20th International Conference on Ground Control in Mining

NUMERICAL SIMULATION ON MICROSEISMICITY DUE TO MINING AT ONE
OF THE COLLIERIES IN AUSTRALIA
Fujii, Y., Associate Professor, Hokkaido University, Sapporo, Hokkaido, Japan
Balusu, R., Senior Mining Engineer, CSIRO Exploration and Mining, Brisbane, QLD, Australia
Deguchi, G., Manager, JCOAL, Chiyoda-ku, Tokyo, Japan
Ishijima, Y., Professor, Hokkaido University, Sapporo, Hokkaido, Japan

ABSTRACT
Numerical simulation for microseismicity due to mining at one of thecollieries in Australia was carried out and the results were compared to theobserved microseismicity.
3-D Stress analysis showed stress concentration on the edges of themined areas and on the coal pillars. The stress concentration ahead of thecoal face was biased to the tailgate. This is similar to the observed
distribution of microseismic events. Stress reduction above and below themined area was seen. Tensile stress was observed mainly above the minedarea. It was realized that compressive failure due to nearly verticalmaximum principal stress and nearly horizontal minimum principal stress inretreat direction (NS) might occur ahead of the face. Tensile failure due tonearly vertical minimum principal stress above the mined area was expected.
Compressive failure due to nearly horizontal maximum principal stress in
EW direction and nearly vertical minimum principal stress above and below
the mined area were also expected.
Numerical simulation based on Seismic Moment Method was carriedout first with a standard initial stress and strength data set. Broken elements
were mainly distributed 0 to 100 m ahead of the face with more suchelements in the roof than in the floor. Distribution of the broken elementswith relatively small maximum shear seismic moments in plan view wasbiased to the tailgate. On the other hand, broken elements with relatively
large maximum shear seismic moments distributed evenly. This behavior issimilar to the observation that weak microseismic events showed a biased
distribution while the strong ones were distributed evenly. The height of theyield zone was initially 30 m. It increased gradually up to 200 m until theface reached 500 m from the setup entry and then it was kept almost
constant. These features are comparable to the microseismic observation.
Effects of strength on the simulated results were then investigated. It
was confirmed that the larger the strength data, the smaller the yield zone.
Similar features to microseismic observation were obtained for the resultswith smaller strength data although there was no similarity between thesimulated results with larger strength data and the microseismic observation.
The selected best strength data values were much smaller than thelaboratory values.
Effects of initial horizontal stress were then investigated. The biased
distribution of broken elements was not observed when smaller horizontal
stresses were assumed. Broken elements appeared behind the face whenlarger horizontal stresses in the retreat direction were used.
Finally, effects of panel layout were investigated by carrying out
simulation for three imaginary panel layouts. Stress concentration on theedges of mined area was significantly small when there were no gobs
around the panel of interest. Neither the biased distribution of the stressconcentration nor that of broken elements was seen. No significant

difference in the height of the yield zone and its growth behavior from thestandard case was observed. In the case where the setup entry was set
parallel to that of LW3, results were almost the same as the standard case. In
the case where the setup entry position was 210 m ahead of that of LW3,
biased stress concentration area behind the setup entry was observed.
Broken elements concentration on the setup entry was observed in an initialstage of mining.
INTRODUCTION
Gas drainage is one of the problems that should be solved at one of thecollieries in Australia. The project group of Mining and Exploration, CSIRO
carried out microseismic monitoring for LW4 at the mine in order to obtain
information for effective gas drainage. It will be useful if microseismicity
can be numerically predicted because a prediction provides suggestions
about microseismic monitoring and gas drainage operations. Moreover,comparison between prediction and monitoring results will be useful tounderstand deformation and failure mechanism of rock mass aroundlongwall mining panels.
Many attempts have been made to predict rockbursts at coal faces aswell as microseismicity in underground mines. For instance, Cook et al. [1]found a correlation between rockburst occurrence and calculated energy
release rate. Sugawara et al. [2] compared microseismic intensity to thecalculated energy release rate. However, no prediction on location could begiven since failure could not be considered in the analyses by energy release
rate. The excess shear stress concept was developed by South African
researchers (McGarr & Wiebols [3], Ryder, [4], Spottiswoode, [5, 6]). This
index also could not indicate the location and intensity of microseismic
events or rockbursts.
Areas where microseismic events concentrate and the variation of
microseismic intensity with respect to face advance were successfully
predicted by the strain energy release rate method proposed by Fujii and
Ishijima [7]. The value of their computed seismic energy release rate was,however, ten thousand times the observed one. In 1991, Fujii and Ishijima[8] proposed a new method. The predicted and monitored maximum shear
seismic moment release rates were now having the same magnitude andshowing the same behavior. In the above method, only failure of the rockmass was considered and that in coal seam was ignored. Therefore, a further
modification was done by Fujii et al. [9] and the modified method was
proposed as Seismic Moment Method. Case studies in Horonai Coal Mineand in Miike Coal Mine, Japan were described in the paper. Rockbursts that
occurred at both mines as well as microseismicity were successfully
predicted in the case studies.
This paper will first outline Seismic Moment Method with a slight
modification in the evaluation of the maximum shear seismic moment. Thenthe results of 3-D elastic stress analysis on LW4 by using Displacement

20th International Conference on Ground Control in Mining
Discontinuity Method [10] will be shown. Normal stress distribution will becompared to the distribution of the observed microseismic events. Detailed
stress distribution around the face will also be shown considering possible
failure mode. Next, the results of numerical simulation on microseismicity
by using Seismic Moment Method will be shown. Occurrence of elements'
failure and its variation with face advance will be displayed. The
distribution of broken elements will be compared to that of the observed
microseismic events. Effects of strength and initial stress data on simulated
results will then be investigated. Finally, simulation for several imaginary
panel layouts will be carried out in order to investigate effects of panel
layout on failure around longwall mining panels.
SEISMIC MOMENT METHOD
Definition of maximum shear seismic moment
The seismic moment tensor, Mpq, is a fundamental physical quantity inseismology and is related to the displacement in the rock mass, un, in
xn-direction by the following equations for a fault source [11].

 dM pq ∂ Gnp  dA*
un = 
 dA∂ x q ∫(1)

Mpq is represented by the following equations,∫ [ui ]n j cijpq dA(2)

where,
cijpq : elastic constants;[ui] : displacement discontinuity;
nj : unit normal vector of the fault.
The maximum shear seismic moment, Ms, is a scalar quantity, definedfor each microseismic event as follows:

Ms =M1 − M 3
2

The simulation procedure is as follows (Fig. 2).
(1) The coal seam is divided into square elements (coal seam elements)
having the side lengths of 2h (30 m); while the rock mass aboveand below the coal seam is divided into cubic elements (rock masselements) having side lengths of 2h. The thickness of the region ofinterest was set at ±270 m from the coal seam.
(2) A failure parameter of 0 is assigned to both the coal seam elements
and rock mass elements as initial values and the stress state at thecenter of each element is calculated for the case where only minedpanels exist. The stress states are then compared to a tensile failure
criterion. The failure parameter is changed to 1 if the tensile failure
criterion is satisfied. If the element remains intact, the stress state isthen compared to Coulomb's criterion. The failure parameter ischanged to 2 if the criterion is satisfied.
(3) The stress state at the center of elements whose failure parameter is
0 is calculated for the case where the coal face advances by 2h (30m) and is compared to the failure criteria. The maximum shear
seismic moment is calculated and the failure parameter is changed
if an element breaks.
(4) The third step is iterated until the mining sequence is terminated.
Maximum shear seismic moment due to failure of element

where*: convolution symbol;
Gnp : Green function;A: fault area.

M pq =

with the value of stress drop [7]. An evaluation procedure for the stress statein the coal seam, which is required in order to consider coal seam failure, isgiven in Appendix.

(3)

where M1 and M3 are the maximum and minimum principal values of theseismic moment tensor. The maximum shear seismic moment represents amagnitude of the seismic moment tensor. It becomes equivalent to theseismic moment, M, a scalar quantity which is used in seismology if the
focal mechanism is of the double-couple model.
Numerical procedure
The brief outline of Seismic Moment Method is described (Fig. 1). The
3-D elastic stress analysis by using Displacement Discontinuity Method isprogressively carried out first for each mining step of 30 m in order to makea data file on displacement discontinuity of each element. And then software
for simulation is run to evaluate element breakage and maximum shear
seismic moment.
Strictly speaking, a 3-D elasto-plastic code is required for Seismic
Moment Method. The reasons why the authors used an elastic code are as
follows; (1) the authors do not have access to a 3-D elasto-plastic code; and(2) the authors do not think that 3-D elasto-plastic analyses for longwall
mining have already become a practical means. Actually, CPU time just for
the elastic analysis described later was about 4 hours by a super computer. It
would take at least ten times the CPU time if an elasto-plastic analysis iscarried out. The following assumptions were required using an elastic
analysis code; (1) the stress state is determined using an elastic analysis;
and, (2) any element whose stress state is satisfied by a given failure
criterion is regarded as broken. The error due to these assumptions increases

This quantity was evaluated considering stiffness of the surrounding
medium in the previous paper [9]. This was because the broken element was
located only ahead of the face and it was easy to classify their boundary
conditions according to their location. However, failure of rock elements
above and below the mined area was also anticipated for Mine A case andthe boundary condition for each broken element varies significantly
depending upon the location of the element. It would be difficult to estimate
precisely the stiffness of the surrounding medium for Mine A case. The
following method was therefore used instead of the published method.
Namely, it was assumed that a circular rupture plane, with radius h andcenter located at the center of the element, appeared when a failure criterionof the element was satisfied.
The method for a tensile failure of a rock element is described first.
Miyamoto [12] shows displacement discontinuity due to normal stress ∆σ atdistance r from the center of a penny-shaped crack, which is perpendicular
to z-axis and has radius h, as the following equation.

[u z ] =8∆σ (1 − ν 2 )h2 − r2πE(4)where ν is Poisson's ratio. Average displacement discontinuity can beevaluated as follows.

[u z ] =∫ [u z ] dA = 16∆σ (1 − ν 2 )h
3πE

A

(5)

For a sufficiently small fault source, Eq. (2) can be rewritten as thefollowing equation.

M pq = [ui ]n j cijpq A(6)Non-zero components of seismic moment tensor are therefore as follows;

M xx = M yy =M zz =

16∆σ (1 − ν )νh 3
3(1 − 2ν )16∆σ (1 − ν ) 2 h 3 .
3(1 − 2ν )Substituting Eqs. (7) and (8) into Eq. (3), we get,

(7)
(8)

20th International Conference on Ground Control in Mining

Panel layout, mehcanical properties, etc.

3-dim DDM program

Location and Ms of broken element

Simulation program

Displacement discontinuity

Normal stress on coal seamStress state in rock mass

Fig. 1

Procedure of Seismic Moment Method
Start

Initialize failure parameter to 0

No
Failure parameter = 0?

Yes
Calculate stress stateTensile failure?

Yes

No

Compressive failure?

Failure parameter = 1
NoCalculate seismic moment

Yes
Failure parameter = 2

Calculate seismic moment

Iterate until mining ends

Iterate for all elements
Stop

Fig. 2

Flow chart of the simulation program

20th International Conference on Ground Control in Mining
8∆σ (1 − ν )h 3
Ms =
3

(9)

Assuming that a sheet like inclusion rock in the crack fails in a tensile
manner and the stress drop is ∆σ, Eq. (9) can be used to evaluate maximum
shear seismic moment due to tensile failure of a rock element containing thecrack.
The method for a compressive failure of a rock element is given below.
Sato & Kinoshita [13] showed displacement discontinuity due to shear
stress ∆τ at distance r from the center of a penny-shaped crack, which isperpendicular to z-axis and has radius h, as the following equation.

[u x ] =
8∆τ (1 − ν ) 
r2

h−

π ( 2 − ν )G 
h + h2 − r2 the mining area is located at 200 m below the ground surface and theaverage dip is 5°. Rock masses above and below the coal seam wereassumed to be homogenous. The mechanical properties shown in Table 1
were used in the simulation.
The calculation took about 30 min by a Pentium II 450 MHz PC. Fig. 4
shows an example of the normal stress distribution on the coal seam. Stressconcentration can be seen on the edges of the mined panels. The highest
stress concentration can be seen on the coal pillars between LW2 and LW3.
There appears rather wide area of stress concentration ahead of the face of
LW4. The normal stress is higher near the tailgate than the maingate. In the
microseismic observation, most events locate between the coal face and 100
m ahead of it (Fig. 5a). The center of the distribution is biased to the tailgate.
This feature is similar to the calculated stress concentration.

(10)

y'

O

Average displacement discontinuity can be evaluated as follows.

[u x ] =232∆τ (1 − ν )h3π ( 2 − ν ) E

LW1 (mined)

Non-zero components of seismic moment tensor and the maximum shear
seismic moment can be evaluated in the similar way to the tensile failurecase.

M xz = M zx = M s =16∆τ (1 − ν )h 3
3( 2 − ν )(12)Assuming that a sheet like inclusion rock in the crack fails in a shear
manner and the stress drop is ∆τ, Eq. (12) can be used to evaluate
maximum shear seismic moment due to shear failure of a rock element
containing the crack. Basically, Eqs. (9) and (12) can also be used for the
failure of a coal seam element.
ELASTIC STRESS ANALYSIS
The mining area was divided into 40 x 85 elements (Fig. 3). The sizeof each element is 30 x 30 m. It was assumed that the shallower edge of

Fig. 4MPa.

LW2 (mined)(11)

LW3 (mined)
LW4

x'

N
Fig. 3

1 km
Panel layout

Table 1
Parameters used in the elastic analysis
2
Erock / Ecoal
Poisson' ratio
0.2
Vertical stress
Product of specific gravity and depth
Specific gravity
27 kN/m3
σx (EW)
1.20 σVσy (NS)
0.48 σV
Working height
4.5 m

An example of normal stress distribution on the coal seam. The gray scale between black to white denotes the normal stress of 0 to 40

20th International Conference on Ground Control in Mining
Let's investigate the stress distribution in detail. Fig. 6 shows stress
distribution along y"-axis (O" is taken at the center of the coal face) at 1020m face advance. The vertical stress concentration at the coal face can be
seen. Rock elements near the coal seam and/or coal seam elements would
break ahead of the face if the compressive strength is small. The verticalstress decreases above and below the mined area. The vertical stress in thefloor remains as compressive (Fig. 6b). On the other hand, some of thevertical stress data in the roof are tensile (Fig. 6a) because of the effects ofthe gravity force. Tensile failure due to vertical tensile stress might occur
above the mined area if tensile strength of rock element is small.
Compressive failure may occur due to the maximum principal stress in EW
direction and the nearly vertical minimum principal stress even if the tensile
failure does not occur.

NUMERICAL SIMULATION
Panel layout is already shown in Fig. 3. Numerical simulation was
carried out for several cases. Each case is specified as Case X-n(-x) where
X, n and x denote initial stress variation (Table 2), strength variation(Table3) and variation in panel layout, respectively. The results for Case A-1,
which will be referred later as a standard case, are shown in this chapter.
Simulation took about four hours by a super computer (Hitachi SR8000,
320 GB memory, 32 nodes, 1.1 TB HD, 256 GFLOPS) at Hokkaido
University Computing Center, Japan. Broken elements are shown in Fig. 7.
In the initial stage of mining, the broken elements locate in the vicinityof the face. The broken rock elements that locate at the edges of mined area
are due to the nearly vertical maximum principal stress and the minimum
principal stress nearly in the retreat direction. On the other hand, the broken(a) Plan view(b) Sectional viewFig. 5Observed distribution of microseismic events relative to LW face after Luo et al. [14] (intentionally rotated)

20

20

σz'

10

σx'
σy'

0
-100

Stress (MPa)Stress (MPa)σz'0y" (m)(a) 15 m above coal seamFig. 6
Stress distribution at 1020 m face advance

100

10

σx'
σy'

0
-100

0
y" (m)(b) 15 m below coal seam100

20th International Conference on Ground Control in Miningrock elements above and below the mined area are due to the maximum
principal stress in nearly EW direction and the reduced nearly verticalminimum principal stress. The distribution of the broken elements in theplan view is gradually biased to the tailgate. This can be clearly seen when
the face reaches 390 m from the setup entry. This biased distribution ismainly due to broken elements with relatively small maximum shear
seismic moments since the biased distribution is not observed in Fig. 8
which shows broken elements with relatively large maximum shear seismic
moments. A biased distribution similar to the simulated results can be seenin the observed microseismic events (Fig. 5a). However, relatively strong
events distributed rather evenly.
The maximum height of broken elements in the roof increases graduallyas the face advances up to 500 m and is kept almost constant (Fig. 9).
Cluster of broken elements just as forming a plane often appear in the roof
as the face advances up to 240 m (Figs. 7b and 7c). This might correspondto the observed main fall of the roof when the shape of the mined area
became almost square. No cluster was observed in the following faceadvance because the yield zone ahead of the face is high enough and there
are no unbroken elements left behind the face.
EFFECTS OF STRENGTH
Strengths in Case A-0.5 are as low as a half of those in Case A-1. The
behavior of the broken elements' distribution (Fig. 10a) is similar to theobservation. However, the height of the yield zone is larger than theobservation. Strengths, which are five times larger than Case A-1, were usedin Case A-5. The size of the yield zone is significantly smaller than theobservation (Fig. 10b). The biased distribution is not observed.
EFFECTS OF HORIZONTAL STRESSResults of case B-1, where both the horizontal components of initialstress are 0.48 times the vertical component, are shown in Fig. 10c. There isno similarity between behavior of the simulated broken elements and theobserved biased distribution of microseismic events. This is because therock elements, which have to be kept intact in order to exhibit the biased
distribution, had already been broken before the mining of LW4.
Horizontal stresses in EW and NS directions are both 1.2 times thevertical stress in Case C-1. All broken elements were located behind theface (Fig. 10d). Compressive failure of rock mass, which is due tohorizontal maximum principal stress and vertical minimum principal stress,
dominates. This is because the larger horizontal stress in the retreat direction
prevents failure of rock mass ahead of the face. This feature is totallydifferent from the observation. A few tensile failure of rock mass were also
observed.

EXISTENCE OF COAL ROOF
Numerical simulation was carried out for Case A-1'. This is based on
Case A-1, however, strength for coal seam was assigned to the immediateroof strata. Stress distribution is not correct since the elastic constants forthe immediate roof is still for rock mass.
The intensity of microseismic events in the immediate coal roof weremuch weaker than that in the main roof in the microseismic observation.
This was not observed in the simulated results. This discrepancy mightsuggest that this numerical model is still not precise enough. It can also be
pointed out that the calculated fracturing intensity for microseismic eventsfrom the coal roof in the observation would be less than the reality sinceelastic wave from a severely fractured coal roof would be much more
attenuated than that from rock mass.
EFFECTS OF PANEL LAYOUT
Numerical simulations were finally carried out for three imaginary panel
layouts (Fig. 11). Case A-1-a is for a single mining panel without any othermined areas. Setup entry is excavated parallel to that of LW3 in Case A-1-b.
The position of the setup entry is 210 m ahead of that of LW3 in Case
A-1-c.
Case A-1-a
Normal stress distribution is shown in Fig. 12a. Stress concentration on
the edges of mined area is significantly less than Case A-1. The biased
distribution of the stress concentration area cannot be seen. Results ofnumerical simulation on microseismicity are shown in Fig. 13a. The size ofthe yield zone is smaller than Case A-1 and the biased distribution of broken
elements cannot be seen. However, the height of the yield zone and its
growth behavior were almost the same as Case A-1 (Fig. 9).
Case A-1-b
Both the normal stress distribution and the distribution of broken
elements are almost the same as case A-1, except that the biased distributionof stress concentration area and that of broken elements can be observed
earlier than Case A-1. Height of the yield zone is almost the same as caseA-1.

Table 2
Initial stress variation
X
A
B
C
Erock / Ecoal
2
Poisson' ratio
0.2
Vertical stress
Product of specific gravity and depth
Specific gravity
27 kN/m3
σx (EW)
1.20 σV
0.48 σV
1.20 σVσy (NS)
0.48 σV
0.48 σV
1.20 σV
Working height
4.5 m
Table 3nStrength variation

Tensile strength (MPa)Residual tensile strength (MPa)
Uniaxial compressive strength (MPa)Residual compressive strength (MPa)Angle of internal friction (degree)

0.5
Rock
0.25
0.225
0.5
0.45
30

Coal
0.125
0.1125
0.25
0.225
30

1
Rock
0.5
0.45
1
0.9
30

Coal
0.25
0.225
0.5
0.45
30

5
Rock
2.5
2.25
5
4.5
30

Coal
1.25
1.125
2.5
2.25
30

20th International Conference on Ground Control in Mining(a) 30 m(b) 120 m(c) 240 m(d) 390 m(e) 1170 m(f) 2280 m
Fig. 7
Distribution of broken elements in Case A-1. There are three rows of drawings - the top ones are plan view (x'-y'), the middle ones are
longitudinal cross-section (y'-z', along mining direction) view, and the bottom ones are end view (x'-z'). The radiuses of the circles are proportional
to the logarithm of maximum shear seismic moment. Locations of the circles are displaced randomly up to h / 2 in order to avoid overlapping. Grayand black circles denote compressive failure of rock mass and coal seam, respectively. Tailgate is right side and maingate is left side in the x'-z'
section.

20th International Conference on Ground Control in Mining

Fig. 8Broken elements having maximum shear seismic
moments which are greater than 5 GN⋅m (Case A-1, 1170 m). The
top ones are plan view (x'-y'), the middle ones are longitudinalcross-section (y'-z', along mining direction) view, and the bottom
ones are end view (x'-z').

(a) Case A-0.5 (weak case)(b) Case A-5 (strong case)

Height (m)300Case A-1
200
Case A-1-a
100
0Fig. 9

0

1000
2000
Face advance (m)Maximum height of broken elements(c) Case B-1 (small stress case)(d) Case C-1 (large stress case)Fig. 10Broken elements for various strengths or under various initialstresses. The top ones are plan view (x'-y'), the middle ones are
longitudinal cross-section (y'-z', along mining direction) view, and thebottom ones are end view (x'-z').

20th International Conference on Ground Control in MiningOyx(a) Case A-1-a

LW4

O

y

Case A-1-a

1 km

LW1 (mined)(b) Case A-1-c
Fig. 12
Normal stress distribution for two imaginary panel layouts

LW2 (mined)
LW3 (mined)
x

LW4

O

y

Case A-1-b

1 km

LW1 (mined)
LW2 (mined)
LW3 (mined)x

LW4

Case A-1-c
Fig. 111 km

Three imaginary panel layouts.
(a) Case A-1-a(b) Case A-1-c
Fig. 13
Distribution of broken elements for two imaginary panel layouts. The top
ones are plan view (x'-y'), the middle ones are longitudinal cross-section (y'-z', along
mining direction) view, and the bottom ones are end view (x'-z').
distribution of microseismic events.

Case A-1-c
Normal stress distribution is shown in Fig. 12b. The biased stressconcentration area can be seen behind the setup entry as well as ahead of theface. The stress distribution pattern is almost the same as case A-1 except
for the biased distribution behind the setup entry. The results of numericalsimulation are shown in Fig. 13b. Broken elements concentrate on the setup
entry as well as on the coal face. The biased distribution is observed even
earlier than Case A-1-b. Height of the yield zone is almost the same as Case
A-1.
CONCLUSIONS
Numerical simulation for microseismicity due to mining at one of thecollieries in Australia was carried out and the results were compared to theobserved microseismicity.
3-D Stress analysis showed stress concentration on the edges of themined areas and on the coal pillars. The stress concentration ahead of thecoal face was biased to the tailgate. This is similar to the observed

Distribution of stress components around coal face was investigated.
Compressive failure due to nearly vertical maximum principal stress andnearly horizontal minimum principal stress in the retreat direction ahead of
the face were expected. It was expected that tensile failure due to nearlyvertical minimum principal stress might occur above the mined area.
Compressive failure due to nearly horizontal maximum principal stress in
EW direction and nearly vertical minimum principal stress above and below
the mined area was also expected.
Numerical simulation on microseismicity due to the mining of LW4 was
carried out with a standard initial stress and strength data set. Broken
elements were mainly distributed 0 to 100 m ahead of the face with moresuch elements in the roof than in floor. Distribution of the broken elementswith relatively small maximum shear seismic moments in plan view wasbiased to the tailgate. On the other hand, broken elements with relatively
large maximum shear seismic moments were distributed evenly. Thisbehavior is similar to the observation that weak microseismic events
showed a biased distribution while the strong ones were distributed evenly.
The height of the yield zone was initially 30 m and increased gradually as20th International Conference on Ground Control in Mining
the face advances up to 500 m. It was thereafter kept almost constant.
Cluster of broken elements just as forming a plane often appeared in theroof as the face advanced up to 240 m. This might correspond to theobserved main fall of the roof when the shape of the mined area becamealmost square.
Effects of strength on the simulated results were investigated. It was
confirmed that the larger the strength data, the smaller the yield zone.
Similar features with microseismic observation were obtained for the resultswith smaller strength data although there was no similarity between thesimulated results with larger strength data and the microseismic observation.
The selected best strength data values were very small compared to thelaboratory values. This might suggest that the observed microseismicity
reflects slip and opening of planes of weakness.
Effects of initial horizontal stress were investigated. It was clarified thatthe biased distribution of broken elements was not observed when smaller
horizontal stresses were assumed. Broken elements appeared behind theface when larger horizontal stresses were used. The results suggest that theinitial horizontal stress affects significantly on failure around longwall
mining panels.
Effects of panel layout were investigated by carrying out simulation forthree imaginary panel layouts. It was realized that stress concentration on
the edges of mined area was significantly small when there was no gobs but
LW4 itself. Neither the biased distribution of the stress concentration nor
the biased distribution of broken elements was seen. No significant
difference in the height of the yield zone and its growth behavior from thestandard case were observed. In the case where the setup entry was set
parallel to that of LW3, no significant difference from the standard case wasobserved in the normal stress distribution or distribution of broken elements
except that the biased distributions were observed earlier than the standard
case. The height of the yield zone did not seem to be affected by the change
in panel layout. In the case where the setup entry position was 210 m aheadof that of LW3, a biased stress concentration area behind the setup entry
was observed. Broken elements concentration on the setup entry wasobserved in the initial stage of mining. The biased distribution of broken
elements ahead of the face was observed even earlier than the previous case.
The behavior of the yield zone height was similar to the standard case.
The above results are basically reasonable comparing to themicroseismic observation. It is considered that they are offering valuableclues to understand deformation and failure around longwall mining panels.
Some of the results can only be obtained by using 3-D analysis. Thissuggests the importance of a 3-D numerical stress analysis for strata control
in longwall coal mining. However, the simulation method should be further
sophisticated to give even better results. For instance, such inhomogeneity
as multi strata structure, dykes, etc. cannot be simulated at present.

8. Fujii, Y. and Ishijima, Y. (1991), Numerical Simulation on
Microseismicity Induced by Deep Longwall Coal Mining, Min. Sci. &Tech., Vol. 12, No. 3, pp. 265-285
9. Fujii, Y., Ishijima, Y. and Deguchi, G. (1997), Prediction of Coal Face
Rockbursts and Microseismicity in Deep Longwall Coal Mining, Int. J.
Rock Mech. Min. Sci. & Geomech. Abstr., Vol. 34, No. 1, pp. 85-96
10. Crouch, S.L. and Fairhurst, C. (1973): The Mechanics of Coal Mine
Bumps and the Interaction between Coal Pillars, Mine Roof and Floor,
U. S. B. M. Contract Report, H0101778
11. Aki, K. and Richards, P. G. (1980), Quantitative Seismology, Theory
and Methods. 1, W. H. Freeman and Company, San Francisco
12. Miyamoto, H. (1967 in Japanese), Elastic Theory in Three Dimension,
Shokabo, Tokyo
13. Sato, K. and Kinoshita, S. (1976 in Japanese), Griffith Locus inCompressive Deformation, J. MMIJ., Vol. 92, pp. 409-414
14. Luo X., Ross J., King A. (2000), Microseismic Monitoring at
Dartbrook Mine for Underground Gas Emission Control, CSIRO
Report No. 675.
APPENDIX

Evaluation of stress components in coal seam apart from coal face by
several times the working height
Assume a homogeneous, isotropic elastic medium and that Young's
modulus, rigidity, Poisson's ratio are represented by EC, GC, νC for coalseam and ER, GR, νR for rock mass. The normal of the coal seam is taken as
z-axis. Traction of coal seam τxz, τyz, σz and stress component σ xR, σ yR, τxyR
on the surface of rock which contacts to coal seam are given by DDM.
Since a coal seam element is of a tabular shape, strain components in rockmass εxR, εyR, γxyR are equivalent to those in coal seam εxC, εyC, γxyC assumingthat there is no slip on the boundary between coal and rock. By using thisrelationship, stress components in coal seam σxC, σyC, γxyC can be
represented as follows.

σ xC = ( λC + 2 µ C )ε xR + λCε yR + λCε zC(A1)

σ yC = ( λC + 2 µ C )ε yR + λCε xR + λCε zC(A2)

τ xyC = µ Cγ xyR(A3)

ε zC =

σ xC + σ yC
σz
−ν C
EC
EC

(A4)

Where λC, µC are Lame's constants for coal seam, εzC is the vertical strain
component in coal seam.
Hooke's law for rock mass can be represented as follows.

REFERENCES
1. Cook, N. G. W., Hoek, E., Pretorius, J. P. G., Ortlepp, W. D. andSalamon, M. D. G. (1966), Rock Mechanics Applied to the Study ofRockburst. J. South Afr. Inst. Min. Metall, Vol. 66, No. 10, pp. 435-528
2. Sugawara, K., Obara, Y. and Kaneko, K. (1986 in Japanese), Predictionof Microseismicity Associated with Longwall Coal Mining, Proc.
MMIJ Spring Meeting, pp. 283-284
3. McGarr, A. and Wiebols, G. A. (1977), Influence of Mine Geometry
and Closure Volume on Seismicity in a Deep-Level Mine, Int. J. Rock
Mech. Min. Sci. & Geomech. Abstr, Vol. 14, pp. 139-145
4. Ryder, J. A. (1988), Excess Shear Stress in the Assessment of
Geologically Hazardous Situations, J. S. Afr. Inst. Min. Metall., Vol.
88, No. 1, pp. 27-39
5. Spottiswoode, S. M. (1988), Total Seismicity, and the Application of
ESS Analysis to Mine Layouts. J. S. Afr. Inst. Min. Metall, Vol. 88, No.
4, pp. 109-116
6. Spottiswoode, S. M. (1988), Volume Excess Shear Stress andCumulative Seismic Moments. Pre-printed Papers, 2nd. Intl. Symp.
Rockbursts and Seismicity in Mines, Minneapolis, Minnesota, pp. 1-12
7. Fujii, Y. and Ishijima, Y. (1988 in Japanese), Microseismicity Inducedby Deep Coal Mining and Its Numerical Simulation, J. MMIJ, Vol.
104, No. 1204, pp. 345-351

ε xR =ε yR =

σ xR
ER

σ yR
ER

γ xyR =

−ν R
−ν R

τ xyR

σ yR + σ z

(A5)

ER

σ xR + σ z

(A6)

ER
(A7)

G

The following equations on stress components in coal seam are obtained bysubstituting Eqs.(A4) to (A6) into Eqs.(A1) and (A2), Eq.(A7) into Eq.(A3)and by assuming νC= νR= ν.

EC{σ xR − ν (1 − ν )σ z } + ν (1 − ν )σ z
ERE
σ yC = C {σ yR − ν (1 − ν )σ z } + ν (1 − ν )σ z
EREτ xyC = C τ xyR
ER

σ xC =

(A8)
(A9)
(A10)