Revisiting total, matric, and osmotic suction in partially saturated geomaterials

Abstract

An accurate quantification of negative pore pressure (commonly referred to as 'suction') in the pore network is necessary for modeling the mechanical response of unsaturated geomaterials. Traditional definitions and formulations of total, matric, and osmotic suction suggest incorrect pore fluid pressures under certain conditions. In this paper, the notion of suction is revisited by deriving an expression for pore fluid pressure in a simple osmotic, capillary tube using the framework of mixture theory in conjunction with the fundamental laws of thermodynamics. Based on the derived expression for the tube, expressions are derived for total, matric, and osmotic suction for partially saturated geomaterials. Particular attention is given to osmotic suction since confusion regarding its mechanisms has apparently contributed to its misapplication in geomechanics. The new expressions derived herein adequately explain behavior that is incorrectly explained by the traditional formulations and unifies two approaches to modeling osmotic suction previously considered to be in contradiction.

Drying, Shrinkage, and Cracking of Cementitious Materials

“…This equation, based on the Derjaguin-Broekhoff-de Boer theory, shows that the sum of the disjoining and capillary forces is known from the relative vapor pressure. The saturation pressure used in this equation must take into account the amount of solute (Grasley and Rajagopal 2012;Lura et al 2003), which is significant in the pores of cement paste, and increases as drying proceeds. Equation (2) describes the forces in the saturated portion of the body, but Coussy et al (2004) pointed out that the shrinkage is influenced by the surface forces present in the unsaturated part of the body.…”

Irreversible desiccation shrinkage of cement paste caused by cement grain dissolution and hydrate precipitation

“…A modified version of Eq. (6) should be used if one wishes to simulate evolving chemistry of the pore solution during desiccation [39].…”

“…One thing to note is that, in reality, the rate of hydration reaction has a close relationship with the 1 The initial RH (i.e., prior to any desiccation) in cement-based materials is suppressed by the reduction in water activity caused by the presence of dissolved species in the pore fluid [38]. Such effects can be accounted for as in [39], whereby the dissolved species and their concentrations are explicitly accounted for in a modified version of Eq. (6).…”

An extension of Biot's theory with molecular influence based on mixture coupling theory: Mathematical model

“…Mixture theory maintains the individuality of the solid and fluid phase and takes account of phase interaction effect (Grasley et al, 2011;Grasley and Rajagopal, 2012;Rajagopal and Tao, 2005;Rajagopal and Tao, 1995). It adopts an 'energy approach' to build a link between microscale molecular force and macro-level pressure.…”