TWO NEW THAMES TUNNELS
When the Brunels, father and son, drove the first ever subaqueous tunnel beneath the Thames in 1825-43, the project was beyond the leading edge of current expertise and became a icon of Victorian technological endeavour against impossible odds. The river broke through the roof and completely flooded the tunnel five times; work was abandoned for seven years with the shield bricked up behind a mirror and the half-completed tunnel open as a tourist attraction (the 'Great Bore'); at least 12 men were killed and many more died from cholera caught from the constant rain of river water; the total cost was £486,249 and Sir Mark Isambard Brunel's health was irreparably damaged. But its fame was stupendous, twice as many people visited it in August 1851 than the Great Exhibition of the same year, and Miss Julia Pardoe, visiting Constantinople, was closely questioned on it by Mustafa Reshid Pasha.
The huge spiral descent ramps were never constructed, and so this herculean project lingered as a seedy footway cum-funfare connecting Rotherhithe and Wapping until bought by the East London Railway in 1864, never having paid a dividend to its backers. With this dreadful precedent, subaqueous tunnelling was abandoned as beyond contemporary techniques for 43 years until Barlow/Greathead's shield-driven segmental cast-iron Tower Subway set the pattern for all subsequent London Clay tube tunnels.
One hundred and thirty one years later the Greater London Council's Thames Barrier, seven miles downstream from Brunel's tunnel, was its true successor. One of the largest and most difficult civil engineering projects in Britain, it is the world's largest movable flood barrier and took 4,000 persons eight years to construct at a cost of £500,000,000 - only the Channel Tunnel will surpass it. A little known feature of its construction are a further two sub-Thames tunnels, bringing the saga of the Thames Tunnel full circle.
The Barrier consists of ten movable steel gates spanning a third of a mile (530 m] across the tidal Thames at Silver-town. The four largest gates, each 61 m wide, span the navigable channel. They are sectors of cylinders which can be rotated about a horizontal axis, so that they either lie flat on the river bed in a concrete cill, or rise to form a vertical wall against the tide. Designed to resist all but a 1,000-year tide, the barrier should cope with 50 years of rising average tide levels.
The gates are rotated by a pair of hydraulic rams at each end rocking a massive crank. This machinery is housed in nine piers, 13 stories high with foundations 15 m into the chalk, built within deep water coffer dams. They are linked underwater by pre-constructed cellular concrete cills which were floated to the site and flooded to sink them into position. All the piers and their machine rooms evidently have to be connected by power and control cables and to allow human access, and by an absolutely fail-safe method, considering that the barrier will be operated for real in virtually disaster conditions.
The solution was to incorporate two completely independent and unconnected tunnels in the concrete cills, each carrying a duplicated cable network. Each tunnel therefore connects both banks, allowing any pert of the barrier to be reached, under any conditions, from either end and by two separate routes. My schematic section, compiled from the various publicity handouts available from the Barrier Information Centre, gives an idea of the stupendous scale of the undertaking from the size of the man in one of these tunnels.
