The immersed tube method involved dredging a 12 metre-plus deep trench in the bed of the River Tyne and placing four pre-fabricated concrete sections (caissons) into it. These sections were built three kilometres upstream in a dry dock restored especially for the project in Walker, Newcastle, by VolkerStevin Marine.
Each caisson weighed 10,000 tonnes, was approximately 90 metres in length, 15 metres wide and 8.5 metres high.
Once they were all completed, ballast tanks inside the caissons were filled with just enough water so they could be floated downstream without being too high or too low in the river.
The dry dock was then flooded and the units carefully floated out, one unit at a time during January and February 2010 at high tide before beginning their journey downstream with the aid of three tug boats.
The units were temporarily moored at Howdon Basin before the delicate operation to lower them into the pre-dredged trench.
The bulk head of each unit was fitted with a compressible seal at one end and a receiving gasket at the other end. Once submerged the seal and gaskets were married up and the water between the bulk heads was released. Hydrostatic forces then came into play and forced the units together and so formed a watertight seal. They were connected to the cut-and-cover sections of the tunnel on either side of the river via dedicated transition structures on each river bank.
The units were fitted with two 25 metre high towers – one to provide access to workers once they were submerged, the other for surveying purposes (contractors required millimetre accuracy and the help of skilled divers to position the units correctly).
The river was closed to shipping for 48 hours whilst the immersion of each of the four units took place.
Sand was injected below each tunnel element, to secure it in place, using a sand flow technique. Once all tunnel elements were in place, a closure joint was constructed in situ to seal the elements together. Rock armouring was placed above the tunnel along its entire length to protect it. The temporary internal bulk head walls and ballast tanks were then removed. While the units were being constructed, dredging was taking place to prepare the 12 metre-plus deep trench across the river to accommodate them.
Thanks to an innovative partnership between main contractor, Bouygues Travaux Publics, and the Port of Tyne, dredging was carried out using a cutter suction technique. All the spoil was pumped direct from the river bed via a 2.6 kilometre long pipeline to Tyne Dock, to provide infill as part of the Port’s redevelopment plans.
Around one million tonnes (520,000m3) of material excavated from the New Tyne Crossing site was used to fill the Victorian Tyne Dock, with approximately 800,000 tonnes of material arising from dredging and the remaining 200,000 tonnes supplied from land excavations. The partnership brought benefits for the environment, for local communities, and for regeneration, thus justifying the transport authority’s case at Public Inquiry that flexibility was needed on the issue of waste disposal so that the contractor could explore the best environmental solution available once construction began.