Lin-ming Dou

Prevention and forecasting of rock burst hazards in coal mines

Rock bursts signify extreme behavior in coal mine strata and severely threaten the safety of the lives of miners, as well as the effectiveness and productivity of miners. In our study, an elastic-plastic-brittle model for the deformation and failure of coal/rock was established through theoretical analyses, laboratory experiments and field testing, simulation and other means, which perfectly predict sudden and delayed rock bursts. Based on electromagnetic emission (EME), acoustic emission (AE) and microseism (MS) effects in the process from deformation until impact rupture of coal-rock combination samples, a multi-parameter identification of premonitory technology was formed, largely depending on these three forms of emission. Thus a system of classification for forecasting rock bursts in space and time was established. We have presented the intensity weakening theory for rock bursts and a strong-soft-strong (3S) structural model for controlling the impact on rock surrounding roadways, with the objective of laying a theoretical foundation and establishing references for parameters for the weakening control of rock bursts. For the purpose of prevention, key technical parameters of directional hydraulic fracturing are revealed. Based on these results, as well as those from deep-hole controlled blasting in coal seams and rock, integrated control techniques were established and anti-impact hydraulic props, suitable for roadways subject to hazards from rockbursts have also been developed. These technologies have been widely used in most coal mines in China, subject to these hazards and have achieved remarkable economic and social benefits.

Case Study of Passive Seismic Velocity Tomography in Rock Burst Hazard Assessment During Underground Coal Entry Excavation

Rock bursts, which are sudden dynamic events induced by deformation and fracture of the coal-rock mass, pose a serious threat to the production and safety of underground coal mining throughout the world. With increasing mining depths, rock bursts have become a common safety problem in underground coalmines (Dou et al. 2006; Zhu et al. 2016). In recent years, many rock bursts have occurred during coal entry excavation. Results show that rock bursts are mainly located 30 m behind the excavation face, the lengths of damaged section of entries exceed 100 m, and these areas are also those where rock bursts occur most often during coalface mining. Moreover, it is difficult to determine when a rock burst may occur, and where. For example, a rock burst in the Junde Coal Mine, Hegang mining area, on 18 January 2013 resulted in four deaths and the destruction of 200 m of gateway (Li et al. 2015). The investigation showed that rock bursts also occurred in the damage zone during entry excavation and that rock bursts occurred repeatedly in multiple areas of the coalface during excavation and mining. However, if accident statistics are analysed in more detail, it becomes apparent that stress concentrations were present in these rock burst risk areas because the occurrence of rock bursts mainly depends on the stress distribution around the excavation face. It is well known that the majority of rock bursts are related to geological structures, such as: faulting, folding, and igneous intrusion. Based on field observations of the stress in certain areas, the stress distribution is shown to have been abnormal, where stress concentrations are present. Therefore, stress anomaly information identification is a key issue for use in the monitoring of, and warning system for, rock burst hazards, especially during entry excavation. The prediction of rock bursts in a longwall panel has been researched for over 20 years, and the foci have been: pulverised coal drilling parameters (Gu et al. 2012), electromagnetic emission (He et al. 2011; Wang et al. 2011), borehole exploration (Zhang et al. 2014; Mark and Gauna 2016), and microseismicmonitoring (Cai et al. 2014; Iannacchione andTadolini 2016), with less attention paid to entry excavation. Moreover, the aforementioned conventional methods are not sufficient to meet the needs of engineering projects owing to their shortcomings of being time-consuming, difficult to implement, or causing disruption to production. Seismic velocity tomography allows large-scale rock burst assessment and bestows the advantages of high resolution, high reliability, and clear imagery: it is, therefore, of interest to mining engineers. Depending on the type of source used, tomography is classified as either ‘‘active’’ or ‘‘passive’’ (Swanson et al. 1992). Active tomography, in which the seismic wave is created artificially, is preferred for the detection of stress state (Meglis et al. 2005; Mitra and Westman 2009) and hidden structural defects (Zhao et al. 2000; Cai et al. 2014) in the pre-mining coalface. Passive tomography, which uses mining-induced seismic events as its source, is always feasible for long time-lapse & Lin-ming Dou lmdou@cumt.edu.cn

Roadway Stagger Layout for Effective Control of Gob-Side Rock Bursts in the Longwall Mining of a Thick Coal Seam

Currently, rock bursts pose a serious threat to the safety of miners and equipment in underground coal mining operations, especially in China. The number of coal mines with rock burst hazards is increasing year by year with no signs of letting up. By 31 December 2013, there were 142 coal mines in China which had experienced rock bursts. Each year, rock bursts cause considerable economic loss and enormous casualties. For instance, a rock burst induced by a large thrust fault caused 10 deaths and trapped 75 people on 3 November 2011 during the headgate excavation of LW21221 in Qianqiu coal mine, Yima City, China (Cai et al. 2014a, b; Li et al. 2014). Rock bursts are serious not only because the hazard itself can cause damage, but because it can cause a series of secondary disasters, such as coal and gas outbursts, and gas explosions. The most serious gas explosion recorded to date killed 214 people, injured 30 people, and caused a direct economic loss of U49.689 million. It happened on 14 February 2005 in Sunjiawan coal mine, Fuxin City, China. Investigation revealed that the gas explosion was induced by a rock burst (State Administration of Work Safety, State Administration of Coal Mine Safety 2005). Existing research mainly concentrates on the monitoring, prediction, and prevention of rock bursts (Adoko et al. 2013; Cai et al. 2014a, 2014b; Kornowski and Kurzeja 2012; Mu et al. 2013). Control measures against rock bursts are usually passive and include: de-stress blasting (Konicek et al. 2013), directional fracturing (He et al. 2012a), large-diameter drilling (Li et al. 2014), etc. These measures are time-consuming and only reduce rock burst potential without complete elimination of the hazard. The latest statistical data (Pan et al. 2013) show that 87 % of rock bursts occurred in roadways in China’s coal mines. Compared with the 72.6 % seen from previous statistics (Dou and He 2001), this proportion has increased. Among roadway rock bursts, gob-side rock bursts (GRBs) (i.e., rock bursts occurring in gob-side roadways) account for the majority. For example, the eight rock bursts in the No. 17 coal seam of Xing’an coal mine, Hegang City, China, caused damage to the tailgate and the longwall face eight and two times, respectively. However, the headgate was not damaged at all (see Fig. 1a). The 22 rock bursts in the No. 17 coal seam of Junde coal mine, Hegang City, caused damage to the tailgate, the longwall face, and the headgate 18 times, five times, and once, respectively (see Fig. 1b). Both tailgates in the two coal mines are gob-side roadways. It is common in other coal mines that GRBs account for the majority of rock bursts because gob-side roadways bear a higher stress. If GRBs are controlled effectively, rock burst hazards will be significantly mitigated. In this work, a case study of Yuejin coal mine (YCM) in Yima City, China, was analyzed to ascertain whether, or not, roadway staggered layouts could control GRBs. The aim of this study was to deduce whether, or not, this & Lin-ming Dou lmdou@cumt.edu.cn; kdlzl2010@163.com

Evolution and effect of the stress concentration and rock failure in the deep multi-seam coal mining

Supports crushing accident occasionally occurs in the protected seam exploitation of deep multi-seam coal mining structure and results in adverse effect to the production. To prevent its recurrence in a newly developed working field, a 3D numerical extraction model was built based on the geologic and mining conditions of Jining coal mine to evolve the changes, state and characteristics of the reconstructed vertical and lateral stress in rock interlayer after protective seam exploitation. Stress release and increase zones of this mining structure were separated. Mining-induced localized stress concentration and the interlayer rock failure behavior were explored. The action of concentrated stress on the hydraulic supports in protected seam was discussed upon the major stress redistribution. Using the infinitesimal strain method, a mechanical model was created to further explore, from the vertical and lateral directions, the cause and mechanism of localized stress concentration and rock failure behavior in rock interlayer. The field investigation was finally performed to verify the numerical and mechanical results, and the essential control measures were proposed to prevent this accident. Key findings of this study bring some new insights into the deep multi-seam coal extraction and help to promote a more reliable underground mining.

Microseismic Precursory Characteristics of Rock Burst Hazard in Mining Areas Near a Large Residual Coal Pillar: A Case Study from Xuzhuang Coal Mine, Xuzhou, China

Identification of precursory characteristics is a key issue for rock burst prevention. The aim of this research is to provide a reference for assessing rock burst risk and determining potential rock burst risk areas in coal mining. In this work, the microseismic multidimensional information for the identification of rock bursts and spatial–temporal pre-warning was investigated in a specific coalface which suffered high rock burst risk in a mining area near a large residual coal pillar. Firstly, microseismicity evolution prior to a disastrous rock burst was qualitatively analysed, and the abnormal clustering of seismic sources, abnormal variations in daily total energy release, and event counts can be regarded as precursors to rock burst. Secondly, passive tomographic imaging has been used to locate high seismic activity zones and assess rock burst hazard when the coalface passes through residual pillar areas. The results show that high-velocity or velocity anomaly regions correlated well with strong seismic activities in future mining periods and that passive tomography has the potential to describe, both quantitatively and periodically, hazardous regions and assess rock burst risk. Finally, the bursting strain energy index was further used for short-term spatial–temporal pre-warning of rock bursts. The temporal sequence curve and spatial contour nephograms indicate that the status of the danger and the specific hazardous zones, and levels of rock burst risk can be quantitatively and rapidly analysed in short time and in space. The multidimensional precursory characteristic identification of rock bursts, including qualitative analysis, intermediate and short-time quantitative predictions, can guide the choice of measures implemented to control rock bursts in the field, and provides a new approach to monitor and forecast rock bursts in space and time.

Evolution of Stress Concentration and Energy Release Before Rock Bursts: Two Case Studies from Xingan Coal mine, Hegang, China

Since the first recorded rock burst occurred in England in 1738, more than 20 countries have reported rock bursts (Kabiesz and Makowka 2009; Ortlepp and Stacey 1994; Patynska and Kabiesz 2009; Patynska 2013; Uszko 2009), including Germany, South Africa, Poland, the Czech Republic, Canada, Japan, France, etc. In China, rock bursts have become a common safety issue in underground coal mining. The number of coal mines experiencing rock bursts has increased annually (Jiang et al. 2010; Li et al. 2015). To date, 142 coal mines in China have suffered rock bursts which resulted in large economic losses and heavy casualties. For instance, the rock burst on 15 March 2013 in Junde Coal mine, Hegang City, caused the closure of a 200 m gateway, trapped 24 people, and killed four (Lu et al. 2015). The rock burst on 3 November 2011 in Qianqiu Coal mine, Yima City trapped 75 people underground and killed 10 people (Li et al. 2015). The rock burst on 14 February 2005 in Sunjiawan Coal mine, Fuxin City, caused a serious gas explosion and killed 214 people (State Administration of Work Safety, State Administration of Coalmine Safety 2005). The temporal and spatial evolution of mining-induced tremors reveals the process of initiation, development, and expansion of micro-fractures inside the coal-rock mass together with energy accumulation and release. This process may develop to cause either a tremor or a rock burst (Li et al. 2014; Wang et al. 2013). Tremors, rock bursts, or both are more likely to be induced in high-stress regions and the energy released therein is much higher. Therefore, rock burst risk can be evaluated by locating high-stress, and high-energy, regions. Microseismic (MS) monitoring is applicable when detecting the location and energy of mining-induced tremors. Recently, seismic velocity tomography (SVT) has been widely used for inference of high-stress distribution zones in underground mines by introducing seismic signals received by MS monitoring systems. For instance, Luxbacher et al. (2008) and Hosseini et al. (2012, 2013) conducted SVT by introducing mining-induced seismic signals, and found that high-velocity regions agreed well with high-abutment stress regions as predicted by numerical modelling, both of which were observed to redistribute as the coalface advanced. Dou et al. (2012), Banka and Jaworski (2010), and Lurka (2008) conducted SVT at regular time intervals during longwall mining, and found that rock bursts, or strong seismic events (i.e. tremors in underground mining), mainly occurred in high-velocity regions. Meanwhile, the bursting strain energy (BSE) index, which views rock bursts as a process of energy accumulation and release in the coal-rock mass as mining activities disturb the in situ stress field, was proposed here to characterise the spatial distribution of tremors [refer to Cai et al. (2015) for more details]. It was found that the & Si-yuan Gong libcumt@163.com

Frequency spectrum analysis on micro-seismic signal of rock bursts induced by dynamic disturbance

Blasting and breaking of hard roof are main inducing causes of rock bursts in coal mines with danger of rock burst, and it is important to find out the frequency spectrum distribution laws of these dynamic stress waves and rock burst waves for researching the mechanism of rock burst. In this paper, Fourier transform as a micro-seismic signal conversion method of amplitude-time character to amplitude-frequency character is used to analyze the frequency spectrum characters of micro-seismic signal of blasting, hard roof breaking and rock bursts induced by the dynamic disturbance in order to find out the difference and relativity of different signals. The results indicate that blasting and breaking of hard roof are high frequency signals, and the peak values of dominant frequency of the signals are single. However, the results indicate that the rock bursts induced by the dynamic disturbance are low frequency signals, and there are two obvious peak values in the amplitude-frequency curve witch shows that the signals of rock bursts are superposition of low frequency signals and high frequency signals. The research conclusions prove that dynamic disturbance is necessary condition for rock bursts, and the conclusions provide a new way to research the mechanism of rock bursts.

In Situ Test Study of Characteristics of Coal Mining Dynamic Load

Combination of coal mining dynamic load and high static stress can easily induce such dynamic disasters as rock burst, coal and gas outburst, roof fall, and water inrush. In order to obtain the characteristic parameters of mining dynamic load and dynamic mechanism of coal and rock, the stress wave theory is applied to derive the relation of mining dynamic load strain rate and stress wave parameters. The in situ test was applied to study the stress wave propagation law of coal mine dynamic load by using the SOS microseismic monitoring system. An evaluation method for mining dynamic load strain rate was proposed, and the statistical evaluation was carried out for the range of strain rate. The research results show that the loading strain rate of mining dynamic load is in direct proportion to the seismic frequency of coal-rock mass and particle peak vibration velocity and is in inverse proportion to wave velocity. The high-frequency component damps faster than the low-frequency component in the shockwave propagating process; and the peak particle vibration velocity has a power functional relationship with the transmitting distance. The loading strain rate of mining dynamic load is generally less than class 10−1/s.

Focal mechanism caused by fracture or burst of a coal pillar

As a regional, real-time and dynamic method, microseismic monitoring technology is quite an appropriate technology for forecasting geological hazards, such as rock bursts, mine tremors, coal and gas outbursts and can even be used to prevent or at least reduce these disasters. The study of the focal mechanisms of different seismic sources is the prerequisite and basis for forecasting rock burst by microseismic monitoring technology. Based on the analysis on the mechanism and fracture course of coal pillars where rock bursts occur mostly, the equivalent point source model of the seismicity caused by a coal pillar was created. Given the model, the seismic displacement equation of a coal pillar was analyzed and the seismic mechanism was pointed out by seismic wave theory. The course of the fracture of the coal pillar was simulated closely in the laboratory and the equivalent microseismic signals of the fractures of the coal pillar were acquired using a TDS-6 experimental system. The results show that, by the pressure and friction of a medium near the seismic source, both a compression wave and a shear wave will be emitted and shear fracture will be induced at the moment of breakage. The results can be used to provide an academic basis to forecast and prevent rock bursts or tremors in a coal pillar.

Mechanical Analysis of Static Stress Within Fault-Pillars Based on a Voussoir Beam Structure

List of symbols A, B, C Rock blocks A0, B0, C0 Action points of lateral thrust H, L, W Thickness, length and width of rock block (m) h0 Thickness-to-length ratio of rock block (h0 = H/L) Lx Length of fault-pillar (m) a Thickness of voussoir arch (m) d0, d1 Vertical deflections of rock blocks B and C (m) aA, aB, aC Rotation angles of rock blocks A, B, and C ( ) h Angle of fault plane in vertical direction ( ) u, uf Friction angles of rock and fault plane ( ), 0.85 for most rocks and faults c Unit weight of rock (N/m), 2.5 9 10 N/m for most rocks p Applied stress on rock block from overlying strata (Pa) q Total applied stress on rock block from overlying strata and the rock block (q = p?cH) (Pa) rc Uniaxial compressive strength (UCS) of rock (Pa) r, s Compressive stress and shear stress at fault plane (Pa) f(lx) Static stress within fault-pillar (Pa) P Total load on rock block (P = pLW ? cHLW = qLW) (N) T Lateral thrust (N) R0–0, R0–1, R1–2 Shear forces between rock blocks (N) R, R1 Resistance forces of collapsed and broken strata (N) Tf, Rf Normal force and shear force at fault plane (N)