Abstract: The finite difference procedure FLAC3D was adopted to simulate the recovery of the residual pillars in Tianmashan gold mine till the destruction of the roof strata. When the roof damaged, the displacement mutation occurred and the calculations did not converge. Therefore, how the displacement mutation and calculation convergence happened can be as a criterion to judge the roof damage. According to the surrounding rock stress and displacement variation rules in stoping process, and combined with the on-site monitoring devices and numerical simulation results, a theoretical foundation is provided for the monitoring points layout optimization and forecasting for the sudden collapse of mined-out area, thereby enhancing the direction and accuracy of on-site monitoring. It is concluded that: ①Destruction occurred from roof first. This area is the most sensitive area of surrounding rock displacement and can be as the monitoring points for judging displacement mutations, and is a preferred location for multi-point displacement layout. ②After pillar recovery, the top surrounding rocks affected by mining induced the stress transferring to the terminal of the mined-out area. With the exposed area of mined-out area increasing, the bearing stress in the terminals becomes larger. However, when the roof strata damage in mined-out area occurred, the path of this transferring was blocked. So the top rock weight in top of mined-out area no longer delivered downward to the terminals and then the stress in supporting area decreased, showing a rebound phenomenon of stress. This phenomenon provides a theoretical foundation of predicting the sudden collapse of mined-out area for on-site stress monitoring. Therefore, layout of stress monitoring in the terminal point of mined-out area is superior to other locations.

Key words: Displacement mutation, Stress reaction, Forecast on collapse of mined-out area