Showing papers in "International Journal for Numerical and Analytical Methods in Geomechanics in 2011"
TL;DR: In this paper, two rotational failure mechanisms are proposed for both the active and passive cases, and an extension of the proposed collapse mechanism to include a tension cut-off in the classical Mohr-Coulomb failure criterion is presented and discussed.
Abstract: The aim of this paper is to determine the collapse and blow-out face pressures of a circular tunnel driven by a pressurized shield. The analysis is performed in the framework of the kinematical approach of the limit analysis theory. Two rotational failure mechanisms are proposed for the active and passive cases. These mechanisms have two significant advantages with respect to the available ones: (i) they take into account the entire circular tunnel face instead of an inscribed ellipse to this circular area, and (ii) they are more consistent with the rotational rigid-block movement observed in the experimental tests. For both the active and passive cases, the three-dimensional failure surface was generated 'point by point' instead of simple use of the existing standard geometric shapes such as cones or cylinders. This was achieved by employing a spatial discretization technique. The numerical results have shown that the present rotational mechanisms provide, in the case of frictional soils (with or without cohesion), a significant improvement with respect to the translational mechanisms. Finally, an extension of the proposed collapse mechanism to include a tension cut-off in the classical Mohr-Coulomb failure criterion is presented and discussed.
300 citations
TL;DR: In this paper, a 3D model of a lattice spring model was used to simulate dynamic failure in a 3-dimensional model of an elastic lattice model with 3D lattice springs.
Abstract: Keywords: 3D model ; lattice spring model ; microstructure ; dynamic failure ; Numerical-Model ; Particle Model ; Fracture Model ; Concrete ; Composites ; Defects ; Rock ; Simulations ; Aggregate ; Behavior Reference GEOLEP-ARTICLE-2009-013doi:10.1002/nag.930View record in Web of Science Record created on 2009-05-08, modified on 2017-12-10
245 citations
TL;DR: In this paper, the authors used the discrete element method (DEM) to investigate the evolution of fabrics of virtual granular materials consisting of 2D elongated particles, and found that a common ultimate/critical state is eventually reached by all specimens regardless of their initial states.
Abstract: In an effort to study the relation of fabrics to the critical states of granular aggregates, the discrete element method (DEM) is used to investigate the evolution of fabrics of virtual granular materials consisting of 2D elongated particles. Specimens with a great variety of initial fabrics in terms of void ratios, preferred particle orientations, and intensities of fabric anisotropy were fabricated and tested with direct shear and biaxial compression tests. During loading of a typical specimen, deformation naturally localizes within shear bands while the remaining of the sample stops deforming. Thus, studying the evolution of fabric requires performing continuous local fabric measurements inside these bands, a suitable task for the proposed DEM methodology. It is found that a common ultimate/critical state is eventually reached by all specimens regardless of their initial states. The ultimate/critical state is characterized by a critical void ratio e which depends on the mean stress p, while the other critical state fabric variables related to particle orientations are largely independent of p. These findings confirm the uniqueness of the critical state line in the e − p space, and show that the critical state itself is necessarily anisotropic. Additional findings include the following: (1) shear bands are highly heterogeneous and critical states exist only in a statistical sense; (2) critical states can only be reached at very large local shear deformations, which are not always obtained by biaxial compression tests (both physical and numerical); (3) the fabric evolution processes are very complex and highly dependent on the initial fabrics. Copyright © 2010 John Wiley & Sons, Ltd.
144 citations
TL;DR: In this article, the initial conditions and the development of suction due to desiccation and varying material properties were inputted to UDEC, a distinct element code, using its internal programming language FISH.
Abstract: The ability to model and predict the formation of desiccation cracks is potentially beneficial in many applications such as clay liner design, earth dam construction, and crop science, etc. However, most studies have focused on statistical analysis of crack patterns and qualitative study of contributing factors to crack development rather than prediction. Because it is exceedingly difficult to capture the nonlinear processes during desiccation in analytical modelling, most such models handle crack formation without considering variation of material properties with time, and are unattractive to use in realistic modelling. The data obtained from laboratory experiments on clay soil desiccating in moulds were used as a basis to develop a more refined model of desiccation cracking. In this study, the properties, such as matric suction, stiffness and tensile strength of soil, and base adhesion, could be expressed approximately as functions of moisture content. The initial conditions and the development of suction due to desiccation and the varying material properties were inputted to UDEC, a distinct element code, using its internal programming language FISH. The model was able to capture some essential physical aspects of crack evolution in soil contained in moulds with varying lengths, heights, and materials of construction. Extension of this methodology is potentially beneficial not only for modelling desiccation cracking in clay, but also in other systems with evolving material properties such as concrete structures and road pavements. Copyright © 2010 John Wiley & Sons, Ltd.
102 citations
TL;DR: In this article, the inclination angle of the bedding plane with respect to the shear plane was used as a reference geometrical descriptor of inherent fabric anisotropy.
Abstract: This paper investigates shear strength of granular materials with inherent fabric anisotropy. Most previous studies have described strength of these materials in the principal stress space, and the orientation of the bedding plane with respect to the principal stress directions was used as the reference geometrical descriptor of inherent fabric. The present study has found that it is theoretically more convenient and practically more useful to use instead the inclination angle of the bedding plane with respect to the shear plane for the same purpose. Direct shear tests and biaxial compression tests with different loading directions with respect to the bedding planes were simulated with discrete element method (DEM) models consisting of ellipse-shaped particles. Key mechanical behaviors of natural sands reported in the literature were successfully captured in the numerical simulation. A shear failure criterion was determined as a function of the inclination angle based on the direct shear simulation results, and was used to successfully predict the results of the biaxial compression simulations. Microstructural inspection of deformation and strain localization of the biaxial compression simulations found that the proposed shear failure criterion can reasonably predict the orientations of the initial failure planes. It was also discovered that shear bands in directions conjugate to the initial failure plane orientations can develop and dominate specimen deformation at larger strain levels. Considering the availability of biaxial compression test equipment and historical data, two methods for back-calculating inclination angle-dependent shear strength from biaxial compression results were proposed, and validated using DEM simulation results. Copyright © 2010 John Wiley & Sons, Ltd.
97 citations
TL;DR: In this paper, the authors present a synthesis of the works performed by various teams from France, Italy and Canada around the question of second-order work criterion in geomechanics.
Abstract: This paper presents a synthesis of the works performed by various teams from France, Italy and Canada around the question of second‐order work criterion. Because of the non‐associative character of geomaterials plastic strains, it is now recognized that a whole bifurcation domain exists in the stress space with various possible modes of failure. In a first part these failure modes are observed in lab experimental tests and in discrete element modelling. Then a theoretical study of second‐order work allows to establish a link with the kinetic energy, giving a basis to explain the transition from a prefailure (quasi)static regime to a postfailure dynamic regime. Eventually the main features of geomaterials failure are obtained by applying second‐order work criterion to five different constitutive rate‐independent models—three being phenomenological and two micromechanical. As a whole this paper tries to gather together all the elements for a proper understanding and use of second‐order work criterion in geomechanics.
97 citations
TL;DR: In this article, a 3D smooth particle hydrodynamics (SPH) simulator for modeling grain scale fluid flow in porous media is presented, and the accuracy of results for such low Reynolds number flows is highly dependent on the implementation of no-slip boundary conditions.
Abstract: In this paper, a three-dimensional smooth particle hydrodynamics (SPH) simulator for modeling grain scale fluid flow in porous media is presented. The versatility of the SPH method has driven its use in increasingly complex areas of flow analysis, including the characterization of flow through permeable rock for both groundwater and petroleum reservoir research. SPH provides the means to model complex multi-phase flows through such media; however, acceptance of the methodology has been hampered by the apparent lack of actual verification within the literature, particulary in the three-dimensional case. In this paper, the accuracy of SPH is addressed via a comparison to the previously recognized benchmarks of authors such as Sangani and Acrivos (Int. J. Multiphase Flow 1982; 8(4): 343–360), Zick and Homsy (J. Fluid Mech. 1982; 115:13–26) and Larson and Higdon (Phys. Fluids A 1989; 1(1):38–46) for the well-defined classical problems of flow through idealized two- and three-dimensional porous media. The accuracy of results for such low Reynolds number flows is highly dependent on the implementation of no-slip boundary conditions. A new, robust and numerically efficient, method for implementing such boundaries in SPH is presented. Simulation results for friction coefficient and permeability are shown to agree well with the available benchmarks.
94 citations
TL;DR: In this paper, the Lattice Boltzmann method (LBM) has been used to study the effects of permeability and tortuosity on flow through saturated particulate media and identify the relationships between permeability with other parameters such as particles diameter, grain specific surface, and porosity.
Abstract: In this paper, Lattice Boltzmann method (LBM) has been used to study the effects of permeability and tortuosity on flow through saturated particulate media and identify the relationships between permeability and tortuosity with other parameters such as particles diameter, grain specific surface, and porosity. LBM is a simple kinematic model that can incorporate the essential physics of microscopic and mesoscopic processes involved in flow through granular soils.
The obtained results indicate that the 2D LB model, due to its inherent theoretical advantages, is capable of demonstrating that the porosity and specific surface are the most influential parameters in determining the intrinsic permeability of granular media. The obtained results show that particles' size diameter has a two-fold effect on the coefficient of permeability: one is through specific surface and the other is by tortuosity factor. Numerical study also reveals that tortuosity of granular soils decreases almost linearly with increasing the porosity. Copyright © 2010 John Wiley & Sons, Ltd.
92 citations
TL;DR: In this article, an interaggregate contact law is introduced relating contact forces to aggregates relative displacements, and the deformation of the assembly can be obtained by integrating the movement of the inter-aggregate contacts in all orientations.
Abstract: This paper examines the mechanical behavior of a sensitive marine clay. Various laboratory tests on intact and reconstituted samples of Guinea Gulf marine clay were performed under isotropic compression and drained triaxial compression at constant confining stresses. Microstructure analysis on intact and reconstituted samples was also carried out under different loading conditions. The effect of inter-aggregates bonding on mechanical properties is discussed. Based on experimental analysis, a new modeling method is proposed. In this approach, the clay is regarded as an assembly of aggregates of clay particles. An inter-aggregate contact law is introduced relating contact forces to aggregates relative displacements. The deformation of the assembly can be obtained by integrating the movement of the inter-aggregate contacts in all orientations. Thus, the effect of inter-aggregates bonds and debonding is considered in a direct way. The model is evaluated through comparisons between the predicted and measured results on Guinea Gulf marine clay. The evolutions of local stresses, strains, and bonds in inter-aggregates planes are discussed to explain the anisotropy induced by the applied loading. Copyright © 2010 John Wiley & Sons, Ltd.
87 citations
TL;DR: By using the lower bound limit analysis in conjunction with finite elements and linear programming, the bearing capacity factors due to cohesion, surcharge and unit weight, respectively, have been computed for a circular footing with different values of phi.
Abstract: By using the lower bound limit analysis in conjunction with finite elements and linear programming, the bearing capacity factors due to cohesion, surcharge and unit weight, respectively, have been computed for a circular footing with different values of phi. The recent axisymmetric formulation proposed by the authors under phi = 0 condition, which is based on the concept that the magnitude of the hoop stress (sigma(theta)) remains closer to the least compressive normal stress (sigma(3)), is extended for a general c-phi soil. The computational results are found to compare quite well with the available numerical results from literature. It is expected that the study will be useful for solving various axisymmetric geotechnical stability problems. Copyright (C) 2010 John Wiley & Sons, Ltd.
84 citations
TL;DR: In this paper, an inverse micromechanics approach was used to interpret nanoindentation results to deliver cohesivefrictional strength behavior of the porous clay binder phase in shale.
Abstract: An inverse micromechanics approach allows interpretation of nanoindentation results to deliver cohesivefrictional strength behavior of the porous clay binder phase in shale. A recently developed strength homogenization model, using the Linear Comparison Composite approach, considers porous clay as a granular material with a cohesive-frictional solid phase. This strength homogenization model is employed in a Limit Analysis Solver to study indentation hardness responses and develop scaling relationships for indentation hardness with clay packing density. Using an inverse approach for nanoindentation on a variety of shale materials gives estimates of packing density distributions within each shale and demonstrates that there exists shale-independent scaling relations of the cohesion and of the friction coefficient that vary with clay packing density. It is observed that the friction coefficient, which may be interpreted as a degree of pressure-sensitivity in strength, tends to zero as clay packing density increases to one. In contrast, cohesion reaches its highest value as clay packing density increases to one. The physical origins of these phenomena are discussed, and related to fractal packing of these nanogranular materials. Copyright 2010 John Wiley & Sons, Ltd.
TL;DR: In this article, the authors proposed a strain space multiple mechanism model to model the deformation of granular materials. But the model is not suitable for the modeling of macroscopic deformation and dilatancy of micromechanical components.
Abstract: A granular material consists of an assemblage of particles with contacts newly formed or disappeared, changing the micromechanical structures during macroscopic deformation. These structures are idealized through a strain space multiple mechanism model as a twofold structure consisting of a multitude of virtual two-dimensional mechanisms, each of which consists of a multitude of virtual simple shear mechanisms of one-dimensional nature. In particular, a second-order fabric tensor describes direct macroscopic stress–strain relationship, and a fourth-order fabric tensor describes incremental relationship. In this framework of modeling, the mechanism of interlocking defined as the energy less component of macroscopic strain provides an appropriate bridge between micromechanical and macroscopic dilative component of dilatancy. Another bridge for contractive component of dilatancy is provided through an obvious hypothesis on micromechanical counterparts being associated with virtual simple shear strain. It is also postulated that the dilatancy along the stress path beyond a line slightly above the phase transformation line is only due to the mechanism of interlocking and increment in dilatancy due to this interlocking eventually vanishing for a large shear strain. These classic postulates form the basis for formulating the dilatancy in the strain space multiple mechanism model. The performance of the proposed model is demonstrated through simulation of undrained behavior of sand under monotonic and cyclic loading. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, the authors present an overview of the stabilized procedures that in their view are most promising, including fluid pressure Laplacian stabilization (FPL), a stabilization that uses bubble functions to resolve the fine-scale solution within elements, and a method derived by using finite increment calculus (FIC).
Abstract: Numerical solutions for problems in coupled poromechanics suffer from spurious pressure oscillations when small time increments are used. This has prompted many researchers to develop methods to overcome these oscillations. In this paper, we present an overview of the methods that in our view are most promising. In particular we investigate several stabilized procedures, namely the fluid pressure Laplacian stabilization (FPL), a stabilization that uses bubble functions to resolve the fine-scale solution within elements, and a method derived by using finite increment calculus (FIC). On a simple one-dimensional test problem, we investigate stability of the three methods and show that the approach using bubble functions does not remove oscillations for all time step sizes. On the other hand, the analysis reveals that FIC stabilizes the pressure for all time step sizes, and it leads to a definition of the stabilization parameter in the case of the FPL-stabilization. Numerical tests in one and two dimensions on 4-noded bilinear and linear triangular elements confirm the effectiveness of both the FPL- and the FIC-stabilizations schemes for linear and nonlinear problems. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, the authors revisited the localized failure mode, replacing the well-known Rice criterion within the wider context of bifurcation, and established that localized failure is a particular case of failures observed within the so-called BIFURcation domain: the incremental strain within the localization band is associated with a vanishing value of the second order work.
Abstract: The concept of failure is one of the most debated in soil mechanics, for two reasons essentially. First, this is a crucial issue in the engineering of structures and geotechnical project design. Second, this is still a challenging academic issue mobilizing significant scientific interest in the development of a unique framework to describe the different failure modes. In this respect, this paper revisits the localized failure mode, replacing the well-known Rice criterion within the wider context of bifurcation. Considering a micro-mechanical model, the main theoretical results are covered. In particular, it is established that localized failure is a particular case of failures observed within the so-called bifurcation domain: the incremental strain within the localization band is associated with a vanishing value of the second-order work. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, finite element methods are used to predict failure in long slope profiles where the critical mechanism is not necessarily at the base of the soil layer, and the influence of slope angle is also examined in long slopes.
Abstract: Interest in the mechanics of landslides has led to renewed evaluation of the infinite slope equations, and the need for a more general framework for estimating the factor of safety of long and infinite slopes involving non-homogeneous soil profiles. The paper describes finite element methods that demonstrate the potential for predicting failure in long slope profiles where the critical mechanism is not necessarily at the base of the soil layer. The influence of slope angle is also examined in long slopes, leading to some counter-intuitive conclusions about the impact of slope steepness on the factor of safety. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, a discrete-based coupling method for problems in porous media is proposed and compared to idealized packed assemblies and results are presented for simulations of steady flow through porous media.
Abstract: Some of the most challenging problems in geomechanics involve the coupling between fluid flow and solid deformation. In this paper we briefly present an overview of existing coupling methods to problems involving fluid flow and deformation and describe testing of a new discrete-based coupling method for problems in porous media. Modeled permeability and porosity distributions are compared to idealized packed assemblies and results are presented for simulations of steady flow through porous media. Previously published results for the coupled model focused on few or multiple particles in a fluid, whereas our results show good agreement to packed assemblies of particles (i.e. porous media). Finally, the two-dimensional model is applied to sand production, a common problem in geomechanics. Sand production is defined as the co-production of both a fluid and solid phase in oil and gas wells. In our models, we capture initial sand production associated with early-time drawdown. Later-time results show episodic sanding rates associated with formation stability and instabilities. Both observations are qualitatively consistent with laboratory and field observations. We find that high confining pressure inhibits the production of sand, through elevated interparticle contact forces. It is argued that these physically based models have a use in testing and evaluating competing hypotheses of sand production but their applicability is currently limited to small spatial and temporal scales. We believe these models bridge an important gap between the underlying physics of micro-mechanical interactions of fluid and solid grains and the continuum descriptions of those systems. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, a generalized plasticity state parameter-based model is proposed to reproduce the hydro-mechanical behavior of unsaturated soils, which is based on two pairs of stress-strain variables and a suitable hardening law taking into account the bonding-debonding effect of suction and degree of saturation.
Abstract: The aim of this paper is to extend the generalized plasticity state parameter-based model presented in part 1 to reproduce the hydro-mechanical behavior of unsaturated soils. The proposed model is based on two pairs of stress–strain variables and a suitable hardening law taking into account the bonding—debonding effect of suction and degree of saturation. A generalized state parameter for unsaturated state is proposed to reproduce soil behavior using a single set of material parameters. Generalized plasticity gives a suitable framework to reproduce not only monotonic stress path but also cyclic behavior. The hydraulic hysteresis during a drying—wetting cycle and the void ratio effect on the hydraulic behavior is introduced. Comparison between model simulations and a series of experimental data available, both cohesive and granular, are given to illustrate the accuracy of the enhanced generalized plasticity equation. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: By extending Darcy's law to the dry domain above the free surface and specifying the boundary condition on the potential seepage surfaces as Signorini's type, a Partial Differential Equation (PDE) defined in the entire domain of interest is formulated for non-steady flow problems with free surfaces as discussed by the authors.
Abstract: By extending Darcy's law to the dry domain above the free surface and specifying the boundary condition on the potential seepage surfaces as Signorini's type, a partial differential equation (PDE) defined in the entire domain of interest is formulated for non-steady seepage flow problems with free surfaces. A new parabolic variational inequality (PVI) formulation equivalent to the PDE formulation is then proposed, in which the flux part of the complementary condition of Signorini's type in the PDE formulation is transformed into natural boundary condition. Consequently, the singularity at the seepage points is eliminated and the difficulty in selecting the trial functions is significantly reduced. By introducing an adaptive penalized Heaviside function in the finite element analysis, the numerical stability of the discrete PVI formulation is well guaranteed. The proposed approach is validated by the existing laboratory tests with sudden rise and dropdown of water heads, and then applied to capture the non-steady seepage flow behaviors in a homogeneous rectangular dam with five drainage tunnels during a linear dropdown of upstream water head. The non-steady seepage flow in the surrounding rocks of the underground powerhouse in the Shuibuya Hydropower Project is further modeled, in which a complex seepage control system is involved. Comparisons with the in situ monitoring data show that the calculation results well illustrate the non-steady seepage flow process during impounding and the operation of the reservoir as well as the seepage control effects of the drainage hole arrays and drainage tunnels. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, the main ingredients of the generalized plasticity model, such as rule, loading-unloading discriminating direction and plastic modulus, are modified to include a dependency on the state parameter.
Abstract: The behavior of granular materials is known to depend on its loose or dense nature, which in turns depends both on density and confining pressure. Many models developed in the past require the use of different sets of constitutive parameters for the same material under different confining pressures. The purpose of this paper is to extend a basic generalized plasticity model for sands proposed by Pastor, Zienkiewicz and Chan by modifying the main ingredients of the model flow—rule, loading–unloading discriminating direction and plastic modulus—to include a dependency on the state parameter. The proposed model is tested against the available experimental data on three different sands, using for each of them a single set of material parameters, finding a reasonably good agreement between experiments and predictions. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, a numerical tool for detecting instabilities in elasto-plastic solids (with an emphasis on soils) and inserting a discontinuity at these instabilities allowing the boundary value problem to proceed beyond the instabilities.
Abstract: This paper is concerned with developing a numerical tool for detecting instabilities in elasto-plastic solids (with an emphasis on soils) and inserting a discontinuity at these instabilities allowing the boundary value problem to proceed beyond these instabilities. This consists of implementing an algorithm for detection of strong discontinuities within a finite element (FE) framework. These discontinuities are then inserted into the FE problem through the use of a displacement field enrichment technique called the extended finite element method (XFEM). The newly formed discontinuities are governed by a Mohr–Coulomb frictional law that is enforced by a penalty method. This implementation within an FE framework is then tested on a compressive soil block and a soil slope where the discontinuity is inserted and grown according to the localization detection. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, a macroelement model for shallow foundations encompassing the majority of combinations of soil and foundation-soil interface conditions that are interesting for practical applications is presented. But the model is not explicitly used in the formulation of the model, the obtained force states by the model are always contained within it.
Abstract: The scope of this paper is to present a macroelement model for shallow foundations encompassing the majority of combinations of soil and foundation–soil interface conditions that are interesting for practical applications. The basic idea of the formulation is to raise the common assumption that the surface of ultimate loads of the foundation is identified as a yield surface in the space of force parameters which the footing is subjected to. Instead, each non-linear mechanism participating in the global response of the system is modelled independently and the surface of ultimate loads is retrieved as the combined result of all active mechanisms. This allows formulating each mechanism by respecting its particular characteristics and offers the possibility of activating, modifying or deactivating each mechanism according to the context of application. The model comprises three non-linear mechanisms: (a) the mechanism of sliding at the soil–footing interface, (b) the mechanism of soil yielding in the vicinity of the footing and (c) the mechanism of uplift as the footing may get detached from the soil. The first two are irreversible and dissipative and are combined within a multi-mechanism plasticity formulation. The third mechanism is reversible and non-dissipative. It is reproduced with a phenomenological non-linear hyperelastic model. The model is validated with respect to the existing results for shallow foundations under quasi-static loading tests. It is shown that although the ultimate surface of the foundation is not explicitly used in the formulation of the model, the obtained force states by the model are always contained within it. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, the effect of relative density, initial confinement and number of jacking strokes on the unit shaft resistance of piles jacked into sand is studied using one-dimensional finite element analysis.
Abstract: In recent years, pile jacking has become a viable alternative installation method for displacement piles. Pile jacking produces minimal noise, vibration and air pollution during installation. In addition, it is possible, at the end of jacking, to have a good estimate of the ultimate static capacity of the pile. In this paper, the shaft resistance of piles jacked into sand is studied using one-dimensional finite element analysis. The finite element simulations, using a two-surface plasticity model, demonstrate the effects of relative density and confinement on the unit shaft resistance of piles jacked in sand. The impact of the number of jacking strokes on the unit shaft capacity is also assessed. Based on the numerical results, we developed equations for shaft resistance quantifying the effects of relative density, initial confinement and number of jacking strokes. Predictions using these equations are compared with data obtained from centrifuge tests and field tests. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this article, a new thermo-mechanical model is developed, applicable to large-scale, deep-seated landslides consisting of a coherent mass sliding on a thin clayey layer.
Abstract: In this work, a new thermo-mechanical model is developed, applicable to large-scale, deep-seated landslides consisting of a coherent mass sliding on a thin clayey layer The considered time window is that of catastrophic acceleration, starting at incipient failure and ending when the acquired displacement and velocity are such that the sliding material begins to break up into pieces The model accounts for temperature rise in the slip zone due to the heat produced by friction, leading to water expansion, thermoplastic collapse of the soil skeleton, and subsequent increase of pore water pressure The model incorporates the processes of heat production and diffusion, pore pressure generation and diffusion, and an advanced constitutive law for the thermo-mechanical behavior of soil An analysis of the Vajont landslide is presented as an example A sensitivity analysis shows that friction softening is the mechanism most affecting the timescale of the final collapse of a slide, but also that the mechanism of thermal pressurization alone can cause a comparably catastrophic dynamic evolution It is also shown that, all other factors being equal, thermo-mechanical collapse will cause thicker slides to accelerate faster than shallow ones
TL;DR: In this paper, a virtual multidimensional internal bond (VMIB) model was developed to simulate the propagation of hydraulic fractures using the finite-element method, and the results showed the method to be very effective.
Abstract: A virtual multidimensional internal bond (VMIB) model developed to simulate the propagation of hydraulic fractures using the finite-element method is formulated within the framework of the virtual internal bond theory (VIB) that considers a solid as randomized material particles in the micro scale, and derives the macro constitutive relation from the cohesive law between the material particles with an implicit fracture criterion. Hydraulic pressure is applied using a new scheme that enables simulation of hydraulically driven cracks. When the model is applied to study hydraulic fracture propagation in the presence of a natural fracture, the results show the method to be very effective. It shows that although the in situ stress ratio is the dominant factor governing the propagation direction, a natural fault can also strongly influence the hydraulic fracture behavior. This influence is conditioned by the shear stiffness of the fault and the distance to the original hydraulic fracture. The model results show that when the fault is strong in shear, its impact on hydraulic fracture trajectory is weak and the hydraulic fracture will likely penetrate the fault. For a weak fault, however, the fracture tends to be arrested at the natural fault. The distance between the fault and the hydraulic fracture is also important; the fault influence increases with decreasing distance. The VMIB does not require selection of a fracture criterion and remeshing when the fracture propagates. Therefore, it is advantageous for modeling fracture initiation and propagation in naturally fractured rock. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this article, a fixed grid algorithm was proposed to simulate the propagation of shallow hydraulic fractures, under plane strain or axisymmetric conditions, by combining an implicit finite volume method and the displacement discontinuity method for solving the elasticity equation.
Abstract: This paper describes a fixed grid algorithm to simulate the propagation of shallow hydraulic fractures, under plane strain or axisymmetric conditions. Because of the low stress environment that exists near a free surface, these fractures are generally characterized by a fluid front that lags behind the fracture edge. The simulation of shallow hydraulic fractures requires therefore the tracking of two distinct fronts. The proposed algorithm, which traces its roots to the one described by Zhang et al. (Int. J. Numer. Anal. Methods Geomech. 2005; 29:1317–1340), advances the fracture by a constant step and computes the corresponding time required to reach this new fracture configuration as well as the matching location of the fluid front within the fixed grid. For any trial value of the time and of the front position, a non-linear system of algebraic equations is solved using either Newton's method or a fixed point iteration scheme, with preconditioning. The non-linear system is formulated by combining an implicit finite volume method for solving the lubrication equation and the displacement discontinuity method for solving the elasticity equation. The critical difference with the algorithm of Zhang et al. lays in the approach chosen to update the fluid front position. Rather than using a filling fraction method, the new algorithm adopts a velocity treatment of the fluid-front location that is akin to a one-dimensional implementation of a level set algorithm for front tracking. This conversion from a fluid volume to a fluid velocity-based approach to update the front position increases the degree of implicitness of the algorithm, which is responsible for a significant reduction of the CPU time by about two orders of magnitude. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this article, an analysis of individual and collective force chain bucklings is undertaken using data from a discrete element simulation. And the results from the simulation are used to guide the development of a lattice model of collective, localized force chain failure by buckling.
Abstract: The evolution of internal structure plays a pivotal role in the macroscopic response of granular materials to applied loads. A case in point is the so-called 'stress-dilatancy relation', a cornerstone of Soil Mechanics. Numerous attempts have been made to unravel the connection between stress-dilatancy and the evolution of fabric and contact forces in a deforming granular medium. We re-examine this connection in light of the recent findings on force chain evolution, in particular, that of collective force chain failure by buckling. This study is focussed on two-dimensional deformation of dense granular assemblies. Analysis of individual and collective force chain bucklings is undertaken using data from a discrete element simulation. It is shown that the kinematics of force chain buckling lead to significant levels of local dilatation being developed in the buckling force chain particles and their confining first-ring neighbors. Findings from the simulation are used to guide the development of a lattice model of collective, localized force chain buckling. Consideration of the statics and kinematics of this process yields a new stress-dilatancy relation. The physics of buckling, even at its simplest form, introduces a richness into the stress-dilatancy formulation in a way that preserves the essential aspects of fabric evolution, specifically the buckling mode.
TL;DR: In this article, the concept of test controllability is extended to a hydro-mechanically coupled framework, in order to better describe suction effects on the mechanical behaviour of soil.
Abstract: The paper presents a theoretical approach to deal with mechanical instabilities in unsaturated soils. Towards this goal, the concept of test controllability is extended to a hydro-mechanically coupled framework. A constitutive approach based on the introduction of hydraulic generalized stress–strain variables is first adopted, in order to better describe suction effects on the mechanical behaviour of soil. The mathematical consequences of hydro-mechanical coupling are presented next and two indices are defined to identify the onset of an instability. Possible instability modes linked to saturation processes are discussed. It is shown that the way in which the hydraulic variables are controlled in tests on unsaturated soil specimens is the key factor for the possible occurrence of instabilities and the consequent loss of test controllability. It is shown in particular that unsaturated soil specimens are prone to instability when an externally controlled water flux is injected into the specimen (inundation). This result could in part explain the sudden collapse of soil specimens subjected to wetting under constant applied stresses, which is observed both in the laboratory and in the field. Copyright © 2009 John Wiley & Sons, Ltd.
TL;DR: In this article, a three-phase medium model is proposed in describing the dynamic property of filled rock joints and an analytical study on longitudinal wave transmission normally across a three phase rock joint is presented.
Abstract: A three-phase medium model is proposed in describing the dynamic property of filled rock joints and an analytical study on longitudinal wave transmission normally across a three-phase rock joint is presented. Parameters in the three-phase medium model were determined by a series of modified split Hopkinson pressure bar (SHPB) tests, where a sand or clay layer was used to represent an artificially filled rock joint. The effect of the unloading path on the transmitted wave was discussed by comparing the analytical and SHPB test results. The derived wave transmission coefficients across the filled joint agreed very well with those from the test results. Both the analytical and the test results showed that the wave transmission coefficients were affected by the mechanical properties of the fillings. Parametric studies with respect to the volume ratios of water and air in the three-phase medium and the type of filling material have also been performed. Copyright (C) 2010 John Wiley & Sons, Ltd.
TL;DR: In this paper, the authors analyzed the relationship between the behavior of a clay material at the macroscopic scale and its microfabric evolution and proposed an approach based on the study of clay particles orientation by SEM picture analysis after different phases of triaxial loading.
Abstract: The goal of this study was to analyze the relation between the behaviour of a clayey material at the macroscopic scale and its microfabric evolution. This may lead to a better understanding of macroscopic strain mechanisms especially the contractancy and dilatancy phenomena. The approach proposed in this paper is based on the study of clay particles orientation by SEM picture analysis after different phases of triaxial loading. In the initial state of the samples (one-dimensional compression), the SEM observations highlight a microstructural anisotropy with a preferential orientation of the particles normal to the loading direction. During isotropic loading, densification of the clayey matrix occurs related to a random orientation of particles indicated by the term ‘depolarization’. In the earlier stages of constant σ3 drained triaxial path on slightly overconsolidated specimens, the microstructural depolarization seems to persist inside a macroscopic domain, in which only the volumetric strains due to the isotropic part of the stress tensor evolve. Then, a rotation mechanism of the particles towards preferred directions seems to be activated. The phenomenon appears directly linked to the evolution of the deviatoric part of the stress tensor. Copyright © 2010 John Wiley & Sons, Ltd.
TL;DR: In this article, a mathematical model for the analysis of dynamic fracture propagation in the saturated porous media is presented, which incorporates a discrete cohesive fracture model, coupled with the flow in porous media through the fracture network.
Abstract: In this paper, a mathematical model is presented for the analysis of dynamic fracture propagation in the saturated porous media. The solid behavior incorporates a discrete cohesive fracture model, coupled with the flow in porous media through the fracture network. The double-nodded zero-thickness cohesive interface element is employed for the mixed mode fracture behavior in tension and contact behavior in compression. The crack is automatically detected and propagated perpendicular to the maximum effective stress. The spatial discretization is continuously updated during the crack propagation. Numerical examples from the hydraulic fracturing test and the concrete gravity dam show the capability of the model to simulate dynamic fracture propagation. The comparison is performed between the quasi-static and fully dynamic solutions, and the performance of two analyses is investigated on the values of crack length and crack mouth opening. Copyright © 2010 John Wiley & Sons, Ltd.