Study on the Rock Mass Caving and Surface Subsidence Mechanism Based on an In Situ Geological Investigation and Numerical Analysis

To deeply understand the mechanism of the rock mass caving and associated surface subsidence during sublevel caving mining, the Xiaowanggou iron mine was selected as an engineering project case study. The study area was analyzed by means of an in situ geological investigation and numerical simulation. First, a borehole television (BHTV) system and a GPS monitoring system were used to monitor the caving process of the roof rock mass and the development of the surface subsidence; the monitoring time was thirteen months. Then, a numerical simulation was used to analyze the damage evolution of the rock mass. Research shows the following: in situ geological monitoring results indicate that the caving process of the roof rock mass presents intermittent characteristics, where slow caving and sudden caving are conducted alternatively and the arched-caving trend is more pronounced with continuous mining. The surface subsidence, horizontal displacement, and horizontal deformation of the hanging wall are higher than that of the footwall, and the subsidence center gradually deflects to the hanging wall in the late stage of the +45m sublevel mining. Numerical simulation results indicate that the extension and penetration of the shear and tensile cracks along the joints and intact rock bridges are the main factors causing the rock mass caving and the existence of the stress arch and its evolution process is the fundamental reason for the intermittent caving of the rock mass. The rapid development of damage to the hanging wall (the damage angle reduced) is the main cause of the deflection of the subsidence center affected by joints and the mining size. In the future of mining, large-scale subsidence will occur on the surface of the hanging wall.