GEOLOGY

Main factor inducing mining dynamic disasters: Fault activation in mining disturbance

Peer-Reviewed Publication

KEAI COMMUNICATIONS CO., LTD.

 

IMAGE: FIG.1 COAL SEAM ROOF DISPLACEMENT AND STRESS CHANGE CURVE NEAR FAULT ACTIVATION DURING LOWER PLATE MINING view more 

CREDIT: CHEN J, SHI K, PU Y, ET AL.

In underground geological structures, faults activation is frequently encountered during the process of coal mining. These geological structures have seriously damaged the continuity and integrity of the rock strata, in many parts of the world. The activation of faults has consistently been a critical factor impacting the safety of coal mining operations. Consequently, there is an urgent need to investigate the instability and failure of surrounding rock caused by fault activation.

To that end, a team of researchers from China conducted a comprehensive study on the impact of disturbance stress and roof abscission layer monitoring within zones affected by fault activation. The aim was to establish a theoretical foundation for effective roadway support.

“We utilized the discrete element 3DEC numerical analysis method to construct a model that simulates the unstable fracture of the surrounding rock resulting from fault activation,” explained Jie Chen, lead author of the study. “Specifically, we focused on the excavation of the upper and lower side walls of the faults, examining the characteristics of unstable fracture and stress variations in the surrounding rock induced by fault activation.”

The team found that as the coal working face progresses, the mining stress progressively intensifies. A zigzag wave pattern was observed on the relationship curve between coal mining and roof displacement in the vicinity of the fault (Figs. 1 and 2).

“This pattern indicates that the surrounding rock in the fault activation affected zone experiences a combination of static and dynamic loads,” added Chen.

“Simulation results further demonstrate that the stress and displacement of the surrounding rock near the fault increase as the coal mining face advances,” said co-corresponding author Yuanyuan Pu. “The recommended safe distance when approaching the fault is 30 meters. Conversely, the numerical tests indicate a slightly shorter safe distance of 26 meters when approaching the fault.” (Fig. 3)

The team hopes that their latest findings, published in the KeAi journal Rock Mechanics Bulletin, can get more attention in the field of mining safety to improve the safe and efficient mining of coal mines.

Fig. 2 Variation curve of positive stress and shearing stress of fault plane

Fig. 3 horizontal and vertical slip variation curves of fault plane

CREDIT

Chen J, Shi K, Pu Y, et al. 

Contact the author: Jie Chen, School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China, jiechen023@cqu.edu.cn; Yuanyuan Pu, School of Resources and Safety Engineering, Chongqing University, Chongqing 400044, China, yuanyuanpu@cqu.edu.cn.

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JOURNAL

Rock Mechanics Bulletin

DOI

10.1016/j.rockmb.2023.100037 

METHOD OF RESEARCH

Experimental study

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Study on instability fracture and simulation of surrounding rock induced by fault activation under mining influence

Research on Fault Activation and Its Influencing Factors on the Barrier Effect of Rock Mass Movement Induced by Mining

by 

Yanhui Guo

 1,*,

Luo Luo

 1 and

Chuangye Wang

 2

1

Faculty of Public Safety and Emergency Management, Kunming University of Science and Technology, Kunming 650093, China

2

Engineering Technology Research Institute of PetroChina Coalbed Methane Co., Ltd., Xi’an 710082, China

*

Author to whom correspondence should be addressed.

Appl. Sci. 2023, 13(1), 651; https://doi.org/10.3390/app13010651

Received: 14 October 2022 / Revised: 20 December 2022 / Accepted: 21 December 2022 / Published: 3 January 2023

(This article belongs to the Special Issue Fracture and Failure of Jointed Rock Mass)

Abstract

For the study of the driving forces behind fault activation and its influencing factors on the barrier effect of rock mass movement under the influence of mining, the discrete element numerical simulation software 3DEC was used for the analysis of the impact on the distance to mining area from fault, the buried depth of the upper boundary of the fault, the dip angle of fault, the size of the mining area and the thickness of the fault zone respectively. The results show that the mining areas are closer to the fault as distances decrease, the burial depth of the upper boundary of the fault increases, and the size of the mining area increases, the fault is easier to activate, and fault activation has a stronger barrier impact on displacement field and stress field propagation. When the fault is cut into the goaf, the difference of rock displacement in both directions of the fault increases when the dip of the fault increases, and the fault is more susceptible to instability and activation. The barrier strength grows with the increase of the thickness of the fault fracture zone. The results of this study have important implications for the guard against and control of deep mining-related fault activation disasters.

Keywords: 

fault activation; mining impact; rock mass movement; barrier effect; 3DEC

1. Introduction

With the acceleration of urbanization and industrialization, the demand of human society for mineral resources continues to increase [1]. With the increasing scale of underground mining, mine disasters are becoming more and more serious [2,3]. A fault is an inevitable major geological structure in the mining of underground mines, which will alter the stress and displacement fields of the surrounding rock, destroying the continuity and stability of the rock layers [4,5]. During deep underground mine extraction, various influencing factors will lead to fault activation, resulting in mining disasters. For example, rock bursting, mine water gushing, and mining area collapse on the surface [6,7,8,9]. Thus, one of the key elements impacting underground safe mining has been fault activation [10,11,12]. At the same time, during large-scale underground mine extraction, the faults distributed within the mining influence range will become a barrier for rock mass movement, deformation, and mining stress propagation [13]. In addition, due to the unclear understanding of the barrier effect of faults on rock mass movement, the unreasonable design of underground mine shaft and roadway engineering will lead to the abandonment of shaft and roadway engineering. This could have a great influence on the green, efficient as well as safe exploitation of mines [14,15]. The key factors affecting the barrier effect of fault activation rock mass movement include the distance between the mining area and the fault, the burial depth of the upper boundary of the fault, the fault dip angle, the size of the mining area, and the thickness of the fault zone. Therefore, the analysis of influencing factors on the barrier effect of rock mass movement under the influence of mining is of great significance in preventing and controlling the related disasters caused by the fault activation in deep mining of underground mines.

Numerous academics have studied that the influencing factors of fault activation and its barrier effect caused by mining in recent years. In terms of fault activation law and mechanism analysis, Jiang et al. [16] studied and analyzed the stress and rock mass movement characteristics in the process of fault activation of coal seams through three-dimensional numerical calculations. Nguyen and Rutqvist et al. [17,18,19] developed a mathematical model to simulate the fault activation during the controlled water injection experiment of the fault in the “Mont Terri Underground Research Laboratory” in Switzerland. Based on this, they also carried out a series of controlled fault activation experiments in clay. Jiang et al. [20] studied the space-time evolution law of the normal and shear stresses at the contact surface of the fault as well as the law of movement in the fault hanging wall and footwall through simulation. Zhao [21] studied the ground pressure behavior and dynamic response characteristics before and after fault activation under the influence of mining by establishing a similar model based on the actual project. Some scholars have studied fault activation through indoor experiments, Ohno et al. [22] conducted injection tests and repeated packer tests on siliceous mudstone faults, which can effectively evaluate the sensitivity of fault hydraulic fracturing to fault activation. Chen and Gong et al. [23,24,25] used acoustic emission technology to unite RFPA software to simulate and analyze the failure process of deep rock joints, and revealed the failure mechanism of deep jointed rock mass during excavation. Mngadi and Nguyen et al. [26,27] carried out friction experiments of different groups on rocks from deep mining in high-stress areas, and described the propagation process of the fracture along the underground brittle shear zone, which is of great significance to the study of rockburst. Li et al. [28] studied the stress environment and conditions of various fault activation and think that mining would cause uneven changes in vertical stress and horizontal stress in the fault, which was easy to induce fault activation.

The formation of faults is related to historical evolution, so some scholars predict the possibility of fault activation by analyzing the historical evolution of faults [29,30]. The farthest period can be traced back to the penultimate Glacial period for analysis [31,32], and some scholars have assessed the risk of fault reactivation through hundreds of years of historical data on the mine [33]. Hong et al. [34] even determined the time of fault activation by measuring the age of fault rocks. Molina et al. [35] used integral transformation technology and pressure derivative to estimate the occurrence time of fault reactivation. Clendenin et al. [36] studied a fault in the southeast of Missouri. Through surveying and mapping, they showed that the Grace Corner fault zone also has a complex history of polyphase reactivation, involving three Paleozoic reactivation periods of Late Ordovician, Devonian, and Post Mississippi.

With the rapid development of computers in the 21st century, the development of numerical simulation technology is relatively mature, so many scholars have conducted some research on fault activation in the process of deep mining through various numerical simulation software. Guo et al. [37,38,39] revealed the fault activation mechanism attributed to successive deep mining through numerical simulation and field monitoring. Ghosh et al. [40] detected geologic structure stress states such as faults in the Rajendra underground coal mine in India to realize safer mining operations and obtain an understanding of the changes in stress distribution. Li et al. [41] researched the mechanical mechanism of fault activation through three-dimensional numerical tests and microseismic monitoring. Zuo et al. [42] studied the distance between the working face and the maximum displacement of the fault plane when the fault is activated combined with the digital speckle correlation method, discontinuous deformation method, and numerical simulation method. Taking the Barapukuria coal mine in Bangladesh as the research background, Islam et al. [43] employed the boundary element method (BEM) to research the characteristics of stress as well as deformation around the fault and the influence of groundwater on fault activation. Sainoki and Wei et al. [44,45] based on the secondary development feature of the finite difference program FLAC 3D, investigated the effects of stress wave, fault surface roughness, and other parameters on fault activation. Lv et al. [46] established a thermal water mechanical damage coupling model (THMED) with faults based on FLAC 3D. Analysis and research of the fault activation barrier effect. Haddad et al. [47] used a three-dimensional, fully coupled porous elastic finite element simulation to evaluate the possibility of reactivation of barrier normal fault under normal fault stress state in different production scenarios

No matter through numerical simulation, indoor experimental research, or theoretical analysis, it is inevitable that some simplifications will be made to the real situation on the spot. Combined with the actual situation of the site, the analysis of fault activation can maximize the reliability of the results. In order to avoid the influence of fault activation on the mining safety of underground mines, Islavath and Liu et al. [48,49] not only established a complete monitoring system for fault activation in the underground, but also analyzed the safety of goaf near the fault during longarm mining through practical cases. Van Balen and Delogkos et al. [50,51] studied the possibility of fault reactivation based on the field activities of the Rohr Valley Rift System (RVRS) fault and the normal fault exposed by Kardia lignite mine in the Ptolemy basin, northwest Greece, respectively.

To sum up, although predecessors have carried out a great deal of fruitful studies on fault activation patterns and mechanisms under mining influence, however, there are still some deficiencies in the existing research results. First, in the past, there was much research regarding the activation of faults induced by coal extraction, while relatively little research regarding the activation of faults induced by metal mine extraction. As we all know, the deposit of metal and coal have different occurrence states and mining methods, so the fault activation effect and disaster mechanism caused by mining are also different. Second, the barrier effect of rock mass movement caused by faults is not clearly understand, which will lead to the unreasonable design of underground mine shafts and roadway engineering, resulting in the abandonment of some shaft and roadway engineering, and will significantly affect the safe, effective, and environmentally friendly mining of mines. In China, the safety production of metal mines still maintains a severe situation. On the premise of advocating safe and efficient production, it is vital to do research on the related problems faced in the mining process of underground metal mines. Therefore, based on the key factors such as the distance between the fault and the mining area, the buried depth of the fault of the upper boundary, the fault dip angle, the size of the mining area, and the fault fracture zone thickness, this paper studies the fault activation and its influencing factors on the barrier effect of rock mass movement under the influence of mining.

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Mechanism of Coal Burst

 

Triggered by Mining-Induced

 

Fault Slip Under High-Stress

 

Conditions: A Case Study

ORIGINAL RESEARCH article

 Earth Sci., 27 May 2022

This article is part of the Research Topic

Deep Rock Mass Engineering: Excavation, Monitoring, and Control

Jinzheng Bai1,2*, Linming Dou1,2*, Jiazhuo Li1,3, Kunyou Zhou1,2, Jinrong Cao1,2 and Jiliang Kan1,2

• 1School of Mines, China University of Mining and Technology, Xuzhou, China
• 2State Key Laboratory of Coal Resources and Mine Safety, China University of Mining and Technology, China
• 3School of Mining Engineering, Anhui University of Science and Technology, Huainan, China

Coal burst disaster is easily triggered by mining-induced fault unloading instability involving underground engineering. The high-static stress environment caused by complex geological structures increases the difficulty in predicting and alleviating such geological disasters caused by humans. At present, the mechanism of coal burst induced by mining-induced slip fault under high-stress conditions still cannot be reasonably explained. In this study, the burst accidents occurring near mining-induced slip fault under high-stress conditions were carefully combined, and the “time–space–intensity” correlation of excavation, fault, and syncline and anticline structure of the mining areas was summarized. On this basis, the rotation characteristics of the main stress field of the fault surface subjected to mining under high-stress conditions and the evolution law of stress were analyzed. Last, based on the spectrum characteristics of mining-induced tremors, the first motion of the P-wave, and the ratio of Es/Ep, the source mechanism behind mining-induced fault slip under high-stress conditions was revealed. The results demonstrate that the coal burst triggered by the fault slip instability under high-stress conditions is closely related to the excavation disturbance and the fold structure. Mining activities trigger the unloading and activation of the discontinuous structural surface of the fault, the rotation of the stress field, and the release of a large amount of elastic strain energy and cause dynamic disasters such as coal bursts. The research results in this study are helpful to enrich the cognition of the inducing mechanism of fault coal burst.

Introduction

Coal burst can generally be classified into three types, i.e., the fault-induced type, the coal pillar-induced type, and the strain-induced type (Kaiser et al., 2000), in which fault-induced coal burst is caused by the superposition of the mining-induced quasi-static stress in the fault coal pillar and the seismic-based dynamic stress generated by fault activation (Cai et al., 2020). Coal burst triggered by mining-induced fault slip (CBTMIFS) refers to the dynamic phenomenon that the deep excavation activities lead to the fault’s transformation from a locked state to an activated state, consequently resulting in sudden instability accompanied by violent energy release (Pan, 1999). Unlike natural earthquake induced by fault activation, mining activities are a key factor in the occurrence of CBTMIFS (Ortlepp and Stacey, 1992). A strong mining tremor of magnitude 5.2 in 1997 is considered one of the largest seismic events recorded at the Klerksdorp mine in South Africa, and the analysis result of ground motion parameters indicates that the violent earthquake was attributed to an existing fault slip in the region (McGarr et al., 2002). In 2005, 112 shallow earthquakes were recorded during the construction of the MFS Faido tunnel in Switzerland, which were felt strongly on the ground and caused considerable damage to the tunnel. The focal mechanism solution was consistent with the strike and tendency of natural fault (Husen et al., 2013). On November 3, 2011, the F16 thrust fault was activated at the Qianqiu coal mine in Yima, Henan Province, China, causing 10 fatalities and trapping 75 miners. On March 27, 2014, another devastating burst accident of magnitude 1.9 in this coal mine caused 6 fatalities and trapped 13 miners. The accident investigation report pointed out that the key factor of the accident was slip activation of the thrust fault (Cai et al., 2018). The abovementioned dynamic disasters closely related to human mining activities have attracted extensive attention from the media and the public. If the internal mechanism of CBTMIFS can be revealed, important ideas can be provided for predicting and remitting the risk of such engineering disasters.

Different from the brittle shear deformation of faults, the fold structures such as syncline and anticline reflect the continuous ductile deformation of rocks under crustal movement and sedimentation (Suppe, 1983). Both faults and folds are widely distributed in nature, often in the same tectonic unit. For large-scale crustal movements, multiple fold and fault structures interact and mutually transform through interlayer slip, uplift, and fold during the long historical tectonic movement and sedimentation process, and the specific forms include fault-related fold, fault-transition fold, fault-propagation fold, fault-detachment fold, imbricate structure, wedge structure, and interference structure(Bieniawaki, 1967). For the medium- and small-scale production range of mining areas, the frequent geological movement dominated by ancient stress leads to the complex regional tectonic stress field. Therefore, it will be more difficult to investigate the disaster-triggering mechanism of the mining-induced fault slip under a high-stress engineering background.

In order to clarify the occurrence mechanism of CBTMIFS in geological anomaly areas, plenty of studies have been carried out through theoretical analysis, laboratory experiment, numerical simulation, and field experiment,including the mechanical response and mineral composition of fault gouge (Morrow and Byerlee, 1989), hydraulic pressure and stress state of the fault zone (Segall and Rice, 1995), slip and failure criterion of fault (Fan and Wong, 2013), and energy accumulation and release law of the fault surface (Zhao and Song, 2013). On this basis, the key scientific issues condensed include the following: 1) How engineering dynamic disturbances, such as blasting, TBM excavation, hydraulic fracturing, geological drilling and rockburst, natural earthquake, driving load, and continuous explosion, will lead to slip, failure, and even instability of faults in high-stress geological anomaly areas? 2) What response characteristics will be caused to the stress field, vibratory field, and energy field of surrounding rock in the adjacent production area once the fault instability occurs in the high-stress geological anomaly area?

Relevant studies suggest that local high-stress concentration is likely to occur and develop when the mining working face or the excavation boundary is close to the fault in the high-stress geological anomaly area, and the corresponding burst risk increases (Cook, 1976; Blake and Hedley, 2003; Yin et al., 2014). When the fault approaches the critical stress state, the normal stress and the shear stress decrease sharply due to the reduction of intergranular force and the contact fracture of particles, and the evolution of fault state depends on the initial stress condition and excavation process (Wu et al., 2017; Yin et al., 2012). Field observations and theoretical analysis show that the development height of mining-induced fault rupture and slip is controlled by the magnitude and direction of principal stress, while the intensity of seismic events is related to the stratum matrix and local fractures involved in the rupture process (Duan et al., 2019). At the same time, many investigations have explored the response behavior of faults to static and dynamic load disturbances by changing stress conditions in laboratory tests. Marone (1998) pointed out that static friction and aging strengthening of faults are systematic responses that depend on loading rate and elastic coupling. Li et al. (2011) simplified the normal behavior of faults to elastic stiffness, adopted the coulomb-slip model to characterize the shear behavior of faults, and conducted a quantitative study on the propagation and attenuation law of seismic waves in discontinuous rock masses. Bai et al. (2021) introduced the displacement-related moment tensor method to reproduce the phenomenon of mining-induced fault slip of coal mine site in numerical simulation.

To sum up, the stress distribution and evolution characteristics of conventional fault activation instability have been well researched on. However, there are few studies on CBTMIFS under high-stress environments, and the existing research results ignore the influence of mining quasi-static loading and unloading stress paths and ground motion stress on the fault slip instability. Therefore, it is necessary to further study the mechanism of CBTMIFS under high-stress conditions, for providing guidance for the monitoring and prevention of coal bursts induced by fault instability.

International Journal of Rock Mechanics and Mining Sciences

Volume 123, November 2019, 104096

Microseismic investigation of mining-induced brittle fault activation in a Chinese coal mine

Author links open overlay panel, , , , , , 

https://doi.org/10.1016/j.ijrmms.2019.104096

Abstract

The process whereby a brittle fault was activated by mining is not only significant for disaster prediction, but also for earthquake prediction. Thus, it was necessary to study their activation process. In this study, stress state, AE (acoustic emission)/MS (microseismic) activities during brittle fault activation, stages of brittle fault activation, and the relation between AE/MS activities and brittle fault parameters were analysed and it was feasible to identify buried brittle faults, determine their parameters and activation process based on MS monitoring. The No. 22517 working face in the Dongjiahe Coalmine was taken as an example. Firstly, a buried brittle fault was identified in No. 22517 working face by the MS monitoring method. Secondly, the buried brittle fault parameters were determined. Its azimuth angle was 307.5° in the orbital roadway and 357.5° on the haulage roadway. Its dip angle was 73° and it was a reverse fault. Thirdly, the activation process of the buried brittle fault was determined. The buried brittle fault was not affected by the mining activities when the distance (from mining activities to the buried brittle fault) was over 300 m, the ‘disturbance stage’ was from 300 m to 80 m, and the ‘local activation stage’ from 100 m to 30 m, and the ‘whole activation stage’ around 30 m. The b value decreased during the process of brittle fault activation and reached a minimum value at the whole activation stage; it was concluded that these changes are related to the stress distribution around the brittle fault. The results demonstrated that MS monitoring technology can detect small buried brittle faults ahead of the working face, and is also an effective method to study the process and mechanism of brittle fault activation.

Introduction

The activation and instability of brittle faults contributes greatly to the preparation and occurrence of natural earthquakes. However, the scale of brittle faults causing natural earthquakes is usually large and the activation cycle very long. Thus, it is difficult for seismologists to systematically study large-scale and long-period brittle fault activation. In recent years, underground engineering (especially underground mining engineering) has developed rapidly. In the process of underground mining, it was inevitable to encounter some small- and medium-scale brittle faults. The presence of brittle faults can be a cause for serious problems such as rockbursts,1, 2, 3 water outburst,4, 5, 6, 7 coal and gas outbursts8 and roadway damage9 in underground mines as well as tunnels of civil engineering projects. The study of the activation mechanism of small- and medium-scale brittle fault is significant not only for the prediction of disasters caused by mining, but also for studying the process of earthquake preparation.

Many scholars have carried out laboratory experiments to study the shear slips process of brittle faults. Lockner10 and Lei11,12 pointed out that the aggregation of AE events on the plane of the future through-going rupture was often observed prior to the macroscopic rupture in experiments, and such AE events were more abundant in a heterogeneous sample than that in a homogeneous sample. Ohnaka et al.13 performed a series of laboratory experiments on the nucleation of propagating slip failure on pre-existing brittle faults with different surface roughness. They found that the nucleation process consisted of two phases and the nucleation process was greatly affected by geometric irregularities on the rupturing surfaces. Thompson et al.14,15 carried out a stick-slip experiment on a saw-cut Westerly granite sample by means of AE and CT (computed tomography). They stated that these large-amplitude AE events can be used to determine the nucleation site of slip and a small number of AE were recorded prior to each macro slip event. Based on a stick-slip experiment on a saw-cut, Goebel et al.16,17 concluded that geometric asperities were connected to regions of low b values, increased event densities and moment release over multiple stick-slip cycles. Moradian et al. 18, 19, 20 stated that the adhesive bond between concrete and rock had the most important effect on shear mechanism and AE had enough accuracy to monitor the shear behaviour of the joints and localize the points and the intensity (energy) of the asperities’ failure. Meng et al.21,22 studied the effects of shear history, fault-surface roughness, fault gouge, interface of different lithology and normal pressure on the shear strength and AE characteristics. The knowledge gained from laboratory experiments was expected to be applicable to MS characteristics around natural active brittle faults.

However, due to the difference between laboratory experiments and in-situ situation, it was essential to study the brittle fault activation based on in-situ monitoring. However, the brittle fault activation cycle, which drives earthquakes, is too long and the scale was too large for practical in-situ monitoring and study. The brittle fault activation induced by mining activity provided an opportunity to study brittle fault activation process based on in-situ monitoring. Donnelly et al.23 divided the surface deformation process into three stages based on the effect of brittle faults on surface deformation. Naoi et al.24,25 studied the relation between the development characteristics of planar AE clusters and brittle fault activation. They deduced that the observed expansion of the two AE clusters could be attributed to the expansion of the aseismic slip patches along the brittle fault. Yabe et al.26 observed foreshock activity of an Mw 2.2 earthquake (main shock) in a deep gold mine in South Africa and found that the foreshocks during 3 months before the main shock were concentrated in three clusters, and that the locations of foreshock clusters did not change with the location of mining area. Liu et al.27 pointed out that MS monitoring technique was useful for detecting geologic tectonic activities, such as brittle fault activations buried ahead during excavation activities. This paper took the example of the No. 22517 working face in the Dongjiahe Coalmine as an example for studying the method for identifying a buried brittle fault and determining the buried brittle fault parameters and exploring the activation process of the buried brittle fault based on the in-situ MS monitoring.