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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.
Medway Tunnel maintenance
Maintenance closures occur every three months for three nights
with restrictions being in place from 8pm to 6am.
Advance warning signs are displayed advising drivers of future
maintenance closure dates on the approach roads to the tunnel, such
as the A289. Further advance warnings are also displayed on the
motorway's Variable Message Signs (VMS).
For current and planned roadworks information, please visit
http://medway.roadworks.org/
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