A deeper understanding of tunnels

August 2009, International Railway Journal - Keith Barrow

 August 4, 2009

FOR MOST infrastructure managers, the essential and perpetual task of measuring tunnels and other clearances requires a major commitment of both human and financial resources not to mention the rising and pressing requirements regarding safety and tunnel accessibility. In the best cases, data is collected by manually operated measuring devices (MODs) at very low speed which when in use occupy track capacity which could otherwise be used by revenue-earning trains. On lines with a high traffic density, finding the time to carry out such inspections safely and without disruption to traffic is a perennial challenge. But recent technological advances mean a new generation of more compact measuring systems is now emerging, offering the opportunity to fit such equipment to normal service trains.

Last month MERMEC began testing its latest optical triangulation measuring solution, T-Sight 5000, which consists of two modules, a clearance gauge measuring system and a tunnel wall inspection system. T-Sight 5000 produces an accumulated image, and all objects inside the cross section are scanned by the laser beam and reflected inside the camera (see panel). As the object moves through the laser beam, contour slices are generated and the complete object shape is reconstructed by uniting the acquired sections.

Clearance gauge module is capable of measuring up to 1000 profiles per second, while tunnel wall inspection module acquires 4000 profiles per seconds, meaning T-Sight 5000 can recognize defects with a resolution of 2 mm travelling at 30km/h. The information acquired by the scanner can then be used to produce computer renderings of the infrastructure. The scanner module and software can automatically detect some types of infrastructure defect almost instantly, and profiles can be viewed in real time, although detailed analysis is normally made after the recording run because T-Sight 5000 produces such vast quantities of data – 4 million points are measured per second, and the camera is capable of acquiring 80 megapixel data images.

The system has a 360° measurement field which means it can also be used to measure various other infrastructure clearances such as ballast profiles and track alignment. All measured parameters are referenced from the track centre and track geometry data, while 3D ballast profile acquisition is referenced to the sleepers. This means T-Sight 5000 can detect abnormal swelling or subsidence of the ballast, as well as deviation from the normal 3D profile.

MERMEC says another major advantage of T-Sight 5000 is that it can accurately measure the position of weights used to tension catenary contact wire. Fluctuations in temperature cause the contact wire to expand and contract, which means the height of tensioning weights relative to the ground changes according to climatic conditions. Because of its high-frequency scanning capabilities, T-Sight 5000 can establish whether these fluctuations exceed the operator’s specified limits, allowing catenary to be maintained to a safe standard.

The system is also capable of measuring platform clearances and the alignment of adjacent tracks.
T-Sight 5000 is going to be installed onboard the Swiss Federal Railways (SBB) measuring vehicle developed by MERMEC and it will be put into operation starting from September 2009. SBB’s 3011km network has 307 tunnels totalling 259km, and a dedicated team of engineers carries out inspections on an almost continuous basis.

MERMEC says one of the main technical challenges encountered in the development of T-Sight 5000 was the need to produce a system small enough to mount on conventional railway vehicles. This was deemed an essential requirement because it would allow operators to significantly reduce the cost of measuring by obviating the need for specialist rolling stock. Having achieved this, Mermec is confident that there is a significant untapped market for such technology, particularly among the ever-increasing number of railways with tunnels more than 2km long. Indeed, Metros are considered a key market because of the need to carry out regular inspections on lines with long underground sections and a very high traffic density.

MERMEC also plans to offer a turnkey measurement service using its own rail vehicles and technicians, giving smaller operators access to valuable data about their infrastructure which might previously have been impossible due to the cost of acquiring dedicated measuring equipment.


IRJ - August 2009
A deeper understanding of tunnels

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