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Localizzazione delle Deformazioni nei Mezzi Multifase
No full textDissertazione presentata per il conseguimento del titolo di Dottore di Ricerca in Meccanica delle Strutture.
In questa tesi è stata sviluppata l’analisi numerica della localizzazione delle deformazioni nelle sabbie descritte come continui multifase. E’ stata inoltre studiata l’influenza dell’interazione fra le fasi sullo sviluppo delle bande di taglio, avendo verificato la stabilità delle equazioni che descrivono il comportamento meccanico dei mezzi porosi quando è violato il postulato di Drucker. Il fenomeno è stato studiato in piccole deformazioni in campo dinamico, in deformazioni finite in campo statico
Numerical modelling of a slope stability test by means of porous media mechanics
Full text linkPurpose – The purpose of this paper is to present a finite-element analysis of the initiation of a slope failure in a small-scale laboratory test due to pore pressure variation. To this aim, a fully coupled multiphase model for saturated/partially saturated solid porous materials based on porous media mechanics is used.
Design/methodology/approach – The slope is described as a three-phase deforming porous continuum where heat, water and gas flow are taken into account. The gas phase is modelled as an ideal gas composed of dry air and water vapour. Phase changes of water, heat transfer through conduction and convection and latent heat transfer are considered. The independent variables are: solid displacements, capillary pressure, gas pressure and temperature. The effective stress state is limited by Drucker-Prager yield surface for the sake of simplicity. Small strains and quasi-static loading conditions are assumed.
Findings – The paper shows that the multiphase modelling is able to capture the main experimental observations such as the local failure zone at the onset of slope failure and the outflow appeared in that zone. It also allows understanding of the triggering mechanisms of the failure zone.
Research limitations/implications – This work can be considered as a step towards a further development of a suitable numerical model for the simulation of non-isothermal geo-environmental
engineering problems.
Practical implications – The multiphysics approach looks promising for the analysis of the onset of landslides, provided that the constitutive models for the multiphase porous media in saturated/unsaturated conditions and the related mechanical and hydraulic properties are described with sufficient accuracy.
Originality/value – Elasto-plastic thermo-hydro-mechanical modelling of the initiation of slope failure subjected to variation in pore pressure boundary condition
Coupling equations for water saturated and partially saturated geomaterials
No full textA mathematical model for a saturated and partially saturated non-isothermal porous medium is presented. The porous material is treated as a multiphase continuum with the pores of the solid skeleton filled by water and gas, which may be either vapour alone or a mixture of dry air and vapour. The governing equations at macroscopic level are derived in a spatial setting using averaging theories. Finite kinematics is included in the model. The solid skeleton of the medium can undergo large elastic or inelastic deformations described in the framework
of hyperelastoplasticity. The fluids are assumed to obey Darcy’s law
Implementation and validation of advanced constitutive models for the analysis of hydro-thermo-mechanical interactions in geo-environmental engineering problems
No full textAim of this work is the implementation and the numerical validation of advanced constitutive models for isothermal and non-isothermal water saturated or unsaturated soils in the finite element code COMES-GEO developed at University of Padua.
In this code soils are modelled as non-isothermal multiphase porous media, where interstitial voids of the deforming solid matrix may be filled with liquid water, water vapour and dry air or other gas (e.g. methane). To handle this multiphase system, an analytical multi-scale approach has been used by the general frame of averaging theories in deriving the governing balance equations. These equations have been discretized in space and time by means of the finite element method for a numerical solution.
The following advanced constitutive models for soils have been implemented: ACMEG-T model for water saturated clays in non isothermal conditions; ACMEG-TS model for water saturated and partially saturated clays in non isothermal condition; Pastor-Zienkiewicz model for water saturated sands in isothermal conditions; Bolzon-Schrefler-Zienkiewicz model for partially saturated sands in isothermal conditions; Bolzon-Schrefler model for partially saturated sands in non isothermal conditions.
Validation of the models implementation was performed by comparison between finite element results and results obtained by experimental tests or by model driver. The main results are summarized in this work. In particular, three different tests were simulated: isotropic compression test, oedometric compression test and triaxial compression test in different conditions of confining pressure, temperature and suction and for different kind of soils.
Preliminary results concerning the simulation of environmental engineering problems close this work, pointing out that with a sufficiently general thermo-hydro-mechanical model the main couplings occurring in soils may be reproduced in a relevant manner
Simulation of cavitation in water saturated porous media considering effects of dissolved air
Full text linkAn extension of a mathematical model for non-isothermal multiphase materials to consider the dissolution of air in liquid water and air mass sources during its desorption at lower water pressure is presented. The solid skeleton is assumed elasto-plastic; heat, water and air flows and water phase changes are taken into account. Physics of air dissolution and desaturation due to the air released from liquid water during cavitation in porous media are discussed. A numerical example where cavitation develops during shear band development in undrained water-saturated dense sands is solved with the developed model as discretized in space and time with the Finite Element Method
A multiphase approach for a unified modelling of fully and partially saturated porous materials by considering air dissolved in water
Full text linkA unified mathematical model for the hydro-thermo-mechanical behaviour of saturated and partially saturated porous media is developed to analyze saturated/unsaturated porous media considering the effects of air dissolved in water.
Physics of air dissolution and water cavitation in porous media, as well as different numerical techniques used for modelling the transition between fully and partially saturated state, are briefly discussed. The model equations are discretized by means of the Finite Element method. A correspondingly updated code is used to analyze two examples.
It is shown that considering the dissolved air had a small influence on the results of numerical simulations both for water outflow due to gravity forces (Liakopoulos test), and the fast fluid flows and cavitation accompanying water desaturation in the strain localization zone during compression test of dense sands.
However, the procedure allows for unified modelling of the partially and fully saturated media, without application of any ‘unphysical’ numerical technique
Fluid-structure interaction in dynamic strain localisation of multiphase geomaterials
No full textThis paper is devoted to the analysis of the fluid-solid interaction in dynamic strain localisation of initially water saturated porous media. The experimental results show the importance of the fluids phases during localisation of undrained water saturated dense sands. Therefore the use of a multiphase material model is necessary. Such a model is presented in the paper and is used for the numerical analysis of strain localisation phenomena. The important role of the fluids for localisation analysis is demonstrated also by the existence of an internal length scale contained in the model and strongly dependent, among other parameters, on the permeability of the medium
A finite element model for water saturated and partially saturated geomaterials
No full textA finite element formulation for a saturated and partially saturated porous medium undergoing large elastic or inelastic deformations is presented. This model is derived from the general thermo-hydro-mechanical model for porous materials developed in a previous contribution of the authors to this lecture notes. The porous medium is treated as a multiphase
continuum with the pores of the solid skeleton filled by water and air, this last one at constant pressure. The governing equations at macroscopic level are derived in a spatial setting. Solid grains and water are assumed to be incompressible at the microscopic level for simplicity. The consistent linearisation of the fully non linear coupled system of equations is derived. A spatial finite element formulation of the governing equations conclude this contribution of the authors
Finite element analysis of the initiation of landslides with a non-isothermal multiphase model
No full textFinite element analysis of the initiation of landslides due to capillary
and water pressure variation is presented in this work. To this aim, a non-isothermal
elasto-plastic multiphase material model for soils is used. Soils are modelled as a
three-phase deforming porous continuum where heat, water and gas °ow are taken
into account. In particular, the gas phase is modelled as an ideal gas composed of
dry air and water vapor. Phase changes of water, heat transfer through conduction
and convection and latent heat transfer are considered. The macroscopic balance
equations are discretized in space and time within the ̄nite element method. The
independent variables are the solid displacements, the capillary and the gas pressure
and the temperature. The e®ective stress state is limited by Drucker-Prager yield
surface for simplicity. Small strains and quasi-static loading conditions are assumed.
Numerical simulation of a slope stability experiment is presented assuming plane
strain condition during the computations
A unified approach to numerical modeling of fully and partially saturated porous materials by considering air dissolved in water
Full text linkThis paper presents a unified mathematical approach to model the hydro-thermo-mechanical behavior of saturated and partially saturated porous media by considering the effects of air dissolved in liquid water. The model equations are discretized by means of the Finite Element method. A correspondingly updated code is used to analyze two examples; the first one is the well known Liakopoulos test, i.e. the drainage of liquid water from a 1m column of sand, which is used to validate numerically the model here developed. As second example, a biaxial compression test of undrained dense sands where cavitation takes place at strain localization is simulated.
It is shown that considering the dissolved air has a small influence on the overall results of numerical simulations, while the histories of the fluid variables (gas and capillary pressure and water saturation) differ from other approaches neglecting the air dissolved. This may be important if appropriate constitutive models for partially saturated materials are used. A major advantage of the proposed procedure is that it allows for a unified modeling of partially and fully saturated zones in porous media without application of any `unphysical' numerical technique
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