Denmark has taken a further major step in strengthening climate resilience in the capital region with the launch of the SST04 Cloudburst Tunnel, a large-scale underground flood protection scheme for Greater Copenhagen.

The project forms a central element of the long-term strategy to protect Copenhagen, Gentofte, Gladsaxe and Frederiksberg against increasingly frequent extreme rainfall and cloudburst events. By diverting excess stormwater and combined sewer flows into a new deep tunnel system, SST04 is designed to significantly reduce flood risk in some of Denmark’s most densely built-up urban areas.

Flood management concept

During heavy rainfall, surplus water from the regional sewer networks will be diverted directly into the cloudburst tunnel. Where inflows remain within tunnel capacity, the water will be temporarily stored underground. Once sufficient capacity becomes available in the sewer system and at the treatment works, the stored water will be pumped to the Lynetten wastewater treatment plant via a newly constructed discharge pipeline.

In extreme events that exceed tunnel storage capacity, excess flows will be safely discharged into the Øresund through a dedicated outfall structure at Fiskerihavnen/Skudehavnen.

Scope of works

The SST04 contract includes a substantial package of tunnelling and underground works, comprising:

• Construction of 13 shafts across approximately 15 construction sites
• 12 tunnel sections linking the shafts
• Around 3.2 km of segmental bored tunnel
• Approximately 7.0 km of pipe jacking tunnel
• Underground connection structures tying the existing stormwater network into the new tunnel system
• An outfall structure and pumping station at Fiskerihavnen, including underground technical installations and a small above-ground building
• New drainage pipelines and extensive utility diversions to enable shaft construction

Most works will be carried out in densely populated urban areas, characterised by restricted working space, heavy traffic and complex stakeholder coordination.

Construction methods and technical requirements

The client is seeking contractors with proven experience in major urban tunnelling projects, including:

• Segmental tunnel construction, with associated design responsibility
• Pipe jacking, including design
• Works at railway crossings regulated under CSM-RA (Common Safety Method for Risk Evaluation and Assessment)
• Construction of deep shafts using diaphragm walls and secant pile walls (minimum 1000 mm diameter, minimum 15 m depth)
• Complex utility works in heavily trafficked urban corridors
• In-situ concrete works, including internal shaft structures and pumping stations

Experience in comparable ground conditions and full lifecycle project delivery—from contract award through to handover—will be key selection criteria.

Procurement and prequalification

The project will be procured under a restricted procedure. SST04 is classified as critical infrastructure and is subject to Danish investment screening legislation. Contractors or owners established outside the EU/EFTA will require approval from the Danish Business Authority.

Prequalification requirements include:

• Minimum average annual turnover of DKK 4 billion over the past three years
• Average solvency ratio of at least 25%
• Positive average profit margin over the past three years
• Up to 10 reference projects covering 12 technical categories, with references permitted from the past 10 years due to the limited number of comparable projects in Denmark

Programme outlook

The contracting authority expects to publish the formal tender documents in early 2026. The bid validity period will be nine months, with possible extensions linked to foreign subsidy investigations or investment screening approvals.

Tunnelbuilder comment

With more than 10 km of combined bored and pipe-jacked tunnels, multiple deep shafts and demanding urban interfaces, SST04 represents one of the most significant tunnelling opportunities currently emerging in Northern Europe’s climate adaptation market. The project further underlines Copenhagen’s commitment to large-scale underground solutions for long-term flood resilience.

For tender information please click dk/32. 01/26.