INTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS Int. J. Numer. Anal. Meth. Geomech. 2011; 35:1299–1317 Published online 13 July 2010 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/nag.952 A thermo-hydro-mechanical model for unsaturated soils based on the effective stress concept Matthieu Dumont 1 , Said Taibi 1, ∗, † , Jean-Marie Fleureau 2 , Nabil Abou-Bekr 3 and Abdelghani Saouab 1 1 FRE CNRS 3102 Laboratoire d’Ondes et Milieux Complexe LOMC, Université du Havre, 53 rue de Prony, BP 540, 76058 Le Havre Cedex, France 2 UMR CNRS 8579 Laboratoire MSS-Mat, Ecole Centrale Paris, 92295 Châtenay-Malabry, France 3 Laboratoire EOLE, Université A. Belkaid, B.P. 230, Tlemcen, Algeria SUMMARY A simple thermo-hydro-mechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quanti- tatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way. Copyright  2010 John Wiley & Sons, Ltd. Received 15 November 2009; Revised 11 May 2010; Accepted 14 May 2010 KEY WORDS: unsaturated soils; effective stress concept; THM behaviour; elasto-plastic constitutive law 1. INTRODUCTION Thermo-hydro-mechanical (THM) behaviour of unsaturated soils is a subject of interest for civil engineers, for example, in the use of geothermal energy or for nuclear waste disposal. Over the past three decades, the study of THM behaviour and non-isothermal mass flow in porous media has been a key area of research in environmental geomechanics. 1.1. Experimental A lot of studies have been carried out in the last 40 years concerning the impact of temperature changes on saturated soil behaviour during isotropic and oedometric loadings. Concerning isotropic loadings, some studies investigated the effect of the over consolidation ratio (OCR) on thermal strains [1–7]. Others studies had as the object, the influence of temperature on preconsolidation stress [8, 9], on stiffness parameters (c s or  and c c or ) [7–14], and the evolution of thermal conductivity on a THM path [15, 16]. ∗ Correspondence to: Said Taibi, FRE 3102 LOMC CNRS, Laboratoire d’Ondes et Milieux Complexe, Université du Havre, 53 rue de Prony, BP 540, 76058 Le Havre Cedex, France. † E-mail: Said.Taibi@univ-lehavre.fr Copyright  2010 John Wiley & Sons, Ltd.