The Influence of Grain Size Distribution on Mechanical Compaction and Compaction Localization in Porous Rocks

The modes of formation of clastic rocks result in a wide variety of microstructures, from poorly‐sorted heterogeneous rocks to well‐sorted and nominally homogeneous rocks. The mechanical behavior and failure mode of clastic rocks is known to vary with microstructural attributes such as porosity and grain size. However, the influence of the grain size distribution, in particular the degree of polydispersivity or modality of the distribution, is not yet fully understood, because it is difficult to study experimentally using natural rocks. To better understand the influence of grain size distribution on the mechanical behavior of porous rocks, we prepared suites of synthetic samples consisting of sintered glass beads with polydisperse grain size distributions. We performed hydrostatic compression experiments and found that, all else being equal, the onset of grain crushing occurs much more progressively and at lower pressure in polydisperse synthetic samples than in monodisperse samples. We conducted triaxial experiments in the regime of shear‐enhanced compaction and found that the stress required to reach inelastic compaction was lower in polydisperse samples compared to monodisperse samples. Further, our microstructural observations show that compaction bands developed in monomodal polydisperse samples while delocalized cataclasis developed in bimodal polydisperse samples, where small grains were systematically crushed while largest grains remained intact. In detail, as the polydispersivity increases, microstructural deformation features appear to transition from localized to delocalized through a hybrid stage where a compaction front with diffuse bands propagates from both ends of the sample toward its center with increasing bulk strain. Plain Language Summary In nature, sediments like sands and gravels in rivers, are composed of particles which can be all the same size or, more commonly, can be a mixture of lots of different sizes. Once the particles all hold together and the sediments become sedimentary rocks, the range of sizes of the particles can have an impact on how the rock behaves macroscopically under the pressure conditions of the Earth's crust. To explore this effect, we prepared synthetic rocks by sticking together particles of glass in mixture of different sizes which we then deformed under high pressure in the laboratory. Our results show that rocks made of mixture of sizes are systematically weaker than rocks with particles of a single size