Implementing Concrete Mattress Design for Effective Port Scour Protection
Project Overview
The construction of the Al Faw Grand Port in Iraq marks a significant development in enhancing the country’s maritime infrastructure and expanding its capacity to handle increased shipping traffic between Asia and Europe. As part of a broader strategy to revitalise Iraq’s economy and establish it as a pivotal trade hub, the port’s development involves extensive quay structures designed to accommodate large container vessels. A critical component of this ambitious project was addressing the issue of port scour—the erosion of sediment around foundation piles caused by strong currents and wave action—which posed a threat to the long-term stability and safety of the marine structures.
Challenges with Traditional Scour Protection
Initially, the design for scour protection relied on the use of traditional rock armour. This method, while conventional, presented several significant challenges:
- Material Sourcing and Cost: Procuring approximately 430,000 m³ of high-quality rock armour was both expensive and logistically complex. The limited availability of suitable rock from local quarries meant that sourcing the required material would involve substantial costs and potential delays.
- Construction Scheduling: The placement of rock armour needed to occur after the installation of the piles but before the construction of the deck. This sequence placed scour protection on the critical path of the construction schedule, introducing the risk of delays and bottlenecks in the overall programme.
- Environmental Impact: The quarrying, transportation, and placement of such a large volume of rock armour would have considerable environmental implications, including increased carbon emissions and potential ecological disturbances in both the source and installation locations.
Adoption of In Situ Concrete Mattress Solution
To address these challenges, the project team, in collaboration with engineering firms Proserve and the designer SENet, proposed an innovative alternative: the use of an in situ concrete mattress system for port scour protection. This solution offered multiple advantages over traditional methods.
The concrete mattress system significantly reduced the required material volume—from 430,000 m³ of rock armour to just 26,500 m³ of concrete. This drastic reduction not only lowered material costs but also simplified logistics, as concrete could be sourced and produced locally, eliminating the need for extensive quarrying and transportation.
Furthermore, the installation of the concrete mattresses provided greater construction flexibility. Unlike rock armour, which had to be placed before deck construction and thus remained on the critical path, the concrete mattresses could be installed at various stages—before, during, or after deck construction. This flexibility removed scour protection from the critical path, allowing other construction activities to proceed unimpeded and improving the overall programme efficiency.
The concrete mattresses also offered effective scour protection. By forming a continuous concrete apron over the protection area, the mattresses conformed to the seabed’s contours and sealed against the piles and the structure. This design prevented under-bed erosion by ensuring that no flow could erode the bed material underneath, effectively mitigating the risk of port scour.
Engineering and Design Considerations
Implementing the in situ concrete mattress system required careful engineering and design to address the specific challenges posed by the marine environment and the project’s unique requirements.
Wave Zone Stability
In areas exposed to significant wave action, particularly at the upper sections of the slope, the mattresses needed to withstand increased hydrodynamic forces. To enhance stability in these zones, the design incorporated several key features:
- Increased Mattress Thickness: The mattress thickness was augmented to 0.35 metres in high-energy wave zones to provide additional mass and resistance against wave forces.
- Permeability Holes: Engineered holes were incorporated into the mattress at 1.2-metre intervals. These holes allowed water to move through the mattress, reducing uplift pressures caused by wave run-down and decreasing the potential for destabilising forces.
- Bedding Layer Stone: A graded stone layer consisting of stones sized between 70 and 250 mm in diameter was placed beneath the mattress. The stone size was selected to prevent migration through the permeability holes while providing a stable foundation that accommodated the mattress’s contours and supported its weight.
Differential Settlement Accommodation
The seabed under the jetty was expected to experience settlement of up to 290 mm after the installation of the mattress. To ensure the mattress could accommodate this differential settlement without compromising its integrity or effectiveness, the design included the following features:
- FLEX Mattress Segments: The mattress was designed with thin ‘webs’ every 1.2 metres, creating flexible segments capable of slight rotation. This design allowed the mattress to adapt to uneven settlement by permitting controlled cracking in the thin webs, thereby preventing larger cracks that could lead to erosion of the underlying material.
- Settlement Collars: Steel collars were installed around the piles. These collars positioned the mattress to prevent it from resting directly on the piles, ensuring that the mattress could follow the seabed settlement independently. This approach avoided stress concentrations at the pile interfaces and reduced the risk of damage or large gaps forming between the mattress and the seabed.
Installation Process
The installation of the concrete mattresses was a critical phase that required precision and coordination between the construction teams and divers. The process involved several key steps:
Demonstration Installations, Al Faw Grand Port, Iraq
- Submerging and Positioning the Fabric: Specially trained divers submerged the geotextile fabric that would serve as the formwork for the concrete mattress. They carefully rolled it out along the seabed, ensuring it conformed to the seabed’s contours and extended down the slope as required.
- Securing to Piles and Structures: The fabric was secured to the piles and existing structures using restraining systems. This step was crucial to maintain the correct positioning of the mattress during the filling process and to ensure a tight seal against the piles to prevent under-bed erosion.
- Filling with Micro-Concrete: A highly fluid micro-concrete was pumped into the submerged formwork from the surface. The concrete’s fluid properties allowed for reliable filling over large areas and accommodated long pumping distances, ensuring a homogenous and void-free concrete slab.
- Custom Fabrication of Panels: The mattress panels were custom-fabricated to match the project’s specifications. Panels measuring 52.5 metres in length and 7.5 metres in width were used to facilitate seamless coverage and structural continuity across the protection area.
- Construction Joints: A zip flap joint system was employed to connect the slope mattress to the berth mattress at a later stage. This design allowed for the initial installation of the slope mattress without impeding barge operations that could potentially damage the mattress. It also ensured that the connection between the two sections maintained the integrity of the continuous concrete slab.
Programme Efficiency and Cost Savings
The adoption of the in situ concrete mattress system yielded significant benefits in terms of programme efficiency and cost savings. By removing scour protection from the critical path, the construction of the deck and other key components could proceed without delay. This flexibility in scheduling allowed the project to maintain momentum and reduce the risk of time overruns.
The reduction in material volume from the use of concrete instead of rock armour resulted in considerable cost savings. The simplified logistics of sourcing and transporting concrete, as opposed to large quantities of rock, further contributed to the financial efficiency of the project.
Environmental Benefits
In addition to economic advantages, the concrete mattress solution offered environmental benefits. The decreased need for quarrying and transporting large volumes of rock reduced carbon emissions and lessened the environmental impact associated with material extraction and transportation. Using locally sourced concrete and minimising material movement aligned with sustainable construction practices and environmental stewardship.
Collaborative Efforts
The success of the concrete mattress implementation was underpinned by effective collaboration among the project’s key stakeholders:
- Proserve: Provided engineering expertise, custom design solutions, and ongoing support throughout the installation process. Their experience in designing and supporting the installation of concrete mattresses was instrumental in adapting the technology to the project’s specific needs.
- SENet: As the designer, SENet adopted Proserve’s example design and integrated it into the overall project plans. This collaboration ensured coherence between the design and practical execution, addressing the unique challenges of port scour at the Al Faw Grand Port.
- Huesker: As Proserve’s global partner, Huesker manufactured the geotextile formwork to exacting standards, leveraging over 160 years of weaving expertise. Their role was critical in producing the high-quality materials required for the successful implementation of the concrete mattress system.
Conclusion
The Al Faw Grand Port project demonstrates how innovative engineering solutions can effectively address complex challenges such as port scour. By adopting the in situ concrete mattress system, the project achieved efficient scour protection while optimising costs and improving programme efficiency. The approach not only ensured the structural integrity and longevity of the marine infrastructure but also aligned with environmental sustainability goals.
This case study highlights the importance of considering alternative methods and collaborating across disciplines to find solutions that meet technical requirements, economic constraints, and environmental considerations.
