Mining-induced ground deformation in tectonic stress metal mines: A case study
Introduction
In metal mines, ground deformation is influenced by many factors, such as geological structure, in situ stress, existence of ore bodies, the hydrogeological conditions, and the method of mining (Brady and Brown, 1985, Cai et al., 2000, Bruneau et al., 2003a, Bruneau et al., 2003b, Li et al., 2004, Ma et al., 2007, Liu et al., 2011, Zhao et al., 2013b, Fu et al., 2015), leading to an obvious difference between the ground deformation induced by underground mining of metal and coal mines, and thus has distinctive characteristics. For instance, in Copper Mine, Mount Isa, Australia, under the influence of the W41 and W42 faults, there is a tension zone resulting in vertical displacement around the shaft at level 11, and therein, vertical displacements appear offset (Bruneau et al., 2003a, Bruneau et al., 2003b); In the Jinshandian Iron Mine, China, deep deformation of the rock mass may cause fracture, and is thus discontinuous. However in the ground, the deformation is continuous (Fu et al., 2015); in the Longshou Mine, under the influence of open-pit slope extrusion and underground mining, buckle folding deformation occurs at the pit bottom, and the central part of the pit bottom is rising while the surrounding areas are subsiding (Zhao et al., 2011, Zhao et al., 2013a).
More complex factors cause the ground deformation induced by underground metal mining to be complicated, especially in tectonic stress metal mines which have suffered from many intensive tectonic movements in the formation of their orebodies (He, 2003, Huang, 2008). Such metal mines may frequently be subjected to horizontal tectonic stresses greater than those applied in the vertical direction (up to several tens of times higher), thus the mining-induced secondary stress field can significantly affect the ground deformation characteristics. Chenchao Iron Mine is typical of the tectonic-stress metal mines in China (Chen and Xia, 2013, Huang, 2008). In its western area, as reported by Chen et al. (2012), ground collapse occurred suddenly with no evident pre-warning on 17 April, 2006, and resulted in a 4140 m2 subsidence area and severe economic loss. Furthermore the subsidence area spread rapidly as the mining depth increased (Chen et al., 2013). Excessive ground deformations were measured at greater than 10 metres from the mine road of the footwall: ground subsidence and cracking spread to the southeast area around the transport tunnel: this has cost more than ¥30 million in maintaining the transport tunnel to date. Analysis of monitoring results revealed that ground deformation may differ in mining-influenced areas or may remain relatively unchanged, even showing distinctive characteristics compared to those seen in other mines. Thus comprehensive investigation of mining-induced ground deformation from the ground collapse (17 April, 2006) to date will facilitate more effective safe production in Chengchao Iron Mine, and further may possibly offer a reference to other tectonic stress metal mines.
In this study, in situ monitoring of ground deformation in the western area of the Chenchao Iron Mine was conducted for an eight year period, including horizontal and vertical displacement information. Firstly the distribution characteristics of the ratio of horizontal to vertical displacement and characteristics of the time–displacement curves were investigated. Then, based on the relative position and orientation of the maximum principal stress, the strike of the major structure plane, and the strike of the mined-out area in the footwall, the failure process affecting the rock mass in mining-induced areas was deduced. Eventually three kinds of the time–displacement curves types were found, and the corresponding curve characteristics were studied to deduce the failure mechanism of the associated deep rock masses.
Section snippets
Distribution of rock masses
Chengchao Iron Mine lies in a complex geological environment with either a hilly or mountainous topography, and all the known orebodies mostly existing in gully areas. The rock mass surrounding the hanging wall near the ore body is diorite, with hornstone found at greater distances therefrom; the footwall is high-quality granite, which is also the main rock mass in the mine. The surrounding rock mass, from the top of the ore body to its outcrop between the hanging wall and the footwall, is a
Monitoring method and implementation process
To investigate the ground deformation, firstly measurement was carried out using both levels and total stations (Huang, 2008). With mining activities extending both horizontally and vertically, the mining-influenced area spread rapidly: due to the steep, changing topography this proved too expensive. A global positioning system (GPS) monitoring technique (Lv et al., 2008, Mancini et al., 2009, Ng et al., 2010, Wang et al., 2011, Liu et al., 2012, Can et al., 2013), accurate in the horizontal
Discussion
Hence, according to the aforementioned analysis, we can conclude that the main factors affecting ground deformation in tectonic stress metal mines as follows:
- (1)
Underground mining is the critical factor affecting ground deformation: the magnitude of the ground deformation, and the failure mode of the rock mass is primarily determined by the size, shape, and location of the mined-out area (e.g., the different failure modes seen in each rock mass in the north-eastern area and northern area of
Conclusion
By analysis of the ground deformations in the western area of the Chengchao Iron Mine, the following may be concluded:
- (1)
In the northeastern area, with increased distance from the mined-out area, the ratios of horizontal to vertical displacement increased gradually, at a large distance, the horizontal displacement may be even an order of magnitude greater than the vertical displacement, which induced regional horizontal movement. Plots of the ratio of horizontal to vertical displacement over time
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