10th Asian Regional Conference of IAEG (2015)

Slope Stabilization for Coal mining In Prabumulih District, South Sumatera Province,Indonesia
Hilman Damanhuri AL AHFAS1), Febri HIRNAWAN2), Irvan SOPHIAN2),
Nur KHOIRULLAH2), Agus Wiramsya OSCAR2)(1) PT. Soilens. Bandung, West Java, Indonesia.
E-mail:damanhuri.hilman@gmail.com(2) Laboratory of Engineering Geology, University of Padjadjaran, Bandung, West Java, Indonesia
Abstract
The succed of open pit mining is supported by mine slope stability because it islinked with safety issue and mining productivity. To obtain safe and economic slope design,
geotechnical study needed such as geomechanics analysis and slope stability analysis.
Geomechanics analysis conducted from few samples to obtain laboratory result of physicaland mechanical soil and rock properties. The rock characteristic of slope, rock mechanics
are needed to calculate safety factor digitally.
This research is conducted to calculate safety factor of mine slope using seismic load
as external factor of slope stability. The location of this research lies at area that will be coalmining area. The exploration drilling shows that research area consisted of claystone,
carbonaceous claystone, sandstone and coal as Muaraenim Formation.
The result of slope stability analysis shown that mine slope is safe and economic at
50 meters depth with slope angle no more than 45 degrees considering of seismic condition
with α = 0.1 g.
Keywords: Coal Mining, Slope Stability Analysis
1.

Introduction

northwest – southeast trends antiklinorium (Gafoer,et.al, 1986). Muaraenim Formation consisting
lithologies namely sandstones, siltstones, fossiled
claystones with light yellow colored and coal lenses
containe iron oxide as concretion and thin strata.
Coals in this Formation are almost low grade lignit.
Only on several spots it change becomes high grade
coal.
The accelerations in those zones vary between
0.03g to 0.30g (Masyhur Irsyam, et al., 2010). From
the attached map, the site is located in the second
seismic zone giving a seismic coefficient of 0.10g.

The landslide or often called the avalance is ageological phenomena that happened because themovement of the rock or soil with various types. In
general, the landslide caused by two factors which isdriving factor and the triggering factor. Driving factor
are factors that influence the condition of own
material, while the triggering factor is a factor thatcausing the materials moves.
The succed of open pit mining is supported bymine slope stability because it is linked with safetyissue and mining productivity. To obtain safe andeconomic slope design, geotechnical study needed
such as geomechanics analysis and slope stability
analysis. Geomechanics analysis conducted from few
samples to obtain laboratory result of physical andmechanical soil and rock properties. The rockcharacteristic of slope, rock mechanics are needed tocalculate safety factor digitally.
Geology of research area consisting by Late
Miocene – Early Pliocene Muaraenim Formation(Tmpm) and Late Pliocene – Pleistocene KasaiFormation (Qtk). The geological structure are

2.

MethodGeotechnical drilling is carried out to obtain soil
and subsurface rock geomechanics. Rocks and soils
sample are taken from geotechnical drilling data andmechanical properties tested at laboratory. The resultof laboratory tests are used to calculating slope
stability analysis.
Slope stability analysis using slope stability
analysis software and simulating safe mine slope with1

10th Asian Regional Conference of IAEG (2015)

30, 40, 50, 60 and 70 meters height variation; 20o, 25o,30o, 35o, 40o, 45o and 50o degree slope variation;using natural ground water level; static andconsidering seismic load 0.1 g as horizontal seismic
and 0.05 g as horizontal seismic.
Analysis method using circular failure because
rocks in the research area generally can be classified
as soil based on strength index classification (Hoek &Bray, 1977), thus the calculation did by using Bishopcalculation. Safe mine slope is has more than 1.25
value of safety factor in static condition (Bowles. J. E,1986) and more than 1.05 value in seismic condition.

Single slope stability analysis with ground waterlevel 6.5 meters below surface shown the slope is safe
at 30, 40 and 50 meters with overall slope, also at 60
meters depth with 40o slope and 70 meters depth with
20o slope in static condition. While slope stability
analysis using seismic load shown the slope is safe at
30 and 40, also 50 meters with 20o, 25o, 30o, 35o, 40oand 45o of slope angle (figure 3).
The figure 4 show relationship between safety
factor and slope angle with various depth in static
condition and seismic condition. Safety factor valueof slope are inversely proportional with depth/slope
height and slope, safety factor value will be little
worth where the depth or slope height and slope angle
are getting greater.

3.

Result
The lithologies found in research area areclaystones, sandy claystones, clayey sandstones,
sandstones and coal from Muaraenim Formation.
Subsurface profile made from several geotechnicaldrilling data to determine and reconstructedsubsurface rock strata that will be used for slope
stability analysis profile. Rock strata in researcharea has 5o dipping to south east (Figure 2).
Physical and mechanical properties of soil andsub-surface rocks has various value based on thelaboratory test results. The table 1 show the physicaland mechanical properties used for mine slope
stability analysis calculation. Soil and rock properties
method are obtained by physical test (unit weighttest) and mechanical test (direct shear) in laboratory.

4.

Conclusion

Slope stability analysis results show that mineslope safe (stable condition) at 60 metres depth with
40o slope angle maximum in static condition, while inseismic condition mine slope safe at 50 metres depthwith 45o slope angle and 60 metres with 20o slope
angle maximal. The mine slope with depth and slope
angle less than 50 metres with 45o slope angle ishighly recommended result based on slope stability
analysis using seismic coefficient, but it is suggested
if mine slope opened less than these slope stability
result because not all of the other factors both ofinternal and external factor have considered in theseslope stability calculation.

Table 1. The physical and mechanical properties ofsoil and rocks used for slope stability analysis.
NoMaterials
γC (kPa)φ (°)(kN/m3)
1
Soil
16,34
12,00
10,25
2
Claystone 1
18,26
85,26
11,92
3
Claystone 2
18,23
105,8413,52
4
Claystone 3
15,18
150,9229,27
5
Claystone 4
17,17
199,92
23,83
6
Sandstone
15,68
350,84
32,15
7
Coal
13,00
182,00
21,00
8
Sandy
14,21
85,26
19,01
Claystone 1
9Sandy
16,82
70,56
22,30
Claystone 25.

Acknowledgment

We would like thank to PT RMK, Head ofLaboratory of Engineering Geology of University of
Padjadjaran for giving discussion and advice, PT
RMK, the director of PT Komunitas Bangun Bersamaand all sponsors for supporting until this research isfinished.
6.

Note : γ = Unit weight
C = Cohession
Φ = Angle of internal friction

References

Gafoer, S. Cobrie, T and Purnomo, J. 1986. Geologyof Lahat Quadrangles, Sumatera. Bandung : Centerof Research and Development Geology.
Hoek, E ., dan Bray, J, W . 1977. Rock SlopeEngineering. London : The Institution of Mining2

10th Asian Regional Conference of IAEG (2015)

and Mettalurgy.
Irsyam, Masyhur, et al. 2010. Peta Hazard Gempa
Indonesia 2010. Jakarta : Kementrian Pekerjaan
UmumBowles, J.E., 1986. Physical and Geotechnical
Properties of Soil (Soil Mechanics). Jakarta :Erlangga Publisher.

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10th Asian Regional Conference of IAEG (2015)

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Figure 1. Location of research area (taken from Google Earth, not to scale)Figure 2. Sub-surface correlation bassed on two geotechnical drilling boreholes (not to scale).

Figure 3. Mine slope stability analysis result in static condition (left) and seismic condition (right).

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10th Asian Regional Conference of IAEG (2015)

Figure 4. Comparison between safety factor and slope angle at static condition and seismic condition.

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