Intrinsic stochasticity in soil-rock mixtures: Meso-to-macro mechanisms and random elastoplastic damage model

The mechanical behaviour of soil-rock mixtures (SRMs) under shear is governed by meso-structural transitions driven by spatial heterogeneity in particle arrangement, geometry, and force chain topology—a source of non-negligible intrinsic stochasticity within SRM structure that remains poorly captured by existing constitutive models. This study establishes an integrated stochastic framework that links fabric evolution to constitutive response through discrete element simulations and a meso-structurally informed elastoplastic model. We identify rock content Rc and its shape as the two dominate factors: characteristic Rc demarcates soil- from rock-dominated fabric, while irregular particles enhance geometric interlocking, shifting the dominant mechanism from contact sliding to skeletal stability control. These meso-scale transitions are embedded into a damage-plasticity constitutive model, where damage-softening and plastic dilatancy emerge as co-manifestations of meso-structural instability, quantitatively regulated by fabric parameters. Crucially, the macro-scale shear stress response exhibits intrinsic uncertainty arising directly from the meso-stochasticity of SRMs, rigorously decomposed into two components: (i) structural stress fluctuations—abrupt, event-like changes due to force chain reorganization and geometric locking during shear; and (ii) stochastic stress noise—irreducible background variation from contact sliding and particle motion. Notably, intermittent stress bursts (6-10% of peak strength) intensify with increasing Rc and may trigger shear localisation. The framework provides, for the first time, a unified interpretation of SRM behaviour spanning coupled stochastic fabric evolution and constitutive response, along with physically meaningful stress uncertainty, establishing a physically grounded basis for reliability-based geotechnical analysis of SRMs.