Tunnel Condition Monitoring

A way out of manual inspection

Modern transport infrastructure would be inconceivable without tunnels. In Germany, there are hundreds of road, rail and underground tunnels requiring regular maintenance. Many of these tunnels have been in operation for decades, so regular maintenance and repair are becoming increasingly important in order to avoid safety risks and consequential damage. Today, inspections are largely performed manually in a slow, costly and unobjective process, while the degree of digitization is low.  The inspections are lengthy, expensive and do not provide any objective condition data. In the near future, however, Building Information Modeling (BIM) will also become the standard in tunneling requiring complete digitalization of the building information. Fraunhofer IPM develops technical measurement solutions for a cost- and time-efficient inspection process during regular operation.

Measuring systems for tunnel inspection

The Clearance Profile Scanner CPS-Plus by Fraunhofer IPM enables fast and efficient inspection of tunnel structures. The optical measurement system provides absolutely referenced and high-resolution information on the 3D geometry, surface structure and wall moisture of tunnels in a single measurement process.

We are currently developing an advanced system with a new type of light deflection unit, which for the first time enables a full 360° scan of the structure. Measuring speed is four times faster than with commercially available systems, while data quality remains the same. This will enable simultaneous recording of relevant condition characteristics even at high driving speeds of the inspection vehicle. This makes it unnecessary to block the structure. The compact and flexible system design also allows the measuring system to be used on a wide variety of mobile platforms such as pigs, trains or road vehicles.

© Fraunhofer IPM
The CPS-Plus measuring system provides BIM-compliant data for monitoring the condition of tunnels. This includes geometric data, data on surface structure and moisture.

Optical hammer impact test for detecting surface damage and delamintions

For tunnel inspection, we are working on a laser-based method for delamination testing on tunnel walls, which is intended to replace the state-of-the-art hammer impact test. The hammer test involves manually tapping the tunnel wall with a hammer to provoke resonance vibrations which hint at detect hidden defects, such as delamination. In the future, a pulsed laser will be used to do this: The laser creates plasma in the air right above the object’s surface without damaging it, which will excite characteristic resonance vibrations on the surface – similarly to a hammer – if delaminations, cavities or defects are present underneath the surface. The mechanical vibration of the concrete surface is measured directly via the frequency shift of the backscattered light using a laser Doppler vibrometer (LDV). The amplitudes and frequencies of the vibrations provide information about the size and depth of the cavities and defects. During test measurements in the laboratory, the laser-based method was able to detect defects up to a few centimetres in size that could not be detected using the classic hammer impact test. Initial test measurements under real conditions in various tunnels also delivered promising results.