Introduction

Viaduct of Millau monitoring was designed by a team of experts already involved with the structural design of the viaduct. With the owner, they have identified objectives and priorities. Data management, supervision and maintenance of the Monitoring System were studied as well. This paper describes the monitoring program and results measured during of the construction phase are presented in the full paper.

1. Aims of the Monitoring

Monitoring is divided into three phases: construction, acceptance and long-term survey. The aims of the monitoring of the viaduct are different at each phase:

During construction, measurements are necessary to control the geometry and the displacements, mainly during launchings of the deck. Topographic techniques are mainly used; they allow to check that the behaviour of the viaduct is in conformity with the predicted values.

At the Acceptance of the viaduct the reference state of the viaduct is recorded. This will be used as the "starting point" for the long term monitoring.

Long-term survey includes three aspects: control of specific parameters related to the safety of vehicles and passengers, control of the behaviour of the viaduct and control of the aging of the structure.

2. Monitoring Design

Monitoring measurements correspond to precise goals. After priorities were agreed, detailed technical specifications were written. A first three years period for a continuous monitoring of the Viaduct of Millau is predicted. After this time, knowledge of the behaviour of the viaduct, particularly under windstorms ought to be complete. Monitoring will then be revised for very long term monitoring.

Data processing and analysis of monitoring measurements must be easy. It means that they could be analysed by specialised and even by non-specialised people. The first analysis after data processing is the comparison with levels with predefined acceptable deviations. Then, if necessary, a detailed analysis will be carried out by a group of engineers and experts.  Management of thousands of data requires careful attention. Data were organised at the beginning of the monitoring. Recorded data and data files are structured to be directly exported to a spreadsheet software instead of a database for a better efficiency.

3. Monitoring program

The viaduct monitoring has been voluntarily separated from the monitoring systems used for safety purpose. The structural monitoring system has no warning or alarm use. Most of the instruments for the monitoring - sensors, data acquisition modules, and network - are industrial devices. Except for concrete behaviour of foundations, we did not select fibre optic extensometers, because of their poor frequency response and their price: long base extensometers were preferred. The meteorological instruments used for the traffic supervision includes cup anemometers, vanes, ice detectors, visibility, temperature and hygrometry, rain and snow height, pavement temperature and state. The monitoring program includes survey with automatic levels, total station and prisms, GPS (fig.1), temperature and displacement sensors, extensometers, tiltmeters, accelerometers, load cells and meteorological instruments.

4. First results during construction

During launching, many parameters are recorded and compared with the designers’ calculations; the position of the temporary “neck” of the deck, measured with a total station and a DGPS receiver is shown on figure 1 or the effect of the wind on the deck (figure 2).

Modelisation of temperature effect on concrete piers deformation from numerous automatic temperature measurements, allows to predict the displacements of the top of the piers, during the construction.

5. Conclusion

In order to verify the calculations and to be able to judge the behaviour of the structure during construction and in particular during launching operations, an instrumentation programme has been put in place, considerably more through than would normally be the case for a cable-stayed structure. This programme allows monitoring of the behaviour of all the elements of the structure during construction (foundations shafts, foundations slab, piers, temporary piers, deck, pylons, stay-cables).

The operations associated with the official handover of the structure will enable baselines to be established which will subsequently act as references for the later monitoring of the viaduct during its operation.

The analysis of the monitoring shows that good design, organisation and management are the essentials to the monitoring.

Fig. 1 Computed and measured levels of the deck during launching (GPS)

Fig. 2 Wind speed during launching

Millau Viaduct after launch L6 north