Cellcore from Cordek addresses the issues of ground uplift caused by deep excavation on the New Tyne Tunnel Crossing
(03/05/2010)

When completed in 2011, the New Tyne Tunnel Crossing will comprise a two lane immersed tube tunnel stretching between East Howdon on the North bank and Jarrow on the South bank of the River Tyne. The four giant concrete tunnel segments are being created up river from the construction site and will be floated down the River Tyne before being immersed to form the new tunnel on the riverbed. The main contractor for the project is French company Bouygues Travaux Public. Engineers on the New Tyne Tunnel Crossing have specified Cellcore from Cordek to address the issues of ground uplift caused by deep excavation on the access road leading to the tunnel.

Expanded polystyrene manufactured by Jablite is supplied to Cordek to produce the patented Cellcore system. Cellcore from Cordek is a honeycomb of interlocking expanded polystyrene sections, which are designed to collapse at a pre-determined load.

It was anticipated that ground uplift could cause a problem on this project, due to the deep cut and cast excavations on the access roads to the tunnel. If this movement is restrained, the lateral and uplift forces produced are huge and would eventually result in damage to the 1.2m reinforced concrete slabs above. Cellcore allows the EPS to compress and deform to accommodate this uplift, thereby reducing the forces that would otherwise be transmitted to the concrete slab foundations. This provided civil and structural engineers Aecom and High Point Rendell with a solution for dealing with the overburden recovery associated with the deep excavations on the North and South side of the tunnel.

Rodney White, Managing Director of Cordek, explains: “Type CC Cellcore in four grades was used on this project, which is available in a standard panel size of 2.44m x 1.22m. The product features a heavy-duty polypropylene top, although on the New Tyne Tunnel, an additional 50mm blinding layer of concrete was used above and below the Cordek heave protection layer.”

Cordek’s Type CP Cellcore was developed for use beneath suitably reinforced concrete slabs and includes a 50mm thick polystyrene top, protected by a 2mm thick polypropylene sheet that provides a high degree of insulation.

As Cellcore uses EPS, it absorbs expansion, unlike some other anti-heave products, that rely on material degradation, which means the system is immediately effective. This was a huge benefit for contractors on the New Tyne Tunnel, as once the Cellcore panels were laid and the joints sealed, the surface was ready for the next stage of construction.

Water pressure was a potential problem for the engineers due to the proximity to the river, which resulted in excavations beneath the water table. This meant that a number of Cellcore panels were supplied with drainage apertures.

Richard Lee, Sales Director at Jablite, said: "This was a very prestigious project to be involved with and we are delighted to have been able to work with Cordek to provide a solution that counteracts the ground uplift on this major new structure. Our technical team worked closely with Cordek and the engineers to come up with a solution that will ensure protection of the concrete slab.”

In addition to its physical properties, expanded polystyrene is easy to handle and compatible with rapid construction techniques, which is a major benefit for contractors. As part of modern methods of construction, it can be transported to site in the minimum number of loads, producing less waste and with no negative environmental impact.

Expanded polystyrene has environmental benefits too as it is HCFC-free and non-toxic. The material is also extremely versatile as its closed-cell formation inhibits absorption of water and therefore avoids any problems of waterlogging and drainage.