PRELIMINARY
Land subsidence is a natural phenomenon that often occurs in big cities that stand on layers of young sediment, including Jakarta. The results of a survey conducted using the flat leveling method presented at the exposure of the Jakarta Coastel Defense Strategy, 2013 (Figure 1), the area of DKI Jakarta has experienced significant land subsidence.
Based on the study, the causes of land subsidence were identified by several factors, namely:
Figure 1. Land subsidence survey using leveling method
- Intake of ground water
- Building expenses (settlement)
- Natural consolidation of soil layers
- Decrease due to tectonic force
GEOLOGY CONDITIONS OF DKI JAKARTA
At the location of the Kamal Village Office and its surroundings, including Aluvium (Qa) in the form of clay, silt, sand, gravel, crust and lumps.
Some are included in coastal embankment deposits (Qbr) in the form of fine-coarse sand, sorting, with mollusk shells (Figure 2).
Figure 2. Geology at the work location
WORK LOCATION
The job location is in the yard of the Kamal Muara Village Office, Penjaringan District, North Jakarta City, DKI Jakarta Province, Indonesia.d
EXTENSOMETER
In this extensometer construction work consists of 2 tools that are strung together in one extensometer system, namely:
- Extensometer
- Piezometer
With the reclamation activity on the north coast of Jakarta, in 2016 Budget the Department of Industry and Energy plans to build a monitoring point for land subsidence with the Extensometer System on the west coast of Jakarta. This extensometer station can be used as a measurement point to compare land subsidence that will occur on the reclamation island and as a consideration in planning the construction of the Giant Sea Wall.
The development of an extensometer on the west coast of North Jakarta is expected to be able to complete data from previous monitoring stations and to monitor land subsidence in the area. The decline is estimated to be largely due to land consolidation, tectonics, building loads and groundwater abstraction. The influence of groundwater abstraction on subsidence in this area is small but cannot be ignored, as is the effect caused by building loads. Construction of a groundwater subsidence monitoring station with an extensometer system consists of an extensometer and piezometer. The role of the piezometer becomes very important, the piezometer being the only tool used to observe the decrease in pore pressure due to groundwater extraction in the area of the observation site.
Based on the explanation above, the monitoring system for land subsidence or also known as the Extensometer System has been built by a system consisting of:
Two extensometer wells along with monitoring equipment installed in 8 different layers with a depth of the first well as deep as 150 meters and a second well as deep as 300 meters serve to determine the deformation of the soil / rock layers in the eight identified layers.
A piezometer well with a depth of 150 meters along with monitoring equipment to monitor pore pressures in the four aquifer layers.
The control room follows the extensometer data recording and processing network.
EXTENSOMETER TOOLS
Figure 3. Data Logger.
Data Logger
Data logger (data recorder) is a special device that is capable of storing data within a certain period. This data logger unit is connected with rotary prots), which then records the data can be directly connected directly to a computer device (Figure 3).
Rotary Ports
Rotary pots unit functions as a reader (sensor). A potentiometer with rotary ports that will rotate follows each anchor position movement embedded in the observed soil layer depth (Figure 4).
Figure 4. Rotary Ports.
Anchor
The anchor functions as a hook or holder on the ground layer, this anchor which will give a signal in the form of movement that is affected by the movement of the ground which is then connected to the rotary ports (Figure 5).
Figure 5. Groutable Anchor
PIEZOMETER TOOLS
Piezometer Sensor Piezometer sensor is a sensor that functions to measure pore pressure at the position where the tool is planted at a certain depth (Figure 6).
Figure 6. Sensor Piezometer
Vibrating Wire Readout
Vibrating Wire Readout is a portable device for reading numbers / values from piezometer instruments. This device is categorized as simple because there is no need to mine other accessories (Figure 7).
Figure 7. Vibrating Wire Readout
INSTALLATION RESULTS
150m Depth Piezometer
Based on the results of well logging measurements, the piezometer sensor installation position can be determined at a depth of 45m, 85m, 115m and 140m. Next is the reconstruction of the drill hole and the position of the tool (Figure 8).
Figure 8. 150m Piezometer Reconstruction
Extensometer 150m Depth
Well logging that has been done in this hole shows the installation position of the groutable anchor extensometer, which is at a depth of 36m, 56m and 124m. Following the reconstruction of the drill hole and the position of the tool (Figure 9).
Figure 9. 150m Extensometer Reconstruction
Extensometer 300m Depth
Based on the results of logging measurements made at this drill hole with a depth of 300m, then the position of the extensometer installation is determined at a depth of 190m, 215m, 167m and 298m. Following the reconstruction of the drill hole and the position of the tool. (Figure 10)
Figure 10. Reconstruction of a 300m Extensometer
DEVELOPMENT OF PROTECTED HOUSES
Construction of protected homes is intended to protect and secure a series of extensometer devices. This house was built with an area of 8 × 6 meters and can protect the three tools (Figure 11).
Figure 11. Protected House Building
INSTALLATION OF DATA LOGGER
Installation work is carried out by installing a data logger located in the guardhouse as an instrument for receiving data from extensometers and piezometers that have been installed. Furthermore, the cable in the extensometer and piezometer head frame is connected to the data logger, while for the operation of the electric power logger data is obtained from a battery connected to the solarcell as a battery charger while the data logger is operating (Figure 12).
Figure 12. Data Logger Installation
CONCLUSION
Based on the results of the construction work of a land subsidence monitoring station with an extensometer system that has been carried out by drilling, taking samples and analyzing fracture formations of soil layers (cutting drilling and geophysical logging results by radioactive electrical well logging methods), it can be concluded as follows:
- Installation of an extensometer as a monitoring instrument for land subsidence with 150m drilling is placed at depths of 36m, 56m, 94m and 124m.
- Installing an extensometer as a monitoring instrument for land subsidence with drilling 300m is placed at depths of 190m, 215m, 267m and 298m.
- Piezometer installation as a pressure monitoring instrument in the soil and porous rock layers with 150m drilling is placed at depths of 45m, 85m, 115m and 140m.
- Installation of data loggers with a software-based system as an instrument for processing and storing extensometer data including piezometers, a way of retrieving data directly through a data logger and a USB cable converter and wireless network.