Static liquefaction: The role of grain size polydispersity from a micro-structural perspective

Static liquefaction, defined as the loss of strength after strain softening under undrained shear, is often the cause of catastrophic failures in loose earthfills, waste dumps and tailings storage facilities. The number of failures does not cease to increase yearly, revealing operational faults, construction defects, and a lack of knowledge regarding liquefaction triggering. While liquefaction is well understood at the scale of a representative elementary volume of soil, the triggering mechanisms at the particle scale are seldom studied. This study aims to analyze the links between the micro- and macro-mechanisms controlling static liquefaction by means of numerical simulations using the Discrete Element Method. We consider mono to highly size polydisperse 2D samples of discs, prepared at varied densities from loose to dense packings. As expected, loose samples liquefy, while dense ones continue to exhibit resistance under undrained shearing. Some medium-dense cases liquefy only temporarily, recovering their shear strength at larger strains. We reveal a dual mechanism for liquefaction through multi-scale analysis: a collapse of the contact network—marked by the coordination number dropping below 3—and the emergence of low-density regions. Temporary liquefaction involves transient connectivity loss with minor density fluctuations, enabling stress recovery. In contrast, full liquefaction combines both effects, leading to an irreversibly disconnected and heterogeneous microstructure.