Clays and rocks containing anhydrite possess the property of increasing their volume by absorbing water. Technically, one speaks of rock swelling.

In tunnelling, as is well known, swelling can be observed through base heave or, if it is prevented, by swelling pressure on the invert arch. With an intact invert arch the tunnel tube can be lifted blockwise as a whole, so that heave of the bottom is also accompanied by displacement of the tunnel roof. In many cases the invert arch cannot resist the pressure, and it is heavily damaged.

The shape of the invert arch has to satisfy two criteria: bearing capacity and serviceability. In the tunnels for high speed trains, due to low admissible differences in ground movement along and across the tunnel axis the second criterion may be decisive. But also during construction problems can arise, which are caused by the swelling phenonemon. The question, under which conditions when advancing the tunnel using blasting may one drill dry or wet, has to be carefully examined.

Since in the future a large number of underground structures will have to be excavated in swelling rock, a well founded and correct procedure in the planning and execution phases is of great economic importance. The Belchen Tunnel on the National Highway N2 is just one example of possible set-backs resulting from ignorance or misjudgement of the swelling phenomenon. The current state of knowledge concerning rock swelling in underground construction as well as about the appropriate measures to adopt in the planning and during execution is incomplete and has never been treated in depth in the literature. Therefore, in practice when confronted with important decisions one often proceeds according to engineering judgement and not according to verified scientific findings.