The First Immersed Tunnel in the Middle East

Engineering a Monolithic Pour

Immersed tunnel construction places high demands on concrete quality, dimensional accuracy, structural integrity and watertightness.

For the Al Faw project, the walls and roof slab of each 21-metre segment were designed to be cast simultaneously in a single concrete pour. This minimized additional construction joints and supported the required long-term structural performance and watertightness.

The challenge was to develop a formwork system capable of handling segment heights ranging from approximately 9.85 m to 11.05 m, while also allowing the system to be efficiently repositioned for repetitive casting operations.

PERI engineers approached the formwork not simply as a structure carrying concrete loads, but as an integral part of the complete construction process.

Seven Formwork Systems and a Customized Carriage Concept

The project utilized seven immersed tunnel formwork systems, combining proven PERI components with purpose-designed steel structures.

The solution incorporated the VARIOKIT Engineering Construction Kit, SRU girders, VT 20 K timber beams, PERI Steel Formwork and specially engineered steel carriage systems.

The carriage concept was particularly important for the internal formwork. Instead of relying heavily on cranes to dismantle and relocate major assemblies after each pour, the system enabled the formwork to be efficiently moved between casting positions.

This helped reduce crane dependency, simplify site logistics and create a repeatable production process - important advantages for a project of this scale.

From 12 Days to a Seven-Day Cycle

One of the project's key targets was a seven-day production cycle per tunnel segment.

The initial construction cycle required approximately 12 days. Reaching seven days required more than formwork equipment alone.

Through close collaboration between the contractor and PERI engineering teams, the complete construction sequence was continuously reviewed and optimized, including formwork operations, repositioning, construction procedures and site logistics.

The result was a reduction from the initial 12-day cycle to the targeted seven days, significantly improving productivity and supporting faster project delivery.

It demonstrates an important principle in repetitive infrastructure construction: efficiency comes not only from the concrete pour itself, but from optimizing every operation around it.

Adapting to Different Tunnel Geometries

PERI's engineering scope extended beyond the immersed tunnel sections.

Dedicated formwork solutions were also developed for the Cut & Cover, Open Cut and transition sections, allowing the systems to adapt to changing tunnel geometries while maintaining construction quality, dimensional accuracy and productivity.

Detailed engineering and structural verification were carried out using PERI CAD and RSTAB, supporting the analysis of construction loads, customized assemblies and the special carriage systems before implementation on site.

For safe and efficient access and support throughout the construction cycle, PD 8 shoring towers and PERI UP Flex scaffolding were also incorporated into the solution.