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July 9, 2015
Geotechnical engineers have successfully completed a new road tunnel in the Norwegian city of Trondheim after overcoming challenges of difficult ground and the preservation of the city’s cultural heritage.
Construction work on the eastbound E6 motorway between Trondheim and Stjørdal in Norway is on track to meet the 2014 completion deadline. However, the project has had its challenges, not least in the construction of a 2.6 km road tunnel on the approach to central Trondheim.
Under normal circumstances, this would have been a routine job. But the engineers soon found that this site was far from ordinary. Firstly, the planned tunnel path ran into 300 m of loose, watery clay, including 100 m of extremely sensitive “quick clay,” and secondly, the route was close to a number of historical wooden buildings, precariously perched on wooden piles.
As a result, when it came to installing retaining walls on either side of the tunnel’s eastern portal, conventional piling with a Hydraulic Impact Hammer could not be employed as this would have created vibrations in the clay, possibly leading to disturbance and potential damage to the historical buildings.
To protect the city’s cultural heritage, the Trondheim authorities arranged for five of the most endangered buildings to be removed from the area. These were simply lifted off their wooden foundations by a giant crane and carried away intact to a safe location.
While this precautionary work was under way, NCC, the project’s geotechnical engineers, concentrated on finding a way to stabilize the ground for the installation of retaining walls, to be drilled into the bedrock with cement injection of the rock socket.
With the site consisting of layers of clay between 9 m and 23 m thick, as well as a groundwater level only about 1.5 m below the surface, all activities that could disturb the clay or lower the groundwater were strictly prohibited.
The Norwegian roads administration, in cooperation with the contractors, decided to test a new method of using drilled casings equipped with an interlocking system along the length of the pipes. Three casings were drilled into rock directly and three were drilled through the quick clay layer and then into the rock.
Extensive measuring was conducted including piezometer, total pressure and inclinometer tests as well as geotechnical testing of samples taken from the soil surrounding the casings. It soon became clear that this was a favorable solution and that Atlas Copco’s large diameter casing system was the only one capable of meeting all of the requirements.
The system, called Elemex, has been developed by Atlas Copco specifically for installations in difficult ground. It consists of a ring bit attached to a casing tube and a pilot bit attached to an inner drill string.
The casing was advanced with the Elemex pilot bit driving the ring bit and with gentle air flushing across the bit face. Each pipe was gradually “pulled” down through the clay until it reached bedrock. Then the pilot bit was used to drill into the bedrock in the normal way. Once the pipe was securely installed in the bedrock, the inner drill string was removed leaving the pipe ready for cement injection.
To drill the casings in place, two modified sheetpiling units were used as DTH (down-the-hole) drill rigs, each equipped with Atlas Copco QL200 hammers, 610 mm casing and 674 mm oversized ring bits. The oversized ring bit allowed the locking elements of each pipe to slide into each other easily and also drilled holes that were large enough for each new casing that followed.
With this setup, a total of 349 interlocking pipes were installed to depths ranging from 11–33 m and 1.2–4.6 m in solid rock. The result is retaining walls that are both watertight and load bearing, horizontally as well as vertically.
The installation work was carried out by Hallingdal Bergboring as a sub-contractor to Kynningsrud Fundamentering which, in turn, was contracted by NCC. Geir Veslegard, Project Manager for Hallingdal Bergboring recalls: “To meet the special demands on this job the Elemex system was a good choice. It was a continuous solution whereby we were able to get through the clay and into the bedrock in one operation. Once we got it right, it became routine.” He adds that the solution fully met NCC’s demands. “In these conditions, I don’t think it would have been possible to install the walls in any other way. They are rigid, watertight and secure and the job was completed on time.”
Comments Trond Imset, Manager of Kynningsrud Fundamentering: “We prepared this solution in partnership with Atlas Copco and Ruuki [the pipe system manufacturer] as an alternative to sheet piling. It has not been done anywhere in the world on such a large scale. The start was slow but when we became more familiar with the method we were installing 14 tubes per day with an average length of 20 meters and boring 1.5 to 4 meters in rock. Overall it went according to plan thanks to good planning and cooperation.”
Gunnar Schmidt, Business Line Manager for Atlas Copco in Norway, explains that the normal procedure when using Elemex is to lock the pilot bit into the ring bit, then rotate and flush to penetrate the soil and bedrock. This normally works perfectly, but in Trondheim a slightly different procedure had to be adopted.
“No percussion was allowed and the friction in the interlocks proved difficult to overcome with rotation and flushing alone,” he said. “Instead we used resonance-free, vibro-rotation, which means that there was just enough force applied to the pipe to get it through the clay without causing vibrations. This was then followed by drilling and flushing with the pilot bit once we got to the bedrock.”
The cut-and-cover Trondheim tunnel was completed at the end of 2011 after which the five historical buildings that had previously been removed were replaced on their original sites. The tunnel was subsequently opened to traffic at the beginning of 2012 – right on schedule.