HomeNewsTurning stone into a sea sanctuary: BESIX’s Nordhavn project champions marine biodiversity

7 October 2025

Turning stone into a sea sanctuary: BESIX’s Nordhavn project champions marine biodiversity

A practical approach to sustainability is taking shape beneath the waves in Copenhagen, Denmark, as the BESIX – MT Højgaard joint venture turns surplus stone from the Nordhavn Tunnel works into a new habitat for marine life.

Raising biodiversity

Excavation on the tunnel has produced large volumes of stone. Rather than treating this material as waste, the project team has worked with the Danish Road Directorate and the Køge Bugt Stenrev association to put it to use where it can do the most good. More than 2,000 tonnes are being shipped to Køge Bay to form the ‘Klintekongen’ stone reef just off Mosede Harbour.

The aim is straightforward: create conditions that help nature recover. The reef is designed to support the spread of seaweed plants, improve shelter for local species, and, in time, raise biodiversity and water quality in the bay. Cod and other marine species are expected to benefit as the structure matures, finding cover and food among the stones.

For us and our partner, this is sustainability in action: we are have been making responsible choices about resources from day one. We have reused some of the excavated stone directly within the tunnel project. What we cannot incorporate on site is now being repurposed to strengthen coastal ecosystems.

Xavier Debruche

Resident Manager , BESIX Denmark

Mette Stender, senior project manager at the Danish Road Directorate and responsible for the tunnel construction, calls it a “win-win for all parties and most of all for the marine environment”. Her assessment reflects a simple truth: with planning and partnership, the by-products of infrastructure can create lasting value.

Surplus as an opportunity

The approach also builds on experience. In the summer of 2023, approximately 1,000 tonnes of Nordhavn stone were placed off Solrød Strand to create the first stone reefs in the area. That early effort set a useful precedent and informed the logistics now bringing the final shipments to Køge Bay.

What stands out in Nordhavn is not spectacle but discipline: measure the material streams, prioritise reuse on site, and redeploy what remains where it can deliver a clear environmental outcome. It is a model that treats surplus as opportunity, aligning construction needs with regional restoration goals.

As the last loads head out from the harbour, this initiative underscores a wider lesson for the industry. “When engineering and environmental stewardship are aligned from the start, sustainable outcomes become part of delivery… not an add-on!” insists Xavier Debruche.

In Køge Bay, the result will be visible over time, as seaweed takes hold, fish return, and a working tunnel in Copenhagen quietly helps to restore life along the coast.