Study on Surface Deformation and Movement Caused by Deep Continuous Mining of Steeply Inclined Ore Bodies

Abstract

In order to study the surface movement and deformation law of deep continuous mining of steeply inclined orebodies in high-stress areas, the surface movement and deformation law of deep continuous mining by caving method in the Shizishan mining area was studied based on the field fissures investigation, GPS monitoring, and large-scale geotechnical engineering numerical simulation software FLAC3D 5.0. The results show that with deep continuous mining of the orebody, surface fissures, and monitoring displacement are rapidly increasing. After the stoping of different sublevel orebodies, there will be an obvious settlement center on the surface, and the horizontal surface displacement also shows a trend of gradual increase. The results indicate that surface subsidence at the mine site is in an active development phase. The research results are of great significance to the prevention and control of surface rock movement disasters in mining areas.

1. Introduction

As the mining depth of metal mines deepens and the number of goafs increases, geological disasters frequently occur [1]. The mining of deep orebodies causes environmental changes, especially the mining of steeply inclined orebodies [2,3], which lead to mine disasters, such as surface subsidence, landslides, mine earthquakes, water, mud inrush, etc. These disasters cause a large number of casualties, equipment destruction, and loss of mineral resources [4,5,6], causing huge losses of life and property to the country, mining enterprises, and the masses of people in the mining area. This results in adverse social effects, seriously restricting the sustainable development of the national economy and mining enterprises [7,8].

In recent years, many scholars have conducted research into the surface deformation caused by deep mining of steeply inclined orebodies in high-stress areas. Some progress has been made in studying surface deformation and movement caused by mining [9]. As the depth of underground metal mining increases, the subsidence of surface displacement will also change. Gao, Xu, and Li et al. [10,11,12] analyzed the monitoring results of the surface movement caused by deep mining, thereby revealing the mechanism of surface deformation caused by deep mining. With the progress of science and technology and the increasing attention to the surface deformation caused by underground mining, Eray and Zhao et al. [13,14] monitored, analyzed, and evaluated the surface deformation caused by the mining process of deep orebodies based on GPS measurement technology. Because the mined terrain is relatively complex, some mines cannot monitor the surface deformation using GPS technology, so Chen, Agnieszka, and Fan et al. [15,16,17] monitored the surface deformation caused by deep mining using InSAR monitoring technology, revealing the distribution and deformation law of surface deformation in time and space. Until today, InSAR monitoring technology has been widely used in the study of surface deformation caused by deep orebody mining and has derived different data processing methods such as PS-InSAR [18], AD-InSAR [19], SBAS-DInSAR [20,21], etc., but these methods are based on historical data for analysis and cannot study future deformation development trends. Numerical simulation technology has developed from the last century to today and is gradually maturing. Through the method of numerical simulation, the surface deformation caused by deep mining can be understood in advance by establishing a model. Wang et al. [22] used the discrete element software PFC 5.0 to study the mechanisms of surface collapse caused by underground mining. Guo et al. [23,24] used the discrete element software 3DEC 4.0 to study the movement and deformation law of the surface and surrounding rock caused by the dip angle of the orebody to the mining of the steeply inclined orebody under a tectonic stress environment. Svartsjaern and Yuan et al. [25,26] analyzed the generalized transmission mode of rock-mass movement caused by orebody excavation and the characteristics of surface rock-mass movement through numerical simulation and actual monitoring. Since monitoring is only an analysis of past displacement, it is more critical to predict surface-displacement changes caused by underground mining based on monitoring data. Mateusz et al. [27] used the finite element method to predict the surface movement and deformation caused by underground mining. Zhao et al. [28] proposed a method for predicting surface subsidence in mining areas based on a probabilistic integral model that combines the principles of geotechnical movement in different study areas. Takashi et al. [29] used the finite difference method (FLAC3D) combined with the surface monitoring data to predict the surface deformation caused by underground mining. James et al. [30] established the strain prediction equation based on the two-step method and predicted the relative horizontal movement of the mining area surface through the strain prediction equation. Nie et al. [31] proposed the arctangent function prediction model to predict the surface displacement caused by the excavation of underground ore bodies based on the “S” type settlement curve of the collapse pit monitoring points and the failure mechanism of the goaf. The prediction method of the numerical simulation model is easily affected by the small changes in different formation properties, fractures, weak intercalations, rock-mass structure, etc., which affects the reliability of the prediction results [32,33]. Therefore, when establishing a model through numerical simulation to predict the surface deformation and movement caused by deep orebody mining, it is necessary to fully understand the stratum properties and rock-mass structure of the predicted area so as to improve the reliability of the prediction result.

In conclusion, although there are many studies on surface movement and deformation caused by underground orebody mining, most research methods are relatively single. It is easily influenced by various factors within the study area, thereby reducing the accuracy of research results. In addition, there are few studies on the law of surface movement and deformation caused by deep continuous mining of steep, thick, and large orebodies under high stress. Therefore, this paper takes into account various influencing factors, such as the topographic feature and rock properties of the mining area. The continuous mining of the deep part of the Shizishan Copper Mine is the research background. Using three different methods for joint research, the on-site investigation of ground fissures and GPS monitoring of surface rock movement are applied to analyze the characteristics of surface deformation. Furthermore, the numerical simulation software FLAC3D is used to predict and analyze the law and trend of surface movement and deformation caused by continuous mining of steeply inclined orebodies under high stress. The research results are of great significance for surface movement and deformation disasters such as mountain landslides and surface subsidence caused by deep continuous mining in mines.

2. Engineering Geology of the Mining Area

Shizishan Copper Mine is situated in Xiaojie Town, Yimen County, Yunnan Province, China, right at the intersection of Yimen, Shuangbai, and Lufeng Counties. Conveniently located 51 km away from Yimen County, 89 km from Kunming City, and 52 km away from the Lufeng station of the Chengdu-Kunming railway, the mine is surrounded by the Yuanjiang fault depression basin to the south, the Kunming depression belt to the east, and the Luwu fault depression basin to the north. The strata within the area range from Sinian to Paleozoic and Mesozoic, with Kunyang group strata outcropping in over 90% of the area, partially covered by the Mesozoic Red Beds, while the Yuanmou-Xinping ancient land is situated to the west of the Lvzhijiang fault. Figure 1 shows the geographical location of the Shizishan Copper Mine.

The deposit in the Shizishan mining area belongs to the famous Dongchuan-type sedimentary, metamorphic copper deposit. It is mainly distributed in two ore belts. The eastern ore belt, which is mainly distributed in the Minluoxue strata and the light purple alternating zone of dolomite and argillaceous dolomite, with a small amount distributed in sandstone, is characterized by wide distribution and a large number of ore occurrences, though the overall situation is poor and not confined to this area. The Shizishan deposit is distributed in this belt. The western ore belt, the Shishan layer at the bottom of the Lvzhijiang River, is also composed of light purple interactive argillaceous dolomite, mainly distributed in the Yimen area and accounting for more than 70% of the proved reserves. The magmatic activity in the mining area is relatively simple, and the primary lithology of the mining area includes cyan-grey dolomite, faded dolomite, purple slate, and carbonaceous slate. The typical sublevel section of the Shizishan Copper Mine is shown in Figure 2.

The dip angle of the orebody in the Shizishan mining area is 70~80°, belonging to the steeply inclined orebody [35,36], and the thickness of the orebody is 20~160 m. The typical ore body profile of the Shizishan Copper Mine is shown in Figure 3. The designed beneficiation capacity of the mining area is 1700 t/d, and the actual production scale is 1750–2200 t/d. The mine began production in October 1977. The highest surface elevation of the mine is 2143 m, and the design elevation of the project’s first phase is 1787~1587 m (sublevels 4 to 8). The design of the mine capacity is 1750 t/d by using an adit-chute, auxiliary shaft, and cableway transportation scheme, and mining has been completed. The design elevation of the project’s second phase is 1587~1337 m (sublevels 8 to 13). The blind shaft development scheme has been adopted, and the designed ore capacity is 1000 t/d. The design elevation of the project’s third phase is 1337~1237 m (sublevels 14 to 15). A blind inclined shaft development scheme has been adopted, and the designed ore capacity is 1000 t/d. The basic stoping of the project’s third phase is completed. The design elevation of the project’s fourth phase is 1237~787 m (sublevels 16 to 24). At present (June 2015), the mining of orebody in sublevel 16 and the development of sublevel 17 of the deep continuous mining project are being carried out [37]. According to the measured results of in-situ stress in the mining area, the indoor rock mechanics test results, and the quantitative standard of high in-situ stress [38,39,40], the mining area should belong to the category of high in-situ stress areas.

3. The Change Situation of Ground Surface Fissures Disasters in Mining Areas

With continuous mining for many years, the problem of surface damage characterized by ground fissures in the Shizishan Copper Mine has become increasingly prominent. In order to have a more intuitionistic understanding of the actual changes in ground fissure disasters in mining areas, the author and the staff of the mine’s surveying and mapping department conducted a field survey on the surface ground fissures in the mining area in 2013 and 2015. A comparison map of the development and distribution of surface ground fissures in the mining area was drawn according to the field survey results (Figure 4). Contrasted with the distribution of surface fissures in 2013, it was observed that the zone of surface subsidence extended outward. The fissures in the north (footwall direction) and the south (hanging wall direction) have undergone significant changes from the previous stage. Several large fissures appear to be interconnected in the direction of the orebody, and their widths have obviously increased, their extensions have been intensified, the depths of the surface fissures have deepened, and their affected range has become wider. In the western part of the surface, there is a scarp of several hundred meters, and its height is continually increasing. New surface fissures seem to be diffusing outwards in the eastern part of the surface.

Three new fissures have been created east of the surface between sections 10 and 5. The first fissure is nearly E-W, with an extended length of 80 m and a maximum width of 50 cm. The second fissure is NW 49°, with an extended length of about 45 m and a maximum width of 20 cm. The third fissure has a turning length of about 65 m, towards NE 2°~NE 10°, and the maximum fissure width is about 30 cm. Fissures in the eastern part of the surface extend outward 40 m compared to the previous period. Figure 5 shows ground fissures in the eastern part of the surface.

There are NW 48° step fractures located in the west of the surface, between section lines 55 and 60, and a step height of approximately 2.5 m. It is connected with the NE 9° fractures in the north and the trend in the north. Two new fissures were generated 15 m to 25 m away from the lateral side of the step fissure, and the two fissures were not connected. The direction of the two fissures was approximately parallel to the step fissure, and the maximum width was about 30 cm (Figure 6).

There are two approximately parallel fissure zones in the north of the surface (the footwall direction of the orebody, between profile lines 55 and 60, near X-direction coordinate 24°51′17″ N). The fissure strike is about NE 42°. The length of the first fissure is 67 m, the length of the second fissure is 114 m, and the distance between the two fissures is about 11 m, belonging to the tension fissure. The measurement suggests that no new ground fissures have appeared in the north of the surface (the direction of the footwall of the orebody), and the width and depth of many early tension fissures are further expanded and deepened (Figure 7).

On the south of the ground surface (the hanging wall direction of orebody), the first fissure zone lies between section lines 40 and 60; in the early stage, four new fissures were generated in the south of the surface, and the fissure trend was nearly E-W, as a set of echelon cracks. The outermost fissure is about 64 m away from the outermost fissure in the early stage, indicating that the fissure in the first fissure area has an obvious tendency to expand towards the hanging wall. Located in the second fissure zone between section lines 15 and 35, it strikes NE 44°. In the early stage, the two fissures have been wholly penetrated and extended to both ends in parallel. The extension length is more than 300 m. From section lines 15 to 40, the direction is close to the direction of the orebody. There is a stepped distribution between section lines 30 and 40. Compared with the early stage, the maximum step height difference increases to 1.9 m. East of profile line 30, affected by topography, an opening fissure with a maximum width of about 0.8 m is formed (Figure 8).

From 2013 to 2015, the development of ground fissures did not stabilize with the downward depth of mining. On the contrary, the deformation of the ground fissures in the mining area increased damage to the surface, especially in the hanging wall direction (southeast of the surface), and the trend continued to grow or deteriorate. With deep continuous mining, the continuous collapse of overlying rock mass caused by mining further affects surface deformation [41]. In the future mining process, after section 17, it is very likely a collapse pit will form on the surface within a short time, which must be paid attention to it.

It can be seen that in the fissure development zone, the surface mountains show signs of tearing, collapse, and other phenomena. From the morphology of the ground fissures, section lines 55 to 60 in the western part of the surface form stepped geomorphic features, similar to the typical fault type. The fissures in the footwall direction to the north and east of the surface are mainly tensile fissures, which are characterized by wide, deep, steep, and almost vertical fissures. In the direction of the hanging wall of the south surface, east of profile 40, the ground fissures are in the direction of south and east, showing an echelon arrangement. The outermost ground fissures are in step distribution, suspected to be the boundary of a sinking basin, while in the west of profile 40, there is an echelon arrangement in the south direction.

The measured data of ground fissures show that each fissure has deformation. There are not only opening displacements but also descending and horizontal staggering displacements to one side of the goaf. The displacement in different directions has the same order of magnitude. In the north and east of the surface, the horizontal dislocation displacement is the largest on both sides of the surface fissure of the cyan-grey dolomite and the faded dolomite, followed by the vertical subsidence displacement. The surface fissure settlement of the faded dolomite in the west is the largest, followed by the horizontal displacement. The deformation of the first fissure zone located in the purple slate in the hanging wall is smaller than that of the second fissure zone, but the southward development trend of the first fissure zone is more pronounced, and the surface displacement of the second fissure zone is mainly located on the side of the step sinking, while the other side is not apparent.

It is found from the investigation of the disaster situation of ground fissures that with the continuous deep mining by caving method in the Shizishan mining area, the ground fissures are serious, and the ground fissures obviously expand, which can easily cause ground collapse, mountain landslides, and other disasters. It will seriously affect safe production in the mining area as well as the safety of the surrounding residents. The method of on-site investigation only studies the macroscopic trends of surface movement and deformation in mines. Therefore, it is necessary to conduct more detailed on-site monitoring and numerical simulation research on the surface movement and deformation caused by continuous mining with the caving method in deep mining areas. This can provide a more comprehensive understanding of the risks of surface movement and deformation as well as help with the analysis and prediction of potential geological disasters.

4. Monitoring of Surface Rock Movement and Analysis of Deformation Characteristics in Mining Area

4.1. Design and Implementation of GPS Monitoring of Surface Rock Movement in Mining Area

The vegetation in the subsidence area of the Shizishan mining area is dense, with an average vegetation height of approximately 2.5 m, resulting in poor natural visibility. It is not suitable for conventional instrument monitoring. GPS monitoring is not affected by terrain and can meet the requirements of surface deformation monitoring in terms of accuracy [42,43]. Therefore, the GPS monitoring method is used to monitor the surface rock movement in the mining area. In 2011, in order to monitor the situation of the surface subsidence area, four monitoring points were established that had a comprehensive perspective of the subsidence area, namely C1, C2, C3, and C4. Three reference base points were established outside the subsidence area to observe the deformation of the subsidence area, as shown in Figure 9.

Considering the convenience of the permanent use and erection of instruments and equipment, monitoring datum points is not easy to produce their deformation and displacement. The design of the monitoring point utilizes C30 reinforced concrete buried at a depth of more than 1.3 m. According to the China C-class GPS measuring point layout requirements, the top is equipped with a forced centering device. Monitoring piles in surface subsidence areas is mainly used to monitor settlement and horizontal displacement. In terms of reliability and anti-damage, “table-type” piles are directly arranged in subsidence areas, and the height angle of obstacles in the field of view is no more than 15° to meet the GPS requirements observation angle. The accuracy error of GPS monitoring displacement changes can be controlled within 1 mm. Figure 10 compares GPS monitoring points of surface rock movement and main mining activities in the mining area. Figure 11 shows some on-site monitoring points and monitoring base points.

4.2. Analysis of Surface Rock Movement of Monitoring Results and Deformation Characteristics in Mining Area

Since the monitoring point was arranged in 2011, six phases of monitoring have been conducted with the monitoring data from the first phase as the reference benchmark. Data collection was carried out on average every 6 months, and the last set of data collection was in April 2015. The X, Y, and Z differences indicate the changes in the X, Y, and Z directions between the current and previous periods, while the cumulative differences reflect the cumulative changes along the X, Y, and Z directions.

4.2.1. Analysis of Horizontal Displacement Change of Monitoring Points

The analysis of the time effect of displacement of the deformation monitoring points reflects the change in the mining of the deformation monitoring points. By establishing the corresponding relationship between the deformation monitoring point and time, the dynamic evolution trend of movement and deformation of each monitoring point can be determined. Based on the surface monitoring data from the mining area, the relationship curves of cumulative deformation and deformation rate with time in the X direction (Figure 12 and Figure 13) and the relationship curves of cumulative deformation and deformation rate with time in the Y direction (Figure 14 and Figure 15) were drawn to analyze the horizontal surface movement and deformation law of the mining area.

• Analysis of surface horizontal movement and deformation in the X direction

  (1)

  According to the evolution curve of the displacement in the X direction of each monitoring point over time, it can be observed that the horizontal displacement of the surface in the X direction of the four monitoring points shows an increasing trend over time, and the horizontal movement rate of the surface in the X direction has significantly increased after November 2014.

  (2)

  The maximum cumulative deformation in the X direction is at the measurement point C4. The accumulated displacement of the surface in the X direction at this measurement point reached 695 mm in the last phase of monitoring in April 2015. The second is the measurement point C3 and C2, and the cumulative deformation in the X direction is 538 mm and 430 mm, respectively. The monitoring results of measuring points C4, C3, and C2 in the X direction are all positive, indicating that the three measuring points all have a horizontal displacement component due north; they all have a displacement pointing toward the footwall of the orebody. The cumulative displacement of the measurement point C1 in the X direction during the last phase of monitoring is −320 mm, indicating that the horizontal displacement component in the X direction points south, namely the direction of the hanging wall of the orebody.

  (3)

  From the perspective change rate of X-direction displacement, the deformation rate increases with time in the X direction, displaying a smooth change with an increasing trend. Phases 1 to 3 belong to the deformation rate increase, while phases 3 to 5 show a slower change. However, during the monitoring period of phase 6, the overall X-direction rate showed a sudden change. Overall, the horizontal displacement rate of measuring point C4 in the X direction was the highest, reaching 1.58 mm/d on April 14, 2015. The deformation rates of C3 and C2 in the X direction were 1.16 mm/d and 1.07 mm/d, respectively, while C1 was the lowest, at −0.48 mm/d.

Figure 12. Cumulative horizontal displacement in X direction of each measuring point.

Figure 12. Cumulative horizontal displacement in X direction of each measuring point.

Figure 13. Deformation rate during X direction at each measuring point.

Figure 13. Deformation rate during X direction at each measuring point.

2.

Analysis of surface horizontal displacement and deformation in the Y direction

(1)

According to the evolution curve of the displacement in the Y direction of each monitoring point over time, it can be seen that the horizontal surface displacement in the Y direction of monitoring points C2, C3, and C4 all showed an increasing trend over time, while the displacement of monitoring point C1 in the Y direction remained unchanged in the first to fifth periods, but began to increase in the sixth period.

(2)

The last phase of monitoring was on 14 April 2015. Measuring point C3, with the largest cumulative deformation in the Y direction, reached 713 mm. This was followed by measuring point C2 and measuring point C4, with a cumulative deformation in the Y direction of 638 mm and 180 mm, respectively. The monitoring results of measuring points C2, C3, and C4 in the Y direction are positive, indicating that the three measuring points have horizontal displacement components in the due east direction, i.e., they all have displacement pointing to the goaf. The cumulative displacement of measuring point C1 in the Y direction monitored in the last phase is −43 mm, indicating that the horizontal displacement component in the Y direction points to the west, but the displacement was small, and the change was not significant.

(3)

From the displacement change rate in the Y direction, the deformation rate of measuring points C2 and C3 increased linearly from August 2012 to October 2013, while the deformation rate decreased from October 2013 to October 2014. However, after October 2014, the deformation rate showed a sharp growth trend, in which measurement point C3 increased from 0.41 mm/d in the fifth phase to 1.34 mm/d, and measurement point C2 increased from 0.55 mm/d to 1.00 mm/d. The deformation rate of measuring point C4 was 0 mm/d in October 2014 but increased to 0.31 mm/d in April 2015.

Figure 14. Accumulated horizontal displacement in Y direction of each measuring point.

Figure 14. Accumulated horizontal displacement in Y direction of each measuring point.

Figure 15. Deformation rate during Y direction at each measuring point.

Figure 15. Deformation rate during Y direction at each measuring point.

According to the law of surface horizontal displacement and deformation, the horizontal displacement of each measuring point points to the direction of goaf. The cumulative absolute displacement of measuring point C2 is the largest, followed by measuring points C3 and C4. The displacement component in the X direction is much larger than that in the Y direction, indicating that the displacement component in the footwall direction is large. From the perspective of the deformation rate, the overall horizontal deformation rate at each measuring point increased slowly from August 2010 to October 2013. From October 2013 to October 2014, the deformation rate changed steadily. Between October 2014 and April 2015, the horizontal movement rate of the surface increased sharply, indicating that the surface movement was in an active stage.

4.2.2. Analysis of Surface Subsidence Change at Monitoring Point

(1)

According to the vertical displacement (Z-direction) of monitoring points C1 to C4 and the evolution curve of the deformation rate with time (Figure 16 and Figure 17), the cumulative displacement of the surface in the vertical direction of the four monitoring points showed an increasing trend with time. The measuring point C3 had the largest cumulative deformation in the vertical direction. As of April 14, 2015, the largest cumulative deformation in the vertical direction is the measuring point C3, and the cumulative displacement has reached -1131 mm. This is followed by the cumulative displacement in the vertical direction, from large to small, of measuring point C2, measuring point C4, and measuring point C1, which are −878 mm, −732 mm, and −461 mm, respectively. The cumulative vertical displacement of each monitoring point increased almost linearly in the first five periods, while the vertical displacement rate increased significantly in the sixth period.

(2)

From the vertical displacement change rate, the subsidence rate of measuring points C3 and C4 decreased from August 2012 to May 2014 and then increased from October 2014 to April 2015. The surface settlement rate of measuring point C3 increased from −0.83 mm/d to −2.71 mm/d, and measuring point C2 increased from −0.83 mm/d to −1.28 mm/d. The surface subsidence rate of C4 and C1 increased from −0.62 mm/d and −0.004 mm/d to −0.95 mm/d and 0.86 mm/d in the fourth period.

According to the law of surface subsidence and deformation, the largest cumulative subsidence is at measuring point C3, followed by measuring point C2 and measuring point C4. The smallest subsidence is at measuring point C1. The cumulative subsidence of measuring point C3 and measuring point C4 can be divided into three subsidence stages as a whole. From August 2012 to May 2014, the rate of surface subsidence increased slowly. Between May 2014 and October 2014, the rate of surface subsidence decreased. From October 2014 to May 2015, the rate of surface subsidence increased sharply. The rate of surface subsidence of measuring point C2 has been increasing. The surface subsidence rate of measuring point C1 began to increase after October 2014, but the previous subsidence rate fluctuated slowly.

4.3. Mechanism Analysis of Surface Movement and Deformation Based on Monitoring Results

After the mining of the orebody forms a certain scale of goaf, there is a subsidence center in the area of section 25 to section 40. The surface of the subsidence area moves towards the subsidence center under the tensile action, and the surface monitoring points move slightly towards the subsidence center in different moving directions. With the downward extension of orebody mining, the exposure span of goaf, the stress and stress difference of surrounding rock, and the creep variable of surrounding rock continue to increase, which will lead to an increase in the deformation of surrounding rock in the hanging wall and footwall as well as the deformation force of surrounding rock acting on the overlying rock in the goaf. When the overlying rock is in the process of extrusion deformation, the surface will produce deformation movement towards the goaf [44], and the X, Y, and Z coordinates of surface monitoring points will change accordingly.

With the further mining of the orebody, the span of the goaf continues to increase, the exposed area of the overburden continues to expand, and the pressure on the balance arch gradually increases. The arch’s span and the mechanical and physical properties of the rock mass determine the roof’s stability. If the span value increases to the limit value of collapse, the balance arch cannot support the overlying rock mass, which will lead to the instability of the upper rock mass and gradual collapse. When the underground goaf is expanded to a certain extent, the collapse of unstable rock mass at the top of the goaf will gradually transfer and then affect the ground’s surface, resulting in a subsidence area on the surface. In the case of poor stability of the overlying rock mass of the goaf, the development of the scope of the subsidence pit may be relatively fast. After the subsidence area appears on the surface, the horizontal tectonic stress of the overlying strata of the roof is gradually released. The overlying strata of the roof crack towards the subsidence direction due to tensile deformation, and the affected rock mass collapses under the action of its weight. Therefore, the surface monitoring points also move in the direction of subsidence, with different ranges of change. The movement range of monitoring points near the subsidence center is greater than that of other monitoring points, and the overall change trend is apparent.

The monitoring results of the accumulated three-dimensional surface displacement at monitoring point C3 showed that the three-dimensional coordinate variation trend of the monitoring point was significant. The surface subsidence and horizontal displacement of measuring point C4 were large from the hanging wall direction to the footwall direction. From August 2012 to October 2013, the mining area was in the process of mining the orebody from sublevel 14 to sublevel 15. Thus, the rate of surface deformation increased slowly. Because measuring point C3 is located near the center of the western drift ore mining area, the surface subsidence and horizontal movement of the point were large, and the surface movement of measuring points C2 and C4 gradually increased due to mining. From October 2013 to October 2014, the mining area mainly carried out partial residual mining of eastern slate ore and the development of the deep sublevel 16, while the main orebody was not mined. Therefore, the horizontal deformation rate of the surface is basically constant, and the cumulative horizontal deformation changes slowly. From October 2014 to April 2015, the mining of the deep sublevel 16 orebody and the development and preparation of the deep sublevel 17 orebody were mainly carried out. The deep continuous mining caused a significant increase in the surface movement and deformation rate of measuring points C2, C3, and C4, while the surface movement and deformation rate of measuring point C1 began to increase under the influence of the mining of the orebody of profile 25.

5. Surface Movement and Deformation Law and Deformation Trend Prediction of Continuous Mining in Deep Mining Area

5.1. Numerical Model

FLAC3D is Lagrange finite difference analysis software. Compared with other software, it has the advantages of fast solution speed and more accurate results in simulating the continuous deformation of the rock mass. Therefore, FLAC3D software was selected to predict and analyze the law and trend of surface deformation caused by deep continuous mining through a numerical simulation method. In order to accurately analyze the deformation evolution law of the actual surface in the mining process, considering the surface undulation shape, the model was established according to the actual [45]. The X direction of the model was perpendicular to the strike direction of the orebody, with a length of 1400 m. The Y direction of the model was the strike direction of the orebody, with a length of 1500 m. The Z direction of the model was vertical. The bottom elevation of the model was 687 m, the highest elevation of the top of the model was 2117 m, and the maximum height of the model was 1430 m. The model was divided into 394,713 units and 411,825 nodes. The final generated mesh and orebody model are shown in Figure 18. The Mohr-Coulomb elastoplastic constitutive model was adopted for calculation [46,47]. The two ends of the model in the X direction constrain the displacement in the X direction. The two ends of the model in the Y direction constrain the displacement in the Y direction, and the three directions of X, Y, and Z are fixed at the bottom of the model. The top of the model is a free boundary. The in-situ stress is applied inside the model according to the measured in-situ stress in the mining area. The macroscopic rock mechanical parameters of the lithology are shown in

Table 1. Macroscopic rock mechanics parameters of Shizishan Copper Mine (Reprinted/adapted with permission from Ref. [48]. 2022, Yanhui Guo).

The Lithology	Density $\mathit{\rho }$ (g/cm3)	Elastic Modulus E (GPa)	Poisson’s Ratio	Tensile Strength ${\mathit{\sigma }}_t$ (MPa)	Cohesion C (MPa)	Angle of Internal Friction Φ (°)
Cyan gray Dolomite	2.85	20.0149	0.269	2.1966	2.6110	41.99
Faded Dolomite	2.84	13.0620	0.280	2.1796	2.5504	34.05
Orebody	2.84	17.8847	0.214	2.2577	2.9637	47.42
Purple Slate	2.63	4.5676	0.284	1.0431	1.5710	31.68
Carbonaceous Slate	2.70	2.2416	0.350	0.9298	1.3848	35.42

.

In order to accurately simulate the dynamic evolution characteristics and laws of the surface displacement field during mining, excavation is performed in sequence according to the mining depth and time. The excavation calculation is based on the single excavation of each sublevel. Each excavation height is 50 m and excavated from the upper sublevel 4 (1787 m level) down to sublevel 24 (787 m level), a total of 21 excavations.

5.2. Development and Evolution Analysis of Surface Rock Movement Trend

The absolute vertical and horizontal displacements of surface rock mass also change significantly with the mining depth in the whole process from sublevel 4 to 24. Figure 19 and Figure 20 show the contour maps of the vertical surface displacement (or settlement) and the horizontal surface displacement (in the vertical orebody strike direction) after stoping a typical orebody in the sublevel.

Through comprehensive analysis of the changes of surface vertical displacement and horizontal displacement contour maps after the mining of orebodies in different sublevels, and investigation of the rock movement trend development and evolution characteristics and laws, the following can be observed:

(1)

After the conclusion of orebody mining in sublevel 8, a specific surface subsidence range appears directly above the excavation area. The maximum vertical displacement of the surface is 20 mm, the maximum horizontal movement of the surface along the X direction is 6 mm, and the maximum displacement along the opposite direction of X is 6 mm. The subsidence range is approximately circular, and the maximum subsidence point is located near section 25. The range of surface movement is from profiles 60 to 20 along the orebody strike.

(2)

After mining sublevel 12, the movement range of the surface was expanded. The influence range of the hanging wall direction of the goaf was greater than that of the footwall direction. The movement range extends to the inclined direction of the orebody. The surface movement boundary moves 87 m in the footwall direction, 400 m in the hanging wall direction, 316 m in the west direction, and 24 m in the east direction. The maximum vertical displacement of the surface is 90 mm. The maximum horizontal displacement of the surface along the X direction is 25 mm, and the maximum displacement along the X opposite direction is 20 mm. The subsidence range changed from circular to elliptical, and the maximum subsidence point was near section 35.

(3)

After the stoping of the orebody in sublevel 15, the maximum vertical displacement of the surface is 190 m, the maximum horizontal displacement is 40 mm along the X direction, and the maximum displacement is 30 mm along the opposite direction of X. The movement range of the surface continues to expand, and the moving boundary of the surface moves 133 m toward the footwall direction, and the moving boundary of the hanging wall direction exceeds the range of the model. The strike direction extends 176 m to the west and 24 m to the east. The influence range of the hanging wall direction of the goaf is more significant than that of the footwall direction. With the continuous migration of the mining center, the maximum subsidence point of the surface subsidence basin shifts to the south and east direction, and the maximum subsidence point of the moving basin is located 20 m to the west of section 25.

(4)

After excavating the orebody in sublevel 18, the movement range of the surface is further expanded. The maximum vertical displacement of the surface is 320 mm, the maximum horizontal displacement is 70 mm along the X direction, and the maximum displacement is 50 mm along the opposite X direction. The influence range of the footwall direction is small, while the influence range of the goaf hanging wall direction is further expanded outward. The surface movement boundary moves 49 m to the footwall direction, and the strike direction extends to the west to the model boundary and 48 m to the east. The subsidence range is approximately round, and the maximum subsidence point is located 18 m to the west of section 30.

(5)

After the stoping of the orebody in sublevel 21, the movement range of the surface is further expanded. The maximum surface settlement increases to 520 mm, the maximum horizontal movement along the X direction is 140 mm, and the maximum displacement along the opposite direction of X is 80 mm. The influence range of the footwall direction is small, while the influence range of the goaf hanging wall direction is further expanded outward. The surface movement boundary moves 107 m to the footwall direction, extending 9 m to the east in the strike direction. The subsidence range is oval, and the maximum subsidence point is located near section 30.

(6)

After the stoping of the orebody in sublevel 24, the movement range of the surface is further expanded. The maximum vertical displacement of the surface increases to 800 mm, the maximum horizontal movement along the X direction is 260 mm, and the maximum displacement along the opposite direction of X is 140 mm. The influence range of the footwall direction is small, while the influence range of the goaf hanging wall direction is further expanded outward. The surface movement boundary moves 133 m to the footwall direction and extends 12 m to the east in the strike direction. The subsidence range is oval, and the maximum subsidence point is located near section 35.

(7)

In the process of deep continuous mining, due to the extension of the ore body in the south-by-west direction, the surface movement range develops southward. As a result of the expansion of the subsidence range in the south direction, new fissures will be generated. With the generation of new fissures, the existing fissure width and fracture will continue to expand. The results are consistent with the actual investigation of surface fissures.

6. Conclusions

In this paper, three methods of field investigation, GPS monitoring, and numerical simulation are used to analyze the law of surface movement and deformation caused by deep continuous mining of steeply inclined ore bodies, and the mechanism of surface deformation and movement caused by deep mining is revealed. It can greatly reduce the adverse effects of land subsidence, mountain landslides, and other disasters caused by surface movement and deformation. The results show that through the combination of these three methods, the surface movement and deformation caused by the deep mining process can be well analyzed and predicted, and the main conclusions are as follows:

(1)

Judging from the site situation of surface fissures in the mining area, from 2013 to 2015, the fissures on the surface were damaged to varying degrees, and obvious three-dimensional features appeared. In terms of displacement changes, there are both opening displacement and horizontal displacement, and the displacements in the three-dimensional direction are of the same magnitude. The horizontal displacement on both sides of the surface fissures in the north and east of the surface is the largest, followed by the vertical subsidence displacement, while the trend of displacement in the west of the surface is just the opposite. In the southern part of the surface, two fissure zones appeared, the first fracture zone developed into the hanging wall more clearly, and the second fracture zone increased significantly compared with the previous period. Especially from the current development of ground fissures, the hanging wall direction of the ore body in the southeast of the surface has a trend of continuous growth or deterioration, which must be paid attention to.

(2)

According to the on-site GPS monitoring data, it can be seen that after the formation of a specific scale of goaf, there are subsidence centers in the area of lines 25 to 40, and the surface of the subsidence area is moved to the subsidence center by stretching. The surface monitoring points move slightly to the subsidence center, and the movement direction is different. The surface movement and deformation in the mining area show an upward trend, and the surface subsidence and horizontal movement rate of each measuring point increase rapidly, indicating that the surface subsidence is in an active development period.

(3)

From the prediction results of the deformation trend by numerical simulation software FLAC3D, with continuous deep orebody mining in the mining area, there are apparent subsidence centers on the surface, the subsidence contour is concentric oval, and the long axis direction is consistent with the direction of the ore body. With continuous mining, the subsidence area expands, and the position of the subsidence center shifts to the hanging wall of the orebody. The maximum subsidence center is located near section line 30. Meanwhile, the horizontal displacement of the two movement centers on the surface continues to increase. Due to the terrain, the horizontal displacement of the hanging wall is less than that of the footwall because the maximum horizontal movement position of the footwall is a steep mountain, and the horizontal displacement is large after mining.