Photo: Port of Gothenburg

Unconventional port expansion benefits the environment 

23.01.2018 / Kristina Bernstén

The port of Gothenburg is being expanded using a new, environmentally sensitive method. The binding or alteration of hazardous substances in a chemical way is where the big gains lie. 

As cities grow and become more crowded, one challenge is to develop areas suited for construction. This may require unconventional methods.

In Sweden's coastal cities, expansion has been achieved by regenerating industrial areas, such as in the north of Djurgården, Stockholm, where a residential area has blossomed at a location previously used for the port and gasworks.

In Gothenburg, it is the port that needs more space. By the end of the 2020s, 22 hectares of land suitable for construction will have been created using treated dredging spoils. 

As cities grow and become more crowded, one challenge is to develop areas suited for construction. This may require unconventional methods.
Kristina Bernstén Chief Specialist, COWI

Sustainable alternative to dumping the spoils at sea 

Ports are filled with sediment. Dredging is necessary, but with today's knowledge of the marine environment, dumping the contaminated spoils further out to sea is not an acceptable solution.

Adding the spoils to a landfill also has major disadvantages: there is a risk of the environmental toxins leaking, and it is costly.

A different method was applied in Gothenburg, which provides a more sustainable solution and generates an environmental benefit.

Chemical cleaning of contaminated sediment

So let's zoom in on the key elements of the unconventional solution in Gothenburg. The method, known as Stabilisation and Solidification, means contaminated sediment taken up as a result of dredging can be used to create areas more suited for construction, while at the same time binding environmental toxins.

In short, sediment from dredging is mixed with various binding elements, such as cement. The mixture is then pumped into a banked area where new quays or terminal surfaces need to be created, whereupon it cures to produce a solid body - a monolith - which is then covered with a hard-crafted surface, e.g. asphalt.  The environmental toxins are safely enclosed to create new areas suitable for construction. The binding action means that the risk of any substances leaching is negligible, assuming no physical impact occurs to the monolith. The permeability of water is, like dense clay, small.

The technology itself is not new; mixing in various additives to improve the strength of different spoils is a long known method. The binding or alteration of environmentally hazardous substances in a chemical way is where the big gains lie.  

Hitting the right mix 

Our role was to develop the recipe for the binding agent needed for the spoils in question, as well as the design of the embankment. A particular challenge facing projects like this is identifying the right mix of binding agents. The seabed sediment could be composed of different substances in different places, so the recipe had to be bespoke. 

In this case, common cement was used, along with GGBS, a granular blast furnace slag. As a by-product of steel production, GGBS has excellent binding properties to handle impurities and also has good resistance. Another common ingredient is fly ash, a residual product derived from various methods of combustion.  

This way of using dredging spoils produces a major environmental benefit in the long term, but it does not mean that the method is free from complications. Dredging spoils contain large amounts of organotin compounds such as TBT, highly toxic substances previously applied to the bottoms of boats. There will be an increased leaching of toxins in the short term, both in the dredging itself and when the material is placed out to cure.

Not a landfill but a productive waste disposal facility 

A lot of the focus has been on what happens from the time you place out the spoils until they begin to cure. Adding the cement increases the pH value. This makes the TBT more leachable. At the beginning of the process there will be increased leaching, which will then decrease as the stabilised and solidified dredging spoils cure.

It is important to emphasise that this is not a landfill, but rather a productive waste disposal facility. The successful pilot project was completed in spring 2017. However, there are a number of stages: lab tests, developing recipes etc. and to verify if the solution works on a larger scale, further testing is needed.

The project was a collaboration between the Port of Gothenburg, COWI, Peab and the State Geotechnical Institute. 

Meet the expert

My name is Kristina Bernstén and I am a marine ecologist.

I am highly fascinated by the marine environment, partly because it is so difficult to grasp as you cannot see what is happening below the surface. 

Part of my job is to participate in consultation meetings with the general public, at which a lot of emotions are vented when it comes to dredging and the turbidity it brings. 

Society must move forwards, and this is my way of contributing. Development is positive, although you could be forgiven for thinking it is a little slow sometimes.

Get in contact

KREN

Kristina Bernstén
Chief Specialist
Environment, Sweden

Tel: +46 108501278