Existing Landslide Monitoring Systems and Techniques

Transcription

1 Existing Landslide Monitoring Systems and Techniques P. D. Savvaidis, Department of Geotechnical Engineering, School of Civil Engineering Faculty of Engineering, The Aristotle University of Thessaloniki, Greece Abstract. Phenomena such as earthquakes, volcanoes, landslides, coastal erosion, glacial movements or subsidence following mining, water or oil extraction are all characterized by surface movements ranging from millimeters to meters, over spatial scales of meters to tens of kilometers and temporal scales of hours to years. Detecting the timing and amount of deformation is critical for understanding the physical causes and eventually warning of possible hazards. Monitoring of deformation of structures and ground surface displacements during landslides can be accomplished by using different types of systems and techniques. These techniques and instrumentation that can be classified as remote sensing or satellite techniques, photogrammetric techniques, geodetic or observational techniques, and geotechnical or physical techniques are presented in this paper. Keywords. Landslides monitoring techniques, geodesy, photogrammetry, GPS, geotechnical techniques 1 Introduction Landslides are downslope movements of rock, debris or earth under the influence of gravity, which may cover a wide range of spatial and temporal scales. Most landslides occur at steep slopes, but they can also happen in low relief areas in connection with excavations by rivers or construction work. Landslides can be triggered by natural environmental changes or by human activities. Earthquakes, volcanic activity, heavy rainfalls and changes of ground water level are typical natural triggering mechanisms for landslides, which amplify the inherent weakness in rock or soil. Landslides may result in severe human casualties, property losses and environmental degradation. Therefore, it is well justified that maintaining the stability of slopes is a critical aspect of any geotechnical engineering project. Monitoring the surface displacements of a slope can provide valuable information about the dynamics of the landslide phenomenon. The magnitude, velocity and acceleration of displacements can provide an indication of the stability of the slope. These movements, if detected early enough, can indicate impending catastrophic failure of a slope mass. Landslide hazard mitigation strategies comprise a range of activities including hazard mapping and assessment, real time monitoring and warning systems for active landslides, protective engineering measures, development of public awareness, and emergency planning. Mitigation measures benefit from the understanding of landslide processes and triggering mechanisms which build on the knowledge of geophysical and geological properties of the mass wastes and on models of slope deformation and of failure processes. In general, the information on landslide properties and the understanding of the processes is still inadequate. After an area is suspected of having a potential for failure, an observation campaign is established in order to determine the magnitude, direction and velocity of displacements. This campaign would also help to determine the frequency of the subsequent measurement epochs. 2 Systems and Techniques for Landslide Monitoring The measurement and recording of data related to the physical characteristics of the earth s surface is a critical phase of any major civil engineering project. The information, which these terrain investigations provide, is an essential element of the initial planning stages of a project. Monitoring of deformation of structures and ground surface displacements during landslides can be accomplished by using different types of systems School of Rural and Surveying Engineering, The Aristotle University of Thessaloniki, 2003 From Stars to Earth and Culture In honor of the memory of Professor Alexandros Tsioumis pp

2 and techniques. These techniques and instrumentation can be classified as remote sensing or satellite techniques, photogrammetric techniques, geodetic or observational techniques, and geotechnical or instrumentation or physical techniques. The choice of instruments and methods of measurement or the creation of a dedicated monitoring system depends upon the different types of deformation that will affect the method of stability analysis and consequently the whole deformation monitoring campaign: 1. Remote sensing or satellite techniques with space-derived information have significant potential for landslide hazard assessment and for improved understanding of landslide processes. Similar sensors and methods are of importance for seismic hazards and management of earthquake disasters. 2. Photogrammetric techniques can be an effective tool for monitoring actively moving landslides and for analyzing the velocity or strain-rate fields. These techniques allow the determination of ground displacements over long periods of time, by comparing the corresponding sets of aerial photographs. 3. Ground-based geodetic techniques make use of many instruments and methods of measurement for absolute displacement computations. They are usually employed according to an episodic monitoring program. In some cases, the geodetic sensors are put on control points and perform the measurements during each repetition. In other cases, a sensor can permanently be put on an observation point and perform measurements to targets on control points according to a computer-controlled program. 4. Satellite-based geodetic techniques make use of satellite positioning systems, such as the Global Positioning System, GPS. There are a number of methodologies applied that can guarantee high accuracy, continuous and reliable results. 5. Geotechnical techniques make use of sensors permanently working on or in the structure or region under consideration. They can operate on a 24-hour base and measure the change of geometrical and/or physical characteristics of the deforming item (relative deformation). They can also use a telemetric system for real-time transmission of measurement data to a control center. For accurate landslide inventory mapping and analysis of landslide properties the space-derived data can be integrated with all other available information on landslide occurrence and characteristics, including aerial photos, geotechnical data and geodetic monitoring results as well. In situ observations are needed to obtain detailed information on geological and mechanical properties of landslides. Of importance are subsurface properties, which can be determined accurately only with geophysical exploration techniques and drillings. Aerial photography is also an important source of information, but the update of these images is often a problem. The techniques applied vary from one case to another, depending on the expected risk, the accessibility of the area, the potential for damage, the availability of resources etc. A short review of the existing systems and techniques for landslide monitoring will be given below. 2.1 Remote sensing or satellite techniques for landslide monitoring Remote sensing in the form of the photographic, scanning and processing system is one of the most appropriate means of recording existing ground conditions, of essaying their potential for engineering projects and also of evaluating the effect or potential effects on the environment. Satellite images in the optical region with high spatial resolution are used for producing landslide inventory maps and for mapping factors related to the occurrence of landslides such as surface morphology, structural and lithological properties, land cover, and temporal changes of these factors. The satellite data are used complementary to aerial photography or as substitute if no recent air photos are available. In addition, for hazard zonation at regional and national scales, satellite imagery is a more economic database. Another advantage of satellite-based remote sensing is the capability of repeat observations, which results in more frequent update of information on landslide characteristics than the conventional data sources. At the small scale of satellite imagery, only extremely large landslides can be identified directly. The value of satellite imagery is that the landslide susceptibility of an area can be determined indirectly from some of the features that are identifiable at those scales. Regional physiography, geologic structure, and most landforms as well as land-used practices and distribution of vegetation are evident on the satellite imagery. These features in conjunction with the tonal patterns present on the imagery provide clues to the type of surface materials present, the surface moisture 243

3 conditions, and the possible presence of buried valleys. Correlating these factors to geology and topography and using local experience in region make it possible to rate the susceptibility of various areas to sliding. The combination of aerial photography and infrared imagery provides a more accurate and complete portrayal of terrain conditions than that obtained from either system alone. Infrared imagery provides types of information that is valuable for evaluating existing landslide and landslide - susceptible terrain, like surface and near - surface moisture and drainage conditions, indication of the presence of massive bedrock or bedrock at shallow depths, distinction between loose materials that are present on steep slopes and are susceptible to landslides etc. In addition to optical sensors, which have been used to obtain morphological, lithological and structural properties of landslides, powerful tools for landslide monitoring have been developed in recent years based on imaging radar (Synthetic Aperture Radar, SAR). Also, the technique of spaceborne Interferometric Synthetic Aperture Radar (InSAR) provides an excellent means of observing deformation over broad areas. InSAR has proven to be a powerful tool to characterize large-scale deformation associated with active faults. It also can resolve small-scale deformation features such as shallow creep, postseismic and interseismic deformation. And it is an ideal tool for measuring land subsidence and improving digital terrain models. InSAR offers the possibility to map and monitor the displacement of slopes and even of individual objects. Applications have been demonstrated in different places, based on ERS data, revealing satisfactory accuracy for displacement analysis and time series of motion over many years. Major constraints for the application result from vegetation and from unsuitable orientation of a slope relative to the radar illumination. The problem of vegetation can be partly overcome by the PS technique, if stable objects such as houses etc. are located within these areas. In comparison to ground based GPS or geodetic surveys, InSAR provides the complete motion field of a slope, and is also an economic means for detecting and analyzing unstable slopes over large areas. Space data are important because of the large spatial coverage and the repeat observation capabilities. This is important for landslide inventory maps from regional to national scales because of the lower costs than conventional measurements and the actuality of the data. Particularly in remote areas, the information available from conventional sources is far from satisfactory. In addition, spaceborne InSAR capabilities are unique for providing repeat observations of surface motion over large areas at modest costs. Digital elevation data from space are also of interest for landslide characterization, where the accuracy of the present satellite systems (microwave or optical) is adequate, though improvements are desirable for smallscale landslides. SAR satellites (ERS, Radarsat-1) show excellent capabilities for monitoring surface motion with high accuracy. The InSAR capabilities will be continued and even improved with ASAR on ENVISAT and Radarsat-2, TerraSAR etc.). In addition, InSAR is of interest for topographic mapping. High spatial resolution optical sensors data, (Ikonos, Landsat, SPOT etc.) provide useful information for landslide inventory mapping and damage assessment. For monitoring the motion of active landslides ground-based optical systems (video cameras) are used, but they have limitations in case of fog, rain and darkness. Recently new ground-based remote sensing systems have been developed and tested successfully (Laser-scanners, SAR interferometry). Three-dimensional laser scanning systems (fig. 1) are currently used in high accuracy, small-scale industrial metrology applications as well as for airborne mapping. Metrology applications can be found in the automobile and aircraft assembly industries, as well as in space-borne measurement. Several terrestrial laser-imaging systems have been released. These types of imaging system provide a user with a dense set of three-dimensional vectors to unknown points relative to the scanner location. Given the volume of points and high sampling frequency, laser-imaging systems offer an unprecedented density of geospatial information coverage. For this reason, there is enormous potential for use of this instrumentation in monitoring applications where such dense data sets could provide great insight into the nature of structural deformations. Another important application of Earth observation data is the generation of digital elevation models (DEMs), which is basic information for the characterization of slopes and for numerical modeling of mass waste processes. Data sources are optical stereo images with high spatial resolution and InSAR. 244

4 offers the advantage of cloud penetration, which is an advantage for landslides occurring during adverse weather or in connection with volcanic eruptions. The use of laser scanning for large scale (i.e., greater than a few meters in range) ground-based measurement operations is still at the research and development stage for terrestrial monitoring. It should also be stressed that remote sensing is only an aid to engineering investigations, proving information which is complementary to that obtained from other observation techniques, field measurements, site visits and existing sources of data such as maps and projects reports. 2.2 Photogrammetric techniques for landslide monitoring Fig. 1. Laser 3D scanners Regarding real time monitoring of landslides that represent a risk, the capabilities of Earth observation from space is limited because of the repeat observation intervals. Accurate measurements of surface motion and its temporal changes, as provided by InSAR, are of significance. Depending on the repeat orbit and pointing capability of the sensor, repeat intervals of a few days can be achieved which is suitable only for the observation of slow motion. In case of faster movements ground-based installations are needed. Earth observation sensors are able to provide information for landslide damage assessment. In order to contribute to relief operations, high temporal and high spatial resolution is needed. In principle this can be achieved by sensors with sideways pointing capabilities, but the repeat observation intervals of the present satellites are not yet fully satisfactory. Optical sensors and SAR are suitable for this task, where SAR Photogrammetric techniques have been extensively used in determining ground movements in ground subsidence studies in mining areas, and terrestrial photogrammetry has been used in monitoring of engineering structures. The main advantages of using photogrammetry are the reduced time of fieldwork; simultaneous three-dimensional coordinates; and in principle an unlimited number of points can be monitored. The accuracy of photogrammetric point position determination has been much improved in last years, which makes it attractive for high accuracy deformation measurements. The interpretation of aerial photography has proven to be an effective technique for recognizing and delineating landslides. It is an effective technique for recognizing and delineating the three-dimensional overview of the terrain from which the interrelations of photography, drainage, surface cover, geology materials, and human activities on the landscape can be viewed and evaluated. Aerial photographs present an overall perspective of a large area and boundaries of existing slides can readily be delineated on aerial photographs. Surface and near-surface drainage channels can be traced. Important relations in drainage, topography and other natural and man-made elements that seldom are correlated properly on the ground become obvious on photographs. Furthermore, soil and rock formations can be seen and evaluated in their undisturbed state. Recent photographs can be compared with old ones to examine the progressive development of slides and aerial photographs can be studied at any time, in any place and person. 245

5 Terrestrial photogrammetry and ground-based photography is also being used for local-scale landslide monitoring. Sites that are too steep or too small to be confidently viewed from the air lend themselves to study on the ground, albeit at a distance. Terrestrial photogrammetry can effectively be used at unsafe or inaccessible sites, as road cuttings and landslides. Special cameras with minimized optical and film distortions must be used in accuracy photogrammetry. Cameras combined with theodolites (phototheodolites), or stereocameras have found many applications in terrestrial engineering surveys including mapping and volume determination of underground excavations and profiling of tunnels. The accuracy of photogrammetric positioning with special cameras depends mainly on the accuracy of the determination of the image coordinates and the scale of the photographs. Using a camera with f = 100 mm, at a distance S = 100 m, with the accuracy of the image coordinates of 10 µm, the coordinates of the object points can be determined with the accuracy of 10 mm. Special large format cameras with long focal length are used in close range industrial applications of high accuracy. They can give sub-millimeter accuracy in digitizing objects up to several tens of meters away. Recently, solid-state cameras with CCD (charge couple device) sensors have become available for close range photogrammetry in static as well as in dynamic applications. Continuous monitoring with real time photogrammetry becomes possible with the new developments in CCD cameras and digital image processing techniques. As with aerial photographs, the image can be measured photogrammetrically, to yield quantitative data, or interpreted for geotechnical information, or both. The technique has advantages over conventional surveying methods if site access or time available to complete the survey is limited, or when changes need to be monitored over a long period of time. In terrestrial techniques photographs can be taken over a period of time to record changes in the landscape in a more or less convenient way. This is not the case, though, with aerial photographs where temporal coverage is difficult to be achieved in a cost-effective way. 2.3 Ground-based geodetic techniques for landslide monitoring Conventional ground-based geodetic techniques have been used for deformation monitoring of structures and landslides. Two basic methods for the design of a deformation survey can be used: 1. A horizontal or vertical control network is established in the area under investigation with control points located in the deforming region. 2. Total station instruments are used to measure angles and distances to target-prisms located on the moving mass. In both cases the aim is the computation of targetpoint coordinates and/or heights for each measurement epoch. From the comparison of these coordinates/heights, after all proper statistical and reliability tests, the horizontal and/or vertical displacement vectors of each control point can be determined Triangulation and trilateration horizontal control networks In triangulation and trilateration networks angles/ directions and distances are measured with the proper instrumentation. Manually operated theodolites have been traditionally used for angle measurement in high accuracy deformation surveying. Distances are measured using manually operated electronic distance measurement (EDM) instruments: a. Electronic theodolites. Over the last two decades, the technological progress in angle measurements has been mainly in the automation of the readout systems of the horizontal and vertical circles of the theodolites (fig. 2). Various, mainly photo-electronic, scanning systems of coded circles with an automatic digital display and transfer of the readout to electronic data collectors or computers have replaced the optical readout systems. As far as accuracy is concerned, electronic theodolites have not brought any drastic improvements in comparison with accuracy optical theodolites. Atmospheric refraction is a particular danger to any optical measurements, particularly where the line-of sight lies close to obstructions. b. Electronic Distance Measurement (EDM) Instruments. Short range (several kilometers), electro-optical EDM instruments with visible or near infrared continuous radiation are used widely in engineering surveys. The accuracy (standard deviation) of EDM instruments is expressed in a general form as: s = ± (a + bs) where "a" contains errors of the phase measurement and calibration errors of the so-called zero correction (additive constant of the instrument and of the reflec- 246

6 tor), while the value of "b" represents a scale error due to the uncertainties in the determination of the refractive index and errors in the calibration of the modulation frequency. Typically, the value of "a" ranges from 3 to 5 mm. In the highest accuracy EDM instruments, the "a" value is 0.2 mm to 0.5 mm based on a high modulation frequency and high resolution of the phase measurements in those instruments. One recently developed engineering survey instrument is Leica DI2002 that offers a standard deviation of 1 mm over short distances. Over distances longer than a few hundred meters, however, the prevailing error in all EDM instruments is due to refraction problems during their propagation in the atmosphere. It is interesting to observe that the production of high accuracy EDM instruments (e.g. the Mekometer 5000) was discontinued due to a small demand and an extremely high cost. Fig. 2. Leica TM5100A high precision electronic theodolite c. Dual frequency instruments. Only a few units of a dual frequency instrument (Terrameter LDM2 by Terra Technology) are available around the world. They are not really user-friendly in use but standard deviations of ± 0.1 mm ± 0.1 ppm may be achieved with them. Research in the development of new dual frequency instruments is in progress. d. Three-dimensional positioning systems. Two or more electronic theodolites linked to a PC computer create a three-dimensional (3D) (positioning) system with real-time calculations of the coordinates. The systems are used for the highest accuracy positioning and deformation monitoring surveys over small areas or for measurements for industrial applications. Leica TMS and UPM400 (Geotronics, Sweden) are examples of such systems. Positions (x, y, and z) of targets at distances up to ten meters away may be determined with standard deviations smaller than 0.05 millimeters. These systems can be used for landslide monitoring. In this case high accuracy theodolites measure horizontal and vertical angles to targets on the area under consideration. Standard deviations depend among others upon the accuracy of the instruments used but can be kept in low values Vertical control networks and height determination The traditional technique for height determination is differential leveling. It provides height difference measurements between a series of benchmarks. Vertical positions can be determined to very high accuracy (± mm) over short distances (10-100's of meters) using accuracy levels. Two major classes of accuracy levels commonly used for making deformation measurements are automatic levels and digital levels: a. Automatic levels. The old method of geometrical leveling with horizontal lines of sight is still the most reliable and accurate, though slow, surveying method. With high magnification leveling instruments, equipped with the parallel glass plate micrometer and with invar graduated rods, a standard deviation smaller than 0.1 mm per set-up may be achieved in height difference determination with balanced lines of sight not exceeding 20 meters. In leveling over long distances (with a number of instrument set-ups) a standard deviation of 1 mm per kilometer may be achieved in flat terrain. b. Digital levels. The digital automatic leveling systems with height and distance readout from encoded leveling rods have considerably increased the speed of leveling (fig. 3). There are several contradictory remarks on their performance as high accuracy levels and some improvements are expected to be introduced by the manufacturers but they can effectively be used in landslide monitoring if geodetic leveling is selected as the most suitable technique for height determination. c. Zenith angle methods. High accuracy electronic theodolites and EDM equipment allow for the replacement of geodetic leveling with more economical trigonometric height measurements. An accuracy better than 1 mm may be achieved in height difference determination between two targets 200 m apart using accuracy electronic theodolites for vertical an- 247

7 gle measurements and an EDM instrument. This technique is especially more economical than conventional leveling in hilly terrain, and in all situations where large height differences between survey stations are involved. It can be used in landslide monitoring instead of geodetic leveling. The refraction error is still the major problem with increasing the accuracy of trigonometric leveling. Fig. 3. Leica NA 2003 automated digital level and section from bar-coded invar level rod d. GPS measurements. Height differences can also be computed with the use of GPS receivers. Height determination with GPS has somewhat worse accuracy than horizontal position. GPS control networks have been for land subsistence measurements. It must be noted that GPS provides geometrical height differences that must be transformed into orthometric in order to be compatible to measurements with the other techniques Total station instruments for landslide monitoring In the past, any electronic theodolite linked to an EDM instrument and to a computer was considered to be a total surveying station which allows for a simultaneous measurement of the three basic positioning parameters, distance, horizontal direction, and vertical angle, from which relative horizontal and vertical positions of the observed points can be determined directly in the field. The last years, the manufacturers of surveying equipment produce integrated total stations. Different models of total stations vary in accuracy, range, sophistication of the automatic data collection, and possibilities for on-line data processing. The launch of the motorized theodolites (fig. 4) known also as surveying robots or robotic surveying systems introduced the possibility of collecting 3D positional information for automatic deformation monitoring. They can track a moving target and make automatic measurements of angles and distances to the target in motion. These instruments can make measurements at data rates up to 1 Hz and can operate autonomously once locked to the target that has been manually set by an operator. Current technology provides total stations that are able to measure angles with an accuracy of ± 0.15 mgon, and distances with an accuracy of ±1mm + 1ppm to a range of 3,500 m. Total stations allow the measurement of many points with prism-targets on the surface being monitored within a short period of time. Using Automatic Target Recognition (ATR) technology each prism can be found and its center 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 uses nonactive prisms and hence does not require a power source at each prism, reducing the cost of each prism installation. Early automated vision systems were installed in accuracy theodolites by the 1980's. Its operating components consisted of an external video camera imaging system and a separate servomotor drive. Modern systems are more sophisticated being packaged internally and having an active beam sensing capability. An emitted IR signal is transmitted to the prism that passively reflects the signal back to the instrument. The return spot is imaged on a high-resolution (500 x 500) pixel CCD array. The centroid is located in relation to the current position of the optical cross-hairs (reticule). A series of targets are sighted so the instrument can be trained to their location at least once. With the approximate coordinates of each target prism stored in memory, the ATR system can then take over the pointing, reading, and measuring functions completely within the instrument. The user can program target search radius, data rejection thresholds, and other controls into the operating menus. The first commercial motorized total station was Georobot. Recent advanced systems include for example, the Geodimeter 140 SMS (Slope Monitoring System) and the Leica APS and Georobot III systems based on the motorized TM 3000 series of Leica electronic theodolites linked together with any DI series of EDM. These can be programmed for se- 248

8 quential self-pointing to a set of prism targets at predetermined time intervals, can measure distances and horizontal and vertical angles, and can transmit the data to the office computer via a telemetry link. Similar systems are being developed by other manufacturers of surveying equipment. The robotic systems have found many applications, particularly in monitoring high walls in open pit mining and in slope stability studies. Generally, the accuracy of direction measurements with the self-pointing computerized theodolites is worse than the measurements with manual pointing. Fig. 4. TCA2003 and TDM5005 motorized total stations As with any observational technique, total station systems have both advantages and disadvantages associated with their use. The main advantage of using total station instruments is that they provide threedimensional coordinate information of the points measured. One of the disadvantages is the requirement to have an unobstructed line-of-sight between the instrument and the targeting prism. Another disadvantage is that vertical refraction errors can reduce the accuracy of the height information that may be obtained from the total station measurements. Recently, a few models of EDM instruments with a short pulse transmission and direct measurement of the propagation time have become available. These instruments, having a high energy transmitted signal, may be used without reflectors to measure short distances (up to 200 m) directly to walls or natural flat surfaces with an accuracy of about 10 millimeters. An example is the Leica DIOR 3002 EDM instrument. Reflectorless total stations have also been introduced and can be used in some cases for landslide monitoring. However, the accuracy of distance measurement depends upon the repeatability and reflective properties of the targeting surface or object. It must be pointed out that electronic total station instruments have largely replaced older instruments and techniques, such as theodolites and EDM instruments in many surveying applications. 2.4 Satellite-based geodetic techniques for landslide monitoring The Global Positioning System (GPS) can be used as an alternative surveying tool to assist in geotechnical evaluations of steep slopes by providing 3D coordinate time series of displacements at discrete points on the sliding surface (fig. 5). Current GPS positioning techniques for monitoring applications typically include the use of either episodic techniques for smallscale projects or continuous monitoring for regional scale projects. Each of these techniques has associated trade-offs between system installation and maintenance costs, and the quality of the resulting coordinate time-series. GPS positioning is based on measuring the transit time of radio signals emitted by orbiting satellites. For a receiver to compute its stand-alone position, it must be in view of at least four satellites. Code Phase Positioning is widely used in navigation and low accuracy tracking applications, and relies on the measurement of the modulated GPS signal code phase, which exhibits a resolution of about 1m. The measurement is affected by several perturbations, which bring the achievable accuracy to about 5-10m. GPS offers advantages over conventional terrestrial methods. Visibility among stations is not strictly necessary, allowing greater flexibility in the selection of station locations than for terrestrial geodetic surveys. Measurements can be taken during night or day, under varying weather conditions, which makes GPS measurements economical, especially when multiple receivers can be deployed on the deforming mass during the survey. The accuracy of GPS relative positioning depends on the distribution (positional geometry) of the observed satellites and on the quality of the observations. Several major sources of error contaminating the GPS measurements are: 1. Signal propagation errors--tropospheric and ionospheric refraction, and signal multipath 2. Receiver related errors--antenna phase center variation, and receiver system noise 249

9 3. Satellite related errors--such as orbit errors and bias in the fixed station coordinates. Experience with the use of GPS in various deformation studies indicate that with the available technology the accuracy of GPS relative positioning over areas of up to 50 km in diameter can be of the order of ± 5 mm. Systematic measurement errors over short distances (up to a few hundred meters) are usually negligible and the horizontal components of the GPS baselines can be determined with a standard deviation of 3 mm or even smaller. The accuracy of vertical components of the baselines is 1.5 to 2.5 times worse than the horizontal components. Recent improvements to the software for the GPS data processing allow for an almost real time determination of changes in the positions of GPS stations. Fig. 5. GPS measurements in a landslide area Different types of errors affect GPS relative positioning in different ways. Some of the errors may have a systematic effect on the measured baselines producing scale errors and rotations. Due to the changeable geometrical distribution of the satellites and the resulting changeable systematic effects of the observation errors, repeated GPS surveys for the purpose of monitoring deformations can affect derived deformation parameters (up to a few ppm). Attention to the systematic influences should be made when a GPS network is established along the shore of a large body of water and measurements are performed in a hot and humid climate. The solution for systematic parameters in a GPS network may be obtained by: 1. Combining GPS surveys of some baselines (with different orientation) with terrestrial surveys of a compatible or better precision. 2. Establishing several points outside the deformable area (fiducial stations), which would serve as reference points. These aspects must be considered when designing GPS networks for any engineering project. Although GPS does not require the visibility among the observing stations, it requires an unobstructed view to the satellites that limits the use of GPS only to reasonably open areas. When performing GPS based deformation surveys, the receiver used must be geodetic quality, multichannel, dual frequency, and capable of one second data sampling. The receiver should also be capable of recording the GPS carrier frequency, receiver clock time, and signal strength for each data sample. Typically, a GPS receiver is required for each reference station in the reference network. The same model receiver/antenna combination should be used for each setup. Preprocessing of GPS survey data, at a minimum, must include determination of the 3D coordinate differences and associated variance-covariance matrix in the 3D coordinate system for all baselines observed, and data screening to eliminate possible outliers. Other satellite-related positioning systems such as GLONASS and pseudolites have also been used for deformation measurements but their application to large scale campaigns is still not possible. The GLONASS system does not provide the level of operational capability needed for high precision deformation monitoring. The new GALILEO satellite positioning system introduced by the European Union as a civil satellite system is still in the stage of design Episodic GPS deformation monitoring GPS techniques are used to measure vectors in space among the points of a control network. In order to compute deformations, repeated GPS surveys must be done (for example every few weeks or months). A comparison of their results can give the observed displacements of the network points. Static, rapid-static or real-time kinematic GPS surveying techniques can be used. Precise static positioning using carrier phase differential GPS involves forming double differences to eliminate most errors common to both receivers, and integrating the measurement over time. The method thus requires collecting and post processing a relatively large amount of data, a sufficient amount of 250

10 computing resources, and is by definition non realtime. Real Time Kinematic GPS ( RTK GPS) delivers 3D coordinates with an accuracy of ± (5mm + 2ppm) in real-time with a frequency as high as 0.2Hz. Equipment which provide 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 structures (e.g. bridges and buildings) and landslides. Another possibility is using the rapid static positioning technique. In this way, the time for the measurements at each station is reduced to a few minutes. When using an episodic monitoring system there are lower establishment costs, but there are certain disadvantages as well. These may include: 1. Discontinuous time series that does not always allow the extraction of the trend of the deformations from the noise existing within the observations. 2. Poor systematic error modelling due to short observation times. 3. Safety considerations for personnel re-entering site. Episodic GPS monitoring systems have less cost to deploy but require personnel costs throughout the lifetime of the measurement regime Continuous operating GPS systems GPS networks of regional scale have been established across the tectonic zones at various tectonic plate boundaries. These networks form permanent arrays, which observe continuously GPS phase data mainly for geodynamical studies. In the local scale, continuous monitoring GPS systems can be used for the detailed study of landslide motions. The basic idea is to establish a few GPS stations outside the landslide area as a reference frame. The GPS monitoring stations are established at critical points of the landslide zone in order to measure the landslide motion at discrete points. All GPS stations transmit the GPS data to a master control station where the time series of the motions of the monitoring stations are continuously calculated thus providing the continuous monitoring of movements on the earths surface with a prescribed tracking rate. According to the software being used there can also be an alarm system activated when recorded displacements exceed a pre-determined value. Although that typically L1/L2 GPS receivers are being used, continuous operating systems comprised of low-cost L1 frequency receivers have also been in operation with promising results for landslide monitoring. The actual method of communication between measurement stations and control station may differ depending on the geographic location and the specific requirements of the monitoring project. Where a cellular telephone infrastructure is available and the application is uncritical in both timing and security level, a connection over a cellular modem is ideal. Examples are the long-term, weekly monitoring of a land subsidence, or the daily measurement of a breakwater protection structure. A UHF radio link is a good choice when the application is such that independence from existing telecommunication infrastructures is desirable or essential. This may be the case for landslide or mudslide monitoring, where the same or a possibly unrelated catastrophic event may cause overload or disruption of the cellular telephone infrastructure, thus rendering the monitoring and surveillance system unusable when its operation is needed most. With a radio link, the cost of a single communication is negligible compared to the cellular solution, but the necessary equipment investment may be higher when distances over several km must be bridged between the measurement site and the base station, requiring the installation of radio relay stations. As a third possibility, data are made available through Internet sites. In this way, all elements of a monitoring network may be accessed either locally or remotely from any part of the world. This eases remote diagnostics, as well as applications where more than one network needs to be monitored from a centralized location. The continuous operating GPS deformation monitoring systems, CODMS, are capable to detect horizontal landslide motions with an accuracy of about ± 2-3 mm in near real-time i.e. within a few minutes. The most important advantages for using continuous monitoring permanent GPS: 1. Multiple reference stations can be used. 2. High accuracy can be achieved. 3. Continuous time series is created for a complete study of the phenomena. 4. There is better systematic error modeling capability. 5. There are no set up errors. 6. Minimal user interaction. 251

11 On the other hand, the method has high establishment costs. Often, the cost of installing and maintaining a network of GPS receivers that operate continuously prohibits the use of such systems. Continuous operating GPS systems can give a high level of coordinate precision that can enable the detection of small deformations (in the order of few mm or cm) over long period of time (months or years). This is made possible because of the high temporal density of the resulting coordinate time-series Multi-antenna GPS deformation monitoring systems A multi-antenna GPS deformation monitoring system can provide high precision GPS-derived coordinate solutions of multiple monitoring points using only one GPS receiver (fig. 6). This is achieved by sampling data from a number of GPS antennas using a coaxial switching device. This innovative approach to GPS deformation monitoring reduces the amount of instruments required to measure at multiple locations on the deforming region. As a result, the implementation costs are kept low. The system consists of (at least) one GPS receiver connected to a number of GPS antennas. A coaxial switching device enables data from the antennas to be sampled sequentially. A radio communication system is used to transmit the raw GPS data from the remote stations to the master control station. Fig. 6. Multi-antenna GPS deformation monitoring system The system can be operated on an episodic or on a continuous base. The results of the use of such systems were very promising but the area covered is still small due to the limits in the length of the connection cables of the antennas to the receiver. A dedicated radio system transmitting data from the antenna to the receiver would enhance dramatically the efficiency of this technique for landslide monitoring and other deformation applications. 2.5 Geotechnical techniques for landslide monitoring Geotechnical sensors are used extensively in the monitoring of structures. These sensors are often placed within the structure and out of sight, however they are never out of mind. During construction of the structure 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. The main geotechnical sensors used for deformation monitoring include; extensometers, inclinometers, piezometers, strain meters, pressure cells, geophones, tilt meters and crack meters. Most of 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 analyzable measurements or measurements to calibrate additional sensors, is meteorological sensors. The principal techniques/instrumentation will be briefly discussed below Inclinometers They are instruments installed in boreholes drilled within the landslide mass. They measure the curvature of initially straight boreholes, thus detecting any change in inclination of the borehole casing. They can help for the determination of slip surfaces or zones of 252

12 movement and they reveal the depth of the failure plane(s). This is typically accomplished with the help of a gravity operated tilt sensor. There are many different configurations of borehole inclinometers (fig. 7) available. They are distinguished according to the measurement sensors: vibrating wire transducers, differential wire transducers, servo-accelerometers and gravity activated electrolyte cells. The accuracy attainable can be of the order of ± 0.02 mm over a baseline length of 3 m. The main disadvantage of this type of instrument is that curvature is only observed in one axis Extensometers Various types of instruments, mainly mechanical and electromechanical, are used to measure changes in distance in order to determine compaction or upheaval of soil, convergence of walls in engineering structures and underground excavations, strain in rocks and in man-made materials, separation between rock layers around driven tunnels, slope stability, and movements of structures with respect to the foundation rocks. Depending on its particular application, the same instrument may be named an extensometer, strain meter, convergencemeter, or fissuremeter. The various instruments differ from each other by the method of linking together the points between which the change in the distance is to be determined and the kind of sensor employed to measure the change. The links in most instruments are mechanical, such as wires, rods, or tubes. The sensors usually are mechanical, such as calipers or dial gauges. In order to adapt them to automatic and continuous data recording, electric transducers can be employed using, for instance, linear potentiometers, differential transformers, and selfinductance resonant circuits. Extensometers can measure the axial displacement between a number of reference points in the same measurements axis. They can be installed within a borehole or on the slope surface (fig. 8). The wire extensometer is widely used typically measuring baselines of up to 80 m in length with a accuracy of ± 0.3 mm per 30 m. The actual accuracy depends on the temperature corrections and on the quality of the installation of the extensometer. Maintaining a constant tension throughout the use of the wire extensometer is very important. Steel, invar, aluminum, or fiberglass wires of various lengths, together with sensors of their movements, may be used depending on the application. The main disadvantage of this type of instrument is that the one-dimensional displacement vector cannot measure out of line displacements. The extensometer must also be anchored outside the deformation zone if absolute displacements must be recorded, which can be a problem if the deforming area is large. To determine the total strain tensor in a plane (two normal strains and one shearing), a minimum of three extensometers must be installed in three different directions. A new development in the measurements of extensions and changes in crack-width employs a fully automatic extensometer that utilizes the principle of electro-optical distance measurements within fiber optic conduits. The changes in length of the fiber optic sensors are sensed electro-optically and are computer controlled Piezometers Many landslides are triggered by slope saturation following heavy rainfall. Measurement of pore water pressures and piezometric levels form an important part of slope stability analysis. Threshold levels can be defined to provide early warning of conditions that may lead to catastrophic failure. Fig. 7. Inclinometer 253

13 2.5.4 Geophones They are devices that can measure vibration associated with movement. They are useful as early warning systems (fig. 10). They can detect landslides on the basis of frequency composition, amplitude, and duration of the vibration signal. Fig. 8. An extensometer installed on the slope surface Piezometers measure the pore pressure of groundwater within a geological structure, thus giving an indication of the build up of stresses and strains within the rock mass. Common types of borehole piezometers are the vibrating wire, pneumatic and standpipe piezometers. The vibrating wire piezometers (fig. 9) convert water pressure to a frequency signal via a diaphragm, a tensioned steel wire, and an electro-magnetic coil. So, a change in pressure on the diaphragm causes a change in tension of the wire that can be recorded. They are reliable, accurate and can measure rapid changes in pore water pressure. The data collection can be automated but they have a rather high cost. The other types of piezometers are simple devices, rugged, inexpensive and popular. On the other hand they cannot measure rapid changes in the piezometric head and need to manually measure the pore pressure Tiltmeters The measurement of tilt is usually understood as the determination of a deviation from the horizontal plane, while inclination is interpreted as a deviation from the vertical. There are many reasonably priced models of various liquid, electrolytic, vibrating wire, and pendulum type tiltmeters that satisfy most of the needs of engineering surveys. They are instruments that can measure degrees of rotation. They are based on electrolytic level sensors (fig. 11). They can be more sensitive than and are useful on extremely slow moving rotational failures. Tiltmeters are used to determine the direction of movement, to delimit the areas of deformation and to determine the mechanism of movement (e.g. slumping or slope creep). They can also provide advance warning of accelerated slide movement and quantify the effectiveness of landslide repairs. Fig. 10. An early landslide warning system based on geophones Fig. 9. Sensor of a vibrating wire extensometer Fig. 11. An electrolytic level sensor of a tiltmeter 254