First sheet piled cofferdam section for the foundation of the Forth Replacement Crossing’s central tower was craned out to Beamer Rock in the middle of the Forth Estuary on Monday (5 November) in what is a key milestone for Transport Scotland's project. 

The rectangular sheet piled cofferdam of ten 80t, 10.6m high sections is being constructed inside the (now excavated) rock. Each section is made of a steel wall and a 0.5m deep concrete base. The cofferdam is due to be completed by mid December.

Contractor Forth Crossing Bridge Constructors is erecting three towers for the cable stay structure at a height of 207m above high tide. The crossing will be 2.7km long including approach viaducts.

The north and south towers’ foundations are being formed inside 1200t double skinned steel caissons 30m high and of 30m diameter. Each caisson has a temporary caisson fixed on top so that the structure remains above sea level. This allows dewatering of the caisson and the formation of the tower’s concrete foundations in dry conditions.  

Once the foundations are complete, the temporary caissons are removed for recycling and the permanent caissons remain out of sight below the surface of the water, sunk into the seabed.

The approach viaducts to the new bridge are carried on hollow cross section, reinforced concrete piers – eight on the south side, two on the north. On the south side, the foundations for the pier nearest the bridge (ie the northernmost) will be constructed using a caisson. The majority of the remaining piers will have their foundations constructed using sheet piled cofferdams. 

Sections of the central tower cofferdam which is now under construction out on Beamer Rock.

Deputy First Minister, Nicola Sturgeon and project director David Climie viewing the cofferdam.

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