Time effects in geomaterials

In this paper, time effects, seen as strain rate effects and creep phenomenon, on the mechanical behaviour of geomaterials are considered. Coupling between these effects and partial saturation of geomaterials is more particularly studied. An elasto-visco-plastic model, called RASTRA, is presented and validated from the comparison with experimental data.Résumé : Dans cet article, les effets du temps, vus comme les effets de la vitesse de chargement et le phénomène de fluage, sur le comportement mécanique des géomatériaux sont abordés. Le couplage entre ces effets et le caractère partiellement saturé des géomatériaux est plus particulièrement étudié. Un modèle élasto-visco-plastique, appelé RASTRA, est présenté puis validé sur la base d’une comparaison avec des données expérimentales.Mots clés : Vitesse de chargement, fluage, succion, saturation partielle, élastoplasticité

Constitutive equations and numerical modelling of time effects in soft porous rocks

peer reviewedA constitutive model is developed within the framework of Perzyna's viscoplasticity for predicting the stress-strain-time behaviour of soft porous rocks. The model is based on the hyperelasticity and multisurface viscoplasticity with hardening. A time-stepping algorithm is presented for integrating the creep sensitive law. An example of application to one-dimensional consolidation is presented. The objectives are to: 1. present a soft rock model which is capable of taking into account the rate sensitivity, time effects and creep rupture; 2. to discuss the use of an incremental procedure for time stepping using large time increments and 3. to extend the finite element code Lagamine (MSM-ULg) for viscoplastic problems in geomechanics

Elasto-viscoplastic modelling of unsaturated soils under static and dynamic loading in 3D stress space

Consolidation, dynamic analysis, and wave propagation are some of the topics in geomechanics in which a complete characterization of coupling of the solid skeleton deformation and fluid flows is necessary for an accurate evaluation of material response. Dynamic behavior of soils is widely investigated in the past decades; however, they have mainly concerned the behaviour of dry or fully saturated porous media. Considering a three-phase continuum system which accounts for the interactions between the phases is crucial for investigating dynamic behavior of real soils which are invariably in an unsaturated state. Deposits located near the surface of the earth with relatively low water content, highly plastic clays which undergo changing environment or loose silty sands which collapse under wetting process are examples of unsaturated soils and experience severe situations especially under dynamic conditions. This thesis presents an elasto-visco-plastic flow-deformation model for dynamic analysis of unsaturated soils including mechanical and hydraulic hysteresis. Governing equations of fluid and solid phases are derived based on theory of continuum mechanics considering phase interaction, and nonlinear deformation of solid skeleton subject to dynamic loading. A numerical scheme is developed using a robust Finite Element method as the global solution to solve various boundary value problems. For the local solution, a comprehensive bounding surface viscoplastic model is presented for unsaturated soils which accounts for suction hardening and rate effects and can simulate monotonic and cyclic loading paths. Consistency condition theory is used to describe the viscosity behaviour of the material. A unique relationship between stress, strain, and strain rate of the material is also defined to perfectly describe the effect of the strain rate hardening. Several examples are solved to validate the model and demonstrate the capability of the proposed framework for investigating behaviour of soils in complex hydro-mechanical conditions