CONSTRUCTION of the £1.5 billion road bridge over the Forth has begun in earnest with the first of several giant steel tubes that will house the foundations for its three towers being sunk into the river bed.

The 23 metre-high structure, known as a caisson, that will eventually envelope the concrete foundation for the north tower was lowered onto the silt bed of the Forth on June 7 after being delivered by boat from a construction yard in Poland last month.

Over the next two months it will sink a further downwards as soil is excavated from its hollow interior and concrete pumped into its hollow sides until it makes contact with the rock bed, 40m below the river surface.

Managers at Forth Crossing Bridge Constructors, the consortium commissioned by government agency Transport Scotland to build the crossing, said the foundation work taking place for the remainder of this year was one of the most crucial phases in what is the biggest construction project in a generation, and the one most fraught with risk.

An even bigger caisson, at 30m high and weighing nearly 1200 tonnes, is floating further upstream in the Forth, held in place by six anchors, and is due to be lowered into position at the end of next week when similar excavation work will take place to help sink it onto the rock bed of the river.

Once in place, a water-tight seal will be built at the base and underwater concrete poured into each caisson to form the foundation on which the north and south tower of the bridge will sit. The central tower which at 210m is also the biggest, will sit atop Beamer Rock in the middle of the river, which has already undergone blasting to level the surface.

Carlo Germani, project director for FCBC, said: "We're into the real construction work. We've had some preparation and site installation work but what you're seeing now is the real construction of the bridge. The foundations are probably the most critical of all the operations we've got to perform."

Around 800 people are currently employed at the site headquarters in Rosyth, more than double the number three months ago, with the total number size of the workforce set to peak at 1200 at the end of the year.

The three towers which will hold up the cable-stayed bridge are due to take shape next year and the bridge deck will be built outwards from each tower in 2014, with each section due to join up the following year. The bridge is due to open to traffic in 2016.

Project leaders said the public could expect to see an "elegant" structure emerge over the next two years, partly as a result of a unique system of overlapping the cable stays, giving the structure more stability.

This has allowed the towers to be more slender than in similar bridges where "H" or diamond-shaped structures have been needed to support the weight of the bridge and traffic.