In a project to monitor Britain's railway tunnels, Halcrow Group Limited in London employed 3D laser scanning technology to efficiently evaluate deformations in the tunnel walls. The acquired data will aid Halcrow's client, Network Rail, the owner of Britain's railway infrastructure, in monitoring the potentially dangerous bulges for worsening conditions. "These are all brick-lined, Victorian-era tunnels," says Thomas Greig, project manager for tunnels and underground space at Halcrow. "There are two issues. If there's a bulge, it might require remedial work to repair, based on our back-analysis of how much it can bulge without collapsing. The other issue is it might be approaching or encroaching on the clearance envelope [the minimum space required for rolling stock to pass without clashing] thus requiring specific monitoring."
What drove Halcrow to investigate laser scanning? "The traditional approach, which is to rip it out and rebuild, is no longer considered cost-effective," Greig reports. Seeking a better solution for its client, in 2001 Harcrow began experimenting with existing laser data that Network Rail had collected for clearance analysis. In 2003 it selected Z+F's (Zoller + FrÃ¶hlich USA Inc., Duquesne, Pa.) Imager 5003 3D scanner to collect the data for comparison of the portions of the tunnel known to be undeformed to portions that are deformed. This process is then used to predict the need for either remediation or future monitoring.
Why 3D Laser Scanning?The data from Network Rail that Halcrow first experimented with had been collected using laser technology from SpaceTec Datengewinnung GmbH (Frieburg, Germany), according to Greig. But Halcrow concluded that "using SpaceTec for one-off jobs would be prohibitively expensive because of the requirement to work from a road-rail vehicle and [because of] the prohibitive costs of mobilizing from Germany to the UK." Halcrow, however, had several options for data collection: visual inspection, as relied upon for the last two hundred years; conventional surveying with a total station, picking up points at intervals; use of a 2D profiler (a laser scanning device for collecting 2D cross-sections of the inner surface of the tunnel lining) aboard a custom trolley; use of a 3D scanner aboard a trolley platform; and use of a 3D scanner aboard a train.
Of these, Halcrow decided that 3D laser scanning was the solution that best met Network Rail's needs to compare the profile of the tunnel at a specific time with the profile of the tunnel at a later or earlier time; to capture data at a minimum rate of 100 m per hour due to limited track possession times; and to achieve high data density to ensure validity of deformation analysis.
The specific solution chosen by Halcrow was to mount the scanner on a trolley platform. "Halcrow had already been engaged with Z+F for laser scanning in earthwork surveying such as unstable slope and slip monitoring," Greig reports. After evaluating some other scanner manufacturers, the company selected Z+F for deformation analysis because it had a local presence and was previously used on Britain's railways. Z+F was "very willing to adapt processes to suit [Halcrow's] needs," and offered competitive prices, according to Greig.
Why a custom trolley rather than a train-based system? Greig explains that laser scanning of tunnels from onboard passenger trains can be done, but does not make sense for tunnel deformation analysis. "Train-based scanning is cheaper if you have lots of length to do," he says, citing tunnel clearance analysis as an example. But in deformation analysis, Greig says, "we only get particular tunnels or small sections of tunnels that clients ask us to look at."
Key BenefitsWhat are the key benefits of 3D laser scanning? "The improvement in dimensional control [over the old methods of visual inspection or traditional survey using total stations] lets us monitor better and have better control over when remedial works are needed," Greig reports. "We can tailor our responses more finely, which potentially saves a lot of money for our client." Why is this? "We like the high density of data from 3D scanners. I don't think it would be practical to do it using any of the other alternatives." Important benefits of the technology also include accuracy and the acquisition of visual records of the tunnel lining condition.
Greig also reports that high-density 3D images are useful as "visual records of what's going on inside the tunnel," something a 2D profile scan does not provide. "We use that [visual record] as a comparison between two surveys to tell where the bulges or cracking are getting worse. We just visually compare two images, not automatically." Automatic comparison to detect crack formation is available in software from SpaceTec, according to Greig, "but this will only work for new, concrete-lined tunnels. I believe that the image data from brick-lined tunnels is far too complex for current technology to distinguish cracks from other features. We currently don't know of any software to compare profiles automatically, but believe that if we can get the funding, this could be developed relatively easily."
Work ProcessHalcrow outlines its work process:
- Z+F Imager 5003 mounted on simple platform trolley
- Scanner position fixed by survey to permanent ground markers at tunnel portals
- Scanner position tracked inside tunnel by use of total station
- Scanner stopped at intervals*, and a 3D 360-degree by 312-degree scan taken
- Rail positions surveyed conventionally for route proving (not required for just deformation analysis)
- 3D point clouds registered together in Z+F LFM software
- 3D point clouds organized automatically into consecutive 2D slices in LFM software
- Data output in 2D linear view format into SpaceTec's TuView software through direct link from LFM to TuView
To begin, "we assume an as-built profile, taken elsewhere in the tunnel and use it to measure deformations," Greig reports. As-built profiles are taken from portions of the tunnel deemed not to be deformed. Traditionally these profiles were taken from paper as-built records, but as Greig notes, these "tend to be poor or unavailable, as the tunnels were constructed [more than] 150 years ago." Once an as-built profile has been acquired, the scanner collects data from the portion of the tunnel in question. "The [3D point cloud] as collected is converted by Z+F as a service into slices through the point cloud for the client to use on its choice of software."
Halcrow then uses SpaceTec's TuView software to compare the reference profile with the scanned portion of the tunnel and identify differences between the two profiles. With this information, Halcrow identifies the locations of tunnel deformations for Network Rail, which can then inspect these areas manually. Another benefit, Greig reports, is that "we do back-analysis of the laser profile to determine possible causes of the deformation." This information also lets Halcrow gauge the rate of movement, which Greig notes is generally "not that great. We're talking in terms of years, not months."
What's next? To date, Greig reports, "something in the order of 4,000 meters of tunnels have been scanned using the 3D laser scanning method, and it appears to be accepted as the appropriate technology for this application. We are looking forward to future projects to enable every tunnel in the network to be scanned."
Spar Point Research LLC provided this report. For information on Spar Point LLC, visit www.sparllc.com.
*The intervals are whatever is needed to yield coverage of the sections requested to be monitored by the client. For example, scanning one specific bulge in a tunnel wall may require only a single stop. If a longer segment of tunnel needs to be scanned, the intervals are determined by the useful range and field of view of the scanner: one segment of tunnel is scanned, then the trolley is rolled down the track to a new scan position, making sure that there will be enough overlap between the two scans to allow the two scan data sets to be registered (i.e., accurately merged together).