Abstract: Thorough researches on unsaturated soils and special soils including fill, collapsible loess, expansive soil, bentonite and sandy clay, are carried out, and a series of novel results are obtained. A lot of instruments and equipments suitable for unsaturated soils and special soils, such as oedometer, direct shear apparatus, gas permeation device, multifunction triaxial apparatus and temperature-controlled triaxial apparatus as well as CT-triaxial apparatus, are successfully developed. Important mechanical properties and laws of water flow, air flow, deformation, strength, yield, moisture and meso-structure of unsaturated soils and special soils are revealed. An axiomatics theoretical system of geomechanics and the stress state variables of unsaturated soils are established. The formulae for effective stresses isotropic unsaturated soils and anisotropic porous media are proposed. The constitutive hierarchy including nonlinear model, elasto-plastic model, coupled temperature-mechanical model and meso-structure damage model as well as the general model for soil-water characteristics curve are established. The 3-D consolidation theory based on the theory of mixture and the hierarchy of consolidation model are created, the relevant software to solve 2-D consolidation problems is programmed independently, and the analytical solutions to 1-D problems and the numerical solutions to 2-D problems are obtained. Many difficult problems such as deformation and stability of dams and embankments are solved using the above achievements. It is indicated that the proposed theories can be adopted to guide engineering practices and to be as the theoretical basis for engineering decision.