Evolution Mechanism of Seepage-Induced Sand Inrush in Broken Rock Masses under the Coupled Effects of Particle Gradation and Outlet Boundary Conditions

Abstract To elucidate the mechanism of seepage-induced sand inrush in broken rock masses under the coupled effects of particle gradation and outlet boundary, the seepage-induced sand inrush tests of broken rock mass under different Talbot gradation and permeable-plate aperture combinations were carried out with broken mudstone and aeolian sand as test materials. The variation rules of equivalent permeability, total sand discharge and the time to first sand emergence were analyzed. On this basis, the seepage-induced sand inrush mode and mechanism of broken rock mass were discussed in combination with experimental phenomena. The results show that : (1) With the increase of Talbot exponent or permeable-plate aperture, the equivalent permeability of broken rock mass shows a nonlinear growth trend, with an increase of 46.3% -125.4% and 106.4% -182.3%, respectively. (2) With the increase of Talbot exponent or the aperture of permeable plate, the total sand discharge in broken rock mass increases, and the time to first sand emergence is shortened. The total sand discharge increased by 410.0% ~ 1366.7% and 259.5% ~ 994.5%, respectively. (3) The seepage-induced sand inrush mode of broken rock mass can be divided into three types : Type I migration–deposition–clogging–slow-release mode with limited fine particle migration, Type II local clogging–staged release mode with fine particle stage release due to local instability of clogging structure, and Type III channel connectivity–sustained sand-discharge mode with local clogging difficult to form and rapid penetration of seepage channel.(4) Particle gradation affects particle filling and local clogging by regulating the content of the small-particle fraction (< 0.8 mm) in the broken mudstone and the pore-filling capacity, whereas the outlet aperture regulates particle migration and channel formation through sieving constraints and outlet release conditions. Their coupled effects jointly alter the competitive relationship between local filling–clogging effects and outlet release capacity, causing the sand-inrush mode to progressively evolve from stable local clogging (Type I) to staged breakthrough (Type II) and sustained connectivity of dominant channels (Type III). The results can provide an experimental basis for risk identification and differentiated prevention and control of water–sand inrush in shallow coal seams.