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LOWWALL STABILITY ANALYSIS, CASE STUDY AT BINUNGAN COAL OPEN-PIT
PT BERAU COAL, EAST KALIMANTAN, INDONESIA
Budi SULISTIANTO1*, Ginting J. KUSUMA1, Ridho K. WATTIMENA1,
Irwandy ARIF1,
Lukman HAKIM2, Yogi DANIZAR2, Arief WIEDHARTONO2 1 Department of
Mining Engineering, Institute Technology Bandung, Bandung 40132,
Indonesia
2 PT Berau Coal *Corresponding author: [email protected]
ABSTRACT PT Berau Coal is planning to mine the coal at Binungan
Site, Pit E-7 up to elevation of -120msl, where the elevation of
the outcrop is about 40 msl. A slope failure has occurred at
lowwall in eastern part of the pit. This failure was triggered by
bedding plane which is parallel to the slope face and the
occurrence of ground water in the sandstone layers. It is,
therefore, important that a geotechnical study has to be conducted
to guarantee the stability of lowwall. This paper describes the
site investigations and slope stability analysis. The analysis
result reveals that the slope is not stable until pit-floor reaches
the elevation of -75msl. In order to solve this problem, simulation
by lowering the ground water is carried out. The result shows that
the water level should be lowered using vertical drainholes at RL
-60 msl before the pit floor reached.
Keywords: slope failure, lowwall, site investigations, slope
stability analysis.
INTRODUCTION
One of the coal open pit run by PT Berau Coal (PTBC) at Binungan
Mine Site is E7 Pit. A slope failure has occurred at eastern block
lowwall of the pit on October 2008 (Figure 1). This type of failure
was indicated as plane failure due to bedding plane and the
occurrence of ground water in the sandstone layers. Currently, PTBC
is planning to carry out the coal mining until the elevation of
Reached Level (RL) -120 msl, which consequently will create a
higher lowwall since the elevation of coal outcrop is about 40 msl.
As reported by the mine engineer, after slope failure, the mine
floor heaving occurred, and also seepage occurred at some point at
the lower part of lowwall. Therefore, a geotechnical study is
necessary to be conducted in order to guarantee that the lowwall is
stable.
The analysis is conducted using the limit equilibrium method
which is Janbu method by computer program Slide ver 3.0 from
Rocscience. A simulation by applying the ground water pressure is
carried out in order to know the effect of ground water. The slope
stability analysis was conducted in a cross sections as
representative of the site condition.
SITE INVESTIGATION
Geotechnical Investigation
Rock stratification at Binungan E7 pit is dominated by sandstone
and mudstone. The dip of the bedding planes vary from 17o 21o. The
rock slope lithology was obtained directly by visual observation
from site investigation and also interpreted from exploration
borehole given in cross-section as shown in Figure 2.
The lowwall slope is formed after excavating the exposed coal
seam E, and almost single slope. It was found that the rock layer
below coal seam E is still stratified rock which consist of the
following material (from upper to lower) : fine Sandstone (called
sandstone-1); Mudstone ; coarse Sandstone (called sandstone-2) ;
thin coal seam; Mudstone.
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(a)
(b) (c)
Figure 1: Failured Lowwall Slope
Geological structure mapping was focused on the opened crack and
exposed joint on the ramp. The mapping result shows that the
lowwall slope was relatively stable and there was no disturbance
from macro structure even though, relatively large number of micro
structures were observed and these tend to be varied. Figure 3
shows the distribution of joint orientation.
: Seam E : Fine Sandstone : Mudstone : Coarse Sandstone : Seam
D
Figure 2:Rock stratification [1]
East West
Sliding Direction
Lowwall
Highwall
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Figure 3:Distribution of Joint Orientation
The direction of the bedding planes was found to be parallel
with the slope orientation, and the joint directions intersect each
other and also intersect with the slope orientation. Nevertheless
from site investigation, it can be identified that the joint acted
only as a trigger in slope failure, and the main cause of the
failure was due to the bedding planes. Based on the lithology found
in the site, rock samples were then taken for laboratory testing.
The test result is shown in Table 1.
Table 1. Rock Parameter [1]
No Lithology (kN/m3) c (kPa) (.....0) 1 Mudstone 22.4 126 23.5 2
Coal 13 200 35.0 3 Sandstone 24.9 119 22.4
Ground Water Investigation
The sandstone-2 layer is not as compact as the sandstone-1
layer. Because of sandstone-2 position which is confined by
impermeable layers (mudstone and thin coal seam), it has potential
as water carrying materials with high water pressure. The water is
believed played an important role in the slope failure. In order to
reduce the water pressure, some vertical drain holes were drilled
in the slope face. The 100mm diameter of drain holes were drilled
vertically at the location above RL -30 - -40msl within 50m
interval laterally. The depths of the holes were varied from about
80m. The result of water flow rate measurement showed that there
was a decreasing of the flow rate with time.
Ground water table was being monitored periodically by
piezometer. The lowering of ground water caused by horizontal drain
holes, gives positive effect on slope stability. Figure 4 shows the
drain hole, and Figure 5 shows water flow rate monitoring
result.
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Figure 4:Vertical Drain Hole
Figure 5:Water Flow Rate Monitoring Result
Rain Intensity
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LOWWALL SLOPE STABILITY ANALYSIS Slope stability analysis in
lowwall was conducted to determine the FoS of designed slope which
considered the rock stratification at cross section 65 (Figure 6a
and 6b), and to find safe level of ground water during the mining
process. The calculation was based on limit equilibrium using Janbu
method [2]. The rock parameter input for this slope stability
analysis is obtained from laboratory test (Table 1) and the weak
layers parameter is obtained from back analysis, which are cohesion
is 60 kPa and internal friction angle is 130. Figure 6c and 6d show
the result of analysis, which the sliding plane is simulated as a
part of existing bedding plane and the ground water condition is
modeled as distributed pressure applied at the bottom of mudstone
layer.
(a) (b)
(c) (d)
Figure 6:Slope Stability Analysis Result for Final Design; (a)
Designed lowwall map, (b) A-A cross section, (c) Analysis result
without ground water lowering,
(d) Analysis result with ground water lowering
When the ground water is not drained by vertical drainhole, the
water pressure shown in Figure 6(c) and the result of FoS is 0.787
that means the slope is not stable and mudstone layer will be
sliding. Based on ground water flow rate measurement (Figure 5)
which showed that ground water still exist in the sandstone layer
even the vertical drainhole is constructed, so the model of water
pressure acting at mudstone layer will be changed like shown in
Figure 6(d) and the FoS is 1.269. It means that the slope is in
critical condition because FoS is still lower than 1.3. Considering
this condition and the occurrence of discontinuities (Figure 3),
therefore, in order to assure the mining process could be done
until level RL -120 msl, it is suggested to construct additional
vertical drain hole on level RL -60 and -90 msl. These vertical
drainhole construction should be immediately conducted when mining
pit floor has reached those each level to maintain tolerable
groundwater pressure.
Vertical Drainhole
A
A Vertical Drainhole
RL +40
RL -30
RL +40
RL -30
RL -120
1.2690.787
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CONCLUSION
From the above analysis, it is concluded that the targeted pit
floor can be reached by applying the vertical drain hole due to the
fact that the ground water is exist. Based on simulation which has
been carried out, it is revealed that to reach mining elevation of
RL -120 msl, vertical drain hole must be constructed at RL -30. In
order for assuring the stability condition, additional vertical
drain hole should be constructed at RL -60 and -90msl.
ACKNOWLEDGMENTS
The author would like to acknowledge PT. Berau Coal management
for the possibility and opportunity to conduct this study.
REFERENCES [1] PT Lapi ITB, 2008, Lowwall Stability Binungan Site
E7 Pit, East Kalimantan Indonesia, Report
submitted to PT Berau Coal. [2] Hoek,E. and Bray,J.W, 1981, Rock
Slope Engineering, Institution of Mining and Metallurgy,
London.
