Prediction and modeling of tensile stresses and shrinkage

Volume change as a result of drying is often neglected in soil mechanics and soil hydrology, despite the important influence it has in the change of mechanical stability and water flow. Therefore, processes which lead to volume change have to be understood. Tensile stresses as the main parameter for shrinkage are a result of hydraulic and mechanical mechanisms in unsaturated soils or soil substrates. Both mechanisms have to be recognised as dependent processes. Unsaturated soils are defined as 3-phase systems. Capillary forces in soil pores act as contractive forces of the liquid phase on the solid phase. The resulting tensile stress caused by water increases with decreasing degree of water saturation. This causes shrinkage in a given soil volume, including soils with small plasticity. Mechanical stress parameters will simultaneously be changed with shrinkage, which as a result also change the hydrological parameters altering the pore system. The separation of the mechanical from the hydraulic stressis difficult. Therefore, a method was developed, which allows the determination of tensile stress under defined boundary conditions and is based on the general stress equation. Also a method is described by which this information is used for general modeling of volume change by hydraulic stress and general empirical functions used in hydraulic modeling.