Applications

Ports & Harbours

Europort, Rotterdam

Open stone asphal mattresses - Scour protection

Open Stone Asphalt (OSA) is a robust material consisting of aggregate bound together with asphaltic mastic.

Above water OSA is placed in-situ whilst underwater pre-fabricated mattresses are used. The OSA is placed on a geotextile and a reinforcing steel mesh, onto which the lifting hooks are secured.

Mattress thickness is designed to suit the hydraulic design loads.

Europort, Rotterdam

As ships become larger the depth of water required alongside the quays becomes greater. Previously rock layers approximately 1.1m thick have been used to combat the scour produced by ships as they manoeuvre adjacent to the quay.

To increase the depth of the excavation in front of the quay will often undermine the stability of the structure, so instead, a thinner armour layer is required. By removing 1.1m of rock and replacing it with OSA mattresses of 0.25m thickness increases the water depth without decreasing the stability of the sheet piles.

Placing Mattresses

The OSA mattresses are produced away from the quay so as to minimise any disruption to the port's activities. The mattresses are produced directly on pontoons which are towed to the placing location.

Divers check the new formation and guide the mattresses into place. At Europort, mattresses of 20m x 10m were placed, each weighing 100 tonnes.

The joints between the mattresses and between the mattress and the sheet piling are filled with asphaltic mastic. The mastic is placed hot underwater using a purpose built frame (see picture) or an insulated mastic pipe.

Advantages

• Off-site production of mattresses minimised disruption to the port.
• Installation is relatively quick.
• Thin layers increase water depth without undermining pile stability.
• OSA is permeable (~10-2), eliminating hydraulic gradients and therefore possible uplift of mattress.
• The robust, bound scour protection leaves a regular surface which is easily maintained (dredged when necessary).
• Flexible material follows any settlement and allows a secure tie-in with outer rock layer.

< back