Modeling Volume Change and Mechanical Properties with Hydraulic Models

TLDR

Soil volume change results from both mechanical and hydraulic stresses, so a complete description must include both stress types. The authors combine mechanical and hydraulic stress theories into a hydraulic function based on soil water suction, enabling continuous prediction of water volume change and derivation of mechanical parameters such as preconsolidation stress and Young’s modulus from curvature points, which are then used to calculate preconsolidation stress via Casagrande’s method across various soils and tests. Using the van Genuchten equation in RETC, the method achieves high accuracy in modeling volume change and deriving mechanical parameters, outperforming graphic Casagrande and statistical approaches, especially when data sets are variable, samples heterogeneous, or stress–strain curves flat, yielding reliable preconsolidation stress estimates.

Abstract

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.