Incipient particle motion in the expansion of water-inrush channel by considering the 3D relative exposure degree

Water inrush is a high-risk disaster in the construction of water-rich tunnels. The loss of particles on the sidewall of the water inrush channel is the key factor. To study the incipient particle motion in the expansion of water-inrush channel by considering the 3D relative exposure degree, the analysis for incipient flow velocity of the water inrush channel is conducted. The BP neural network and triaxial test are used to get the mesoscopic parameters. Through the DEM–CFD coupling, numerical simulation for the incipient particle motion is implemented. The effects of the 3D relative exposure degree on the incipient flow velocity are analyzed. Finally, the incipient flow velocity in the expansion of water inrush channel is verified by the laboratory test and engineering field test. The results show that: (1) the large particles have greater incipient flow velocity; (2) the particles in large dip angle are easier to move; (3) the 3D relative exposure degree of particle conforms to the Weibull distribution and the exponential distribution; (4) the incipient flow velocity of particles is closer to the critical flow velocity of rolling instability; (5) the incipient flow velocity is determined by the particles at the bottom of expansion channel.