Abstract: Based on design data of a Pingwu tailing reservoir of lead-zinc concentrator in Mianyang city，a series of flume model experiments were conducted to study seepage damage (piping) of tailing dam foundation by simulation experiment platform of tailing dam-break.The whole development process from seepage damage (piping) incipit to dam failure was observed and analyzed，and it means that piping damage of tailing dam foundation can be generalized into four stages:no seepage deformation stage，forming stage，development stage and overall damage stage.Evolution mechanism of seepage damage (piping) was revealed:piping channel towards the upstream develops only within a certain area and eventually stops when horizontal average hydraulic gradient is under critical hydraulic gradient.Piping channel will persistently propagate towards the upstream and finally cause dam foundation structure to fail or break as the horizontal average gradient exceeds the critical value.The experiment result showed that piping damage of tailing dam foundation only occurs in shallow area at the top of tailings; Horizontal average critical hydraulic gradient of piping damage is 0.484.

Key words: Dam foundation, Seepage damage, Piping, Evolution mechanism, Critical gradient