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When modelling the phenomenon of strain localisation in strain-softening soils with the finite element (FE) method, nonlocal approaches have been commonly employed to avoid mesh dependency and numerical instability. This paper first presents the FE formulation of a critical state model for highly overconsolidated clays incorporating a nonlocal method. The performance of the nonlocal strain regularisation is subsequently assessed through a series of coupled hydro-mechanical (HM) analyses of undrained and drained triaxial compression tests on London clay. The mechanism behind the evolution of strain localisation in triaxial tests is investigated and a comparison with equivalent plane strain analyses is discussed. Finally, a comprehensive sensitivity study is presented, investigating the influence of the two nonlocal parameters, in the adopted nonlocal algorithm, on the predicted stress–strain responses. A key outcome is the derived linear relationship between the two parameters, which enables a unique stress–strain response to be achieved in either axisymmetric or plane strain analyses with multiple combinations of the two parameters. Such a modelling capability is essential in applications of the proposed nonlocal strain regularisation in large scale boundary value problems in which restrictions on element size exist. [ABSTRACT FROM AUTHOR]

This paper provides a numerical framework for seismic analyses for layered saturated ground under inclined plane P1-SV waves. A unified stiffness matrix method (SMM) formula for both u - w - p and u - p formulations is first derived to obtain the dynamic response of saturated fields. Then, the domain reduction method (DRM) is extended to two-phase media to reproduce the target wavefield within the numerical models. The accuracy of the proposed SMM-DRM framework is verified by comparisons with analytical solutions and other numerical methods. Its applicability in complex boundary value problems is also proven by solving a typical SSI problem. The numerical framework is especially advantageous in the analyses of complex subdomains under obliquely incident seismic waves. With the procedure given in the paper, the framework can be implemented into existing finite element models straightforwardly. [ABSTRACT FROM AUTHOR]

The dynamic behaviour of saturated coarse-grained soils has recently received wide attention because of its impact on the seismic performance of geotechnical and structural systems. This is due to the peculiarities of their cyclic response (e.g., liquefaction, ratcheting and volumetric-deviatoric coupling). Consequently, the seismic risk mitigation of the built environment requires efficient predictive models applicable in design and assessment. To this end, several constitutive models have been developed for a realistic description of the cyclic soil behaviour. From this perspective, this paper describes the implementation and testing of the bounding surface plasticity model developed by Papadimitriou and Bouckovalas (2002) in OpenSees as a means for advanced assessment of soil-structure systems. The implemented model includes essential features of the cyclic soil response. Moreover, a modified fabric tensor evolution equation is introduced for improving the response and numerical stability in boundary value problems, at the cost of an extra model constant. The workflow concerning the integration of the model into OpenSees is presented, followed by instructions about its use in boundary value problems. A comprehensive verification of the model response is discussed. The numerical simulations demonstrated the robustness of the implemented code in capturing soil behaviour from small to large strain levels. [ABSTRACT FROM AUTHOR]

A new static equilibrium mechanical model on noncircular shallow tunnelling considering initial stress field is proposed in this paper by constraining far-field ground surface, and a mixed boundary value problem thus forms with elimination of displacement singularity at infinity. By applying analytic continuation, the mixed boundaries are transformed to a homogeneous Riemann–Hilbert problem, which is subsequently solved via an efficient and accurate iterative method with boundary conditions of static equilibrium, displacement single-valuedness, and traction along tunnel periphery. The Lanczos filtering technique is used in the final stress and displacement solution to reduce the Gibbs phenomena caused by the constrained far-field ground surface to obtain more reasonable results. Several numerical cases are conducted to intensively verify the proposed solution by examining the utilization of Lanczos filtering, the elimination of displacement singularity, and solution convergence of constraining arc and truncation number, and comparisons with existing solutions, and all the results are in good agreements. Finally, more numerical cases are conducted to investigate the stress and deformation distribution along ground surface and tunnel periphery considering different tunnel geometries, lateral coefficients, and tunnel depths. Further discussions on the defects of the proposed solution are also conducted for objectivity. [Display omitted] • A new mechanical model is proposed for noncircular shallow tunnelling. • Far-field ground surface is constrained to equilibrate unbalanced resultant due to tunnelling. • Displacement singularity at infinity caused by unbalanced resultant is eliminated. • The solution simultaneously gives correct stress and reasonable displacement. [ABSTRACT FROM AUTHOR]

During shield tunneling in sand strata, variations in the state of sand can lead to localized instability and strength loss, which brings about numerical difficulties. This paper numerically investigates how nucleation and propagation of plastic deformation zones, including those leading to localized banding, possibly promote failure of shield tunnel faces excavated in sand. To this end, a material state-dependent model is used to capture the mechanical responses of sand. Nonlocal enhancement is introduced, through the volumetric strain increment that drives the change of void ratio and related material hardening/softening behavior, to regularize ill-posed boundary value problems caused by the activation of strain localization. Simulations of tunnel face failure indicate that lower initial void ratios result in more concentrated plastic deformation in the vicinity region near the face and a strain-softening trend in global deformation responses. Conversely, initial looser states drive the deformation zone to propagate spatially to the ground surface, and the resultant global deformation responses exhibit strain-hardening. The second-order work is used to explain the link between material instability around the tunnel faces and initial density. It was found that material instability usually occupies a fraction of plastic deformation regions, and shrinks as sand initial density increases. [ABSTRACT FROM AUTHOR]

Conventional finite element analyses generally suffer from mesh dependency when considering softening effects. Non-local strain regularisation techniques have been developed to address this issue, which are generally complex to implement. In view of this, this paper introduces an more efficient procedure for implementation of a non-local method in Abaqus for Coupled Eulerian-Lagrangian (CEL) large deformation finite element undrained analyses. Results from a series of biaxial compression simulations demonstrate that the non-local method with a strain-softening Tresca model in CEL avoids mesh dependency. Owing to the stationary Eulerian element and the built-in mapping algorithm in CEL, high computational efficiency is achieved, adding no more than 12% to the computational cost. Guidance is provided on selection of internal length scales, element sizes and the search radius to ensure efficient non-local calculations, and it is shown that a softening scaling rule can also be used to allow use of practical mesh densities for some boundary value problems. Simulations of a number of classical geotechnical problems demonstrate the type of boundary value problems where the non-local method can effectively mitigate the mesh dependency, whilst also highlighting that the method fails to control the strain localisation that develops at soil-structure interfaces due to geometrical issues. [ABSTRACT FROM AUTHOR]

Abstract Cyclic stress paths in geotechnical experiments can generate considerable principal stress rotation (PSR) in the saturated soil. The PSR without changes of principal stress magnitudes can generate additional excess pore water pressures and plastic strains, thus accelerating liquefactions in undrained conditions. This paper simulates a series of laboratory tests considering the PSR using two types of sand. The impact of PSR is taken into account by using an elastoplastic soil model developed on the basis of a kinematic hardening soil model with the bounding surface concept. The soil model considers the PSR by treating the stress rate generating the PSR independently. The capability of this soil model is verified by comparing the numerical predictions with and without PSR, as well as experimental results. The comparative results indicate that the simulation with the soil model considering the PSR can better reproduce the test results on the development of shear strain, reduction of effective confining pressure and liquefaction than the soil model without PSR. Therefore, it is important to consider PSR effects in simulations of geotechnical experiments under cyclic loadings. [ABSTRACT FROM AUTHOR]

Abstract A reliable and accurate stability analysis in Hoek-Brown rock masses under fully three-dimensional geometries requires an efficient and powerful computational method. This paper presents a computational implementation of a three-dimensional lower bound finite element limit analysis using semidefinite programming for Hoek-Brown rock masses. Its computational performance is investigated extensively through a variety of boundary value problems in rock mechanics, namely triaxial compression, bearing capacity and cavity contraction. New lower bound solutions are numerically derived, including bearing capacity of square footings, uniaxial compression of cube and cylindrical rock specimens with fully rough platens and tension test on double notched rock specimens. [ABSTRACT FROM AUTHOR]

The capacity of open-ended piles strongly depends on the potential plugging occurring during installation. The Discrete Element Method (DEM) is particularly suitable to the study of pile plugging, as it can model large soil deformation. However, DEM simulation of the 3D boundary value problems is computationally expensive, and particle upscaling is usually used to reduce the number of particles modelled. In this paper, two granular beds were created with the same average porosity, initial stress field and contact parameters, but different particle scales. Two pile geometries were installed by plain and rotary jacking. Normalised results show that the pile penetration mechanism is strongly affected by the particle scale. Larger particles lead to earlier pile plugging, higher shaft resistance and require a greater force to penetrate the ground. This effect can be linked to a modified penetration mechanism, with larger shear zones and less well defined "nose cone" under the pile tip for larger particles. Changing particle scaling has a neutral effect on the total penetration resistance of a rotary jacked pile, but reduces the base penetration and increases the plug resistance. Maximising the ratio of particle scale to wall thickness is key to adequately capturing the pile coring mechanism. [ABSTRACT FROM AUTHOR]

This paper presents semi-analytical solutions to two-dimensional plane strain consolidation of unsaturated soils under different initial and boundary conditions. By applying the finite sine and Laplace transforms, the partial differential equations are converted to the ordinary differential equations. The semi-analytical solutions are obtained by using the finite sine inversion transform. Crump’s method is adopted to perform the inverse Laplace transform to obtain analytical solutions in time domain. The present solutions are more general and can be degenerated into conventional solutions to one-dimensional consolidation of unsaturated soils. Several examples are provided to investigate two-dimensional plane strain consolidation behavior of unsaturated soils. [ABSTRACT FROM AUTHOR]

The scaled boundary method (SBM) is an effective numerical approach for analyzing elasto-statics of bounded and unbounded media. To enhance abilities of the scaled boundary approach, this method can be coupled with mesh free technology. In this paper a point interpolation based SBM is proposed to analyze seismic soil-tunnel interaction problems. In the proposed approach, boundary of the domain is modelled with the scaled boundary point interpolation method while the interior domain is modelled by the conventional finite element method (FEM). This is the first time that a mesh-free scaled boundary method is used to analyze seismic problems. The presented method has some advantages over previously presented mesh-free SBMs. In the scaled boundary point interpolation method, the shape functions have the Kronecker delta function property and do not require radial basis functions to discretize the boundary of 2D problems. A shaking table test is designed and used to verify the proposed method. It is shown that the proposed numerical approach leads to results that are in a good agreement with those of the designed shaking table tests. [ABSTRACT FROM AUTHOR]

In this paper, a three-dimensional simulation of fully coupled multiphase fluid flow and heat transfer through deforming porous media is presented in the context of EFG mesh-less method. Spatial discretization of the system of governing equations is performed using EFG and a fully implicit finite difference scheme is employed for temporal discretization. Penalty method is used for imposition of essential boundary conditions. The developed numerical tool is employed to simulate two problems of nuclear waste disposal and CO 2 sequestration in deep underground strata. The obtained results demonstrate the capability and robustness of the developed EFG code. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The paper presents the Finite Element formulation of the equations proposed by Tsiampousi et al. (2016) for coupled consolidation in unsaturated soils. Their coupling is discussed in relation to a conceptual model which divides soil behaviour into zones ranging from fully saturated to dry states. The numerical simulation of a laboratory experiment involving drainage of water from a vertical column of sand is used to validate the equations. Finally, the example of rainfall infiltration into a cut slope highlights how aspects of the conceptual model are reflected in the numerical analysis of boundary value problems involving unsaturated soils. [ABSTRACT FROM AUTHOR]

The paper presents an Asymptotically Matched Layer (AML) formulation in a moving frame of reference for transient dynamic response of a multi-layer 2D half-space. A displacement based finite element formulation of the convected domain problem is presented together with the AML formulation in which the original convolution integrals are represented via two auxiliary displacement-like state-space variables. A parametric study of the AML parameters is conducted for optimizing the absorbing properties. The performance is demonstrated on a single- and a two-layered half-space for various velocities of an impulse Ricker load. Excellent absorbing properties are demonstrated in both half spaces. [ABSTRACT FROM AUTHOR]

The governing equations for the one-dimensional consolidation of double-layered structured soils under time-dependent loading are established in this paper. Using simplified k – σ ′ and m v – σ ′ models, the double-layered structured soils can be analyzed with three- or four-layered soils in which the thicknesses of the upper and lower layers change gradually. Approximate solutions for the governing equations are then obtained for two types of boundary conditions, and a computer program is developed. Using these solutions and the computer program, the consolidation behavior of double-layered structured soils is investigated. [ABSTRACT FROM AUTHOR]

Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutive modeling of unsaturated soils. However, this type of tests requires sophisticated equipment and is time-consuming. This has been one of the major obstacles to the implementation and dissemination of unsaturated soil mechanics beyond the research context. In contrast to suction-controlled triaxial tests, the suction-controlled oedometer test requires simpler equipment and a shorter testing period. Oedometer tests represent the at-rest earth pressure ( K 0 ) condition, which is an important stress state in any simulation. The major disadvantage of the oedometer test is that its lateral stress is controlled by the condition of zero lateral strain and remains unknown during the testing process. At present, no well-established, simple, and objective methods are available that take advantage of oedometer test results for constitutive modeling purposes. This paper derives an explicit formulation of the at-rest coefficient for unsaturated soils and develops an optimization approach for simple and objective identification of material parameters in elasto-plastic models for unsaturated soils using the results from suction-controlled oedometer tests. This is achieved by combining a modified state surface approach (MSSA), recently proposed to model the elasto-plastic behavior of unsaturated soils, with the quasi-Newton method to simultaneously calibrate all parameters governing virgin behavior in elasto-plastic models. The Barcelona Basic Model (BBM) is used to demonstrate the application of the proposed explicit formulation and calibration method. Results predicted using obtained parameters are compared with laboratory test results for the same stress paths in order to evaluate the simplicity and objectivity of the proposed method. [ABSTRACT FROM AUTHOR]

Experimental data have demonstrated that a strong relationship exists between the state parameter and the peak strength and dilatancy characteristics of sands. This paper proposes a way of reproducing this behaviour using a modified Mohr-Coulomb failure criterion, which retains its simplicity while improving substantially its modelling capabilities. The formulated constitutive model is calibrated for Nevada sand following a well-defined procedure and used in the prediction of four centrifuge tests investigating the behaviour of axially loaded footings. It is shown that the proposed model reproduces well both the element tests and the more complex footing problems, demonstrating its usefulness for engineering practice. Moreover, a simplified version which does not require the definition of the Critical State Line is proposed for situations when this aspect of soil behaviour cannot be determined with confidence. It is shown that such simplification results in only slightly less accurate predictions than the full version of the model, while simulating aspects of soil response that cannot be reproduced using constant values for strength and dilatancy parameters. [ABSTRACT FROM AUTHOR]

This paper uses the finite element upper and lower bound limit analysis methods to investigate the three-dimensional (3D) slope stability of two-layered undrained clay slopes. The solutions obtained from the slope stability analyses are bracketed to within ±10% or better. For comparison purposes, results from two-dimensional (2D) analyses based on the numerical limit analysis methods and the conventional limit equilibrium method (LEM) are also discussed. This study shows that 3D boundary of a slope can have significant effects on the slope stability. In addition, the results are presented in the form of stability charts which can be convenient tools for practicing engineers. [ABSTRACT FROM AUTHOR]

This paper proposes a novel optimization-based finite element method with a mixed constant-stress smoothed-strain element to solve elastoplastic problems. The proposed computational framework has three advantages: (1) in the proposed mixed constant-stress smoothed-strain element, the strain is smoothed on the edge-based smoothing domain, and the stress is assumed to be constant in each smoothing domain; (2) the cubic bubble shape function is introduced at the centroid of the element to enrich the displacement field. This makes the proposed approach accurate and free of volumetric locking for low-order elements; (3) the discrete elastoplastic boundary-value problem is formulated as a standard second-order cone problem based on the generalized Hellinger–Reissner variational principle, which is quickly and efficiently solved by the primal-dual interior-point algorithm. The proposed approach is validated on several classical problems of geotechnical engineering: the bearing capacity of footing and pipe under small deformation and slope stability. All the obtained results demonstrate that the proposed method adopting the novel mixed constant-stress smoothed-strain element offers competitively high computational accuracy and efficiency. [ABSTRACT FROM AUTHOR]

This paper presents a semi-analytical solution to one-dimensional consolidation of viscoelastic unsaturated soils with a finite thickness under oedometric conditions and subjected to a sudden loading. The solution is obtained by using Lee’s correspondence principle based on the semi-analytical solution to one-dimensional consolidation of elastic unsaturated soils. The boundary contains the top surface permeable to water and air and the bottom impermeable to water and air. A typical example is given to show the evolution of excess pore-air and pore-water pressures as well as the total degree of consolidation of the soil layer with time for different ratios of air–water permeability coefficient, elastic modulus and viscoelastic coefficient. The one-dimensional consolidation behavior of viscoelastic unsaturated soil is discussed according to the semi-analytical solution. These results contribute to a better understanding of the consolidation behavior of viscoelastic unsaturated soils. [ABSTRACT FROM AUTHOR]

This paper presents numerical treatments of elastoplasticity relations for a previously developed critical state two-surface plasticity model and its implementation into a bespoke finite element code as well as the commercial software Abaqus via the user defined subroutine, UMAT. The model is implemented with an efficient integration scheme based on the explicit modified Euler scheme with automatic substepping and error control. Validation of the implemented model is assessed through simulation of several laboratory tests comprising a variety of initial states and loading conditions followed by a study on the accuracy and efficiency of the integration scheme. Subsequently, the model is employed to analyse some sophisticated boundary value problems involving finite deformations, inertia effects, soil-structure interactions and saturated porous media. The constitutive model captures particular features of clay behaviour, such as the prediction of strain rate dependence, small-strain stiffness degradation, the development of residual strength at very large shear strains and stress anisotropy. The effects of these features on the behaviour of two important geotechnical problems – including pipe-seabed interaction under lateral motion and dynamically installed anchors – are specifically investigated using two advanced finite deformation schemes: one based on the Arbitrary Lagrangian Eulerian (ALE) method and another on the Particle Finite Element Method (PFEM). The study illustrates the robustness of the proposed integration scheme and successful application of the soil model, indicating that the use of such complex soil models may be useful for realistic analyses of geotechnical problems. [ABSTRACT FROM AUTHOR]

Fabric anisotropy underpins the mechanical response of granular soils pertaining to a wide range of practical geotechnical applications. This paper presents a multiscale computational study on a strip footing resting on an anisotropic soil foundation. The focus of this study is placed on examining the cross-scale links of key grain-scale mechanisms in the soil that underscore interesting macroscopic observations of the footing problem over a full loading range from peak to large deformation regimes until its failure. We employ a hierarchical coupling of Material Point Method and Discrete Element Method (MPM-DEM). Mesoscale ensembles consisting of elliptical particles with specific alignments to represent bedding planes in anisotropic soils are generated. They are embedded into the material points of the MPM and serve as Representative Volumetric Elements (RVEs) with solutions by DEM to extract nonlinear material responses in solving the footing as a boundary value problem that may undergo large deformation to failure. The study confirms experimental observations that the bearing capacity of the strip footing decrease with the bedding angle α. It shows that ignoring fabric anisotropy for soil may lead to a significant overestimation or underestimation of the bearing capacity in extreme cases. The final failure patterns for all anisotropic cases feature general failure modes with two major slip surfaces, and they are predominantly in an asymmetric manner except the horizontal bedding case and the isotropic case. The degree of asymmetry in the failure pattern shows a correlation with the bedding angle. These observed features are further corroborated with microstructural analyses on the evolution of different sources of fabric anisotropy in slip surface. [ABSTRACT FROM AUTHOR]

The paper presents a finite deformation, isotropic hardening, non–associative elastic–plastic constitutive model (FD _ Milan model) for describing the mechanical behavior of a wide range of bonded natural geomaterials such as stiff overconsonsolidated clays, porous soft rocks or bio–improved soils. The formulation of the model is based on the multiplicative split of the deformation gradient and on the assumption of hyperelastic behavior. To deal with the occurrence of strain localization, typically observed in this class of geomaterials, the model has been equipped with a non–local version of the hardening laws. This approach is capable of regularizing the pathological mesh dependence occurring in the post–localization regime when adopting classical plasticity models. In view of its application to practical geotechnical problems characterized by large displacements and deformations within a hydro-mechanical coupled environment, the model has been implemented in the recently developed Particle Finite Element code G–PFEM for geomechanics applications. To demonstrate the effectiveness of the numerical implementation, a series of numerical simulations has been performed considering two representative boundary value problems: the modeling of shear localization in plane strain biaxial tests and the simulation of CPTu tests in a saturated porous soil. The results of biaxial test simulations have highlighted the role of the characteristic length in controlling the thickness of the localized zone and the effect of the confining pressure in determining the pattern of shear band formation. An interesting feature emerging from the partially drained CPTu simulation results is the progressive formation of persistent shear bands, which originate from the cone tip and propagate outwards along the entire penetration depth. [ABSTRACT FROM AUTHOR]

The proper solution of geotechnical boundary value problems requires robust constitutive models that can describe the mechanical behavior of geomaterials under various loading conditions, while also accounting as closely as possible for the different material scales of interest. This is even more relevant to granular media where the complexity of the mechanical behaviour is not limited to the nature of the contact law between grains, and instead originates from the multiplicity of contacts oriented along all the directions of the physical space to form distinctive mesostructures. This paper revisits the so-called H-model, which belongs to the broad family of micromechanical approaches whereby an intermediate scale (mesoscale) is explicitly introduced into the formulation. One great advantage of the model is that it can be extended by accounting for further multi-physical couplings, as for example the presence of capillary bridges between grains. This versatile model was implemented within an explicit finite difference based computational software (FLAC), and the present work demonstrates its ability to analyze engineering problems with a microstructural viewpoint, while also providing new insights in microstructural mechanisms of failure difficult to capture with standard phenomenological models. [ABSTRACT FROM AUTHOR]

This paper presents a framework of the closest point projection method (CPPM) based on the unified hardening (UH) model. The UH model based on the Cam-clay model and the concept of the subloading yield surface was rewrote as a concise form of classical elasto-plastic model. Then the CPPM algorithm combining the exact elastic property and the spatially mobilized plane (SMP) strength criterion by the transformed stress method could be conveniently derived for the numerical implementation of the UH model. According to this algorithm, the consistent tangent operator corresponding to the final stress state of the integration increment was deduced formally. And the mean stress probed by the elastic predictor was avoided to be negative or zero for the usage of exact elastic property. Eventually, the accuracy, robustness and efficiency of the proposed integration scheme were demonstrated by the numerical examples including the boundary value problems of the single integral point and multi-elements of geotechnical engineerings, which was respectively compiled as the single integral point program and the user defined subroutine (UMAT) of the commercial finite element program ABAQUS. [ABSTRACT FROM AUTHOR]

This paper presents a semi-analytical solution for undrained cylindrical cavity expansion in modified Cam-clay soils under biaxial in-situ stresses based on the small strain and large strain assumptions in the elastic and plastic regions, respectively. The cylindrical cavity expansion problem is treated as a boundary value problem and formulated by solving a system of first-order differential equations with four stress components as basic unknown variables. The validity of the proposed solution is examined by comparing with the existing solution, where the in-situ stress is uniform. Extensive parametric studies are conducted to investigate the effects of biaxial in-situ stresses on the stress distribution, the expansion process and the shape and size of elastic-plastic boundary. The results indicate that the elastic-plastic boundary is in the shape of an ellipse due to the shear stress induced by biaxial in-situ stresses. The major axis of the elliptical elastic-plastic boundary is in accordance with the direction of the larger in-situ stress. The present solution is free of the limitation that the cavity expands under the uniform in-situ stress, therefore it is expected to provide a more general method for practical geotechnical and petroleum problems. [ABSTRACT FROM AUTHOR]

In this paper, an original J 2 -deformation type model coupled with state-dependent dilatancy is further extended to a multiaxial stress space. Compared with most existing models, the proposed model only requires ten parameters, which can be easily calibrated, and does not involve any abstruse concepts in elastoplasticity theory. The model is then implemented into ABAQUS through a user-defined subroutine (UMAT), and the capability of the extended model in capturing the mechanical behavior of sand is demonstrated by element tests, including conventional triaxial tests and true triaxial tests. To demonstrate the robustness of the model, two typical boundary value problems are illustrated, i.e., the biaxial compression test and the strip footing test. The model is further incorporated with anisotropic critical state theory to simulate anisotropic soil behavior. The biaxial compression test with smooth and rough boundaries is simulated with the anisotropic model. The results confirm the impact of fabric anisotropy and boundary constraints on the shear strain distributions and patterns of shear bands. The simulation results show the capability of the proposed model, which can be applied in practical analyses and geotechnical engineering designs. [ABSTRACT FROM AUTHOR]

Triaxial compression experiments on dry Ottawa sand are numerically simulated using Finite Discrete Element Method (FDEM) in conjunction with X-ray CT imaging. A novel framework is introduced and validated where particle shape is captured in FEM simulations using shell elements for simulating triaxial boundary value problem. The proposed approach is well suited for load cases with low/intermediate confining pressures where particle breakage is negligible. The numerical model allows for grain deformation as well as robust particle contact fabric evolution for the realistic grain morphology. For this research, the confining latex membrane is modelled as hyper-elastic material to provide flexible boundary condition on the lateral surface of the cylindrical sand specimen. Volume change measurements are obtained directly based on the convex hull of an assembly of sand grains, with no contributions or associated errors from latex membrane effects. In this paper, we attempt to explore the FDEM modeling aspect from different perspectives considering macroscale and microscale response. The influence of the frictional and frictionless boundary on the strength deformation response and localization in displacement field is discussed. Furthermore, sensitivity of mesh refinement, interparticle friction coefficient and critical shear stress on strength and volume change response is quantified. [ABSTRACT FROM AUTHOR]

The paper focuses on the rapid drawdown of earth dams in the hypothesis that a prompt reservoir lowering is needed soon after a strong earthquake. The interest on such a topic comes from the fact that in Italy and worldwide there are many large dams placed very close to active faults. In this case, the dam may be asked to face the earthquake first and the rapid drawdown later to face an emergency state. The problem has been numerically investigated with reference to a zoned earth dam (Campolattaro Dam) placed in a highly seismic area of Southern Italy. To solve the overall boundary value problem, encompassing several stages of the dam lifetime, a 2D finite difference model has been solved through the FLAC2D code, implementing a coupled transient seepage formulation, which accounts for partial saturation of the soil during the drawdown stage. Unlike past literature studies, in which the drawdown stage was modelled without accounting for the past loading history of the dam, in the performed simulation the drawdown stage was placed at the end of a quite real sequence of events, encompassing the embankment construction, the reservoir impounding, and earthquake scenarios compatible with the dam site seismic hazard. The performed analyses pointed out that a seismic stage previously experienced by the dam could further contribute to decreasing dam stability during the drawdown especially during the first stages of reservoir lowering. In addition, the faster the drawdown, the smaller the dam safety factor against stability (FOS) with more prominent effects of the initial (pre-drawdown) soil conditions. [ABSTRACT FROM AUTHOR]