12 March 2019
Chengchao Iron Ore Mine: severe subsidence induced by mining in China
There is a nice article in a recent edition of the journal Engineering Geology (Xia et al. 2019) examining subsidence induced by the Chengchao iron ore mine, which is located in Ezhou City, Hubei Province, China. The paper describes the dramatic subsidence structures and explains their formation. The subsidence pits are remarkable – this is a recent Google Earth image that shows the craters that have formed:-
The location is 30.319, 114.918 if you want to take a look. Mining started at the Chengchao iron ore mine in 1969, with the mining method focusing on sub-level caving. Subsidence started to be observed in the construction period for the mine (between 1958 and 1969), and it has extended thereafter. The subsidence is manifested on the surface primarily through the formation of large, deep pits, shown in the image below from Xia et al. (2019):-
Xia et al. (2019) note that subsidence has occurred over an area measuring 1200 m x 300 m. Whilst the pits are the obvious manifestation of the subsidence, the authors note that the ground between the craters has also subsided substantially. In the centre of the worst of the pits the total vertical movement is up to 60 metres! The largest of the pits have a diameter of about 70 metres, and subsidence is continuing to develop across the area, with smaller, new pits forming in addition to the broad subsidence bowl.
Interestingly, Xia et al. (2019) conclude that the processes of subsidence at Changchao iron ore mine are complex. The geology consists of a faulted mixture of granite, marble, diorite and hornstone. Some of the collapses are located within the karst (marble) rocks, primarily as a result of changes in the groundwater conditions during dewatering for the mine. But in many other cases the caving technique has removed support from the overlying rocks, which have collapsed into the void. This process is ongoing – compare the image below, from 2012, with the one at the top of the page (from 2018). Several new pits have developed, and others have enlarged substantially:-
The authors note that the subsidence has led to expensive works to relocate mining infrastructure. Fortunately, there are few non-mining assets in this area.
Kaizong Xia, Congxin Chen, Yun Zheng, Haina Zhang, Xiumin Liu, Yangyang Deng and Kuoyu Yang 2019. Engineering geology and ground collapse mechanism in the Chengchao Iron-ore Mine in China. Engineering Geology,
249, 129-147. https://doi.org/10.1016/j.enggeo.2018.12.028.