Xishimen iron mine is located in Wuan County Hebei Province. There is a river, named Mahe River, in the mine zone. In order to maintain the stability of Mahe riverbed, an ore pillar of more than 200m in width was reserved under the riverbed. But the ore pillar had been exploited illegally. In July 2004, the ground subsidence of the Mahe River trench had been discovered. To make the riverbed stability, a technology treating the goaf under riverbed was proposed. Firstly the engineering geology mapping, geophysical prospecting and drilling technology were utilized to investigate the present situation of goafs; Secondly, the mechanism of ground subsidence was analyzed by using finite element method, and the influence on subsidence induced by the following mining was forecasted. Finally, based on the analysis result, a reinforcement scheme, including backfill in the goaf, grouting reinforcement in the superficial crevasse belt and waterproof layer construction on the surface of the riverbed, was completed. Moreover, ground settlement survey was used to examine the effectiveness of the treatment. The survey results showed that new surface subsidence did not arise. The treatment scheme can be referred to similar engineering.
In mining mineral resources, the natural environment will inevitably be changed[1]. Improper exploitation will cause geological hazards such as surface subsidence, landslide, roof falls[2- 4]. Once a geological accident occurs, it would seriously affect the mine's production. The research on the occurrence mechanism of geological accident provides a reasonable basis of reinforcement measures, which can effectively prevent similar accidents and protect the mine's safety[5, 6]. In the process of the research of the geological mechanism of the accident, the study of the status of goaf, geological structure, mechanical properties of rock are the bases, numerical calculation is a common means to simulate the development of goaf, and on-site monitoring can effectively observe changes in the development of geological disasters and strengthening effect[7–11]. Because the civilian illegally exploited the ore pillar which was reserved to make Mahe riverbed stability, local land subsidence occurred in the mining area of Xishimen iron ore plant which is located in Wuan County of Hebei Province. Figure 1 shows the pre- and post- subsidence riverbed location of Mahe River. The subsidence of the Mahe river trench has destroyed the anti-seepage structure of the riverbed, and would jeopardize the normal mine activities. In this paper, we investigated the mechanism of ground subsidence and forecast the development of settlement. Then, a reinforcement scheme was proposed to make the riverbed stability.
To find out the present situation of goaf and develop a reliable reinforcement scheme, three types of geology surveys were carried out. These included engineering geology mapping, geophysical prospecting and drilling technology. Engineering geology mapping investigated the minerals distribution, terrain topography, goaf and monitored ground surface settlement. The investigation results showed that there would be four big goafs(see in Fig. 2). Then we used the geophysical prospecting and drilling technology to check these goafs.
