1,721,080 research outputs found
DEM numerical tests on dry granular specimens: the role of strain rate under evolving/unsteady conditions
No full textIn this paper, the mechanical behaviour of an ideal dry granular material under both evolving and steady conditions has been studied. Triaxial loading both constant volume and constant pressure Discrete Element (DEM) tests on periodic cells have been performed. The role played by strain rate, void ratio and imposed pressure has been analysed. The aim of the paper is to obtain a numerical data set, at present totally absent in the literature, useful for the definition of constitutive relationships capable of reproducing phase transitions (from solid-to-fluid and vice versa) taking place in granular materials. The novelty of the data set obtained derives also from the type of loading tests performed, referred as heating, cooling and cyclic, in which different strain rate histories are imposed, inspired to what locally occurs in flowing granular masses, accelerating and decelerating according to the slope inclination. The numerical results are interpreted by considering state variables (granular temperature and fabric) and the energies evolution (elastic and kinetic fluctuating) to put in evidence phase transition processes. Again, this interpretation of the results is fundamental to conceive upscaled constitutive relationships based on thermodynamic theories. The numerical results have allowed to highlight the role of the strain acceleration/deceleration, and the evolution of the directional properties of the microstructure even in the collisional regime (never described up to now). Cooling constant volume tests results provide a description, not available in the literature, of the evolving response of the material under pure collisional conditions
Deep tunnel fronts in cohesive soils under undrained conditions: a displacement-based approach for the design of fibreglass reinforcements
No full textThe fronts of tunnels excavated under particularly difficult ground conditions by employing conventional tunnelling methods are commonly supported: the stabilization is usually achieved either by improving the mechanical properties of the soil (injections, jet grouting, soil freezing, etc.) or by introducing linear inclusions. This last technique, consisting in the introduction of pipes (usually made of fibreglass reinforced polymers) in the front, is particularly popular since it is very simple to adapt the reinforcement geometry, length and number to the different conditions encountered during the excavation. The design of this reinforcement technique is nowadays based on very simplified approaches: on either empirical formula or the limit equilibrium method. In a previous paper, the authors numerically studied the mechanical response of unreinforced fronts in cohesive soils and defined a non-dimensional front characteristic curve. In this paper, the authors intend to take into consideration the role of reinforcements by following the same approach. A procedure allowing the definition of the reinforced non-dimensional front characteristic curve, once the reinforcement pattern is assigned, is introduced. The practical use of this curve is described
Dry granular masses impacting on rigid obstacles: numerical analysis and theoretical modelling
Get PDFThe assessment of the time evolution of the impact force exerted by dry flowing masses on rigid obstacles is mandatory for the dynamic design of sheltering structures and the evaluation of the vulnerability of existing structures. In this paper, the results of an extensive numerical campaign performed by employing a discrete element method (DEM) code are presented and the role of different geometrical factors (flow length, height and front inclination) and state parameters (porosity and velocity) on the impact force–time evolution is investigated. The impact process is studied to correlate local information with the macroscopic response and a physically based force–time function, generalising the formula already introduced by the authors for the assessment of maximum impact force, in which each parameter is correlated with the previously mentioned factors, is proposed
Three-Dimensional Constitutive Model for Dry Granular Materials Under Different Flow Regimes
No full textThe numerical simulations of granular materials, in the framework of continuum mechanics, is quite challenging since the constitutive model should be capable of reproducing the transition from solid- to fluid like regimes and vice versa. In this paper a constitutive model, valid under general three dimensional evolving conditions, and capable of describing the material response under both quasi-static and dynamic regimes is presented. The model is calibrated by employing a series of true triaxial DEM numerical simulations performed on a periodic cell. The comparison between model predictions and DEM results highlights that the capability of the constitutive relationship of taking into account the dependence of the mechanical behaviour of the dry granular material on Lode angle, strain rate, void ratio and confining pressure
Modelling phase transition in granular materials: From discontinuum to continuum
No full textThis work focuses on the behaviour of granular materials under unsteady, simple shear conditions and, in particular, on from solid- to fluid-like phase transition. The authors introduce a theoretical model, based on continuum mechanics, able of predicting the mechanical behaviour of granular media under both quasi-static and dynamic conditions. The model assumes a parallel scheme where confining and shear stresses are computed as the sum of two contributions: the quasi-static and the collisional one. The quasi-static contribution is obtained by employing an elastic–plastic model including the critical state concept, while the collisional one is derived from the kinetic theory of granular gases. In order to test the model under unsteady conditions, DEM numerical simulations of time evolving homogeneous shear flows have been performed by considering an assembly of frictional, deformable spheres, under constant volume conditions. Simulations have been performed by systematically changing both void ratio and shear rate. The comparison between theoretical model predictions and DEM results is done in terms of time evolution of stresses and granular temperature. Suitable initial conditions are imposed to reproduce both solid to fluid (liquefaction) and fluid to solid (solidification) phase transitions
Modelling Phase Transition in Saturated Granular Materials in MPM
No full textDescribing solid to fluid and viceversa granular material phase transition is crucial when simulating many natural phenomena involving soils and characterized by large displacements, such as landslides or soil liquefaction. Despite the large number of available numerical codes developed to deal with large displacements, such as those based on MPM, PFEM or SPH formulations, evident in the literature is the lack of constitutive approaches capable of simulating phase transition. This work focuses on the implementation of a constitutive model, recently conceived by the authors, in the open source MPM code ANURA3D, modified to account the hydro-mechanical coupling for. Phase transition occurrence is ruled by both void ratio and granular temperature, being this latter a state variable measuring granular agitation. The evolution of granular temperature is governed by the energy balance equation. Under saturated conditions coupling terms are added to account for water damping effects on granular agitation. The implementation is tested by simulating two undrained triaxial tests, performed on an ideal monodisperse granular specimen by changing the imposed strain rate
Impact of saturated granular masses against rigid obstacles: the role of fluid compressibility and front inclination
Get PDFIn recent years, the impact of saturated granular flows against rigid obstacles has been studied by using different numerical approaches. The very low compressibility of water causes numerical instabilities when impact problems are simulated. In this work, a sensitivity analysis has been done by using a Material Point Method code to assess the influence of fluid compressibility and front inclination on numerical results. When the mass front is inclined, fluid bulk modulus does not significantly affect the solution and can be reduced to speed up the computations and reduce spurious numerical oscillations
Numerical analysis of the mechanical interaction among adjacent foundations: towards a simplified design approach
No full textThe increasing demand of sustainability of the construction of new structures and the necessity of verifying/retrofitting the existing ones require reliable design tools. To this aim, fundamental is gathering new insights in the mechanical response of foundations. As far as shallow foundations are concerned, one aspect commonly disregarded in the current engineering practice is the influence of the mechanical interaction among adjacent foundations. The authors approach this problem by performing a parametric finite element non-linear numerical study on a squared shallow foundation, loaded by a vertical centred load under fully drained conditions. The numerical results put in evidence that the interaction, for any spacing value, causes an initial increase in settlements, whereas the dependence of bearing capacity on spacing seems to be, according to the case, either beneficial or detrimental. For particular geometries and mechanical properties, the reduction in bearing capacity may reach about the 30%. These dependences are justified in the light of the findings at the local scale, i.e. discussing the spatial distributions of irreversible strain, displacement and stress fields as the applied load increases. Finally, the authors have proposed a simplified design approach to derive not only the bearing capacity, but also a preliminary prediction in terms of displacements accounting for the reciprocal foundation interaction
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