The following is an excerpt.
A submerged, floating tunnel in the Sognefjord on Norway’s west coast will withstand powerful explosions. Tests in SIMLab’s shock tube at NTNU show concrete to be tougher than assumed.
“We carried out the first tests with five centimetre thick, unreinforced concrete plates. We subjected them to a shock wave of seven bar. We chose this pressure because it was stated in the guidelines of the Norwegian Public Roads Administration (NPRA). To our surprise, very little happened,” says Martin Kristoffersen.
He is one of the many researchers who work to establish the foundations for a decision about a future ferry-free coastal highway, the new E39, along the west coast of Norway from Kristiansand to Trondheim. According to Kristoffersen, there is an increasing probability that we will soon see the world’s first submerged, floating tunnel.
Very good margin
The NTNU researcher has one year left of the three-year project he is engaged in. The task is to find out what happens when a submerged, floating tunnel is subjected to an inner explosion.
The findings so far will soon be presented in a peer-reviewed journal. They are based on a large number of tests with varying shock waves in the shock tube. The conclusion so far is that a submerged, floating tunnel will withstand most plausible explosion scenarios with good margin, including tank lorry incidents like the recent one in the Oslofjord tunnel.
It should be noted that there is a big difference between the five centimetre thick plate in the first test and the final dimensions. The tunnel wall under consideration will be between 80 and 100 centimetres, which is 15 to 20 times as thick, and reinforced. Even the car bomb Anders Behring Breivik set off outside the government administration complex in Oslo in 2011 would have had problems. It had an explosive force equivalent to 700 kilos of TNT.
Since NTNU’s six million NOK shock tube was in place in 2014, steel, aluminium and glass have been subjected to strong shock waves. Now the same has been done with concrete.
“It has been a win-win situation. On one hand, we have been able to test the shock tube on a new material. On the other, the tube is unique when it comes to performing repeatable loads under controlled circumstances. This has proven very useful as we want to learn how concrete tackles strain,” Kristoffersen says.
The tube is quite solid. In the pressure chamber in one end, it is possible to build a pressure of 170 bar. That equals the pressure at 1.7 kilometres below sea level. In practice, most tests are carried out with much lower pressure. Kristoffersen actually holds the record so far, with 80 bar. When the shock wave hit the test plate in the other end of the 20-metre long tube, the level was still 30 bar. That was more than the five centimetre thick concrete plate could take.
It is far from coincidental that the tests are carried out in the lab linked to the SIMLab research group at NTNU. SIMLab is world-leading on the behaviour of materials and structures subjected to sudden strain.
Nor is it coincidental that in 2015, SIMLab was granted their second SFI – Centre for Research-based Innovation – with substantial financial support from the Research Council of Norway. The new SFI carries the name CASA, Centre for Advanced Structural Analysis. The 16 partners include five of the world’s leading car manufacturers, the Norwegian Defence Estates Agency, the Norwegian National Security Agency and NPRA.
“To be able to do the job in this environment is crucially important because of the access to the lab, but above all because the research group has relevant experience. It is most central to know what tests to perform and what results to look for,” Martin Kristoffersen underlines.
Kristoffersen defended his PhD at SIMLab. He still works there; now as a postdoc financed by the large E39 project of the NPRA.
When a submerged, floating tunnel is being considered in the Sognefjord, it is because the fjord is 3.7 kilometres wide at the crossing point: this is nearly twice as long as the world’s longest existing suspension bridge. A tunnel under the fjord is unfeasible: it is 1 300 metres deep. Thus the need for ground-breaking research: submerged, floating bridges of this kind simply do not exist today.
The solution is also being considered for two other fjord crossings on the new E39: in the Sulafjord, as part of the Hafast connection, and in the Digernes strait, in combination with the underwater tunnel in the Bømlafjord.
Concrete tubes next
In addition to the tests in the shock wave, Kristoffersen has shot at the concrete plates with a rifle in the lab of SIMLab. These tests are relevant because an explosion is more than a shock wave: it also involves physical objects being hurled out at great speed.
As mentioned, both the NPRA and the Norwegian Defence Estates Agency (NDEA) are partners in SFI CASA. This greatly simplifies the execution of Kristoffersen’s remaining tests:
“NDEA provides testing grounds, staff and explosives so we can carry out the next phase. What we will do, is to subject concrete tubes of varying dimensions to blast loads.”
“You have worked with this for two years now. Has a submerged, floating tunnel become more or less likely during this time?”
“I think it has become more likely. When we started, this solution was under consideration for five fjord crossings. Now we may be down to three. At the same time, it is more likely at these three locations than it was at the outset, partly because we know better what will give a safe solution. In the end, I think other factors will decide whether a submerged, tubular bridge will be chosen or not. Such a process involves a great variety of considerations.