Refurbishing a 60-Year-Old Port
A jetty that underwent refurbishment at Eilat Port required scour protection under the already existing deck. Given the undulating slope, high density of piles with its irregular spacing, a geotextile formed concrete mattress possessed the unique capabilities for a solution.
Ease of Installation
The geotextile function is to cast concrete in underwater conditions. The lightweight geotextile fabric is negatively buoyant meaning that a single diver can easily handle and manoeuvre it into place underwater.
Fabrication
Proserve tailored the fabric formwork to suit the required dimensions. Due to the irregular spacing of piles, five distinct mattress panel designs were developed. Each panel measured 24m x 5m and was fully filled on site. Zip connections along the top and bottom edges of each panel ensured a sealed connection between adjacent units, allowing the formwork to achieve continuous protection with interlocking concrete aprons.
The internal vertical ties were woven directly into the fabric and anchored between the top and bottom layers of the formwork. These ties offer several key advantages:
- Automatic thickness control: The system inherently maintains mattress thickness, reducing the need for diver intervention to adjust or reposition the formwork.
- Reduced concrete usage: Minimised surface undulations result in a more consistent thickness, lowering the volume of concrete needed to meet minimum scour protection requirements.
- Improved hydrodynamic stability: A smoother concrete surface reduces the risk of uplift from water forces acting on the mattress.
Concrete Mattress vs Rock Armour In Piled Revetments
In-situ concrete mattress is the preferred method for scour protection due to its high reliability, particularly in environments where maintenance access is limited, such as piled revetments. While rock protection offers certain advantages—such as porosity and ease of repair on level beds—it becomes significantly more costly and challenging to install on sloped, piled structures.
Furthermore, increased flow velocities and turbulence around piles on inclined surfaces can compromise the stability of rock armour, increasing the risk of failure.
Loss of Bed Materials at Piles
A key principle of in-situ concrete mattress is sealing around the structure to prevent bed material being lost from under the concrete apron.
For construction on complex undulating slopes, the mattress formwork and pile seals must be engineered with surplus material to ensure the protection conforms to the arrangements of piles and slopes.
Pile bolsters are used to ensure a sand-tight seal is achieved first time. Sealing around the piles was also essential to prevent scour from propagating down and around each pile – a vulnerability in many traditional scour protection systems.
These pile seals:
Prevent water jetting down the pile line, which would otherwise initiate focussed scour holes.
Maintain the integrity of the apron, ensuring continuity even at pile penetrations.
Installed with close diver supervision and filled as part of the main pour sequence, ensuring both visual inspection and structural integrity during placement.
On this project, the slope levels could vary to +0.1/-0.3m, with piles being driven over 400mm out of tolerance at bed level in some locations.
Porosity
An increased thickness of 575mm was used in the wave zone to manage a design wave height of 3m and a peak period of 6.5 seconds.
This area of the mattress also had open holes of 80mm diameter at 1.4m centres to achieve the required porosity and resist uplift during wave rundown.
A stone layer with a larger diameter than the porosity holes is laid under the mattress in the wave zone, ensuring no loss of bed material.
The additional thickness was not required outside of the wave zone. To optimise concrete usage, a transition compartment of 300mm thickness was added, transitioning to 220mm for the remainder of the slope.
Protecting the Edges and Preventing Underscour
Underscour / loss of bed material at the edges can lead to progressive failure of the mattress.
Embedment of the mattress edge is fundamental to prevent underscour of all mattresses, especially when combined with a failing edge detail of mattress blocks or rock armour.
A rock falling edge apron was used to protect the edges on this project.
The edge trench was deep enough to contain rock armour and large enough not to be displaced by flow.
Without this deadweight, the displaced rock armour may roll onto the mattress protection area, impacting draft clearance for vessels.
When edge scour occurs, the rock armour should settle into the localised scour holes.
Should the scour exceed the designed depth, additional rock armour can be added.