GEOLOGY
Main factor inducing mining dynamic disasters: Fault activation in mining disturbance
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.
The publisher KeAi was established by Elsevier and China Science Publishing & Media Ltd to unfold quality research globally. In 2013, our focus shifted to open access publishing. We now proudly publish more than 100 world-class, open access, English language journals, spanning all scientific disciplines. Many of these are titles we publish in partnership with prestigious societies and academic institutions, such as the National Natural Science Foundation of China (NSFC).
JOURNAL
Rock Mechanics Bulletin
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
1. Introduction
Mechanism of Coal Burst
Triggered by Mining-Induced
Fault Slip Under High-Stress
Conditions: A Case Study
- 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.
- 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.
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.
Microseismic investigation of mining-induced brittle fault activation in a Chinese coal mine
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.
No comments:
Post a Comment