SWRC Modeling in Unsaturated Soils: A Pore Network Approach

Abstract

The soil water retention curve (SWRC) represents the relationship between capillary pressure (matric suction) and water content. It is one of the fundamental characteristics of unsaturated soils. Basic properties of unsaturated soils such as shear strength, volume change behavior and coefficient of permeability are closely related to the SWRC. SWRC is, usually, determined at zero confining pressure and is used, afterward, to describe soil behavior such as shear strength at both zero and nonzero confining pressures. As determination of SWRC is a time consuming task, there is a great appeal to find suitable numerical schemes and simulations tools which can estimate the retention behavior of soil at different mechanical and hydraulic conditions. One of the newly introduced simulation techniques for two phase flow in porous media is pore network modeling. Pore network modeling has not yet been much introduced in unsaturated soil mechanics. In this study, pore network modeling is used to study the effect of stress level on SWRC. For this purpose, first, the main characteristics of a 3-D pore network model which affect its efficiency in SWRC modeling are found. Coordination number, mean pore radius, aspect ratio, and pore distribution standard deviation were found to highly influence the simulated SWRC. Based on these findings a pore network is constructed so that it can resemble the real soil pore structure. The best matching parameters were obtained by comparison of the simulated SWRC and the experimental one. This was done by an error minimization process. In order to generate the pore network, available experimental data of SWRC (at zero confining pressure) is used and SWRC at other stress levels is subsequently generated using the initial pore network model. The SWRC results of the pore network model for each stress level are in good agreement with the experimental data. Thus, a suitable pore network generated from SWRC (at zero confining pressure), can be used to simulate SWRC at other stress levels. It is observed that the structural parameter controlling throat radii as well as coordination number have to be changed when stress level changes in order to be able to match SWRC at other stress levels.