London's congestion created the need for tunnels and its booming economy provided the financing. But what made them feasible was the city's location. The clay on which most of the city is built provided an excellent tunnelling medium. It is soft enough to be excavated easily, but impermeable enough to stay dry. Once it is burrowed through it will not crumble. It has a "stand-up time", says Roger Bridge of the British Tunnelling Society—when the first Crossrail tunnel was being built, parts of the section could be bored out and then explored as the clay stayed in place. Harder rock would require more powerful machines or explosives to dig through it.
In contrast, cities such as Dublin are built on a mixture of sandstone and boulder clay. The boulders make it difficult to mine through, while the sand absorbs water, making it less sturdy. Before the development of better technology, such as pressure-balance machines, such conditions made tunnelling tricky. Indeed, parts of south London have fewer tunnels than the centre because fewer of its neighbourhoods are built on clay.
Other countries are catching up with London. The longest tunnel in the world was built in Switzerland in 2010 while Bertha, the world's biggest boring machine, is currently whirring under Seattle.
Otros países están alcanzando a Londres. El túnel más largo del mundo fue construido en Suiza en 2010, mientras que Bertha, la taladradora más grande del mundo, actualmente está perforando bajo Seattle.
Otras ciudades se están emparejando con Londres. El túnel más largo del mundo fue construido en Suiza en el año 2010 mientras que Bertha, la taladradora más grande del mundo, esta actualmente trabajando bajo Seattle.
Otros aíses se estánponiendo al corriente con Londres. Los túneles más largos en el mundo fueron construídos en Suiza en el año 2010 mientras Bertha, la máquina taladradora más grande del mundo, está actualmente zumbando bajo Seattle.