Anchors Stabilize Vital Harbor Works on the Island of Alderney
Located south of Great Britain, Alderney Island is the northernmost island of the Channel Islands. The key lifeline between the island and Great Britain is the Commercial Quay dock, which is located in Braye Harbour. The harbor receives essential materials such as food, fuel, or building materials, and it is the island’s most important passenger harbor.
Comprehensive rehabilitation of the more than 100 year old quay was necessary in order to accommodate vessels measuring 95 m in length. In addition, for security reasons, freight and passenger operations will have to be separated in the future. Furthermore, the old quay offered inadequate mooring security for oil tankers and had experienced damage from corrosion.
In May 2008, a specialist team sent by the civil engineering firm Geomarine began cleaning off the existing seabed sand down to sound bedrock. Afterwards, a steel former was placed on the seabed with the help of six divers and filled with concrete up to the level of the underside of the pre-cast concrete blocks.
A structural steelwork skeleton was then erected to offer temporary support to blocks. The new quay wall consists of 842 pre-cast concrete blocks, each weighing 12 t, and 2,500 m³ of concrete. GEWI® anchors were used to anchor the quay wall to the seabed. The GEWI® anchors were passed through the apertures of the precast block stack and drilled into an 8m rock socket below the seabed prior to being stressed to 120 t. Drilling of the rock socket was achieved by using the Down The Hole hammer method, with flush velocity a key factor in ensuring the removal of the drill spoil. It was essential that the adjacent borehole be sealed and grouted before drilling could commence on the next borehole in order to prevent blow-through between boreholes. All in all, 40 No. Ø 63.5 mm GEWI® anchors in lengths of 18m were installed, together with 44 No. Ø 63.5 mm GEWI® high shear dowels.
The grouting of the anchors required specialized measures to ensure that the boreholes were completely filled. A system of tremie tubes with integrated grout level indicators ensured that the 8m long borehole could be fully grouted. This allowed prestressing of the anchor tendon prior to bonding with the precast concrete segments, ensuring that the prestressing force was permanently transferred to the concrete segment quay wall.
The realization of this project on Alderney Island presented a logistical challenge to everyone involved. Despite the isolated location of the island, delayed shipping, adverse weather conditions and bad conditions for working underwater, work at the new quay wall was successfully completed.