Simulation of swelling pressure measurements on expansive soils

Abstract

Numerous methods have been proposed to predict the swelling pressure and the amount of swell of an expansive soil. These methods generally involve the use of a one-dimensional consolidation apparatus (i.e., oedometer). A large amount of test results and experience involving these methods have been reported. In contrast, little attempt has been made to formulate a theoretical framework to simulate these testing procedures and to visualize the different stress paths used in the various methods. The primary objective of this research program is to formulate a theoretical framework which can embrace all swelling testing procedures. The formulations are to accommodate various boundary conditions and to simulate the stress paths that have been followed using various testing procedures. The research program commenced with a literature review which provided a summary of the research which have been conducted on laboratory swelling pressure measurement and the theoretical simulation of swell testing methods. A theoretical model for describing the pore-water pressure and volume change behavior during various swelling oedometer tests is formulated. The theory is based on the equilibrium equation, the constitutive equations for unsaturated soils and the continuity equation for the pore fluids. A computer program, SWELL, based on the theoretical model is developed using finite element method. The presented theory is used to describe the behavior observed during the experimental program. Several types of laboratory tests (i.e., falling head permeability test, Free Swell oedometer test, pressure plate test, shrinkage test and constant suction consolidation test) were performed to identify the appropriate soil properties and variables which control the swelling behavior of an unsaturated soil during the swelling oedometer tests. Several empirical equations were proposed to describe the soil properties. The proposed theory was used to simulate the results from the Free Swell oedometer tests, the Constant Volume oedometer tests, constant water content (i.e., undrainage loading) oedometer test and the Loaded Swell oedometer tests. In general, good agreement was found among the computed and measure values of volume change, vertical total stress and pore-water pressure. Additional analyses were conducted using various values for the saturated coefficient of permeability and the imposed boundary conditions. The computed rate of swelling were found to be quite sensitive to the coefficient of permeability of the soil and the length of the drainage path. The final stress state and surface evaporation were also found to influence the rate of swelling, the percent swell, and the swelling pressure.