Compressibility of undisturbed silt loam soil – measurements and simulations

The protection of soils against compaction requires knowledge of the mechanical behavior and properties of structured unsaturated soils and their dependence on soil moisture. This study of an agricultural silt loam soil is based on undisturbed specimens from the plow layer (7-17 cm depth) and two subsoil layers (47-57 cm and 67-77 cm depth) conditioned to five initial suctions (s = 1, 3, 6, 16, 32 kPa) per layer and compressed under drained, confined uniaxial stress. From the resultant total axial stress versus void ratio curves, effective axial stress versus void ratio curves were calculated using a modified form of van Genuchten’s equation (van Genuchten, 1980). The effective axial stress versus void ratio curves were fitted using the unsaturated soil mechanics model by Casini (2012). The effective initial precompression stress, compression, and recompression indices and the soil mechanical parameters N and b were then determined for the plow and subsoil layers. Based on Casini (2012), a model was developed to predict the effective initial precompression stress from the initial void ratio and suction or saturation degree. The Casini (2012) model was successfully fitted to the effective axial stress versus void ratio curves recorded in the confined uniaxial compression tests on the specimens, as well as the relationship of the effective initial precompression stress to the initial void ratio plus the initial degree of saturation and suction. The results show that some measure of soil density (void ratio) is equally as important as soil moisture to describe compression behavior for structured, undisturbed soil.