Monitoring analysis and environmental assessment of mine subsidence based on surface displacement monitoring

Abstract

Over the past several years, a metal mine by block caving method has experienced a long-term and progressive surface deformation and fracturing, and then we start our investigation based on this background. The location of surface rupture was based on a series of mapping activities and the deformation data was collected by GPS from 2013 to 2016. In this paper, emphasis was put on the analysis of the fissures, deformation and stress of surface subsidence. Results reveal the diversity magnitude and structural features of surface deformation and ground fissures. In addition, the time dependent behavior is comprehended and the subsidence zone reflects different types of time-displacement curve – regressive phase, steady phase and progressive phase, all these achievements indicate the complexity and diversity of the subsidence zone. On the other hand, stress calculation which inspired from the mechanical model of the cracking of hole wall is carried out, it is meaningful to understand the relation between fracture features, displacement vectors and horizontal stress.

Keywords

Block caving surface deformation ground fissures stress analysis

1. Introduction

Block caving is initiated by blasting an extensive horizontal panel, stress redistribution and gravity combine to trigger progressive fractures in the vicinity of the mining area. The ground surface does not subside immediately until caving thinned the overlying cap rock and cannot transfer the load effectively. Caving progressively extends upwards with a further extraction, causing significant surface depression or subsidence eventually, above the goaf and adjacent areas. The complex temporal-spatial process is controlled by geological structures, dip angle of orebody, joint orientation and fault location, and so on [1, 2, 3].

In general, two types of deformation zones can be observed on the ground surface – continuous and discontinuous deformation zones [4]. Block caving mining is typically characterized by discontinuous subsidence. Discontinuous deformation zone is characterized by irregular failure surface and large disturbances affected regions, Features such as tension cracks, steps and chimney caves normally appear in this area [5, 6]. Continuous deformation zone is a relatively regular and smooth lowering of the ground surface and shows only elastic deformation or continuous non-elastic strain [7].

According to Benko et al. [9], surface subsidence above longwall operations where the topography is relatively flat tends to be symmetric while surface subsidence above mines where a slope is present shows a pattern of greater subsidence developing in the upper part of the slope. Kyu-Seok [11] showed that caving-induced surface deformations tend to be discontinuous and asymmetric due to large movements around the cave controlled by geologic structures, rock mass heterogeneity and topographic effects. Villegas and Nordlund [8] proved the time-dependent movement of the hangingwall surface is described based on the surveying data and three different stages of the time-displacement behavior is identified. Stephenson et al. [10] performed a detailed analysis of the subsidence by three-dimension physical modelling, concluded that the failure mode is a combination of topping and shear failure. Du and Guo [12] pioneered detailed step-path analyses of rock slopes with the development of a limit equilibrium approach that incorporated shear failure along joints, shear through intact rock, and tensile failure of rock bridges. Brummer et al. [13] indicated a plausible evolution path of the footwall fracturing that was subsequently described by conceptual numerical models created in 3DEC. Vyazmensky et al. [14] evaluate the significance of geological structure on surface subsidence combined FEM/DEM-DFN (discrete fracture network) modeling. Also, many scholars have usednumerical simulation to study metal mines/tunnels. For example, codes such as PHASE2 [16, 17], FLAC3D [18], PFC2D [19], UDEC [20], 3DEC [21] have been used to study break and limit angles, ground surface subsidence, caved rock zone deformation, and failure mechanisms in discontinuous rock masses.

In this paper, the main objective is improving the understanding of subsidence phenomena and figuring the mechanism of block caving by a particular case study of Jingerquan. On the other hand, little attention has been paid to tectonic and stress patterns of the subsidence area, which is significant to fracture propagation. Hence there is a genuine need for an inclusive study on stress redistribution of surface subsidence.

2. Basic condition

The Jingerquan mine, located in the Xinjiang province in northeast China, is a typical metal mine with steep dip angle. The ore deposit is approximately 2.0 km long, 700 meters wide and extends to more than 500 meters underground, the strike of the orebody is almost 330 ${}^{\circ}$ and dips southwardly with dip angle range from 35 ${}^{\circ}$ in the deep excavating orebody to about 65 ${}^{\circ}$ which located in shallow orebody excavated before. as parts of the orebodies were quite near the surface (some were even directly exposed on the surface), many private mining sites were set up. The mining activities at Jingerquan commenced in 2009, a total of 2Mt ore was excavated and roughly 800 thousand square meters mined-out areas was produced up to 2015. In 2012 which being mined with block caving mining method for about 3 years, large scale ground movement and ground fissures appeared, as a result, we start our fundamental investigation in 2013.

The mechanical properties and corresponding parameters of the rock mass are the fundament of stability evaluation of engineering rock mass. A large quantity of laboratory tests on various kinds of rock mass have been conducted in Jingerquan mine, the results of tests are shown in Fig. 1. It can be seen from the text result that the principal strengths of rock mass improve sharply with the increasing of mining depth, except a slight decline of poisson ratio from plane 965 to plane 981. On the other hand, the features of strong alteration and weathering widely develop in the superficial and shallow of rock mass, which greatly decrease the mechanical properties of the rock mass.

Figure 1.

Mechanical properties of different depth.

Besides the engineering geology and rock mechanical properties, tectonic stress is another prominent factor inducing a large scale deformation of ground surface. In terms of Jingerquan mine, field tests and measurements have been taken on the virgin stresses by the stress releasing technique. According to the in situ stress results, the distribution characteristics of the virgin stresses in the Jingerquan mine area are as follows:

The horizontal stress is the maximum principal stress of the mine area which trends NE with the value of 34.9 MPa.

The maximum principal stress is basically parallel to the regional tectonic stress field in Xinjiang region.

The vertical stress of rock mass with value of 18.5 MPa is the intermediate principal stress, which exceed the weight of overlying strata.

The shear stress reaches 8.55 MPa which is liable to damage the roadway roof.

3. Monitoring

3.1 Fissures

In 2013, a brief visual inspection and surface mapping was conducted and the mapping was firstly focused on the surface fissures. The investigation results showed that all of the visual surface deformation and failure situated in the hanging rocks, a trough subsidence formed over a large area, the overburden of the center part of trough moves vertically downward and the adjacent moves towards the center and downwards at the same time, those movements have enough cumulative effect to form fissures. The fissures can be divided into two groups according to their distribution characteristics: Fissure Zone No. 1 and Fissure Zone No. 2.

Fissure Zone No. 1 is located above the goaf and two kinds of fissures are visible compression fissures and tension fissures. The main fissures of this area are compression fissures of X-shape whose average extended length is approximately 50–100 meters. The angle of X-shape is controlled by maximum principal stress and minimum principal stress which are formed by squeezing action of the trough. There are many branch fissures parallel to the main compression fissures in this zone. The tension fissures often appear around the trough and steps are likely formed with the continuous and intensive subsidence. Fissure Zone No. 2, formed by tension stress, is located 300 m northward away from Fissure Zone No. 1. It has a length of 30–40 m average and a width of 10–20 mm. The fissures in the zone trend to be connected. Fissures have often been associated with geological discontinuities and faults and the extending orientation of the fissure zones appears to be approximately parallel to the trend of the ore body.

The measurement results by the fissure gauges revealed that the relative displacement of the two sidewalls of a ground fissure usually appears to be of three-dimensional features. The relative displacement of monitoring points around fissure No. 4 kept inconsistent and increased continuously with the mining deep, it means fissure No. 4 is still in activity. Similarly, the data of monitoring points around fissure No. 5 and No. 6 shows that fissure No. 5 and No. 6 appeared a tendency of expansion, while it’s much gentler than fissure No. 4. We can deduct from the results above that the deep extension section below Fissure Zone No. 1 had wrecked and separated from each other. According to the monitoring data, the monitoring points around fissure No. 1, No. 2 and No. 3 moved to subsidence center as a whole and the width of fissures remained stable in general. We could deduce from above that the deep extension section below Fissure Zone No. 2 keeps a state of bending instead of wrecking.

Figure 2.

Schematic diagram of surface cracks, red lines represent visible fissures, blue triangle represent the monitoring points around the typical fissures.

Figure 3.

Scene photographs of fissure No. 4 and No. 5.

3.2 GPS

In 2013, a monitoring network of 101 stations, which are marked bars mounted in cylindrical concrete bases buried 0.5 m into the ground, are distributed along the hanging wall in eight surveying lines oriented north-south (94, 95, 96, 97, 98, 99, 100, 101) and monitoring data are updated every four months. Besides, seven additional monitoring stations are applied around the subsidence center for safety reasons.

As shown in the vertical displacement contours, the vertical subsidence zone was an irregular cycle which similar to tubular-shaped and the affected area extended 300 meters along the dip direction of the ore body, 350 meters along the strike. By the latest monitoring data of 2016.4, the maximum vertical displacement reached 1650.6 mm. According to the comparison of each stage of subsidence curve, the magnitude of the subsidence increase with the long-term and large-scale repeated excavation, however, the location of the subsidence center are not changed obviously in spite of the center of the orebody moved with the mining depth. It means that the ground surface subsidence center was mainly decided by the geologic weaknesses in the rock mass underground mainly.

In terms of horizontal displacements, the horizontal displacement zone was an irregular cycle similarly, the maximum horizontal displacement reached 473.99 mm which was less than vertical displacement, it means the vertical displacement was more prominent but the horizontal displacement can’t be ignored simultaneously. On the other hand, the subsidence center sink continuously during the excavation, we can speculate that new air faces appeared and horizontal structure stress released coincidentally, which manifested as tensile cracks. Consequently, the horizon deformation zone expanded to north by the influence of tectonic joints and fracture zones. The horizontal displacement of expanded area was roughly 20–30 mm from 2013.10 to 2016.4.

Figure 4.

Accumulative vertical displacement from 2013.7–2016.4, on the base map of Google Earth (the red pointer represent the monitoring points).

Figure 5.

Accumulative horizontal displacement from 2013.7–2016.4, on the base map of Google Earth (the red pointer represent the monitoring points).

4. Methodology of horizontal stress calculation

Trough is the surface expression of subsidence due to collapse of support pillar where large voids are created by the mining activity, stress redistribution and gravity combine to trigger progressive fracturing and caving of the rock mass between surface collapse trough and underground mined area. The stress around the collapse trough has varied during the conversion from rock-intact to rock-fractured downwards and we put emphasis on the horizontal stress on ground surface in this paper.

Before the excavation, the horizontal stress of the in-situ stress can be expressed by tectonic stress and horizontal component caused by gravity stress.

$\displaystyle\sigma_{1}=\sigma_{1}^{k}+\sigma_{H}=\sigma_{1}^{k}+k\gamma_{0}h=% \sigma_{1}^{k}+\frac{\mu}{1-\mu}\gamma_{0}h$ (1) $\displaystyle\sigma_{2}=\sigma_{2}^{k}+\sigma_{H}=\sigma_{2}^{k}+k\gamma_{0}h=% \sigma_{2}^{k}+\frac{\mu}{1-\mu}\gamma_{0}h$ (2)

In which $\sigma_{1}$ and $\sigma_{2}$ are the horizontal stress of the in-situ stress, $\sigma_{1}^{k}$ and $\sigma_{2}^{k}$ are horizontal tectonic stress, $\sigma_{H}$ is the horizontal component caused by gravity stress, $k$ is side pressure coefficient, $\mu$ is Poisson ratio, $\gamma_{0}$ is rock bulk density, $h$ is depth to surface ground.

Due to the formation of collapse trough, the stress point to the wall of the collapse trough is defined as $\sigma_{0}$ , therefore, the stress around the collapse trough has changed, compared to the in-situ stress.

$\displaystyle\nabla\sigma_{1}=\sigma_{1}-\sigma_{0},\nabla\sigma_{2}=\sigma_{2% }-\sigma_{0}$ (3)

In which $\nabla\sigma_{1}$ and $\nabla\sigma_{2}$ are the horizontal stress following the collapse.

The horizontal slice around the surface trough is in a state of plane stress, a large quantity of experimental research shows that the stress state is in accordance with plane solution of elastic theory, by a condition of the depth of mine-out area is much greater than the diameter of collapse trough. The radial stress ranges from in-situ stress away from the collapse trough to lateral stress of the wall of the collapse trough, the normal shear-stress concentrate and reach the maximum value.

The stress calculation model of Jingerquan is inspired from the mechanical model of the cracking of hole wall, we take a hypothesis that the caving zone is a cavity filled with debris of rock mass, hence $\sigma_{0}=$ 0, $\sigma_{H}=$ 0 on the surface ground of the caving zone, the Eqs (1) and (2) turn into Eq. (3) as below:

$\displaystyle\nabla\sigma_{1}=\sigma_{1}^{k}\ \ \ \nabla\sigma_{2}=\sigma_{2}^% {k}$ (4)

We obtain the correlation formula between radial and tangential displacement $u, v$ and horizontal tectonic stress $\sigma_{1}^{k},\sigma_{2}^{k}$ based on the solution of G-Kirsch, as follows:

$\displaystyle u=\frac{1+\mu}{2E}\left[{\left({\sigma_{1}^{k}+\sigma_{2}^{k}}% \right)\left({r+\frac{R^{2}}{r}-2\mu r}\right)+\left({\sigma_{1}^{k}-\sigma_{2% }^{k}}\right)\left({r+4\frac{R^{2}}{r}-4\mu\frac{R^{2}}{r}-\frac{R^{4}}{r^{3}}% }\right)\text{cos}2\theta}\right]$ (5) $\displaystyle v=\frac{1+\mu}{2E}\left[{\left({\sigma_{2}^{k}-\sigma_{1}^{k}}% \right)\left({r+\frac{2R^{3}}{r}-8\mu\frac{R^{2}}{r}+\frac{R^{4}}{r^{3}}}% \right)}\right]\text{sin}2\theta$ (6)

In which $R$ is the radius of collapse trough, $r$ and $\theta$ are the polar coordinates of the monitoring points, $E$ and $\mu$ are the elastic modulus and poisson ratio of rock mass.

The formula of surrounding rock displacement before excavation is as follows:

$\displaystyle u_{0}=\frac{1+\mu}{2E}r\left[{\left({\sigma_{2}^{k}+\sigma_{1}^{% k}}\right)\left({1-2\mu}\right)+\left({\sigma_{2}^{k}-\sigma_{1}^{k}}\right)% \text{cos}2\theta}\right]$ (7) $\displaystyle v_{0}=\frac{1+\mu}{2E}r\left({\sigma_{2}^{k}-\sigma_{1}^{k}}% \right)\text{sin}2\theta$ (8)

The final formula of displacement in the subsidence area is as follows:

$\displaystyle u_{r}=u-u_{0}=\frac{1+\mu}{2E}\left[{\left({\sigma_{1}^{k}+% \sigma_{2}^{k}}\right)\frac{R^{2}}{r}+\left({\sigma_{1}^{k}-\sigma_{2}^{k}}% \right)\left({4\frac{R^{2}}{r}-4\mu\frac{R^{2}}{r}-\frac{R^{4}}{r^{3}}}\right)% \text{cos}2\theta}\right]$ (9) $\displaystyle v_{\theta}=v-v_{0}=\frac{\sigma_{1}^{k}-\sigma_{2}^{k}}{2E}\left% ({1+\mu}\right)\left[{\frac{R^{4}}{r^{3}}-2\frac{\left({1-\mu}\right)R^{2}}{% \left({1+\mu}\right)r}}\right]\text{sin}2\theta$ (10)

5. Results and discussion

It is a normal practice to analyze the surface displacements, because the displacements can be acquired directly by GPS or InSAR, thus less effort is done to reflect on stress reaction coupled with continuous ore extraction, which is the key factor of quantity and direction of displacement vector, as well as the fracture distribution. A horizontal stress calculation based on Eqs (9) and (10) is conducted by MATLAB and the input parameters are $u,v,r,\theta$ . $r,\theta$ represent the position of the monitoring points in polar coordinate system, which the zero point of the polar coordinate system is the subsidence center according to the previous monitoring results, $u, v$ represent the radial-displacement and tangential-displacement surveyed by GPS from 2013–2016. The radial-stress and tangential-stress are calculated and the detailed features can be summarized in the following.

Figure 6.

Schematic diagram of radial-stress, positive value represents the direction pointed to the subsidence center.

Figure 6 presents the quantity and distribution of radial-stress and shows a completely distinct trend compared with distribution of displacement. The extension can be divided into two regions parallel to the strike of the orebody, located in the north and south of the subsidence center. The maximum radial-stress of south region is 31.77 Mpa and concentrates on two monitoring points on both sides of a compression fissure of X-shaped located in Fissure Zone No. 1, the radial-stress of other area ranges from 2.9 Mpa to 6.7 Mpa and extends to the mining direction. The north region is unconnected and located in Fissure Zone No. 1 and Fissure Zone No. 2 respectively, the maximum radial-stress of north region is 11.43 MPa located Fissure Zone No. 1, the radial-stress lies in Fissure Zone No. 2 range from 2.1 MPa to 3.6 MPa.

Figure 7.

Schematic diagram of tangential-stress, positive value represents the direction of clockwise.

Figure 8.

Schematic diagram of the relationship between radial stress, surface fissures and displacement vectors.

Figure 9.

Schematic diagram of the relationship between tangential stress, surface fissures and displacement vectors.

As shown in Fig. 7, the distribution of tangential-stress shows a similar trend with the radial-stress, two regions of north and south and parallel to the strike of the orebody. While the bigger stress of 30.31 MPa lies in the north region compared with the south region, and the north region just extends a narrow area compared with the radial-stress, we cannot observe the concentration of tangential-stress in Fissure Zone No. 2. The maximum tangential-stress of south region is 9.92 Mpa and extends to the mining direction which is similar to the radial-stress.

In order to understand the relation between horizontal stress, fissures and displacement direction fully, we draw the stress distribution graph as shown in Fig. 8. Fissure No. 5 and No. 6 located in the two wings of X-shaped fissure, corresponding to the concentration of radial-stress. The X-shaped is irregular, the further wing is about 100 meters and in north-south direction which is almost parallel to the spread of radial-stress, the other extends just about 40 meters. Fissure No. 1, No. 2 and No. 3 are tensile cracks and related to the radial-stress in north region, these fissures existed before we begin our investigation and monitoring in 2013, the maximum radial-stress of these fissures is 3.6 MPa and less than the ultimate tensile strength, it means that the maximum radial-stress reached the ultimate tensile strength at some time in the past and reduced rapidly following the appearance of tensile fissures. Additionally, we cannot observe tensile fissures in the south of the subsidence center corresponding with Fissure No. 1, No. 2 and No. 3 at present, but the maximum of tangential-stress in this area is 6.7 MPa and we can deduce that this area is the potential breakage with tensile failure coupled with continuous ore extraction. As to fissure No. 4, there are numerous secondary fissures parallel to fissure No. 4 which is unique in the subsidence area, these secondary fissures situated around the concentration region of tangential-stress, affected and dominated by tangential-stress.

Prophase research have proved that displacement vector pointed to the underground mine and changed with the distance from the monitoring points to the center of the ore body. The conclusion emphasized the effort of radial-stress while the influence of tangential-stress is ignored. As shown in Fig. 9, the direction of displacement vectors doesn’t point to the subsidence center absolutely but deflect to the direction of the tangential-stress, especially in the area of stress concentration. On the other hand, it does not mean that the stress focus on all angle of the subsidence center, the position of stress concentration is controlled and influenced by the joint orientation and faults.

6. Conclusions

The long-term GPS monitoring of ground deformation revealed the spatial distribution characteristic and the reaction to the large-scale excavation, the maximum vertical displacement reached 1650.6 mm and the maximum horizontal displacement reached 473.99 mm in the past four years. As to the fissures, two groups are divided according to the distribution characteristics, one of them is located above the goaf and constituted of compression fissures of X-shaped, the other are tension fissures and lie on the border of subsidence area. Moreover, the failure of deep extension section is speculated by the relative displacement of the fissures.

The time-dependent deformation goes through three different stages – regressive, steady and progressive and represent different failure mechanism respectively, it could be an important tool to estimate the movement rate of different regions. We classified monitoring points by different subsidence stages and strengthen the understanding of the whole surface subsidence.

Inspired from the mechanical model of the cracking of hole wall, we acquired the calculation formula of horizontal stress on ground surface based on a serious of hypothesis, and then the stress distribution of surface subsidence accompanied with underground mining is distinct. Based on the achievement above, we got a more reasonable explanation of the displacement vectors and fissures – X-shaped fissures, tensile fissures and secondary fissures.

Although the long-term GPS monitoring of ground deformation is laborious, this research illustrates that it is beneficial for a deeper understanding of the deformation features and fracture mechanism. Furthermore, the mine construction engineering can be arranged rationally to avoid these disasters.

Footnotes

Acknowledgments

The research was supported by the Natural Science Foundation of China (Grant Nos. 51774184) and the Research Initiation Fund Project of Northern University of Technology (Grant Nos. 51774184).

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