Mechanisms of osmotic flow and volume change in clay soils

Abstract

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 repulsive-minus-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. Key words: physicochemical, osmosis, volume change, NaCl salt, montmorillonite, clay, stress state variables, R – A.

What is the significance of pore pressure in a saturated shale layer?

“…The introduction of the swelling was considered in soil sciences by, e.g., Barbour and Fredlund [1989] or Hueckel [1992] or in rock mechanics by, e.g., Sherwood [1992Sherwood [ , 1994, Dormieux et al [2003], Gajo et al [2002], or Loret et al [2002]. Although the developments proposed by the rock mechanists are rigorous, their practical use for hydrogeologic applications is puzzling.…”

“…In order to simplify the notation, we omit the angle brackets and the subscript OXYZ and denote the macroscopic (anisotropic) "pressure" p. Considering the notion of effective stress introduced by Terzaghi and extending this notion to the electrochemical processes [Barbour and Fredlund, 1989], one obtains…”

An experimental and constitutive investigation on the chemo-mechanical behaviour of a clay

“…The so-called 'osmotic processes' resulting from these properties have been widely described in the literature (e.g. Marine & Fritz, 1981;Fritz, 1986;Barbour & Fredlund, 1989;Loret et al, 2002).…”

“…Various authors use different terminology for these phenomena: Barbour & Fredlund (1989) mention osmotic consolidation and osmotically induced consolidation, whereas Kaczmarek & Hueckel (1998) discuss chemical consolidation and chemico-osmotic consolidation. Barbour & Fredlund (1989) interpreted the results of a series of four experiments conducted by Mesri & Olson (1971), and presented additional experimental results on two different types of soil to quantify 'osmotic compressibility and permeability'. The main conclusion of their work was that the dominant mechanism of volume change was osmotic consolidation, referred to as chemical consolidation here.…”

“…In the 1970s and 1980s, chemically induced consolidation, permeability and shear strength variations, and osmosis and osmotic effects were extensively studied (e.g. Mesri & Olson, 1970, 1971Mitchell et al, 1973;Sridharan & Venkatappa Rao, 1973, 1979Marine & Fritz, 1981;Fritz, 1986;Barbour & Fredlund, 1989). The late 1990s and 2000s brought forth coupled chemo-mechanical models, in which the concentration or mass fraction of certain species in the pore liquid played a major role (e.g.…”