogenization equation is proposed for the small strain stiffness of gap-graded granular soils to reproduce the small-strain stiffness observed in numerical simulations and further validated by laboratory test data from the literature.
Abstract: Sandstone usually disintegrates into gap-graded granular materials with a matrix-sustained structure due to weathering factors. This paper presents an investigation on the small strain stiffness of this type of granular materials based on Discrete Element Method (DEM) simulations. Our numerical results indicate the percentage of sliding contact is negligibly small within the small strain range, and the small strain stiffness of fines is well consistent with the widely recognized Hardin’s equation. Both findings confirm the validity of DEM simulations on the study of small strain response of granular materials. The simulation results are further analyzed based on mixture theory. A structure variable is introduced to correlate with the evolution of inter-aggregates structure. This variable is found to increase with the volume fraction of coarse aggregates but is rather independent of the confining stress and the initial void ratio of the fine matrix. Based on the insights drawn from DEM simulations, a homogenization equation is proposed for the small strain stiffness of gap-graded granular soils to reproduce the small strain stiffness of gap-gaped materials observed in our numerical simulations and is further validated by laboratory test data from the literature. The equation can be conveniently incorporated into classical elastoplastic models to model gap-graded granular materials.
TL;DR: In this paper, a random field-based approach for modelling the stratigraphic uncertainty is presented, in which, the spatial correlation of the stratum existence between different subsurface elements is characterized by an autocorrelation function, and the probability of the existence of a particular stratum in a given non-borehole element is determined according to the derived spatial correlations.
Abstract: Due to the complexity and inherent spatial variability of the strata and the limited availability of borehole information, the subsurface stratigraphic configuration at a site is hard to characterize accurately, which gives rise to stratigraphic uncertainty. This paper presents a random field-based approach for modelling the stratigraphic uncertainty, in which, the spatial correlation of the stratum existence between different subsurface elements is characterized by an autocorrelation function, and the probability of the existence of a particular stratum in a given non-borehole element is determined according to the derived spatial correlations. With the existence probabilities calculated, Monte Carlo simulation (MCS) is used for sampling the possible realizations of the stratigraphic configuration. The number of the MCS samples is determined based upon an information entropy analysis. To illustrate the effectiveness of this new approach, a set of hypothetical examples (with different stratigraphic settings) are studied; and, the advantages of this new approach over the existing stratigraphic uncertainty modelling approaches are depicted through a comparative analysis. Finally, this new approach is applied to Majiagou Landslide for a probabilistic stability analysis. The influences of the stratigraphic uncertainty on the stability of this landslide and the location of the slip surface are revealed.
TL;DR: In this paper, a finite element model was developed to simulate the flexible pavement structure by considering the nonlinear characteristics of subgrade soils which are related to humidity and stress, and a UMAT was programmed to characterize nonlinear moisture and stress-sensitive relationship of sub grade soils.
Abstract: This study aimed to develop a finite element model to simulate the flexible pavement structure by considering the nonlinear characteristics of subgrade soils which are related to humidity and stress. First, an analytical model was proposed and proven to quantify the effect of the humidity and stress on the resilient modulus of subgrade soils. Second, a UMAT was programmed to characterize nonlinear moisture and stress-sensitive relationship of subgrade soils. Then, the humidity field values of subgrade were calculated by GEOSTUDIO and verified by field measurement. Meanwhile, the humidity field values were imported into ABAQUS through MATLAB. Finally, the UMAT was implemented in the finite element model of flexible pavement structures. The finite element simulation indicated that the distribution of moisture content of subgrade significantly affects the resilient modulus distribution of subgrade and critical response of pavement structures. Resilient modulus of subgrade soils in the vicinity of load area are significantly larger than those far away the load area. The surface deflection, the tensile stress at the bottom of inorganic binder base, and the compressive strains on the top of subgrade increase greatly with the initial humidity field condition to the equilibrium humidity field condition.
TL;DR: In this article, the authors examined the seismic slope failure mechanism of rotational sliding mass and calculated the permanent displacement based on Newmark's sliding block theory, with extension to compute the rotational displacement in the presence of horizontal ground acceleration history.
Abstract: Assessing the seismic stability of earth slopes has long been a challenging task in geotechnical engineering mainly because of two reasons: (1) the mechanism of slope movement triggered by an earthquake is not fully understood, and (2) the criteria of seismic performance or failure state are hard to specify. As an attempt to address these concerns, this paper examines the seismic slope failure mechanism of rotational sliding mass. Permanent displacement is analyzed based on Newmark’s sliding block theory, with extension to compute the rotational displacement in the presence of horizontal ground acceleration history. A simplified relationship between the rotational and horizontal motions of a circular failure mass is also obtained. By comparing case studies, the possible reasons whether and how the seismic slip surface differs from the static one are explained. In addition, we discuss the allowable displacement when a quantitative judgment of the slope performance is required. In this regard, the underlying uncertainties of the soil properties are introduced to probabilistically relate this threshold value to the reliability index, a substitute for the probability of failure or risk acceptance level. The results from the parametric studies indicate the reliability-based design of the allowable displacement is a promising approach for seismic slope analysis.
TL;DR: In this paper, a series of monotonic and cyclic triaxial shearing tests on Toyoura sand are simulated by discrete element method (DEM) incorporating rolling resistance.
Abstract: A series of monotonic and cyclic triaxial shearing tests on Toyoura sand are simulated by discrete element method (DEM) incorporating rolling resistance. The input parameters are calibrated against laboratory triaxial test data considering different initial states and monotonic loading paths. The calibrated DEM model successfully captures the density and stress dependency of sand behavior under both monotonic and cyclic conditions. It is shown that when approaching liquefaction, contact yielding alters from sliding-dominant to rolling-dominant. The expression of mechanical redundancy index (IR) is derived considering both effects of contact sliding and rolling. The IR = 1.0 condition is shown to be a unified criterion distinguishing between solid state and liquid state upon both monotonic and cyclic liquefactions. A unique micro critical state line (CSL) is obtained in MCN-p′ plane. The cyclic resistance ratio is exponentially correlated with both the macro state parameter (ψe0) derived from the e-p′ CSL and the micro state parameter (ψMCN0) determined from the MCN-p′ CSL. The specimens with the same ψe0 may have distinct cyclic behaviors, while cyclic behaviors of specimens with the same ψMCN0 are close to each other, indicating that ψMCN0 is a more plausible state variable for characterizing the behavior of sand than ψe0.
TL;DR: In this article, the authors use the Discrete Element Method (DEM) to model concrete samples and assess the roles of the strength of aggregates and of the aggregate/mortar interfacial transition zone (ITZ) on concrete strength.
Abstract: This study aims to understand the micro-mechanisms that drive fracture propagation in concrete and to assess the roles of the strength of aggregates and of the aggregate/mortar interfacial transition zone (ITZ) on concrete strength. We use the Discrete Element Method (DEM) to model concrete samples. Mortar is represented by a volume of bonded spherical elements. Bonds are governed by a new displacement-softening law. Aggregate centroids are randomly placed in the DEM sample. We use CT scan images of real aggregates to plot 3D aggregate contours. The spherical elements that are contained in 3D contours around the randomly placed centroids are replaced by clusters with aggregate properties. The number and the size of the clusters are determined from the experimental Particle Size Distribution. The DEM concrete model is calibrated against uniaxial compression tests and Brazilian tests of both mortar and concrete. It is found that: At same aggregate volume fraction, a concrete sample with randomly placed aggregates and ITZ bonds is stronger; Concrete strength is linearly related to aggregate tensile strength; A linear relationship exists between the contact ratio in the mortar/aggregate ITZ and concrete strength; The ITZ has more influence on concrete strength than aggregate tensile strength.
TL;DR: In this article, a new fluid-solid coupled numerical approach by combining the Dynamic Fluid Mesh method with DEM (discrete element method) is applied to simulate suffusion, which accounts for permeability and porosity changes due to soil skeleton deformation and suffusion.
Abstract: A new fluid–solid coupled numerical approach by combining the Dynamic Fluid Mesh method with DEM (discrete element method) is applied to simulate suffusion. The fluid mesh is generated according to the soil skeleton formed by coarse particles and updated at regular intervals. Seepage forces are calculated and applied on solid particles in the DEM model. The new approach accounts for permeability and porosity changes due to soil skeleton deformation and suffusion. A model of soil sample under triaxial stress condition is developed to simulate the suffusion process. The results show that the erosion process can be divided into three stages: initial stage when numerous fine particles are washed away and the flow rate increases with more eroded particles, deceleration stage when the erosion rate decreases and the flow rate tends to reach a steady state, and stabilization stage when the erosion rate levels off to zero gradually. Parametric studies show that the increase of hydraulic gradient and coarse particle size ratio both increase the eroded particle mass. However, high confining stress will decrease the eroded particle mass. Hydraulic gradient affects the whole process of suffusion whereas confining stress and particle size ratio mainly affect the second and third stages.
TL;DR: In this paper, the effect of particle shape on particle breakage was investigated through triaxial tests on sand assemblies mixed with non-spherical agglomerates using the discrete element method.
Abstract: The effect of particle shape on particle breakage is investigated through triaxial tests on sand assemblies mixed with non-spherical agglomerates using the discrete element method. Mixed assembly is generated in terms of cumulative distribution of sphericity of real sands. Comparable macro-mechanical results are found between DEM and experimental observations, including non-linear stress-strain relationship, peak deviatoric stress, and peak friction angle. More broken bonds and agglomerates numbers are generated in mixed assembly than that in spherical assembly. The hyperbolic relationship between particle breakage and energy input is found to have relevance to particle shape, and the fitting result of mixed assembly is in better accordance with real sands.
TL;DR: In this article, a three-dimensional discrete element grain-based stress corrosion model incorporating the theories of subcritical crack growth and chemical reaction rate was built to explore the time-dependent behavior of damage evolution and fracture patterns of brittle rocks on a mesoscopic scale.
Abstract: A three-dimensional discrete element grain-based stress corrosion model incorporating the theories of subcritical crack growth and chemical reaction rate was built to explore the time-dependent behavior of damage evolution and fracture patterns of brittle rocks on a mesoscopic scale. The model was first validated and the model accurately captured the evolution of damage (tensile and shear microcracks) on the mesoscopic scale and the macroscopic mechanical behavior (the strength, and failure patterns) observed in laboratory experiments. The subcritical parameters of the model were calibrated to match the time-dependent damage deformation behavior observed in laboratory experiments. The time-dependent numerical results replicated the typical decelerating-accelerating axial strain behavior seen in laboratory brittle creep experiments. The crack propagation pattern in the simulation indicated that tension cracks were dominant. The results of numerical simulation showed that the time-to-failure during brittle creep decreased with the increase of the stress level, while the initial strain value, initial damage value, and minimum creep strain rate increased, as observed previously in the laboratory. We conclude therefore that the presented model supports a rich set of grain-scale discontinuities that can be related to microstructural features and provides a deeper understanding of the evolution of time-dependent damage on the mesoscale.
TL;DR: In this article, a 3D numerical mesoscale model is proposed to describe the time-independent deformation and fracturing of brittle rocks that accounts for material heterogeneity and local material degradation.
Abstract: A new 3D numerical mesoscale model is proposed to describe the time-independent deformation and fracturing of brittle rocks that accounts for material heterogeneity and local material degradation. The concept of renormalization at the mesoscale was introduced to capture the co-operative interaction of microcracks. The maximum tensile stress criterion and the Drucker-Prager criterion (which considers all three principal stresses) were used to evaluate the damage of specimen-forming elements in tension and shear, respectively. Simulations are presented alongside experimental data to show the capabilities of the model in tackling the failure process of intact rock, with an emphasis on microcrack initiation and propagation. We demonstrate that our model is capable of simulating the initiation and propagation of microcracks in deforming rocks and captures the mechanical behavior, strength, and failure patterns seen in laboratory experiments. Our 3D model therefore emerges as a powerful tool to study the evolution of failure in brittle rocks.
TL;DR: In this article, the behavior of a prefabricated vertical drain unit cell with clays of high initial water content has been investigated by laboratory model tests and finite element analysis (FEA).
Abstract: The behaviour of a prefabricated vertical drain (PVD) unit cell with clays of high initial water content has been investigated by laboratory model tests and finite element analysis (FEA). The results indicate that there is considerable horizontal soil movement towards the PVD. The soils in the zone near the PVD are in horizontal compression, and the other zones are in horizontal tension. The surface settlements are not uniform, and the soil at the periphery of the unit cell settles more than the soil near the PVD. The model test results indicate that the phenomenon of soil particle separation does not occur, and it implies that the main mechanism of apparent “clogging” is due to non-uniform consolidation. The results of the FEA indicate that at the end of vacuum consolidation, the soil adjacent to the PVD has a higher effective vertical stress as well as undrained shear strength, but the stress state is very close to the critical state line. Then, a new explicit consolidation analysis method has been established/verified for considering the variations of consolidation properties, including apparent clogging during the consolidation process.
TL;DR: This paper presents machine learning aided stochastic reliability analysis of spatially variable slopes, which significantly reduces the computational efforts and gives a complete statistical description of the factor of safety with promising accuracy compared with traditional methods.
Abstract: This paper presents machine learning aided stochastic reliability analysis of spatially variable slopes, which significantly reduces the computational efforts and gives a complete statistical description of the factor of safety with promising accuracy compared with traditional methods Within this framework, a small number of traditional random finite-element simulations are conducted The samples of the random fields and the calculated factor of safety are, respectively, treated as training input and output data, and are fed into machine learning algorithms to find mathematical models to replace finite-element simulations Two powerful machine learning algorithms used are the neural networks and the support-vector regression with their associated learning strategies Several slopes are examined including stratified slopes with 3 or 4 layers described by 4 or 6 random fields It is found that with 200 to 300 finite-element simulations (finished in about 5 ~ 8 h), the machine-learning generated model can predict the factor of safety accurately, and a stochastic analysis of 105 samples takes several minutes However, the same traditional analysis would require hundreds of days of computation
TL;DR: In this paper, a unique anisotropic double porosity elastoplastic framework was developed to describe the coupled solid deformation-fluid flow in the transversely isotropic fissured rocks.
Abstract: Geomaterials containing fissures such as some sedimentary rocks often exhibit a bimodal pore size distribution, and they are also inherently anisotropic due to the distinct bedding planes. Hydromechanical modeling of solid deformation and fluid flow of such geomaterials remains a significant challenge. In this paper, we have developed a unique anisotropic double porosity elastoplastic framework to describe such processes. Furthermore, for the solid constitutive model, because of the loss of stress tensor coaxiality between the trial state and the final state, we have derived a new implicit return mapping algorithm to obtain the updated effective stress, history parameters and consistent tangent operator for any given strain increment efficiently, followed by a uniaxial strain point simulation to provide benchmark results. Subsequently, 3D stress point simulations are carried out to calibrate the projection and plasticity parameters using triaxial experimental data as well as to illustrate the strain-softening phenomenon. Initial boundary value problem simulations have been conducted to analyze the impacts of fluid flow and solid constitutive model on the resulting geomaterials’ responses. The overarching goal of this paper is to better understand the coupled solid deformation-fluid flow in the transversely isotropic fissured rocks.
TL;DR: In this paper, a lower bound finite element limit analysis is presented for the undrained stability of unsupported rectangular excavations in non-homogeneous clays, and a new design equation of the problem is also presented for practical use by practising engineers.
Abstract: Solutions for the undrained stability of unsupported excavations are important in practice as they can be used for assessing the safety of temporary excavations associated with various civil works. Even though the previous stability solutions of unsupported excavations namely infinitely long trenches and circular excavations are available in the literature, there is a lack of the stability solution of unsupported rectangular ones. In this paper, the lower bound solutions for the undrained stability of unsupported rectangular excavations in non-homogeneous clays are presented for the first time. A three-dimensional lower bound finite element limit analysis is developed in order to investigate the stability of this problem. The undrained shear strength of non-homogeneous clays are considered as a linearly increasing one with depth. The effects of aspect ratios of rectangular excavations, excavated depth ratios, and normalized strength gradients on the stability number of the problem and its associated failure mechanisms are examined parametrically. A new design equation of the problem is also firstly presented for practical use by practising engineers.
TL;DR: In this paper, a combined finite-discrete element method (FDEM) was developed to simulate solid fracture by defining a traction-separation damage law, which is capable of simulating breakage behaviors of sand particles.
Abstract: Particle morphology is an inherent characteristic that has a significant influence on breakage behaviors of natural sands. Based on X-ray micro-computed tomographic scanning and image-processing, the three-dimensional (3D) particle surfaces of natural sand particles were first reconstructed by the spherical harmonic (SH) analysis. The traditional morphological parameters of the particles, including sphericity, roundness, and aspect ratio were then calculated based on the SH-reconstructed particle surfaces. Furthermore, a 3D local roundness descriptor was used to characterize the local angularity of the contact area between a particle and its loading platens. To model particle breakage, this study developed a combined finite-discrete element method (FDEM) in which the cohesive interface elements were inserted between the finite elements to simulate solid fracture by defining a traction-separation damage law. By using the developed FDEM, a series of single particle crushing simulations were conducted for ellipsoid particles and two types of sand particles. The results show that the developed FDEM model is capable of simulating breakage behaviors of sand particles. Statistical analysis showed that local roundness is the most significant factor in determining the particle fracture pattern, with the strongest positive correlation with the particle crushing strength of sand particles subjected to uniaxial loading.
TL;DR: In this article, the effects of irregular particles on macroscopic and microscopic behavior under triaxial compression tests for a range of various roundnesses were investigated, and the microscopic characteristics, including the coordination number, particle rotation and percentage of sliding contacts, were examined.
Abstract: This report investigated the effects of irregular particles on macroscopic and microscopic behaviour under triaxial compression tests for a range of various roundnesses. A multi-sphere method was employed to model the three-dimensional irregular particles generated by rotation-invariant spherical harmonics. After being prepared in the densest conditions, all assemblies were subjected to axial compression until a critical state was reached. The macroscopic characteristics, including the shear strength and dilatancy response, were investigated. We found that the shear strength generally decreased and the volumetric strain linearly and monotonically decreased with increasing roundness. Then, the microscopic characteristics, including the coordination number, particle rotation and percentage of sliding contacts, were examined. An increase in the coordination number obstructed the rotation of particles and led to an increase in the percentage of sliding contacts. Finally, analysis of the related anisotropy coefficients of the entire contact network was carried out to probe the microscopic origins of macroscopic behaviour. A detailed analysis revealed the microscopic mechanism of the dependence of the peak and residual shear strengths on roundness.
TL;DR: In this paper, the impact of yaw excavation loadings on ground displacement and segmental stress for a curved shield tunnel is investigated through finite element models, the reliabilities of which are validated through comparisons to field data and analytical solutions.
Abstract: This paper describes the key influences of yaw excavation loadings on ground displacement and segmental stress for a curved shield tunnel. The influences are investigated through finite element models, the reliabilities of which are validated through comparisons to field data and analytical solutions. Multiple case studies of different curvature tunnels and their comparison to straight-line tunnels are presented. Under the dual action of overcutting and construction loadings, the surface settlement of the curved tunnel is larger than that of the straight-line tunnel. The horizontal displacements at the inner and outer sides of the curved tunnel are asymmetric with respect to the tunnel axis. This asymmetry can increase significantly during yaw excavation of over one ring width. Yaw excavation loadings have a significant influence on the horizontal and vertical displacements of the ground within a span of shield length starting from the position of the hydraulic jacks until the back. The circumferential compressive stress, axial tensile stress, and axial compressive stress of newly installed segment of the curved tunnel are greater than those of the straight-line tunnel. Interestingly, the stress increments increase linearly with yaw severity. The results are of benefit to suggest improvements for practical construction procedures.
TL;DR: In this article, the particle flow code is used to analyze the dynamic process of the Xinmo landslide, including: disintegration, entrainment, granular flow and accumulation, and the results show that the kinetic characteristics of the collapse mass were more sensitive to the bond strength than to the friction coefficient.
Abstract: As a useful tool to simulate the movement of granular materials, the Particle Flow Code is used to analyze the dynamic process of the Xinmo landslide, including: disintegration, entrainment, granular flow and accumulation. The microparameters of the landslide rock mass were calibrated with uniaxial compression and landslide deposition simulations. The results show that the kinetic characteristics of the collapse mass were more sensitive to the bond strength than to the friction coefficient. For the colluvium on the runout path, the influence on the kinetic characteristics was insignificant in the case of the lower bond strength and friction coefficient. The whole process lasted approximately 100 s, and the main sliding time was 65 s. The peak velocity was 72.4 m/s, and the maximum displacement was approximately 2.5 km. The results indicate that the gravitational potential energy was mainly converted into friction energy and kinetic energy in the acceleration stage. However, the proportion of the collision energy increased gradually in the deceleration stage. The entrainment effect plays an important role in the Xinmo landslide movement. The high gravitational potential energy of the collapse mass was the basis of the high mobility, and the mobility was further enhanced because of the entrainment effect.
TL;DR: In this article, a modified Holmquist-Johnson-Cook (HJC) model was proposed to capture the compressive behavior but not the tensile behaviour of brittle materials.
Abstract: The original Holmquist-Johnson-Cook (HJC) model, which can only capture the compressive behaviour but not the tensile behaviour of brittle materials, is modified in this study. First, the Lode-angle function is introduced to the yield strength surface to consider the changes in the substantial shear strength between the tensile and compressive meridians. A hyperbolic function is used to describe the strain rate effect under dynamic tensile loading. An exponential model is introduced to express the tensile softening stage of brittle material and define the tensile damage variable. Then the modified HJC model is implemented in LS-DYNA via user subroutine UMAT. Further the modifications are validated by single element tests and rock mechanical laboratory experiments. The verification results show that the modified HJC model can effectively describe the tensile responses of brittle material under static and dynamic loading. Last, a comparison between a blasting-crater field test and the corresponding numerical prediction by the modified HJC model is carried out. The comparison results demonstrate that the shape and size of the crater predicted by the modified HJC model show a good agreement with the field test data. Therefore, the modified HJC model has the capacity to capture the tensile and compressive behaviours and damage evolution of brittle material.
TL;DR: The results indicate that the proposed approach has both good accuracy and efficiency in slope reliability analysis involving the copula-based cross-correlated random fields of c and ϕ at low failure probability levels.
Abstract: This study proposes a subset simulation (SS)-based approach for efficient slope reliability analysis involving copula-based cross-correlated random fields of cohesion (c) and friction angle (ϕ) of soils. First, the copula theory for modeling the cross-correlation between c and ϕ is briefly introduced. The algorithms for generating the copula-based cross-correlated random fields of c and ϕ are detailed. Then, the SS for efficient slope reliability analysis involving copula-based cross-correlated random fields of c and ϕ is explained. Finally, two slope examples with the same geometry but different sources of probability information are presented to illustrate and demonstrate the proposed approach. The results indicate that the proposed approach has both good accuracy and efficiency in slope reliability analysis involving the copula-based cross-correlated random fields of c and ϕ at low failure probability levels. The copula theory for characterizing the cross-correlated random fields can consider both the Gaussian and non-Gaussian dependence structures between c and ϕ. The copula selection has a significant impact on slope reliability with spatially variable c and ϕ. The probabilities of slope failure produced by different copulas differ considerably. This difference increases with decreasing probability of slope failure. The commonly-used Gaussian copula may lead to a significant underestimate of the probability of slope failure. The reasonable identification of the best-fit copula for characterizing the cross-correlated random fields of c and ϕ based on the test data is highlighted in practical slope reliability analysis.