The second vehicle tunnel was built using immersed tube technology in the river section. This has involved dredging a deep trench in the bed of the Tyne for the immersion of pre-fabricated concrete sections.
The river section of the new vehicle tunnel connects to the cut-and-cover sections of the tunnel, either side of the River Tyne, via dedicated transition structures on each river bank.
Each transition structure consists of a shaft which was constructed with three diaphragm wall panels (underground reinforced walls) and one steel ‘combi’ pile wall on the river side of the structure. The bottom of the shaft has an inbuilt concrete slab which acts as a support for the walls of the structure. The combi pile wall was removed to enable connection with the river tunnel sections.
The course of the new tunnel across the River Tyne has been dredged using techniques to minimise potential impacts on river ecology, and shipping.
The river section of the tunnel comprises four concrete pre-cast tunnel elements, which were constructed at Walker dry dock, approximately 3km upstream of the tunnel site. Each tunnel element is approximately 90m long, 15m wide and 8.5m high. The elements were built within dry dock number 4, on a free draining material, and each is made up of four separate segments to enable a more robust final product to be built. Temporary ballast tanks were placed within each tunnel element to enable final transportation.
The dry dock was then flooded and the tunnel elements were floated out to the river, and downstream towards the tunnel site. Before being lowered into place the tunnel elements were berthed at Howdon Dock so that they could be fitted with immersion equipment.
Each element was lowered into place, sequentially, once it was ready for immersion. Each element was towed into the river channel and secured in position where the internal ballast tank was filled to enable the element to sink.
Using cables the element were carefully lowered into place on the dredged river bed. Once in place the tunnel elements closest to the river banks were pulled towards the transition structures so that the connection between the two could be made.
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 the tunnel structure.