Immersed tube elements for the New Tyne Crossing are been floated into place. Gemma Goldfingle reports.
As residents in Wallsend, Newcastle flock to the banks of the Tyne to watch a giant grey object float gracefully downstream, they can be forgiven for uttering “what on earth is that?”. A submarine? Or has Nessie ventured south of the border?
In fact, the sight the crowds are witnessing is the penultimate piece of the second Tyne road tunnel, a 90m long concrete structure, drifting gracefully to its final resting place.
For many in the crowds it is about time. The current Tyne Tunnel, built in 1967, was designed to carry 24,000 vehicles a day - in fact it takes 38,000. Tailbacks have become a regular experience for the weary travellers of Tyneside.
Trevor Jackson, managing director of concessionaire TT2, which is part financing, designing, building, operating and maintaining the new 1.6km crossing, agrees the project is long overdue.
“This project was stuck in planning for over 10 years while the existing tunnel is running hugely over-capacity. The bureaucracy involved in getting a project off the ground in this country is ridiculous,” says Jackson, who comes from South Africa. “After 10 years of planning, the construction process only takes four years.”
The £260M tunnel is the final stretch of the A19 from Yorkshire to Northumberland to be dualled. When fully operational, the new crossing will carry southbound traffic while the older tunnel will carry northbound vehicles, effectively doubling the capacity of the crossing.
A number of techniques are being used to construct the 1.6km tunnel, including the aforementioned immersed tube.
Rather than boring below the seabed, a stretch of watertight tube forms the tunnel. It is laid down and covered with backfill in a dredged trench. Despite it being a common technique on the Continent, this is only the third immersed tunnel to be built in the UK.
“An immersed tube was the least risky method. It removes the need to tunnel under the river”
Trevor Jackson, TT2
“This was the least risky method. It removes the need to tunnel under the river which is inherently risky. To minimise risk here we would have had to dig deep, which would be expensive,” says Jackson.
Bouygues Travaux Publics (part of the TT2 consortium) is managing the tunnelling works. It brought in Volker Stevin Marine, a joint venture of Dutch firms Volker Stevin, Volker Construction International and Bam Civiel. All three have vast experience of building and installing immersed tunnels.
Four concrete tubes, or caissons, will join together under the Tyne to form the 360m main crossing just downstream of the existing tunnel, and will link East Howdon on north Tyneside with Jarrow, south of the river. The four elements have been constructed at the Neptune dry dock in Wallsend, some 3km from the final location.
Manoeuvring the colossal concrete tube downstream is done with pinpoint precision. There is a limited window to transport the elements. It must be done during high, or spring tide, to ensure the element passes over the seal of the dry dock. The team has sought permission from the Port of Tyne to close the navigation channel for 48 hours during the move.
“There is actually only an hour-and-30-minute window in which the tide is high enough for the caisson to cross the dry dock seal,” says Volker Stevin project engineer Wesley de Jong. “The process of getting the element out of the dry dock is a slow one. We have limited room in the dock and we can’t risk damaging the tube through collision.”
A tug boat winched to the tube gently pulls it out of the dock. With only 380mm of space either side this is a delicate process. It takes more than an hour to move the initial 230m to get the caisson out of the doc, while the remaining 3km takes only 45 minutes.
Once the tricky reverse out of the dock is completed, a further two tug boats are winched on either side of the tube and it is smooth sailing as it travels gracefully to a holding dock in Howdon Basin, only metres from its permanent location.
“It may look smooth but that is due to months of preparation. It’s just like cooking. A good meal takes a lot of pre-planning,” says Volker Stevin Marine project manager Gerrit Smit. Working out how and when to make the big move was planned by the Volker team for months.
“Of course, spring tides had to occur in the coldest winter in the UK for years,” says Smit. “But despite the snow, the weather has been kind. It was heavy wind and rain we were worried about. Currents here can reach 1m per second.”
Constructing the tubes
Finding a dry dock to construct and store the vast tubes was a challenge. Fortunately the team found a suitable yard a short distance from the site. The Neptune Yard was formerly home to Swan Hunter, one of Newcastle’s most famous shipbuilding firms.
“There are few docks of that size available and even Neptune was a snug fit,” says Smit. “We had initially considered building the elements as far afield as Holland or Belgium. This option not only saved a huge amount of transportation, but brought jobs to the region.”
Snug fit is an underestimate. When all four caissons are in the dry dock there is only 380mm between the edge of the elements and the dock wall at the closest point. Space is so tight that all construction materials have to be placed inside the dry dock using tower cranes from the dockside.
Building each 90m long, 15m wide element takes two months and is an intricate process. The tubes must be builtto the same profile as the riverbed, which means not only the main structure but internal walls, which divide the main tunnel and the escape passage, are constructed at a carefully worked out angle.
Gravel is laid on the dry dock to fit the profile of the riverbed, before the base of the tube is poured, over a plywood blinding layer, with concrete.
The 90m length could not be cast in one single action so travelling formwork has been designed by RMD Kwikform.
The 21m system sits on two train tracks so it can easily glide out when the frame and roof concrete has set. The device can hold up to 300m3 of concrete over 173m2.
“Creating a structure so large requires a lot of material. Masses of reinforcement are wrapped around the formwork, along with fire protection, before concrete is poured,” says Smit. “Over 3,250t of reinforcement bar and 10,000t of concrete is needed per element.”
As the floor and wall slabs are cast separately, there is some concern that there may be cracking.
Volker Stevin Marine weaved a system of tubes throughout the reinforcement bar that pumped cold water, which cooled the concrete and quickened the curing process, preventing cracking.
Each tube is sealed with a temporary bulkhead so that it floats with the tide. This will eventually be broken when in its final place.
In contrast to the floating, immersing the tubes must take place during neap tide, where the tidal range is at a minimum.
This occurs approximately one week after the tube is taken to the holding dock at Howden Basin. The first element was submerged on 15 January and the final section will be sunk. Vital preparation takes place over that week before the tube takes its place in a pre-dredged trench in the Tyne.
Four 4m by 4.5m concrete foundation pads are positioned on the riverbed using one of Europe’s largest floating piling barges - Dutch firm Stemat’s 400t Dina M. Two 25m towers are attached to the top of the barge - one for accessing to the tube while it lies on the riverbed and the other containing surveying and monitoring equipment.With only 30mm variance on tube position allowed, there is little room for error.
The tube is anchored to a steel catamaran that is attached to lifting points on the element’s roof. When the tide is sufficiently low, water is added to internal ballast tanks which weigh the caisson down. These tanks have a capacity of 2M litres.
“Creating a structure so large requires a lot of material. Masses of reinforcement are wrapped around the formwork, along with fire protection before concrete is poured”
Gerrit Smit, Volker Stevin Marine
“This is a complicated and challenging part of the project, especially at this time of the year when the weather can be very harsh and the daylight hours are limited,” says Bouygues Travaux. Publics project managing director Nicolas Caille. “We have highly specialised equipment and skilled divers in the river to make sure things go smoothly within this limited window.”
Three to four hours pass as the tube is precisely lowered. The team works through the night, when the tide is at its most tranquil. When the element reaches its position, steel jacks on each corner connect with pre-placed foundation pads.
The first element was laid at the Jarrow approach on the south of the river and connects to a specially built 6m long transition structure which connects the immersed section to the cutand- cover section on land.
Each transition structure consists of a shaft which is constructed with three diaphragm wall panels and one steelpile wall on the seaward side of the structure. The bottom of the shaft has an inbuilt concrete slab. After its construction the steel pile wall is removed to enable connection. The transition structure mirrors the immersed caisson sections and connects in the same way. It also has a temporary bulkhead wall which is broken through when all sections are in place.
“Connection is done using pressure. Once the tunnel element is jacked firmly against the preceding element or transition structure it makes contact with a rubber Gina gasket,” explains de Jong. “When connected, water between the bulkheads is pumped out. This creates a pressure difference between the bulkheads and the hydrostatic pressure on the outside of the tunnel. Suction compresses the gasket and seals the joint.
A catch on top of the element is also pinned together with the adjacent caisson.”
After this process, sand is pumped to the river to fill the dredged area. Some 6,000m2 is delivered from pipes at the base of each element, then rock armour is piled on top of the tubes for protection.
When all the tunnel units are immersed at the end of February the temporary internal bulk head walls and ballast tanks will be removed. By spring there will be a clear route from across the river. The new vehicle tunnel is due to open to traffic in February 2011, at which point the existing tunnel will close for major refurbishment. Both tunnels will be operational by 2012.
The immersed tube tunnel is only half the Tyne Tunnel story. Complex geology and issues concerning practicality of construction across the 1.6km route means that a combination of tunnelling techniques is required to complete the crossing.
Transition structures at either end of the immersed tube connect it with cut-and-cover sections which make up the majority of the route. These were built using diaphragm walls or secant piles, depending on ground conditions.
In shallower sections at the southern entrance, secant piles were sufficient to form the cut and cover tunnel wall.
Secant piles of 1.05m diameter were installed to form 185m of tunnel. Beyond this section, as the tunnel moves deeper underground, diaphragm walling was necessary.
The majority of the 1km cut-and-cover section is constructed using diaphragm walls of 1.2m thickness to depths of 32m.
Subcontractor Bachy Solentanche installed the last sections of piling and diaphragm wall construction in late July. An army of 10 piling rigs and seven grabs filled the narrow stretch of land across the site.
Unforeseen ground conditions were encountered on the southern side where harder-than-expected material was found.
Bedrock was so strong that drills were broken during construction. A hydrofraise machine had to be used to scratch through the hard material. The drilling rig has tungsten carbide-tipped cutters which rotate in opposite directions to break up soils.
Hard rocks of strengths up to 100MPa can be crushed using the machine without the need for vibration.
After the hard rock is scratched away, a pump above the cutting drum sucks up the loosened soil and carries it to the surface.
Once the material was removed, bentonite was poured into the hole to hold the shape until drilling was complete and concrete pumped in.
The banks of the well-worn river Tyne have seen much development which has left the legacy of a route riddled with services, many of which have few records describing their location and others that are no longer in use.
Bachy had to carefully work around them, but in two 50m sections where the services could not be diverted or navigated a mined tunnel section was built. Once the rock is excavated, the wall is lined with sprayed concrete.