Kyushu University Global COE Program
Journal of Novel Carbon Resource Sciences, Vol. 7, pp. 47-52, Feb. 2013

Surface Subsidence Analysis Due to Longwall Underground
Coal Mining Method in PT. Gerbang Daya Mandiri,Tenggarong, East Kalimantan
Tri Karian, Timbul M. P. Habeahan, Budi Sulistianto, Suseno Kramadibrata
Institut Teknologi Bandung, Bandung 40132, Indonesia(Received November 16, 2012; accepted January 7, 2013)
PT Gerbang Daya Mandiri will apply longwall underground coal mining method on its mine site, which islocated in Kutai Region. Hence, surface subsidence would potentially occur at the surface area and consequently
subsidence analysis including the likelihoods of subsidence profile, maximum value of subsidence and area of surface
subsidence is needed. The analysis reveals that the maximum value that is calculated using empirical formulae of
the UK National Coal Board differs to that of numerical analysis using distinct element method and the later alsosuggests that the coverage area of surface subsidence would be 1.7 × 106 m 2.

1. Introduction

4AF 5
AFThe mine site of PT Gerbang Daya Mandiri (GDM) islocated in Kutai Kertanegara, East Kalimantan, Indonesia(Fig. 1). GDM coal consists of several seams which arepart of Kutai Basin Coal with average dip of 3o-13o
and coal seam thickness varies from 0.1 to 9.8 meters.
Despite open pit coal mine of GDM has been operating
since October 2009, longwall underground coal mine
method however should now be applied to keep the minein operation and this is due to environmental constraints.
It has been well accepted that the application of longwall
underground coal mine may expect surface subsidence.
Since most of the surface area is made use of farm land,
rice field and settlement it is therefore important to carryout a comprehensive research of subsidence analysis.
The research embraces prediction of maximum value ofsubsidence, subsidence profile and the area of surface
subsidence from which potential remedial action could
be prepared in early stage. In order to gain a complete
picture of the subsidence analysis, the research is carriedout in panel AF4 & AF5 and panel NB F3 & NB F4 on A
seam and BC upper seam respectively (Fig. 2).

LEGEND
Mine DesignSeam AConcession
Boundary

Mine Design
Seam BC-Upper

Pipeline

F3NB 4FNB

Fig. 2 Mine design of AF4, AF5, NB F4, and NB F4Panel1).

2. Panel dimension and site investigation

Fig. 1 Location of GDM Coal Mine1).

The geometry of panels AF4, AF5, NB F3 and NB F4 isgiven in Table 1. The lithology of the mining area at GDM
is dominated by sandstone and claystone as can be seenin the bore-log of NED 02 borehole (Fig. 3). Highwall
side of former open pit mine which shows interburden
between seam 1, seam A and seam B is shown in Fig. 4.

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Journal of Novel Carbon Resource Sciences, Vol.7, Feb. 2013

Laboratory tests on rock samples obtained fromthe NED-02 borehole were conducted to determine
the physical and mechanical properties and they areavailable in Table 2. Whereas the mechanical propertiesof the material of the bedding planes such as cohesion
and internal friction of angle are given in Table 3.
The subsidence analysis applies both numerical andempirical methods and uses the data of Tables 2 and3 as well as data provided by the GDM GeotechnicalDepartment.
Table 1
Seam

PanelTilt(°)

Width(m)Height(m)Length(m)Depth(m)

A F4

8.7

140

2.8730

200

A F5

8.7

1402.8920

235

NB F3

8.1

140

2.81250245

NB F48.1

140

2.81030

285

Length (m)Depth (m)ABC upper

Panel geometry1)

Depth (m)Surface025

Fig. 4 Interburden of Seam 1, A, and B from exposed
highwall of open pit area.
Table 2
Material

0.2

0.5

38.5

1.9

0.9

400

Sandstone
OB 1

2.2

0.5

0.2

3.7

21.2

0.5

0.3

425

Claystone
IB 1-A

2.0

0.3

0.2

0.5

38.5

0.4

0.2

Claystone
IB A-BC

2.2

0.1

0.2

0.5

38.5

0.1

0.1

Sandstone
IB A-BC

2.2

0.2

0.2

7.4

21.6

0.3

0.1

Coal

1.3

0.2

0.3

2.6

45.6

0.5

0.1

75

475

100

500
225.6

A

550
575

D

600

225625

250

3.4

2.8

11.6
12.1

2.8
3.9

66 low

300
325

16.6

5.0 0.8

5

19.7

4
32 up
375 2
350

400

2.3
4.4
1.7 3.4
10.91

4.1 1.4
0.9
9.4
0.7
7.4
5.9 0.6
0.8
8.1
6.5
12.8
2.8

Table 31.30.5
4.0 1.3
15.7
1.0

Contact plane characteristics1,2)

Contact Plane

Cohesion(MPa)

Internal Friction Angle(°)

Sandstone- Coal

1.328

Sandstone-Claystone

0.128

Claystone-Claystone

0.228

Sandstone-Sandstone

1.828

Coal-Coal

1.3

28

650

25.3
275

E: modulus of elasticity; v: Poisson ratio; γ: density; c: cohesion; ϕ: internalfriction angle; G: shear modulus; B: bulk modulus; σt: tensile strength.

20.6
EF

200

8.1
9.3
19.2

BC525

10.9
7.8G(GPa)

1.5450175CϕB(MPa) (…°) (GPa)v

2.1

50150Eγ(gr/cm3) (GPa)Claystone
OB 1

Length (m)

73.1

125

Rock physical and mechanical properties1)

3. Empirical analysis

675700

212.6

725
750
775800Claystone
Sandstone
Coal SeamFig. 3 Bore-log of NED 02 Borehole.

The maximum value of surface subsidence causedby extraction of an underground longwall panel can bepredicted empirically using graphical method developedby the U.K. National Coal Board. By using this method
the maximum surface subsidence (S) can be predictedfrom surface subsidence factor (F) times the thicknessof coal seam (m), where the factor is obtained from therelationship between the factor to width and depth of theopening graph.
Predicted surface subsidence (s) at various points above
the longwall panel can be determined using the graphof the U.K. National Coal Board. The data used in thismethod consists of; h (depth of the panel center point),w (width of opening panel) and d (distance of various
points along subsidence profile to the panel center point).

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Surface Subsidence Analysis Due to Longwall Underground Coal Mining Method in PT. Gerbang Daya Mandiri, Tenggarong, East Kalimantan

A correlation table between w/h and d/h is used thenthe values of s at various points are then calculated fromthe s/S ratio times S (maximum surface subsidence).
Based on the dimension data of each panel the values
F and w/h for each panel are obtained and presented inTable 4.
Table 4

onto the model is equivalent to the horizontal in-situ
stress (σh) and is defined as 2/3 of σv for 300 m depth3).
The load works towards the x and z directions with g of10 m/s2.
N

Values of surface subsidence factor and w/hof panel

PanelSurface Subsidence Factor (F)w/h

AF40.65

0.72

AF50.58

0.60

NB F30.55

0.58

NB F40.46

0.50

4. Numerical analysis

Seam A outcrop

EZ

Seam BC outcrop

The numerical modeling is run based on 3-D Distinct
Element Method using 3DEC Ver. 2.0 software. The firststep is to model the geometry that is defined in three
dimensions (see Fig. 5) and this is based on the GDM
mine design as mentioned earlier. The model geometry
is constructed by combining mine design, topographic
maps, scanline data, mining situation maps, and the rockstratification. Different colors in Fig. 5 represent eachrock strata found in borehole NED 02.
The model of panel geometry is referred to the panels
AF4 & AF5 and panels NB F3 & NB F4 of seam A andseam BC upper respectively (Figs. 6 and 7).
After the model geometry has been defined, thesecond step is to model the material and discontinuities
characteristics in the numerical analysis which consistof :1. Building up discrete elements. The elements close to
the panel are smaller mesh size than that of element
further from the panel and that of close to the panel
must have a mesh size smaller than that of panel
dimension.
2. The material characteristics comply with the MohrCoulomb failure criterion.
3. The material properties of rock body include
modulus of elasticity (E), Poisson ratio (v), density(γ), cohesion (c), internal friction angle (ϕ), shear
modulus (G), bulk modulus (B) and tensile strength(σt).
4. The joint characteristics are represented by normal
stiffness (Kn), shear stiffness (Ks), cohesion (C),tensile strength (σt) and internal friction angle (θ).
The third step is to determine the load and boundaryconditions. The boundary conditions for the model aredefined as follows; displacements in the western andeastern of the model as well as in northern and southernof the model are only permitted in the y and z directions
as well as in the x and y directions. At the lower limitof the model the displacement is only permitted in thedirection of x-direction and z-direction. The load applied

Fig. 5 Overall geometry model for numerical analysis.

AF4AF5Fig. 6 Geometry model of panel AF4 and AF5 on
Seam A.

NB F3NB F4Fig. 7 Geometry model of panel NB F3 and NB F4 on
Seam BC upper.

5. Result and discussion

Table 5 indicates the values of the maximum surface
subsidence for every single panel extraction based on
calculation of the numerical and empirical methods.
The subsidence profiles associated with the data given
in Table 5 is exhibited in Figs. 8 to 11. The profiles are
made by taking cross section perpendicular to each panel
extraction direction and through each center of the panel.

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Journal of Novel Carbon Resource Sciences, Vol.7, Feb. 2013

Table 5Maximum value of surface subsidence due tosingle panel extraction

Extracted
PanelMaximum Surface Subsidence (m)EmpiricalMethod

Numerical
Modeling

Difference
m%

A F4

1.8

1.0

0.8

44%

A F5

1.6

1.00.638%

NB F3

1.5

0.8

0.7

46%

NB F4

1.2

0.7

0.5

41%

Fig. 8 Surface subsidence profile due to panel AF4 extraction.

The difference values of the maximum surface
subsidence calculated by both methods are attributed
to the following conditions: the empirical method
assumes that the entire material above the coal panel
being isotropic, homogeneous and continuous. Inaddition, the panel geometry is considered flat and thecalculation is merely based on depth, width and heightof a panel only. On the other hand, the numerical method
obviously allows the calculation using an-isotropic,
non-homogeneous, non-continuous media and complex
geometry conditions. Moreover, the basic physical andmechanical properties used in the both methods remain
relatively the same.
It is however important to note that the use of empiricalmethod is always site specific as any empirical methods
is derived from a certain site that possesses particulargeological and geomechanical properties. Consequently,
the use of any empirical method for other site might
possibly yield unexpected recommendation in which
cautious treatment should be exercised. Apparently,
similar results are also experienced by Adamek etal. (1987) and Munson & Eichfield (1980). Thus, thisresearch has then put special emphasis on subsidence
analysis based on the numerical modeling.
Another advantage of applying the numerical method
is that the subsided area can be calculated and theindividual areas for each panel are given in Table 6.
Table 6Fig. 9 Surface subsidence profile due to panel AF5 extraction.

Fig. 10 Surface subsidence profile due to panel NB F3
extraction.

Area of subsidence due to single panel
extraction based on numerical analysis

Extracted PanelArea of Subsidence (× 106 m2)

AF4

0.6

AF50.8NB F30.9

NB F40.9

Having done the predicted subsided area for each panel,
the angle of draw can also be obtained as shown in Figs.
12 to 15. It can be noted that there will be two values of
the angle of draw once the seam has an inclination oris tilting4), i.e. lower and upper angle of draws. Table 7
provides the values of panel tilt, lower angle of draw andupper angle of draw.
Table 7Fig. 11

Surface subsidence profile due to panel NB F4
extraction.

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Lower limit and upper limit angle of eachextracted panel

Extracted
PanelPanelTilt(°)

Lower limit angle(°)

Upper limit angle(°)

AF48.75549AF58.75450

NB F38.15044

NB F48.14944

Surface Subsidence Analysis Due to Longwall Underground Coal Mining Method in PT. Gerbang Daya Mandiri, Tenggarong, East Kalimantan

brings about 6° departure of the center the maximum
surface subsidence off each panel center. Similarly
happens to panels NB F3 and NB F4 which have thesame panel inclination of 8.7° causes the centers of themaximum surface subsidence move away of 4° and 5°
respectively (Figs. 14 and 15).
In order to determine the surface subsidence due
to extraction of all panels a principle in the numericalmodeling so called superposition will be applied, in which
individual values of the maximum surface subsidence ofeach panel are added up. By applying this principle, themaximum surface subsidence due to all panel extraction
is obtained i.e., 2.5 meters.
Yet, it would be interesting to find out how the valuesof the maximum surface subsidence calculated basedon option-1: individual values of the maximum surface
subsidence of each panel are added up (as mentionedabove) and option-2: value of the maximum surface
subsidence if entire panels are considered as one big
panel extraction.
The results of the exercise of these two options are
given in Table 8 and Figs. 16, 17 and 18. It can be seenthat the maximum surface subsidence of the option-2
is about 88% of that option-1, but the difference of thesubsided areas (option-1 = 1.76 × 106 m2 and Option-2 =1.77 × 106 m2) is only about 1% and insignificant.
Having gone through all exercises it then be said that
applying the superposition principle for a single panel
extraction can be confirmed by that of entire panel
extraction.

Fig. 12 Regional influences of panel AF4.

Fig. 13 Regional influences of panel AF5.

5. Conclusions

Fig. 14

Regional influences of panel NB F3.

Fig. 15

Regional influences of panel NB F4.

Both methods of numerical and empirical can be used
to predict surface subsidence due to an undergroundlongwall coal mining extraction.
From the foregoing analysis it can be noted that despite
the fact many empirical method is usually derived froma specific site, this method suggests greater maximum
surface subsidence, more practical, faster than that of
the numerical one. Due to its site specific, this method
is accordingly deemed inappropriate for application ofthis research.
The superposition principle based on single panel
extraction method and entire panel extraction method
can be used in generating and calculating the subsidence
profile as well as maximum surface subsidence
respectively. The two methods apparently provide
insignificantly different results.
Table 8

Numerical method suggests that panel inclinationinfluences the center of the maximum surface
subsidence4). Regarding this matter, Figs. 12 and 13 show
evidences of inclination of 8.7° of panels AF4 and AF5- 51 -Comparison of maximum surface subsidence
from numerical modeling result

Extracted
Panel
AF4, AF5, NB
F3, NB F4

Maximum Surface Subsidence (m)Single Panel Extraction with
Superposition Principle

Entire Panel
Extraction

2.5

2.2

Journal of Novel Carbon Resource Sciences, Vol.7, Feb. 2013

400.5

Elevation (m)400399.5
399
398.5
398
397.5
397
2400

2600

2800

3000

3200

3400

3600

3800

4000

4200

x-coordinate (m)Single Panel Extraction with Superposition Principle

Fig. 16

Fig. 17Comparison of surface subsidence profile from numerical modeling result.

Surface subsidence area due to the entire panel
extraction.

Acknowledgements: The authors would like toacknowledge PT Gerbang Daya Mandiri (GDM)
management for the possibility and opportunity toconduct this study, Dr. Bukin Daulay, Msc., Headof Research and Development for Mineral and Coal
Technology (TekMira) for opportunity given to use 3DEC
Software for numerical analysis, and also members ofMine Planning Laboratory, also Rock Mechanic andMining Equipment Laboratory ITB for all support anddiscussion during conducting this research.
References
1)
2)

Entire Panel Extraction

PT LAPI ITB, Report submitted to PT Gerbang Daya
Mandiri (2012).
W. Keilich, Coal Operator’s Conference, 313, New South Wales (2006).

Fig. 18 Surface subsidence area due to the single panel
extraction using superposition principle.

3)
4)
5)
6)
7)

8)

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P. R. Shorey, International Journal of Rock Mechanics& Mining Sciences, 31(1), 23 (1994).
S. S. Peng, Coal Mine Ground Control, 1st ed., John Wiley & Sons, New York (1978).
B. N. Whittaker and D. J. Reddish, Subsidence: Occurence, Prediction, and Control, 1st ed., Elsevier SciencePub. Co., New York (1989).
Itasca, 3DEC User’s Guide, version 2.0, Itasca Consulting Group Inc., Minnesota (1999).
V. Adamek And P. W. Jeran, Predictions of Surface
Deformation over Longwall Panels in the Northern Appalachian Coalfield, Report of Investigations 9142 U.S.
Bureau of Mines (1987).
D. E. Munson and W. F. Eichfield, Evaluation of European Empirical Methods for Subsidence in U.S. Coalfields, Springfield, Albuquerque (1980).