Into the void

Crossrail’s Phyllis & Ada (London’s most voracious worms) are the latest in a long line of giant worms that have nibbled their way under the city. The family tree (or should that be root?) goes back at least as far as 1825 to the tunnelling shield used to bore the Thames Tunnel between Rotherhithe and Wapping (pictured).

The square structure may not look very worm-like. But Peter Ackroyd in London Under tells how it was designed by Marc Brunel to mimic the ship-worm Teredo navalis. The Teredo eats timber and passes the wood through its body; the excreta are then used to bolster the fabric of the tunnel it has created. In Brunel’s tunnelling shield, the nibbling was done by men in 34 separate cells who would carve out clay. Workers behind would then line the newly excavated tunnel with brick and stone.

Phyllis and Ada may look rather different, but the principles remain the same. Where men once removed 4 ½ inches at a time, the rotating cutting wheel will bore at a rate of 100m a week. A screw conveyor will carry the loosened material through to the back of the machine. Concrete tunnel segments will slide in to line the void.