At the beginning of the 19th century London badly needed a road tunnel under the Thames to the east of London Bridge.
But no-one had mastered driving a tunnel under water in unstable ground. It was known that a thick band of London clay existed, but above it lay water-bearing gravels to the river bed and below the clay were equally treacherous sands under artesian water pressure.
In 1818, IK Brunel's father Marc Brunel patented a cylindrical tunnelling shield with six compartments at the front, where tunnel miners could excavate the ground. At the rear was a circular 'tail' in which cast iron segments could be assembled to form the tunnel lining.
Each compartment could be moved forward independently as a miner advanced the face using wooden boards or 'polings' to secure the ground against collapse. When all had been advanced, the rear of the cylinder was moved forward by screw or hydraulic jacks thrusting off the completed lining. The next ring of lining elements would then be assembled within the protection of the tail.
For the Thames Tunnel, which started in 1823 with IK as his father's assistant, Marc Brunel developed a rectangular shield, constructed in wrought iron, working on the same principle.
By the standards of any age the Thames Tunnel was a remarkable feat of engineering and it provided the method by which many road, rail, sewer and other tunnels are constructed to this day.