J. Cent. South Univ. (2020) 27: 2160-2172

DOI: https://doi.org/10.1007/s11771-020-4438-3

Ground subsidence induced by pillar deterioration in abandoned mine districts

LUO Rong(罗容)^{1, 2}, LI Guang-yue(李广悦)¹, CHEN Lu(陈璐)^{2, 3},YANG Qi-yi(杨琪毅)², ZANG Chuan-wei(臧传伟)⁴, CAO Wen-zhuo(曹文卓)⁵

1. School of Resources Environment and Safety Engineering, University of South China,Hengyang 421001, China;

2. School of Civil Engineering, Changsha University of Science & Technology, Changsha 410114, China;

3. Engineering Research Center of Catastrophic Prophylaxis and Treatment of Road and Traffic Safety of

Ministry of Education, Changsha University of Science & Technology, Changsha 410114, China;

4. Key Laboratory of Mining Disaster Prevention and Control, Qingdao 266590, China;

5. Department of Earth Science and Engineering, Royal School of Mines, Imperial College,London SW72AZ, UK

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: When roadways are constructed above or adjacent to heavily mined regions, the ground subsidence caused by pillar collapse inflicts severe damage on these roadways. In this study, some surface subsidence events were first reviewed to present roof caving characteristics caused by pillar failure. The bearing characteristic and failure pattern of a single pillar with or without effect of discontinuity were further numerically simulated using distinct element code (3DEC). It was found that the spalling of pillar or slippage of discontinuity would damage the bearing capacity of pillar during the failure process. The stress at the pillar core could be greater than uniaxial compressive strength of the pillar. However, when a discontinuity runs through a pillar, the slippage of discontinuity would significantly degrade the bearing capacity of the pillar. In pillar support system, if any pillar unexpectedly degrades or loses its bearing capacity, the load transferred from the degraded pillar acts on neighboring pillars, and the shear force also increases at relevant positions. However, the roof cutting and surface subsidence characteristics would perform in different patterns. In some cases, surface subsides slowly; in the worst scenario, shock bump may be induced by pillar and roof collapse.

Key words: subsidence; deteriorating pillar; failure process; roof cutting

Cite this article as: LUO Rong, LI Guang-yue, CHEN Lu, YANG Qi-yi, ZANG Chuan-wei, CAO Wen-zhuo. Ground subsidence induced by pillar deterioration in abandoned mine districts [J]. Journal of Central South University, 2020, 27(7): 2160-2172. DOI: https://doi.org/10.1007/s11771-020-4438-3.

1 Introduction

Transportation infrastructures are developing quickly due to the rapid economic development, the spanking urban expansion and the fast industrial progress in China [1-5]. Some transportation networks have to be occasionally constructed above or adjacent to some abandoned mines, as a great deal of goaf, formed by the violent underground mining activities over the past decades, has existed in the lines that the roadways pass through. However, the abandoned underground mine has many potential negative effects on the traffic networks, among which the possible ground subsidence above or near the abandoned mine is one of the most severe hazards for the traffic systems [6, 7]. Once the ground subsidence is induced by pillar deterioration, the damages of roadways, such as pavement damages, high-filling embracement damages, deep-cutting slope failures, etc, are difficult to avoid [8, 9]. To decrease the possibility of roadway damage, the stability of the abandoned mines needs to be guaranteed. Unfortunately, the pillar, used to sustain the stability of the goaf, is easily to become weakened and lose its bearing capacity under the direct or indirect impact of the discontinuity, dynamic disturbance, underground water, etc, then resulting in the ground subsidence [10-15]. Owing to the vulnerability of the support pillar, the ground subsidence has become a serious geotechnical and environmental problem [16, 17]. To provide some guidelines for controlling the damage of ground subsidence on the traffic systems, it is necessary to investigate the ground subsidence behavior during the deterioration of the support pillars.

In past decades, the evaluation about the stability of the pillars in the underground mining has been conducted. Many formulas have been widely used to estimate the strength of a pillar [18]. Meanwhile, the stress of the pillar is estimated according to the pressure arch or the tributary area method [19]. Then, the safety factor of the pillar can be calculated by the pillar strength divided by average pillar stress, which is utilized to design the size of pillar or estimate the stability of a pillar [20, 21]. However, some pillars with high safety factor usually fail unexpected, resulting in cascading pillar collapse. That is because the failure of any pillar must disturb the adjacent pillars through transferring the additional load from the failed pillar to other pillars in a multiple pillar-roof system [22]. Due to the propagation of the interference from the failed pillars, the ground subsidence may be caused. Sometimes, only the slight roof deformation is induced by the failure of a few pillars. In the worst scenario, a large area of surface subsidence could occur when a large number of pillars collapse [23]. For instance, the progressive pillar failure induced a catastrophic ground subsidence with an area of 53000 m² on November 6, 2005 in China, which resulted in 38 injuries (12 on the surface and 26 underground) and 37 deaths (17 residents on the surface and 20 miners underground) [24]. In Collingwood Park, Australia, two subsidence events occurred due to the pillar failure, on December 7, 1988 and the second failure on April 25, 2008, respectively. In the first, up to 570 mm of ground subsidence was monitored after the initial subsidence, and the total ground subsidence was estimated to be around 1.7 m. In the second, the maximum total subsidence was recorded to be around 1.4 m, and approximately 30 to 40 houses within the immediate ground subsidence area were damaged at the varying degrees [25, 26]. These accidents pose a serious risk to engineering structures. However, the subsidence behavior induced by the deteriorating pillar has not been illuminated systematically.

In this study, some subsidence events were exhibited and analyzed firstly. Numerical simulation using the distinct element code (3DEC) was performed to demonstrate the pillar deterioration characteristic considering or not considering the influence of joint. Then, the subsidence behaviors induced by the pillar failure were investigated.

2 Surface subsidence events

2.1 Some subsidence events in Yulin coal mining district, China

Yulin coal mining district is located in the northeast of China, which is one of the largest coal production bases in China, extending from the southern part of Inner Mongolia Autonomous Region to the northwest of Shaanxi Province, China. This coalfield covers 5 to 6 mineable seams, and part of coal seams have been exploited by some small mines using the room and pillar mining method in the past few decades. Consequently, a great number of pillars have been left to support the overlying rock strata [27]. Unfortunately, the surface subsidence events caused by the pillar failure frequently occurred in recent years. For example, the room and pillar mining method was adopted in Changxing mining coal region, and a goaf area of 4.905 km² had been formed since 1987. The subsidence events covering an area of 0.63 km² occurred as shown in Figure 1(a). The panel barrier pillar failed, even though it was expected to remain stable. The subsidence to the north extends to the mine boundary, and the south subsidence extends to the barrier pillar. A subsidence event covering an area of 0.018 km² also occurred within a 4.009 km² goaf area in Changlebao coal region as shown in Figure 1(b). The subsidence area was limited within one or two panels, in which most of the panel barrier pillars were still stable.

According to some statistics, similar subsidence events also occurred in the other mines in the Yulin coal district. Table 1 shows the pillar conditions and collapsed area. A total of 10 coal mines experienced the localized surface subsidence during past decades. And the majority subsidence events were characterized by the slow or residual subsidence, few of which triggered shock bump during the collapse process.

2.2 Subsidence event in Linyi gypsum mining district

Linyi gypsum mining district is an important gypsum production base located in eastern China. Since large scale mined-out area has been formed, subsidence events sometimes were caused by the pillar failure. On December 25, 2015, cascading pillar failure and abrupt ground subsidence occurred unexpectedly, causing 29 workers trapped in underground among which only 15 workers were rescued, one died, and the 13 workers disappeared until February 6, 2016 [29]. According to the accident survey, the engineering condition and the process of accident can be summarized as follows: the Wanzao Mine was adjacent to Yurong Mine in Linyi gypsum mine district as shown in Figure 2;the #3 gypsum seam was extracted by those mines at different time. The immediate roof of both mine was mudstone with low strength. However, the main roof was characterized by limestone with thickness from 30 to 120 m, and the limestone stratum was strong and intact. The extracting depth of Wanzao Mine changed from -110 m to +115 m during July 1996 to March 2012, forming a goaf with area more than 0.12 km². The Yurong Mine was extracting gypsum from -322 m to +120 m from 2008. Unfortunately, part of barrier pillar was extracted. As the pillar failure and the roof collapse occurred in Wanzao Mine, the energy released and transmitted to neighbor mine, and then the pillars were overloaded and roof collapsed in Yurong Mine. Magnitude 4.0 shock bump was recorded in this subsidence event.

Figure 1 Subsidence events occurr in Changxing and Changlebao coal mine:

Table 1 Some collapse events occurring withinYuling coal mining regions

Figure 2 Layout of Wanzao and Yurong mine [29]:

3 Bearing and failure characteristic of single pillar

The important function of the pillar is considered to support the overlying strata in mines and many other underground engineerings. However, the stability of the pillar is affected by underground water, discontinuity, engineering activity, etc. The effective size of the pillar could be diminished, resulting in the reduction of bearing capacity and then the ultimate collapse. Generally, the failure pattern can be summarized as follows: 1) Pillars failed progressively under the direct or indirect impact of the stress and atmosphere. The crack or failure zone may initially form at the middle position of pillar edge, and propagate or spall to the center (Figure 3(a)); 2) Large discontinuities typically extend from the roof to the floor in a pillar or pillar ribs. Sliding can occur along these discontinuities, causing the significant weakening of these pillars (Figure 3(b)).

Figure 3 Failure pattern [30]:

To reveal the effect of weakening factors on the pillar bearing capacity and the failure process, numerical analyses with distinct element code (3DEC) were performed. As shown in Figure 4, pillar models with or without discontinuity were established. Two models have the same pillar size of 10 m×10 m×10 m, the thicknesses of the roof and the floor were set to 3 m in the numerical model. In this study, the strain softening model based on the Mohr-Coulomb criterion was utilized as failure criterion of pillar. The friction angle and cohesion can be softened upon the onset of plastic yield by a user defined piecewise linear function. And the elastic model was chosen for roof and floor. The mechanical parameters and softening rate are listed in Table 2. The normal stiffness K_n of discontinuity was set to 10 GPa/m, and shear stiffness K_s was set to 5 GPa/m. More detail information can be found in our previous studies [31, 32]. As shown in Figure 4, during the loading process, five stress measurement points were installed at the centre line of the pillar. Horizontal section in the middle of the vertical direction of the pillar was chosen to reveal the stress state. Vertical section in the middle of the horizontal direction was monitored to investigate the distribution of plastic zone.

Figure 4 Sketch of numerical model

Table 2 Mechanical parameters of numerical model

3.1 Failure process of pillar without influence of discontinuity

The compressive test of the pillar without influence of discontinuity was conducted according to the method as shown in Figure 4. Figure 5 shows the stress-strain curves of monitoring points and average stress of the pillar. Figure 6 exhibits some typical stress states of horizontal section of the pillar, and the plastic zone of vertical section of the pillar is exhibited in Figure 7. It can be found that, during the beginning stage of loading, the distribution of the stress was uniform in the pillar (state a in Figures 5 and 6). Then the plastic zone occurred at the edge (state b in Figure 7), and the stress of those zones started to decline. However, the average stress of the pillar continued to increase. After state c, the volume of the pillar core decreased gradually, resulting in the degradation of the pillar bearing capacity, due to the plastic zone expanded from the edge to the centre of the pillar (state b to state e as shown in Figure 7). The strength of the pillar (the maximum value of average stress) was 8.23 MPa. However, the monitoring points #2, 3 and 4 still increased, and the maximum stress values were greater than the uniaxial compressive strength of the pillar. It is necessary to note that, the uniaxial compression strength of material used in the numerical model is 7.92 MPa, which is estimated based on Mohr-Coulomb criterion using the parameters as shown in Table 2. Hence, the strength of the pillar is slightly greater than that of the material.

Figure 5 Stress-strain curves

As previously discussed, the spalling or plastic zone would evolve from the edge to the centre of the pillar, and the stress of those zone reduced,resulting in the gradual degradation of the bearing capacity. Consequently, if the spalling speed of the pillar can be reduced, the stability would improve.

Figure 6 Stress characteristics during failure process of:

Figure 7 Plastic zone characteristics during failure process of:

3.2 Failure process of pillar with influence of discontinuity

Generally, the bearing capacity and the ability to resist the deformation of the pillar can be significantly affected by the discontinuity. And it is impossible to avoid constructing the pillar in the joint or fracture area in underground mining. To understand the effect of the discontinuity on the pillar, the compressive test of the pillar impacted by the discontinuity was conducted, as shown in Figure 4. Figure 8 shows the stress–strain curves of monitoring points and average stress of the pillar. Figure 9 exhibits some typical stress states of horizontal section of the pillar, and the plastic zone of vertical section of the pillar is exhibited in Figure 10. It can be found that, although a discontinuity contained in pillar, the stress was non-uniform distribution in the pillar even in the beginning stage of loading (state a in Figure 8). With the increasing of loading, the sliding of the discontinuity induced the declining of the stress, of which the first stress step was exhibited in Figure 8.

Figure 8 Stress-strain curves of pillar effected by discontinuity

And the stress of the monitoring #3 close to the discontinuity dropped significantly. The plastic zone evolved at the position of the pillar corner in slip direction as shown in Figure 10(d). The stress concentered on the pillar at the lower part of the discontinuity (Figure 9(e)), and the degree of the non-uniform load bearing of pillar augmented. After state e, the stress of the pillar at the position below of the discontinuity also declined, resulting in the degradation of the pillar bearing capacity. And the strength of the pillar (the maximal value of average stress) was 6.51 MPa. Moreover, the volume of elastic zone (pillar core) was remaindered about half of pillar and the pillar lost its bearing capacity unexpectedly. It is necessary to note that, the strength of this pillar is approximately 79% of that of the pillar not impacted by the discontinuity.

Figure 9 Stress characteristics impacted by discontinuity during failure process of:

As previously discussed, the slipping of the discontinuity would cause the significant degradation of the bearing capacity. Consequently, if the discontinuity can be restricted to slip using rock bolting or grouting, the stability of the pillar would be reinforced.

4 Subsidence characteristics

4.1 Subsidence scope

When the room-and-pillar mining method is used to extract mineral resources, the pillar will be left to support the overlying strata and to prevent roof collapse. Hence, if any individual pillar loses the bearing capacity, its load would be transferred to the neighboring pillars though the roof and the shear force of the roof would increase correspondingly as shown in Figure 11(a). Consequently, the additional load on these pillars and the roof may lead to their failure during the load redistribution, and the associated roof subsidence will be caused. As discussed in Section 2, in some cases, quite a few of pillars fail, and then the strong barrier pillar will bear the additional load, and roof crack occurs, inducing the surface subsidence (Changlebao coal mine). However, in the worst scenario, if the roof is stronger and the barrier pillar is unable to support the additional load, hundreds of pillars will fail progressively and the collapse area will extend to the boundary pillar or boundary of mine district (Changxing coal mine). Those results indicate that failure of any individual pillar can overload adjacent pillars, in which the pillars failed progressively. And the roof crack can also be caused, losing its function to transfer load as shown in Figure 11(b). Consequently, the failure propagation will be restricted and the collapsed area cannot extend to the boundary of mine. Moreover, the load of remnant pillar will drop again, which contributes to the increasing of the stability of the goaf. Above all, it can be concluded that the collapse is easily caused by the failure of any pillar, but the collapse area is controlled by the condition of the roof.

Figure 10 Plastic zone and discontinuity slipping characteristics during failure process of:

Figure 11 Collapse area:

4.2 Intensity of large mined-out areas subsidence

During the subsidence process, the great majority mines were characterized by slow or residual subsidence. It maybe take a few months or even longer time. For example, up to 570 mm of subsidence was monitored among around twelve month after the initial subsidence in an abandoned mine in Collingwood Park, Australia [24]. The main reasons of slow ground subsidence were weak enough overburden, shallow depth of cover, geological discontinuities and dissolution of rocks. In those engineering conditions, the roof may be easily cracked, which is induced by pillar deformation and degradation. The ground subsidence mechanism may be presented as follows: the degradation or collapse of the pillars causes the load redistribution, and the additional load from the weakening pillars is transferred to the adjacent barrier pillars which are considered strong enough. The shear force would concentrate on the roof above the barrier pillar, as shown in Figure 11(a). Thus, the roof shear crack initiates easily at the corresponding position in the same time. Then the area of the roof circled by shear cracks would subside slowly.

Unfortunately, some subsidence events occurred in a few seconds, causing shock bump. In Shibadun coal mine, twice mine subsidence events caused shock bump with magnitude 2.8 and 3.2, respectively. Subsidence area of 0.27 km² was formed. Furthermore, from September 2009 to December 2015, a total of 13 shock bumps were detected in the Yulin District, with magnitudes of 2.1 to 3.3 [28]. Moreover, magnitude 4.0 shock bump was triggered by the pillar failure and the roof collapse in Linyi gypsum mining district [29]. The subsidence mechanism may be described as follows: with the increase of time, the pillars became deteriorated in Wanzao Mine as illustrated in Figure 12(b), the pillar spalled, and local roof caving occurred in mudstone stratum firstly;however, the limestone was intact and strong. When the pillar continued to collapse and the immediate roof caved gradually, the span of main roof increased, the stress and energy would concentrate at the position of the end of the roof, as shown in Figure 12(c). Once the span of the main roof in limestone stratum approached to the critical stable state, any pillar collapse or load disturbance will cause the shear slip at the end of the roof, resulting in abrupt roof collapse and energy release, and the shock bump may be induced. Furthermore, the violent shaking would propagate to neighbor mine, inducing the occurrence of cascading pillar failure and roof collapse in Yurong Mine. Ultimately, shock bump was triggered, resulting from pillar collapse and roof cutting as illustrated in Figures 11 and 12. The failure propagation mechanical of the pillars subjected to dynamic load can be explained in previous paper [22]. Those accidents waeres characterized by the abrupt collapse, posing a great threat to workers safety and environment.

Figure 12 Large-scale failure process (modified after Ref. [29]):

5 Conclusions

Ground subsidence events, which occurred in Yulin coal mining district and Linyi gypsum mining district, were presented firstly in this study. The bearing characteristic and failure pattern of a single pillar with or without the effect of discontinuity were then investigated using distinct element code (3DEC), and the subsidence characteristics are also analyzed. The following conclusions can be drawn:

1) In recent years, ground subsidence events caused by the pillar failure frequently occurred. Generally, the subsidence area was limited to one or two mining panels, and the ground subsided slowly. Sometimes, the pillar failure and the roof collapse occurred unexpectedly, triggering underground hazards such as shock bumps.

2) The spalling of the pillar or the slippage of the discontinuity would damage the bearing capacity of the pillar during the failure process. Moreover, the stress at the pillar core could be greater than the uniaxial compressive strength of the pillar. However, when a discontinuity runs through a pillar, the slippage of the discontinuity would significantly degrade the bearing capacity of the pillar.

3) If any individual pillar fails, its load will be transferred to the neighboring pillars through the roof, and the shear force of the roof accordingly increases. When the roof was cracked, the associated roof subsidence may be caused, and the collapsed area will be restricted by roof cracks.

4) When the degradation of the pillars and the initiation of shear cracks in the roof occur almost at the same time, the whole roof circled by the shear cracks would subside slowly. However, if the pillar continues to collapse and the immediate roof caves, the span of the main roof in strong stratum increases gradually. Once the span of the main roof approaches the critical stable state, any pillar collapse or load disturbance may cause the abrupt collapse of the whole roof and associated energy release, and shock bump may be induced.

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(Edited by ZHENG Yu-tong)

中文导读

矿柱劣化诱导矿区沉降规律研究

摘要：采空区塌陷易造成上跨公路受损而影响其安全运行。首先，探讨了矿柱失稳条件下顶板的变形坍塌特征。其次，采用离散元软件(3DEC)，对矿柱的承载特征及失稳模式进行了数值分析。结果表明，矿柱表层剥落或弱面滑移均会造成承载能力的劣化。且在矿柱群-顶板系统中，单个或局部矿柱的失稳，均会引起其相邻矿柱应力及顶板剪切应力的增加，进而诱发地表沉降。然而，地表沉降的范围及坍塌程度可能差异较大，部分矿区表现为小范围缓慢沉降，有些矿区则会出现大规模坍塌并诱发矿震。

关键词：地表沉降；矿柱劣化；失稳过程；顶板剪切

Foundation item: Projects(51838001, 51878070, 51904101) supported by the National Natural Science Foundation of China; Project(2019SK2171) supported by the Key Research and Development Program of Hunan Province, China; Project(kfj190402) supported by the Open Fund of Engineering Research Center of Catastrophic Prophylaxis and Treatment of Road & Traffic Safety of Ministry of Education(Changsha University of Science & Technology), China

Received date: 2020-03-25; Accepted date: 2020-04-27

Corresponding author: CHEN Lu, PhD, Lecturer; Tel: +86-13575140241; E-mail: chenlu@csust.edu.cn; ORCID: 0000-0002-3385-9335; ZANG Chuan-wei, PhD, Associate Professor; Tel: +86-13730982029; E-mail: chuanweizang@163.com; ORCID: 0000-0002-3228-4621