Evolution of the water retention characteristics of granular materials subjected to grain crushing

This paper reports a series of experiments aimed at studying the effect of grain crushing on the water retention capacity of granular soils. Specimens of granular materials have been subjected to oedometric compression at high pressures, revealing that crushing causes significant alterations of both grain-size distribution (GSD) and soil water retention curve (SWRC). In particular, the experiments have shown that the suction air-entry value (s_AEV) changes considerably during crushing, thus controlling the shape of the SWRC in proximity of saturated conditions. Such evidence has been interpreted through a number of GSD-dependent retention models available in the literature. In particular, the results have been used to verify the hypotheses of a recently proposed hydromechanical model based on the breakage mechanics framework, which enables the prediction of simultaneous variations in void ratio, GSD, and SWRC through constitutive relations linking the s_AEV to the predicted degree of particle breakage. Although all models suggest an upward shift of the SWRC with the accumulation of crushing, the change of its shape and location are captured by each model with different levels of accuracy. Most notably, the analytical relations predicted by the breakage mechanics theory are able to capture satisfactorily the observed changes of the suction air-entry point, therefore representing a convenient tool for the analysis of geotechnical systems made of unsaturated soils susceptible to breakage, such as transportation infrastructures and rockfill dams.