Experimental study on the mechanisms of water-sand inrushes induced by mining under thin bedrock of Neogene unconsolidated strata

Aiming at the engineering safety issues of water-sand inrush during thin bedrock coal seam mining under extremely thick Neogene unconsolidated strata in eastern China, this paper seeks to reveal the fracture evolution laws of overlying strata and the chained disaster mechanism of water-sand inrush, providing theoretical support for disaster prevention. Based on the engineering geological characteristics of typical mining areas, novel solid-fluid coupling similar materials are developed by introducing calcium carbonate fine aggregates to optimize aggregate gradation, achieving collaborative regulation of low strength, low permeability, and anti-disintegration properties. A three-dimensional geological test model incorporating thick aquifers, thin bedrock, and clay aquicludes is constructed, revealing that periodic weighting stages represent high-risk periods for water-sand inrush, with bedrock thickness and the combination of unconsolidated layer clay strata significantly influencing failure modes, while mining-induced fractures exhibit characteristic “OX” shaped distribution. Experimental results demonstrate that after water-conducting fracture zones penetrate the upper bedrock boundary, they evolve through three stages: “water seepage-fracture zone propagation-sand particle migration”, where abrupt hydraulic gradient changes during periodic weighting stages constitute critical triggers for crossing sand inrush thresholds. Initial seepage establishes hydraulic connections between dynamic fracture networks and aquifers, followed by intermediate water-rock interactions reducing surrounding rock strength and forming preferential seepage channels, ultimately leading to late-stage hydraulic gradients exceeding critical values to induce non-Darcian flow water-sand inrush. Clay content in strata shows a negative exponential relationship with fracture aperture, whereas sand particle flux in unconsolidated strata exhibits exponential growth with increasing fracture aperture. The research proposes spatiotemporal evolution patterns of overburden failure and the chained disaster mechanism of water-sand inrush, identifying key evolutionary characteristics including exponential growth of water seepage, strength degradation of surrounding rocks, and hydraulic gradient increases triggering non-Darcian flow, thereby providing theoretical foundations for waterproof coal pillar design and disaster early warning systems.