Project Requirement
To support increased vessel traffic on the Dunkirk-Dover route, a new RORO berth (RORO6) was constructed at Dunkirk Ferry Terminal. The berth accommodates ferries operating with powerful twin propellers, berthing up to every 45 minutes. This highly repetitive berthing pattern generates extreme propeller wash, leading to rapid and localised seabed erosion.
At a nearby existing berth, fine sand was scoured up to 5 metres below berth level in just a few years. The new berth’s scour protection system needed to:
- Limit maintenance dredging over the design life;
- Be installable beneath the completed piled deck structure;
- Offer resilience to cyclic scour loading without requiring excessive material;
- Minimise the risks associated with propeller strikes or rock displacement.
Minimising environmental disturbance and managing the construction within a constrained operational ferry terminal added to the complexity.
Concrete Mattress Solution
An in situ concrete mattress system was selected to meet the stringent technical requirements. The mattress offered a continuous protective layer that prevents localised scour around piles and the berth structure.
Key features of the solution included:
- Hinged Edge Design:
A pivotal aspect of the system was the use of a hinged edge (HE), which enables the mattress to flex and settle gradually as scour develops beneath it. Rather than resisting the natural tendency of the seabed to erode, the hinged edge accommodates it, allowing the apron to remain in contact with the seabed and continue providing protection even as local levels drop.
The HE is anchored into a 4-metre-deep trench, deep enough to intercept early scour and provide sufficient embedment. If scour continues beyond the depth of the hinged edge, the system is designed so that rock armour can be added at a later stage, but only in defined locations, significantly reducing initial material use.
This design reduces the risk of undercutting and toe failure, a common issue with rigid or shallow-anchored protection systems. It also allows faster installation, as the formwork is easier to place and fill without requiring complex shaping or deep backfill. The ability to conform to scour conditions over time is a major reason the hinged edge design is considered high-performance in dynamic berthing environments.
- Pile seals:
Sealing around the piles was essential to prevent vertical scour from propagating down around each pile shaft — a vulnerability in many traditional scour protection systems. The concrete mattress system incorporated tight pile seals, using tailored collars and infill fabric to wrap around the piles and tie back into the main mattress body.
These seals:
- Prevent water jetting down the pile line, which would otherwise initiate focused scour holes;
- Maintain integrity of the apron, ensuring continuity even at pile penetrations;
- Reduce the risk of prop wash interaction at the pile-mattress interface, which could otherwise lead to early failure.
The pile seals were installed with close diver supervision and filled as part of the main pour sequence, ensuring both visual inspection and structural integrity during placement.
- Sliding restraint compartment:
The central compartment of the mattress provided sliding restraint to prevent longitudinal movement of the mattress in the deep toe trench. This was especially important given the sloping profile and potential for the fabric to shift under hydraulic loads before the concrete cured. By filling this internal restraint zone first, the installation team ensured that the mattress remained securely anchored before completing the hinged edge and upper fills.
- Modular, fill-on-site approach:
The use of Incomat formwork allowed the mattress to be supplied as flexible, rolled units that could be positioned, restrained, and pump-filled on site with standard concrete. This was crucial given the restricted working space beneath the deck and the proximity to operational ferry movements.
These features combined to create a highly adaptable, high-integrity scour protection system capable of performing under some of the most demanding marine construction conditions.
This approach provided a durable, adaptable scour protection system that could be installed using conventional plant while ferry operations continued nearby.
Design Requirements
The mattress was designed to:
- Resist erosion under cyclic propeller loading;
- Be quickly filled and restrained using a reliable and repeatable process;
- Offer long-term durability without shifting under hydraulic loads;
- Accommodate installation tolerances and irregular seabed profiles.
The dimensional stability of the Incomat formwork was critical for this performance. It enabled precise placement and efficient filling of long mattresses (up to 55m), even on variable gradients under deck structures.
Installation Details and Challenges
Installation was carried out by Eiffage Génie Civil, just 50 metres from an active ferry lane, with no disruption to service. Divers carefully ballasted the empty mattress to prevent displacement from ferry wash.
Each mattress had internal compartments. Installation began by filling the central sliding restraint compartment (Compartment 1), followed by hinged edge compartments (Compartment 2) for stability. Filling always began at the lowest point of each section to ensure even distribution.
- Total concrete volume: ~6,100 m³
- Installation pace: 72 mattresses placed over 15 weeks
- Filling rate: 100 m³/day
- Working hours: Standard construction hours (typically 10–12 hours/day)
The installation team carried out shore-based training and dry runs to refine techniques and mix design prior to live placement, allowing them to finish the mattress works one week ahead of programme.
Advantages of the Concrete Mattress System
- Material efficiency: The concrete mattress provides full-bed protection at reduced thickness compared to rock armour. An equivalent rock solution could require 9,000–12,000 tonnes of material, depending on placement density.
- Construction simplicity: Mattresses were installed using conventional plant and dive teams, with no need for large rock deliveries or marine craneage.
- Reduced dredging: The trenched and hinged design pre-empted early scour, reducing the need for ongoing dredging in the high-energy zone.
- Propeller safety: Unlike loose rock, the mattress offers a stable surface with reduced risk of propeller strike or movement under jet wash.
- Storage and sourcing: Rock sourcing was constrained, and large stockpiles weren’t feasible on site. The concrete solution avoided these issues entirely.
- Maintainability: The HE design supports targeted rock addition by the port authority if scour eventually exceeds trench depth — only in isolated areas and long after construction.
Conclusion
The Dunkirk RORO6 berth demonstrates the technical strength of in situ concrete mattresses for high-frequency, high-scour environments. With tailored trenching, sliding restraint, and a hinged edge design, the solution delivered long-term protection under challenging operational conditions — with logistical, cost, and maintenance advantages over traditional rock armour.
Construction of the scour protection works was successfully completed in 2023, delivering a resilient and future-proof solution to support the continued growth of the Dunkirk-Dover ferry service.
