Urban Area Soft Ground Tunnelling

It has already been said and written many times that the construction of tunnels in urban areas has proved more and more indispensable, if not vital. The subsequent development of tunnelling for 15 or 20 years has brought large and sometimes spectacular improvements in the fiel of execution methods. But, particularly in urban areas, the difficulties to be overcome have been and still remain very important. As a matter of fact, most of big cities, have been settled near rivers or lakes; therefore the geological nature of the subsoil is soft ground and the level of ground water is high. As a consequence, tunnelling is often faced with many well-known difficulties and dangers issued from decompression or loosening of the ground, sometimes accompanied by a lowering of the water table. This may compromise the safety of the excavation itself and of the workers, and also cause settlements which many severely damage nearby buildings or structures. This is the reason why it is always necessary to work out specially adapted methods and even to develop new ones. In most cases, each work is a particular one and has to be specifically studied since numerous data are to be taken into account, related not only to geology, cohesion and behaviour of the different layers concerned, ground water, the activity of it, but also to