Cost saving with Concrete Mattress
Concrete mattress is being installed as scour protection at Al Faw Grand Port as an alternative to the original rock design. Concrete mattress can be installed below the finished deck taking the works off the critical path. It is also an effective alternative when sufficient quality rock is expensive to source. These factors were beneficial to the contractor.
Installation
In-situ concrete mattress is installed by divers who submerge the fabric, roll it out, restrain to the piles and fill to form the concrete mattress. A highly fluid micro concrete is used to pump fill the in-situ concrete mattress formwork allowing for reliable filling over large areas and long pumping distances. Concrete mattress panels are fabricated to suit the requirements of the project. 52.5m long 7.5m wide panels have been installed to each pile bay to form a continuous slab of plain concrete.
Design
Proserve provided an example design which was adopted by the designer SENest. The design and mattress fabrication is fully customized to the project.
Service
Proserve engineered and fabricated the mattresses for the project, and provide 24/7 remote support, on site demonstrations, training and support to the construction team.
Project Engineered Details
Wave Zone Permeability
The mattress arrangement at the top of the slope was engineered for stability, this involved flattening the slope at the top, increasing the mattress thickness, adopting 70-250mm stone grading under the mattress and permeability holes at 1.2m centres. These holes are fabricated into the mattress and are automatically formed when the fabric is filled.
Differential Settlement
The slope under the jetty is set to settle up to 290mm after mattress installation. Several bespoke details were designed and engineered to provide effective scour protection with this large level of settlement:
Flex mattress – Incorporating lines of very low thickness every 1.2m for rotational capacity
Bedding stone – Sized up to 250mm diameter that cannot escape through cracks in the mattress
Settlement collars – Steel collars were provided at the piles to ensure rotational and settlement capacity by positioning the mattress at the piles to ensure a gap on the downslope side so that the mattress can follow the settlement of the slope rather than supporting itself on the piles. This prevents large cracks and gaps in the mattress which would allow erosion of slope material through those large cracks.
Construction Joints
The mattress to the slope is to be installed before the mattress to the berth so that large barges with spud legs can operate without damaging the mattress. A zip flap joint was incorporated which allows the berth mattress to be connected to the slope mattress several months later, with no loss in sealed and continuous concrete slab construction. The joints to the mattresses on the slope are positioned on the pile lines for ease of diver installation.
