About the Medway Tunnel
The river and its estuary form one of Medway's greatest assets. This is a major focus for the regeneration of the Rochester and Chatham riverfronts, with the development of Chatham Maritime, the Historic Dockyard and the Medway City Estate.
The Medway Tunnel is the first immersed tube tunnel to be built in England and only the second of this type in the UK, the other being at Conway, North Wales. It was built in three distinct sections. The centre part of the tunnel is the 370 metres of immersed tube, which is linked to cut and cover tunnels on both the east and west banks of the river.
The casting basin required the excavation of 600,000 tonnes of earth. It was 300 metres long, 150 metres wide and 15 metres below ground level. This is the equivalent of a hole covering an area of nine football pitches and deeper than the height of an average house. The volume of the basin was 400,000 cubic metres and it was flooded with 50,000,000 gallons of water when the tunnel sections were ready to be floated into place. During the tunnel section casting, the basin was kept dry by a cofferdam (a sheet-piled wall) restricting river water access, supplemented by a series of pump wells.
Three immersed tube sections were built. They were all 23.9 metres wide and 9.15 metres high but one section was 118 metres long and the other two were 126 metres in length. They were made of concrete and each section weighed around 30,000 tonnes. The sections were built 20 metres at a time. The floor was built first, then the central dividing wall and finally the side walls and roof were cast in one continuous concrete pour. The concrete was specially prepared on site. The floors, walls and roof of the sections contained a mass of cooling pipes which were used to control how the concrete set. This was to make sure that it did not crack when it hardened and therefore remained waterproof. When they were completed, the ends were sealed with a bulkhead and ballast tanks fitted. With the flooding of the casting basin, the ballast tanks allowed the sections to be floated, just like large submarines.
To get the sections into place across the river bed, cables were stretched across the river from the west bank. Four pontoons were moored close to the east bank and cables attached to the each of the tunnel sections. The section was then slowly pulled out into the river. As it moved into the river, the pontoons were relocated to provide directional stability. When the section was clear of the casting basin, the sinking rig and towers were fitted to it. The towers allowed the section to be placed accurately above the dredged trench in the river.
Once in place the section was secured using mooring cables. The ballast tanks were flooded and the section very carefully lowered onto temporary foundations placed on the bed of the river in the dredged trench. Locating brackets (see diagram) were used to align each the section with the adjoining one. Large hydraulic jacks made sure the sections were level and in exactly the right place.
The seal between each section was formed by a heavy-duty bulbous gasket, which spanned the whole end of the section and which was specifically prepared for this type of application. The ends of each section were faced with steel plates and the gasket was attached to one of the plates and compressed against the other. A secondary sealing gasket was also used, which spanned the two adjoining sections on the inside of the structure.
The tunnel sections were initially joined by winching them together and, when they were precisely in place, water was pumped from between the bulkheads, hydrostatic pressure forcing the units together, ensuring that the gasket was fully compressed. Apart from the main and secondary gasket, no other seals or links were needed between the sections. Sometimes, the concrete between these sections expands and contracts, when this happens leaks may occur.
When the three sections were in place, sand was pumped underneath them to form permanent foundations. The dredged channel was back-filled and a layer of rock placed over the tunnel roof. This acts as a protection from shipping and the wearing effect of the tides. The road was built through the completed tunnel and service ducts and emergency walkways were provided.
The work was started in May 1992 and the Medway Tunnel was opened by the Princess Royal on 12 June 1996.