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.