GEOLOGY AND GEOPHYSICS INVESTIGATION LANDSLIDE AREA

GEOLOGY AND GEOPHYSICS INVESTIGATION LANDSLIDE AREA IN
BLANGTEMUM BADAK UKEN, DABUN GELANG, GAYO LUES DISTRICT
Gartika Setiya Nugraha ST, M.Si(1), Ir. Faizal Adriansyah, M.Si(2), Muchlis S.Si,
M.Sc(3), Ibnu Rusydy S.Si, M.Sc(4)
1)
Jurusan Teknik Geofisika, Fakultas Teknik, Universitas Syiah Kuala
Jl. Tgk. Syeh Abdul Rauf No. 7, Darussalam Banda Aceh 23111, email: [email protected]
2)
Ketua Ikatan Ahli Geologi Indonesia Pangda Aceh, email: [email protected]
3)
Jurusan Teknik Pertambangan, Fakultas Teknik, Universitas Syiah Kuala
Jl. Tgk. Syeh Abdul Rauf No. 7, Darussalam Banda Aceh 23111, email: [email protected]
4)
TDMRC-Unsyiah Jl. Tgk. Abdurrahman kompong Pie Kec. Meuraxa Banda Aceh 23111,
email: [email protected]
ABSTRACT
Based on geologic sides, gayo lues regency had a complex history,whiches marked by
deformation rock cycle, that dissaparated of 3 episode. That is kenozoikum deformation
marked with fault and movement. An old rock in this segment filled by high temporation of
metamorphic rock.Above this rock precipitated lamstone rock with volcano rock.In the end of
the permian’s age until the end of the Triassic shallow marine environment. Which truncated
by a fault up takengon which occurs in tertiary. Besides a complex geologic condition, a very
high rock weathering also makes gayo lues district prone to geological disaster such as
landslides. Behind this conditions,Gayo lues regency located in an area with hills and
mountain morphology with a height ranging from 400-1200 meters above sea level (m asl).
The general purpose from this research is to integrated study of landlides in terms of
geology and geophysic. While the goal in particular is knowing subsurface lithology and
stratigraphy of the slope based on the variation of the resistivity, detect the plane skipped
along the slope selected,and knowing the type of landslide that will happen in the futuren will
come in the area of Kampung Blangtemun Badak Uken district of Gayo Lues.
In this research, Analysis that used is geologic field analysis that included : Geologic maping
and geologic technique. Geophysics analysis that going to used covered of steps, that is
resistivity method. This geophysic step looking up the plane skidded with used resistivity
method.
Results of this study was to determine the regional geology condition, 2D cross-section
below the surface, and the depth and slope of the plane skidded on a 1 slope elected. The
analysis concludes from several locations in the field observation point, the soil movement
that occurred in the investigation or Air southwest trending toward N225oE (SW). Ground
movement occurs because of disruption of the stability of slopes, among others, to the
number of fractures so that the rain water into the fracture add weight to the ground. Type of
ground motion that occurs is creep or " Rayapan ".
Keywords : geologic, geophysic, resistivity method, creep, stability of slopes.
INTRODUCTION
In Geology, Gayo Lues district has a complex history characterized by the formation
of three cycles of sediment deposition Pre-Tertiary volcanics. The oldest rocks in the
area are precipitated by a high degree of metamorphic rocks (slates, metaserpih and
metakuarsit arenit) Kluet Formation (Puk) Late Paleozoic age. On it precipitated
limestones alternating with mafic volcanic rocks Peusangan Group (late Permian ~
late Triassic age) are truncated by the fault ride Takengon happened to Tertiary. In
addition to the geological conditions are very complex, very high rock weathering
also makes the Gayo Lues district is prone to geological disasters such as landslides
and. Gayo Lues district located in an area with hilly and mountainous morphology
with a height ranging from 400-1200 meters above sea level.
Disaster risk reduction efforts in areas prone to landslide, it is necessary to measure
in detail and thorough in areas prone to landslide. One indication of a potential
landslide area is formed in the plane skidded along the slope. This slip field can be
detected using geoelectric resistivity because the upper soil layer with impermeable
bottom layer which has a high resistivity contrast value.
Geoelectric Method is one of electrical resistivity method used to study the state of
subsurface properties by studying the flow of electricity in the rocks below the earth's
surface. Geoelectric resistivity method is generally used for shallow exploration,
about 300-500 m. The principle of the method is that the electrical current is injected
into the earth's natural flow through the two electrodes, while the potential difference
that occurs is measured through two potential electrodes. Layer above the slip plane
field will have a very low resistivity values while a field slip layer has a high resistivity
values. Resistivity contrast allows the use of 2D resistivity geoelectric method for
detecting the slip plane.
The survey was carried out aiming to Know subsurface lithology and stratigraphy of
the slope based on the variation of the resistivity. Detect the plane skidded along the
selected slope. Knowing the type of landslide that will occur in the future.
Research Sites
Kabupaten Gayo Lues secara geografis terbentang pada posisi dari 03°40’26” –
03°16’55” LU sampai 96°43’24” - 97°55’2497° BT, dengan batas administratif adalah
sebagai berikut :
Figure 1. Map of Research Sites
BASIC THEORY
Rock formations of the investigation region, successive of the young to the old as
follows
:
Formation
Rampong
(Tlr):
sandstone,
siltstone
and
mudstone
conglomerate, Oligocene age. Formation Kluet (Puk): batusabak, filit, arenit
metamorphic quartz and metamorphic limestone (In the end of the permian’s age
until – Carbon), as shown in the picture below.
Figure 2. Regional Geological Map
METHODS
GEOLOGICAL METHODOLOGY
Field equipment needed in the field, in addition to personal gear in accordance with
the conditions of the field, such as clothing, shoes, hats, etc., the basic supplies
needed are:
•
GPS,
•
Compass Geology,
•
Hammer Geology,
•
Laope / hand-lens,
•
Field Notebook, stationery, ruler, and protractor,
•
Topographic base map,
•
Geological Map,
•
Komporator and scale,
•
Hidrokhlorik acid solution (HCl), and
•
Camera (Documentation).
GEOPHYSICAL METHODOLOGY
General provisions that need to be considered in the implementation of Electrical
Resistivity include:
•
Geoelectric equipment placed on the ground and covered not in direct contact
with the soil.
•
Potential cable is always placed above the current ground level
•
Displacement current electrode and potential electrode performed at the same
time and using a predetermined distance from the zero point
•
Stems currents electrode and potential electrodes should be planted about 20
cm into the ground with both cable and connections to the appliance should be
good, do not have a wiring defect that would cause them to leak.
•
Geoelectric users should avoid wire paths, waterways and streams.
•
Geoelectric users should avoid high-voltage electrical wire lines (SUTET/
SUTEM) and water pipelines.
•
Groundwater research with geoelectric method performed on a straight stretch
of track and the current and potential electrode spacing is measured with
measuring tape.
•
Geoelectric data processing by means of computerized
•
Direction electrode wires should be parallel to the strike layer.
•
geoelectric should avoid areas of water scarce areas.
Electrical Resistivity Equipment
This survey will use the tool Electrical Resistivity SuperSting R8/IP type. In the field
measurements, the equipment should be used, among others:
1. one unit SuperSting R8/IP
2. one unit Switch Box
3. 4 roll of cable eletroda
4. 56 pieces Eletroda
5. 2 units of 12 Volt Batteries
6. Test Box
7. Jumper cables
Figure 3. Research Methodology flowchart.
8. AC adapter
9. Data cable from the computer to the device
10. Meter 2 pieces (100 m)
11. 2 pieces Hammers
12. four units Walky Talky
13. Tent to protect equipment
14. Protective Box for tools Superting R8 and Switch Box
15. Altimeter types Cobra 4 Weather PHYWE.
RESULT
Field observations at ground level many indications memperlihat ground motion that
occurs in this region sepertti shown in the picture below,
Figure 4. Map Location of Geotechnical.
Based on observations of Geotechnical known that there are two areas with the
potential landslide in the future. Results of the analysis of several locations in the
field observation point, southwest trending soil movement (N225oE or SW). There
are two groups of soil mass movement as seen in the image below.
Based on the results of 2D geoelectric measurements, depth of field obtained slip
between 3-6 meters (track 1 top). In track 2 (the bottom of the building residents) slip
plane found at depths between 3-13 meters.
Figure 5. Geotechnical map (showing the two groups of soil mass movement).
Figure 6. 2D Cross Section Geoloistrik Processing Results in Two Observation
Sites.
Figure 7. Geolistrik 2D cross-section combined with geological observational data
indicate a potential landslide areas.
CONCLUSION
1.
Wavy hills geomorphology unit with relief of 21% to 55%
2.
Outcrop ÆWeathered Rocks
3.
Claystone (shale) Å weathered of Formation Kluet (Puk)
4.
Soil silt/clay part of Rampong Formation (Tlr)
5.
Type of ground motion that occurs is creep or " rayapan ".
6.
There are two areas of potential landslide occurrence trending N225oE (Sw).
7.
The first region (N): depth of field slip 3-6 meters and there are three areas of
slip zone.
8.
The second area (s): depth of field slip 3-13 meters and there are two areas
of slip zone.
9.
3-D modeling Æ the volume of soil above the slip plane ranges from 1,978
M3 - 13,470 M 3.
RECOMMENDATIONS
1.
The avalanche danger can be eliminated by contigous bored-pile retaining
wall where the structure of these poles anchored to bedrock.
2.
The installation of Monitor wells around the slopes. The monitoring wells are
also used for pumping water when groundwater levels rise due to high rainfall.
3.
Single root tree planting around the slopes where tree planting could reduce
the risk of landslides and strengthen cliff.
4.
If some of the methods above techniques cannot be implemented because
lack of resources, the people around the village of Blang Temun Badak Uken
should be relocated to a safer place, the original location can still be used for
farming areas (rice fields, plantations, farms and others) .
5.
Area planned for the relocation of the village Blang Temun Badak Uken
feasibility assessment should be done technically to ensure the area is safe
from landslides and other natural disasters.
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Landslide Disaster With 2D resistivity method in the Village Pablengan
Matesih Karanganyar District”, Indonesian Journal of Applied Physics, Vol.2
No.1 hal. 51.
Joko S., 2002, “Introduction to Engineering Geophysics”, ITB, Bandung.
Lilik H., 1990, “Prisoners geoelectric Type”, Earth Physics Laboratory ITB, Bandung
Noor D., 2006, “Environmental geology”, Graha Ilmu, Yogyakarta
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Wiley and Sons Ltd., New York.
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