Abstract Number : 446 INTERNATIONAL SYMPOSIUM ON Bali, Indonesia, June 1 ST 6 TH, 2014 MODERN TOOLS FOR THE MONITORING OF DAMS hhdttjjhkljdjjsgshjhfsdkjhskslsl;s;s;;s;;s;;sjsjkjffffrtttttttfggjfgjgkfkjkjf fffffjfjjfkkfjjj Jean-François SAGEAU, Hervé LANCON & Géraldine CAMP Pierre BROUILLAC & Pierre CARREAUD SITES (HQ), Rueil-Malmaison, France SITES, Ecully, France pierre.brouillac@sites.fr ; pierre.carreaud@sites.fr ABSTRACT: The dams are expected to work continuously during decades. In many countries, monitoring actions are initiated in order to assess their ageing which enables structural engineers to make the right decisions for their maintenance. Our presentation introduces different modern tools developed for global monitoring, such as ScanSITES 3D, SIMon-e, as well as several modern sensors as optical fibers, MEMS chain inclinometers and wireless devices. SCANSITES 3D is a combination of three methods dedicated to long term monitoring. It is based on the latest technology in 3D laser scanning and ultra high definition photogrammetry. It aims to produce a GIS file containing comprehensive information about the geometrical deformations of the structure, the defects (cracks, seepage, corrosion,..) and the visual conditions. The dam is surveyed with a LIDAR system to produce a 3D model and a colored map representing the deflections of up or downstream walls. The surface is also captured with an Ultra High Definition picture which can reach 30 billion of pixels. It is processed to get a scaled picture called orthophotography and useful to perform the visual inspection. All defects are recorded into a data base, in which many features are stored: location, length, width and additional information. The sensing technologies, such as SIMon-e, are useful to record physical parameters with a set of sensors (temperature, crack opening, inclinations, displacements...). The system operates in complete autonomy (energy, wireless data connection). All data are available through a secured full web application using a simple Web browser. The global database contains a set of data dedicated to several dams or several previous surveys. With appropriate queries, the engineer can objectively identify an abnormal condition of a specific dam amongst others, as well as detecting an unexpected ageing in a time frame. To conclude, the technologies developed by SITES provide a value-added for the preventive maintenance of the dams in respect to a durable and sustainable approach. Keywords: monitoring, sensors, 3D laser scanning, photogrammetry, visual inspection
1. INTRODUCTION Lots of Large structures like, dams, cooling towers, tunnels, were built in the 60 s. Lots of them are still here and working 60 years later. Some of these industrial projects might be categorized in the next decades as historical heritage for their rareness, their innovation or their size. It would be catastrophic if these structures were to fail, as they have too great an importance: economic or emblematic significance, or just for safety reasons. In this way and regarding their size and constitution, which are exceptional, the structural health of dams, viaducts, or nuclear containment has to be monitored in order to avoid heavy and expensive repairs that might be done too late and using methods which would require shutdown of the facility. For decades, among the existing monitoring devices and methodologies applied to different types of large structures, three are widely used for safety management: visual inspection, geometric survey and sensor monitoring. The first is usually carried out with empiric methods, the second is realized using accurate but discrete methods such as geodetic micro-triangulation and the third, realized with sensors instrumentation is most of time manually collected. This paper introduces a new approach, using an exhaustive and numeric method called SCANSITES 3D and SIMon-e. The SCANSITES 3D is based on the combination of the SCANSITES method, an advanced tool which provides numeric defect inspection of large structures, a new wide ranged LIDAR technology aiming to deliver 3D exhaustive geometric mapping, and very high resolution photogrammetric coverage. SIMon-e is a unique sensor data storage, visualization and remote management platform offered to administrators and customers. It enables them to know the behaviour of their dams, buildings, bridges, monuments, accessing monitoring information, 24/7 and everywhere. This multiple applications system collects automatically data from sensors set inside and on the structure, processes and highlight data through a secured full web application using a simple Web browser. 2. SCANSITES3D OVERVIEW Due to concrete or metal material use, knowledge of the structure cracking, corrosion and visual evolution is necessary for monitoring. In the past, the owners of cooling towers were not completely satisfied with the traditional defect mapping process, using binoculars or rope access. The main drawback is the difficulty to produce a scaled defects map enabling an accurate and reproducible monitoring (crack evolution, opening measurement ). To answer this problem, the SCANSITES was developed in the 1990 s for nuclear cooling towers. This system aims to produce a digital defects mapping connected to a database which is working as a true real-time G.I.S. (Geographic Information System) Figures 1 and 2. It is composed of: - a hardware tool with a robotized video head (focal length from 200 to 4000 mm) and its controllers Figure 1, - a software suite including a database and dedicated inspection tools (defects localization, cracks opening measurement, geometric characteristics, mapping tools) Figure 2.
The whole system is designed to operate on-site, without heavy carriage. Several dozens of dams, cooling towers and nuclear containment building, viaducts and chimneys have been examined by the SCANSITES, in France and across the world, by SITES Company team. 3D model Spatial intersection Picture of a defect observed at 208 meters (122 meters high above the ground) Video and position Motors control Motorised head Control equipment in a van Fig. 1. SCANSITES in operation Fig. 2. Defects mapping (conical projection) The evolution of this system, the SCANSITES3D includes a geometric monitoring with the LIDAR and an entire visual coverage with high definition photogrammetry. If, for the first system, the entire inspection was done on field during 2 to 8 weeks, for the second, after some days on field for the acquisition, the inspection is done at the office with the help of thousand of photos projected and assembled into one scaled mosaic. 3. LIDAR OVERVIEW The LIDAR Figure 3 is a device which produces some high density surveying in 3D coordinates. This technology is mature and can be used for long range applications since less than 10 years. It is based on two angular coders linked to mirrors (horizontal and vertical) and a remote laser distance measurement device. The system works with enough velocity to acquire more than 100 000 points each second. For the majority of large structures, a wide range LIDAR is used. It is able to scan structures, up to 1000 meters onto surfaces. The result of a LIDAR survey, called point cloud Figure 4, is composed of tens of millions points known in XYZ and laser intensity. The average density is 1 point each 5 to 20 mm and the accuracy of the modeling surface is few millimeters (5-10 mm). Figure. 3. LIDAR in operation Figure. 4. Point cloud of dam - intensity colored
4. HIGH DEFINITION ORTHOPHOTOGRAPHIC COVERAGE The photogrammetric coverage delivers exhaustive and high definition pictures of the structure, including the parts without any defects. It is possible to produce a visual inspection, using long focal lens and high definition digital camera in order to produce referenced pictures. The camera and lenses used Figure 5 can give a pixel equivalent to few millimeters onto the structure, which makes it possible to detect the main defects. As the photos positions and orientations are known (XYZ and Euler angular), each photo can be projected on the 3D mesh to texture it. The next step is to project the textured 3D mesh on a primitive (plane, cone, cylinder) to obtain a map. This projected assembly of image is called orthophotography. Figure 6 At each point of view, a complete photo coverage is done with a given focal lens: more than 500 photos can be done for one point of view that s represents about 1200 photos for the complete detailed inspection of a dam. After the projection and assembly of each photo, the size of the orthophotography can reach up to 10 billion of pixels. More and more, with the digital photography and computing progress, the inspection based on orthophotography can completely replace the old SCANSITES inspection. The field operations and associated treatments were designed in order to be fully compatible and replaceable: the tools used to do the inspection, database, mapping and results are the same. Photo coverage Photos mosaic Orientation and referencing Projection Texturing Mapping Figure 5. Camera, lens and panoramic head Figure. 6. Diagram of orthophotography operations 5. OPERATIONS The SCANSITES 3D monitoring requires all data are known in a single XYZ referential. The method can use the one established for the traditional survey (targets, pillars, geodetic points). Regarding the visual inspection, each structure owner has its own requirements. It deals with defects, which have to be surveyed, and the associated classification. One of the most important parameter is the minimum opening for a crack that needs to be surveyed. It mainly impacts the focal length used during the inspection (up to 4000 millimeters) and widely, the total number of defects stored. All those considerations help to prepare the mission, mainly the database and the inspection software. The LIDAR, and the photogrammetric head are set at different locations in order to cover the structure s surface. With the LIDAR, a complete scan is realized. Based on this point cloud, a triangular meshing Figure 7 is generated and converted in a 3D shape. The first
use of this 3D shape is to allow to locate the defects in 3D and/or to project the photos in order to create the orthophotography. Figure. 7. Triangular meshing With those incoming data, the visual inspection is processed. With the SCANSITES, the operator inspects the entire wall by moving the inspection head with a joystick. When a defect is found, it is caught. The 3D map is updated in real time with defects and the database is filled with its characteristics and coordinates. In parallel, and most of time, in replacement of this operation, a complete high definition photogrammetric coverage is done. The operations done to inspect the orthophotography uses the same software than the real time tool but this inspection is realized at the office. 6. TREATMENTS The treatments aim to produce, on a multilayer file, a map containing all defects caught, the geometric deflections and the photogrammetric coverage. The first step is to compare the structure 3D shape to the theoretical shape or to a previous survey. The 3D deflections are extracted and a map is generated. Figure 9, 10 Two ways of representation are possible. One is a coloured map: each colour depending on deflection value. The other way is to carry out a contour line representation. The second step is to overlay the defects surveyed with the SCANSITES or orthophotography, using the same referential network. The last step is to overlay the pictures directly on the structure s 3D shape enabling to produce an orthophotography. With these files, many views can be generated such as composite views: defects/deflections, defects/pictures, or thematic views (based on database queries). 7. CASE STUDY: A CONCRETE ARCH DAM IN FRANCE The SCANSITES3D method was applied on a large dam located in France. Its main figures are 95 meters height and 200 meters crest length. The aim of this job was to connect the geometric deflections to the defects surveyed. The LIDAR survey recorded 70 millions of points, and the defects total quantity was near 4000. The focal lens of photogrammetric coverage has been adjusted between 100 and 500 mm in order to produce an orthophotography of 1.5 billion pixels with a pixel size of 5 mm. Figure 8. The detailed inspection was performed by the SCANSITES tool with a focal length of 1000 mm. The defects have been mapped using a cylindrical projection based on the mean cylinder of the downstream wall. This monitoring was helpful to locate the parts to repair and to store the visual aspect of the wall. The same inspection has been done on the upstream wall in the same referential, enabling the detection of similar cracks from up to downstream.
Defects number Figure. 8. Orthophotography of the downstream wall Figure. 9. Map overlaying deflection/defects Figure. 10. Deflections map with deflection threshold 8. RESULTS 8.1. Statistics SCANSITES3D provides some numerical results: a scaled defect map and defects database. The first result is to produce an accurate report emphasizes defects evolution between two inspections, and the number of defects classified by zone Figure 11. 1800 1744 1600 1443 1478 1523 1400 1313 1200 1076 1145 1000 800 811 600 400 200 0 A B C D E F G H Cracks Seepage Corrosion Miscanellaneous Total by zone Figure 11. Number of defects by family and zone The LIDAR coverage is a guideline for defects analysis, for instance to see if cracks are correlated, or not, with geometrical distortions. 8.2. Use of the data for repairing and utility of geometry The data extracted from the visual inspection are useful to establish an accurate bill of quantities for restoration works, like total crack length to be treated, total corroded bar amount for passivation treatment Providing exact length and exact position of cracks and corrosions allows preparing intervention of reparation with all needed information: quantity of product to use,
localization (high from the floor or the crest, ) and simulations for the access (length of rope and inclination, high of positive or negative crane, ) As the accurate 3D shape of the structure is known, data for planning sensor installation or wells location can be easily computed. 8.3. Advantage of LIDAR to traditional survey Traditional geometric survey uses theodolites and well-known micro-triangulation methods. It presents the advantage to provide some results close to the best possible accuracy (near 1 mm). However, the drawback is it is a discrete method, since it focuses on limited number of points, usually few tens (target, reflectors), not necessary placed on critical parts. In addition, the geometrical surveys with LIDAR were much quicker to carry out than the traditional geodesic method. The survey obtained in this way does not suffer of the wall movements and deflections due to thermal variations during the survey. 8.4. GIS (Geographic Information System) consultation All data (photo, geometric survey, defects maps and evolutions) are overlaid on a same file. The engineer gets a faster way to make his diagnosis compared to the fastidious data fusion imposed by separated reports. The last advantage is to store all collected data in a database, offering efficient tools to measure the structure ageing and widely a fleet of structures. 9. USE OF UAV The treatment chain is fully compatible with digital images acquired with light UAV. Figure 12. Some quad, hex or octo-copters are able to maintain a fixed altitude and to stay at a fixed distance of the wall, enabling us to capture a mosaic of the wall with about 80% of overlapping between two adjacent images. The use of SFM software (Structure From Motion) is very useful and fast to get internal and external orientation and position of each photo inside the dam coordinate system. Next, the presented treatments can project each image on the 3D real shape, then on the plane, cylinder or cone, and finally do the assembly and inspection. Figure 13. About 1500 photos have to be shot for a traditional 200m wide by 100m high dam. There is no limit in term of size of structure and resolution for the treatments. The shooting flight is hard as the pilots must work with irregular wind and GNSS signal loss Figure 12. UAV (MicroDrone) in operation Figure 13. OrthoMosaic and inspection of a part
10. SIMON-E MONITORING SYSTEM SCANSITES3D is a non continuous monitoring method. In the next decades, with the reduction of the equipment costs, it might be possible to install permanent Lidar and high resolution camera on front of structures and access the data with our actual web-based system SIMon-e. At the moment, this tool is useful to locate pertinent positions on the structure to be monitored with sensors as the 3D shape is known. 10.1. SIMon-e Monitoring System overview The objective of Structural Health Monitoring (SHM) is to improve safety and reliability of infrastructures by detecting damages before they reach a critical state and allow rapid post-event assessment. More and more structures are equipped with structural health monitoring systems. They provide a means to monitor performance of key parameters under real environmental conditions. Monitoring systems incorporate a Data Acquisition System (DAS) that processes data from sensors and allows getting parameters in live such as cracks displacement, inclination, deformation, temperature, hygrometer, wind speed Figure 14. Figure 14. SIMon-e Monitoring System overview (Crackmeters on a dam) The purposes of the Data Acquisition System are the followings: Centralize data from the whole sensor network, Make available and manage data with appropriated and organized classification, Enable a fast and easy visualization of any data arriving from any sensor (digital value, curve, etc), Record and manage data file according to a specified organization. All these functions are carried out by a server linked to a control screen allowing the data visualization and the data management via an interface specially developed for each project. These functions have to be available since any computer connected to the web through a secure web interface by defined users (with login and password). So, when an event happens, the Data Acquisition System provides real-time information to quickly identify damaged area(s) which can lead to a reduction in discovery time and break in service.
For the operator, the Monitoring System provides important benefits: Provide real-time sensor data, Alerts owners to any potential problems, Can be used to plan maintenance schedules, Provides instant notification when thresholds are exceeded. The main functionalities of SIMon-e Monitoring System are: Access to monitoring information 24/7, Secure connexion via an internet connexion, Friendly Web interface, Synthesis of all monitoring indicators, Graphical data visualization Figure 16, Threshold configuration (Alerts via Email or SMS message), Data export in multiple formats, System diagnosis. Figure 15. SIMon-e Monitoring System Figure 16. Graphical data visualization 10.2. Specific case: Concrete Dam SIMon-e Monitoring System was used on a concrete dam by using sensors inside and outside the structure such as strain gages for local deformation, optical fibers for global deformation, crackmeters, temperature probe, hygrometer probe, wind speed, tiltmeter for inclination The system functions in complete autonomy (energy by solar panels, wireless data connection). All data are available through a secured full web application using a simple Web browser Figure 15. In our case, the expansion of concrete caused by alkali-aggregate reaction (AAR) causes significant deformation. Several technologies are used to monitor the deformation behaviour, among which are inverted pendulums. The inverted pendulums provide critical information regarding the overall tilt of the dam. The expenses associated with installing pendulum technology and the relatively short life expectancy have led engineers to investigate the feasibility of using ShapeAccelArray (SAA) Figure 18 as a supplementary technology. The micromachined electromechanical system (MEMS) accelerometers used to measure tilt within SAAs are capable of precision approaching that of inverted pendulums, SAA offers also the ability to monitor along a non-straight path.
Figure 17. Inverted pendulum and SAA Figure 18. SAA on reel The data here show good agreement between the SAA and the inverted pendulum. The differences between the two sets of readings, expressed as standard deviations, were within the stated accuracy of the inverted pendulums (0.3 mm) Figure 19. The SAA could become a valuable tool in both automating long term deformation readings, and in avoiding the need for precision drilling to enable inverted pendulum installations. 11. CONCLUSION Figure 19. SAA vs Inverted pendulum We ve presented a new and modern approach for visual inspection, geometric survey and web based consultation. Theses technics are here focused on dams. This method is particularly convenient for every structure which needs the resuming of its monitoring program, because it provides an exhaustive inventory. It also allows the readjustment of an existing monitoring program by completing the partial survey done by conventional approaches. Not only is this method is adapted to concrete structures, but it can also be used on old constructions, masonry-works, clay works Finally, the correlation between defects and deflections is precious information used to locate the areas where geodic surveying and sensors have to focus on. Moreover, besides useful results for the monitoring, the SCANSITES3D provides as-existing mappings which are often lacking on old structures. The second tool, SIMON-e is a web-based application, able to collect data from any kind of sensor like the new SAA based on MEMS.