Scour Protection with In Situ Concrete Mattress
In situ concrete mattresses protect vital marine and coastal infrastructure from scour and erosion. These mattresses have become a preferred solution in marine and geotechnical engineering, offering adaptability and long-term protection for structures such as ports, bridges, and revetments. This article explains the technology behind in situ concrete mattresses, their advantages over traditional methods, and the various types available for different applications.
Proserve has been at the forefront of this technology, pioneering in situ concrete mattresses since 1966, offering proven, long-lasting solutions across a range of marine engineering projects.
What Are In Situ Concrete Mattresses?
In situ concrete mattress is created by pump-filling a specially woven fabric formwork with concrete on-site. The formwork functions as a lost shuttering system, like traditional formwork, where it serves its purpose once the concrete has set. The formwork produces uniform, plain concrete aprons that are highly durable and can range in thickness from as little as 10 cm to as much as 1 metre.
How the Technology Works: Key Principles of In Situ Concrete Mattresses
High Strength Plain Concrete
The design of the fabric formwork allows water to bleed out during the filling process while retaining the sand and cement particles, resulting in a high-strength, durable concrete apron. This method ensures that the mattress conforms closely to the underlying seabed, providing continuous, robust protection against scour.
Tailored Protection Extent
A significant benefit of in situ concrete mattresses is the ability to tailor the formwork to the specific area needing protection. By casting a concrete apron over the exact extent of the area at risk, engineers can ensure that scour is completely mitigated, preventing erosion from compromising the structure. Each mattress can form a shear joint with its neighbour producing a continuous concrete apron over the protection area.
Reduced Material and Deadweight
In situ concrete mattresses require less concrete material and thickness than traditional methods like rock armour. This reduction in deadweight and material usage makes the solution not only more efficient but also environmentally friendly, as less material needs to be transported and placed.
Advantages of In Situ Concrete Mattresses:
Adaptability to Seabed Conditions
One of the main advantages of in situ concrete mattresses is their ability to conform to a wide range of seabed conditions, whether flat, sloped, or uneven. This adaptability ensures full contact between the mattress and the seabed, which is critical for effective scour protection. The fabric formwork can be adjusted to fit complex and irregular underwater surfaces, ensuring no gaps are left unprotected.
Reduced Material Usage and Deadweight
Unlike rock armour or pre-cast concrete blocks, in situ mattresses use less concrete while achieving the same, if not better, level of protection. This efficiency in material usage leads to lower transportation and installation costs and reduces the environmental impact of projects.
Efficient Transportation and Installation Process
The installation process of in situ concrete mattresses is straightforward and streamlined. The lightweight fabric formwork is easy to handle and position, and once installed, it is pump-filled with concrete. As the water bleeds through the porous fabric, a high-strength concrete layer is formed, creating a durable protective apron. This method reduces installation time, labour requirements, and overall project costs compared to traditional methods.
Types of In Situ Concrete Mattresses & their Applications:
In situ concrete mattresses come in various types, each tailored to address different environmental and structural needs. Below is an overview of the main types and their applications.
1. Constant Thickness Mattresses
Constant thickness mattresses maintain a uniform thickness across the installation area, making them particularly suited for flat seabeds or areas that require even protection. These mattresses provide reliable scour prevention with reduced material usage.
Applications:
- Port Scour Protection: Protecting quay walls, jetties, and other port structures from tidal forces and currents.
- Bridge Scour Protection: Preventing erosion around bridge piers, which can compromise the stability of these structures.
- Canal Lining: Offering long-term erosion protection along canal beds and walls, helping to maintain their structural integrity.
2. Flex Mattresses
Flex mattresses are designed to adapt to uneven or sloping seabeds. Their flexible formwork allows them to conform to irregular surfaces, providing close-fitting protection in dynamic environments where the seabed profile may change over time.
Applications:
- Piled Jetty Protection: Protecting the foundations of piled jetties from scour caused by shifting sediments and tidal forces.
- Revetment Protection: Used at the base of revetments to absorb wave energy and prevent undercutting and erosion along the shoreline.
3. Open Hole Mattresses
Open hole mattresses incorporate voids or apertures within the formwork, allowing water and sediment to pass through while still providing structural protection. This makes them ideal for areas where maintaining natural water flow and drainage is critical.
Applications:
- Shoreline Protection: Providing effective erosion control along coastlines while allowing natural sediment transport and drainage.
- Port Scour Protection: Allowing sediment movement in port areas while safeguarding structures against scour.
4. CT Bulk Mattresses
CT Bulk (Constant Thickness Bulk) Mattresses are designed for heavy-duty applications where additional mass and stability are required. They provide extra weight, making them suitable for large fill projects or areas requiring significant deadweight.
Applications:
- Breakwater Protection: Deployed around breakwaters to prevent scour caused by intense wave action, ensuring the structure’s long-term durability.
- Large Fill and Deadweight Projects: Providing the necessary mass to stabilise infrastructure, especially in areas with strong environmental forces.
Customising Mattress Thickness for Specific Projects:
One of the key advantages of in situ concrete mattresses is the ability to customise the thickness of the concrete layer to suit specific environmental conditions and structural needs. Thicker mattresses may be required in areas exposed to strong currents or high sediment transport, while thinner mattresses may be sufficient in less demanding conditions.
Factors influencing mattress thickness include:
- Water Flow and Speed: Higher velocities may require thicker mattresses to resist the erosive forces of moving water.
- Structural Loads: Heavier structures, such as bridge piers or breakwaters, may require thicker mattresses to provide adequate support and protection.
By customising the thickness of the mattress, engineers can ensure that the level of protection is optimised for the specific needs of the project.
Conclusion:
In situ concrete mattresses offer a versatile and effective solution for protecting marine and coastal infrastructure from scour and erosion. By utilising porous yet grout-tight formwork and the ability to tailor both the coverage and thickness of the concrete apron, these mattresses provide reliable protection with less material than traditional methods.
Whether it’s constant thickness, flex, open hole, or CT bulk mattresses, this technology allows for highly customised, site-specific solutions that ensure the long-term stability of structures like ports, jetties, revetments, bridges, canals, and breakwaters.
This adaptable technology has been successfully installed by Proserve since 1966, offering engineers a proven method to safeguard critical infrastructure.
