Bertrand François

ACMEG-T: Soil Thermoplasticity Model

This paper addresses an advanced and unified thermomechanical constitutive model for soils. Based on experimental evidence showing the nonlinear and irreversible thermomechanical responses of saturated soils, the constitutive equations of the developed model, Advanced Constitutive Model for Environmental Geomechanics-Thermal effect (ACMEG-T), are presented. In the context of elastoplasticity and critical state theory, the model uses the multimechanism plasticity and bounding surface theory. Nonlinear thermoelasticity is joined with two coupled thermoplastic dissipative processes. The yield functions, the dissipative potentials and the plastic multipliers are introduced. Attention is particularly focused on the coupling between both plastic mechanisms, an isotropic and a deviatoric one, which are in agreement with the consistency condition for multiple dissipation. As far as isotropic mechanism is concerned, a unique thermomechanical yield surface reproduces the thermoplasticity observed at low and intermediate overconsolidation ratios, as well as the plasticity under mechanical loading in an framework unifying mechanical and thermal hardenings. Finally, the efficiency of ACMEG-T is proven by validation tests on drained and undrained thermomechanical paths.

Constitutive modelling of the thermo-plastic behaviour of soils

ABSTRACT Research interest in the thermo-mechanical behaviour of soils is growing as a result of an increasing number of geomechanical problems involving thermal effects. This paper addresses various issues concerning the thermo-mechanical behaviour of soils. Starting from experimental evidence of non-linearity and hardening that heating causes in clayey soils, a general thermo-plastic formulation is then introduced for the derivation of the constitutive equations. After that, constitutive modelling is presented and treated in the context of elasto-plasticity. Numerical simulations support the theoretical aspects of the paper by showing performances of the constitutive model.

Numerical modelling of the hydrogeological and geomechanical behaviour of a large slope movement: The Triesenberg landslide (Liechtenstein)

Using advanced hydrogeological and geomechanical finite element modelling, it has been possible to model the mechanical behaviour of a large slope movement, the Triesenberg landslide. This slope is located along the Rhine valley in the Principality of Liechtenstein and covers an area of around 5 km 2 , which includes two villages. Pore-water pressure fields calculated by the hydrogeological model were used as input for the geomechanical model. The results obtained through numerical simulation agree fairly well with field measurements of peak velocity, spatial and temporal distribution of velocity, and total displacements. Such results were obtained using a modified Cam-Clay elastoplastic constitutive model for which the required material parameters were obtained through careful geotechnical tests. These finite element models were carried out in two and three dimensions to gradually improve the understanding of the physical phenomena governing the hydrogeological conditions and the movements of the slope.

Compression tests on a sandy silt at different suction and temperature levels.

This paper presents a unified thermo-mechanical experimental study on a remoulded unsaturated sandy silt and brings a contribution to the understanding of the fundamental mechanics of unsaturated soils in non- isothermal conditions. The experimental program was carried out at four temperatures and four suction levels using two thermo-hydro-mechanical (THM) cells, one isotropic and the other oedometric. The effect of suction and temperature on the compressibility and on the apparent preconsolidation pressure of the soil is addressed. Finally, an analytical expression of the evolution of the apparent preconsolidation pressure with respect to temperature and suction is proposed.

An oedometer for studying combined effects of temperature and suction on soils

This paper reports the development of an advanced thermo-hydro-mechanical (THM) oedometer in order to characterize the behavior of soils under combined non-isothermal and unsaturated conditions. The simultaneous control of temperature, suction and stress states within the sample required rigorous calibration. This THM oedometer accommodates samples 80 mm in diameter and 23 mm in height. The applied vertical stresses can reach values up to 1 MPa, the controlled temperature ranges from 20 °C to 80 °C, and the applied suction is up to 500 kPa. The paper thoroughly discusses the calibration of the device and presents some results of tests performed on a sandy silt.

A stress-strain framework for modelling the behaviour of unsaturated soils under non-isothermal conditions.

This paper addresses a new, unified thermo-mechanical constitutive model for unsaturated soils through a coupled study. In the context of elastoplasticity and the critical state theory, the model uses the concepts of multimechanism and bounding surface theory. This advanced constitutive approach involves thermo-plasticity of saturated and unsaturated soils. Bishop’s effective stress framework is adopted to represent the stress state in the soil. This stress is linked to the water retention curve, which is represented by an elasto-plastic model. Attention is focused particularly on the coupling relations inferred from this unified thermohydro-mechanical (THM) study. Finally, the theoretical aspects of the paper are supported by comparisons between numerical simulations and experimental results.

Lime-stabilisation of high plasticity swelling clay from Ethiopia

Abstract In the present study, highly expansive clay soils from the Highlands of Ethiopia were studied to evaluate the efficiency of lime treatment to improve their mechanical properties for road subgrades. Soils treated with quick lime at 5, 7 and 9% by dry weight of the soil were cured for seven days under controlled temperature of 40 ± 2 °C and geomechanical laboratory tests were conducted to evaluate its impact on the engineering properties of the soil. Test results show substantial improvements in the properties of the soil after lime treatment. Addition of lime significantly reduces the plasticity index and swelling potential of the soil. Similarly, despite the reduction of optimum proctor dry density due to lime treatment, the unconfined compressive strength and the California bearing ratio show considerable improvements. Based on the current study, expansive soils of the studied area can be effectively stabilised for road subgrade works with the addition of 7% quick lime by dry weight of the soil. For very problematic soil, as the one investigated here, the drastic reduction of swelling potential is of particular interest for a possible application of road subgrade.

Experimental and Analytical Evaluations of Freezing Effects in Borehole Heat Exchangers

The paper addresses the evaluation of the integrity and the durability of borehole heat exchangers when submitted to freezing temperature. Three grouting materials are investigated experimentally and the obtained results demonstrate that the freezing impact strongly depends on the thermal, hydraulic and mechanical properties of the grout. Radial cracks are obtained due to the induced ice pressure that generates tensile effective stress in the grout. The prediction obtained from a thermo-mechanical analytical solution correlates very well with the experimental observations. It is demonstrated that the cracks are induced by an excess of tensile tangential stress around the pipe.

Role of the Soil Mineralogy on the Temperature Dependence of the Water Retention Curve

This paper presents experimental results of water retention curves of a compacted clay silt at different temperature. For limited suction values (up to 220 kPa), the results show a significant effect of the temperature variation (from 22°C to 50°C) on the water content at constant suction. This effect cannot be exclusively attributed to the thermal decrease of the surface tension of water. Through this study, the thermal decrease of the thickness of the diffuse double layer around the clay particles is assumed to be the most significant effect that explains the decrease of water retention capability when temperature increases. With this approach, the temperature effect on the water retention curve of any clayey soil can be estimated from the mineralogical composition of the soil. This theory is applied on five different clayey soils.

Modelling the thermo-plasticity of unsaturated soils

This paper presents a highly-coupled thermo-plasticity model for unsaturated soils. The effect of temperature on the mechanics of unsaturated soils is briefly addressed. Then the equations of the developed model, so-called ACMEGTS, are detailed. Finally, the model is validated by the means of comparisons with experiments. This constitutive model constitutes an effectivetoolfor modelling the thermo-hydro-mechanical (THM) behaviour of geomaterials involved in the confinement of nuclear waste disposal.