An investigation into the joints of the ore body and rock mass at Chengchao iron mine is performed with combination of digital panoramic borehole camera system and in-situ statistics. Based on software Surpac

a 3D model for the gob is built. By subdividing the 3D model

a 2D discrete element model based on Universal Distinct Element Code is developed. The model is utilized to simulate the law of displacement fields and the rock-fall patterns of the overlying rock at different elevations when ore bodies are mined. The variations of the rock movement angle are analyzed. It is shown that: (1)When the ore bodies at the elevations between -430m and -500m are mined using back filling method; the maximum displacement for the gob is 205.6cm

which is smaller than the 215.2cm measured with the caving method. By comparing the settlement values and the horizontal displacement values of the auxiliary shaft

it is found that the incline rate of surface is 0.56mm/m for back filling method

smaller than the 1.22mm/m for the caving method; the horizontal deformantion rate is 1.03mm/m

smaller than the 1.31mm/m for the caving method; it is shown that the back filling method has a good effect on controlling overlying rock displacement and ground movement. (2) The movement of the overlying rock behaves in the manner of intermittence and jump when using the caving method. A self-balancing arch can be formed in the process of the overlying rock falling; hence

a number of hidden gobs may occur. Based on the comparison between the practical monitoring data and the numerical simulation

it is found that when a long period mining is carried out at a given depth

the decrease rate of movement angle exhibits a nonuniform variation mode

namely “slow-acceleration” cycle

implying the occurance of the self-balancing arches and gobs.