The Medway Tunnel provides a dual carriageway road crossing under the River Medway between Gillingham and Frindsbury. The tunnel, 725 metres long from portal to portal, consists of three sections.
At either bank is a cut and cover section with diaphragm walls and roof and floor slabs, 260 metres long on the east bank and 95 metres long on the west bank. Between the east approach and the west approach, the central section is an immersed tube tunnel, only the second road tunnel in the UK to use this method of construction. The central section was precast in three units at a casting basin on the east bank of the river. Each of the rectangular units measured 23.9 metres wide and 9.15 metres deep. Two units were 126 metres long, and the other was 118 metres long.
Great care was taken during the concrete pouring. Each unit was constructed in 20-metre sections. The floor of each unit was constructed first, then the central dividing wall, and finally the side walls and the roof which were cast in one continuous concrete pour. A network of cooling pipes controlled the temperature at which the concrete set, making sure that it remained waterproof and that it did not crack as it hardened.
When the three units were completed, the ends of the units were sealed with temporary bulkheads. The casting basin was flooded, and the units were floated out into the river and positioned over a pre-dredged channel in the riverbed, which had been dredged through the alluvial silts and sands into the underlying chalk.
Each of the units contained ballast tanks, so that engineers could control the height at which the units floated and could sink the units into the channel when they were in position. The units were then joined together through hydrostatic pressure compressing the gaskets at the ends of the units, and the bulkheads were removed to form the tunnel.
When the tunnel sections were in place, sand was pumped underneath to form a foundation, and the trench was backfilled. A layer of rock was placed over the filled trench to provide protection from the scour of the tidal current.