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.
The project has following objectives:
- scientific and methodical treatment of the problem of swelling in rock with interpretation of the swelling phenomena,
- description of the forces acting in the region of the invert, constructional measures, design charts
- recommendations for the determination of the swelling potential.
Summary in the form of a handbook for consulting engineers and contractors. The handbooks content is as follows:
The purpose of the handbook is that of providing a consistent representation of the state of the art in the specific field of tunnel construction.
The book is divided into two main part: Basic knowledge and engineering activities in practice. In the basic knowledge a brief historical retrospective found on observations and assumptions on which it was based until the early 20 century. Then there is a brief review of specific geological aspects that always recur in subsequent discussions. The core chapter of the "Basics" brings together several separate treaties on the process of swelling in the clay minerals and rocks containing anhydrite, as well as the role played by water. The proportion varies according to laboratory conditions and terrain.
The second part of the work is devoted to the peculiarities of the project and execution of work in swelling rocks. In this section, the reader is made aware of the many gaps in our today knowledge, gaps that go against the validity of the static calculations. It may also notice that the knowledge gaps in geotechnical data and insufficient measures may still lead to calculations using a suitable model.