Abstract: In order to study the water inrush mechanism of Karst collapse column in different depths with coupled underground water pressure and compaction degree, a modified variable mass seepage experiment system on broken rock was used to conduct variable mass seepage experiments for broken mudstone considering particle migration. The duration time of seepage burst, weight of gushed particle, change rate of gushed particle weight and change rate of porosity were recorded and calculated, and the change rules under seepage pressure and initial porosity were obtained. It shows that in the initial seepage stage, seepage pressure and flow are stable, they change rapidly when seepage burst happens, and the change range is related to weight of gushing particle and the process of pore structure adjusting. With the initial porosity and seepage pressure increasing, the duration time of seepage burst gets shorter, weight of gushed particle gets larger, and these two parameters are fitted by logarithmic functions relating to seepage pressure and initial porosity; change rate of gushed particle weight and change rate of porosity get faster and both are fitted by exponential functions relating to seepage pressure and initial porosity. With seepage pressure increasing, the influence of initial porosity on the duration time of seepage bursts, weight of gushed particle, change rate of gushed particle weight and change rate of porosity are gradually weakened.

Key words: broken mudstone, variable mass, seepage pressure, initial porosity, seepage burst