Early Warning and Real-time Slope Monitoring Systems in West and East Malaysia

Transcription

1 Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing Early Warning and Real-time Slope Monitoring Systems in West and East Malaysia William H. T. Fung, Richard J. Kinsil, Suhaimi Jamaludin and Sashi Krishnan Abstract This paper presents the application of slope safety warning systems on two slopes along strategic roads in Malaysia. The systems are used as an alternative to further structural reinforcement or remedial work. Their purpose is to provide long-term monitoring and early warning to facilitate the understanding of slope behaviour, and to manage landslide risk faced by road users. The first application is in West Malaysia. It is a slope cut that is located at the 46 th kilometre of the Simpang Pulai Kuala Berang Highway. The second case is an embankment fill located at the 50.4 th kilometre of the Federal Road 500 between Penampang and Tambunan of Sabah. The first slope has had a long history of continual slow deformation, while the behaviours of the latter are closely linked with the rainfall conditions. In view of the distinct slope movement patterns, on-site instrumentation techniques are different, geodetic, geotechnical and hydrogeological monitoring techniques are utilised. As the slopes are situated in mountainous terrain, unfavourable weather conditions and remoteness of the sites poses serious challenges to the specification of lightning protection, power supply and data communication systems. Although the on-site system components are site specific, they are controlled by the same web-based instrumentation data management system. To permit that representative indices of slope instability are used in the warning module of the systems, diverse criteria are adopted. Apart from the conventional ones that make use of rainfall, groundwater level and soil deformation/movement, for the slope in Perak, creeping as described by landslide velocity is incorporated as a warning indicator. To reduce likelihood and consequences of catastrophic collapse, understanding of slope behaviour is a pre-requisite. Such understanding is possible with the systems that provide timely measurement of actual slope performance, and early warning for impending danger. Keywords Early warning and real-time slope monitoring system, web-based instrumentation data management system, warning criteria Introduction Conventionally, monitoring data in slope instrumentation are taken manually on site using portable equipment. In some projects where continuous data are required, automatic devices are installed to capture the data. However, it is still common to manually retrieve the data from the instruments. In recent years, the instrumentation, and communication technology has advanced considerably. Real-time continuous slope monitoring systems have become feasible. Such technologies have found growing importance in slope monitoring. In particular, this is extremely useful for slopes where they are active in movement and landslides are a hazard, while the processes controlling the movement are not clear enough. Real-time slope monitoring is a means for better understanding of slope behaviour, and, just as importantly, used to give early warning of impending danger. This paper deals with application of early warning and real-time slope monitoring systems, or termed herein as slope safety warning systems, in West and East Malaysia. This has been applied on two slopes that have been of concern. The first application is implemented on a slope cut along the second East- West Coast Highway in West Malaysia. It is located in

2 W. Fung, R. Kinsil, S. Jamaludin and S. Krishnan Slope Safety Monitor Perak at the 46 th kilometre of the Simpang Pulai Kuala Berang Highway. The second case is an old fill embankment slope located at the 50.4 th kilometre of the Federal Road 500 between Penampang and Tambunan of Sabah. The former slope cut has been moving since its completion in 2004, while the behaviours of the latter are closely linked with the rainfall conditions. The distinct movement patterns prescribe specific instrumentation techniques for their monitoring. Slope monitoring in West Malaysia The slope cut on the second East-West Coast Highway, has shown signs of distress since the completion of highway in A series of remedial works were done, but this could not inhibit instability appearance. The development of the instability was detailed by Jamaludin et al. (2012) in conjunction with the geological setting and landslide processes operating at the site. A general view of the slope is presented in Fig. 1. It is considered that a large mass of ground has been activated, and further remedial work is not economically justifiable without a better understanding of the slope behaviour. The extent of the unstable portion of the cut is approximately 150m long and 24 to 60m high. monitoring using robotic total station has been put in place. It was based on the observed large deformation with manual equipment during the construction stage. An additional advantage over other common geotechnical sensors is that instruments installed on the slope are not of electrical type that are vulnerable to lightning damage. The measurements are taken by a robotic total station with laser beams. The monitoring is run by an automated robotic total station, controlled by in-house software suites for programming, and data acquisition and processing. In respect of data communications, the remoteness and adverse weather conditions preclude the use of conventional mobile and wireless techniques. To provide reliable communication and power systems, specially designed satellite communications and power systems were deployed. Fig. 2 presents the layout of the monitoring targets, while the physical setup of the monitoring station at the site can be seen in Fig. 3. Fig. 1 Overview of slope at 46 th kilometre Simpang Pulai Kuala Berang Highway. Fig. 2 Layout of monitoring prisms. In the present situation where complete safety is exceptionally expensive if not impossible and the use of the highway by the public is to be maintained, a risk management approach would definitely help decision making. A key element of the approach is the implementation of an early warning and real-time slope monitoring system to provide the authority with timely actual slope movement measurements and advanced warning for impending danger. Instrumentation installed on a continuously moving slope The continuous movement observed in the slope is too extensive to be taken care by any conventional geotechnical instrumentation. Instead, geodetic Fig. 3 Setup of robotic total station (left), and antenna for satellite communications (right). To permit the monitoring system to assume its role as a slope safety warning system, the on-site components require to be integrated with a web-based instrumentation data management system. An in-house product is used for this purpose. Its core is a relational 2

3 Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing database system that runs on a geographic information system platform. This provides rapid data retrieval/storage, processing, presentation, and diverse automatic alert triggers. Fung et al. (2010) discussed details of the operating software suites, and the instrumentation installation. Since the installation of the complete system at the end of 2009, the system has been running up to the present, which provides valuable data to enable better comprehension of the slope performance. In addition to deformation monitoring, further use of the data has been made to reduce the landslide risk as discussed below. Monitoring of slope movements and their velocity Prior to implementation of the current monitoring system, manual optical survey of the slope already recorded a total slope movement >5m at several locations with a maximum of 7m from December 2007 to December This was associated with the failure of a series of slope remedial work. Since the current system was in place at the end of 2009, the slope has moved further by maximum of 10m as illustrated in the Fig. 4. Emphasis is placed on the uppermost target prisms located within the unstable slope cut, named TP11 to TP51. increases dramatically. Indeed, the movement rate has formed a key part of the warning system as will be discussed. The rate of surface movement observed in concerned target prisms is shown in Fig. 5. Except for TP21, the slope movement stays in the Slow velocity class (Cruden and Varnes 1996). Fig. 5 Velocity versus time plot. Fig. 6 presents only the results within the Slow velocity class. The slope movement rate patterns resemble those as depicted by Leroueil et al. (1996) for active landslides in the reactivated stage. It is considered that the recorded patterns are related to minor local failures that have been seen on the site. Fig. 6 Velocity versus time plot for activation of local failures. Fig. 4 Total slope displacement of uppermost prisms. Jamaludin et al. (2008, 2012) have discussed the development of the observed movement patterns of slopes in the area, and suggested that the patterns follow the creep model proposed by Varnes (1978). In essence, the model consists of three stages: primary, secondary and tertiary creep. Primary creep is associated with high displacement rate, while a quasistatic rate of movement is in the secondary creep stage. The tertiary creep is associated with an increasing rate of movement. Jamaludin et al. (2012) suggested that the slope is currently in the secondary creep phase. As such, monitoring should be continued until occurring of tertiary creep signs when the rate of slope movement Warning criteria based on movement rate Since completion of the slope cut on the highway in 2004, continuous deformations have been observed. Moreover, the slope deformations obtained from both manual and automatic optical survey have been large. Conventional warning criteria based on magnitude of movement alone are not meaningful from the perspective of the risk mitigation. As previously discussed, the creep model would be applicable for describing the development of slope failure. Catastrophic failure cannot be arrested. However, landslide risk for road users can be largely mitigated by incorporating a pre-failure warning framework based on slope movement rate into the monitoring system as suggested by Fung et al. (2010). Such framework was 3

4 W. Fung, R. Kinsil, S. Jamaludin and S. Krishnan Slope Safety Monitor proposed by Jamaludin et al. (2012), and included into the monitoring system. Towards accelerating creep, the movement rate will be increasing, and high. Jamaludin et al. (2012) suggested warning criteria for the failure phase of the slope, based on different velocity threshold values, adapted from those proposed by Cruden and Varnes (1996) with due consideration of the measuring technique accuracy. The criteria are summarised in Tab. 1. Table 1 Proposed warning criteria for movement rate. Alarm Level Velocity Limit Level 1: < 3mm/hr Normal (72mm/d) Level 2: 3-9mm/hr Advisory (216mm/d) Level 3: Watch Level 4: Danger 9-18mm/hr (432mm/d) > 18mm/hr (432mm/d) Proposed Response Daily data monitoring by JKR staff Continuous monitoring, data analysis & review, field observation Increase preparedness, continuous data analysis, inform police/ preparedness team Continuous monitoring, decision to be made (to evacuate/close the road) The threshold levels have also been presented in Fig. 5 and 6. As shown, the slope has been very active with continuous movement, whereas the movement rate generally stays below the first threshold. Slope monitoring in East Malaysia The embankment fill is located along the Federal Road 500, between Penampang and Tambunan of Sabah. A general view of the slope is presented in Fig. 7. The road is a strategic one and connects the low land areas of the west coast to the interior regions of Sabah. The stretch of the road, where the slope fill lies, is constructed across the steep slopes of the Crocker Range, which is dominated by sedimentary rocks. As identified in the National Slope Master Plan (CKC 2009), the area is characterised as a landslide prone area. The occurrence of landslides along the stretch is quite common during monsoons every year, in the southwest and northeast directions. Fig. 7 Panoramic view of embankment fill at Federal Road between Penampang and Tambunan. Many of the slope fills, along the mountainous stretch of the road, are old embankments. They usually block the natural drainage, and rely on postconstruction man-made drainage to help discharge rainwater away from inside the fills. In addition, surface runoff is another main cause of slope failure, as it allows rainwater to penetrate and percolate into the slope face and weaken the soil due to high moisture. Surface drainage and protection is necessary to maintain the slope safety. Although routine maintenance is in place, sign of distress, such as surface cracks and displaced channels, have been observed on the embankment fill as shown in Fig. 8. To facilitate the understanding of the fill embankment behaviour, and provide an alert mechanism for road users and the authority about its stability, a safety monitoring system is now operational. Fig. 8 Surface cracks (inset showing a displaced channel). This is an autonomous system that allows real-time automatic capturing of rainfall, groundwater level and slope movement. It provides data availability in all weather conditions through the same web-based system. Most importantly, alerts will be triggered automatically by the system to provide early warnings to the authority so that preventive action can be taken accordingly. Instrumentation for an intermittently triggered slope fill Instrumentation based on geotechnical and hydrogeological monitoring techniques is utilised to investigate the slope behaviour and to provide early warning alert. Automatic instruments include rain gauge station, two sets of piezometers, tiltmeters, and two manual inclinometers to establish subsurface soil movements. The system was operational in early March 2013 before the approaching southwest monsoon (May to September). As shown in Fig. 9, there have been five peaks up to mid-june of the same year. The rolling 24-4

5 Proceedings of World Landslide Forum 3, 2-6 June 2014, Beijing hr rainfall exceeds the first threshold level of 50mm in 24 hours for landslide warning in April. As it will be highlighted, the rolling 24-hr rainfall has been found to be a useful indicator for landslide occurrence. For completeness the recorded hourly rainfall is also presented on the same figure. slope failure rate and maximum rolling 24-hour rainfall. Its applicability to the present case may need further justification. However, it is still worthwhile to follow the approach as rainfall induced landslides are common in both places. When, for example, the rain gauge records rolling rainfall of more than 100mm in 24 hours, consultation between concerned authorities should begin. Fig. 9 Graphs of hourly and rolling 24-hr rainfall. A graph showing rainfall against water level is presented in Fig. 10 for easy comparison. The change in water level was not abrupt except for the first rainstorm. It is also interesting to note the corresponding slope movement during the same period. As illustrated in Fig. 11, the slope was activated whenever the rolling 24-hr rainfall exceeded the trigger levels. Fig. 11 Rolling 24-hr rainfall and slope movement. The original intent of tiltmeters installation was to provide a triggering device for slope instability. However, as an indicator for road maintenance, such as surface cracks and damaged drainage channels, further use of the measured parameters is made. Warning criteria for slope movement as inferred from the tiltmeter measurements has been adopted. In addition, similar criteria have been made for the rise of water level to attract the attention of the authority to the change in the groundwater regime. Tab. 2 summarises the proposed warning criteria for the site. Table 2 Proposed warning criteria for rainfall, water level and movement Fig. 10 Rolling 24-hr rainfall and groundwater level. Warning criteria based on rainfall and movement Brand et al. (1984) reported on the work for the understanding the relationship between rainfall and man-made slope failures in Hong Kong. The number of causalities was very small unless the maximum hourly rainfall approached 70mm/hr. In addition, Brand (1984) found that a threshold rainfall of 175mm in the preceding 24 hours would warrant the issue of the landslide warning. The current landslip warning system in Hong Kong has gone further as summarised by Chan and Pun (2004). It is based on the correlation between Parameters Advisory Watch Warning Hourly RF (mm) Rolling 24-hr RF (mm) Rise in Water Level (m) Soil Movement (mm) Web-based data management system The data collected on site are managed by the same web-based data management system. Individual system was prepared for each site and installed in the Public Works Departments of the federal and Sabah governments in Kuala Lumpur and Kota Kinabalu, respectively. The system is accessible to authorised users as web service through the client software. It is important to note that the data of different nature are coming from geodetic, geotechnical and 5

6 W. Fung, R. Kinsil, S. Jamaludin and S. Krishnan Slope Safety Monitor hydrogeological instrumentation, but they are directly handled by the same system. The system is a complete information hub that can manage projects gathering data of different types and analyse them together. This is particularly important for handling large-scale projects or managing several project sites with the same hub. Summary and conclusion To reduce the likelihood and consequences of catastrophic collapse, understanding of slope behaviour is a pre-requisite. Such understanding is important to the two reported cases where the slope movement may not be stopped 0r it is not justifiable economically by means of civil engineering works to arrest it. As the slopes are part of the strategic roads, a risk management approach must be adopted to mitigate the risk faced by road users, and help the authority make decision. Nowadays, instrumentation and computer information technologies have advanced considerably. Such technologies have found their growing importance in real-time slope monitoring. By incorporating appropriate slope activity models into the monitoring system, the measurement of slope performance and hydrogeological conditions will provide more insight into the underlying slope behaviour. Just as importantly, the system permits threshold levels for different slope performance indices to be predefined and refined in the system as advanced warning criteria. Apart from conventional criteria making use of rainfall, groundwater level and soil deformation/movement, landslide velocity is engaged as a warning indicator. In the end, the system becomes a slope safety monitoring system as well as a risk management tool, which provides real time slope monitoring and early warning services. Acknowledgments This paper is published with the permission of the Heads of the Public Works Department of the federal and Sabah state governments for use of the information on the slope monitoring projects. References Brand E W, Premchitt J, Phillipson H B, (1984) Relationship between Rainfall and Landslides in Hong Kong. Proceedings of 4 th International Symposium on Landslides, Toronto. Vol. 1, pp CKC, Cawangan Kejuruteraan Cerun, (2009) National Slope Master Plan Cawangan Kejuruteraan Cerun, Malaysia. Chan R K S, Pun W K, (2004) Landslip Warning System in Hong Kong. Geotechnical Instrumentation News, 41: Cruden D M, Varnes D J, (1996) Landslide Types and Processes. Landslides: Investigation and Mitigation. Transportation Research Board National Research Council, Special Report 247, Washington D C. pp Fung W H T, Le Goff D, Krishnan S, (2010) Instrumentation Data Monitoring System for Slope Safety Monitoring in Malaysia. Proceedings of International Conference on Slope 2010 Geotechnique and Geosynthetics for Slopes, July Chiang Mai, Thailand. pp Jamaludin S, Abdullah C H, Jaafar K B, (2012) Monitoring of slow moving landslide at 46 th km of Simpang Pulai Gua Musang Highway, Malaysia. Proceedings of 11th International Symposium on Landslides and 2nd North American Symposium on Landslides, 2-8 June Banff, Canada. Vol. 2, pp Jamaludin S, Jaafar K B, Abdullah C H, Mohamad A, (2008) Landslide Warning System for Mount Pass, Malaysia Based on Surface Monitoring Technique. Proceedings of International Conference on Management of Landslide Hazard in the Asia-Pacific Region, November Sendai, Japan. pp Leroueil S, Locat J, Vaunat J, Picarelli L, Faure R, (1996) Geotechnical Characterisation of Slope Movements. Proceedings of 7 th International Symposium on Landslides. Trondheim, Norway. Vol. 1, pp Varnes D J, (1978) Slope Movement Types and Processes. Landslides Analysis and Control, National Academy of Sciences Transportation Research Board Special Report 176, Washington D C. pp William H. T. Fung ( ) Fugro Geotechnical Services Ltd., Unit 8, 10/F Worldwide Industrial Centre, Shan Mei Street, Fotan, Shatin, N.T., Hong Kong wfung@fugro.com.hk Richard J. Kinsil Slope Engineering Branch, Public Works Department Headquarters of Malaysia, Jalan Sembulan, Kota Kinabalu, Sabah, Malaysia Richard.Kinsil@sabah.gov.my Suhaimi Jamaludin Slope Engineering Branch, Public Works Department Headquarters of Malaysia, Jalan Sultan Salahuddin, Kuala Lumpur, Malaysia suhaimij@jkr.gov.my Sashi Krishnan Konsortium MyStar Communications Sdn. Bhd., 23-3, Jalan 4/109F, Plaza Danau Desa 2, Taman Danau Desa, Kuala Lumpur, Malaysia sashi.krishnan@mystar-msc.com 6