A case study on the characteristics of footwall ground deformation and movement and their mechanisms

The characteristics of ground deformation induced by the excavation of underground orebodies are summarized based on an analysis of over 10 years of deformation data recorded in the eastern mining area of the Chengchao mine in China. These data are combined with laws governing the development of ground-surface cracking and collapse to study the corresponding failure mechanism. The results indicate that the ground deformation in most mining-affected areas increases at a low rate of acceleration. Furthermore, the displacement–time curves are exponential in nature (that is, the relationship between displacement and time can be expressed using an exponent). Curves representing the ratio of the horizontal: vertical displacement over time show different behaviors including convergence, fluctuation, increase, fall-back, exponential, and linear trends. The originally designed scope of the ground movement is significantly smaller than the scope of the actual ground movement suggested by the in situ deformation data (the difference in movement angle amounts to 18°; the difference in break angle to 20°). After rapidly decreasing in the initial mining stage, the break angle remained constant (~ 50°) for a long period of time. The surrounding rock mass conformed to a failure model in which the rock mass topples toward the mined-out area leading to failure surfaces being formed in the deep parts of the rock mass. The inclination angles of these failure surfaces are different in different excavation-affected areas. The excavation-affected area in which the failure surfaces occur can be viewed as a large-scale surface-cracking zone. Outside this zone, small-scale surface cracking can be observed and this area can be viewed as a small-scale surface-cracking zone. In the area investigated, the occurrence of ground movement that is significantly larger than predicted is attributed to the combined effect of engineering-geological conditions, hydrogeological conditions, and the burial depth and geometric configuration of the underground orebodies of the mine. More specifically, the surrounding rock mass is continuously subjected to large-scale unloading during the process of karst collapse (during the construction stage) and ground collapse (during the mining stage) of the mine. Then, the surrounding rock mass progressively undergoes toppling failure and becomes unstable, making the region subjected to ground movement progressively extend outwards.