Abstract: Hydrogeological conditions of the coal mining are becoming increasingly complicated, especially, the water pressure in a confined aquifer is getting higher and higher. When the disaster of water-inrush occurs, under the condition of the high hydraulic gradient, it brings the problem of high-velocity non-linear water flow through the water-inrush channel of fractured rock mass. An experimental apparatus was designed to study the behaviour of high-velocity non-linear water flow through the porous media under the condition of the high hydraulic gradient (up to 600). One-dimensional (1D) uniform flow in the homogeneous porous media was experimentally investigated. The porous media was constructed by six types of smooth balls with the diameters ranging from 1 mm to 6 mm which were used to simulate the broken rock mass. Experimental results indicated that, for the porous media with the porosity between 0.44~0.45, when the hydraulic gradient is greater than 145, the process of water flow through the porous media could be divided into three stages: the linear laminar flow, nonlinear laminar flow and turbulence by analyzing the curve of hydraulic gradient and velocity, the curve of hydraulic gradient and Reynolds number. Moreover, the obtained critical flow velocity of the transition from linear laminar flow to non-linear laminar flow is 0.23~0.78 cm/s, the critical hydraulic gradient is 3~8. While the critical flow velocity of the transition from laminar flow to turbulent is 1.6~4.8 cm/s, the critical hydraulic gradient is 90~145. As the particle size increases, the critical flow velocity increases gradually, but the critical hydraulic gradient decreases. The results also show that the permeability is a linear positive correlation with the square of the particle size as well as the non-Darcy flow influence coefficient with the reciprocal of particle size. Moreover, with the increase of permeability, the non-Darcy flow influence coefficient decreases exponentially. Finally, the conclusions can provide some significant references for the problems of water-inrush in high water-pressure confined aquifers in practical engineering and the theoretical study of non-linear seepage flow through the porous media.

Key words: porous media, particle diameter, critical velocity, hydraulic gradient, non-linear flow