The stress strain processes in structured unsaturated arable and forest soils depend very much on the internal soil strength and on the existing hydraulic and mechanical boundary conditions. These variables affect soil deformation by compaction and shearing to a great extent. As soon as the internal soil strength, defined as the precompression stress value, is exceeded by external forces, an intense virgin compression process occurs in combination with shearing forces at high pore water pressure values. These processes result in a complete homogenisation of the soil profile down to a certain depth. Consequently both the hydraulic and the gas fluxes are affected which results in an intense alteration of ecological and mechanical properties of the site due to enhanced swelling and shrinkage. The consequences of such compaction and shearing on soil hydraulic functions and on the rigidity of pore systems must be analysed and considered also with respect to model water fluxes. Key words: mechanical strength, shear stress, precompression stress, swelling and shrinkage, structured soils, pore size distribution, hydraulic conductivity, modelling