Rock slope stability monitoring system in the Batu Mountain area, Lembang, West Bandung

. Gunung Batu is placed in Lembang, Langensari, West Bandung right of Lembang Fault line. Lembang Fault is an active fault with a slip rate between 1.95 – 3.45 mm/year and is expected to initiate an earthquake of Mw 6.5 – 7. Earthquakes in the Lembang Fault zone occurred in the 15th and 60th centuries BC, with an earthquake cycle every 170 – 670 years. From this, earthquake cycle estimates indicate that the Lembang Fault is in the final stage of the earthquake cycle. Gunung Batu is a populous area and has grown as a recreational area. Gunung Batu has a complex geological structure with potential rock slides. The observation and kinematic analysis results of the Gunung Batu area suggested that this area partly has an unstable slope and potentially has wedge and toppling failure. This condition of instability requires further monitoring to minimize damage and losses to populations around the Gunung Batu area. The development of slope stability monitoring tools has progressed significantly recently. Selecting the proper monitoring tool is the main component of data input to prevent possible hazards. The monitoring elements include rainfall, slope movement, and tilt movement. Sensor types that will be used in this monitoring system are integrated wireless monitoring systems. This system was developed to monitor the activity of the slopes of Gunung Batu, Lembang, West Bandung. The monitoring results for the last six months show the system is running well without errors.


Introduction
Gunung Batu is placed in Lembang, West Bandung right on active Lembang Fault line (Figure 1).The area of Gunung Batu is densely settled and used as a recreation area.The stability of the Gunung Batu slope is considered Gunung Batu's safety support.Slope stability is affected by rock characteristics, slope geometry, geological structure, rainfall, and external forces such as earthquakes.
Gunung Batu has a complex geological structure that potentially occurs for rock slides.Geologically, Gunung Batu is an outcrop of old volcanic rock layers forming rock slopes due to the movement of the Lembang Fault [1].The displacement of the Lembang Fault caused the rock layers to experience extensive fractures that caused a strength decrease and rock slope instability.Slope stability is controlled by discontinuity due to the joint of the rock mass [2][3][4][5][6].Joint mapping to determine and analyse rock slope stability is needed.Joint mapping results obtained information on rockslide type that maybe occurred [2,7].Based on previous study observation and kinematic analysis results, the Gunung Batu area is partially unstable and can potentially experience wedge and toppling failure [7].The Gunung Batu residents stated that rockfalls had occurred several times.This instability condition requires further monitoring to minimize damage in the community around the Gunung Batu area.Slope stability analysis is also used as an unstable area with the worst failure due to the Lembang Fault movement.
Gunung Batu is located in the active Lembang Fault zone with a slip rate of 1.95 -3.45 mm/year and is expected to produce an earthquake of Mw 6,5 -7 [1].Earthquakes in the Lembang Fault zone happened in the 15 th and 60 th BC, with an earthquake cycle every 170 -670 years.From the earthquake, the cycle estimates that the Lembang Fault is in the final stage of the cycle.Earthquakes will cause significant damage to the community around Gunung Batu, causing rock slope instability.Rock stability monitoring system developed to mitigate rock failure due to geological structure and Lembang Fault activity.In addition, creating a monitoring system can be used as research and education to increase public knowledge and awareness of the Lembang Fault hazard.
The development of slope stability monitoring tools has experienced rapid growth.The selection of suitable monitoring tools is a prominent part of producing proper input data used to prevent hazards that will arise.Factors that must be monitored include rainfall, slope movement, and tilt movement.This system was developed to monitor the slope movement of Gunung Batu, Lembang, West Bandung.

Method
Slope stability monitoring systems have experienced rapid development.Proper location determination and tools are needed to monitor the movement of Gunung Batu effectively.

Location determination
Based on previous research, the Gunung Batu slope has partly unstable conditions and potentially has wedge and toppling failure.The condition of complex geological structures can increase instability in addition to the natural conditions and possibility of earthquakes due to the active movement of the Lembang Fault.Observation results of the two types of rock failure used to determine the placement of monitoring tools.

Wedge failure
Wedge failure is classified as translational slides.Wedge failure often occurs in rocks that have discontinuity planes that intersect to form geological structures [8][9][10][11].Wedge failure occurs most frequently in rocks with the intersection of two discontinuity planes (Figure 2a).The discontinuity plane areas can be in the form of faults, joints, or bedding areas.Wedge failure can occur in a rock mass with two or more sets of discontinuities whose lines of intersection are approximately perpendicular to the strike of the slope and dip towards the plane of the slope [12].This mode of failure requires that the dip angle of at least one joint intersection is greater than the friction angle of the joint surfaces and that the line of joint intersection intersects the plane of the slope (Figure 2b).Wedge failure depends on the peak and residual shear strength.Wedge failure can appear fast, within minutes, or a much longer time of several months.Wedge failure occurrence depends on the structure and lithology of rock masses [13].After all, lithology is not the only parameter that causes wedge failure.The term trend (  ) and plunge (  ) describe the intersection line position.The plunge of the intersection line must be less than the dip slope (  ) and more significant than the mean value of friction angle (φ) in both discontinuity planes   >   > φ [5,12] (Figure 3).Discontinuity measurement can be obtained by the scanline method.Measurement results of the scanline method were analysed using stereographic projections.Toppling failure is a rock condition that experiences forward rotation and falls from the rock mass.The instability of toppling failure occurs by sliding down, tumbling down to forward, and sliding and tumbling down.Toppling failure will occur on rock slopes where the dip direction is opposite to the dip direction of the discontinuity plane.This situation can be described by blocks placed on an inclined plane (Figure 4).

System monitoring devices
The sensor type selection is based on the parameters of slope movement, that is rock displacement and tilt displacement, to be monitored during periods of extreme rain.Sensor types that will be used in the development of this integrated wireless monitoring system include: a.A rain gauge is used to measure the intensity level of rainfall.The rain gauge tool is a tipping bucket type of 0.2 mm accurate.Selection of a rain gauge is necessary for the Gunung Batu area because the rainfall data of West Bandung belong to the medium (100-300 mm) to high (300-500 mm) rainfall category.b.Tiltmeter is used to measure changes in slope that occur on the moving part of the soil slope either at the soil surface or the movement of the soil layers in the slow-moving.The accuracy of the tiltmeter is up to 0.1 o .c. Extensometer continuously measures the relative displacement between the measured and fixed reference points.The extensometer has an accuracy of 0.1 mm and a measurement range of up to 40 cm in slow-moving deep soil movement areas.d.Gateway used as a relay for data collectors

System monitoring devices
The slope stability monitoring system implemented in the Gunung Batu area is a wireless sensor network system.Several sensors are connected wirelessly with a gateway device, which sends monitoring data to a server.Each device also has a battery and solar panel to operate continuously.The sensor devices used in the slope stability monitoring system include a tiltmeter, extensometer, and rain gauge.One or more devices for each type of sensor are installed at different locations in the monitoring area to measure several variables related to slope stability in the Gunung Batu area.
A tiltmeter sensor is used to measure changes in slope that occur on rock slopes (Figure 6a).At the same time, the extensometer sensor is used to measure displacement on the surface of rock slopes (Figure 6b).Both sensors have a waterproof plastic enclosure containing a microcontroller, radio transceiver, battery, and elements sensing.The tiltmeter uses a Micro Electro Mechanical System (MEMS)based sensing element.MEMS is a technological process used to integrate devices in a small size.MEMS is a system that combines mechanical and electrical components.The extensometer uses a wire potentiometer.Gateway devices have a central role in the network as connecting devices for wireless sensor devices to transmit monitoring data to servers (Figure 7).A gateway is a unit that dispatches the instruction code to compile the data from all monitoring sensors.After the gateway accepts the data from the monitoring sensors, this data is gathered and sent to the server by the GSM/GPRS module.Gateway devices connect wirelessly with sensor devices in the network using the short-for-long-range (LoRa) protocol.LoRa is a spread spectrum modulation technique derived from chirp spread spectrum (CSS) technology.Meanwhile, the gateway device uses a cellular modem device to connect to the internet network for communication with the server.Apart from being connected wirelessly to the tiltmeter and extensometer sensors, the gateway device is also physically connected to the rain-measuring device.

Monitoring results
The server in the monitoring system stores data from sensor devices in the study area.In addition, it also has a role in presenting monitoring data in the form of tables and charts to facilitate analysis (Figure 8).The status of the installed monitoring system, such as battery status and data communication, can also be seen in the web application developed and installed on the server (Figure 9).
The monitoring results for the last six months show the system is running well without errors.Stability analysis based on monitoring tools showed that the Gunung Batu slopes are still stable and will continue to be monitored.Monitoring results will be carried out continuously and are expected to be used as initial data for predictions of instability in the Gunung Batu area.

Conclusion
The monitoring system for the stability of the Gunung Batu slope was carried out based on the results of previous studies.The results showed that the Gunung Batu slope has the potential for wedge and toppling failure.Determination of tool installation points based on positions with the potential for a rock slide.The monitoring tools installed are a rain gauge, tiltmeter, extensometer, and gateway.This monitoring tool is installed to monitor displacement and slope movements compared to particular rainfall.The monitoring result for the last six months shows that the system is running well without errors.Monitoring results show that the slopes of Gunung Batu are still in stable condition.The following research is carried out by monitoring continuously as initial data to predict the occurrence of instability.