Mathieu Nuth

An introduction to the constitutive modelling of unsaturated soils

ABSTRACT An introduction to the constitutive modelling of unsaturated soils is presented. To facilitate understanding by the inexperienced reader, the paper considers the basic aspects of modelling of the behaviour of unsaturated soils from a qualitative point of view. First, the stress framework is discussed by analysing the various possible stress and strain variable combinations. The main concepts of a constitutive model for unsaturated soils using Bishops generalised effective stress are then presented. The constitutive modelling uses the Cam-Clay framework. The conception of an unsaturated model is linked to the experimental analysis of a soil response to typical hydraulic and mechanical loading cycles.

Numerical Modeling of Some Features of Heat Exchanger Pile

Note: Sols Reference LMS-CONF-2006-006 Record created on 2006-11-09, modified on 2016-05-30

A model for the water retention behavior of deformable soils including capillary hysteresis

Experimental findings on the hysteresis of the soil water retention curve, relating the degree of saturation to the matric suction, have to be superimposed with the aspects due to the soil matrix deformation. In order to set a comprehensive model for the retention behavior, we propose to review the main mechanisms of the capillary hysteresis. The paper also quantifies the effects of mechanical straining on the retention curve on the basis of experimental data. The intrinsic shape of the soil water retention curve is first defined, followed by the empirical relationship between air entry value and void ratio. The retention sub-model of a complete constitutive model for unsaturated soils is described, the mathematical formulation being based on kinematic hardening and featuring direct coupling with the mechanical stress-strain module. Model capabilities are assessed on complex retention outlines, displaying the added value of the proposed framework for prediction issues.

Implications of a Generalized Effective Stress on the Constitutive Modelling of Unsaturated Soils

The major consequences of the application of the effective stress concept to unsaturated soils are investigated. Several experimental data sets are re-examined in the light of a generalized effective stress, intended to describing the mechanical behaviour of porous media. The critical state lines at different saturation states tend to converge remarkably toward a unique saturated line in the deviatoric stress versus mean effective stress plane. The effective stress interpretation is also applied to isotropic paths and compared with traditional net stress conception. The accent is finally laid on the key feature for constitutive frameworks based on a unified stress, namely the sufficiency of a unique mechanical yield surface.

Modeling the Onset of Shallow Landslides in Partially Saturated Slopes Subjected to Rain Infiltration

Heavy rainfall can lead to shallow slips on slopes which are often initially in a state of partial water saturation. A multiphysics approach and advanced constitutive modeling are necessary to take into account the physical key processes in partially saturated soils during rainfall events, such as water flow through the solid matrix, soil water retention behavior and the effects of matric suction on the mechanical behavior. The elasto-plastic constitutive model ACMEG-s that includes an elasto-plastic water retention model with wetting-drying hysteresis is used in the framework of finite element modeling for the assessment of destabilizing, transient processes in a steep slope during rain infiltration. It is shown that at the onset of failure, wetting pore collapse and plastic shear strains occur in the lower part of the slope and develop upwards towards the slope surface to delimit a probable failure mechanism.

Numerical modelling of the behaviour of a heat exchanger pile

ABSTRACT The geothermal use of concrete geostructures (piles, walls and slabs) is an environmentally friendly way of cooling and heating buildings. With such geothermal structures, it is possible to transfer energy from the ground to fluid-filled pipes cast in concrete and then to building environments. To improve the knowledge in the field of geothermal structures, the behaviour of a pile subjected to thermo-mechanical loads was studied in situ. The aim was to quantify the thermal influence on the bearing capacity of heat exchanger piles. Coupled multi-physical finite element modelling was carried out to simulate the observed experimental results. It is shown that the numerical model is able to reproduce the most significant thermo-mechanical effects.

Chapter 37 Experimental and numerical investigations of the behaviour of a heat exchanger pile

Abstrac The geothermal use of concrete geostructures (piles, walls and slabs) is an environmentally friendly way of cooling and heating buildings. With such geothermal structures, it is possible to transfer energy from the ground to fluid-filled pipes cast in concrete and then to building environments. To improve the knowledge in the field of geothermal structures, the behaviour of a pile subjected to thermo-mechanical loads was studied in situ. The aim was to quantify the thermal influence on the bearing capacity of heat exchanger piles. The thermal increment applied to the pile was on the order of 21°C and the mechanical load reached 1300 kN. Coupled multi-physical finite element modelling was carried out to simulate the observed experimental results. It is shown that the numerical model is able to reproduce the most significant thermo-mechanical effects.

Anisotropic Mechanical Response of a Shale

The purpose of the study is to characterize the main features of the anisotropic mechanical behaviour of a shale (Opalinus Clay), and to identify an anisotropic constitutive framework adapted to the response of the material under mechanical perturbations. Such a constitutive model is required for the simulation of triaxial tests which were done on OC samples. Firstly, the large quantity of available laboratory tests which characterize the mechanical response of OC has been analyzed. The test series provide clear evidence for anisotropic mechanical behaviour of Opalinus Clay. It appears that the stress history and the micro-structure of the material induce a typical response of the material which is more overconsolidated (higher rigidity, less ductile behaviour…) for loading direction parallel to the bedding plane than perpendicular to it. Concerning numerical investigations, the capabilities of the model of Hujeux have been assessed in reproducing the anisotropic features of behaviour of OC. The constitutive model uses the theory of multi-mechanisms plasticity. Two fundamental observations account for the adequacy of the chosen constitutive framework: the elastic moduli depend on the direction of shearing, and induced anisotropy will affect the way the material hardens or softens depending on the angle of shearing.

Analysis of the swelling pressure development in opalinus clay – experimental and modelling aspects

In the context of nuclear waste geological storage, deep argillaceous formations are likely to be subjected to complex mechanical, hydraulic, and thermal loads. In particular, the argillaceous material can be firstly dried, and then re-wetted. During the latter process, the material experiences swelling and can develop swelling pressure if swelling deformations are constrained. In this contribution, the results of swelling pressure tests on shale performed in totally constrained conditions (isochoric tests) are presented. A constitutive model (ACMEG-S) is used to predict the value of the swelling pressure in such conditions. The model is made of two parts. The mechanical part addresses the stress-strain behaviour of the material, as a result of effective stress variation. An elasto-plastic approach is employed, and Bishops unsaturated effective stress, which is a function of the degree of saturation, the suction and the externally applied stress, is used as the mechanical stress. The water retention part of the model defines the relation between the degree of saturation and the suction within the material. The results put into light some factors that control the swelling pressure value, in particular the degree of saturation and the plastic behaviour of the material.

Unified Stress Framework for Modelling Unsaturated Subsoil Behaviour

ABSTRACT Traffic loads combined with moisture variations are the most harmful for road structures. Even if the pavement is assumed to have an elastic behaviour, the partially saturated subsoil can present irreversible strains that endanger the integrity of the road structure. It is proposed to define a proper stress framework for modelling the mechanical behaviour of subgrade layers submitted to both moisture changes and mechanical loads. The major consequences of the application of the effective stress concept to unsaturated subsoils are investigated. Several experimental data sets are re-examined in the light of a generalized effective stress, intended to describing the mechanical behaviour of porous media. The critical state lines at different saturation states tend to converge remarkably toward a unique saturated line in the deviatoric stress versus mean effective stress plane. The effective stress interpretation is also applied to isotropic paths and compared with traditional net stress conception. The accent is finally laid on the key feature for constitutive frameworks based on a unified stress, namely the sufficiency of a unique mechanical yield surface.