Bouygues employs host of construction techniques on River Tyne tunnel

Inside the transition structure, where massive hydraulic struts provide bracing support.

To ease the passage of an ever-increasing amount of traffic crossing the River Tyne, a second vehicle tunnel is under construction. Being built by Bouygues, the geology of the site means several techniques must be used along the tunnel’s length. Juliet Davies explains.

In the 1930s a pedestrian and cycle tunnel was cut beneath the Tyne, giving the local workers an easy and free river crossing.

By 1967, car and lorry traffic had increased to such levels on the road bridges that a road tunnel was constructed.

Now, 40 years later, an additional crossing is needed to alleviate the traffic using this ageing tunnel.

A bridge was considered, but less land would be lost to a tunnel.

So, to resolve the transport issues, concession company Tyne Tunnel 2 (TT2) has been tasked with designing, building, part-financing, and operating all of the tunnels old and new until 2037. Even though a new tunnel would require less land, for the New Tyne Crossing one of the difficulties has been the new project's proximity to the existing tunnel and road network.

Trevor Jackson, TT2 managing director, explains: "A key challenge associated with this project is maintaining traffic flow at the existing vehicle tunnel, while construction of the new tunnel takes place alongside it.

"We have around 38,000 vehicles a day travelling through the original tunnel, and we need to keep them moving."

Journey times

Despite construction activity taking place so close to the road network, TT2 has managed to maintain pre-construction journey times. "In fact, thanks to the layout changes required in order to accommodate parts of the construction site we have successfully reduced journey times for southbound traffic, through improved lane merging," Jackson adds. "It's counter-intuitive, but rather than the construction site creating delays, it has helped us to speed up tunnel traffic."

Local traffic will not benefit from the new tunnel until about 12 months after its completion, as the existing tunnel is to be closed for refurbishment as soon as the new tunnel can take vehicles.

The geology of the site dictates that this is no simple one-method structure, and main contractor Bouygues has had to incorporate several techniques along the length of the tunnel.

Daniel Clert, Bouygues' project manager, describes it as a "compact linear project, only 2.5km long, but very dense. Because of the number of services we sometimes had 10 subcontractors working at the same time - it was very difficult, but we managed it."

South Junction

The South Junction will be realigned to pass over the existing and new tunnel entrances, the existing bridge demolished, and a large sewer diverted. Major roundabout works will allow the A19 traffic priority through the tunnels, which local traffic can join.

This newly created Jarrow Interchange will also house the ventilation plant, while the current ventilation chimney will be improved when the existing tunnel's remedial works are undertaken. The South Portal is carried through from the box section to the secant piled cut and cover.

"Construction of the new south junction presents a particular challenge as the interchange will be completely redeveloped," says Jackson.

He adds: "The existing Howard Street road bridge that spans the top of the existing tunnel will be demolished and replaced by a new loop road for local traffic. This work will take place directly above and around the tunnel entrance, and very careful planning is required to ensure that impacts on traffic flow are minimised, whilst maintaining the highest possible safety standards."

The South Approach at Jarrow uses cut and cover, in a trench down 25m to the ground platform, with 1m thick diaphragm walls to a depth of 35m into the rock bed.

The depths of the diaphragm walls mean massive ground pressures, and a series of hydraulic struts provide temporary bracing support against this.

Once the tunnel's base and roof slabs are in place, the struts can be removed as the backfill is compacted offering permanent lateral support.

Groundforce provided the struts and Tony Gould, the firm's technical director, explains that providing a propping system using propriety hydraulic struts was an immense technical challenge.

"The design loads were very large due to the depth of the dig and poor alluvial ground conditions," he adds.

Several additional situations such as loss of prop, 5t of accidental load, and the potential 38°C rise in ambient temperature also had to be considered. "This combined to provide a set of challenging load cases to be accommodated," says Gould. "We dealt with this using a combination of standard, specially modified, and purpose-designed equipment including the new 500t capacity MP500 strut.

"This strut uses a combination of hydraulic expansion, for ease of installation and pre-loading, with the added security of twin mechanical locking screws."

Buried gas mains

A part of the south section was bored, where buried gas mains meant diaphragm walls were unworkable, so two smaller tunnels, 50m by 50m, were dug using traditional methods. Sprayed concrete lining is used in these sections - an outer sprayed concrete primary lining and an inner cast in-situ permanent lining. The North approach at East Howden involves moving the existing roundabout, and constructing a new Toll Plaza.

The North Section utilises the same cut and cover techniques as the south side.

On this side though, the new tunnel crosses over the existing, deeper tunnel, with only 3m separating them - the main engineering challenge here will be when the weight of the ground above is temporarily removed for the new tunnel, with a risk of ground thrust rising the existing tunnel.

Transition structures (a diaphragm wall and a steel combi pile wall), marry the land tunnels to the river section at both sides, while protecting the excavations from the water.

The total volume of ground excavated is estimated to be around 400,000m³, although over 90% of this will remain on site as backfill and landscaping.

Very little contaminated land had to be addressed, although an old landfill site on the northern site meant that strict environmental policies had to be put in place.

Wildlife protection

For all the highly technical work at the site, wildlife protection legislation allowed a broody seagull to bring work to a halt in one area.

"On the south side, the Gaslight Pub had to be demolished," explains Greulich. "But just prior to the demolition, a seagull nested in the chimney pot.

"We weren't allowed to demolish it until the young birds had flown the nest - while at the same time the local press were running stories about seagulls making people's lives a misery!"

Although taking less than four years, and only 2.5km long, this project packs in more than many other schemes that are several times its size.

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FactFile

Project: New Tyne Crossing

Client: Tyne and Wear Integrated Transport Authority

Concessionaire: TT2

Main contractor: Bouygues Travaux Publics

Subcontractors: VolkerStevin Marine; Groundforce

Specialist consultants: HPR Consult; Parsons Brinkerhoff; Faber Maunsell; Woodholmes Group

Project value: £260m

Start date: February 2008

Finish date: December 2011

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Tugging a tunnel across the Tyne

The river section will not be ‘tunnelled’ in the traditional sense. Some 370,000m³ of the river bed will be dredged, and four 12m by 10m by 90m caissons brought in from their manufacturer VolkerStevin Marine, built in the A & P dry dock at Wallsend.

Floated into their river positions by tug boat, the ballast tanks will be filled, sinking them in to the dredged channel, where a shield of rock armour will prevent damage from dragging anchors.

Throughout this time an operational shipping channel must be kept open.

And it’s not only the shipping that has to be considered - the Tyne’s migratory salmon dictate that no dredging can take place as the fish return up the river to spawning grounds.

"Dredging should take place after October," said Tamsin Greulich, TT2’s partnering manager.

"However, there are exceptions, and with the appropriate management controls in place it can be possible to dredge earlier than this, even during September.

"We’re currently exploring our dredging dates with the Environment Agency and other stakeholders."