experimental study of coal and gas outbursts related to...

ORIGINAL PAPER

Experimental Study of Coal and Gas Outbursts Relatedto Gas-Enriched Areas

Qingyi Tu1,2,3 • Yuanping Cheng1,2,3 • Pinkun Guo4 • Jingyu Jiang1,2,3 •

Liang Wang1,2,3 • Rong Zhang1,2,3

Received: 8 June 2015 / Accepted: 12 April 2016 / Published online: 30 April 2016

� Springer-Verlag Wien 2016

Abstract A coal and gas outburst can lead to a catas-

trophic failure in a coal mine. These outbursts usually

occur where the distribution of coal seam gas is abnormal,

commonly in tectonic belts. To study the effects of the

abnormal distribution of this gas on outbursts, an exper-

imental apparatus to collect data on simulated coal seam

outbursts was constructed. Experiments on specimens

containing discrete gas-enriched areas were run to induce

artificial gas outbursts and further study of these outbursts

using data from the experiment was conducted. The

results suggest that more gas and outburst energy are

contained in gas-enriched areas and this permits these

areas to cause an outburst easily, even though the gas

pressure in them is lower. During mining, the disappear-

ance of the sealing effect of a coal pillar establishes the

occurrence conditions for an outburst. When the enriched

gas and outburst energy in the gas-enriched area is

released suddenly, a reverse unloading wave and a high

gas pressure gradient are formed, which have tension

effects on the coal. Under these effects, the fragmentation

degree of the coal intensifies and the intensity of the

outburst increases. Because a high gas pressure gradient is

maintained near the exposed surface and the enriched

energy release reduces the coal strength, the existence of a

gas-enriched area in coal leads to a faster outburst and the

average thickness of the spall is smaller than where is no

gas-enriched area.

Keywords Coal and gas outburst � Gas-enriched area �Outburst energy � Experiment simulation � Energy analysis

1 Introduction

As a type of non-renewable energy, coal plays an important

role in the global energy structure. With the mining depths

have increased, a variety of disasters and accidents have

become more serious, which greatly influence the safety of

coal mining. Among these disasters, coal and gas outburst

(hereinafter referred to as outburst) has attracted worldwide

attention regarding mine safety (Lama and Bodziony

1998). An outburst is an unstable releasing process of gas

energy and strain energy in a coal seam, emitting a large

amount of coal (rock) and gas into a production space in a

short amount of time (Chen et al. 2013). Numerous factors

influence outbursts, and the causes are complex. Currently,

the mechanisms of outbursts under various geological and

mining conditions have not been fully understood, and

most of the perspectives are empirical or semi-empirical

hypotheses (Choi and Wold 2001; Singh 1984; Wold et al.

2008).

A number of studies on gas outbursts in coal mines have

been done and most have assumed that the distribution of

gas in a coal seam is uniform. However, coal seam gas is a

gaseous geological body. The effect of geological and

tectonic evolution is to change the stress environment and

& Yuanping [email protected]

1 Key Laboratory of Coal Methane and Fire Control, Ministry

of Education, China University of Mining and Technology,

Xuzhou 221116, China

2 Faculty of Safety Engineering, China University of Mining

and Technology, Xuzhou 221116, China

3 National Engineering Research Center for Coal and Gas

Control, China University of Mining and Technology,

Xuzhou 221116, Jiangsu, China

4 State Key Laboratory of Coal Mine Disaster Dynamics and

Control, Chongqing University, Chongqing 400030, China

123

Rock Mech Rock Eng (2016) 49:3769–3781

DOI 10.1007/s00603-016-0980-6

http://crossmark.crossref.org/dialog/?doi=10.1007/s00603-016-0980-6&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1007/s00603-016-0980-6&domain=pdf

the crack and pore systems in the coal seam (Cai et al.

2011; Jiang et al. 2011; Li et al. 2003, 2014). Thus, geo-

logical processes are vital to the storage of coal seam gas

and this causes the gas to be unevenly distributed (Li et al.

2011). This gas is abnormally enriched in some portions of

the coal seams (Groshong et al. 2009; Pashin 1998; Yao

et al. 2009). The recent gas outbursts in China indicate that,

in most cases, outburst accidents occur in tectonic belts.

Gas storage is commonly abnormal in these areas (An and

Cheng 2013).

Because outbursts are unpredictable and very dangerous,

it is difficult to observe and study outbursts directly (Hu

et al. 2015). By simplifying conditions, a laboratory sim-

ulation can largely reconstruct the failure of coal and the

gas emissions during an outburst (Li et al. 2009; Peng et al.

2012; Yin et al. 2009). A simulation can be also used to

study the effect of different factors on outbursts qualita-

tively and the relationships among each factors (Cai 2004;

Skoczylas 2012). Additionally, using energy analysis,

outburst occurrence conditions and the relationship

between the primary and secondary inducing factors at

different stages of an outburst can be quantitatively

explained (Wang et al. 2011; Yu et al. 2015).

As above analysis, a further study of coal and gas out-

bursts related to gas abnormally enriched has been lacking.

For this study, an experimental model containing discrete

gas-enriched areas (uneven distribution of coal seam gas)

to produce outbursts was constructed. The tests examined

the gas storage capacity and mechanical properties of

outburst materials. The outburst occurrence conditions

were then presented based on the energy analysis, the

formation mechanisms for outburst phenomena were

explained and the influence of gas-enriched areas on an

outburst was studied.

2 Experiments

2.1 Materials and Specimen Preparation

Based on the literature (Cheng et al. 2010; Lu et al. 2015a;

Ning et al. 2012; Prasetyo and Do 1998), it is known that

both activated carbon and coal adsorb gas by physical

adsorption. However, with a large specific surface area and

abundant pores of activated carbon, the gas storage

capacity of it is higher than that of coal (Lozano Castello

et al. 2002; Matranga et al. 1992). Therefore, activated

carbon was used as the analog material for the gas-enriched

area (GEA) and coal was used as the material for the

normal area (NA).

Powdered wood activated carbon prepared by the Xintai

Water Purification Material Company in Henan Province,

China, was adopted in the experiment. The coal was

collected from the No. 10 coal seam at the Wolonghu Coal

Mine in Anhui Province, China. Prior to the outburst

simulation experiments, a proximate analysis, a 25 �Cisothermal adsorption test, a pressured-mercury test, and a

triaxial compression test were conducted. The proximate

analysis, the adsorption constants a and b, the porosity u,apparent density q, and the particle size of materials areshown in Table 1 (The material properties listed in Table 1

are the properties of the molded coal specimen).

The outburst experiment specimens used molded coal

with physical properties similar to tectonic coal (Lu et al.

2015b; Skoczylas et al. 2014). The specimens were molded

in an outburst simulation experiment chamber. For these

specimens, six percent water was added to the materials

before molding. The molding pressure was 64 MPa and

was applied for 40 min. The final specimen size was

0.25 m 9 0.25 m 9 0.25 m, as shown in Fig. 1.

2.2 Experimental Equipment

Coal and gas outbursts were simulated in a triaxial coal and

gas outburst simulation system (Guo 2014) that was com-

posed of the outburst simulation experimental chamber, a

loading system, a temperature control system, a gas

injection/vacuum pumping system, and a data collection

system. The simulation system is shown in Fig. 2. The

axial direction of the outburst port is defined as the x axis,

the lateral direction as the y axis, and the vertical direction

as the z axis (Fig. 2). Using the loading system, the spec-

imen can be loaded rx, ry, and rz independently. By beingtransmitted through a rigid bearing plate, the concentrated

load is transformed into a uniformly distributed load before

loading into the specimen. Diagrams of the loading system

and the outburst simulation experimental chamber are

shown in Fig. 3. The main performance parameters for the

system are: (1) rz B 80 MPa, rx and ry B 27 MPa; (2) gaspressure B10 MPa; and (3) temperature B60 �C.

2.3 Experimental Procedure

Nine outburst simulation experiments were conducted

under different experimental conditions. Experiments #1–

#5 were conducted with no GEA, Experiments #6–#9 had a

GEA as described below. According to the results of

Experiments #1–#5, a 0.02-m-wide coal pillar was main-

tained between the GEA and the front wall of the experi-

mental chamber for Experiments #6–#9. The GEA had

dimensions of 0.125 m 9 0.115 m 9 0.2 m, as shown in

Fig. 1b. The GEA was filled with activated carbon and the

NA was filled with coal before the specimen was molded.

Field monitoring data of coal pillar loading suggest that

average confining pressure in a typical underground coal

3770 Q. Tu et al.

123

pillar is approximately 4 MPa and that the confining

pressure typically ranges from 0 to 10 MPa (Medhurst and

Brown 1998). Therefore, the triaxial loading pressures for

rx, ry, and rz were 5.5, 5.5, and 4.8 MPa, respectively. Thegas pressures were in the range of 0.25–0.5 MPa. For

safety, CO2 was used rather than CH4 in all the experi-

ments. The outburst experiments were conducted at

atmospheric pressure, and the experimental scheme is lis-

ted in Table 2.

Before testing the specimens, each was outgassed into a

vacuum for 48 h. Through air intake pipe (Fig. 2), CO2was injected to reach the desired gas pressure and this

pressure was maintained for no less than 48 h to ensure that

the specimen was saturated with gas. Then, the specimen

Table 1 Basic parameters of the experimental materials

Materials Particle size (mm) Density q (kg/m3) u (%) a(m3/t)

b

(/MPa)

Proximate analysis (%)

Mad Aad Vad

Coal \0.25 1.40 9 103 25.61 58.78 1.44 9.61 25.12 7.36Activated carbon \0.25 1.06 9 103 40.39 124.75 2.94 10.56 13.42 15.54

Fig. 1 Experiment specimens.a Experiments #1–#5.b Experiments #6–#9

Fig. 2 Structure diagram of the triaxial coal and gas outburstsimulation experimental system. 1 outburst simulation experimental

chamber; 2 hydraulic jack; 3 axial loading testing machine; 4

horizontal loading pumping station; 5 temperature control system; 6

gas cylinder; 7 triple valve; 8 vacuum pump; 9 control console; 10

data acquisition instrument; 11 testing machine pumping station; 12

outburst port; 13 measuring hole; 14 air intake pipe; 15 upper cover

plate; 16 piston; 17 bearing plate

Experimental Study of Coal and Gas Outbursts Related to Gas-Enriched Areas 3771

123

was loaded rx, ry, and rz by the loading system. Afterequilibration, the outburst port was opened, the specimens

were tested and the results were recorded. After testing,

polyurethane molds were made from the outburst holes.

3 Results and Analyses

3.1 Gas Storage and Mechanical Properties

of the Material

3.1.1 Gas Storage Capacity

The gas stored in the materials can be divided into free gas

and adsorbed gas. The total gas content of a unit volume

material (q) can be calculated from the following equations

(Wang et al. 2002; Yu et al. 2014; Zhang 2008):

q ¼ q1 þ q2; ð1Þ

q1 ¼PuT0P0Tn

; ð2Þ

q2 ¼abPq1þ bP e

nðT1�TÞ 1� Aad �Mad1þ 0:31Mad

� 10�3; ð3Þ

where q1 and q2 are the content of free gas and adsorbed

gas (m3/m3), respectively; P and P0 are the gas pressure of

the coal seam and working face (MPa); T and T0 are the

temperature of the gas before and after the outburst (K); T1is the temperature of the isothermal adsorption test (K); nand n are coefficients that depend on the gas pressure of the

coal seam.

Because the outburst experiments were conducted at

atmospheric pressure, the following values are assumed:

P0 = 0.1 MPa, T1 = T = T0 = 298.15 K, n = 1, and

Fig. 3 Loading system and outburst simulation experimental chamber

Table 2 Experimental scheme of coal and gas outburst

Experiment number Gas pressure (MPa) Ambient temperature (�C) Triaxial stress Existence ofGEA (Y/N)

rz (MPa) rx (MPa) ry (MPa)

#1 0.30 25 4.8 5.5 5.5 N

#2 0.35 25 4.8 5.5 5.5 N

#3 0.40 25 4.8 5.5 5.5 N

#4 0.45 25 4.8 5.5 5.5 N

#5 0.50 25 4.8 5.5 5.5 N

#6 0.25 25 4.8 5.5 5.5 Y

#7 0.30 25 4.8 5.5 5.5 Y

#8 0.35 25 4.8 5.5 5.5 Y

#9 0.40 25 4.8 5.5 5.5 Y

3772 Q. Tu et al.

123

n = 1. According to the parameters in Table 1 and using

Eqs. (1), (2), and (3), the gas contents q, q1, and q2 have

been calculated and are shown in Table 3.

As shown in Table 3, the gas content of the materials

increases with increased gas pressure, but there is a sig-

nificant difference between the two materials. The acti-

vated carbon in the GEA has a greater storage capacity than

the coal in the NA. At the same gas pressure, the total gas

content (q) in the GEA is 2.70–2.92 times that of NA, the

free gas content (q1) is 1.58 times that of NA and the

adsorbed gas content (q2) is 2.76–3.06 times higher.

3.1.2 Mechanical Properties

The axial stress–strain curves for the outburst materials from

the triaxial compression tests at different confining pressures

are shown in Fig. 4. The mechanical parameters are listed in

Table 4. The bond forces (c) and internal friction angles (h)were obtained by constructing Mohr’s stress circles and

Coulomb’s strength curves (Liu et al. 2014).

The way in which the outburst materials react to the

stress–strain curves can be divided into three stages: the

elastic stage, the elastic–plastic stage, and the failure stage

after peak stress. As shown in Table 4, the peak strength

(r1;p) and the elastic modulus (E) increase with increasingconfining pressure (r3). The strength parameters of the twomaterials (r1;p, c, and h) are not significantly different.However, the elastic modulus of the GEA material is lower

and the Poisson’s ratio is greater than these parameters for

the NA material, indicating that deformation of the GEA

material is more obvious.

3.2 Experimental Results

3.2.1 Outburst Intensity

According to the experimental scheme, nine outburst

simulation experiments were conducted and the results are

shown in Table 5. Based on the dynamic phenomenon of

outbursts, the results can be divided into three classes:

outburst, stripping, and no outburst. There is a gas pressure

threshold for an outburst (Wang et al. 2010). The threshold

value is 0.30–0.35 MPa for the experiments with a GEA,

lower than the threshold for the experiments with no GEA

(0.35–0.40 MPa). When the gas pressure exceeds the

threshold, the outburst occurs and the outburst intensity

(explained below) is greater with higher gas pressures.

Experiment #3 is compared with Experiment #9 (Table 5),

and the experiments with a GEA have greater intensity than

those with no GEA, even though the gas pressure are same.

3.2.2 Outburst Hole Characteristics

Outburst holes are shown in Fig. 5. When an outburst does

not occur (Fig. 5a), only the coal near the exposed surface

of the specimen is stripped and no outburst hole is evident.

When an outburst occurs (Fig. 5b–f), there is an obvious

outburst hole and its edges are spherical. It is found that

Table 3 Gas storage capacityof the outburst materials

Materials Gas pressure (MPa) q1 (m3/m3) q2 (m

3/m3) q (m3/m3)

Coal 0.25 0.64 13.81 14.45

0.30 0.77 15.73 16.50

0.35 0.90 17.47 18.37

0.40 1.02 19.06 20.08

0.45 1.15 20.51 21.66

0.50 1.28 21.83 23.11

Activated carbon 0.25 1.01 42.24 42.25

0.30 1.21 45.62 46.83

0.35 1.41 49.36 50.78

0.40 1.62 52.61 54.22

Fig. 4 Stress-axial strain curves of the outburst materials underdifferent confining pressures


123

some smaller fragments which have not been thrown out by

the escaping gas remain inside the hole.

The holes from outbursts produced under different

experimental conditions had different shapes. In experi-

ments #3 through #5 (Fig. 5b–d), the hole extended con-

tinuously from the initial exposed surface to the internal

and the coal that was in the front of hole was completely

expelled. In experiments #8 and #9 (Fig. 5e, f), the residual

coal was found in front of the hole. The hole from exper-

iment #8 was extended in the range of GEA, whereas the

hole from experiment #9 was greatly expanded and pene-

trated into the coal adjacent to the GEA.

Molds of the outburst holes were made using poly-

urethane (Fig. 6). The hole shapes were varied. In Exper-

iment #4, the hole was cone shaped (Fig. 6a). In

Experiment #8, the hole was cylindrical (Fig. 6b). In

experiment #9, the bottom of the hole was approximately

spherical (Fig. 6c).

Figure 7 shows the relationship between the volume of

the outburst holes and gas pressure. V0 represents the actual

volume of the hole as measured by determining the volume

of the polyurethane mold (by immersing the mold in

water). V1 represents the calculated volume of the hole

determined by the density of the molded coal and the

outburst intensity. It is clear that the volume of the holes

increases with increased gas pressure, and V0 is always less

than V1. As expected, the volume of the outburst holes for

the experiment with a GEA is larger than that of the

experiments without a GEA. It should be noted that the

difference in volumes between V0 and V1 is a real differ-

ence and, as will be explained in Sect. 4.2, is a result of the

hole formation mechanisms.

3.2.3 Spall Characteristics

After an outburst, the upper surface of the specimen was

cleaned so that the spall from the outbursts could be exam-

ined (Fig. 8). When an outburst does not occur, the coal is

almost intact (Fig. 8a). However, when an outburst occurs

(Fig. 8b–d), there are a large number of arcuate cracks in the

coal behind the outburst hole and arcuate cracks parallel to

the edges of the hole. The coal is broken into spherical shells

plates with a specific thickness (As shown in Table 6).

Additionally, short cracks are produced in the coal that could

cut the coal plates into smaller fragments. These short cracks

are widely distributed near the outburst hole. The fragmen-

tation degree of the coal decreases away from the outburst

hole. In contrast to experiment #2 (Fig. 8a), although the

outburst hole from experiment #8 is located in the range of

the GEA, it is found that the outburst of the coal in GEA has

an impact on a larger area including the coal adjacent to the

GEA and spall appears there as well, as shown in Fig. 8d.

Table 4 Mechanicalparameters and strength of the

outburst materials

Materials r3 (MPa) E (MPa) Poisson’s ratio (l) r1;p (MPa) c (MPa) h (�)

Coal 0 91.4 0.193 1.09 1.83 25.01

2 312.4 0.107 10.75

5.5 670.4 0.192 19.44

Activated carbon 0 86.5 0.455 0.785 1.73 25

2 166.8 0.128 10.37

5.5 331.0 0.248 18.91

Table 5 Results of the coal and gas outburst simulation experiments

Experiment

number

Results of

outburst

The mass

of filled coal (kg)

Absolute intensity

of outburst (kg)

Relative intensity

of outburst (%)

The distance

of outburst (m)

#1 Stripping 20.967 0.009 0.043 0.10

#2 Stripping 21.078 0.026 0.12 0.21

#3 Outburst 21.198 4.126 19.46 14.47

#4 Outburst 21.039 5.295 25.17 16.98

#5 Outburst 20.920 6.366 30.43 18.14

#6 No outburst 20.695 – – –

#7 Stripping 20.725 0.014 0.07 0.13

#8 Outburst 20.817 4.375 21.02 15.32

#9 Outburst 20.760 4.742 22.84 16.65

Annotation: absolute intensity of outburst is equivalent to the mass of outburst in this paper

3774 Q. Tu et al.

123

3.2.4 Outbursts Propagation

Table 6 lists the outburst propagation parameters, among

whichL0 represents themaximum length of the outburst hole in

the x direction; t represents the total time of an outburst and is

obtained by recording beginning time/ending time of an

outburst; L1 represents the width of the spall area; n represents

the number of arc cracks; L0=t is defined as the propagation

velocity of the outburst (v) and L1=n is defined as the average

thickness of the spall (d). Figure 9 shows the relationship

between the propagation velocity of the outburst (v), the aver-

age thickness of the spall (d), and the gas pressure (P).

Fig. 5 Outburst hole characteristics. a Experiment #2. b Experiment #3. c Experiment #4. d Experiment #5. e Experiment #8. f Experiment #9


123

The propagation velocity of the outburst is higher and

the average thickness of the spall is smaller with higher gas

pressures. At the same gas pressure, the propagation

velocity of the outburst is faster and the average thickness

of the spall is smaller for the experiments containing dis-

crete GEA than for the experiments that had no GEA.

4 Discussion

4.1 Occurrence Conditions for Outbursts

For an outburst to occur, three conditions are required: an

initial condition of instability, a continuous failure condi-

tion, and an energy condition. The energy is required for

the entire process of an outburst, and it can be described as

a necessary condition for the outburst. Before an outburst,

the outburst energy of the coal-contained gas includes gas

energy and strain energy, and they can be expressed as (An

2014):

W ¼ W1 þ Es; ð4Þ

W1 ¼1

2Er2x þ r2y þ r2z � 2l rxry þ ryrz þ rxrz

� �h i; ð5Þ

Es ¼P0q

cn � 1P

P0

� �cn�1cn

�1" #

; ð6Þ

where cn is the process index, cn = 1.30 for CO2; W , W1,and Es are the outburst energy, the strain energy, and the

gas energy of a unit volume coal (MJ/m3), respectively.

It is difficult to measure the volume of gas involved in

the outburst. In addition to the free gas, adsorbed gas also

plays an important role in the outburst (Valliappan and

Zhang 1999). In this study, the gas energy is also thought to

include the free gas energy and the adsorbed gas energy.

Based on that assumption and using Eqs. (4), (5), and (6),

the various types of energy are calculated, as shown in

Table 7.

Table 7 shows that the outburst energy increases as the

gas pressure increases. For the same gas pressure, the

outburst energy from a unit volume of coal in the GEA is

greater than that for a unit volume coal in the NA; the ratio

is 2.68–2.88. The outburst energy distribution for the dif-

ferent experiments is determined and is shown in Fig. 10.

The outburst energy dissipation represents the minimum

energy required for an outburst. According to the

mechanical properties of the outburst materials

Fig. 6 Outburst hole models. a Experiment #4. b Experiment #8. c Experiment #9

Fig. 7 Relationship between the volume of the outburst holes and gaspressure

3776 Q. Tu et al.

123

(Sect. 3.1.2), the value of the outburst energy dissipation

for coal in the GEA should be slightly lower than that for

the coal in the NA. Nevertheless, these values are assumed

to be the same for this study.

For the experiments with no GEA, the distribution of the

outburst energy in the coal is uniform as shown in Fig. 10a.

The necessary condition for the outburst is that the outburst

energy be sufficient to overcome the outburst energy dis-

sipation. Once that condition is met, the outburst energy in

the coal near the initial exposed surface is released first and

the outburst develops continuously from that surface. If the

remaining outburst energy cannot overcome the outburst

Fig. 8 Spall characteristics of the coal after an outburst. a Experiment #2. b Experiment #3. c Experiment #4. d Experiment #8

Table 6 Parameters regardingthe outbursts propagation

Experiment number L0 (m) t (s) v (m/s) L1 (m) n d (m)

#1 0.012 – – – – –

#2 0.023 – – – – –

#3 0.109 1.477 0.074 0.141 11 0.0128

#4 0.132 1.65 0.080 0.118 10 0.0118

#5 0.167 1.69 0.099 0.083 9 0.0092

#6 – – – – – –

#7 0.016 – – – – –

#8 0.139 2.104 0.066 0.100 9 0.0111

#9 0.155 1.808 0.086 0.095 9 0.0106


123

energy dissipation, the outburst stops. The intensity of the

outburst is greater and the range of the outburst hole

extends if there is more outburst energy in the coal.

For experiments with a GEA, the outburst energy is no

longer uniformly distributed in the coal and more energy is

enriched in the GEA. Even though the gas pressure is

lower, the outburst energy in the GEA can be higher than

the outburst energy dissipation, as shown in Fig. 10b. The

0.02-m-wide coal pillar in the experiments acts as an anti-

deformation and low outburst energy layer and is equiva-

lent to a low-permeability zone (An and Cheng 2013; Lu

et al. 2016). It has a sealing effect on the enriched gas and

outburst energy in the GEA. The disappearance of the

sealing effect establishes the conditions for the outburst.

Once the condition is satisfied, the outburst energy in the

GEA is released quickly and the outburst from the coal in

the GEA occurs immediately. When the outburst reaches

the perimeter of the GEA, the energy condition of sur-

rounding coal changes, if the remaining outburst energy

cannot overcome the outburst energy dissipation, then the

outburst stops at the perimeter of the GEA. Otherwise, the

outburst continues to develop in the coal around the GEA.

4.2 Formation Mechanisms for Outburst

Phenomena

As the coal-contained gas is suddenly unloaded, a gas

pressure gradient appears near the exposed surface and has

a tensile effect on the coal. The coal near the exposed

surface is under tension and a crack appears in the coal. A

large amount of adsorbed gas desorbs and quickly enters

the crack. A high gas pressure is maintained in the crack,

and the crack extends along a direction perpendicular to the

maximum gas pressure gradient. Finally, the crack devel-

ops into an arcuate crack that cuts the coal into a spherical

shell plate with a specific thickness (Jin et al. 2009).

After the coal which is exposed earlier experiences an

unstable failure and the gas is emitted. Newly coal is

exposed, and a new gas pressure gradient appears near the

newly exposed surface. The unstable failure of coal then is

transferred progressively to the internal coal. During the

outburst, the coal that has failed is driven by the gas and

moves forward. The coal is loosened, leading to shrinkage of

the outburst hole. Therefore, the actual volume of the out-

burst hole (V0) is always less than the calculated volume (V1).

With the gas emissions and the energy releasing, the gas

pressure gradient and the outburst energy decrease con-

tinuously during the unstable failure of coal. Meanwhile,

the strength of the coal increases with gas desorption (Ates

and Barron 1988). For these reasons, the degree of frag-

mentation of the coal decreases as the failure proceeds

(from crush to spall) as shown in Fig. 8.

4.3 Influence of a Gas-Enriched Area

on an Outburst

Under the condition of uneven distribution of coal seam

gas, more gas and outburst energy are presented in a GEA

and an outburst is easily initiated even though the gas

pressure in the GEA is lower. However, the coal pillar in

front of the GEA is equivalent to a low-permeability zone

Fig. 9 Relationship between the propagation velocity of the outburst,the average thickness of the spall, and the gas pressure

Table 7 The outburst energy of a unit volume coal

Experiment number W11 (MJ/m3) W21 (MJ/m

3) E1s (MJ/m3) E2s (MJ/m

3) W1 (MJ/m3) W2 (MJ/m3)

#1 – 0.039 – 1.587 – 1.626

#2 – 0.039 – 2.053 – 2.092

#3 – 0.039 – 2.524 – 2.563

#4 – 0.039 – 2.996 – 3.035

#5 – 0.039 – 3.465 – 3.504

#6 0.064 0.039 3.316 1.134 3.380 1.173

#7 0.064 0.039 4.504 1.587 4.568 1.626

#8 0.064 0.039 5.674 2.053 5.738 2.092

#9 0.064 0.039 6.814 2.524 6.878 2.563

Annotation: superscript ‘‘1’’ represents the energy of a unit volume coal in GEA; Superscript ‘‘2’’ represents the energy of a unit volume coal in

NA

3778 Q. Tu et al.

123

and this has a sealing effect on the enriched gas and out-

burst energy in the GEA. According to the experimental

results (Table 6), when the gas pressure is less than

0.35 MPa, L0 B 0.016 m. The 0.02-m-wide coal pillar

cannot be penetrated, and still has a sealing effect. The

enriched gas and outburst energy in the GEA cannot be

released quickly, thus the outburst does not occur. When

the gas pressure reaches 0.35 MPa, then L0 = 0.023 m.

The coal pillar is penetrated, and the sealing effect disap-

pears. The enriched gas and outburst energy in the GEA are

released quickly leading to the outburst from the coal in the

GEA occurs immediately.

The GEA is surrounded by normal coal, the enriched gas

and energy in GEA cannot be released because of the

sealing effect. Before the GEA is exposed, this area is

similar to a gas cylinder with high gas pressure and energy

(Guo 2012). As the coal pillar in front of the GEA is

penetrated, the GEA is suddenly exposed. The enriched gas

and energy in the GEA are released wildly and the outburst

is like a physical explosion. The high gas pressure gradient

causes a serious failure of the coal, firstly. Additionally, a

strong shock wave forms with the release of the outburst

energy and the shock wave reflects and changes into a

reverse unloading wave (Zhou et al. 2015). The reverse

unloading wave also has tension effects on the coal and

reduces the strength of the coal (Wu and Jiang 2011).

Under the dual effects of the gas pressure gradient and the

reverse unloading wave, the fragmentation degree of the

coal intensifies and the intensity of the outburst increases.

The coal around the GEA spalls at a low gas pressure, as

shown in Fig. 8d.

The propagation velocity of the outburst and the spall

thickness of the coal are affected by the gas pressure gra-

dient and the coal strength (Fang et al. 1995). The gas is in

a state of dynamic change nearby the exposed surface. Free

gas emissions cause a reduction in gas pressure but the

quick desorption of the adsorbed gas retards the reduction.

Compared with the NA, the amount of free gas and

adsorbed gas are greater in the GEA. This augments the

supply of gas and also retards the reduction in gas pressure.

Therefore, a high gas pressure gradient is maintained near

the exposed surface in the GEA. Additionally, the high-

pressure energy releasing from the GEA reduces the coal

strength. For the same gas pressure, the existence of a GEA

in coal leads to a faster outburst and the average thickness

of the spall is smaller than if there were no GEA.

5 Conclusions

A coal and gas outburst is one of the major disasters that

can occur during coal mining. They commonly occur in

tectonic belts where the gas storage is abnormal. With a

detailed study of coal and gas outburst related to gas-en-

riched areas (uneven distribution of coal seam gas), this

paper concludes the following:

For the effect of gas pressure gradient, the coal is tensile

failure leading to a crack appears in it. As a high gas

pressure is maintained in the crack, the crack extends and

develops into an arcuate crack that cuts the coal into a

spherical shell plate. Driven by the gas, the coal that has

failed moves forward and is loosened leading to shrinkage

of the outburst hole. The unstable failure of coal is trans-

ferred progressively from the initial exposed surface, and

the fragmentation degree of the coal decreases as the

failure proceeds.

Because more gas and outburst energy are contained in a

GEA, it is easy for a coal in GEA to cause an outburst.

However, the coal pillar in front the GEA has a sealing

effect on the enriched gas and outburst energy in the GEA.

Fig. 10 The outburst energy distribution for the different experiments before uncovering the coal. A normal area; B gas enrichment area


123

The disappearance of the sealing effect initiates the

occurrence conditions for the outburst. Once the coal pillar

is penetrated, the outburst from the coal in the GEA occurs,

even though the gas pressure is lower.

Before the GEA is exposed, the GEA is similar to a gas

cylinder. When the GEA is suddenly exposed, the enriched

gas and energy in GEA are released wildly and the outburst

is like an explosion. The high gas pressure gradient causes

a serious failure of the coal. Additionally, a reverse

unloading wave forms with the release of the energy and it

has a tension effect on the coal. Under dual effects, the

fragmentation degree of the coal intensifies and the inten-

sity of the outburst increases.

Compared with a NA of coal, a high gas pressure gra-

dient is maintained near the exposed surface in the GEA.

Additionally, the high-pressure energy release from the

GEA reduces the coal strength. For the same gas pressure,

the existence of a GEA in coal leads to a faster outburst and

the average thickness of the spall is smaller than they

would be if there were no GEA.

Acknowledgments The authors are grateful for the support from theFundamental Research Funds for the Central Universities (No.

2015XKMS004), the National Science Foundation of China (No.

51574229), and A Project Funded by the Priority Academic Program

Development of Jiangsu Higher Education Institutions (PAPD).

Quanlin Yang and Jun Dong are gratefully acknowledged for their

assistance with the language revision. Comments by all of the

anonymous reviewers are highly appreciated.

References

An F (2014) Preparation and initiation of coal and gas outburst and its

numerical modelling study. China University of Mining and

Technology

An F, Cheng Y (2013) An explanation of large-scale coal and gas

outbursts in underground coal mines: the effect of low-perme-

ability zones on abnormally abundant gas. Nat Hazards Earth

Syst Sci Discuss 1:4751–4775

Ates Y, Barron K (1988) The effect of gas sorption on the strength of

coal. Min Sci Technol 6:291–300

Cai C (2004) Experimental study on 3-D simulation of coal and gas

outbursts. J China Coal Soc 29:66–69

Cai Y, Liu D, Yao Y, Li J, Qiu Y (2011) Geological controls on

prediction of coalbed methane of No. 3 coal seam in Southern

Qinshui Basin, North China. Int J Coal Geol 88:101–112

Chen P, Wang E, Ou J, Li Z, Wei M, Li X (2013) Fractal

characteristics of surface crack evolution in the process of gas-

containing coal extrusion. Int J Min Sci Technol 23:121–126

Cheng Y, Wang H, Wang L, Zhou H (2010) Theories and engineering

applications on coal mine gas control. China University of

Mining and Technology Press, Xuzhou

Choi S, Wold M (2001) A mechanistic study of coal and gas

outbursts. In: Proceedings 38th US Rock Mechanics Symposium,

Washington DC, pp 343–350

Fang J, Yu S, Tan Q (1995) A lamination separation and fragmen-

tation model of coal and gas outbursts. J China Coal Soc

20:149–153

Groshong RH, Pashin JC, McIntyre MR (2009) Structural controls on

fractured coal reservoirs in the southern Appalachian Black

Warrior foreland basin. J Struct Geol 31:874–886

Guo B (2012) Pressure vessel physical explosion hypothesis for coal

and gas outburst. J Henan Polytech Univ (Natural Science)

31:261–267

Guo P (2014) Research on laminar spallation mechanism of coal and

gas outburst propagation. China University of Mining and

Technology

Hu Q, Zhang S, Wen G, Dai L, Wang B (2015) Coal-like material for

coal and gas outburst simulation tests. Int J Rock Mech Min Sci

74:151–156

Jiang J, Cheng Y, Wang L, Li W (2011) Petrographic and

geochemical effects of sill intrusions on coal and their implica-

tions for gas outbursts in the Wolonghu Mine, Huaibei Coalfield,

China. Int J Coal Geol 88:55–66

Jin H, Hu Q, Liang Y (2009) Coal damage mechanism in the

developing process of coal and gas outburst. J Coal Sci Eng

(China) 15:138–142

Lama R, Bodziony J (1998) Management of outburst in underground

coal mines. Int J Coal Geol 35:83–115

Li H, Ogawa Y, Shimada S (2003) Mechanism of methane flow

through sheared coals and its role on methane recoveryq. Fuel

82:1271–1279

Li J, Yan J, Xie Y, Chen P, Wang L (2009) Dynamic action

simulation system and preliminary experiments of coal seams

and gas. Min Sci Technol (China) 19:473–478

Li W, Cheng Y-P, Wang L (2011) The origin and formation of CO2gas pools in the coal seam of the Yaojie coalfield in China. Int J

Coal Geol 85:227–236

Li W, Cheng Y-P, Guo P-K, An F-H, Chen M-Y (2014) The evolution

of permeability and gas composition during remote protective

longwall mining and stress-relief gas drainage: a case study of

the underground Haishiwan Coal Mine. Geosci J 18:427–437

Liu Q, Liu K, Zhu J, Lu X (2014) Study of mechanical properties of

raw coal under high stress with triaxial compression. Chin J

Rock Mech Eng 33:24–34

Lozano Castello D, Cazorla Amoros D, Linares Solano A, Quinn D

(2002) Activated carbon monoliths for methane storage: influ-

ence of binder. Carbon 40:2817–2825

Lu S, Cheng Y, Qin L (2015a) Gas desorption characteristics of the

high-rank intact coal and fractured coal. Int J Min Sci Technol

25:819–825

Lu S, Cheng Y, Wang L (2015b) Pore structure and its impact on CH4

adsorption capability and diffusion characteristics of normal and

deformed coals from Qinshui Basin. Int J Oil Gas Coal Technol

10:94–114

Lu S, Cheng Y, Li W (2016) Model development and analysis of the

evolution of coal permeability under different boundary condi-

tions. J Nat Gas Sci Eng 31:129–138

Matranga KR, Myers AL, Glandt ED (1992) Storage of natural gas by

adsorption on activated carbon. Chem Eng Sci 47:1569–1579

Medhurst T, Brown E (1998) A study of the mechanical behaviour of

coal for pillar design. Int J Rock Mech Min Sci 35:1087–1105

Ning P et al (2012) Adsorption equilibrium of methane and carbon

dioxide on microwave-activated carbon. Sep Purif Technol

98:321–326

Pashin JC (1998) Stratigraphy and structure of coalbed methane

reservoirs in the United States: an overview. Int J Coal Geol

35:209–240

Peng S, Xu J, Yang H, Liu D (2012) Experimental study on the

influence mechanism of gas seepage on coal and gas outburst

disaster. Saf Sci 50:816–821

Prasetyo I, Do D (1998) Adsorption rate of methane and carbon

dioxide on activated carbon by the semi-batch constant molar

flow rate method. Chem Eng Sci 53:3459–3467

3780 Q. Tu et al.

123

Singh J (1984) A mechanism of outbursts of coal and gas. Min Sci

Technol 1:269–273

Skoczylas N (2012) Laboratory study of the phenomenon of methane

and coal outburst. Int J Rock Mech Min Sci 55:102–107

Skoczylas N, Dutka B, Sobczyk J (2014) Mechanical and gaseous

properties of coal briquettes in terms of outburst risk. Fuel

134:45–52

Valliappan S, Zhang W (1999) Role of gas energy during coal

outbursts. Int J Numer Meth Eng 44:875–895

Wang H, Xian X, Du Y (2002) Analyticmethod of calculating gas

content in coal seams in deep mining. J China Univ Min Technol

31:367–369

Wang W, Tao Y, Xu J, Peng S (2010) Simulation of coal and gas

outburst with different gas pressure. J Chongqing Univ 33:82–86

Wang G, Cheng W, Xie J, Zhou G (2011) Analysis of the gas content

in the coal and gas outburst. J China Coal Soc 36:429–434

Wold M, Connell L, Choi S (2008) The role of spatial variability in

coal seam parameters on gas outburst behaviour during coal

mining. Int J Coal Geol 75:1–14

Wu A, Jiang C (2011) Research on the propagation of shock waves

from coal and gas outburst. J China Univ Min Technol

40:852–857

Yao Y, Liu D, Tang D, Tang S, Che Y, Huang W (2009) Preliminary

evaluation of the coalbed methane production potential and its

geological controls in the Weibei Coalfield, Southeastern Ordos

Basin, China. Int J Coal Geol 78:1–15

Yin G, Zhao H, Xu J, Wang W (2009) Experimental study of

simulation of coal and gas outburst. Chin J Rock Mech Eng

28:1674–1680

Yu H, Jing R, Wang P, Chen L, Yang Y (2014) Preferential

adsorption behaviour of CH 4 and CO 2 on high-rank coal from

Qinshui Basin, China. Int J Min Sci Technol 24:491–497

Yu B, Su C, Wang D (2015) Study of the features of outburst caused

by rock cross-cut coal uncovering and the law of gas dilatation

energy release. Int J Min Sci Technol 25:453–458. doi:10.1016/j.

ijmst.2015.03.020

Zhang Q (2008) Adsorption mechanism of different coal ranks under

variable temperature and pressure conditions. J China Univ Min

Technol 18:395–400

Zhou A, Wang K, Wang L, Du F, Li Z (2015) Numerical simulation

for propagation characteristics of shock wave and gas flow

induced by outburst intensity. Int J Min Sci Technol 25:107–112


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http://dx.doi.org/10.1016/j.ijmst.2015.03.020http://dx.doi.org/10.1016/j.ijmst.2015.03.020

Experimental Study of Coal and Gas Outbursts Related to Gas-Enriched AreasAbstractIntroductionExperimentsMaterials and Specimen PreparationExperimental EquipmentExperimental Procedure

Results and AnalysesGas Storage and Mechanical Properties of the MaterialGas Storage CapacityMechanical Properties

Experimental ResultsOutburst IntensityOutburst Hole CharacteristicsSpall CharacteristicsOutbursts Propagation

DiscussionOccurrence Conditions for OutburstsFormation Mechanisms for Outburst PhenomenaInfluence of a Gas-Enriched Area on an Outburst

ConclusionsAcknowledgmentsReferences

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