A. Gens

Numerical formulation for a simulator (CODE_BRIGHT) for the coupled analysis of saline media

Presents numerical aspects of the program CODE_BRIGHT, which is a simulator for COupled DEformation, BRIne, Gas and Heat transport problems. It solves the equations of mass and energy balance and stress equilibrium and, originally, it was developed for saline media. The governing equations also include a set of constitutive laws and equilibrium conditions. The main peculiarities of saline media are in the dissolution/precipitation phenomena, presence of brine inclusions in the solid salt and creep deformation of the solid matrix.

Nonisothermal multiphase flow of brine and gas through saline media

We propose a general formulation for nonisothermal multiphase flow of brine and gas through saline media. The balance equations include mass balance (three species), equilibrium of stresses and energy balance (total internal energy). Salt, water and air mass balance equations are established. The balance of salt allows the establishment of the equation for porosity evolution due to solid skeleton deformation, dissolution/precipitation of salt and migration of brine inclusions. Water and air mass balance equations are also obtained. Two equations are required for water: total water in the medium and water present in solid phase brine inclusions. The mechanical problem is formulated through the equation of stress equilibrium. Finally, the balance of internal energy is established assuming thermal equilibrium between phases. Some general aspects of the constitutive theory are also presented.

Modelling the mechanical behaviour of expansive clays

A simple formalism is presented to model the behaviour of expansive clays. Two levels of structure are considered. The behaviour of the macrostructure follows the model developed for unsaturated materials by Alonso et al. (Geotechnique 40 (3) (1990) 405-430). The behaviour of the microstructure is adapted from the work of Gens and Alonso (Can. Geotech. J. 29 (1992) 1013-1032) in order to include the possibility of the micropores being partially saturated. Mechanical coupling between both levels of structure are defined through two functions, one for wetting and the other for drying. They express the change in macrostructural void ratio due to a change in microstructural void ratio, and their value depends on the state of compaction of the macrostructure. The general shape of these curves is discussed on the basis of experimental evidence. Phenomena such as the dependency of strain on stress- suction path, accumulation of expansion strain during suction cycles at low confining stress, accumulation of compression strain during suction cycles at high confining stress, strain fatigue during drying-wetting cycles, macropore invasion by expanded microstructure and development of macroporosity during strong drying can be represented. A mathematical formulation of the model is described and its performance finally assessed by comparison with laboratory tests.

Analysis of a full scale in situ test simulating repository conditions

Coupled thermo-hydro-mechanical (THM) analyses have been used to examine the interacting phenomena associated with the simultaneous heating and hydration of an engineered bentonite barrier placed in a drift excavated in granite. The specific problem studied is an in situ test being carried out in the underground laboratory at Grimsel (Switzerland). After describing the test and the theoretical formulation, the results of a coupled THM analysis using the best parameter estimation currently available are presented and discussed. The ffect of various features of analysis are explored by means of additional analyses in which each of those features are varied, one at the time. Finally, sensitivity analyses have been carried out to examine some critical aspects of the in situ test design. Performance of coupled THM analyses has led to a better understanding of the various inter-related phenomena occurring during heating and hydration of the engineered clay barrier.

AN INTERFACE ELEMENT FORMULATION FOR THE ANALYSIS OF SOIL-REINFORCEMENT INTERACTION

Abstract The formulation of a family interface elements belonging to the zero-thickness class is briefly described. The formulation is isoparametric and can be applied to 2- D and 3- D analysis. A flexible constitutive law, based on elastoplasticity, describing the mechanical behaviour of the interface is presented. In order to assess the suitability of the elements for the analysis of soil-reinforcement interaction, they are employed in the simulation of a pull-out test in two and three dimensions. It is shown that the performance of the elements can depend strongly on the type of integration rule adopted. The use of integration schemes, such as that of Newton-Cotes, which result in the uncoupling of the degrees of freedom of the element is shown to be very advantegeous. An application to the analysis of a problem involving softening behaviour of the interface is also included.

Temperature effects on the hydraulic behaviour of an unsaturated clay

The influence of temperature on the hydraulic properties of unsaturated clays is of major concern in the design of engineered barriers in underground repositories for high-level radioactive waste disposal. This paper presents an experimental study centred on the investigation of the influence of temperature on soil hydraulic properties related to water retention and permeability. Laboratory tests were conducted on artificially prepared unsaturated fabrics obtained from a natural kaolinitic-illitic clay. Special attention is given to the testing procedures involving controlled suction and temperature oedometer cells and the application of the vapour equilibrium technique at high temperatures. Retention curves at different temperatures show that total suction tends to reduce with increasing temperatures at constant water content. Temperature influence on water permeability is more relevant at low matric suctions corresponding to bulk water preponderance (inter-aggregate zone). Below a degree of saturation of 75% no clear effect is detected. Experimental data show that temperature dependence on permeability at constant degree of saturation and constant void ratio is smaller than what could be expected from the thermal change in water viscosity. This behaviour suggests that phenomena such as porosity redistribution and thermo-chemical interactions, which alter clay fabric and pore fluid, can be relevant.

Estimation of parameters in geotechnical backanalysis — I. Maximum likelihood approach

The estimation of soil and rock parameters based on field instrumentation data is a common procedure in geomechanics. The use of system ident

Vapour Transport in Low Permeability Unsaturated Soils with Capillary Effects

cation and optimization techniques allows the performance of this type of analyses in a more rational and objective manner. In this paper a probabilistic formulation for the backanalysis problem is presented. The procedure described involves the evaluation of the measurement covariance matrices, which are derived for some geotechnical instruments used in jield instrumentation. The algorithm used to solve the mathematical problem of optimization is also presented, as well as its coupling to aJinite element code. The algorithm requires the computation of the sensitivity matrix, which can be evaluated “exactly” in terms of thefinite element method. Finally, a synthetic example, based on the excavation of a tunnel, is presented in which the elastic modulus E and the Ko parameter of the material are identljiedfrom measured displacements. The eflect of the number of measurements and their error structure is also discussed.

A constitutive model for rock joints formulation and numerical implementation

A discussion of water phase change in unsaturated soils that develop capillary effects is first carried out in the paper. A distinction between the GR (geothermal reservoir) and the NUS (nonisothermal unsaturated soil) approaches is performed. Several aspects concerning advective and nonadvective fluxes of vapour are described secondly and some relationships concerning the case of mass motion in a closed system subjected to temperature gradients derived. Since the structure of unsaturated clays changes with moisture content, in order to correctly simulate the coupled phenomena induced by temperature gradients a model for intrinsic permeability as a function of humidity is required. A preliminary version of the model is presented and applied to interpret a laboratory test by means of a numerical simulation using CODE_BRIGHT

Estimation of parameters in geotechnical backanalysis — II. Application to a tunnel excavation problem

Abstract An elastoplastic constitutive law for describing the three-dimensional mechanical behaviour of rock joints is presented. The model is intended for use in numerical analysis and is formulated with sufficient flexibility so that it can reproduce a wide range of observed joint stress-strain behaviour. An hyperbolic failure criterion is shown to fit well reported joint strength data and the same type of function is adopted to define the family of yield surfaces. The evolution of hardening/softening is controlled by the total length of the plastic tangential strain path. Locking behaviour under normal loading and dilatancy varying with stress and strain level are also accounted for. Examples of application of the model to the reproduction of test results are presented. The model can be generalized to include anisotropy effects. Finally, some aspects of the procedures used in the numerical implementation of the constituive law are described.