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Lining Up Caisson to Old Ferry Pier

June 2014

Following its arrival under tow from Inchgreen to Ullapool, the caisson was berthed at the fish quay to allow the fenders and deck furniture to be fitted prior to being moved into its final home on the ferry pier.   In order to manoeuvre the caisson into its final position, powerful winches were installed at the bow end and two 10-tonne anchor moorings were placed on the sea bed to hold the structure in place.   A further winch was installed behind the Ice Plant at the round head to pull the caisson back onto a prepared docking point.  On Friday June 13th, the caisson was floated by pumping out water ballast and towed into deep water by the workboat Orcadia which had sailed down from Kirkwall for the work.  Divers then inspected the underside of the caisson to ensure it was clear of any debris remaining from the construction work.   Following the departure of the Saturday evening ferry, the all clear was given.  The structure was slowly winched across the harbour and held off the end of the ferry pier awaiting favourable tidal conditions.  Final touchdown was achieved at 5:00 am when the caisson was filled with 5000 cubic metres of sea water to anchor it in place prior to the local quarry rockfill ballast being delivered later this month.

Having ascertained that it was in the correct position and the ferry had successfully berthed, all winches and moorings were retrieved.  Preparations are now underway to grout the base of the caisson onto the seabed foundation pad.  This will be done using 20 grout tubes built into the dividing walls of the caisson through which cementitious grout is pumped to fill the 60mm space between the caisson base and the seabed foundation.  Once the grouting has been completed (3800 bags of grout) and the divers have ascertained that the grout is continuous, the compartments will be filled with over 10000 tonnes of rock from Morefield Quarry.   This will replace the water ballast and fill the cells of the caisson completely prior to the casting of the 500mm deck slab.   All in there will be a further 6 weeks work to complete the pier extension -thankfully throughout that period the berth is fully operational.