Modeling volume change and mechanical properties with hydraulic models

Volume change of soils may be caused either by external (mechanical) or internal (hydraulic) stresses or a combination of both. A complete description of volume change must therefore include both mechanical and hydraulic stresses. By combining theories of mechanical and hydraulic stress states, a hydraulic function, which predicts the change of water volume as a function of the stress state parameter soil water suction (water retention curve), is adopted to model volume change. The utilization of such a continuous function also enables the derivation of soil mechanical parameters (e.g., preconsolidation stress, Youngs modulus) by determining mathematically the point of maximum Curvature and inflection point. This information can then be used to calculate the preconsolidation stress according to the method of Casagrande. The presented calculation has considerable advantages compared with the graphic method of Casagrande or other methods. On the basis of stress-strain relationships of various textured and structured soils and soil substrates and various test procedures (oedometer test, triaxial test, shrinkage test), volume change is modeled using the described method. It is shown that modeling volume change by the van Genuchten equation using the software RETC is possible with high accuracy. Soil mechanical parameters are derived using the parameters of the van Genuchten equation. The comparison of results of this method with the Casagrande and a statistical method shows that these methods have deficiencies when the data sets have a high variability, the samples are not homogeneous, and when the stress-strain curve is flat. The accuracy of the mathematical method in contrast is very high and the calculated preconsolidation stress very reliable.