Assessment and mitigation of coal bump risk during extraction of an island longwall panel

International Journal of Coal Geology 95 (2012) 20–33

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International Journal of Coal Geology

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Assessment and mitigation of coal bump risk during extraction of an islandlongwall panel

Yaodong Jiang a, Hongwei Wang a,⁎, Sheng Xue b, Yixin Zhao a, Jie Zhu a, Xufeng Pang a

a School of Mechanics, Architecture and Civil Engineering, China University of Mining and Technology, Beijing, 100083, Chinab CSIRO Earth Science and Resource Engineering, PO Box 883, Kenmore QLD 4069 Australia

⁎ Corresponding author. Tel.: +86 13488689275.E-mail address: [email protected] (H. Wan

0166-5162/$ – see front matter. Crown Copyright © 20doi:10.1016/j.coal.2012.02.003

a b s t r a c t
a r t i c l e i n f o
Article history:Received 2 September 2011Received in revised form 3 February 2012Accepted 7 February 2012Available online 15 February 2012

Keywords:Island longwall panelCoal bumpBump-prone zonesStress reliefBoreholes

This study presents an integrated approach for field tests and numerical investigations to assess the risk ofcoal bumps. This approach produces a stress-relief technology using boreholes to mitigate risk during theextraction of an island longwall panel. The field tests were conducted in an island longwall panel in theTangshan coal mine in the city of Tangshan, China. In these tests, roadway roof displacement and electromag-netic radiation (EMR) of roadways in the panel were investigated to determine the zones of intensive roofdeformation. A numerical model FLAC3D (Fast Lagrangian Analysis of Continua in 3 Dimensions) was estab-lished to understand the results of the field tests and to map the zones in the panel with a high risk forcoal bumps. The results of the field tests and the numerical modeling show that the roof deformation startsto occur at more than 30 m ahead of the longwall face and the deformation starts to accelerate after a dis-tance of 10 m in front of the longwall face. Large and rapid roof deformation is considered to be an importantprecursor of coal bump occurrence during the extraction of an island longwall panel. Based on these results, astress-relief technology using boreholes was investigated through numerical methods. The modeled resultssuggest that the abutment stress could be released by drilling boreholes in the zones prone to coal bumps.The effectiveness of the stress release increased with the borehole length and decreased with the boreholespacing.

Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.

1. Introduction

Coal bumps have been a major safety concern in underground coalmines in China for more than 50 years. In general, a coal bump refersto a sudden and violent failure of a coal seam that releases containedelastic energy and expels a large amount of coal and rock into theroadway or working face where men and machinery are present(Mohamed, 2003; Zhao and Jiang, 2010). It can cause fatality, injuryand significant economic loss for the coal mining industry. Coalbumps occur more frequently in an island longwall panel, which issurrounded by previously mined panels, than in other longwallpanels because of its high-stress predominance. Therefore, the studyof coal bumps in the island longwall panel is of significant importancein terms of coal mine safety and productivity.

Numerous studies have been conducted in the past to obtain acomprehensive understanding of coal bumps in an island miningpanel. Several aspects of this research are discussed here. Distributionof abutment stresses has always been a major topic in the study ofcoal bumps in an island mining panel. To control the stability of thesurrounding rock mass and the caveability of the top coal in a fully

g).

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mechanized island panel mined by top coal caving method, Huanget al. (2007) and Liu et al. (2007) analyzed the distribution character-istics of abutment stress in the panel. In their studies, the position andmagnitude of abutment stress in the island panel were determinedby numerical modeling with FLAC3D (Fast Lagrangian Analysis ofContinua in 3 Dimensions). Wang et al. (2002) analyzed the peakand declining areas of abutment stress in an island mining panelusing ground radar. Based on in-situ observations of stress in inclinedcoal pillars and the elasto-plastic limit equilibrium theory, the distri-bution laws of the peak value of abutment stress were obtained byXie et al. (2006).

The forecasting of coal bumps in an island mining panel has beenwidely researched over the last 20 years. The coal bumps were ana-lyzed by Dou et al. (2003) and He et al. (2003, 2004) to determine ahazard index for coal bumps and to provide a method to instanta-neously forecast the initial stage of the coal bump using electromag-netic radiation (EMR) and drillings. Liu et al. (2010) characterizedthe EMR precursor time series of the coal bump and provided re-searchers with a means to obtain precursory information for a coalbump in an island mining panel.

Many attempts have been made to control and prevent the occur-rence of coal bumps in underground coal mines. A widely used ap-proach is to conduct an active stress-relief program in the miningcycle (Mohamed, 2003). The aim is to destroy the structural integrity

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Fig. 1. Location of the Tangshan coal mine.

21Y. Jiang et al. / International Journal of Coal Geology 95 (2012) 20–33

of the coal seam where elastic energy is stored. Iannacchione andZelanko (1995) proposed a method of shot firing to fracture thecoal seam to release stress, and Varley (1986) applied auger drillingto release stress in the coal seam by drilling large diameter holes(100 mm in diameter). Infusion of water into coal seams has alsobeen used in many coal mines to control coal bumps (Iannacchioneand Zelanko, 1995; Mohamed, 2003; Varley, 1986).

In underground mining engineering, the island longwall panel is aspecial type of coal pillar. Due primarily to the concentration of stressin the pillar, one of the most challenging engineering problems inisland longwall mining is to design coal pillars that can avoid coalbumps and catastrophic failure at the working face (Shen et al.,2008). Coal pillar design must take the mechanical and physical

Fig. 2. Layout of T2193A i

properties of the coal into account (Wang et al., 2011). In caseswhere coal is mainly extracted using the longwall mining method,empirical data on the relationship between the strength of the coalpillar and the size of the pillar are essential for the design of coal pil-lars. For this purpose, Bieniawski (1968) and Bieniawski et al. (1994)established an empirical relationship between the in situ strength ofcoal, the pillar size and the pillar strength. Furthermore, in a compre-hensive study aimed at estimating the in situ strength and deforma-tion properties of coal pillars for a range of width-to-height ratios,Medhurst and Brown (1998) investigated the effects of sample sizeon strength of coal by performing a series of triaxial compressiontests to provide engineers with a practical and systematic methodfor estimating the mechanical properties of coal seams.

sland mining panel.

Fig. 3. Simplified panel stratigraphy and geotechnical parameters of the seam, roof and floor strata.

22 Y. Jiang et al. / International Journal of Coal Geology 95 (2012) 20–33

The research on coal pillar strength is fundamental to coal pillardesign. Salamon and Munro (1967), Salamon (1970), and Salamonet al. (1998) proposed an empirical model based on pillar width andheight for the calculation of pillar strength in South African coalmines. The approach has found wide application internationally. Inthe 1990s, a database of coal pillars from Australia (Galvin, 2006;Galvin et al., 1999) was analyzed both in isolation and in combinationwith a database from South Africa and an empirical strength formulasimilar to that of Salamon and Munro (1967) was proposed. To inves-tigate the complete load deformation behavior of coal pillars, a setof rectangular and square-shaped coal pillars was tested in situ by

Fig. 4. Position of the observation

Wagner (1974, 1980). In this research, the distribution of stress in acoal pillar was studied and the importance of the core shape of ayielded coal pillar that may remain effective at peak strength washighlighted.

Empirical and theoretical methods have been developed for deter-mining the distribution of abutment stress of an island mining panel.However, there is limited information in the current literature forthe prediction of coal bumps, the determination of the zone wherethe deformation of the roadway roof is most intense, the classificationof the bump-prone zone in an island mining panel and coal pillars,and stress release by boreholes. The studies conducted by Bieniawski

station in the T2193A panel.

image of Fig.�3

image of Fig.�4

Fig. 6. Results of the roof displacement rate against the mining face measured at the630 m and 589 m stations.


(1968) and Salamon andMunro (1967) focused on coal pillar strengthin coal mining, and the dynamicmechanical state of coal pillars duringcoal mining is seldom reported on.

This paper presents a study of the determination of the zones ofthe most intensive roadway roof deformation using field tests and nu-merical investigation. The numerical modeling was designed to locatebump-prone zones in an island mining panel and coal pillar and toverify the results of the field tests. The dynamic mechanical state ofa coal pillar during coal mining, and the stress-relief technology thatinvolves drilling boreholes into in roadway rib are also investigatedin this paper.

2. Island longwall mining panels

The Tangshan coal mine is located in Tangshan City, China (Fig. 1).It is well known for its large number of island longwall mining panels.The Tangshan coal mine district has a length of 14.55 km and width of3.50 km. The areas of mineable zones are 54.60 km2. There are eightmineable coal seams in the Tangshan coal mine. The T2193A panel se-lected for this study is11 m thick and has a dip angle of 9°. As shownin Fig. 2, both sides of this panel were previously extracted, thus mak-ing it a typical island longwall panel where the #9 coal seam wasextracted. Protective coal pillars were left on both sides of the panelto ensure the stability of the panel (the thick red lines shown inFig. 2). The T2193A panel has an overburden depth of 700 m and is860 m long in the strike direction and 124 m wide in the dip direc-tion. The panel is mined using the top coal caving mining method.The height and width of the panel ventilation and haulage roadwayare 4.0 m and 3.0 m respectively. These two roadways employ a sup-port system consisting of anchor-wire-shotcrete combined with asteel arched yielding frame. The length of the anchor is 2.4 m. The im-mediate roof of the panel is composed of sandy mudstone and has anaverage thickness of 2.9 m and the main roof is dark gray sandy mud-stone with an average thickness of 3.1 m. The immediate floor of thepanel is composed of mudstone and is approximately 4.5 m thick. Themain floor is composed of dark gray sandy mudstone and is approxi-mately 2.0 m thick. The panel stratigraphy and other important geo-technical parameters of the seam, roof and floor strata are shown inFig. 3.

3. Assessment of bump-prone zones by field tests

Field tests were conducted in the two roadways and on the miningface of the T2193A. panel to measure the roof displacement and elec-tromagnetic radiation (EMR). A total of 17 monitoring stations were

Fig. 5. Results of the roof displacement against the mining face measured at the 630 mand 589 m stations.

set up to monitor the roof displacement: 10 in the ventilation road-way and 7 in the haulage roadway. Another 20 monitoring stations,which included 10 stations (Nos.1 to 10) in the ventilation roadway

Fig. 7. Bed separations of the ventilation and haulage roadways: (a) ventilation road-way and (b) haulage roadway.

image of Fig.�5

image of Fig.�6

image of Fig.�7


and 10 stations (Nos.11 to 20) in the haulage roadway were set upat 10 m intervals to monitor EMR in the panel. The location of thesestations in the panel is shown in Fig. 4. The aim of the field testswas to determine the zones of intensive roof deformation.

3.1. Results of the roof displacement monitoring

Two key indicators were studied during the panel extraction: thetotal roof displacement and the acceleration/decelerations rate ofthe roof displacement. To identify these two key indicators, the datafrom the monitoring stations located at 630 m in the ventilationroadway (referred as the 630 m station hereafter) and 589 m in thehaulage roadway (referred as the 589 m station hereafter) wereused for analysis because they represent typical roof displacementtests results. The total roof displacement and acceleration rate of theroof displacement at the 630 m station and the 589 m station areshown in Figs. 5 and 6 respectively. Both Figs. 5 and 6 show a pointof acceleration in the roof displacement curve at both stations whenthe mining face is approximately 30 m away from the monitoring sta-tions, and the roof displacement achieves maximumwhen the miningface is within 10 m. When the mining face is displaced beyond 10 m,there is an acceleration in the roof deformation until the roof col-lapses. These results indicate the location of a bump-prone zone atapproximately 30 m in front of the island longwall mining face.

Fig. 8. EMR intensity in the ventilation and haulage roadways against the distance tothe mining face: (a) ventilation roadway and (b) haulage roadway.

3.2. Results of the bed separation monitoring

The monitored data at the 630 m and 589 m stations were usedagain for analysis of the typical bed separation in the roadway roofs.There are two points in the bed separation monitoring borehole:shallow point and deep points (see Fig. 4). Fig. 7 presents the resultsof the ventilation (Fig. 7a) and haulage (Fig. 7b) roadways at thesestations.

From Fig. 7, the maximum bed separation at a shallow point ofthe ventilation roadway is 134 mm, while at a deep point, it is102 mm. The maximum bed separation occurs when the miningface is approximately 10 to 30 m. The bed separation of the haulageroadway is similar: the maximum value is 127 mm at a shallowpoint and 75 mm at a deep point.

3.3. Results of electromagnetic radiation (EMR) tests

As numerous investigations have shown, there is a relationshipbetween the dynamic failure of rock and coal and the electromagneticradiation (EMR) emitted from the failure (Frida and Vozoff, 2005;Lichtenberger, 2005; Liu and He, 2001; Mallik et al., 2008; Songet al., 2010; Voigt et al., 2004; Zhao and Jiang, 2010). Therefore,in this study KBD5 electromagnetic radiation (EMR) sensors were

Fig. 9. EMR average impulse in the ventilation and haulage roadways against thedistance to mining face: (a) ventilation roadway and (b) haulage roadway.

image of Fig.�8

image of Fig.�9

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4 5 6 7 8 9

Plastic strain (%)

Coh

esio

n (M

Pa)

20

22

24

26

28

30

32

Fri

ctio

n A

ngle

(°)

Cohesion (MPa)

Friction Angle (°)

10

Fig. 12. Variation of the cohesion and friction angle with plastic strain.

0

0

100

200

300

400

500

600

700

22

24

26

28

30

32

34

36

38E

MR

inte

nsit

y of

ven

tila

tion

roa

dway

(m

V)

Roo

f di

spla

cem

ent

(mm

)

Distance to mining face (m)102030405060708090100110

Fig. 10. Comparison between the roof displacement and EMR intensity.


installed in the two roadways of the T2193A mining panel to monitorthe failure characteristics of the coal and rock. The location of thesesensors is shown in Fig. 4. These sensors measure the electromagneticradiation (EMR) intensity and impulse within a specified duration.

As the longwall panel retreats, the EMR intensity (mV) and aver-age impulse (Hz) are continuously recorded at the monitoringstations located in both the ventilation and haulage roadways, asshown in Figs. 8 and 9. These figures show that the EMR intensityand impulse reach their maximum value when the mining facepassed the station Nos. 1 and 20. The intensity and impulse detectedat the other stations decreases with the increase of the distance to themining face. Of note, the recorded data are strongly correlated withthe rock and coal fracturing or failure, rather than with the machinenoise. The latter was filtered out using appropriate triggering criteria(Shen et al., 2008).

To apply the EMR intensity and average impulse for the assess-ment of coal bump risks, these data were compared with the resultsof the measured roof displacement. Fig. 10 illustrates the EMR inten-sity at station No.1 and the total roof displacement at the 639 mstation located in the ventilation roadway. It should be noted that sta-tion No.1 for EMR monitoring and the 639 m station for monitoringthe total roof displacement are both located in the ventilation road-way and are only 9 m apart. The results from Fig. 10 suggest that

Fig. 11. Sketch of the FLAC3D m

the EMR intensity starts to increase steadily when the mining face is30 m away from the station No.1 and peaks when the mining face iswithin 10 m of the station No.1. This trend is consistent with that ofthe total roof displacement recorded at the 639 m station.

These field-monitored results indicated that a coal bump is mostlikely to occur in the roadways when the island mining face is within10 m from the longwall face. Therefore, to mitigate the risk of a coalbump, appropriate prevention measures should be undertaken inthe bump-prone zones of the roadways.

4. Stress distribution in the island longwall panel

4.1. Numerical model and simulation scheme

FLAC3D was employed to simulate the stress distribution in theT2193A panel. The numerical model assumes a length of 700 m inthe dip direction, a width of 500 m in the strike direction and a heightof 300 m, as shown in Fig. 11. In this model, the simulated height ofthe coal seam is 3 m. The element size of the coal seam is kept con-stant as 1.0×1.0×1.0 m, and the element size of the roof and flooris selected to be 2.0×2.0×2.0 m in the numerical model. The width

esh for the T2193A panel.

image of Fig.�11


of the coal pillar is 15 m and the roadways are 4 m wide and 3 mthick. To obtain the stress distribution of the panel, reasonableboundary conditions were set for the numerical model. The horizon-tal displacements of the four vertical planes of the model are restrict-ed in the normal direction, and the vertical displacement at the baseof the model is set zero. At the top of the model, a vertical load(p=γH) was applied to simulate the overburden weight. Based on

Fig. 13. The abutment stress of the island mining panel during periodic weighting: (a) theweighting.

extensive data of in-situ stress measurements at the Tangshan coalmine, both stress coefficients along the x and y directions (horizontalplane) are set to be 0.8. To simulate the falling of the roof during thepanel extraction, both the coal seam and the immediate roof areextracted in the process of modeling. During the extraction, the goafarea was filled by a very soft elastic material to approximately simu-late the support capability of the fallen rock from the roof. The

first periodic weighting, (b) the second periodic weighting and (c) the third periodic

Fig. 13 (continued).

Fig. 14. Abutment stress distribution of the panel along the strike.


Young's modulus of this material is set as 190 MPa and the Poissonratio is 0.25 (Cheng et al., 2010).

The elasto-plastic Mohr–Coulomb model with non-associatedflow rules has been chosen for the failure criterion of the coal, roofand floor strata. (Fama et al., 1995; Hoek, 1990; Itasca ConsultingGroup Inc., 2006; Pietruszczak and Mroz, 1980; Singha et al., 2002).The strain-softening model based on the Mohr–Coulomb failuremodel was employed for the coal seam. In the strain-softeningmodel, the cohesion and friction angle can be made to soften afterthe onset of plastic yield by a user-defined piecewise linear function(Jiang et al., 2009; Zhou et al., 2009). In the standard Mohr–Coulombmodel, these properties remain constant. The mechanical parametersapplied in this simulation are shown in Fig. 3. The variation of thecohesion and friction angles with plastic strain is illustrated in Fig. 12.

During the simulation, the procedure below was followed:

Step 1: Calculation of the magnitude of the mining-induced stress,which is formed when the mining face is extracted;

Step 2: Calculation of the peak mining-induced stress when the peri-odic fall occurs;

Step 3: Determination of the bump-prone zones in the panel.

4.2. Abutment stress distribution of the panel

Fig. 13 shows a 3D view of the abutment stress distribution in thevicinity of the panel during periodic weighting. It can be seen fromthe figure that the abutment stress induced by the working longwallface is relatively low near the face and increases quickly in front of theface. The peak abutment stress occurs at the intersection of the min-ing face and roadway. The location of this peak stress region movesforward with the occurrence of every periodic weighting of the im-mediate roof. In the coal pillars, the peak stress occurred at twozones of the pillars along the strike. One zone was at the front ofthe mining face while the other zone was located behind the miningface. Because the focus of this study was the front of the mining face,

the peak-stress zone behind the mining face was ignored in thisstudy. The abutment stress in the pillars decreased gradually with in-crease of the distance from the mining face.

The magnitude of the mining-induced stress, which is formedwhen the mining face is extracted, gradually increased during themining process. The peak mining-induced stress is reached whenthe periodic fall occurs. The abutment stress distribution during peri-odic weighting in the panel along the strike is shown in Fig. 14. It canbe seen from Fig. 14 that the stress is relatively low near the miningface and increases sharply in front of the mining face. These resultssuggest that the abutment stress reaches a peak value at a distance

image of Fig.�14

Fig. 15. Classification of the bump-prone zones in an island mining panel and coal pillar.


of 7.5 m in front of the mining face. The length of influence zone in-duced by mining is approximately 30 m in front of the island longwallface. These results were fairly consistent with the total roof displace-ment observed in the field tests.

4.3. Determination of bump-prone zones in the panel

As analyzed in Section 4.2 of this paper, there is a peak stress zoneat the intersection of the mining face and the roadway. This zone is lo-cated approximately 10 m in front of the mining face in the strike di-rection and 7 to 8 m from the edge of the roadway. Another peakstress zone was also observed in the coal pillar at a distance of 14 to

Fig. 16. Arbitrary poi

20 m in front of the mining face, as shown in Fig. 15. These resultswere in accordance with those of the roof displacement obtained inthe field tests: there are two zones of peak stress in the panel aroundeach roadway during extraction of the longwall face, and these zonesare classified as bump-prone zones.

5. Dynamic mechanical state of the coal pillar

To investigate the dynamic mechanical state of coal pillars in is-land longwall mining, an arbitrary point in the coal pillar was selectedto analyze the vertical stress and plastic state, as shown in Fig. 16.

nt in coal pillar.

image of Fig.�16

image of Fig.�15


In this study, the coal pillar was 5 to 30 m wide and 3 m thick. Thevertical stress and plastic state of the arbitrary point during the ad-vance of coal mining are presented in Fig. 17. The results in this figureshow that the maximum in the vertical stress increases with theincrease of the coal pillar width. The larger the coal pillar width, thegreater the width of the elastic core. Based on these results, a coal pil-lar with a large width could form a greater elastic core and with theadvance of coal mining, the vertical stress is expected to be large inmagnitude. Because of this high stress and its associated stored elasticenergy, the risk of coal bumps in a coal pillar with a large width isgreater than that with a small width.

6. Numerical investigation of stress relief by boreholes

6.1. Determination of the numerical scheme

To mitigate or eliminate bump risk during island longwall mining,a number of simulated boreholes were drilled in the bump-prone

Fig. 17. Vertical stress and plastic state during the fourth periodic weighting: (a) 5 m wide c(e) 25 m wide coal pillar and (f) 30 m wide coal pillar.

zones, (Fig. 18). A literature review of stress-relief methods withboreholes has shown that the boreholes can be used to reduce thepeak stress concentration in front of mining faces (Dou et al., 2003;He et al., 2003, 2004). However, sufficient amounts of numerical sim-ulation have not been conducted on the effect of borehole length andspacing on stress-relief. Therefore, in this study, boreholes withlengths varying from 10 to 25 m and borehole spacing varying from1 to 3 mwere simulated. The simulation scheme and borehole config-uration are listed in Table 1. The boreholes are drilled within 30 m ofthe mining face because the influence area induced by coal excava-tion was approximately 30 m in front of the mining face. In schemesNos. 1 and 2, the variables modeled were the borehole length andspacing.

6.2. Effect of the borehole length on stress relief

The effect of various borehole lengths for stress relief is presentedin Fig. 19. The modeled results show that the abutment stress could

oal pillar, (b) 10 m wide coal pillar, (c) 15 m wide coal pillar, (d) 20 m wide coal pillar,

image of Fig.�17



be released by boreholes of various lengths. As shown in Fig. 19, theabutment stress after the relief is 46.2 MPa at a distance of 7.5 m infront of the mining face. The peak stress moved forward 24 m infront of the mining face when the boreholes were placed at the ribof the roadway. Table 2 lists the abutment stress and distance to themining face before and after placing the stress-relief boreholes. Themodeled results predicted that the effectiveness of stress relief in-creased with borehole length. For example, when the borehole lengthwas 10 m the stress reduction was 28.9 MPa, while the released stresswas 61.9 MPa when the length was 25 m.

6.3. Effect of the borehole spacing on stress relief

The variable in this scheme was the borehole spacing, which var-ied from 1 to 3 m. The effect of borehole spacing on stress relief is pre-sented in Fig. 20. The modeled results suggested that the abutment

stress could be released by boreholes of various spacing. It was alsoobserved that the abutment stress after the stress relief was46.2 MPa and that it occurred at a distance of 7.5 m in front of themining face. The peak stress moved forward 24 m in front of the min-ing face when the boreholes were placed at the rib of the roadway.Table 3 lists the abutment stress and distance to the mining face be-fore and after placing stress-relief boreholes. The results also showthat stress reduction will decrease with an increase in borehole spac-ing. For example, when the spacing was 1 m, the released stress was34.7 MPa, while the stress reduction was 25.1 MPa when the spacingwas 3 m.

7. Conclusions

Field tests and numerical investigations were undertaken in theT2193A island longwall panel of the Tangshan coal mine to assess



the risk of coal bumps and to use boreholes as a stress-relief technol-ogy to mitigate the risk. The main conclusions drawn from the inves-tigations are summarized below:

(1) The results of roof displacement, bed separation and electro-magnetic radiation (EMR) measurements showed that therewas an acceleration point in the roof displacement curvewhen the mining face was approximately 30 m away and theroof displacement reached its peak value when the miningface was within 10 m. When the mining face passed this10 m point, the roof deformation accelerated until the roof col-lapsed. This implied that coal bump was most likely to occur inthe panel roadway approximately 30 m in front of the miningface.

(2) The model results predicted that the abutment stress in thepanel was relatively low near the mining face and increasedsharply in front of the mining face. The peak abutment stressoccurred near the interaction between the mining face androadways at a distance of 7.5 m to the mining face. The peakstress zones moved forward with the occurrence of eachperiodic weighting event of the immediate roof. The region ofinfluence induced by the panel extraction was located approx-imately 30 m in front of the mining face. These results wereconsistent with those observed in the field measurements.

(3) Analyses of field measurement data and numerical modelingrevealed that there were two bump-prone zones around eachpanel roadway during panel extraction. One zone was within7 to 10 m ahead of the mining face and near the edge of the

Fig. 18. Numerical configuration of boreholes.

Table 1Numerical scheme of the borehole configuration.

Scheme Scheme no. 1 Scheme no. 2

Interval (m) Length (m) Interval (m) Length (m)

Parameter of boreholes 1 10 1 151 15 2 151 20 3 151 25 – –

Table 2Abutment stress for different lengths of boreholes.

Length(m)

Abutmentstress beforestress relief(MPa)

Abutmentstress afterstress relief(MPa)

Stressreduction(MPa)

Distance tomining facebefore stressrelief (m)

Distance tomining faceafter stressrelief (m)

10 46.2 45.6 0.6 7.5 31.515 46.2 40.9 5.3 7.5 31.520 46.2 34.7 11.5 7.5 31.525 46.2 28.9 17.3 7.5 31.5

Fig. 20. Effect of the borehole spacing on stress relief.Fig. 19. Effect of the borehole length on stress relief.


image of Fig.�18

image of Fig.�20

image of Fig.�19

Table 3Abutment stress for different spacings of boreholes.

Interval(m)

Abutmentstress beforestress relief(MPa)

Abutmentstress afterstress relief(MPa)

Stressreduction(MPa)

Distance tomining facebefore stressrelief (m)

Distance tomining faceafter stressrelief (m)

1 46.2 34.7 11.5 7.5 31.52 46.2 28.0 18.2 7.5 31.53 46.2 25.1 21.1 7.5 31.5


roadway, and the other zone was in the coal pillars at a similardistance.

(4) The model results suggested that the abutment stress could bereleased by boreholes drilled from the rib of the panel road-ways parallel with the mining face. The effectiveness of thestress relief will increase with the increase of the boreholelength and will decrease with an increase in borehole spacing.

Acknowledgements

This research is financially supported by the Major State BasicResearch Development Program Fund (No. 2010CB226801), NationalNatural Science Foundation of China (No. 51174213), New CenturyExcellent Talents in Ministry of Education Support Program of China(No. NCET-10-0775), and Fundamental Research Funds for the Cen-tral Universities. The authors would like to thank Prof. Shen Baotangfrom CSIRO Earth Science and Resource Engineering for his valuablecontribution during this study.

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Documents

Assessment and mitigation of coal bump risk during extraction of an island longwall panel