A conceptual constitutive model unifying slurried (saturated), compacted (unsaturated) and dry states

Abstract

Reconstituted and compacted soils are commonly assumed to exhibit a fundamentally different behaviour due to different microstructure. However, inspection of pore size distribution of the same soil in compacted and reconstituted states suggests that the boundary between these two states is more blurred. This paper explores the continuity between the microstructure of reconstituted and compacted states of kaolin clay and formulates a conceptual constitutive model unifying these states. Clay samples were prepared by saturating the pore space with different fluids (water, acetone, and air) and the effect of pore-fluid fraction on the micro-and macroscale response of the clay was investigated experimentally. A conceptual constitutive model for unsaturated clays for quasi-isotropic stress states was therefore formulated, which allows modelling various unsaturated hydromechanical paths based on constitutive parameters only derived from the compression behaviour of clay under dry and saturated conditions (testing on samples formed from dry powder and slurry respectively).

Effective stresses for unsaturated states stemming from clay microstructure

“…The straightforward conclusion drawn by Pedrotti and Tarantino (2018) is that, in compacted samples, macro-pores are filled with air and micro-pores are filled with water (Figure 2b). Accordingly, the microstructure of a compacted sample can be assumed to be the 'sum' of the PSDs of the wet (saturated) and dry part as shown in Figure 3.…”

“…Tarantino and De Col (2008) observed that the characteristic pore size associated with micro-and macro-porosities in compacted kaolin remain the same from very low water Submitted to Geomechanics for Energy and Environment 6 contents (dry-side of optimum) up to the water content at optimum. Pedrotti and Tarantino (2018) carried out an experimental investigation to explore the microstructure of compacted kaolin clay and its interplay with the microstructure of clays reconstituted from slurry and compressed from dry powder. They showed that the PSD of kaolin reconstituted from slurry (formed water-saturated) and kaolin compressed from dry powder (formed air-saturated) overlap astonishingly with the modal sizes associated with the micro-and macro-pores of the compacted samples (Figure 2a).…”

“…No attempts have been made to reconcile the 'granular' framework commonly used to justify the proposed pairs of effective stresses with the complex microstructure of compacted clays as recognised from Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscope (SEM) testing. Pedrotti and Tarantino (2018) have revisited the microstructure of clays encompassing both compacted and reconstituted clays. They formulated a conceptual microstructural model for unsaturated soil, alternative to the traditional 'aggregate-based' microstructural model, based on the assumption that macro-pores are filled with air and micro-pores are filled with water.…”

“…The practical advantage of these two effective stresses is that the modelling of complex hydro-mechanical paths in compacted unsaturated clays only requires constitutive parameters derived from clay reconstituted from slurry (and tested under saturated state) and clay formed from dry powder (and tested under dry conditions). This paper further explores the conceptual microstructural model presented by Pedrotti and Tarantino (2018) by showing its capability to interpret a variety of MIP from the literature. It then shows how the two pair of proposed effective stress and the model derived therefrom is capable of modelling volume change behaviour by considering additional hydromechanical paths from the literature different from those examined by Pedrotti and Tarantino (2018).…”

“…This paper further explores the conceptual microstructural model presented by Pedrotti and Tarantino (2018) by showing its capability to interpret a variety of MIP from the literature. It then shows how the two pair of proposed effective stress and the model derived therefrom is capable of modelling volume change behaviour by considering additional hydromechanical paths from the literature different from those examined by Pedrotti and Tarantino (2018). In addition, it demonstrates that this pair of effective stresses is also capable of modelling shear strength behaviour of compacted unsaturated clays.…”