Volcanoes are inherently unstable structures that are built haphazardly, in both space and time, from the products of successive effusive and explosive eruptions and endogenous growth. These materials have highly variable physical and mechanical properties (Heap & Violay, 2021) and often form oversteepened and unstable slopes. As a result, volcano deformation (such as volcano spreading; Borgia et al., 2000) and mass wasting events (such as debris avalanches resulting from partial flank collapse, lahars, and rockfalls;

The fundamentals of our understanding of the mechanical compaction of porous rocks stem from experimental studies. Yet, many of these studies use natural materials for which microstructural parameters are intrinsically coupled, hampering the diagnosis of relationships between microstructure and bulk sample behavior. To probe the influence of porosity and grain size on the mechanical compaction of granular rocks, we performed experiments on synthetic samples prepared by sintering monodisperse populations of glass beads, which allowed us to independently control porosity and grain diameter. We conducted hydrostatic and triaxial compression tests on synthetic samples with grain diameters and porosities in the ranges 0.2–1.15 mm and 0.18–0.38 mm, respectively. During hydrostatic compaction, sample porosity decreased suddenly and substantially at the onset of inelastic compaction due to contemporaneous and extensive grain crushing, a consequence of the monodisperse grain size. During triaxial tests at high confining pressure, our synthetic samples failed by shear‐enhanced compaction and showed evidence for the development of compaction bands. Critical stresses at the onset of inelastic compaction map out linear‐shaped yield caps for the porosity‐grain diameter combinations for which the critical stress for inelastic hydrostatic compaction is known. Our yield caps reinforce the first‐order importance of porosity on the compactive yield strength and show, all else being equal, that grain size also exerts a first‐order control and should therefore be routinely measured. Our study further reveals the suitability of sintered glass beads as analogs for crustal rocks, which facilitate the study of the deconvolved influence of microstructural parameters on their mechanical behavior.

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