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Forth Replacement Crossing caissons arrive on site

Arrival this week in the Forth Estuary of two up to 1,200t steel foundation caissons signalled the start of full scale construction of the 2.7km long Forth Replacement Crossing cable-stayed bridge.

The double-skinned, up to 30m tall, caissons are now in Rosyth docks being fitted with pumps, lighting and a temporary 10m high upper extension.

Air in the annulus between the two skins acts as a buoyancy aid allowing each caisson to float and, in the next few weeks, the first larger unit will be towed to the site of the bridge’s southern tower just upstream of the existing Forth Road Bridge.

Here concrete will be pumped into the annulus allowing the crane-supported caissons to sink under their own weight to the sea bed.

Sediment over the estuary’s bedrock will be excavated by grab before 16,000m3 of concrete is poured underwater to form a heavy plug. After dewatering above the plug, the bridge’s concrete tower base will be cast in the dry.

The second smaller caisson will help form foundations for the bridge’s northern tower, with the third central tower founded in the dry on a mid-estuary rock island.

Main contractor, Forth Crossing Bridge Constructors (FCBC) – a consortium of Hochtief Solutions, Dragados, American Bridge International and Morrison Construction – was awarded the £790M bridge contract for Transport Scotland in spring last year. “We are on budget and on schedule for completion in 2016,” said FCBC project director Carlo Germani.

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