Many old buildings are supported by wooden pilings that might have been in place for hundreds of years. On an assignment from the Swedish Transport Administration, Charlotte Björdal will examine pilings from ten sites along the route of the Västlänken railway tunnel. The extent to which they are affected by fungi and bacteria can provide an indication of how stable the buildings are.
Västlänken, or the West Link, is about to be built in Gothenburg, and large underground areas will be exposed. This presents an opportunity to research things that otherwise are difficult to study. The wooden pilings on which the city is built, for example.
‘We have gained access to the Västlänken area to monitor what is happening with the wooden foundation pilings under old buildings’, says Charlotte Björdal, who is a professor at the Department of Marine Sciences. ‘This is a general problem in old cities when new construction is to occur, particularly if it exposes a large area and excavates a large pit. What happens then?’
Along with a doctoral student, she will examine how foundation pilings are affected during construction of this calibre. A critical point is the water table. Wood pilings should be covered with water, and if they are, they can last for several hundreds of years. But during dry periods, the upper part of the piling can dry out and be attacked by bacteria or wood decay fungi. In the event of serious attacks, the pilings weaken and the building can subside.
‘We would like to know how fast this process happens. We don’t know much about that today. We might be exaggerating our unease and believing that it happens all at once. How long can we tolerate a little subsiding of the water table before it affects the pilings?’
In the project ten sites with foundation pilings will be monitored for several years. It will include both houses and larger buildings when agreed upon with the owners. Many people have contacted us and want to be included. They are worried about their houses now that Västlänken rail tunnel is being built.
Reaching the pilings is not a simple procedure, so there is close collaboration with the Västlänken project. If you are unable to gain access from the outside, you have to enter the basement, make holes in the concrete, find a piling and dig a deep pit. Björdal then goes there to take samples that are studied under a microscope to see if the piling has been attacked, and if so, how much and how far into the wood the incursion has progressed.
‘Then we can get a picture of the decomposition that has proceeded up to now and thus already has occurred.’
At these sites, new, smaller wooden pilings will be dug down, and researchers will monitor what happens after two, four, six years and maybe even longer. They will also measure the water table through pipes in the ground. During the construction period, Västlänken’s contractors will maintain the water level through infiltration of tap water, which has a different oxygen content than groundwater. The question is also whether oxygen content is a factor.
‘Today we can’t say anything about it, because we don’t know. Nobody has gotten to the bottom of it. These are typical interdisciplinary questions. They concern hydrogeology, land, water transport, oxygen, microorganisms, wooden structures and cultural heritage.’
Björdal is a curator at heart. She has a doctorate in wood science and loves wood. She notes that it takes time to learn how to assess the decomposition of wood when you look at it under a microscope. It is not something that can be measured in a simple way.
‘You have to know a lot about what wood looks like when it is healthy, when it is decomposed by fungi, when it is broken down by bacteria and how different woods are affected.’
Last week she had the opportunity to take samples on pilings sticking up behind Gothenburg Central Station that had been exposed in connection with construction. It is a large area where an old railway had been and a bit down in the earth a whole system of pilings came into view.
‘Most of the pilings were amazing, as healthy as can be. If Gothenburg stands on such fine pilings, nobody needs to be concerned.’
There is a lot of clay in Gothenburg and that is good for wood. Clay soaks up water and retains it. Therefore, clay and pilings are a perfect blend. Clay can keep pilings strong for several hundred years.’
She emphasises that the project results will also be applied to other wood, such as archaeological remains that are buried beneath the city. If pilings do not fare well, neither will these objects, which usually are much thinner than stout pilings.
‘This is research that will be a source of joy to many, not only in Sweden but also abroad. No one has previously studied pilings from a research perspective, where you look at how they are affected under a large building. There are a lot of things that we don’t know and we have to find out. Come back in four years, and I hope we will be able explain how it all fits together and how we can make things better.’