structural 3d – a new approach for the monitoring of the structural health evolution of dams

8/18/2019 Structural 3D – a New Approach for the Monitoring of the Structural Health Evolution of Dams

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Scansites 3D®: a new approach for the monitoring of thestructural health evolution of dams

H. Lançon, Technical ManagerSites

Espace Européen – Bât C4 allée Claude Debussy, 69130 EcullyFrance

S. Piot, Project ManagerSites

2 bis avenue du Centre, 92500 Rueil-MalmaisonFrance

Abstract

Dams are essential structures for economic development, usually for water supply and electricity production.Their preventive and predictive maintenance is necessary to extend their lifespan but their accessibility remainsdifficult. Issued directly from Sites 25-year experience in the implementation of remote methods of investigation based on the follow-up of visual and dimensional defects on these structures, Scansites 3D® is now the tool of

reference for the diagnosis, follow-up and monitoring of dam using current technologies.

1. Introduction

Concrete dam is usually a massive structure and failure is not allowed due to its importance in terms of safety,economic and emblematic position. In this way and regarding its size and constitution which are exceptional,structural health of dams need to be monitored in order to avoid heavy and expensive repairs if done too late and

with methods which do not require shutdown of the facility.Since decades, among the existing monitoring devices and methodologies applied to dams, two are widely usedfor large structures safety management: visual inspection and geometric survey. The first is usually carried outwith empiric methods, and the second is realized using accurate but discrete methods such as geodetic micro-triangulation.This paper introduces a new approach, using an exhaustive and numeric method called “Scansites 3D®”.

The Scansites 3D®

is based on the combination of the Scansites®

method, an advanced tool which providesnumeric defects inspection on large structures, a new wide ranged LIDAR technologies aiming to delivergeometric exhaustive mapping, and photogrammetric coverage.In the first part of this paper, we will introduce the Scansites® method, in a second part the LIDAR coverage andin the third one, the photogrammetry. We will explain how the combination is performed and which data can beextracted on large structures. Before concluding, we will extend this paper with additional data which could be

overlaid, such as thermographic pictures.

2. Scansites® overview

Due to concrete material use, the knowledge of its cracking and evolution is necessary for monitoring.In the past, many owners weren’t completely satisfied with the traditional defects mapping process, using

binoculars or rope access. The main drawback is the difficulty to produce a scaled defects map enabling anaccurate and reproducible monitoring (crack evolution, opening measurement…). To answer this problem, theScansites

® was developed in 1990’s. This system aims to produce a numeric defects mapping connected to a

database which is working as a true real-time G.I.S. (Geographic Information System) “Fig. 1, 2 and 3”. It iscomposed of:

- an hardware tool with a robotized inspection head and its controllers,

- a software suite including a database and several dedicated inspection tools (defects localization, cracksopening measurement, geometric and pathological characteristics…).

The whole system is designed to operate in-field, without heavy carriage. Several dozens of dams have beensurveyed by the Scansites®, in France and across the world, by Sites Company team.


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Figure 1: Scansites® in operation

Figure 2: Defects mapping (conical projection)

Figure 3: Picture of a defect captured with SCANSITES®

3. Lidar overview

The LIDAR “Fig. 4” is a device which aims to produce some high density surveying in 3D coordinates. It is based on two angular coders and a remote electronic distance measurement device. The system works withenough velocity to acquire more than 100 000 points each second. For the majority of dams, a wide rangeLIDAR is used. It is able to scan structures, up to 1000 meters onto surfaces.

The result of a LIDAR survey, called “point cloud” “Fig. 5”, is usually composed of tens of million pointsknown in XYZ. The average density is 1 point each 5 to 20 mm and the accuracy of the modeling surface is fewmillimeters (5-10mm).

Structure 3D model Picture of defect observed at 208

meters (122 meters high above

the ground)

Spatialintersection

Motors control

Video position


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Figure 4: LIDAR in operation on a survey pillar

Figure 5: Point cloud of dam - intensity colored

4. High definition orthophotographic coverage

In this case, the photogrammetric coverage aims to deliver exhaustive and high definition pictures of thestructure. The goal is to be able to produce a visual inspection, using 3D referenced pictures. The camera and

lenses used can give a pixel equivalent to few millimeters onto the structure, which makes it possible to detectthe main defects. As the photos orientation is known, each photo can be projected on 3D mesh to texture it. Nextstep is to project the textured 3D mesh on a primitive projection (plane, cone, cylinder) to obtain a map. This projected image is called orthophotography.

5. Operations

In this section, we will explain the different steps required to produce a Scansites 3D® survey. As previously

mentioned, all data are known in 3D referential. For that, the method can use the one established for thetraditional survey (targets, pillars). In case where there is no available network, it is necessary to create one, based on singular points on the structure and determined with traditional survey operations.

Regarding the visual inspection, each dam 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 fora crack that needs to be surveyed. It mainly impacts the focal length used during the inspection (up to 4000millimeters!) and widely, the total number of defects stored. All those considerations help to prepare the mission,mainly the database and the inspection software.

At this step, in-field operation can begin.


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Figure 10: Deflection map (deflections are in centimeter)

Figure 11: Map overlaying deflection/defects and magnifying


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As the accurate 3D shape of the dam is known, data for planning sensor installation or wells location can beeasily computed.

Traditional geometric survey uses theodolites and well-known microtriangulation methods. It presents theadvantage to provide some results close to the best possible accuracy (near 1 mm). However, the drawback is itis a “discrete” method, since it focuses on limited number of points, usually few tens (target, reflectors), notnecessary placed on critical parts.

Even if Scansites 3D®

is less accurate, the high density scan produces some surface definitions near to 3-4 mmuncertainty. Usually working with few million points, we get global information. It widely improves the

sensitivity of the geometric diagnosis, showing all details.In addition, the geometrical surveys with LIDAR were much quicker to carry out than the traditional geodesicmethod. The survey obtained in this way does not suffer of the wall movements and deflections due to thermalvariations during the survey.Another advantage is these methods work on every structure, even if there is no surveying equipment such astargets.

All data (photo, geometric survey, defects maps and evolutions) are overlaid on a same file. The engineer gets afaster 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 structureageing and widely a “fleet” of structures.All these jobs are performed without rope access, increasing dramatically the safety conditions.

8. Conclusion

We’ve presented a new and modern approach for visual inspection and geometric survey, here focused on dams,with Scansites 3D®. This method is particularly adapted to every structure which needs the resuming of itsmonitoring program, because it provides an exhaustive inventory. It also permits to readjust an existing

monitoring program by completing the lacks forgotten by classical approaches. Not only is this method adaptedto concrete structures, but it can also be used on old construction, masonry-work, clay works… The correlations between defects and deflections are finally some precious information to locate the areas where geodic surveyingand sensors have to focus on. Moreover, besides useful results for the monitoring, the Scansites 3D® provides as-existing mappings which are often lacking on old structures. This method is widely applicable on large structuressuch as dams, but also skyscrapers, chimneys, cooling towers... The next step, in progress, is to overlay a high

accuracy thermographic imagery survey. The aim is to study the possible gain in diagnosis, mainly on cracks.

The Authors

Hervé Lançon, graduated in surveying from ENSAIS, the French engineering university, in 1985. He worked two years inthe European Nuclear Research Center (CERN), in Geneva, where he was in charge of underground works controls of the

27km tunnel accelerator during the excavation and civil work. Then, he was the Production Manager in a photogrammetriccompany. In 1988, Hervé Lançon joined Sites where he developed tools and activities in the core-business of

structures’ durability engineering. Today, he is Sites’ Technical Manager.

Sébastien Piot, graduated in surveying from ENSAIS, the French engineering university, in 1997. That same year, he joinedSITES as an engineer. He was charged to work on large structures both for monitoring (sensors) and surveying. In 2004, hewas MIR Department (Metrology, Instrumentation, Survey) senior executive manager at SITES-Paris. From 2010 to 2012,

Sébastien Piot was in charge of the development and the support of SITES production departments. Since October 2012,Sébastien Piot is the Technical Manager of Sites AFLA, the subsidiary of Sites Company in South Africa.

structural 3d – a new approach for the monitoring of the structural health evolution of dams

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