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HAGI-IAGI Joint Convention Medan 2013
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Landslide Vulnerability on the Main Road of Malino –
Sinjai
Based on Geological Aspects
A. M. Imran1, Busthan Azikin1,2, Sultan1,2
)
Lecturer of Geology Dept., Hasanuddin University,
Kampus Unhas Tamalanrea, Km 10, Makassar
90245 Email: mudimran@yahoo.com, Telf. 0411586227, fax.: 0411-586015
2)
Doctoral Students of Civil Engineering Study
Program, Hasanuddin University.
Abstract
The study area lies on the north-east slope of
Lompobattang Mountian. Due to unstabil landslide often
occurs both in small and large scale, especially in the rainy
season. Morphology consists of steep slope mountaneous
area with lithology of young volcanics.
It was applied detail geological surface mapping that is
involving lithology, morphology and geological structure.
Measuring the resistivity of rocks was done by applying
geoelectric resistivity at two locations. The wire was 500
meter widespread crossing Manipi – Malino road that
parallel to the slope
The result of the study relies that morphology of the study
area is very steep with slope 60O. The are consists of
intercalation between volcanics breccia, tuff and lava. The
rocks are commoly highly altered and found crack/faults.
Based on the analysis, it is concluded that the are three
types of sliding plane: sliding plane lies between soil and
tuff, discontinuities of crack and/or fault, and bedding plane
between lava and overlying rocks.
Key word: Vunerabilty index, volcanic breccias, sliding
plane, geological control
Sari
Wilayah studi terletak pada lereng bagian atas di sebelah
utara timur laut puncak G. Lompobattang. Wilayah ini
sering mengalami longsoran baik skala kecil maupun skala
besar pada setiap musim hujan akibat wilayah tersebut
relatif kurang stabil. Morfologi wilayah ini merupakan
daerah berbukit terjal dengan batuan penyusunnya adalah
vulkanik muda.
Metode yang dilakukan dalam penelitian ini adalah
pemetaan geologi detail secara permukaan yang
menyangkut litologi, morfologi dan struktur geologi.
Pengukuran resistivitas batuan dengan alat geolistrik pada
dua titik dengan jarak bentangan 500 meter. Pengukuran
dilakukan dengan memotong jalan poros Malino – Manipi
atau mengikuti kemiringan lereng.
Hasil penelitin mengungkapkan bahwa morfologi wilayah
studi sangat terjal dengan kelerengan mencapai 60O.
Litologi penyusun wilayah penelitian adalah breksi
vulkanik, tufa dan lava dengan posisi stratigrafi
berselingan. Kondisi batuan telah terlapukkan kuat. Selain
itu kondisi wilayah juga banyak ditemukan rekahan dan
liniement serta gawir-gawir. Berdasarkan analisis tersebut
maka disimpulkan bahwa terdapat bidang-bidang gelincir
antara lapukan tufa dengan batuan asalnya, diskontinuitas
oleh rekahan/patahan dan adanya lapisan lava
(impermeabel) diantara lapisan tufa dan breksi vulkanik.
Kata Kunci: Kerentanan longsor, batuan vulkanik, bidang
gelincir, kontrol geologi
Introduction
Based on the data from Badan Nasional Penanggulangan
Bencana (BNPB), there are 11 regencies in South Sulawesi
which are vulnerable with natural hazard (especially
landslide) namely Tanah Toraja, Luwu, Pinrang, Sidrap,
Wajo, Soppeng, Bone, Sinjai, Bantaeng, Gowa dan
Makassar (Harian Fajar, 5 Nopember 2010). The occurence
of landslide also took place in many main roads in South
Sulawesi. The quick survey of Busthan et al. (2011) found
that the land slide occurred in many palces alaong the main
road connecting of Malino (Gowa Regency) - Manipi
(Sinjai Regency). This is due to the geological condition
consisting the area. The lithology is mainly made up of
young Lompobattang Volcanics which is not well
compacted and highly weathering.
The landslide occured in 2006 triggered flash flooding in
Sinjai Regency, in which material of landslide filled the
upstream of Mangottong River in east side G. Bawakaraeng
and block the water flow of the river for a while. This flash
flood have coused many losses in Sinjai Regency. At least
7 small landslides are identified along the main road of
Malino – Sinjai that have been occurred in 2006.
Study area is located on the northeastern slope of G.
Lompobattang, or laying on the main road from
Gowa Regency to Sinjai Regency. This study aims to
analyze the vulnerability of landslides on the main
road of Malino (Gowa Regency) – Sinjai Regency.
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Peak of G.
Lompobattang
enelitian
Bone Bay
Study Area
structure is relatively parallel to the Walanae Fault which is
the main fault in South Sulawesi. In addition to minor
faults on the field are found many openings or fractures,
with wide openings up to 20 cm, and the escarpment.
Geoelectric resistivity observations at 2 stations indicates
two potential locations of landslide (Figure 2). a) The
potential location that has slip plane between the weathered
tuff (resistivity values from 15 to 29.5 ohm.m) with basaltic
intrusive rock (resistivity 400 - ohm.m 1000). This slip
plane is a crushed zone fault that occur up to 25 meters
depth. b) The second potential landslide locates at the upper
part of the road. The slip plane lies between weathered tuff
(resistivity 15 to 29.5 ohm.m) and its one (resistivity 50200 ohm.m) in the depth of 7.5 meters below surface.
Figure 1 Study area and Topography of G. Bawakaraeng.
Research Methodology
Jalan Poros Manlino-Sinjai
This research was carried out by several methods such as
detail geological mapping in areas that potential for
landslides and in areas that have been occured landslide
including distribution of landslides material, type of
material and material origin.
Furthermore it was applied a geoelectric resistivity
multichannel at two locations. The aim of applying this
method is to determine the water content of subsurface
which is considered as a slip plane.
Result and Discussion
Topographic conditions of the study area has steep slope
upto 85O (Figure 1). Such steep slope is one of the factors
that determine the stability of the slope. If there are enough
triggers such as vibration on the road, or the heavy rain, it
will tend to be unstable and eventually will occure
landslide. Several researches concerning a slope control to
the landslide on the volcanic region are namely by
Kawamura, et al., (2009). Lithology also play an important
role in slope stability. Geology of the study area is
composed of volcanic belonging to the G Lompobattang.
The volcanics consists of Miocene Camba Volcanics
(Tmcv), Pliocene Baturappe-Cindako Volcanics (Tpbv) and
Pleistocene Lompobattang Volcanics (Qlv) (Sukamto &
Surpriatna, 1982).
Measuremen location of the study is in West Sinjai District
is dominanted by volcanic rocks of Lompobattang
Volcanics seires. The rocks consists of volcanic breccia,
tuff, agglomerate and basalt porphyry. According to
Yowono (1989) G.Lompobattang is the youngest volcano
in South Sulawesi and has cone types (strato volcano). This
volcanic type is made up of intercalation of pyroclastic and
lava rock. Geolocical structure developing in this area are
faulting (liniamen) generally trending north - south. This
Figure 2 Result of the geoelectric survey (Res2div) with
cable line across the main road. Interpretation og
this figure in figure 3.
At the seconds station is located at the steep slope.
Interpretation of geoelectric survey shows the potential
landslide lies in the upperpart of the main road within the
weathered tuff (soil). The slip plane lies between the
weathered tuff (resistivity 2,6 – 25 ohm.m) and its fresh
tuff (resistivity 35 – 200 ohm.m) with a depth until 10
meter (Figure 3). Several researches that used geoelectric
survey such as Sule et al. (2007) in Panawangan, Ciamis.
Based on the above data it can be explain that the main
factors of landslide are slope and lithology. The lithology
made up the study area is dominated by the young
volcanics of Lompobattang Volcanics. The rocks are easely
weathered. The field survey shows that many small size of
rock fall found in the study area. Furthermore the high
weathered rocks also have a high swelling factor (Nurjamil,
et al.. (2005).
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Rainfall in the study area were high with an average of 200
mm / month. Thus rainfall is very supportive as a trigger
landslides. In addition, there are many springs found in the
study area.
Bidang Gelincir
Figure 3 Interpretation of figure 2 indicating 2 slip plane of
potential landslide.
Based on the geoelectric survey done in 2 locations and
detailed geological mapping, it can be interpreted that the
landslides that have occurred in the study area is debris
slump type (Varnes, 1978) or rotational landslide (USGS.
2003). Cross section presented in figure 2 and 3 has a
semicircular slip plane. Based on the mechanism of
landslides that have occurred in that area supported by field
data, landslide occuring in the study area is slum or
rotational type (Figure 5).
In addition to the lithologic condition, stratigraphy also
play an important role for landslide susceptibility. Study of
Yuwono (1989) found that the lithology of Lompobattang
Volcanics is intercalting between tuff and lava. Lava is an
impermeable layer, while tuff is highly weathered. By the
presence of lava within the tuff dan volcanic breccia it can
be acs as a slip plane due to its impermeable.characteristics.
The second main factor that plays an important role is the
geological structure. The structure found in the study area
such as faults, fractures and bedding plane. Fracture
openings generally have sizes from cm to 20 cm. Such
opening is potential to occur landslide (Figure 4). The
geological structure also contributes to the occurrence of
mass movement. Sumaryono and Triyana (2011) explained
that the distribution of fault around the peak of G
Bawakaraeng is commonly normal faults trending
relatively north – south.
Jalan Raya
Bidang
Gelincir
Massa tanah
potensi bergerak
W cos  = R
W sin = T
g
Massa tanah
stabil
Keterangan:
- R= Gaya Penahan (W cos α
- T = Gaya penggerak (W sin α
- α = sudut kemiringan
bidang geincir
Figure 5 Slope condition along the main road connecting
Malino and Sinjai (above), and simplified of
landslide susceptibility in the study area.
Conclusion
Figure 4. Crack of the weathered tuff as a sign to occur
landslide.
Landslide susceptibility in the main road of Malino - Sinjai
is controlled by geological factors such as slope, lithology
and geological structure. These factors are steep slopes
with slope between 35 - 85O, rock type is young and highly
weathered volcanic rocks, as well as it is found a number of
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HAGI-IAGI Joint Convention Medan 2013
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discontinuous such as fractures / faults and escarpmentescarpment.
There are three types of slip plane namely at the boundary
between weathered tuff and its fresh one, at discontinuity
especially faulting and the presence of lava/rock laharik.
While the type of mass movement is debris avalanches
and/or rotational slump.
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