Choice of surveying methods for landslides monitoring

Transcription

1 Landslides and Engineered Slopes Chen et al. (eds) 2008 Taylor & Francis Group, London, ISBN Choice of surveying methods for landslides monitoring Shao-tang Liu Department of Civil Engineering, Henan Engineering Institute, Zhengzhou, P.R. China Zhi-wu Wang Fourth Team of Henan Province Coal Field Geology Bureau, Xinzheng, P.R. China ABSTRACT: There are a lot of methods and equipments for landslides monitoring. In this paper, the existing landslides monitoring technology, methods and equipments were introduced and then their advantages and disadvantages were discussed. Finally, the principle how to choice an appropriate surveying method in landslide monitoring were proposed. It is concluded that in all cases, measurements have to be made in terms of time, manpower and budget. The type of the landslide, the environmental conditions of the landslide, the expected accuracy and the professionals who use the deformation monitoring techniques are also the important factors to be considered. 1 INTRODUCTION In China, the geological disasters of landslide and mud-rock flows cause losses of over 1000 lives and total economic losses of over 10 billions of RMB each year. There have been about 90,000 identified landslide sites. So landslide monitoring is very important. In achieving an efficient monitoring programme, it may involve one or all of three approaches; visual inspections, surveying and/or sampling and instrumentation. There are a lot of techniques in each approach, Each monitoring technique has its own advantages, disadvantages and its application range. What are their characteristics? Which is the most appropriate method for a certain landslide monitoring? In this paper, the existing landslides monitoring technology, methods and equipments were introduced and then their advantages and disadvantages were discussed. Finally, the principle of how to choice an appropriate surveying method in landslide monitoring were concluded. 2 EXISTING LANDSLIDES MONITORING TECHNOLOGY AND METHODS There are three basic types of monitoring that can be undertaken: visual; instrumentation; and surveying. Visual monitoring may consist of inspection with supporting notes and/or photographs. Air photographs can also be used. It is important to note that the most effective way of monitoring is ground-based visual inspection and sampling on a regular basis. Instrumentation may include installing equipment for periodic reading or instrumentation for remote, intermittent or continuous data collection. instrumentation includes settlement gauges, inclinometers and piezometric groundwater measurements. Surveying includes all types of physical measurement, In the past, a variety of surveying techniques have been used to detect the surface movements of unstable area. For examples, tapes and wire devices have been used to measure changes in distance between points or crack walls. Levels, theodolites, Electronic Distance Measurement (EDM), and total station measurements provide both and changes of target, control points and landslide features (Liu Shao-tang, 2006). In addition, aerial or terrestrial photogrammetry provides point, contour maps and cross-section of the landslides. Photogrammetry compilation enables a quantitative analysis of the change in slope morphology and also the determination of the movement vectors. A comprehensive summary of the main methods and their precisions is shown in Table 1. This paper is concentrated on the discussion of the surveying methods. 3 ADVANTAGES AND DISADVANTAGES The monitoring techniques also can be divided two groups as geodetic and non-geodetic techniques. Each 1211

2 Table 1. Methods and techniques for landslide monitoring [Gili et al., 2000]. Method/technique Results Typical range Typical precision Precision tape distance change <30 m 0.5 mm/30 m Fixed wire extensometer distance change <10 80 m 0.3 mm/30 m Rod for crack opening distance change <5 m 0.5 mm Offsets from baseline Triangulation differences (2D) <100 m mm differences (2D) < m 5 10 mm Traverse/polygon variable, differences (2D) usually <100 m 5 10 mm Leveling height change variable, usually <100 m 2 5 mm/km Precise leveling height change variable, usually <50 m mm/km EDM (Electronic distance change variable, Distance Measurement) usually 1 14 km 1 5 mm ppm Terrestrial photogrammetry differences (3D) ideally <100 m 20 mm from 100 m Aerial photogrammetry differences (3D) H flight <500 m 10 cm Clinometer angle change ±10 0 ± Precision theodolite angle change variable ±10 GPS survey differences (3D) variable 2 5 mm ppm Note: 1 ppm means one part per million or 1 additional millimetre per kilometre of measured line. group has its own advantages and drawbacks. Geodetic techniques, through a network of points interconnected by angle and/or distance measurements, usually supply a sufficient redundancy of observations for the statistical evaluation of their quality and for a detection of errors. They give global information on the behaviour of the deformable landslide while the non-geodetic techniques give localized and locally disturbed information without any check unless compared with some other independent measurements. On the other hand, the instruments, which are used in nongeodetic measurements, are easier to adapt for automatic and continuous monitoring than conventional instruments of geodetic measurements. Geodetic techniques have traditionally been used mainly for determining the absolute displacements of selected points on the surface of the object with respect to some reference points that are assumed to be stable. Non-geodetic techniques have mainly been used for relative deformation measurements within the deformable object and its surroundings (Anonym, 2002). 3.1 Total stations Current technology provides total stations that are able to measure angles with an accuracy of ±0.5 (0.15 mgon), and distances with an accuracy of (1 mm + 1 ppm) to a range of 3, 500 m (Leica Geosystems, 2002a). Total stations allow the measurement of many points on a surface being monitored within a short period of time. For example, a surface that is being monitored by the placement of 200 prisms would take approximately 17 minutes to measure the 3-dimensional co ordinates of each point. Using Automatic Target Recognition (ATR) technology (Leica Geosystems, 2002b) each prism can be found and its centre identified to provide precise target pointing. Such technologies are ideal for precise applications where the removal of error sources is desired. The ATR approach used by Leica Geosystems uses nonactive prisms and hence does not require a power source at each prism, reducing the cost of each prism installation. 3.2 GPS Global Positioning System offers advantages over conventional terrestrial methods. Intervisibility between stations is not strictly necessary, allowing greater flexibility in the selection of station locations than for terrestrial geodetic surveys. Measurements can be carried out during night or day, under varying weather conditions, which makes GPS measurements economical, especially when multiple receivers can be deployed on the landslide during the survey. With the recent developed rapid static positioning techniques, the time for the measurements at each station is reduced to a few minutes (Celebi, M. et al. 1998). 1212

3 RTK GPS delivers 3-dimensional co-ordinates with an accuracy of (5mm+2 ppm) in real-time with a frequency as high as 0.2 Hz (Leica Geosystems, 2002c). Equipment which provides the accuracy achievable with RTK GPS and with the update rates that is possible with modern GPS receivers provide the ideal sensor for monitoring high and low frequency movements in landslides. 3.3 Insar Elevations can be determined from Synthetic Aperture Radar (SAR) images by interferometric methods. This involves the use of two antennas, displaced either vertically or horizontally, installed on the same satellite or aircraft platform. One of the antennas transmits the signal, but both receive it, resulting in two images being created. The most accurate form of interferometric measurement is differential interferometry (InSAR), which involves the determination of elevation differences between two epochs of terrain measurement. In this case, the variations in the radar signal phases are determined between the two epochs, which reveal terrain surface deformations that may have occurred between the two occasions when the images were recorded. It is claimed that height differences as small as one centimeter can be detected by this method. Such a technique therefore has the potential of being a cost effective, near-continuous, remote method of measuring terrain subsidence. 3.4 Pseudolite It is well known that for GPS-based deformation monitoring systems, the accuracy, availability, reliability and integrity of the positioning solutions is heavily dependent on the number, and geometric distribution, of the satellites being tracked. However, in some situations, such as in urban canyons, monitoring in valleys and in deep open-cut mines, the number of visible satellites may not be sufficient to reliably determine precise. Furthermore, it is impossible to use GPS for indoor applications and due to limitations of the GPS satellite geometry; the accuracy of the height component is generally 2 or 3 times worse than the horizontal components. These factors make it difficult to address GPS deformation monitoring applications in areas where the number of visible satellites is limited or satellite geometry is poor, especially where real-time high accuracy height component monitoring is needed, as in such applications as landslide subsidence or deformation monitoring of man-made structures. Therefore, in order to improve the performance of GPS-only deformation monitoring systems, the integration of GPS with other technologies needs to be investigated. Pseudolites (pseudo-satellites), which are groundbased transmitters of GPS-like signals, can significantly enhance the satellite geometry, and even replace the GPS satellite constellation in some circumstances (such as deformation monitoring indoors). The geometry of the satellite constellation can be improved by the careful selection of the pseudolite locations. In the case of GPS, the measurements with low elevation angles are usually rejected in order to avoid serious multipath, tropospheric delay and ionospheric bias. However, this is not necessary in the case of pseudolites. The quality of the measurements with less than half degree elevation angle (from the pseudolite transmitter to the GPS receivers) is still very high. Therefore, high quality pseudolite measurements with low elevation angles, when included in data processing, can be expected to significantly improve the ambiguity resolution performance and solution accuracy, especially in the height component. The availability is also increased because a pseudolite provides an additional ranging source to augment the GPS constellation. 3.5 Photogrammetry If an object is photographed from two or more survey points of known relative positions (known ) with a known relative orientation of the cameras, relative positions of any identifiable object points can be determined from the geometrical relationship between the intersecting optical rays which connect the image and object points. Aerial photogrammetry and terrestrial photogrammetry have been extensively used in determining landslide movements studies. The main advantages of using photogrammetry are the reduced time of field work; simultaneous three dimensional ; and in principle an unlimited number of points can be monitored (Anonym, 2002). 3.6 Laser scanning Existing techniques (e.g., surveying, GPS) used to monitor large structures such as buildings, viaducts, dams and bridges, though very accurate, are greatly hindered by their low point density. Data acquisition time limits monitoring to only a few samples located at strategic points on the structure (LIU Shaotang & ZHAO Zhan-yang. 2007). Ground-based laser canning is a new technology that allows rapid, remote measurement of millions of points, thus providing an unprecedented amount of spatial information. This in turn permits more accurate prediction of the forces acting on a structure. As an emerging technology though, it certainly can be used in landslides monitoring. 1213

4 3.7 Geotechnical sensors Geotechnical sensors are used extensively in the monitoring of landslides. These sensors are often laced within the landslide and out of sight, however they are never out of mind. During monitoring of the landslide geotechnical sensors of the desired type are carefully chosen and placed at strategic locations to ensure that adequate information is provided to verify design parameters, evaluate the performance of new technologies used in construction, verify and control the construction process and for subsequent deformation monitoring (Craig D. Hill & Karl D. Sippel, 2002). The main geotechnical sensors used for deformation monitoring include; extensometers, inclinometers, piezometers, strain gauges, pressure cells, tilt sensors and crack meters. Geotechnical sensors can either store the measured data internally awaiting download, or the measurements can be automatically logged to a connected computer. Connection to a computer offers a number of advantages (e.g. data stored at a remote location; ability to change update rate of measurement data, when changes in measured values are detected; no need to visit site to download data) and disadvantages (e.g. transfer media required between sensor and computer, for example cable/radio/gsm; loss of data possible if transfer media is not operating and internal storage is not activated). Geotechnical sensors provide measurements that are often essential in deformation monitoring. An additional sensor category that completes the portfolio of deformation monitoring sensors, that provide their own analysable measurements or measurements to calibrate additional sensors, is meteorological sensors. 3.8 Meteorological sensors Meteorological sensors are available in a variety of forms that measure one or more of the required meteorological observables, namely: temperature, relative humidity or dewpoint resp., barometric pressure, wind speed, wind direction, global radiation (solar energy) and precipitation. Sensors that provide such information can be used to calibrate other sensors used in the monitoring program (e.g. calibration of total station distance measurements) or provide valuable information that can be correlated with positional information measured from a total station (e.g. large lateral movements in a bridge can be correlated with significant cross bridge wind gusts, etc.). 3.9 Combining systems Landslide monitoring is a overlapping and comprehensive science referring to the integrated approach of geology, geomatics, mechanics, mathematics, physics, hydrometeorology (Ashkenazi, V. et al. 1998). It provides reliable data and a scientific basis for gaining knowledge of and mastering the evolutionary process of landslides, catching the characteristic information of collapses and landslides in time and making a correct analysis, evaluation, prediction and control of landslides. Thanks to the characteristics of landslide hazard, such as temporal-paroxysmic, spatial-randomicity, phyletic-variety, conditionalterribleness, sequential-ponderance, the technologies and methods of landslide monitoring must have the corresponding characteristics of rapidity, flexibility, veracity and integration etc. With the development of science and technology, more and more combing systems are implied in landslides monitoring, some of them are real-time or near real-time. Mr. Zhang Zhenglu etc. puts forward a new way for landslide monitoring, viz. 2G technique and method (the combination and integration of GPS and Georobot), which has less been applied for landslide monitoring in China and other countries, and there are some applications of landslide monitoring with GPS or Georobot solely in these areas. The 2G technique and method has the advantages of GPS and Georobot, such as setting up datum point with GPS and monitoring deformation points with Georobot, and then the deformation monitoring network may be simplified or not be set up (Zhang Zheng-lu et al. 2007). What is meant by 2G technique and method? Corresponding to 3S, it s the combination and integration of GPS (Global Positioning System) and Georobot. It s the modern technique and method for landslide deformation monitoring. In order to test and check the feasibility and validity of 2G technique and method for landslide monitoring, Jinpingzi landslide of Wudongde hydropower station in Yunnan, China, is selected for testing and practical application. This landslide monitoring network is composed of 15 monitoring points (named from TN01 to TN15), control area is 4.2 km 2, the maximal elevation difference among monitoring points is 830 m, the longest side is 1960 m and the average side is 940 m. Some comparisons and analysis are made between the general monitoring and Georobot monitoring of this landslide network, the results are shown in Table 2 (Zhang Zheng-lu et al. 2007). According to the analysis results of comparison in Table 3, Compared with the general monitoring method, monitoring landslide with Georobot can not only save much manpower and resources, but also greatly shorten working time and improve the working efficiency. Another example of Automatic Real-time Monitoring System (ARMS) is developed by Survey Division, Civil Engineering Department, HKSAR table 3 (Kin wah Leung, 2003). 1214

5 Table 2. The analysis results of comparison of a landslide monitoring network. Observation method General monitoring Georobot monitoring Monitoring equipment & 4 T3 theodolites 1 Georobt (Leica TCA 2003) Software 4 EDM equipments 6 8 prisms Georobot_Net Personnel collocation 4 technicians 1 2 technicians 5 8 workers 8 10 workers Outer observation time about 25 days about 7 10 days Inner processing time 5 days 2 days Table 3. ARMS system overview. Activity Major Hardware/Software involved Field Data Capture Leica TCA 1800/2003 Portable Note-book computer (PIII or above) In-house developed ARMs program Field Data Reduction ARMs program Auto wireless data Nokia Cardphone 2.0 (Transfer rate: 14.4 Kbps) communication Symantec PCAnywhere version 9.2 Window 98 Scheduler Graphical result presentation Desktop PIII computer with Win98 or above Excel 97 or above AUTO-MOTION Excel file with built-in macro Instrument Status viewing & Symantec PCAnywhere remote control of Instrument ARMs program The system comprises a motorized automatic total station, which is linked up to the office control unit by telephone line or wireless GSM network. With the ARMs, real-time situation of a dangerous slope can be monitored round-the-clock remotely with minimum staff resources. 4 CONCLUSION The landslide monitoring has being paid to much academic attention by researchers in the world wide; and lots of monitoring techniques has been advanced. Each monitoring technique has its own advantages, disadvantages and the application range. For example if the landslide is situated in the steep mountains, difficult to pass through, Conventional geodetic surveying methods for deformation monitoring include transit traverse survey, triangulation method, levelling survey, total station methods provide reasonable accuracy, but requiring skilled professionals to conduct the work in site, resulting in heavy workload, high personnel risk and low efficiency. Monitoring and timely alarms in case of hazard cannot be realized at night or in continuous rain, and the monitoring method cost more time, manpower and money, and it s difficult to get the deformation information in time when landslide deformation is expedite. Therefore, the combing system has its advantage in this case. Any way the principle of how to chose a proper method is that the designer should consider the following factors carefully as a whole: purpose and objectives of the monitoring programme; type of monitoring to be carried out; monitoring locations; existing equipment and the instrumentation required; frequency of monitoring and the expected duration of the programme; quality assurance and quality control procedures; methods to be used to inspect, record and evaluate the data; parameters to be measured; analytical detection limits of the techniques; the people who carry on the monitoring. procedures for verifying the achievement of the expected conditions as originally planned in the rehabilitation plan. In a word, measurements have to be made in terms of time, manpower and budget. The type of the landslide, the landslide s environmental conditions, the expected accuracy, and the professions who use the deformation monitoring techniques are also the important factors to consider. 1215

6 REFERENCES Anonym, Structural Deformation Surveying (EM ), US Army Corps of Engineers, Washington, DC Ayan, T., General Review of Deformation Analysis in Geodetic Networks, ITU Journal, Vol l., Istanbul, Turkey. Ashkenazi, V., Dodson, A.H. & Roberts, G.W Real Time Monitoring of Bridges by GPS, Proceeding of XXI FIG International Congress, Commission 5: Positioning and Measurement, Brighton, UK, July: Celebi, M., W. Prescott, R. Stein, K. Hudnut, J. Behr & S. Wilson Structural monitoring using GPS. Proceedings of 11th Int. Tech. Meeting of the Satellite Division of the U.S. Inst. of Navigation, Nashville, Tennessee, September: Craig D. Hill & Karl D. Sippel Modern Deformation Monitoring: A Multi Sensor Approach. Proceedings. FIG XXII International Congress. Washington, D.C. USA, April Kin wah Leung Automatic Real-time Monitoring System (ARMS) a Robotic Solution to Slope Monitoring. Proceedings. 11th FIG Symposium on Deformation Measurements, Santorini, Greece, Liu Shao-tang Deformation measurements during the construction of large dam projects. Chinese Journal of Underground Space and Engineering 06(Z2): Liu Shao-tang & Zhao Zhan-yang Deformation Monitoring of 70 m Span Box Girders of Hang-Zhou Bay Sea-Cross Bridge at Construction Stage. World Bridge 07(2): Zhang Zhenglu, Luo Changlin, Mei Wensheng, Deng Yong & Liu Zuqiang Study of 2G Technology and Method for Landslide Monitoring. Strategic Integration of Surveying Services. FIG Working Week Hong Kong SAR, China, May