Constitutive couplings in unsaturated granular media with crushable grains

Crushable granular materials exhibit a complex hydro-mechanical behavior. On the one hand, the hydraulic state alters the crushability of the solid matrix. On the other one, particle fragmentation causes major variations of grain and pore size distributions, thus impacting hydraulic properties such as permeability and water retention curve (WRC). This paper addresses the continuum modeling of grain crushing in unsaturated sands within the framework of the Breakage Mechanics theory. This choice enables us to discuss the role of an evolving grain size distribution (GSD) on both mechanical and hydraulic properties. First, data available in the literature about the evolution of the WRC upon grain crushing are discussed, thus assessing the accuracy of the hypotheses about its evolution. Then, the model is used to simulate various hydro-mechanical loading paths, showing that it is possible to capture a rich set of macroscopic couplings associated with either high-pressure compression or wetting. Finally, the model is used to investigate the grainsize dependency of these coupling effects. Physical considerations at the micro-scale are used to elucidate the effect of grain breakage on the predicted coupling terms, setting a vision for the future application of this modeling approach also to other classes of geomaterials, such as rockfill and granular rocks.