Abstract
As the world's population grows, with rapid economic expansion, there is an increasing demand for better transport links to improve the mobility of goods and people. As a result more and more tunnels are being built. For example, at the turn of the year 1998/99 more than 200 kilometres of tunnel with a diameter over 1 m were under construction in Germany. About three-quarters of this kilometrage is built for road-or rail-based transportation systems. For economic and safety reasons an increasing portion of tunnels are nowadays drilled by tunnel boring machines (TBM's also called tunnelling machines), particularly in soft ground i.e. less consolidated sediments. For example, 11 TBM's have been employed to bore the 50.5 kilometre Eurotunnel between France and England which mainly passes through Lower Cretaceous chalk and clay. TBMs remove soil and rock from the tunnel face as the cutting wheel rotates and the machine pushes forward, In hard rock traditional drill-and blast still dominates over TBM, but in soft ground most tunnels today are bored by tunnelling machines.
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