10th Asian Regional Conference of IAEG (2015)

The Analysis of The Slope Stability Design on Pemali, North Bangka,Bangka Belitung Islands, Indonesia
Kemala WIJAYANTI(1), Dio Yudha PRATAMA(1), Dicky MUSLIM(1),Irvan SOPHIAN(1), and Zufialdi ZAKARIA(1)(1) Faculty of Geological Engineering, University of Padjadjaran, Indonesia
Abstract
Geotechnical study is important in open-pit mining, especially for the determinationof slope stability design. The object of this research is a tin open-pit mine which located inPemali, North Bangka, Bangka Belitung Islands, Indonesia. This research was conductedbecause the safety factor of the actual slope in Pemali open-pit is below 1,2, which means
critical. This is caused by the lack of material strength because of the strong weathering
process. The purpose of this research is to redesign the slope in Pemali open-pit to obtain astable safety factor value. The fieldwork consists of measuring slope geometry, soil
mechanics properties, and disturbed soil sampling. Triaxial test and direct shear test are also
carried out onto samples. Then, redesign the slope geometry and compute the safety factor
value. The results of the slope stability analysis using simplified Bishop method shows thatthe actual slope must be changed to obtain the safety factor value above 1,2. The 25° actual
overall slope must be changed to 15° on GT4, 15° on GT5 and 20° on GT6. Then the 60°
actual bench slope must be changed to 45° on GT4 and 55° on GT6.
Keywords: slope stability, safety factor, geotechnics, Pemali
1.

Introduction

condition in the mine area, in order to avoid the slope
failure that reduces the effectiveness of the mining
activities.
Nowadays, the research area, which is the area oftin mine that have been abandoned for a dozen years,
consist of not solid material or very weatheredmaterial and very steep slopes that makes mine slope
getting slides in some places. Due to the mining
reclamation, the slope should be recalculated in order
to avoid the risk of mining disaster.
2.

Methodology

This research is divided into two phase. The firstphase is field measurement and the second phase isstudio processing. The field measuring data areconsist of slope geometry (length, height and slope)and soil mechanic properties (angle of internal
friction, cohesion, water content and weight pervolume). Triaxial test and direct shear test are also
carried out onto samples. The field measurement datathen processed into a geotechnical zone map based on
geotechnical characteristic in the research area
(Figure 3).
The last phase is slope simulation in order toobtain the slope with the biggest safety factor value.
Bishop’s safety factor method is carried out in theslope simulation because in Arief (2007), Bishop’sFig. 1 Research Location. Pemali Mining
Area, North Bangka, Bangka Island,Indonesia (black square).
The research is located in Pemali mining area,North Bangka, Bangka Island, Indonesia. The Pemali
tin mine has existed since decades ago, and left for adozen years. In 2012, the mining area is reopened for
production (Sujitno, 1990). Therefore, mine slope
review, analysis and investigation is needed because
there is a significant difference in the material1

10th Asian Regional Conference of IAEG (2015)

simplified method is the most often used in slope
stability analysis. Several assumptions were made insimplified Bishop method (Bishop, 1955; Andersonand Richards, 1987):
1. The failure is assumed to occur by rotationof a mass of soil on a circular slip surface
centered on a common point.
2. The forces on the sides of the slice areassumed to be horizontal. Thus, there is noshear stress between slices.
3.
The total normal force is assumed to act at
the center of the base of each slice.
Although, simplified Bishop method does notsatisfy equilibrium, but safety factors derived fromthis method is in agreement with the safety factors
calculated from finite element methods (Wright et al.,1973). The value of safety factor in Bishop’s method
is formulated as follows:out by Timah Ltd. Company (some data not attachedin this paper due to the company copyright). The datathen processed into a geotechnical zone map (Figure3).
In the geotechnical zone map, there are threezones, and each of them has a different characteristic.
Zone 1 consists of kaolin has a white coloured, andstrongly weathered. The soil derived from theweathering of kaolin, feldspar, and less quartz. The
soil physical properties are low toughness, moist, andlow plasticity.
Zone 2 consists of claystone, derived fromstrongly weathered metasedimentary rock, has abrown coloured, low toughness, moist, and mediumplasticity. Below this material, there is a purple
coloured claystone from weathered phyllite, lessquartz, soft, moist, and high plasticity. In this zone,
the weathered material that oxidized and has a
reddish colour can be found. Weathering process will
be reduced when the material is getting deeper.
The lithology in zone 3 is intrusion of granite thatformed into sandstone and claystone with somegravels because of the weathering process. Have alow toughness, low plasticity, and moderately moist.

Fig. 2 Stresses and Forces Acting on a TypicalSlices.

(1)
Wherec’ΔxW
N’NUαΦ: Cohesion: Slice Width: Weight of Slice:N–U: W cos α: Pore Pressure Force: Slice base inclination: Friction Angle
Fig. 3 Geotechnical Zone Map of Research Area

3.

Results and Discussion
The actual mine slope in Pemali mining area has
certain standards and identical on all sides. Theslopes are 25° in overall slope and 60° in bench slope.
Then the slope redesign refers to the drilling dataLithology characteristic, angle of internal friction,cohesion, water content and weight per volume data
is obtained from the field measurement that carried210th Asian Regional Conference of IAEG (2015)

Table 1 Calculations Result of Safety Factor Value on GT4

Table 2 Calculations Result of Safety Factor Value on GT5

Table 3 Calculations Result of Safety Factor Value on GT6

from GT4, GT5 and GT6 borehole by changing theslope into several slope with a certain interval toobtain the slope with the highest safety factor value.
The slope simulations use 5° interval from 15° to 30°in overall slope and 5° interval from 40° to 60° in
bench slope (Table 1, 2, and 3).
In GT4, from surface to 45 meters below thesurface, consist of brown topsoil with metamorphic
gravel which has a value of density 16,474 kN/m³,cohesion value 15,690 kN/m², and angle of internal
friction 12,5°. Yellow clay with quartz and feldspar
which has a value of density 18,435 kN/m³, cohesion
value 6,276 kN/m², and angle of internal friction
18,22°. Weathered phyllite which has a value of
density 18,239 kN/m³, cohesion value 15,690 kN/m²,and angle of internal friction 19,5°. Sandstone which
has a value of density 18,141 kN/m³, cohesion value
12,846 kN/m², and angle of internal friction 19,85°.
Weathered hornfels which has a value of density
18,141 kN/m³, cohesion value 26,084 kN/m², andangle of internal friction 28,5°. And mixing clay withgravel which has a value of density 17,651 kN/m³,cohesion value 36,282 kN/m², and angle of internal
friction 21,9°.
In GT5, from surface to 48 meters below thesurface, consist of brown topsoil which has a value of
density 19,416 kN/m³, cohesion value 3,530 kN/m²,and angle of internal friction 9,66°. Clay with gravel
which has a value of density 18,828 kN/m³, cohesion
value 29,320 kN/m², and angle of internal friction
26,51°. Sandstone with gravel of metamorphic rockweathering which has a value of density 18,926
kN/m³, cohesion value 22,456 kN/m², and angle ofinternal friction 20,12°. Weathered kaolinite which

has a value of density 19,318 kN/m³, cohesion value
6,766 kN/m², and angle of internal friction 29,55°.
Weathered schist which has a value of density 18,337
kN/m³, cohesion value 13,826 kN/m², and angle ofinternal friction 17,42°. And weathered granite which
has a value of density 18,239 kN/m³, cohesion value
9,512 kN/m², and angle of internal friction 23,15°.
In GT6, from surface to 54 meters below surface,consist of reddish brown topsoil which has a value ofdensity 18,886 kN/m³, cohesion value 9,904 kN/m²,and angle of internal friction 23,76°. Sandstone withclay which has a value of density 19,024 kN/m³,cohesion value 8,531 kN/m², and angle of internal
friction 28,25°. Sandstone with gravel which has avalue of density 20,210 kN/m³, cohesion value
18,631 kN/m², and angle of internal friction 28°. Very
weathered phyllite which has a value of density
19,230 kN/m³, cohesion value 15,199 kN/m², andangle of internal friction 25,38°. And weathered
granite which has a value of density 19,230 kN/m³,
cohesion value 12,552 kN/m², and angle of internal
friction 22,02°.
The safety limit of safety factor value is based on
the safety factor value classification by Ward (1976).
Because the area has a loose and very weatheredmaterial, landslide is possible to occur in mine slope
if the safety factor value below 1,2 and it could becritical (Table 4).
The result of the simulations are the actual mineslope’s safety factor value is below 1,2. Those are0,869 on GT4, 1,044 on GT5 and 1,027 on GT6.
Therefore the actual overall and bench slope must bechanged to obtain the safety factor value above 1,2.

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10th Asian Regional Conference of IAEG (2015)

Table 4 Ward Classification of Relation Between Safety Factor and Ground Movement (Lubis, 2012)
FInformation< 1,2High vulnerability, it would be high possibility for ground movement.

1,2 – 1,7

Medium vulnerability, ground movements can occur.

1,7 – 2

Low vulnerability, ground movements can occur.

2<Low vulnerability, rare ground movements or ground movements almost never occur.

The actual slope must be changed to 15° inoverall slope and 45° in bench slope to obtain 1,581
safety factor value on GT4. Then on GT5 the actual
slope must be changed to 15° in overall slope and 60°
in bench slope to obtain 1,223 safety factor value.
Furthermore the actual slope must be changed to25° in overall and 55° in bench slope to obtain 1,249
safety factor value on GT6.
4.

References
Anderson, M.G. and Richards, K.S. (1987): Slope
Stability:
GeotechnicalEngineeringandGeomorphology, New York, USA, John Wiley andSons.
Arief, S. (2007): The Basic Concept of SlopeStability Analysis, INCO Ltd. Company, Sorowako.
(in Bahasa Indonesia)
Bishop, A. W. (1955): The Use of Slip Circle in The
Stability Analysis of Slopes, Geotechnique, Vol. 5,No. 1, pp 7-17.
Lubis, Ichwan Azwardi. (2012): Alluvial Tin Mining,
Timah Ltd. Company. (in Bahasa Indonesia)
Sujitno, Sutedjo. (1990): Pemali Structure as The
Main Issues in Bangka, Timah Ltd. Company. (inBahasa Indonesia)
Ward, T.J. (1976): Factor of Safety Approach toLandslide Potential Delineation. Thesis presented toColorado University, Fort Collins, Colorado.
Wright, S.G., Kulhawy, F.H. and Duncan, J.M.
(1973): Accuracy of Equilibrium Slope Stability
Analysis, Soil Mechanics and Foundations Division,
American Society of Civil Engineers, Vol. 99, No.
10, pp 783-791.

Conclusion

The result shows that the actual mine slope has
0,869 safety factor value in GT4 slope, 1,044 in GT5
slope and 1,027 in GT6 slope. It means the actualmine slope is critical, because the safety factor value
is below 1,2.
Afterwards, the slope simulations is carried out
by simulating the slopes with a certain interval andslope conditions. According to the simulations, toobtain the safety factor value above 1,2, the 25°
actual overall slope must be changed to 15° on GT4,15° on GT5 and 25° on GT6. Then the 60° actual
bench slope must be changed to 45° on GT4 and 55°
on GT6.
The changes in value of the mine slope based on
simulation will reduce the risk of landslide and willstabilize the mine slope. Thus, mining activities will
run more effectively and productively.
Acknowledgements
Sincere gratitude is given to the advisors and thetechnical supervisor in this research. Special
appreciation is also given to Timah Ltd. Company
which has provided the opportunity to do thisresearch in the mining area and those who can not bementioned one by one.

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