Kyushu University Global COE Program
Journal of Novel Carbon Resource Sciences, Vol. 7, pp. 53-59, Feb. 2013

Optimalization of Outpit Dump at Asam-Asam Open Pit Coal
Mine, PT Arutmin Indonesia
Budi Sulistianto*1, Tri Karian*1, Kardiansyah*2, Syarif Hidayatullah*2,
Sudirman Widhy*2*1Institut Teknologi Bandung, Bandung 40132, Indonesia*2
PT Arutmin Indonesia, Jakarta 12960, Indonesia(Received November 16, 2012; accepted January 15, 2013)

PT Arutmin Indonesia (PTAI) has a mine site located in Asam-Asam region. There is approximately 52 millionton reserve of low-rank coal. The failure occurred in the overburden dumping area 1-2 DH on March 2012 andfurther dumping is still planned in this area. Therefore, a geotechnical study was conducted in order to characterize
the cause of failure and to guarantee that dumping activity can be continued. Result of analysis shows that the post
failure condition is in stable even though the volume of dumping material increases. However, the dumping process
should be done layer by layer so that compaction process can be happened.

1. Introduction

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PT Arutmin Indonesia (PTAI) is one of the Indonesiancoal mining companies. One of its sites is located in
Asam-Asam region, Banjarmasin (see Fig. 1). AsamAsam Coal consists of six major seams which are seamB until seam G. The thickness of their coal seams varies
from 2-25 m with strike of N70oE and dip around 25o-35o.
Asam-asam coal mine has approximately 52 million ton
reserve of low rank coal at a relatively low stripping ratio
which is around 4.67.
Low rank coal is categorized as a marginal coal deposit.
Moreover its price has been dropped down around $60
recently. Consequently mining operation at Asam-Asammine site has to be optimized in order to reduce mining
cost. One of the measures is to reduce haulage distanceof overburden removal activity. As the consequences,
overburden dumping in Asam-Asam coal mine is locatednear lowwall area and the mining front.
One of overburden dumping area in Asam-Asam site
is outpit dump DH 1-2 (OPD DH 1-2). This overburden
dump height is around 25 meter. Failure has occurred inthis overburden dump on March 2012 as shown in Fig.
2. Currently, PTAI is planning to continue coal mining
operation in Pit 1-2 DH. Since an optimization of hauling
distance is needed, the availability of OPD DH 1-2 isvery important for pit 1-2 DH dumping area. Therefore,a geotechnical study has to be conducted in order tocharacterized the cause of failure occurred in OPD pit
1-2 DH and to guarantee that OPD pit 1-2 DH can be used
for further dumping activity.
The numerical analysis is conducted by using the limit
equilibrium method which is Janbu method1) for composite
failure analysis using computer program Slide ver 3.0
developed by Rocscience. A simulation by applying weakmaterial at base of outpit dump is carried out in order toknow the effect of the weak material on the stability of
dumping area2,3). The slope stability analysis was conductedin cross sections as representative of the site condition.

EAST MULIA

WEST MULIAASAM ASAM

LegendN0475 950

1,900

2,850

3,800
MetersRoad
Reserves Block
PKP2B PT Arutmin

Fig. 1 Investigation area.

Fig. 2 OPD pit 1-2 DH and direction of failure.

2. Site investigation

Material of OPD Pit 1-2 DH is overburden from Pit 1-2
DH which mudstone and sandstone are dominated. It is
calculated from surveying that approximately 6 million
cubic meter material is slide with around 400 m widthand 900 m length. Geometry of sliding area is shown in- 53 -

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

3. Outpit dump stability analysis

Elevation (m)Fig. 3.
The type of failure is predicted as a composite failure
consists of plane failure and circular one, and the sliding
plane based on field observation is represented by thedash line in Fig. 3. It is also known that dumping activity
is carried out by using free dump technique without aproper compaction in some of OPD Pit 1-2 DH.
All of samples obtained from the Standard Penetration
Test of Asm-SPT1101 and Asm-SPT-1103 have beentested in laboratory in order to obtain physical andmechanical characteristics of rock forming OPD Pit 1-2
DH. Physical and mechanical characteristics of eachmaterial obtained from laboratory test carried out on
Asm-SPT1101 and Asm-SPT-1103 is shown at Table 1.
Based on the result of site investigation, it is found thata weak layer is lying at base of dump material which isswamp material. It is also found that this material is in
wet condition due to submerged by water for a long time.
All physical and mechanical properties from laboratory
test on geotechnical borehole Asm-SPT1101 and AsmSPT-1103 will be used for the following numerical study
for the stability of OPD Pit 1-2 DH.

The calculation of slope stability analysis was basedon limit equilibrium using Janbu method for composite
failure analysis1). Slope stability analysis in OPD Pit
1-2 DH section before failure was conducted in orderto determine the failure mechanism. Cross section 2 asshown in Fig. 4 is analyzed for this purpose. Physical andmechanical characteristics of OPD Pit 1-2 DH as shownin Table 1 is used for this analysis. The result is shownin Fig. 5a. Based on this figure, it is found that OPD Pit
1-2 DH is in stable condition and this result is different
with the actual field condition. Therefore, it might becaused by the reduction of material strength due totheir deterioration. In order to estimate the parameter ofmaterial after deteriorating, back analysis was conducted
and the weak layer’s parameters were obtained fromtrial and error back analysis. The cohesion and internal
friction angle are 1 kPa and 12.8 deg., respectively
Fig. 5b show the result of analysis for OPD Pit 1-2 DHbefore failure from composite failure possibility usingback analysis material for weak layer at the base of inpit
dump as shown in Table 2.

60
40
20
0
-20
Original Topography

Fig. 3

Table 1Sliding Plane

Geometry of failure at OPD pit 1-2 DH.

Physical and mechanical characteristics of dumping material4)
Asm-SPT 1101

Material

Planned Geometry

Outpit Dump Pit 1-2 DH Surfaceρsat(gr/cm3)

Asm-SPT 1103

C (kPa)ϕ (°)

Material

Dump

1

1.90

46

1

12.80

1

Spt1

1.801

803

12.801

Spt2

1.80

1

200

3

12.80

1

Spt3

1.80

1

250

3

12.80

1

Spt4

1.801

1933

12.801

Spt5

1.80

1

180

3

1

Spt6

1.801

Spt7

1.80

1

Spt8
base

ρsat(gr/cm3)

C (kPa)

ϕ (°)

Dump

1.90

1

46

1

12.801

Spt1

1.801

344

12.801

Spt2

1.80

1

4

12.801

Spt3

1.80

1

154

4

12.801

Spt4

1.801

1544

12.801

12.80

Spt5

1.80

1

120

4

12.8011873

12.801

Spt6

1.801

2744

12.801

227

3

12.80

1

Spt7

1.80

1

260

4

12.801

1.801

3203

12.801

Spt8

1.801

2944

12.801

1

2

2

base

1

2

26.952

1.80

175

26.95

1.80

80

175

Source:1
Table 3.8 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
2
Table 4.5 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
3
Geotechnical Drilling Report Asm-SPT 1101
4
Geotechnical Drilling Report Asm-SPT 1103

- 54 -

Optimalization of Outpit Dump at Asam-Asam Open Pit Coal Mine, PT Arutmin Indonesia

Based on the result of analysis, it can be known that a
failure occurs only at the front side of OPD Pit 1-2 DH.
Providing that front side material moving forward after
failure occurred, stability analysis can be conducted byomitting front side material as shown in Figs. 6 and 7. It
can be seen the large failure at material pile was inducedby the failure of front side material.
By using surface condition after failure and using thesame sliding plane, post failure analysis is then conductedin order to know the stability of outpit dump after failure.
Result of analysis can be seen in Fig. 8. From the resultof post failure analysis it can be known that post failure
at OPD pit 1-2 DH is in stable condition. However, inorder to obtained higher certainty of stability, reslopingis still conducted. Fig. 9a shows the actual field conditionof OPD Pit 1-2 DH after resloping activity is conducted

Sect_5

Sect_1

Sect_4
Sect_3
Sect_2

Fig. 4

Cross section line location.

6.578

1.147
0.361

4.152

(a)

Fig. 6 Result of outpit dump failure analysis by omitting the front side material (SF = 1.147).

0.731
0.320
1.075
0.841

Fig. 7 Result of outpit dump failure analysis at center side by omitting the front side material(SF=0.73).

(b)(b)
Fig. 5 Analysis of OPD pit 1-2 DH (a) using SPT test
material (SF =4.15); (b) using back analysis material (SF=0.84).

Table 2

Physical and mechanical characteristics offailure area OPD pit 1-2 DH 4,5)

Materialρsat(gr/cm3)

C (kPa)ϕ (°)

Dump Material

1.90

46

12.80

Soft Base Material

1.90

11

12.80

Hard Base Material

1.80

175

26.95

Note: Based on back analysis
1

2.913

1.995

Fig. 8 Result of OPD pit 1-2 DH failure analysis atblock 10 after failure (SF=1.995).

- 55 -

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

1.531

(a)
Fig. 10

Result of optimized OPD pit 1-2 DH at section
3 for circular sliding analysis (SF=1.53).

1.387

(b)
Fig. 9 Field condition of OPD pit 1-2 DH after resloping
activity (a) in June 2012; (b) in October 2012.
in June 2012 while Fig. 9b shows its condition in October
2012. Based on the result of further field investigation
OPD pit 1-2 DH is in stable condition which is in a goodcorrelation with the numerical analysis on post failure
condition of Pit 1-2 DH.
For optimizing the mining activity, OPD Pit 1-2 DH
is still needed as a short range dumping area. In order to
optimize OPD pit 1-2 DH after failure, Post failure analysis with the addition of another dumping material frompit 1-2 DH is carried out. Optimization of post failure
OPD pit 1-2 DH is carried out by adding dumping material until the overall slope height is 60 meter and angle of
10o while the single slope height is 10 meter and angle of
35o with failure material act as base of Optimized OPD
Pit 1-2 DH model. Figs. 10 and 11 show the result of optimized OPD Pit 1-2 in Section 3 for circular sliding analysis and composite sliding analysis. The same analysis isconducted for OPD pit 1-2 DH in section 5. The result isshown in Fig. 12.
At the north side of Optimized OPD Pit 1-2 DH plan,
there is a roadway which is used by local people aroundAsam-Asam Mine site (see Fig. 2). The distance between
the roadway and the Optimized OPD pit 1-2 DH planis around 150 meter. In order to know the deformationof optimized OPD Pit 1-2 DH numerical analysis using

Fig. 11 Result of optimized OPD pit 1-2 DH at section
3 for composite sliding analysis (SF = 1.38).

1.990

Fig. 12 Result of optimized OPD pit 1-2 DH at section
5 for circular sliding analysis (SF=1.99).

- 56 -

Optimalization of Outpit Dump at Asam-Asam Open Pit Coal Mine, PT Arutmin Indonesia

Finite Element Method is carried out. It must be ensured
that the deformation should not exceed the buffer zone
around 100 meter from the roadway. Numerical analysis
is carried out at Section 2, Section 3 and Section 5 of
Optimized Pit 1-2 DH.
Table 3

Material characteristics for numerical analysis in
OPD pit 1-2 DH for Sections 2, 3 and 6 are shown in
Tables 3, 4, and 5 for Section 2, Section 3, and Section 5
respectively. The results of numerical analysis are shownin Figs. 13-15.

Physical and mechanical properties for optimized OPD pit 1-2 DH analysis using finite element method in
section 2 4)
Materialρsat(gr/cm3)

C (kPa)ϕ (°)

E (MPa)v

Dump Material

1.901

461

12.8011304

0.34Soft Base Material

1.801

12

12.801734

0.34

Hard Base Material

1

3

4

0.34

3

1.80

175

26.95

285

Source:1
Table 3.8 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
2
Based on back analysis
3
Table 4.5 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
4
Table 3.4 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia

Table 4

Physical and mechanical properties for optimized OPD pit 1-2 DH analysis using finite element method in
section 3 4)
Materialρsat(gr/cm3)

C (kPa)ϕ (°)

E (MPa)v

Dump Material

1.901

461

12.801

1304

0.34

Spt1

1.801

802

12.801

734

0.34

Spt2

1

1.80

200

2

12.80

1

285

4

0.34

Spt3

1.801

2502

12.801

2854

0.34

Spt4

1

1.80

193

2

12.80

1

285

4

0.34

Spt5

1

1.80

180

2

12.80

1

285

4

0.34

Spt6

1

1.80

187

2

12.80

1

285

4

0.34

Spt7

1

1.80

227

2

12.80

1

285

4

0.34

Spt8

1

1.80

320

2

12.80

1

285

4

0.34

base

1

3

3

4

0.34

1.80

175

26.95

285

Source:1
Table 3.8 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
2
Geotechnical Drilling Report Asm-SPT 1101
3
Table 4.5 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
4
Table 3.4 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia

Table 5

Physical and mechanical properties for optimized OPD pit 1-2 DH analysis using finite element method in
section 5 4)Materialρsat(gr/cm3)

C (kPa)ϕ (°)

E (MPa)v

Dump Material

1.901

461

12.801

1304

0.34

Spt1

1.801

343

12.801

734

0.34

Spt2

1.80

1

3

12.80

1

285

4

0.34

1.80

1

154

3

12.80

1

285

4

0.34

1.80

1

154

3

12.80

1

285

4

0.34

1.80

1

120

3

12.80

1

285

4

0.34

1.80

1

274

3

12.80

1

285

4

0.34

Spt7

1.80

1

260

3

12.80

1

285

4

0.34

Spt8

1.801

2943

12.801

2854

0.34

base

1

2

2

4

0.34

Spt3
Spt4
Spt5
Spt6

1.80

80

175

26.95

285

Source:1
Table 3.8 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
2
Table 4.5 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia
3
Tabel Geotechnical Drilling Report Asm-SPT 1103
4
Table 3.4 of Final Report Stability Analysis of Final Highwall in Asam-Asam and Mulia Mine PT Arutmin Indonesia- 57 -

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

Fig. 13 Result of optimized OPD Pit 1-2 DH analysis using Finite Element Method at section 2 (SRF=1.33).

Fig. 14

Result of optimized OPD Pit 1-2 DH analysis using Finite Element Method at section 3 (SRF=1.55).

Fig. 15

Result of optimized OPD Pit 1-2 DH analysis using Finite Element Method at section 5 (SRF=1.41).

Deformation obtained by numerical analysis using
Finite Element Method shows that no more than 1.32
meter of deformation will occur at the OPD pit 1-2 DH.
Therefore, based on analysis of optimized OPD Pit 1-2
DH in sections 2, 3 and 5 it can be understood that the
optimized design for OPD Pit 1-2 DH can be applied.
However, it must be noted that dumping process should
be done layer by layer so that compaction process can behappen and water contained inside of the swamp material
can be drained by the compaction process. Moreover,in order to ensure safety dumping operation, after re-Fig. 16 Monitoring point in OPD pit 1-2 DH.

sloping the dumping material pile at failure area, PTAI
has installed some monitoring point as shown in Fig. 16
which is monitored continuously.

Fig. 17 Location of monitoring point in OPD pit 1-2
DH after resloping is carried out.

- 58 -

Optimalization of Outpit Dump at Asam-Asam Open Pit Coal Mine, PT Arutmin Indonesia

Fig. 18

Movement of D3, D4, and D5 monitoring points in OPD pit 1-2 DH.

Fig. 17 shows the position of monitoring point aroundsections 1 and 2. Monitoring data collection is carried out
between early November and late December. Based on
analysis using monitoring data, it can be understand thatmovement of points D3, D4 and D5 are eastward rather
than northward. It might be due to the existence of pond.
The result of movement is shown in Fig. 18. From Fig. 18,
it can be confirmed that a relatively small deformation
is occurred as predicted by numerical calculation using
Finite Element Method.

4. Conclusions

for mining activity in Pit 1-2 DH until reaching 60 mheight with the assist of monitoring.
Acknowledgements: The author would like toacknowledge PT. Arutmin Indonesia management forthe possibility and opportunity to conduct this study.
References
1)
2)

Failure occur at OPD Pit 1-2 DH is caused bydeterioration of its material and soft base lying underneath
the OPD Pit 1-2 DH. The results of a series of analysis
show that at the beginning, the failure occurred onlyaround the front side of dumping area. However, since
the material is deteriorated, a failure developed until thecenter side of dumping area. Based on the result of post
failure analysis, it can be found that ODP Pit 1-2 DH isin stable condition and can be continued as dumping area

3)
4)
5)

- 59 -

E. Hoek and J. W. Bray, Rock Slope Engineering, 2nded., Institution of Mining and Metallurgy, London(1981).
B. Sulistianto, G. J. Kusuma, R. K. Wattimena, I. Arif,
L. Hakim, Y. Danizar, and A. Wiedhartono, The 4thInternational Symposium on Novel Carbon Resource
Science, Environmental Science and Technology,
Shanghai (2009).
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Sulaiman, The 16th International Symposium on Mine
Planning and Equipment Selection, Bangkok (2007).
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