Bacteria are eating away at concrete in underwater tunnels
This is happening in the subsea Oslofjord Tunnel. Experts assert that the tunnel is still safe to use.
A biofilm of bacteria is destroying the concrete in the deep road tunnel under the Oslofjord, as shown here. The red colour has been caused by the bacteria due to rusting reinforcement inside the sprayed concrete.(Photo: Britt-Marie Wilén / Chalmers University of Technology)
"There's a clear bacterial impact in the Oslofjord Tunnel," concrete expert Per Hagelia tells sciencenorway.no.
Several decades ago, the Norwegian Public Roads Administration suspected that bacteria forming colonies and creating what is known as biofilm could become a problem in subsea tunnels. They therefore contacted a research group at Chalmers University of Technology in Sweden.
"With this study, the researchers at Chalmers have taken international research on an important problem in subsea tunnels a big step forward," says Hagelia.
In extreme cases, the researchers observe degradation of up to ten centimetres over five years. More commonly, one centimetre of concrete is damaged each year.
Bacteria barely known to researchers
Annonse
When tunnels are built into rock, the walls and ceilings are sprayed with concrete. The sprayed concrete is reinforced with steel fibres.
This keeps water out and prevents rock from falling from the tunnel ceiling. If there is a lot of water coming from the rock, more sprayed concrete is applied.
Underwater tunnels – like the Oslofjord Tunnel – contain saline groundwater that pushes against the concrete from behind.
Researchers are now seeing how this creates favourable growth conditions for several quite unusual bacteria on the visible part of the tunnel walls. Some of these bacteria are still largely unfamiliar to researchers.
The bacteria cluster together and form biofilm.
Make the concrete porous
The bacteria get their 'food' from substances in the concrete.
In return for the food, they attack the surface of the concrete and make it porous.
Researchers at Chalmers, in collaboration with the Norwegian Public Roads Administration, have provided new insights into how this hidden biological process is unfolding deep within the Oslofjord Tunnel.
Bacterial damage to underwater tunnels can occur quite rapidly, according to this research. Pictured here is the Oslofjord Tunnel.(Photo: Anine Kalmo Larsen / Norwegian Public Roads Administration)
The Norwegian Public Roads Administration has been examining the sprayed concrete at selected locations in the Oslofjord Tunnel since 2004. In a designated test area for long-term exposure, the administration and researchers have closely monitored the concrete.
"We have been taking measurements in the Oslofjord tunnel since 2014, and we can see that the bacteria eat their way into the concrete surface up to one centimetre a year," Frank Persson, associate professor of molecular biology and microbial ecology at Chalmers, says in a press release.
In extreme cases, bacteria can penetrate up to ten centimetres into the concrete in just five years.
"The concrete covered by the biofilm will gradually dissolve," he says.
Persson points to the presence of biofilm on the concrete surface as a fairly clear warning sign.
Annonse
"You need to monitor the water flow and the spread of the biofilm and locate loose and damaged concrete to spray again, if necessary," says Britt-Marie Wilén, professor of environmental and wastewater engineering at Chalmers.
Same problem with freshwater?
Similar degradation of concrete may also occur in tunnels where freshwater seeps in, according to Britt-Marie Wilén.
"However, the problem is probably greater in environments where seawater penetrates, partly because seawater is favourable for bacterial growth but also because the salt accelerates corrosion in the reinforcement," she says.
Retired Norwegian concrete expert Per Hagelia is sceptical of this:
"We haven't seen such effects in connection with freshwater. The Norwegian Public Roads Administration owns an extremely large number of tunnels, and the maintenance of these would've revealed such conditions," Hagelia tells sciencenorway.no.
The tunnels are still safe
It was in the 1990s that sprayed concrete began to be widely used in road tunnels.
Bacterial biofilm on tunnel walls has been observed for some time. But prior to this Oslofjord Tunnel study, there had been few studies on so-called biocorrosion in tunnels located in fjords and marine areas.
"This is an interesting and important piece of work. The Swedish researchers have discovered a lot of new information in the Oslofjord Tunnel," says Hagelia.
He previously worked for the Norwegian Public Roads Administration.
"They've also found bacteria that are completely new in this kind of context. Much of this is foundational research. We’ve answered some key questions, but new ones have emerged as well," he says.
The Swedish researchers emphasise that the Oslofjord Tunnel is safe to use. The same assurance is given by the Norwegian Public Roads Administration.
100-year lifespan
Per Hagelia points out that the Oslofjord Tunnel was designed for a 100-year lifespan. It is now 25 years old.
Lise Bathen from the Norwegian Public Roads Administration tells sciencenorway.no that the results from tests in the Oslofjord Tunnel have formed the basis for current regulations concerning concrete thickness in other underwater tunnels.
"It’s well known that concrete and other materials degrade over time, both inside and outside of tunnels. Maintenance and repairs are to be expected during the service life of any underwater road tunnel. These structures aren’t meant to last 100 years without any form of degradation," says Bathen.
