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 built in England and only the second of this type in the UK, the other being at Conway, North Wales. Work on the Medway Tunnel started in May 1992.
It was built in three different sections. The centre part of the tunnel is 370 metres of immersed tube, which is linked to cut and cover tunnels on both the east and west banks of the river.
Facts about the casting basin
- the casting basin required the excavation of 600,000 tonnes of earth. The basin was 300 metres long, 150 metres wide and 15 metres below ground level
- the basin is equal to a hole covering an area of 9 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 million 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. The cofferdam restricts river water access and is supplemented by a series of pump wells.
The immersed tube sections
There are 3 immersed tube sections built which were all 23.9 metres wide and 9.15 metres high. Facts about the tubes:
- one section was 118 metres long
- the other 2 were 126 metres long
- they're made of concrete and each section weighed around 30,000 tonnes with sections being built 20 metres at a time.
The floors were built first with the concrete prepared on site. The central dividing wall and finally the side walls and roof were cast in one continuous concrete pour. 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 remained waterproof. Once 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, like large submarines.
Building and placing the Medway Tunnel sections
To get the sections into place across the river bed, cables were stretched across the river from the west bank.
4 pontoons were moored close to the east bank and cables attached to each of the tunnel section, which were 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 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 lowered onto temporary foundations placed on the bed of the river in the dredged trench. Locating brackets were used to align each section with the adjoining one. Large hydraulic jacks made sure the sections were level and in the right place.
The seal between each section was formed by a heavy-duty bulbous gasket, spanning the whole end of the section and was specially 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 used, which spanned the two adjoining sections on the inside.
Joining the tunnel sections
The tunnel sections were joined by winching them together. When they were placed, 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, leaks may then occur.
When the 3 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 Medway Tunnel was opened by the Princess Royal on 12 June 1996.