Mechanisms of osmotic flow and volume change in clay soils

The mechanical behavior of compressible clay soils may be strongly influenced by physicochemical effects when concentrated pore fluids are introduced to the soil. Conceptual models have been used to explain the influence of pore fluid chemistry on the mechanical behavior of clays in a qualitative way. In this paper an alternate macroscopic description of the osmotic volume change behavior of a clay soil undergoing changes in pore fluid chemistry is provided. Theoretical descriptions of two potential mechanisms of osmotic volume change (osmotic consolidation and osmotically induced consolidation) are presented. Osmotic consolidation occurs as a result of a change in the electrostatic repulsiveminus-attractive stresses, R A , between clay particles. Osmotically induced consolidation occurs because of fluid flow out of the clay in response to osmotic gradients. A numerical simulation is used to demonstrate the characteristic behavior of a clay soil undergoing either of these volume change processes. The results of a laboratory testing program on two clay soils exposed to concentrated NaCl solutions are used to illustrate that the dominant mechanism of osmotic volume change in surficial clay soils is osmotic consolidation.