Effects of seepage-induced erosion on nonlinear hydraulic properties of broken red sandstones

Water inrush hazard is a major threat to safe mining and tunnel construction. During the water inrush process, nonlinear properties of rock mass can change gradually due to the erosion of flowing water. As a special rock mass, red sandstone subjected to water is prone to be disintegrated, resulting in the severe erosion. To study effects of erosion seepage on hydraulic properties of broken red sandstones, a series of seepage-induced particle erosion experiments were carried out. Forchheimer coefficient was used as an index to investigate the influencing mechanism of erosion on the nonlinear hydraulic properties. Experimental results show that, as erosion seepage processes, porosity and permeability of all samples increase, while non-Darcy factor of that decreases. Base on the variation of Forchheimer coefficient, the erosion seepage process can be divided into four stages, including the growth period, stationary period, decline period and stagnation period. Dual impacts of the erosion process on the Forchheimer coefficient were observed. On the one hand, the flow velocity is increased during the seepage process, and Forchheimer coefficient is promoted. On the other hand, the porosity of the rock mass is increased by the mass loss, and the growth of Forchheimer coefficient of the fluid is inhibited due to the erosion. The large particle rearrangement in the early stages is the main reason for the sharp rise of nonlinear hydraulic properties, and the particle loss can induce the decrease of nonlinear hydraulic properties. Prevention measures in the first two periods are more effective for mitigating water inrush hazard. Last but not least, with the increase of initial porosity and water pressure and the decrease of Talbot index, there are more rising stages of nonlinear hydraulic properties, and the greater difference between the mean value and the actual value.