3-D Scanning Helps Engineers Bore Precise 3.6 km Tunnel

Faro laser scanner keeps tunnel straight, minimizes waste and cost.

Published: Wednesday, December 2, 2009 - 18:59

Most car drivers enjoy road tunnels without giving a second thought to the years of patient effort that go into the construction. In the case of the new 3.6 km-long, newly opened Vintebro tunnel near Oslo, Norway, in three minutes one can drive through a tunnel that took three years to finish and required the removal of 650,000 cubic meters of rock. The company contracted with this 76 million-Euro job was Mika AS of Oslo. Mika is a Norwegian civil engineering company that specializes in hard rock and marine contracting with particular expertise in tunneling.

Ever since the ancient Greeks built the Eupalinian aqueduct through Mount Kastro in 520 BC, tunnel engineers have appreciated the importance of accurate measurements. The Greeks worried about their tunnels meeting in the middle after a mere 500 meters of excavation, but today, tunnels can be many kilometers long and engineers are concerned with fine-tuning the tunnel cross-section for structural and cost efficiency. “Taking out 1 cm more than necessary over the length of the Vintebro tunnel would produce an unnecessary extra 5,000 tons of rock,” explains Lars Murstam, the Mika site manager for the project. “That’s why we invested in FARO’s tunneling solution: the LS Laser Scanner 880 with 'Real Reality Tunnel’ software.”

The Vintebro project was typical for Norwegian hard rock tunnels: First, dynamite was used. Then after removing the debris, hydraulic hammers scaled the walls to remove any loose rocks. Then structural bolts were inserted and shotcrete—a layer of sprayed concrete—was applied to stabilize the rock surface. Thereafter, the drains and the road surface were laid. The final stage was to install precast concrete lining elements. Because these structural and insulating elements are installed after the completion of the road surface, any mistakes in the profile of the tunnel will cause huge problems. Murstam explains the dilemma: “It is vital that we remove enough rock. On the other hand we cannot simply err on the side of caution and remove too much material, because of the costs. That is why we need to get the profile accurate to the centimeter over the full length of the tunnel.”

“The FARO LS Laser Scanner represents great value for money and ease of use,” says Trond Mjanger, Mika’s surveying manager. “In the past, we measured the tunnel section using traditional visual surveying techniques. That gave us only a small number of points and was very time-consuming. The FARO LS Laser Scanner allows us to rapidly gather a huge amount of data, which we process in the ‘Real Reality Tunnel’ software. The software compares the actual profile of the tunnel with the theoretically defined shape to automatically show where too much or too little rock has been removed.”

“We want to send out a surveyor to measure all our previous projects with the scanner. It will only take a couple of days and the combination of the photos and the data will provide us with an incredibly valuable database should any issues arise in the future,” Murstam continues.

Besides using the scanner as an insurance policy, Mika is investigating new applications that could be sold to customers as added-value services. “Because the laser scanner can combine photos with point cloud data, we think we could develop some fairly interesting geological applications,” says Murstam.

About The Author

FARO

FARO develops and markets computer-aided coordinate measurement and imaging devices and software. FARO’s portable equipment permits high-precision 3-D measurement, imaging, and comparison of parts and compound structures within production and quality assurance processes. The devices are used for inspecting components and assemblies, production planning, 3-D documentation, as well as for investigation and reconstruction of accident sites or crime scenes, and to generate digital scans of historic sites. Principal products include the FaroArm, the FARO Laser Tracker ION, FARO Laser ScanArm, FARO Laser Scanner, FARO Gage, and the CAM2 family of advanced CAD-based measurement and reporting software.