Uncoupled and coupled solutions of volume change problems in expansive soils
Hung, Vu Quang
Lightly loaded structures constructed on expansive soils are often subjected to severe distress subsequent to construction, as a result of changes in the pore-water pressures in the soil. The structures most commonly damaged are roadways, airport runways, small buildings, irrigation canals, spillway structures and all near ground surface structures associated with infrastructure development. Changes in the pore-water pressure can occur as a result of variations in climate, change in depth to the water table, water uptake by vegetation, removal of vegetation or the excessive watering of a lawn. An analytical tool for the prediction of heave is extremely valuable to geotechnical engineers. There has been little advancement in the development of such a tool for solving engineering problems. There does not appear to be a computer program that has been written and widely accepted for solving this problem. It is important that such an analytical tool be developed and that it be developed for both one- and two-dimensional problems. The primary objective of this research study is to apply the general theory of consolidation/swelling for unsaturated soils to provide a reliable, practical technique for the prediction of one-, two- or three-dimensional volume change associated with unsaturated, expansive soils. The void ratio constitutive surface of an unsaturated, expansive soil was estimated from volume change indices obtained from conventional oedometer tests. Mathematical equations, which can be applied over a wide range of stress conditions, are proposed to describe the constitutive surfaces for both soil structure and water phase. The elastic parameter functions that are required for the volume change analysis are calculated from the constitutive surfaces with an assumed value of Poisson's ratio. The solutions to the volume change problems associated with an unsaturated, expansive soil are obtained using both an uncoupled and a coupled approach. In the uncoupled approach, the continuity equation for the water phase and the equilibrium equations are solved independently. Uncoupled solutions are obtained using a partial differential equation solver, called FlexPDE. In the coupled approach, the continuity equation and the equilibrium equations are solved simultaneously. Coupled solutions are obtained using a finite element program, called COUPSO. The examples presented in this study represent typical volume change problems that are often encountered in engineering practice (i.e., influence of vegetation on light engineering structures, water leakage under floor slab and infiltration of water from ground surface). The results of the analyses appeared to be reasonable and in accordance with anticipated behaviour. The research results also showed that the answers from uncoupled solutions compared well with those from the coupled solutions. It is suggested that uncoupled solutions are adequate for the analysis of most volume change predictions for unsaturated, expansive soils.