Newcastle’s £260M (USD 202M) New Tyne Crossing project is one of the UK’s largest infrastructure schemes. It involves the construction of a second vehicle tunnel to dual the existing A19 under the River Tyne. The new crossing is positioned to the east of the current Tyne Tunnel which dates from 1967. Upon completion, the total length of the new carriageway will be approximately 2.6 km with the tunnel itself making up 1.5 km of the total length.

At first glance the building of a new traffic tunnel might seem like a reasonably simple job, but in this case the River Tyne running over the middle of the route is throwing up a myriad of challenges for the concessionaire, Tyne Tunnel 2 (TT2).

Owners, the Tyne and Wear Integrated Transport Authority (TWITA), opted for a public private partnership structure to develop the scheme and appointed the TT2 consortium in 2007 for a 30-year concession period. TT2 is made up of Bouygues Travaux Publics, HSBC Infrastructure Fund II and Bank of Scotland Corporate. Bouygues Travaux Publics is the design-and-build contractor.

“It’s a complex little job, it’s not big in its footprint, but it is difficult, there are lots of different technologies involved,” said Trevor Jackson, managing director of TT2.

To combat the precarious nature of the project, planning has been thorough. Although the tunnel is not particularly long, the challenges faced by Bouygues have resulted in splitting the works in to four distinct sections: the 318-m north approach; the 360-m river section; the 840-m south cut-and-cover section with the south junction, each of which are in progress simultaneously.

Due to the site’s varied industrial history (in the past the area has homed breweries, dockyards and collieries), samples were tested for both contaminates and leachates. Stage one of the project comprised a geotechnical and contamination desk study of the proposed tunnel alignment and the immediate surrounding area. The aim of the desk study was to identify past and current potentially contaminative site usages and included a review of the available historical plans, geology, hydrogeology and hydrological information for the site.

Stage two was a preliminary ground investigation (PGI) along the line of the proposed tunnel to confirm the findings of the desk study and to obtain geotechnical and chemical information on the soils, rocks and groundwater. This information was used to assist in the compilation of the reference design for the proposed scheme. The PGI was carried out between May and July 2000 aimed at obtaining geotechnical and chemical information on the soils, rock and groundwater within the study corridor.

These investigations revealed that the geology far from constant. Daniel Clert, project manager, explained, “When you have a glacial location like we do here, under the water you find granular sandy conditions, then as you go up the banks you start finding different natures of soil, then clay and with that the rock at depth.”

North of the river
The 318m-long north section relies mostly on cut-and-cover techniques. The geology of this section generally comprises the following strata: made ground, occasional instances of alluvium, glacial deposits (upper till and laminated clays, middle till, lower laminated clay, lower glacial till and basal sand and gravels) and Carboniferous Middle Coal Measures bedrock with interbedded mudstone, siltstone and sandstone and thin coal seams.

There is considerable spatial variability in the made ground, alluvium and glacial deposits. Typically, towards the northern end of the section (towards the portal) the tunnels are mostly founded in the glacial deposits. As the alignment heads towards the north bank of the river it passes through lower glacial till and localised alluvial sands and gravels.

Following the construction of 600mmthick diaphragm walls, approximately 170,000m3 of soil was excavated from the area between. This was done in phases with temporary struts placed between the walls for support. The tunnel was then constructed in situ, using base and roof slabs to provide lateral support. The temporary support struts were removed as construction of the base and roof slabs proceeded. Following completion of the main tunnel structure, soil was used to backfill the space above the roof slabs to reinstate the necessary ground level above.

For a small section of the tunnel, this process was not suitable as the new tunnel passes over the existing one. For this area, a bottom down construction technique was chosen. This was to avoid any heave in the existing tunnel which would have been caused by removing the weight of ground material lying above it. In this area the roof slabs were constructed first, followed by excavation beneath the roof, and then construction in situ. All the while constant monitoring of the existing tunnel was taking place with highly sensitive movement sensors.

South of the river
The 840m-long south section is also being constructed using cut-and-cover methods, apart from two small sections of 32m and 40m which are being constructed using sprayed concrete lining in order to avoid major utility diversions. These two sections equate to approximately 4 per cent of the total land tunnel excavations. The general stratigraphy south of the river can be described as made ground (granular and cohesive) overlying either alluvium or sequences of glacial till and laminated clay which overlie bedrock of the Carboniferous Middle Coal Measures. The invert of the tunnel is within bedrock from the south transition structure for approximately 230m. The alignment rises up through lower glacial till and lower laminated clay sequences. The southern 600m or so of tunnel is within upper glacial till, laminated clays and made ground as the tunnel alignment runs near to the surface.

Sprayed concrete lining (SCL) sections were separated from the main cut-and cover trench by 600mm-thick diaphragm walls that were constructed when the diaphragm walls for the cut-and-cover trench were built. These dividing walls are being broken through to enable excavations to take place for the SCL tunnels.

To the south of the tunnel interlocking secant piles have been used to reinforce the excavated space where the tunnel lies above the water table. The tunnel is on a gradual incline towards the surface from the river to the south exit of the tunnel. As the piles are not absolutely water-tight, the tunnel box must be constructed in its entirety within this section, whereas in other places, where the cut-and-cover technique is used, the diaphragm walls are used as the permanent walls for the tunnel structure.

At the south exit of the new tunnel, where both the new and the old tunnel run adjacent to one another, the tunnel is being constructed in alignment with the existing tunnel as a reinforced concrete box structure.

The needs of the formworks on both sides of the tunnel were specific and required bespoke solutions. “We needed to present the best system for each application. Each part of the job has its own little challenges,” says Paul Lawton of formwork provider RMD Kwikform. “With top down construction you couldn’t crane handle that formwork so you had to suspend it and that made it very easy to move. You needed be able to adjust the height of the supports of the north side, whilst on the south it was fixed almost all the way.”

On the north side of the tunnel, the base slab rises at a continuous 6 per cent incline whilst the height of the tunnel changes multiple times throughout its length, meaning that at points the soffit slopes by up to 12 per cent. RMD Kwikform engineering director, Ian Fryer summarises the situation: “The challenge was to deliver a whole-slab-area travelling formwork system with 2m height variation which could be operated without the site staff working at height. The traveller also needed to give the users the flexibility to snake the equipment up an incline of 6 per cent, travelling on rails whilst casting a slab that was up to 8.5m off the ground, at the bottom of a 25m deep excavation full of large ground shoring props.”

In transition
The north and south sections are joined to the central immersed tunnel section by two transition structures, which act like coffer dams, creating a dry space within which to prepare the connection between the two types of tunnel. The outer wall of the transition structures was removed once the connection point was constructed and the element was ready to be immersed and connected.

A rarely used approach was taken to break through the redundant outer wall. Clert explains: “We had to saw cut one side of the shaft in order to open the door for the tunnel element to get in. This required cutting through a 1m thick wall that is 25m below water and 25m height in sections. I don’t think this method has been undertaken very often on any projects in the world. For me it was one of the big worries of the job.”

Understandably so because a delay with this activity would have delayed tunnel tube immersion and the whole project.

Cutting through the wall was done using a series of pulleys and ropes that effectively cut through the concrete partition like a cheese wire. “We had to dispatch divers to install pulleys like that in the wall and start sawing the thing horizontally and vertically,” says Clert. As a diver can only stay at the required depth of 25m for a minute or so a huge team was needed. At such depth, visibility was also severely restricted yet precision was key.

Crossing the river
During the construction of the transition structures, the Tyne was prepared for the arrival and placement of the concrete elements that make up the 360m-long immersed-tube tunnel section. November 2009 saw the arrival of the suction cutter barge ‘Vesalius’ which worked for six weeks preparing a trench for the elements to lie in. Around one million tonnes (520 000m3) of dredged material from the river bed was pumped via a 2.6km-long pipeline directly to infill the Tyne Dock as part of existing redevelopment plans.

The four concrete pre-cast tunnel elements were cast by VolkerWessels UK at Walker dry dock, approximately 3km upstream of the tunnel site. “Initially the units were going to be built on the continent, but fortunately upon arrival in Newcastle we found that there was an unused dock that was suitable for fabrication of the units and we kick-started fabrication there soon after,” says Clert.

Each tunnel element is approximately 90m long, 15m wide and 8.5m high. Overall, 14 400m3 of concrete has been used to construct the river tunnel units. Each element is made up of four separate segments to enable a more robust final product to be built. Ballast tanks were placed within each tunnel element to enable final transportation. Once all tunnel elements were constructed the dry dock was flooded, ready for the tunnel elements to be floated out to the river, and downstream towards the tunnel site. The tunnel elements were berthed at Howdon Dock and fitted with immersion equipment, before being lowered into place.

Each element contains five ballast tanks, two primary tanks located in the traffic corridor, and three smaller, secondary tanks in the pedestrian escape passage. The ballast tanks are a temporary construction within the permanent structure of the element. Each tank comprises a polythene membrane mounted on plywood and supported on timber joists. The timber joists are set on steel framework. The volume of the tanks varies according to the phase of activity. Each element is fitted with two tanks with a holding capacity of 1250m3, one tank with a capacity of 212m3, and two tanks that can hold 81m3. The element can also hold up to 30m3 of trimmed concrete.

The units are sealed between one another with a Trelleborg Bakker Gina gasket which compresses under hydrostatic pressure. Once the units are connected in this way, the water-filled space between the bulk heads is pumped out. Each element has two flat steel plates attached to it at either end. At one end of each element the seal is attached to the steel plates so that when the elements are joined together the seal compresses against the steel flat plate on the other unit, and when they are compressed against each other the seal is created. The elements are compressed together with a force of 35t.

The initial primary seal is formed by the Gina membrane. Once in place, Trelleborg Bakker Omega seal is attached at a later date. The Omega seal will typically be placed some weeks after the gina seal, and becomes the primary seal. Should the Omega seal fail the Gina seal will remain as the secondary, permanent seal.

Each element was lowered into place, sequentially, once ready for immersion. Each element is being towed into the river channel by three tug boats and once secured in position the internal ballast tank filled to enable the element to sink. Using cables the element is carefully lowered into place on the dredged river bed. Once in place the tunnel elements closest to the river banks will be pulled towards the transition structures so that the connection between the two can be made.

Sand will be injected below each tunnel element, to secure it in place, using a sand flow technique. Once all tunnel elements are in place a closure joint will be constructed in situ to seal the elements together. Rock armouring will be placed above the tunnel, along its entire length, to protect the tunnel structure. At present three of the four elements have been laid in the Tyne. The new tunnel is due to be completed in February 2011 followed by refurbishment of the old tunnel for both to be operational in December 2011.


Several excavation methods were used, including cut-and-cover with temporaray support struts removed as construction proceeded Special RMD Kwikform formwork installed in cut-and-cover Breaking through a wall in the SCL section Building the four immersed tube elements in Walker dry dock