Abstract: Based on the seepage theory of porous media, the mass conservation equation, momentum conservation equation and the porosity evolution of particles in collapse column with fluid-structure interaction is deduced and a mechanical model for collapse column water inrush is proposed, which takes the mass variation during the seepage into consideration. Then, numerical simulation is carried on using COMSOL Multiphysics, which is a Finite Element Code. By assuming that the porosity in collapse columns obeys the Weibull distribution, the distribution of the porosity and seepage velocity in the collapse column at different times is obtained, and the seepage discharge is calculated. The results show that: 1) the seepage channels initially distribute randomly, however, with the different erosion effect of water on rock with different porosity, they finally form several main channels; 2) the seepage capacity of the collapse column initially grows slowly, while seepage velocity increases significantly with the growth and connectivity of porosity.

Key words: collapse column, water-inrush mechanism, variable mass seepage；numerical simulation