This paper presents the authors’ personal views on current research being conducted by various research groups around the world in the broad area of mechanics of unsaturated geomaterials in general and soils in particular. The topic is of interest to a wide spectrum of scientists and engineers working in diverse areas such as geology and geophysics, powder technology, agricultural, petroleum, chemical, geotechnical, civil, environmental and nuclear engineering. Even if we restrict ourselves to civil, geotechnical and environmental engineering, it is noted that a plethora of hypotheses as well as a number of empirical and semi-empirical relations have been introduced for describing the mechanics of unsaturated porous media. However, many of these proposed advances as well as methods of testing may lack sound theoretical basis.
 TERZAGHI K.V., The shearing resistance of saturated soils and the angle between the planes of shear, Proceedings of the 1st International Conference on Soil Mechanics and Foundation Engineering, 1936, Vol. 1, 54-56.
 LADE P.V., DE BOER R., The concept of effective stress for soil, concrete and rock, Géotechnique, 1997, 47, 61-78
 PIETRUSZCZAK S., PANDE G.N., On the mechanical response of saturated cemented materials - Part I: theoretical considerations, Intern. Journ. Num. Anal. Meth. Geomech., 1995, 19, 555-562.
 PIETRUSZCZAK S., TURU G., PANDE G.N., On the mechanical response of saturated cemented materials, Part II: experimental investigation and numerical simulations, Intern. Journ. Num. Anal. Meth. Geomech., 1995, 19, 563-571.
 NUR A., BYERLEE J.D., An exact effective stress law for elastic deformation of rock with fluids, Journ. Geophys. Res., 2012, 76, 6414-6419.
 BISHOP A.W., BLIGHT G.E., Some aspects of effective stress in saturated and partly saturated soils, Géotechnique, 1963, 13, 177-197.
 MAŠÍN D., Predicting the dependency of a degree of saturation on void ratio and suction using effective stress principle for unsaturated soils, Intern. Journ. Num. Anal. Meth. Geomech., 2010, 34, 73-90.
 LU N., GODT J.W., WU D.T., A closed-form equation for effective stress in unsaturated soil, Water Resources Res., 2010, 46, 1-14.
 KHALILI N., KHABBAZ M.H., A unique relationship for χ for the determination of the shear strength of unsaturated soils, Geotechnique, 1998, 48, 1-7.
 BAKER R., FRYDMAN S., Unsaturated soil mechanics: Critical review of physical foundations, Engineering Geology, 2009, 106, 26-39.
 Computers & Geotechnics, Special Issue on Unsaturated Soils: Models, Algorithms and Applications (eds. A. Gens and D. Sheng), 2008, Vol. 35.
 Vadoze Zone Journal, Special Section: Principle of Effective Stress, 2014, Vol. 13(5).
 BUCKINGHAM E., Studies on the movement of soil moisture, U.S. Department of Agriculture, Bureau of Soils, 1907, 38.
 RICHARDS L.A., The usefulness of capillary potential to soil moisture and plant investigators, Journ. Agric. Res., 1928, 37, 719-742.
 GARDNER R., A method of measuring the capillary tension of soil moisture over a wide moisture range, Soil Science, 1937, 43, 277-284.
 VAN GENUCHTEN M.T., A closed-form equation for predicting the hydraulic conductivity of unsaturated soils, Soil Sci. Soc. Amer. Journ., 1980, 44, 892-898.
 GALLIPOLI D., Constitutive and Numerical Modelling of Unsaturated Soil, Ph.D. Thesis, University of Glasgow, U.K., 2000.
 GALLIPOLI D., WHEELER S.J., KARSTUNEN M., Modelling the variation of degree of saturation in a deformable unsaturated soil, Géotechnique, 2003, 53, 105-112.
 DIEUDONNE A.C., LEVASSEUR S., CHARLIER R., DELLA VECCHIA G., JOMMI C., A water retention model for compacted clayey soils, Proc. ComGeo III (eds. S. Pietruszczak and G.N. Pande), 2013, 1, 23-31. ICEE.
 WANG M., PANDE G.N., KONG L.W., Comparison of pore-size distribution of soils obtained by different methods, Intern. Journ. Geomech., 2015 (submitted).
 JOSA A., ALONSO A.E., LORET A., GENS A., Stress-strain behaviour or partially saturated soils, European Conference on Soil Mechanics and Foundation Engineering, Dublin, 1987, Vol. 9, 561-564.
 ZAKARIA I., Yielding of unsaturated soil, Ph.D. thesis, University of Sheffield, UK 1995.
 SIVAKUMAR V., A critical state framework for unsaturated soil, Ph.D. Thesis, University of Sheffield, UK 1993.
 LINS A.H.P., Discussion on “An alternative framework for unsaturated soil behaviour” S. J. Wheeler (1991), Géotechnique, 1992, 42, 525-527.
 TARANTINO A., DE COL E., Compaction behaviour of clay, Géotechnique, 2008, 58, No. 3, 199-213.
 TARANTINO A., TOMBOLATO S., Coupling of hydraulic and mechanical behaviour in unsaturated compacted clay, Geotechnique, 2005, 55, No. 4, 307-317.
 GRIFFITHS D.V., LU N., Unsaturated slope stability analysis with steady infiltration or evaporation using elasto-plastic finite elements, Intern. Journ. Num. Anal. Meth. Geomech., 2005, 29, 249-267.
 ALONSO E.E., GENS A., JOSA A., A constitutive model for partially saturated soils, Géotechnique, 1990, 40, 405-430.
 TAMAGNINI R., An extended Cam-clay model for unsaturated soils with hydraulic hysteresis, Geotechnique, 2004, 54, 223-228.
 LI X.S., Modelling of hysteresis response for arbitrary wetting/ drying paths, Computers and Geotechnics, 2005, 32, 133-137.
 NUTH M., LALOUI L., Advances in modelling hysteretic water retention curve in deformable soils, Computer and Geotechnics, 2008, 35, 835-844.
 GENS A., ALONSO E.E., A framework for the behaviour of unsaturated expansive clays, Can. Geotechn. Journ., 1992, 29, 1013-1032.
 LIM Y., MILLER G., Wetting induced compression of compacted Oklahoma soils, Journ. Geotech. Geoenviron. Eng., 1999, 130, 1014-1023.
 CHEN Z.H., FREDLUND D.G., GAN J.K., Overall volume change, water volume change, and yield associated with an unsaturated compacted loess, Can. Geotech. Journ., 1999, 36, 321-329.
 PIETRUSZCZAK S., LYDZBA D., SHAO J.F., Modelling of deformation response and chemo-mechanical coupling in chalk, Int. Journ. Num. Anal. Meth. Geomech., 2006, 30, 997-1018.
 HUECKEL T., Chemo-plasticity of clays subjected to stress and flow of a single contaminant, Int. Journ. Num. Anal. Meth. Geomech., 1997, 21, 43-72.
 PIETRUSZCZAK S., HAGHIGHAT E., Assessment of slope stability in cohesive soils due to a rainfall, Int. Journ. Num. Anal. Meth. Geomech., 2012, 37, 3278-3292.
 TADEPALLI R., FREDLUND D.G., The collapse behavior of a compacted soil during inundation, Can. Geotech. Journ., 1991, 28, 477-488.
 JOTISANKASA A., Collapse behaviour of compacted silty clay, Ph.D. Thesis, Imperial College, London 2005.
 SUN D., SHENG D., XU Y., Collapse behaviour of unsaturated compacted soil with different initial densities, Can. Geotech. Journ., 2007, 44, 673-686.
 WHEELER S.J., SHARMA R.S., BUISSON B.S.R., Coupling of hydraulic hysteresis and stress-strain behaviour in unsaturated soils, Géotechnique, 2003, 53, 41-54.
 ROJAS E., CHAVEZ O., Volumetric behavior of unsaturated soils, Can. Geotech. Journ., 2013, 50, 209-222.
 LI X., THOMAS H.R., FAN Y., Finite element method and constitutive modelling and computation for unsaturated soils, Comp. Meth. Appl. Mech. & Eng., 1999, 169, 135-159.
 PIETRUSZCZAK S., PANDE G.N., On mechanical response of partially saturated soils at low and high degrees of saturation, Numerical Models in Geomechanics, NUMOG V, (eds. G.N. Pande and S. Pietruszczak), 1995, 1, 33-39, Balkema.
 WROTH C.P., HOULSBY G.T., Soil machanics: property characterization and analysis procedures, Proc. 11th ICSMFE, 1985, Vol. 1, 1-55.