矿柱劣化诱导矿区沉降规律研究.pdf

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 3. Engineering Research Center of Catastrophic Prophylaxis and Treatment of Road and Traffic Safety of Ministry of Education, Changsha University of Science 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 per 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, 277 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, ed by the violent underground Foundation item Projects51838001, 51878070, 51904101 supported by the National Natural Science Foundation of China; Project2019SK2171 supported by the Key Research and Development Program of Hunan Province, China; Projectkfj190402 supported by the Open Fund of Engineering Research Center of Catastrophic Prophylaxis and Treatment of Road Accepted date 2020-04-27 Corresponding author CHEN Lu, PhD, Lecturer; Tel 86-13575140241; E-mail chenlu; ORCID 0000-0002-3385-9335; ZANG Chuan-wei, PhD, Associate Professor; Tel 86-13730982029; E-mail chuanweizang; ORCID 0000-0002-3228-4621 J. Cent. South Univ. 2020 27 2160−2172 2161 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 uation about the stability of the pillars in the underground mining has been conducted. Many ulas 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 [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 deation 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 pered 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 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 was adopted in Changxing mining coal region, and a goaf area of 4.905 km2 had been ed since 1987. The subsidence events covering an area of 0.63 km2 J. Cent. South Univ. 2020 27 2160−2172 2162 occurred as shown in Figure 1a. 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 1b. 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 ed, 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; Figure 1 Subsidence events occurr in Changxing and Changlebao coal mine a Changxing coal mine; b Changlebao coal mine Table 1 Some collapse events occurring withinYuling coal mining regions Mine Collapse area number Collapsed area/km2 Goaf area/km2 CD/m PH/m RW/m PW/m Shenghe 2 0.011 2.977 113 5 6 8 Dachuangou 1 0.010 1.292 180 2.5 8 6 Bailu 1 0.240 5.390 115 5.6 9 6 Changxing 3 0.630 4.905 170 3.5 8 7 Shibadun 2 0.270 2.728 150 6.5 7 8 Changlebao 1 0.018 4.009 110 5 8 6 Changcheng 1 0.038 4.560 172 2.5 7 6 Wangjiapan 1 0.015 5.000 80 2 8 1.5 Jinniu 1 9.777 130 3.5 7 8 Shatanwan 1 0.0036 4.375 123 6.5 8 8 Note CD−cover depth; PH−pillar height; RW−room width; PW−pillar width modified from Ref. [28] J. Cent. South Univ. 2020 27 2160−2172 2163 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, ing 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 2 Layout of Wanzao and Yurong mine [29] a Plan view; b Cross-section view 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 at the middle position of pillar edge, and propagate or spall to the center Figure 3a; 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 3b. Figure 3 Failure pattern [30] a Progressive failure; b Failure effected by discontinuity To reveal the effect of weakening factors on the pillar bearing capacity and the failure process, numerical analyses with distinct element code 3DEC were pered. As shown in Figure 4, pillar models with or without discontinuity were established. Two models have the same pillar size of 10 m10 m10 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 Kn of discontinuity was set to 10 GPa/m, and shear J. Cent. South Univ. 2020 27 2160−2172 2164 Figure 4 Sketch of numerical model Table 2 Mechanical parameters of numerical model Property Elastic modulus/ GPa Poisson ratio UCS*/ MPa Tensile strength/ MPa Cohesion Friction angle Original value/MPa Softening rate/ Residual value/MPa Original value/ Softening rate/ Residual value/ Pillar 1.1 0.3 7.92 0.8 2.02 2 0.2 36 1 20 Roof 6.0 0.2 Floor 6.0 0.2 Joint 1.5 1.5 0.01 30 20 Note UCS is the uniaxial compressive strength. stiffness Ks was set to 5 GPa/m. More detail ination 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. 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 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