Tracey Logan goes underground to find out how Crossrail is using the latest engineering techniques to create 26 miles of tunnels below London's tube network, sewers and foundations - and through its erratic, sometimes unpredictable geology. She finds out about the latest science being used in Europe's biggest engineering project.
London sits on a varied geology of deposits of fine-grained sand, flint gravel beds, mottled clay, shelly beds which are sometimes mixed with pockets of water. This sheer variety has presented a challenge to London's tunnel engineers since the early 1800s.
Tracey goes on board one of the huge, 150 metre long, 1000 tonne tunnel boring machines as it makes its way beneath London's Oxford Street. At depths of up to 40 metres it can negotiate London's complex geology with incredible precision and can instantly adjust the pressure it applies at the cutting head to ensure there is no ground movement above.
Its precision engineering means it also follows a rout