The idea for a submerged floating tunnel is not new.
In 1882, British naval architect Edward Reed
proposed a floating tunnel
across the English Channel -- an idea that was vetoed.
The term "floating" is perhaps misleading. The tunnels are fixed in position with cables -- either anchored to the seabed or tethered to pontoons which are spaced far enough apart to allow boats to pass through. Made of concrete, they would function like conventional tunnels, transporting vehicles from one end of a fjord to another.
Waves and currents at 100 feet below sea level are less powerful than those at the surface, explains NPRA's chief engineer Arianna Minoretti.
In addition, a floating tunnel minimizes the impact on the landscape since most of the infrastructure is out of sight. It also creates less noise than traffic on a bridge would. "That would be an advantage ... (for) people living in the area," Minoretti says.
The biggest risks in the project are explosions, fire and overloading, says Minoretti -- and so extensive testing is essential.
NPRA is working with the Norwegian University of Science and Technology's Center for Advanced Structural Analysis (CASA), using live explosives to "investigate how tubular concrete structures behave when subjected to internal blast loads," says CASA researcher Martin Kristoffersen.
The tests will help the team to understand what would happen to the tunnel's structure if, for example, a truck carrying dangerous goods exploded inside.
Results so far indicate that the constant water pressure that surrounds the floating tunnels reduces the damage caused by explosions.
Working with the Norwegian navy, the NPRA team is also investigating how the tunnels would fare if submarines crashed into them.
While locations for the submerged floating tunnels have not yet been pinned down, Minoretti says the
will be completed in just over 30 years' time.
The improved E39 will open up more of the west coast to tourism, while the tunnels may become attractions in their own right -- especially if they are a world first.
"As a bridge engineer working on this amazing project," says Minoretti, "one can only hope."