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In situ stress field inversion and its application in mining-induced rock mass movement
International Journal of Rock Mechanics & Mining Sciences 53 (2012) 120–128
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International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
In situ stress field inversion and its application in mining-induced rock mass movement Haijun Zhao n, Fengshan Ma, Jiamo Xu, Jie Guo Key Laboratory of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2011 Received in revised form 13 April 2012 Accepted 1 May 2012 Available online 24 May 2012
Based on a series of experiments in numerical simulation, the model boundary conditions for in situ stress field inversion and excavation are discussed. Study results indicate that roller boundary conditions are reasonable for the in situ stress field inversion before excavation simulation, while, as a closed system, changing the roller boundary conditions to fixed boundary conditions in the subsequent excavation is optimal when the dimensions of the model borders are greater enough than the zone of influence of the excavation. As a case study, a comparative study of the mining-induced ground movement in a steeply dipping mine is carried out in two different stress fields. The results show that the mining-induced ground movement in the high-level tectonic stress field clearly differs from that in the ideal self-weight stress field. Because of the steep occurrence and large thickness of the ore body, the mining-induced ground subsidence exhibits different characteristics at different mining stages in the practical tectonic environment. Further studies elucidate the causes of these differences and clarify the effects of high-level tectonic stresses on rock mass movement and deformation. Finally, based on GPS monitoring results on the ground surface, the current ground subsidence is evaluated and its development trend is predicted. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Inversion Stress field Underground mining Rock mass movement Ground subsidence Mechanism
1. Introduction The geo-stress field is the most basic but also the most important load on rock engineering [1,2]. The in situ stress field is one of the fundamental conditions for the analysis, design calculation, and stability assessment of rock engineering projects. It is also a fundamental factor in the study of rock mass deformation and failure [1,3,4]. The in situ stress is a natural state of stress imposed on rock mass before engineering excavation is initiated, while the in situ stress field is a spatial distribution of geo-stress field at a certain time. Although it is an unstable state that changes with geological time and space, it can also be regarded as a constant stress field, as compared with the engineering construction period [1,4,5]. The in situ stress field involves a highly complicated process and is affected by many factors. Currently, various measuring methods have been proposed [6–11], but determining a precise in situ stress field remains difficult. In situ measurement is the most direct approach to determine geo-stress field. Because of high costs and limited test areas, however, measurement is usually conducted only in local areas with a few test points [12,13]. Thus,
n
Corresponding author. E-mail addresses: [email protected], [email protected] (H. Zhao). 1365-1609/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmms.2012.05.005
previous measurement results merely reflect the local stress field [6,14]. In addition, because of the complex genesis of the stress field and numerous influencing factors [15–17], certain degrees of discreteness are usually reflected in the measurement results. Therefore, on the basis of measurement results and geologic structure conditions, to derive a reasonable stress field with wide applicability using an effective numerical simulation method is quite necessary [18–21]. The degree of approximation between the inversed stress field and the practical stress field is one of the basic conditions for assessing the validity of numerical simulation. The correctness of in situ stress field obtained by numerical simulation directly influences the final results, especially for excavations in highlevel tectonic stress areas [22,23]. In numerical simulations, in situ stress fields are generally acquired by the regression fitting method using limited data on measurement and topography as bases [18,20,24]. Specifically, combined with displacement boundary conditions, the in situ stress values of all elements can be calculated according to the regression equations of in situ rock stresses. Subsequently, they can be assigned as element internal forces to simulate the in situ stress field. Alternatively, the calculated initial stress values can be converted into nodal forces. Then, combined with stress boundary and displacement boundary conditions, the stress field can be inversed [23,24]. To be more specific, if stress boundary condition is set, external forces are usually applied to lateral boundaries to simulate the
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corresponding tectonic stresses. In the simulating process, the external forces are constant, whereas the boundaries are moveable during excavation simulation. However, if the displacement boundary condition is set, tectonic stresses can be assigned as element internal forces in the calculated area. Regardless of which boundary conditions are established, the state of the inversed in situ stress field should correspond to that of the practical stress field. However, previous studies have shown that simulation results usually vary considerably under different boundary conditions. Thus, in the present study, a set of rational boundary conditions for stress field inversion and excavation is discussed based on a series of experiments in numerical simulation. As a case study, a typical steeply inclined mining mine that occurred in the high-level tectonic stress field is analyzed. By comparing mininginduced subsidence in ideal self-weight stress field, the special mechanisms of rock mass movement in the practical high-level tectonic stress field are clarified. Besides, based on GPS monitoring results on the ground surface, the current ground subsidence is evaluated and its development trend is predicted.
2. Comparative study of boundary conditions in numerical simulation 2.1. Model design and research method A typical stress field inversion model was studied in a homogeneous geologic body to clarify the effects of boundary conditions on numerical simulations. As a plane-strain model, we assume that the hypothetical model is 2000 m long and 500 m high. Besides, we assume the density of the linear-elastic model material is 3000 kg/m3, elastic modulus is 5.0 GPa, and the horizontal stresses are gradually increased from 0 at surface to 25 MPa at bottom in the model. Thus, different combinations of boundary conditions were experimented to inverse the hypothetic stress field. In terms of lateral boundary conditions, stress boundary conditions and displacement boundary conditions were adopted in different simulation processes. When stress boundary conditions were adopted, linearly increased external forces were imposed on the lateral boundaries of the model to inverse the aforementioned stress field. When displacement boundary conditions were adopted (lateral roller boundaries), element internal forces, which were calculated based on linear changes of the horizontal stresses, were imposed on all elements of the model to inverse the assumed stress field. Simulation results are shown in Fig. 1. The distribution curves in the models represent horizontal stress contours. 2.2. Results and discussion As shown in Fig. 1, the distribution contours of horizontal stresses are irregular (Fig. 1a–c) under stress boundary conditions. Aside from the horizontal distribution trend near the lateral borders, large fluctuations can still be observed in the inner part of the model. The main reasons for the different stress patterns under stress boundary conditions can be concluded in the following. The first reason lies in the distribution patterns of the lateral external forces. There are shear stresses on every abstract horizontal planes of the model because of the triangular load distribution; thus, the distributions of the compressive stress are irregular. The second reason for the above results may lie in the effect of the top and bottom free surfaces. The path lines of compressive stresses usually chose the shortest route to reach an equilibrium state. The third reason can be considered to be the end effect of the external forces. According to the Saint-Venant principle in elasticity, the influence of the external load decreases significantly from the pressure surface to the inner part of the
Fig. 1. Inversion results of horizontal stresses under different boundary conditions: (a) roller bottom and lateral stress boundary conditions, (b) fixed bottom and lateral stress boundary conditions, (c) roller bottom and unilateral stress boundary conditions, (d) fixed bottom and lateral roller boundary conditions, and (e) roller bottom and lateral boundary conditions.
model. Thus, the transmission of the compressive stresses shows non-equivalent and non-linear characteristics. By contrast, when the element internal forces are assigned to elements under displacement boundary conditions, the simulated horizontal stress contours are horizontal and smooth, and they are gradually increased from 0 Pa on the surface to 25 MPa at the bottom (Fig. 1d and e). These characteristics are consistent with the theoretical stress field. The differences between Fig. 1d and e lie in the boundary conditions at the bottom. Fig. 1d shows the application of a fixed boundary condition, while Fig. 1e shows the use of a roller bottom condition. When these two models are both composed of homogeneous isotropic strata (exactly shown in this study), the boundary conditions appear to have little influence on the simulation results. However, when they are composed of heterogeneous anisotropic strata, relative shear displacements are certain to occur during calculation. In this case, the fixed bottom boundary may affect the normal stress and displacement field distribution near the bottom. Thus, the roller bottom condition can be more appropriate for the stress field inversion, as shown in Fig. 1e. Besides, model borders are often expanded outward from the zone of influence of an excavation to eliminate boundary effects
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or minimize these within an allowable error range in numerical simulations. In a manner analogous to the definition of the zone of influence of an excavation, the near field of an ore body may be taken as the rock contained within the surface distance 3 times of the dimension of excavations from the ore body boundaries. If model borders are far enough from the excavated area, it can be regarded that the model borders have no effect on the normal distribution of the stress and displacement fields after excavation. Thus, in a rational numerical model, therefore, we can assume that no relative displacements occur at model borders after excavation, or that the relative displacements are sufficiently small and equivalent to zero-displacement boundary. Excavation is a dynamic disturbance in the model as it results in a certain degree of energy dissipation and migration [25,26]. According to the Saint-Venant principle, the zone of influence of an excavation is limited and mainly confined to the near field of the extraction. Thus, if the dimensions of the model borders are smaller than the zone of influence of an excavation, the model borders cannot accommodate the disturbance of excavation. On the contrary, the numerical model can be considered as a closed system because the model borders have no effect on the excavation. In this case, the total strain energy of the model is bound to decrease after excavation because it is a constant value in the entire model. Therefore, in a rational model, boundary conditions should be adjusted accordingly at different stages of simulation. In other words, roller boundary conditions are reasonable for the pre-excavation process of in situ stress field inversion, whereas, fixed boundary conditions are optimal for the subsequent excavation in the model (Fig. 2).
3. Application of the stress field inversion and study of the mining-induced ground movement in a steep nickel mine 3.1. Geology and mining conditions Jinchuan Mine is the largest nickel production base in China. It is located in Jinchang City, Gansu Province, northwest China. The mining area is approximately 6.5 km long, 10–570 m wide,
and extends to more than 1000 m underground. The strike of the main ore body is N501W, and the dip angle ranges from 401 to 701. Thus, it is a typical steep metal mine. In the mine area, ultrabasic rocks are the ore-bearing native rock and it was divided into four relatively independent mine fields because of the separation of the oblique faults. Among these mine fields, the No. 2 mine holds 75.2% of the total reserves of the Jinchuan Mine, thus, it was chosen as the research object of this study. In the No. 2 mine, the ore body is about 1600 m long, 100 m thick on average, and extends from 350 m below the surface to over 1000 m. Different scales of faults and various kinds of contact zones caused poor country rock stability in the mine area. Besides, high-level tectonic stress is a prominent characteristic in the mine area. Field tests and measurements were implemented in various kinds of rock mass at different depths to determine their regularities [27,28]. In all, a total of 18 measuring points were measured and the three principal stresses were determined as follows:
sh,max ¼ 0:046H þ 2:767ðMPaÞ sV ¼ 0:024H þ 0:423ðMPaÞ sh,min ¼ 0:020H þ 0:250ðMPaÞ
ð1Þ
where sh,max , sh,min , and sV are the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress, respectively, and H is the depth of the measuring points (units: m). In the mine area, the axis of the maximum principal stress is horizontal, which is basically perpendicular to the strike of the ore body. The vertical stress of the virgin rock mass is the intermediate principal stress, whose value is slightly smaller than the dead-weight of the overburden. With increasing depth, the ratio h,max/ h,min tends to reach 2.5, and the ratio h,max/ v is close to 2. This means that the value of the horizontal maximum principal stress is 2.5 times that of the minimum principal stress, and twice that of the vertical stress in the mine area. These results indicate that the stress differences in the virgin rock mass are substantial. The mine shafts and tunnels are currently used for underground mining (Fig. 3). The abscissas in Fig. 3 are the numbers of the exploratory lines in the mine area at an interval of 50 m. The underhand drift cut-and-fill mining method is employed in the mine area, and sublevels 1250 and 1150 m are mined concurrently to increase mineral production. However, large-scale ground surface movement and fissures occurred on the upper surface of the ore body after 18 years of mining [29]. Some important facilities, such as houses, mine shafts, and tunnels, have been destroyed by ground deformation and rock mass movement. 3.2. Model design and numerical research program
Fig. 2. Boundary conditions during the pre-and post-excavation processes.
In this study, the geological profile of the exploratory line No. 14 is selected and modeled for the subsequent numerical simulation (Fig. 4; minor surface undulations and contact zones are ignored). As a plane-strain problem, the model is 4000 m long, 1050 m high, and mainly composed of rich ore body, lean ore body, ultrasonic rock, marble, and banded migmatite. The mechanical properties of rock mass and their corresponding parameters have been previously studied through laboratory tests [29–31]. The parameters of rock mass applied in the numerical model were determined according to previous experimental results and inversion of ground subsidence based on GPS monitoring results (Table 1). The laboratory tests showed that the diversity of rock mechanical properties occurred not only in different rock types, but also in the same rock type in different places because of the well-developed joints and fractures in the
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mining area. Furthermore, the intact rock mechanical and strength parameters were usually 1–2 orders of magnitude smaller than the results of in situ mechanical tests because of the fractured rock masses. Thus, to determine feasible and verifiable rock mass parameters for the numerical model is a hard work. On the basis of the previous laboratory tests, however, these parameters can be inversed by the GPS monitoring results in the mining area. In such circumstance, we regard that these inversed parameters are proper for the model study though they may be smaller than laboratory test values and may show unnoticeable differences among these different rock types. In other words, as long as the simulation results reasonably reflect
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the practical in situ stress field and the characteristics of the mining-induced ground movement, the accuracy of the defined rock mass parameters has little effect on the research results. In addition, previous studies on ground movement induced by mining steep ore bodies mainly focus on coal mining [32–34]. However, the ground movement induced by mining steep metal mines clearly differs from that induced by coal mining because of the considerable differences in metallogenic type, rock mass structure, and geologic structure. The properties of in situ stress fields, in particular, play an important part in the mechanical behaviors of rock mass after excavation. Thus, a comparative research program was designed to discuss the special regularities and mechanisms of mining-induced rock mass movement. Specifically, the mining operations were simulated in two different stress fields: the self-weight stress field and the high-level tectonic stress field. The former one means the mining operations were simulated only under ideal self-weight conditions, while the latter one means the mining operations were simulated under practical high-level tectonic stress field, which was inversed according to the in situ stress regression equation (1). In the numerical model, the boundary conditions for in situ stress simulation and subsequent excavation illustrated in Fig. 2 are adopted in this study. In addition, the mining sequences are designed to be consistent with the practical mining operations. The ore body ranging from 1280 to 1250 m is gradually mined out from top to bottom, and then sublevels 1250 and 1150 m are mined simultaneously until the sandwiched ore body between sublevels 1250 and 1150 m is mined out (Fig. 3). The mining rate in the upper sublevel (1250 m) is designed to be twice that of the lower sublevel (1150 m) according to the fact. 3.3. Results and discussion
Fig. 3. Cross-sectional profile of the underground development openings and the ore body in the Jinchuan No. 2 mine.
3.3.1. Back-calculated principal stress fields Fig. 5 shows the distribution maps of the inversed principal stress tensors in different stress fields. Under the ideal self-weight condition, the orientations of the maximum principal stress tensors are vertical
Fig. 4. Numerical model simplified from the engineering geological profile of exploratory line No. 14 (1) banded migmatite, (2) marble, (3) granite, (4) ultra-basic rocks, (5) rich ore, (6) lean ore, (7) gneiss, and (8) filling body.
Table 1 Physical properties of rock mass applied in the numerical model. Lithology
Unit weight (kN/m3)
Elastic moduls (GPa)
Poison’s ratio
Cohesion strength (MPa)
Friction angle (1)
Tensile strength (MPa)
Banded migmatite Ore body Ultrasonic rocks Marble Gneiss Granite Filling body
30.0 29.3 29.3 30.0 28.0 27.6 20.0
2.2 2.5 2.2 2.2 2.0 2.2 0.2
0.25 0.23 0.25 0.25 0.27 0.25 0.28
2.0 1.0 1.0 1.0 1.0 1.0 0.3
44 42 42 44 40 38 44
2.0 1.0 1.0 2.0 1.0 1.0 0.3
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and increase with depth (Fig. 5a). However, the orientations of the maximum principal stress tensors are horizontal and also increase with depth in the practical high-level tectonic stress field (Fig. 5b). Overall, the magnitudes and distributions of the two back-calculated stress fields are satisfied for the subsequent mining simulation. 3.3.2. Characteristics of vertical subsidence and horizontal displacement To facilitate the contrastive study of the mining-induced ground movement under different stress fields, the curves of vertical subsidence (Fig. 6) and horizontal displacements (Fig. 7) in different mining stages are shown in the current work. As shown in Fig. 6a, in the ideal self-weight stress field mining, ground subsidence curves are smooth and symmetrical. With the increase in mining depth, ground subsidence increases prominently, and the maximum subsiding center migrates slowly to the upper surface of the ore body (the left side of the coordinate). When the mining depth reaches a certain level, the maximum settlement rate begins to decrease gradually. In the practical tectonic stress field mining, the curves of ground subsidence exhibit different characteristics at different mining stages (Fig. 6b). In the initial stage of single-level mining, although the subsidence curves are similar to those of mining in the selfweight stress field, the maximum ground subsidence is only 75.3% of the amount of the latter. In the double-level mining stage, however, a slight secondary subsiding center appears at the footwall surface of the ore body when the upper sublevel is mined at the level 1160 m and the lower sublevel is mined at the level 1110 m. At this time, the height of the total mined-out space is 140 m, and the average thickness of the mined-out space is 130 m. However, when the ore body between sublevels 1250 and 1150 m is completely mined out, the secondary subsiding center clearly appeared. Here, the height of the total mined-out space is 200 m, and the average thickness of the mined-out ore body is 120 m. It is obvious that the vertical dimension of the mined-out space is much larger than its horizontal dimension under the circumstance. Besides, the maximum subsiding center migrates gradually to the upper surface of the ore body (the left side of the coordinate) with mining depth during mining process. The migration rate is larger in the practical tectonic stress field than in the self-weight stress field, and the final maximum subsidence is only 78.5% of the latter. Although the maximum subsidence decreases compared with the mining in the self-weight stress field, the horizontal scope of the movement area expands prominently in the practical tectonic stress field. As shown in Fig. 7, horizontal displacements changed at different stages of mining in different stress fields. Horizontal displacements are zero near the outcrop of the ultrabasic rock, in which the
Fig. 6. Curves of ground subsidence induced by mining in different stress fields: (a) in ideal self-weight stress field, and (b) in practical high-level tectonic stress field.
Fig. 7. Curves of horizontal displacement induced by mining in different stress fields: (a) in ideal self-weight stress field, and (b) in practical high-level tectonic stress field.
maximum settlement developed. In general, the horizontal displacements in the hanging wall surface of the ore body are much larger than those in the footwall surface. Furthermore, the maximum horizontal displacement in the hanging wall surface of the ore body is 2–5 times that in the footwall surface of the ore body in the selfweight stress field (Fig. 7a). In contrast, the maximum horizontal displacement in the hanging wall surface of the ore body is 1–3 times that in the footwall surface (Fig. 7b), and the zero point of the horizontal displacement migrates slowly toward the footwall surface of the ore body (the right side of the coordinate) because of the occurrence of the secondary subsiding center in the practical tectonic stress field.
Fig. 5. Distribution map of the inversed principal stress tensors in different stress fields: (a) in ideal self-weight stress field, and (b) in practical high-level tectonic stress field.
3.3.3. Mechanisms of rock mass movement Ground subsidence is usually caused by different kinds of rock mass movements and deformations, and large differences usually exist in subsidence troughs in different ore bodies and stress
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fields. There are mainly two types of rock mass movement and deformation in steep mine mining. One is the movement perpendicular to the strata caused by the bending and breaking of surrounding rocks in the hanging wall and footwall. The other is the movement and deformation parallel to the strata caused by the bending and breaking of surrounding rocks on the roof and floor of the mine goaf, as well as the shearing and slipping of the surrounding rocks [34]. In this study, three kinds of mining-induced displacement contours and their general transmission models are presented in Figs. 8 and 10. The mechanisms of rock mass movement are discussed in the following. In the thin-layer mining stage (Fig. 8), the preponderant rock mass movements occur on the roof and floor of the mine goaf because the vertical height is smaller than the horizontal width in the mined-out space. Although there are rock mass movements in both sides of the mine goaf, they are relatively small and occupy only a small proportion of the movements. In this stage, therefore, the characteristics of mining-induced ground subsidence are similar to those of the mining of level coal seam in both stress fields. However, in the thick-layer mining stage (i.e., the vertical height is considerably higher than the horizontal width in the mined-out space), the curves of the mining-induced ground subsidence exhibit different characteristics in the two different geo-stress fields. In the self-weight stress field, although the areas where rock mass movement and deformation are larger in the hanging wall and footwall of the mined-out space than those on the roof and floor, the vertical displacement components in the rock mass are the overriding displacements. Therefore, when these overriding displacements are transmitted to ground surface, the ground subsidence maintains a single subsiding center throughout the mining process (Fig. 9). However, in the practical tectonic stress field, when the vertical dimension is far greater than the horizontal dimension in the mined-out space, horizontal displacement components become the overriding displacements because of the high horizontal stresses and large lateral convergence space. When these
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Fig. 9. Displacement contours and transmission models of rock mass movement induced by large-scale mining in ideal self-weight stress field.
Fig. 10. Displacement contours and transmission model of rock mass movement induced by large-scale mining in practical high-level tectonic stress field.
Fig. 8. Displacement contours and transmission models of rock mass movement induced by underground mining in initial stage of mining.
overriding displacements are transmitted to the ground surface, two different subsiding centers are formed on the ground (Fig. 10). The large subsiding center corresponds to the hanging wall rock mass movement and deformation, whereas the small subsiding center corresponds to the footwall rock mass movement and deformation. On the basis of the analysis above, it can be concluded that the fundamental reason for the special characteristics lies in the effect of the stress field on rock mass performance. In the tectonic stress
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field mining, relatively high normal stresses occur on the preferred structural planes. These stresses cause large sliding friction for structural plane reactivation, resulting in less impact on ground subsidence and deformation. Moreover, in the high-level tectonic stress field mining, horizontal displacement components are the overriding displacements in the hanging wall and footwall of the mine goaf. The overriding displacements eventually result in the expansion of ground movement and deformation.
4. Monitoring results and prediction of ground movement To study the mining-induced ground movement, GPS monitoring technology was introduced to the Jinchuan No. 2 mine in 2001. Since then, field monitoring has been carried out every 6 months. The monitoring net was composed of 100 GPS monitoring points. These points are laid out along seven exploratory lines and an additional monitoring line is laid out parallel to the strike of the deposit. The multi-period GPS monitoring from May 2001 to May 2010 obtained large quantities of displacement data, which quantitatively described the features of ground movement caused by longterm underground mining. The ground subsidence contours monitored from May 2001 to May 2010 are shown in Fig. 11. The subsidence curve and the geological section along exploratory line no. 14 are shown in Fig. 12. The monitoring results show that almost all coordinates of the monitoring points have been changed since 2001. The scope and extent of the ground subsidence developed continuously with mining depth. Overall, the long axis of the subsidence trough is basically parallel to the strike of the ore body, and extends along the exploratory line about 2.3 km long and 1.5 km wide along the strike of the ore body. Under the long-term effect of underground mining, the largest cumulated subsidence reached 1613 mm,
Fig. 11. Ground subsidence contours monitored from May 2001 to May 2010 (unit: mm).
while the largest cumulated horizontal displacement has reached 1050 mm. As shown in Fig. 12, the asymmetrical feature is notable, and the mining-influenced area is clearly larger in the hanging wall surface of the ore body than in the footwall (Figs. 11 and 12). Besides, tensile deformation is remarkable at the edge of the subsidence trough. The ratio of the horizontal displacement to the vertical displacement is generally doubled or increased by orders of magnitude near the edge of the subsidence trough. In the Jinchuan No. 2 mine, the main mining ore body has considerable thickness, with the largest being 200 m. As previously stated, the horizontal dimension of the excavated area is an important factor that leads to different mechanisms of ground subsidence at different mining stages. At present, the average production level is at 1170 m in the upper sublevel, while the average production level is at 1120 m in the lower sublevel. Hence, the average height in the upper mined-out space is 110 m, while the average width is 120 m. The vertical height of the mined-out space is clearly smaller than the horizontal width. Thus, only a single subsiding center currently occurs on the ground surface (Figs. 11 and 12). However, with increasing mine depth, the upper mine goaf and the lower mine goaf will be connected when the residual level ore body is mined out. By then, the vertical height of the mined-out space will be far greater than the horizontal width, and the horizontal movement and convergence deformation will become the overriding displacements in the mined-out space. Subsequently, the second subsiding center will occur when rock mass movement and deformation are gradually transmitted to the ground surface.
5. Conclusions In numerical simulations, roller boundary conditions are reasonable for the pre-excavation process of in situ stress field inversion. while, as a closed system, changing the roller boundary conditions to fixed boundary conditions in the subsequent excavation is optimal when the dimensions of the model borders are greater enough than the zone of influence of the excavation. In the initial stage of thin-layer mining, the mining-induced subsidence is similar to the mining of level coal seam, whether mining is conducted in the self-weight stress field or in the highlevel tectonic stress field. With increasing mine depth, the subsidence trough maintains a single subsiding center throughout mining in the self-weight stress field. However, in the high-level tectonic stress field, when the vertical dimension is far greater than the horizontal dimension in the mined-out space, the horizontal displacements in the hanging wall and footwall of the mine goaf become the predominant displacement components. Thus, when surrounding rock mass movement and deformation are transmitted to the ground surface, the secondary subsiding center is formed on the ground. In the high-level tectonic stress field mining, relatively high normal stresses on the preferred structural planes cause large sliding friction, thereby resulting in lower subsidence and deformation than in the self-weight stress field mining. Besides, the horizontal displacement components are the overriding displacements in the wall rocks, which directly results in the expansion of ground subsidence and deformation. Jinchuan No. 2 mine is a typical steep metal mine situated in a high-level tectonic stress field. The mining-induced ground subsidence exhibits different characteristics at different mining stages because of its steep occurrence, large thickness, and special tectonic environment. Currently, only a subsiding center exists on the ground because the vertical height of the mined-out space is clearly smaller than the horizontal width. However, when the residual level ore body is mined out, the horizontal movement
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Fig. 12. Ground subsidence curve and the geological section along exploratory line No. 14.
and convergence deformation will become the overriding displacements in the mine goaf, and the second subsiding center will subsequently occur on the ground surface.
Acknowledgments The research is supported by the National Natural Science Foundation of China (Grant nos. 41002107, 41172271, 40972197, and 41030750) and the Important Direction Project of Chinese Academy of Sciences Knowledge Innovation Project (KZCX2-YWQ03-02). Grateful appreciation is expressed for these supports. References [1] Guo HZ, Ma QC, Xue XC, Wang DN. The analytical method of the initial stress field for rock masses. Chin J Geotech Eng 1983;5(3):64–75 [in Chinese]. [2] Lu PY. On the horizontal displacement of the top of straight wall of shiplock highslope in the three gorges dam. Chin. J Rock Mech Eng 2000;1:120–5 [in Chinese]. [3] Gong MF, Qi SW, Liu JY. Engineering geological problems related to high stresses at the Jinping I Hydropower Station. Southwest China Bull Eng Geol Environ 2010;69(3):373–80. [4] Hou MX, Ge XR. Study on fitting analysis of initial stress field in rockmasses. Rock Soil Mech 2007;28(8):1626–30 [in Chinese]. [5] Cooling CM, Hudson JA. Importance of in situ rock stress in repository design. In: Proceedings of the international symposium on rock stress and rock stress measurement, Montpellier,1–3 September 1986. p. 647–56. [6] Pine RJ, Ledingham P, Merrifield CM. In-situ stress measurement in the Carnmenellis granite–II. Hydrofracture tests at Rosemanowes quarry to depths of 2000 m. Int J Rock Mech Min Sci Geomech Abstr 1983;20(2):63–72. [7] Amadei B. In situ stress measurements in anisotropic rock. Int J Rock Mech Min Sci Geomech Abstr 1984;21(6):327–38. [8] Duncan Fama ME, Pender MJ. Analysis of the hollow inclusion technique for measuring in situ rock stress. Int J Rock Mech Min Sci Geomech Abstr 1980;17(3):137–46. [9] Martna J, Hansen L. Initial rock stresses around the Vietas headrace tunnels nos. 2 and 3, Sweden. In: Proceedings of the international symposium
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International Journal of Rock Mechanics & Mining Sciences journal homepage: www.elsevier.com/locate/ijrmms
In situ stress field inversion and its application in mining-induced rock mass movement Haijun Zhao n, Fengshan Ma, Jiamo Xu, Jie Guo Key Laboratory of Engineering Geomechanics, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029 Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 April 2011 Received in revised form 13 April 2012 Accepted 1 May 2012 Available online 24 May 2012
Based on a series of experiments in numerical simulation, the model boundary conditions for in situ stress field inversion and excavation are discussed. Study results indicate that roller boundary conditions are reasonable for the in situ stress field inversion before excavation simulation, while, as a closed system, changing the roller boundary conditions to fixed boundary conditions in the subsequent excavation is optimal when the dimensions of the model borders are greater enough than the zone of influence of the excavation. As a case study, a comparative study of the mining-induced ground movement in a steeply dipping mine is carried out in two different stress fields. The results show that the mining-induced ground movement in the high-level tectonic stress field clearly differs from that in the ideal self-weight stress field. Because of the steep occurrence and large thickness of the ore body, the mining-induced ground subsidence exhibits different characteristics at different mining stages in the practical tectonic environment. Further studies elucidate the causes of these differences and clarify the effects of high-level tectonic stresses on rock mass movement and deformation. Finally, based on GPS monitoring results on the ground surface, the current ground subsidence is evaluated and its development trend is predicted. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Inversion Stress field Underground mining Rock mass movement Ground subsidence Mechanism
1. Introduction The geo-stress field is the most basic but also the most important load on rock engineering [1,2]. The in situ stress field is one of the fundamental conditions for the analysis, design calculation, and stability assessment of rock engineering projects. It is also a fundamental factor in the study of rock mass deformation and failure [1,3,4]. The in situ stress is a natural state of stress imposed on rock mass before engineering excavation is initiated, while the in situ stress field is a spatial distribution of geo-stress field at a certain time. Although it is an unstable state that changes with geological time and space, it can also be regarded as a constant stress field, as compared with the engineering construction period [1,4,5]. The in situ stress field involves a highly complicated process and is affected by many factors. Currently, various measuring methods have been proposed [6–11], but determining a precise in situ stress field remains difficult. In situ measurement is the most direct approach to determine geo-stress field. Because of high costs and limited test areas, however, measurement is usually conducted only in local areas with a few test points [12,13]. Thus,
n
Corresponding author. E-mail addresses: [email protected], [email protected] (H. Zhao). 1365-1609/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmms.2012.05.005
previous measurement results merely reflect the local stress field [6,14]. In addition, because of the complex genesis of the stress field and numerous influencing factors [15–17], certain degrees of discreteness are usually reflected in the measurement results. Therefore, on the basis of measurement results and geologic structure conditions, to derive a reasonable stress field with wide applicability using an effective numerical simulation method is quite necessary [18–21]. The degree of approximation between the inversed stress field and the practical stress field is one of the basic conditions for assessing the validity of numerical simulation. The correctness of in situ stress field obtained by numerical simulation directly influences the final results, especially for excavations in highlevel tectonic stress areas [22,23]. In numerical simulations, in situ stress fields are generally acquired by the regression fitting method using limited data on measurement and topography as bases [18,20,24]. Specifically, combined with displacement boundary conditions, the in situ stress values of all elements can be calculated according to the regression equations of in situ rock stresses. Subsequently, they can be assigned as element internal forces to simulate the in situ stress field. Alternatively, the calculated initial stress values can be converted into nodal forces. Then, combined with stress boundary and displacement boundary conditions, the stress field can be inversed [23,24]. To be more specific, if stress boundary condition is set, external forces are usually applied to lateral boundaries to simulate the
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corresponding tectonic stresses. In the simulating process, the external forces are constant, whereas the boundaries are moveable during excavation simulation. However, if the displacement boundary condition is set, tectonic stresses can be assigned as element internal forces in the calculated area. Regardless of which boundary conditions are established, the state of the inversed in situ stress field should correspond to that of the practical stress field. However, previous studies have shown that simulation results usually vary considerably under different boundary conditions. Thus, in the present study, a set of rational boundary conditions for stress field inversion and excavation is discussed based on a series of experiments in numerical simulation. As a case study, a typical steeply inclined mining mine that occurred in the high-level tectonic stress field is analyzed. By comparing mininginduced subsidence in ideal self-weight stress field, the special mechanisms of rock mass movement in the practical high-level tectonic stress field are clarified. Besides, based on GPS monitoring results on the ground surface, the current ground subsidence is evaluated and its development trend is predicted.
2. Comparative study of boundary conditions in numerical simulation 2.1. Model design and research method A typical stress field inversion model was studied in a homogeneous geologic body to clarify the effects of boundary conditions on numerical simulations. As a plane-strain model, we assume that the hypothetical model is 2000 m long and 500 m high. Besides, we assume the density of the linear-elastic model material is 3000 kg/m3, elastic modulus is 5.0 GPa, and the horizontal stresses are gradually increased from 0 at surface to 25 MPa at bottom in the model. Thus, different combinations of boundary conditions were experimented to inverse the hypothetic stress field. In terms of lateral boundary conditions, stress boundary conditions and displacement boundary conditions were adopted in different simulation processes. When stress boundary conditions were adopted, linearly increased external forces were imposed on the lateral boundaries of the model to inverse the aforementioned stress field. When displacement boundary conditions were adopted (lateral roller boundaries), element internal forces, which were calculated based on linear changes of the horizontal stresses, were imposed on all elements of the model to inverse the assumed stress field. Simulation results are shown in Fig. 1. The distribution curves in the models represent horizontal stress contours. 2.2. Results and discussion As shown in Fig. 1, the distribution contours of horizontal stresses are irregular (Fig. 1a–c) under stress boundary conditions. Aside from the horizontal distribution trend near the lateral borders, large fluctuations can still be observed in the inner part of the model. The main reasons for the different stress patterns under stress boundary conditions can be concluded in the following. The first reason lies in the distribution patterns of the lateral external forces. There are shear stresses on every abstract horizontal planes of the model because of the triangular load distribution; thus, the distributions of the compressive stress are irregular. The second reason for the above results may lie in the effect of the top and bottom free surfaces. The path lines of compressive stresses usually chose the shortest route to reach an equilibrium state. The third reason can be considered to be the end effect of the external forces. According to the Saint-Venant principle in elasticity, the influence of the external load decreases significantly from the pressure surface to the inner part of the
Fig. 1. Inversion results of horizontal stresses under different boundary conditions: (a) roller bottom and lateral stress boundary conditions, (b) fixed bottom and lateral stress boundary conditions, (c) roller bottom and unilateral stress boundary conditions, (d) fixed bottom and lateral roller boundary conditions, and (e) roller bottom and lateral boundary conditions.
model. Thus, the transmission of the compressive stresses shows non-equivalent and non-linear characteristics. By contrast, when the element internal forces are assigned to elements under displacement boundary conditions, the simulated horizontal stress contours are horizontal and smooth, and they are gradually increased from 0 Pa on the surface to 25 MPa at the bottom (Fig. 1d and e). These characteristics are consistent with the theoretical stress field. The differences between Fig. 1d and e lie in the boundary conditions at the bottom. Fig. 1d shows the application of a fixed boundary condition, while Fig. 1e shows the use of a roller bottom condition. When these two models are both composed of homogeneous isotropic strata (exactly shown in this study), the boundary conditions appear to have little influence on the simulation results. However, when they are composed of heterogeneous anisotropic strata, relative shear displacements are certain to occur during calculation. In this case, the fixed bottom boundary may affect the normal stress and displacement field distribution near the bottom. Thus, the roller bottom condition can be more appropriate for the stress field inversion, as shown in Fig. 1e. Besides, model borders are often expanded outward from the zone of influence of an excavation to eliminate boundary effects
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or minimize these within an allowable error range in numerical simulations. In a manner analogous to the definition of the zone of influence of an excavation, the near field of an ore body may be taken as the rock contained within the surface distance 3 times of the dimension of excavations from the ore body boundaries. If model borders are far enough from the excavated area, it can be regarded that the model borders have no effect on the normal distribution of the stress and displacement fields after excavation. Thus, in a rational numerical model, therefore, we can assume that no relative displacements occur at model borders after excavation, or that the relative displacements are sufficiently small and equivalent to zero-displacement boundary. Excavation is a dynamic disturbance in the model as it results in a certain degree of energy dissipation and migration [25,26]. According to the Saint-Venant principle, the zone of influence of an excavation is limited and mainly confined to the near field of the extraction. Thus, if the dimensions of the model borders are smaller than the zone of influence of an excavation, the model borders cannot accommodate the disturbance of excavation. On the contrary, the numerical model can be considered as a closed system because the model borders have no effect on the excavation. In this case, the total strain energy of the model is bound to decrease after excavation because it is a constant value in the entire model. Therefore, in a rational model, boundary conditions should be adjusted accordingly at different stages of simulation. In other words, roller boundary conditions are reasonable for the pre-excavation process of in situ stress field inversion, whereas, fixed boundary conditions are optimal for the subsequent excavation in the model (Fig. 2).
3. Application of the stress field inversion and study of the mining-induced ground movement in a steep nickel mine 3.1. Geology and mining conditions Jinchuan Mine is the largest nickel production base in China. It is located in Jinchang City, Gansu Province, northwest China. The mining area is approximately 6.5 km long, 10–570 m wide,
and extends to more than 1000 m underground. The strike of the main ore body is N501W, and the dip angle ranges from 401 to 701. Thus, it is a typical steep metal mine. In the mine area, ultrabasic rocks are the ore-bearing native rock and it was divided into four relatively independent mine fields because of the separation of the oblique faults. Among these mine fields, the No. 2 mine holds 75.2% of the total reserves of the Jinchuan Mine, thus, it was chosen as the research object of this study. In the No. 2 mine, the ore body is about 1600 m long, 100 m thick on average, and extends from 350 m below the surface to over 1000 m. Different scales of faults and various kinds of contact zones caused poor country rock stability in the mine area. Besides, high-level tectonic stress is a prominent characteristic in the mine area. Field tests and measurements were implemented in various kinds of rock mass at different depths to determine their regularities [27,28]. In all, a total of 18 measuring points were measured and the three principal stresses were determined as follows:
sh,max ¼ 0:046H þ 2:767ðMPaÞ sV ¼ 0:024H þ 0:423ðMPaÞ sh,min ¼ 0:020H þ 0:250ðMPaÞ
ð1Þ
where sh,max , sh,min , and sV are the horizontal maximum principal stress, horizontal minimum principal stress, and vertical principal stress, respectively, and H is the depth of the measuring points (units: m). In the mine area, the axis of the maximum principal stress is horizontal, which is basically perpendicular to the strike of the ore body. The vertical stress of the virgin rock mass is the intermediate principal stress, whose value is slightly smaller than the dead-weight of the overburden. With increasing depth, the ratio h,max/ h,min tends to reach 2.5, and the ratio h,max/ v is close to 2. This means that the value of the horizontal maximum principal stress is 2.5 times that of the minimum principal stress, and twice that of the vertical stress in the mine area. These results indicate that the stress differences in the virgin rock mass are substantial. The mine shafts and tunnels are currently used for underground mining (Fig. 3). The abscissas in Fig. 3 are the numbers of the exploratory lines in the mine area at an interval of 50 m. The underhand drift cut-and-fill mining method is employed in the mine area, and sublevels 1250 and 1150 m are mined concurrently to increase mineral production. However, large-scale ground surface movement and fissures occurred on the upper surface of the ore body after 18 years of mining [29]. Some important facilities, such as houses, mine shafts, and tunnels, have been destroyed by ground deformation and rock mass movement. 3.2. Model design and numerical research program
Fig. 2. Boundary conditions during the pre-and post-excavation processes.
In this study, the geological profile of the exploratory line No. 14 is selected and modeled for the subsequent numerical simulation (Fig. 4; minor surface undulations and contact zones are ignored). As a plane-strain problem, the model is 4000 m long, 1050 m high, and mainly composed of rich ore body, lean ore body, ultrasonic rock, marble, and banded migmatite. The mechanical properties of rock mass and their corresponding parameters have been previously studied through laboratory tests [29–31]. The parameters of rock mass applied in the numerical model were determined according to previous experimental results and inversion of ground subsidence based on GPS monitoring results (Table 1). The laboratory tests showed that the diversity of rock mechanical properties occurred not only in different rock types, but also in the same rock type in different places because of the well-developed joints and fractures in the
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mining area. Furthermore, the intact rock mechanical and strength parameters were usually 1–2 orders of magnitude smaller than the results of in situ mechanical tests because of the fractured rock masses. Thus, to determine feasible and verifiable rock mass parameters for the numerical model is a hard work. On the basis of the previous laboratory tests, however, these parameters can be inversed by the GPS monitoring results in the mining area. In such circumstance, we regard that these inversed parameters are proper for the model study though they may be smaller than laboratory test values and may show unnoticeable differences among these different rock types. In other words, as long as the simulation results reasonably reflect
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the practical in situ stress field and the characteristics of the mining-induced ground movement, the accuracy of the defined rock mass parameters has little effect on the research results. In addition, previous studies on ground movement induced by mining steep ore bodies mainly focus on coal mining [32–34]. However, the ground movement induced by mining steep metal mines clearly differs from that induced by coal mining because of the considerable differences in metallogenic type, rock mass structure, and geologic structure. The properties of in situ stress fields, in particular, play an important part in the mechanical behaviors of rock mass after excavation. Thus, a comparative research program was designed to discuss the special regularities and mechanisms of mining-induced rock mass movement. Specifically, the mining operations were simulated in two different stress fields: the self-weight stress field and the high-level tectonic stress field. The former one means the mining operations were simulated only under ideal self-weight conditions, while the latter one means the mining operations were simulated under practical high-level tectonic stress field, which was inversed according to the in situ stress regression equation (1). In the numerical model, the boundary conditions for in situ stress simulation and subsequent excavation illustrated in Fig. 2 are adopted in this study. In addition, the mining sequences are designed to be consistent with the practical mining operations. The ore body ranging from 1280 to 1250 m is gradually mined out from top to bottom, and then sublevels 1250 and 1150 m are mined simultaneously until the sandwiched ore body between sublevels 1250 and 1150 m is mined out (Fig. 3). The mining rate in the upper sublevel (1250 m) is designed to be twice that of the lower sublevel (1150 m) according to the fact. 3.3. Results and discussion
Fig. 3. Cross-sectional profile of the underground development openings and the ore body in the Jinchuan No. 2 mine.
3.3.1. Back-calculated principal stress fields Fig. 5 shows the distribution maps of the inversed principal stress tensors in different stress fields. Under the ideal self-weight condition, the orientations of the maximum principal stress tensors are vertical
Fig. 4. Numerical model simplified from the engineering geological profile of exploratory line No. 14 (1) banded migmatite, (2) marble, (3) granite, (4) ultra-basic rocks, (5) rich ore, (6) lean ore, (7) gneiss, and (8) filling body.
Table 1 Physical properties of rock mass applied in the numerical model. Lithology
Unit weight (kN/m3)
Elastic moduls (GPa)
Poison’s ratio
Cohesion strength (MPa)
Friction angle (1)
Tensile strength (MPa)
Banded migmatite Ore body Ultrasonic rocks Marble Gneiss Granite Filling body
30.0 29.3 29.3 30.0 28.0 27.6 20.0
2.2 2.5 2.2 2.2 2.0 2.2 0.2
0.25 0.23 0.25 0.25 0.27 0.25 0.28
2.0 1.0 1.0 1.0 1.0 1.0 0.3
44 42 42 44 40 38 44
2.0 1.0 1.0 2.0 1.0 1.0 0.3
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and increase with depth (Fig. 5a). However, the orientations of the maximum principal stress tensors are horizontal and also increase with depth in the practical high-level tectonic stress field (Fig. 5b). Overall, the magnitudes and distributions of the two back-calculated stress fields are satisfied for the subsequent mining simulation. 3.3.2. Characteristics of vertical subsidence and horizontal displacement To facilitate the contrastive study of the mining-induced ground movement under different stress fields, the curves of vertical subsidence (Fig. 6) and horizontal displacements (Fig. 7) in different mining stages are shown in the current work. As shown in Fig. 6a, in the ideal self-weight stress field mining, ground subsidence curves are smooth and symmetrical. With the increase in mining depth, ground subsidence increases prominently, and the maximum subsiding center migrates slowly to the upper surface of the ore body (the left side of the coordinate). When the mining depth reaches a certain level, the maximum settlement rate begins to decrease gradually. In the practical tectonic stress field mining, the curves of ground subsidence exhibit different characteristics at different mining stages (Fig. 6b). In the initial stage of single-level mining, although the subsidence curves are similar to those of mining in the selfweight stress field, the maximum ground subsidence is only 75.3% of the amount of the latter. In the double-level mining stage, however, a slight secondary subsiding center appears at the footwall surface of the ore body when the upper sublevel is mined at the level 1160 m and the lower sublevel is mined at the level 1110 m. At this time, the height of the total mined-out space is 140 m, and the average thickness of the mined-out space is 130 m. However, when the ore body between sublevels 1250 and 1150 m is completely mined out, the secondary subsiding center clearly appeared. Here, the height of the total mined-out space is 200 m, and the average thickness of the mined-out ore body is 120 m. It is obvious that the vertical dimension of the mined-out space is much larger than its horizontal dimension under the circumstance. Besides, the maximum subsiding center migrates gradually to the upper surface of the ore body (the left side of the coordinate) with mining depth during mining process. The migration rate is larger in the practical tectonic stress field than in the self-weight stress field, and the final maximum subsidence is only 78.5% of the latter. Although the maximum subsidence decreases compared with the mining in the self-weight stress field, the horizontal scope of the movement area expands prominently in the practical tectonic stress field. As shown in Fig. 7, horizontal displacements changed at different stages of mining in different stress fields. Horizontal displacements are zero near the outcrop of the ultrabasic rock, in which the
Fig. 6. Curves of ground subsidence induced by mining in different stress fields: (a) in ideal self-weight stress field, and (b) in practical high-level tectonic stress field.
Fig. 7. Curves of horizontal displacement induced by mining in different stress fields: (a) in ideal self-weight stress field, and (b) in practical high-level tectonic stress field.
maximum settlement developed. In general, the horizontal displacements in the hanging wall surface of the ore body are much larger than those in the footwall surface. Furthermore, the maximum horizontal displacement in the hanging wall surface of the ore body is 2–5 times that in the footwall surface of the ore body in the selfweight stress field (Fig. 7a). In contrast, the maximum horizontal displacement in the hanging wall surface of the ore body is 1–3 times that in the footwall surface (Fig. 7b), and the zero point of the horizontal displacement migrates slowly toward the footwall surface of the ore body (the right side of the coordinate) because of the occurrence of the secondary subsiding center in the practical tectonic stress field.
Fig. 5. Distribution map of the inversed principal stress tensors in different stress fields: (a) in ideal self-weight stress field, and (b) in practical high-level tectonic stress field.
3.3.3. Mechanisms of rock mass movement Ground subsidence is usually caused by different kinds of rock mass movements and deformations, and large differences usually exist in subsidence troughs in different ore bodies and stress
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fields. There are mainly two types of rock mass movement and deformation in steep mine mining. One is the movement perpendicular to the strata caused by the bending and breaking of surrounding rocks in the hanging wall and footwall. The other is the movement and deformation parallel to the strata caused by the bending and breaking of surrounding rocks on the roof and floor of the mine goaf, as well as the shearing and slipping of the surrounding rocks [34]. In this study, three kinds of mining-induced displacement contours and their general transmission models are presented in Figs. 8 and 10. The mechanisms of rock mass movement are discussed in the following. In the thin-layer mining stage (Fig. 8), the preponderant rock mass movements occur on the roof and floor of the mine goaf because the vertical height is smaller than the horizontal width in the mined-out space. Although there are rock mass movements in both sides of the mine goaf, they are relatively small and occupy only a small proportion of the movements. In this stage, therefore, the characteristics of mining-induced ground subsidence are similar to those of the mining of level coal seam in both stress fields. However, in the thick-layer mining stage (i.e., the vertical height is considerably higher than the horizontal width in the mined-out space), the curves of the mining-induced ground subsidence exhibit different characteristics in the two different geo-stress fields. In the self-weight stress field, although the areas where rock mass movement and deformation are larger in the hanging wall and footwall of the mined-out space than those on the roof and floor, the vertical displacement components in the rock mass are the overriding displacements. Therefore, when these overriding displacements are transmitted to ground surface, the ground subsidence maintains a single subsiding center throughout the mining process (Fig. 9). However, in the practical tectonic stress field, when the vertical dimension is far greater than the horizontal dimension in the mined-out space, horizontal displacement components become the overriding displacements because of the high horizontal stresses and large lateral convergence space. When these
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Fig. 9. Displacement contours and transmission models of rock mass movement induced by large-scale mining in ideal self-weight stress field.
Fig. 10. Displacement contours and transmission model of rock mass movement induced by large-scale mining in practical high-level tectonic stress field.
Fig. 8. Displacement contours and transmission models of rock mass movement induced by underground mining in initial stage of mining.
overriding displacements are transmitted to the ground surface, two different subsiding centers are formed on the ground (Fig. 10). The large subsiding center corresponds to the hanging wall rock mass movement and deformation, whereas the small subsiding center corresponds to the footwall rock mass movement and deformation. On the basis of the analysis above, it can be concluded that the fundamental reason for the special characteristics lies in the effect of the stress field on rock mass performance. In the tectonic stress
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field mining, relatively high normal stresses occur on the preferred structural planes. These stresses cause large sliding friction for structural plane reactivation, resulting in less impact on ground subsidence and deformation. Moreover, in the high-level tectonic stress field mining, horizontal displacement components are the overriding displacements in the hanging wall and footwall of the mine goaf. The overriding displacements eventually result in the expansion of ground movement and deformation.
4. Monitoring results and prediction of ground movement To study the mining-induced ground movement, GPS monitoring technology was introduced to the Jinchuan No. 2 mine in 2001. Since then, field monitoring has been carried out every 6 months. The monitoring net was composed of 100 GPS monitoring points. These points are laid out along seven exploratory lines and an additional monitoring line is laid out parallel to the strike of the deposit. The multi-period GPS monitoring from May 2001 to May 2010 obtained large quantities of displacement data, which quantitatively described the features of ground movement caused by longterm underground mining. The ground subsidence contours monitored from May 2001 to May 2010 are shown in Fig. 11. The subsidence curve and the geological section along exploratory line no. 14 are shown in Fig. 12. The monitoring results show that almost all coordinates of the monitoring points have been changed since 2001. The scope and extent of the ground subsidence developed continuously with mining depth. Overall, the long axis of the subsidence trough is basically parallel to the strike of the ore body, and extends along the exploratory line about 2.3 km long and 1.5 km wide along the strike of the ore body. Under the long-term effect of underground mining, the largest cumulated subsidence reached 1613 mm,
Fig. 11. Ground subsidence contours monitored from May 2001 to May 2010 (unit: mm).
while the largest cumulated horizontal displacement has reached 1050 mm. As shown in Fig. 12, the asymmetrical feature is notable, and the mining-influenced area is clearly larger in the hanging wall surface of the ore body than in the footwall (Figs. 11 and 12). Besides, tensile deformation is remarkable at the edge of the subsidence trough. The ratio of the horizontal displacement to the vertical displacement is generally doubled or increased by orders of magnitude near the edge of the subsidence trough. In the Jinchuan No. 2 mine, the main mining ore body has considerable thickness, with the largest being 200 m. As previously stated, the horizontal dimension of the excavated area is an important factor that leads to different mechanisms of ground subsidence at different mining stages. At present, the average production level is at 1170 m in the upper sublevel, while the average production level is at 1120 m in the lower sublevel. Hence, the average height in the upper mined-out space is 110 m, while the average width is 120 m. The vertical height of the mined-out space is clearly smaller than the horizontal width. Thus, only a single subsiding center currently occurs on the ground surface (Figs. 11 and 12). However, with increasing mine depth, the upper mine goaf and the lower mine goaf will be connected when the residual level ore body is mined out. By then, the vertical height of the mined-out space will be far greater than the horizontal width, and the horizontal movement and convergence deformation will become the overriding displacements in the mined-out space. Subsequently, the second subsiding center will occur when rock mass movement and deformation are gradually transmitted to the ground surface.
5. Conclusions In numerical simulations, roller boundary conditions are reasonable for the pre-excavation process of in situ stress field inversion. while, as a closed system, changing the roller boundary conditions to fixed boundary conditions in the subsequent excavation is optimal when the dimensions of the model borders are greater enough than the zone of influence of the excavation. In the initial stage of thin-layer mining, the mining-induced subsidence is similar to the mining of level coal seam, whether mining is conducted in the self-weight stress field or in the highlevel tectonic stress field. With increasing mine depth, the subsidence trough maintains a single subsiding center throughout mining in the self-weight stress field. However, in the high-level tectonic stress field, when the vertical dimension is far greater than the horizontal dimension in the mined-out space, the horizontal displacements in the hanging wall and footwall of the mine goaf become the predominant displacement components. Thus, when surrounding rock mass movement and deformation are transmitted to the ground surface, the secondary subsiding center is formed on the ground. In the high-level tectonic stress field mining, relatively high normal stresses on the preferred structural planes cause large sliding friction, thereby resulting in lower subsidence and deformation than in the self-weight stress field mining. Besides, the horizontal displacement components are the overriding displacements in the wall rocks, which directly results in the expansion of ground subsidence and deformation. Jinchuan No. 2 mine is a typical steep metal mine situated in a high-level tectonic stress field. The mining-induced ground subsidence exhibits different characteristics at different mining stages because of its steep occurrence, large thickness, and special tectonic environment. Currently, only a subsiding center exists on the ground because the vertical height of the mined-out space is clearly smaller than the horizontal width. However, when the residual level ore body is mined out, the horizontal movement
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Fig. 12. Ground subsidence curve and the geological section along exploratory line No. 14.
and convergence deformation will become the overriding displacements in the mine goaf, and the second subsiding center will subsequently occur on the ground surface.
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