Grainsize dependence of clastic yielding in unsaturated granular soils

We use a recent reformulation of the Breakage Mechanics theory explaining comminution in wet granular assemblies. By using a dataset for sands, we quantify the relation between a geometric descriptor of the assembly (i.e., the mean grainsize) and the model constants that control the suction air-entry value and the stress threshold at the onset of crushing. Such relations are used to define two contrasting scenarios for the coupling between degree of saturation and yielding. In the first scenario, the suction air-entry value scales inversely with the mean grainsize, while the energy input for comminution is assumed to be independent of the size of the particles. The outcome of this assumption is that changes in degree of saturation are predicted to play a more intense role in finer gradings. Conversely, if we assume that also the energy input for grain breakage scales inversely with the size of the particles, the effect of the degree of saturation is predicted to be stronger in coarser assemblies. In other words, the deterioration of the yielding stress due to grainsize scaling effects is predicted to exacerbate the water sensitivity of unsaturated crushable soils. This result provides an interpretation for the evidence that solid-fluid interactions have a noticeable role in the compression response of assemblies made of coarse brittle particles (e.g., gravels or rockfill), while they tend to play little or no role in granular materials characterized by a finer grading (e.g., sands).