Digging deeper > Tunneling
The soft clay under most of London is good for tube tunnelling. It is easy to dig out and does not collapse readily. But there are also unstable sand and gravel beds under London. These can make tunnelling dangerous.
Marc Brunel’s cast-iron shield was the first to be designed for London clay. It could be jacked forward underground but also protected the miners working inside. Brunel used his shield to build the Thames Tunnel (1825–43). Peter Barlow and James Greathead later developed a smaller, more efficient shield.