Abstract：The sunken topography was formed after opencast working was finished in Mujia Hill，and 375 000 m2 continuous and gently inclined abandoned stops were formed because of illegal mining. To prevent illegal mining and the potential danger of air shock wave resulted from sudden roof caving，it is proposed that the abandoned stope near 143 section in Mujia Hill be disposed by ore-pillar dilapidation. Caving angle and displacement angle can’t be determined by FEM for it is applicable to elastic and plastic medium. So the minimal uncontrolled roof-fall span，displacement angle and caving angle are studied by similarity simulation. The results show when the minimal roof span of 110 m is adopted，uncontrolled collapse of phosphorus dolomite roof and dolomite roof can be guaranteed，and surface subsidence and rock movement are not notable. If the big ore pillar of upside is entirely dilapidated，roof span will arrive at 140 m，caving angle of 78° and 65° will be caused，respectively，and displacement angle will arrive at 64.4°. Limit span determined by similarity simulation is between that by FEM and that by the formula proposed in the paper.
