Engineering geology and ground collapse mechanism in the Chengchao Iron-ore Mine in China

Abstract

Abstract The engineering geological and hydrogeological conditions in the eastern mining area of the Chengchao Iron Mine are analyzed in detail. The results are combined with the results of site investigations and in situ monitoring data, allowing the characteristics of the ground-surface collapse events to be summarized. The mechanism by which ground-surface collapse is induced by underground orebody extraction or de-watering activity was then investigated. The results show that the ground-surface in the mining areas experienced three collapse processes: karst collapse, mixed collapse, and mining-induced collapse. Karst-collapse sinkholes were found during the construction stage of the mine and are generally formed in the marble part of the marble–granite contact zone. These sinkholes show a ‘bead-on-a-string’ distribution, and lines connecting their centers are consistent with the strike of the marble. Such sinkholes are mainly formed due to changes in the dynamic conditions of groundwater caused by de-watering activity. Furthermore, the development level of the collapse mainly depends on the location and scale of the karst caves within the marble. Mixed collapse occurs in the initial mining stage and mainly results from the combined effect of underground orebody excavation and de-watering activity. Mining-induced ground-surface collapse pits mostly develop in the diorite and hornstone and generally show a zonal distribution along the strike of the orebodies from east to west. Mining-induced collapse occurs in the form of chimney caving. First, a caved pipe is formed where the rock masses above the caved zone are highly fractured. Then, the rock masses around the caved pipe gradually cave, and finally a large caved-zone is formed above the mined-out area. Mixed collapse is of particular interest as it is a peculiar type of subsidence involving the superposition of collapse processes related to both mining and karst cavities, and it can provide some instructive insight into the collapsing behavior of the ground-surface.