Optical fibres safeguard bridge

The five optical fibres attached to the underside of the almost 70 year old Göta River Bridge in Gothenburg are both thin and fragile, but they are used to confirm the safe use of the bridge and to extend its life with ten to fifteen years.

Photo: Stig Hedström, Swedeish road and Highway Administration (Vägverket)

The fibres are used to measure the load from the daily traffic and to detect formation of tensions and cracks. The technological advancement behind these fibres has given the measurements a level of sensitivity and precision that was previously impossible.
 
-These optical fibres make it possible for us to follow from the office whether tensions or cracks are occurring in the bridge during periods with heavy traffic. If a crack occurs we can track its whereabouts with an accuracy of ten centimetres, Frank Myrvoll explains. He is NGI's project manager for this work.

The Optical fibre cables on the Göta River Bridge thereby provide a uniquely detailed continuous image of the condition along the bridge that would have been impossible just a few years ago. - If we were to create something similar with the old technology we would have had to install at least 50 000 points of strain gauges, or other types of sensors, which would have been out of question due to its enormous cost. Now, on the other hand, five relatively cheap optical fibres have been set up and they continuously give us information about the condition of the bridge, Myrvoll adds.

A Worn out Bridge
The reason why this bridge in downtown Gothenburg is being monitored is that the local traffic authority wanted to make sure that it is safe to use the old bridge until a replacement is made. The bridge is old and quite worn out, but is an important arterial road that carries around 26 000 vehicles a day. A number of minor cracks and several other fatigue damages have occurred. In the period between 1996- 99 the bridge was repaired for 110 million SEK. A detailed investigation has later indicated that the bridge can be used until 2020, but only if the amount of traffic is kept below a certain level and that the condition of the bridge is constantly monitored. This is where the optical fibres come in.

A new fibre-optic measurement method
The surveillance system uses the Brillouin scattering fibre optic measuring method. Basically this method sends light into one end of the fibre and measures the light that comes out in the other end and reflection. If the fibre has been exposed to a mechanical strain or a deformation across the fibre, the strength or the frequency of the light will change. These changes can be used to calculate exactly where the strain has occurred.

In 2005 the traffic authority in Gothenburg assigned to NGI the job of designing, producing and monitoring the surveillance system. This was after NGI had evaluated different surveillance systems. -We evaluated laser based systems, other optical solutions, image processing methods, monitoring through welded or glued strain gauges etc. The traffic authority wanted very detailed and continuous monitoring of the entire bridge, and the Brillouin scattering method was the most precise and the most economic, Myrvoll says. He has been the project manager at NGI from the very start of the project.

- The greatest advantage with the Brillouin scattering method is that it makes it possible to measure the continuous load of the entire bridge in a very cost efficient way. Also, the optical measurements are immune to electric noise and they are very accurate and precise, Myrvoll sums up. The method was developed at a Swiss university and later commercialized through SMARTEC (www.smartec.ch), which is NGI's subcontractor.
 
Makes the bridge safe
The Göta River Bridge was built in 1939 and was expanded in 1958. The middle part of the bridge has a small lifting part that can be opened to let large boats through. The bridge rests on seven large longitudinal steel beams.  - The steel in these beams has relatively poor and very varying quality, which makes it impossible to predict where cracks could occur. Hence it is necessary to monitor the entire bridge, Myrvoll says. He emphasises that cracks, in theory, can develop anywhere along the seven beams.

Optical fibres has now been installed on five of the total seven supporting beams. This adds up to approximately five kilometres of fibre. All the fibres are attached to a data acquisition system which registers and analyses the data every other hour. In principle it is possible to detect changes anywhere on the cable, but for practical reasons it was in this case restricted to an accuracy of ten centimetres. This implies that the system detects cracks between two points of measurement that are ten centimetres apart.

The system automatically alarms if a micro deformation bigger than a 20 microstrain (20 x 10-6 m/m) occurs, even though that not necessarily is dangerous.  - However, if the system detects increasing deformations, like a crack bigger than 0,5 millimetres, the bridge will automatically be closed for traffic. In such instances the bridge cannot be before it has been inspected and found safe. Still this has yet not occurred.  - It is important to point out that this surveillance system makes it safe to use the bridge, Myrvoll says.

Other bridges under surveillance
NGI has previously conducted other surveillance projects for bridges. The Källösund Bridge is a very important bridge for the Swedish economy since the transportation of both Volvo and Saab depend on the bridge to reach their future markets. Therefore, when large cracks were discovered in the bridge structure, it was not an option to shut the bridge down. Rather, they engaged NGI to install a surveillance system for bridge monitoring, until the bridge was adequately improved and strengthened. Other bridges with NGI surveillance and monitoring are the Skarnsund Bridge and the Helgeland Bridge in Norway, as well as the Uddevalla Bridge in Sweden and the new Svinesund Bridge between Norway and Sweden, which opened in 2005.

Fibre optics for tunnelling surveillance?

The Research Council of Norway recently granted funds for a project to examine the possibilities of using optical fibre in monitoring and thereby securing tunnels.

In the last decade the Brillouin scattering method has been used to monitor a number of bridges, buildings and dams across Europe and the USA. It has also been used on the new Årsta railroad bridge in Sweden.

- The technology is also suitable to monitor steel and concrete structures, underground facilities, pile foundations, landslide prone areas and tunnels. For example, by installing a few optical fibres in the sprayed concrete in the ceiling along the entire length of a tunnel, the system would automatically register small deformations which could indicate potential tunnel collapse. It would also tell you exactly where the mischief occurred, Myrvoll adds enthusiastically. In light of the unexpected rock fall in the Hanekleiv tunnel in Vestfold, Norway in December 2006 caused last year, this has great potential.

NGI is now investigating testing the use of optical fibres in tunnels in a research project called: "Tunnel safety - documentation and verification", partially financed by The Research Council of Norway.

- There is an increasing need to monitor the infrastructure, such as bridges, main transportation lines and other major structures, Myrvoll says. The trend is that axle loads and operating speed of trains and cars are increasing, resulting in increased loading and wear and tear.