Numerical investigation of lateral earth pressure for unreinforced masonry walls

For the design of unreinforced masonry walls under lateral earth pressure according to DIN EN 1996‐3 [1], the active earth pressure is used, which is less than the earth pressure at rest. For the consideration of active earth pressure, a sufficient deflection of the wall is needed. It is unknown whether the deflections in reality are large enough to justify a reduction of the active earth pressure. Therefore a numerical model has been developed which considers the load‐bearing behaviour of masonry walls, with several boundary conditions being considered to estimate the effective earth pressure.     

Discrete element method investigation on thermally-induced shakedown of granular materials

Temperature change, as a common kind of internal perturbation performed on granular materials, has a significant effect on the bulk properties of granular materials. However, few studies on thermally-induced shakedown under a long-term thermal cycling were reported. In this work, the discrete element method was used to give insight into the thermally-induced shakedown on the fabric and stress states within non-cohesive, frictional granular assemblies. Assemblies were submitted to thermal cycling at a stationary boundary condition after experiencing a one-dimensional compression. Evolution of coordination number, entropy and anisotropy was investigated as well as boundary forces and contact forces. At the same time, effects of the heating rate, the initial vertical load and the magnitude of temperature change were examined. It demonstrates that thermal cycling induces a significant force relaxation within granular materials, while the corresponding granular fabric has a small change. In addition, the entropy and anisotropy decreases with thermal cycling. Moreover, the initial vertical load can constrain the development of thermally-induced fabric change, thereby limiting force relaxation to some degree. Both high heating rate and larger magnitudes of temperature change contribute to more significant force relaxation. However, they cause smaller fabric changes even though they provide larger perturbations.     

An investigation on loose cemented granular materials via DEM analyses

This paper presents the results of a numerical study carried out by 2D discrete element method analyses on the mechanical behavior and strain localization of loose cemented granular materials. Bonds between particles were modeled in order to replicate the mechanical behavior observed in a series of laboratory tests performed on pairs of glued aluminum rods which can fail either in tension or shear (Jiang et al. in Mech Mater 55:1–15, 2012). This bond model was implemented in a DEM code and a series of biaxial compression tests employing lateral flexible boundaries were performed. The influence of bond strength and confinement levels on the mechanical behavior and on the onset of shear bands and their propagation within the specimens were investigated. Comparisons were also drawn with other bond models from the literature. A new dimensionless parameter incorporating the effects of both bond strength and confining pressure, called BS, was defined. The simulations show that shear strength and also dilation increase with the level of bond strength. It was found out that for increasing bond strength, shear bands become thinner and oriented along directions with a higher angle over the horizontal. It also emerged that the onset of localization coincided with the occurrence of bond breakages concentrated in some zones of the specimens. The occurrence of strain localization was associated with a concentration of bonds failing in tension.     

DEM investigation of particle anti-rotation effects on the micromechanical response of granular materials

The importance of particle rotation to the mechanical behavior of granular materials subject to quasi-static shearing has been well recognized in the literature. Although the physical source of the resistance to particle rotation is known to lie in the particle surface topography, it has been conveniently studied using the rolling resistance model installed typically on spherical particles within the DEM community. However, there has been little effort on assessing the capability of the rolling resistance model to produce more realistic particle rotation behavior as exhibited by irregular-shaped particles. This paper aims to eliminate this deficiency by making a comprehensive comparison study on the micromechanical behavior of assemblies of irregular-shaped particles and spherical particles installed with the rolling resistance model. A variety of DEM analysis techniques have been applied to elucidate the full picture of micromechanical processes occurring in the two types of granular materials with different particle-level anti-rotation mechanisms. Simulation results show that the conventional rheology-type rolling resistance models cannot reproduce the particle rotation and strain localization behavior as displayed by irregular-shaped materials, although they demonstrate clear effects on the macroscopic strength and dilatancy behavior, as have been adequately documented in the literature. More insights into the effects of particle-level anti-rotation mechanism are gained from an in-depth inter-particle energy dissipation analysis.     

Numerical simulation of granular materials by an improved discrete element method

In this paper, we present an improved discrete element method based on the non‐smooth contact dynamics and the bi‐potential concept. The energy dissipated during the collisions is taken into account by means of restitution coefficients. The interaction between particles is modelled by Coulomb unilateral contact law with dry friction which is typically non‐associated: during the contact, the sliding vector is not normal to the friction cone. The main feature of our algorithm is to overcome this difficulty by means of the bi‐potential theory. It leads to an easy implement predictor–corrector scheme involving just an orthogonal projection onto the friction cone. Moreover the convergence test is based on an error estimator in constitutive law using the corner stone inequality of the bipotential. Then we present numerical simulations which show the robustness of our algorithm and the various possibilities of the software ‘MULTICOR’ developed with this approach. Copyright © 2004 John Wiley & Sons, Ltd.     

Mixtures of granular materials

Balance laws are given for a mixture of granular materials of a type described by Goodman and Cowin. Constitutive equations are given for the case of two dry granular constituents, and consequences of the entropy principle are found.     

Creep of granular materials

This paper examines the creep of brittle granular materials subjected to one-dimensional compression. One-dimensional creep tests were performed on aggregates of brittle pasta and compared with the behaviour of sand at much higher stress levels. It was found that for both materials, creep strain is proportional to the logarithm of time. One possible mechanism for creep is particle crushing. However, it is usually difficult to measure changes in the particle size distribution during creep because the fines produced are so small, and the mass of fines is too small to measure accurately unless creep is permitted for a very long time. However, for pasta, the particle fragments produced are large, and it is found that particle crushing does occur during creep for 24 hours. This is consistent with the proposition that the behaviour of all brittle granular materials is essentially the same. A micro mechanical argument is then summarised which predicts that creep strain should be proportional to log time.     

Evolution of at-rest lateral stress for cemented sands: experimental and numerical investigation

We present measurements of the evolution of the at-rest lateral stress coefficient K ₀ for cemented sands in a modified oedometer and provide additional insights into material response using discrete element method (DEM) simulations. A new scheme for the measurement of K ₀ is adapted to obtain horizontal stress for the entire stress history with parallel measurement of shear wave velocity. Results show that the horizontal stress of uncemented sand linearly increases, while debonding in cemented sands is characterized by a non-linear evolution of horizontal stress. Cement content governs the stress regime in which decementation initiates. The at-rest lateral stress coefficient of cemented sands increases during decementation, resulting in higher values for overconsolidated specimens. The recovery of K ₀ values is manifested at the preconsolidation stress during reloading. Cemented sands collapse followed by decementation and subsequent changes in K ₀ values. The DEM simulations reasonably reproduce laboratory specimen-scale response and are used to highlight the evolution of particle contact force, gradual debonding of cement, and the formation of a blocky structure in cemented sands at the particle-scale. These observations are consistent with inferred response of physical specimens at the particle scale, yet this behavior is not directly observable in the laboratory, highlighting the particular effectiveness of an integrated physical-numerical investigation. The interparticle contact stiffness of cemented sands controls the evolution of horizontal stress at low vertical stress, and the decementation causes the convergence of K ₀ values towards those of uncemented sands at high vertical stress.     

The effects of an impact velocity dependent coefficient of restitution on stresses developed by sheared granular materials

Summary Following the granular flow kinetic theory of Lun, Savage, Jeffrey and Chepurniy, a moment method is used to obtain the approximate form for the single particle velocity distribution function for the case of smooth, slightly inelastic, uniform spherical particles in which the coefficient of restitutione depends upon the particle impact velocity. Constitutive equations for stress are derived and the theory is applied to the case of a simple shear flow. Theoretical predictions of stresses are compared with experimental results. The effect of the impact velocity dependente is to cause the stresses to vary with the shear rate raised to a power less than two; this is consistent with the experimental observations. On the basis of the present theory and comparisons with experimental data it is concluded that theoretical models which include both surface friction and an impact velocity dependente will lead to improved agreement between the theoretical predictions and the measurements.With 12 Figures     

Asymptotic behaviour of granular materials

The concept of the asymptotic behaviour of particulate materials is described, including its enhancement by considering asymptotic states in extension. A 3D discrete element model with elastic spherical particles and the granulometry of a real sand is set up. The numerical sample is stretched from different initial states, and the influence of the strain rate direction on the final state is studied within the stress ratio, void ratio and mean stress space. Asymptotic behaviour is clearly observed, although the grains remain intact (no grain crushing is considered). The extension asymptotic states were observed, and the notion of a normal extension line is introduced. The extension asymptotic states coincide with the peak states observed in the shear tests with constant stress path direction in dense samples.     

Gravitational flow of granular materials

The free flow of granular materials through an orifice in a horizontal bottom has been investigated. An equation is proposed for calculating the mass flow rate.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 15, No. 5, pp. 870–874, November, 1968.     

Numerical investigation of particle shape on mechanical behaviour of unsaturated granular soils using elliptical particles

The unsaturated soils mechanic is of significant importance for understanding and predicting the actual behavior of unsaturated soils in plenty of geo-environmental condition including tailing dams and earth dams during construction and operation, soil slopes, and shallow foundations. In this research, a micromechanical model for unsaturated assemblies composed of elliptical particles is presented so that the effect of capillary force, confining stress, and particle shape on unsaturated granular media behavior is analyzed in the pendular regime. As one of the early attempts, the toroidal liquid bridge for elliptical particles is studied in granular assemblies using modified ELLIPSE program, Discrete Element Method based program. The results show that although the strength of spherical particles grows when the saturated degree increases, the strength of elliptical particles rises and then decreases; however, the friction angle remains almost constant. Furthermore, the highest percentage of apparent-cohesion rise takes place at saturation degree of 10% at the highest eccentricity. As long as the eccentricity increases, the effect of capillary force on shear strength grows continuously. Not only do unsaturated assemblies contract more at low strains, but also they expand more after the phase transfer point at high stains. The assembly composed of particles with eccentricity of 0.15 has the highest coordination number. The coordination number constantly increases with the saturation degree for circular particles. In contrast, for elliptical particles assemblies, when degree of saturation increases, the coordination number increases at the beginning then drops.     

Dilatant plastic deformation of granular materials

A geometrical interpretation is given to the modified associated flow rule derived in the previous paper[l]. According to it, the dilatancy must be regarded as an internal constraint of deformation. The modified associated flow rule then gives equations of plastic deformation which exhibits the specified dilatancy. Hardening and elastic strains can also be incorporated. It is shown that the deformation is non-coaxial in general.     

On convexity, normality, pre-consolidation pressure, and singularities in modelling of granular materials

The issues of convexity, normality, pre- consolidation pressure, and singularities of yield surfaces are discussed in the context of granular materials and soil mechanics. We approach those subjects from a rather unusual direction, by expressing yield surfaces in strain space. It is shown that the convexity assumption in strain space is justified when the elastic behaviour is linear, but not otherwise. As the effective bulk modulus of granular matter is generally pressure dependent, strain space yield surfaces are non-convex. However, strain space non-convexity does not necessarily violate the laws of thermodynamics, and by acknowledging that, arguments in favor of strain space elasto-plasticity could be made. We then define the pre-consolidation pressure directly using the total volumetric strain. The new definition offers to combine the advantages of the classical definition based on the void-ratio and a theoretically consistent definition using the plastic volumetric strain. It also allows removing singularities that may occur due to a zero denominator in the definition of the non- negative plasticity multiplier.     

Effective thermal conductivity of granular materials

Some fundamental works whose authors propose formulas to calculate the thermal conductivities of granular materials are presented.