A trio of tunnelling techniques will come together to create the new Tyne Crossing near Newcastle. Alexandra Wynne reports.
Pace of work is now picking up on the £260M River Tyne crossing in the north east of England, after a lengthy planning process.
The new 1.5km long tunnel will create two-lanes running between East Howdon in North Tyneside and Jarrow in South Tyneside for concessionaire TT2, appointed by the Tyne & Wear Passenger Transport Authority.
The completed crossing will help relieve traffic congestion and is part of regeneration plans for South Tyneside. It will also give the operator more flexibility as it refurbishes the existing tunnel under the Tyne.
French design and build contractor Bouygues Travaux Publics and consultant HPR are delivering the project which started on site last November.
“This is a challenging one because there are so many things to get done in the four years,” says Bouygues project director Nicolas Caille. “Technically, the scheme is complicated but it’s made more challenging because of the time and the environmental constraints.”
A 360m immersed tube tunnel forms the central part of the river crossing. It will adjoin a series of cut and cover tunnel approaches either side.
In addition, the project will involve a tricky combination of tunnelling techniques along its modest 1.5km length.
This includes creating a dry dock for casting the immersed tube sections and installing steel combi-piling to protect the riverbanks, placing concrete diaphragm walls for the cut and cover tunnel portion and constructing a mined section of tunnel with a sprayed concrete lining.
There are just three years left on the project and to cram in all this construction activity has meant that simple, robust construction methods have had to be employed.
“There are very few things on this project that are entirely novel,” explains HPR director Vardaman Jones. “But that’s because there is so much to do in a short time.
“Also, everything - the design and construction - has to start at the same time, which is unusual. This leaves little time for learning as you go.”
The contractor is currently dredging silts to prepare the dry dock, where the four 90m-long immersed tunnel elements will be cast in concrete. Once ready, these will be towed down to the site and sunk into position in a dredged trench.
“There are few things on this project that are entirely novel. But that’s because there is so much to do in a short time”
Meanwhile, the contractor has already installed piles to protect the riverbanks near the tunnel portals. This is important, says Jones, because dredging work along the immersed tube alignment would otherwise disturb the embankments.
The tight construction programme also means that as this is going on, the north and south portals will be built at the same time.
Flanking the immersed tube tunnel section to the north and south will be concrete cut and cover tunnels. The plan is to install 1.2m thick concrete diaphragm walls up to 30m deep to create the tunnel lining. Subcontractor Bachy Soletanche will install these and then prop them as it digs out earth down to the base of the tunnel.
Here, a 1.2m thick concrete floor will be cast, followed by construction of the tunnel roof.
It sounds simple, but there are yet more variations on the theme. On the north side, engineers have had to design around the existing tunnel which is just 3m below it in one location.
The southern section has even more variety. At the shallower end, a 185m length of cut and cover tunnel wall will be formed using 1.05m diameter continuous flight auger secant piles.
Secant piling will be used in shallower sections as this will be relatively cheaper than diaphragm walls which will be used at depths approaching 30m. As the south section moves deeper underground, the design reverts to diaphragm walling.
But the risk of disturbing utilities means that in two places the tunnel will be excavated through the rock and lined with sprayed concrete. One of these sections is 31m and the other 40m.
“We decided to do that because it’s simply quicker than disturbing the utilities,” says Caille.
In addition to the pressure of a four-year project plan, environmental constraints make the schedule even tighter. Migrating salmon use the Tyne between March and November. So as not to disturb their path, the team will have to conduct all the work in the river outside of this period in the less than favourable winter months.
The new tunnel is due to be completely operational by December 2010. At that time, traffic from the existing tunnel will be diverted into it so that the old one can be refurbished. The existing 3.1km tunnel opened in 1967 and is a deeper, rock bored tunnel. The upgrade work primarily involves installing a concrete partition wall along one side to provide a safe emergency route out of the tunnel in the event of a fire.
The refurbishment is due for completion in 2011, when both tunnels will operate together for the fi rst time. From then, the old route will carry traffic northbound and the new tunnel will take southbound traffic.
When open, the existing Tyne crossing is expected to be the UK’s first newly built tunnel to befitted with a mist sprinkler.