2 FELSBAU 25 (2007) NO. 4 ANAGNOSTOU: DESIGN UNCERTAINTIES IN TUNNELLING THROUGH ANHYDRITIC SWELLING ROCKS ROCK MECHANICS Design Uncertainties in Tunnelling through Anhydritic Swelling Rocks By George Anagnostou Bemessungsunsicherheiten beim Tunnelbau in anhydrithaltigem quellfähigen Gebirge Deutsche Kurzfassung fehlt. Tunnelling in Gypsum Keuper is one of the engineering tasks still associated today with large inherent uncertain- ties, despite over one hundred years of tunnelling activity in such rock. We are still far from understanding, let alone from quantifying, the mechanisms governing swelling of anhydritic rocks, as a series of physico-chemical and seep- age flow processes come into play (water adsorption by the clay minerals, chemical transport, dissolution of anhy- drite and growth of gypsum crystals). The extremely long duration of the swelling process, taking several years even under laboratory conditions at the scale of rock specimens, makes systematic investigations very difficult. An empiri- cal design, based upon experience from projects carried out under apparently similar conditions, is hardly possible at all as the intensity of the swelling often varies consider- ably over small distances within one and the same tunnel, even in the case of a macroscopicaly homogeneous rock mass. The observed variability of the rock behaviour seems to be a consequence of differences in water circulation. Observations indicate that seepage flow singularities as- sociated with fractures or layer interfaces have a big effect on the swelling phenomenon. The uncertainties concern- ing water circulation and time development of swelling are common to all existing design concepts. T he term “swelling” denotes the property cer- tain rocks have of increasing their volume by absorbing water. In tunnel construction the swelling causes a long-term heave of the tunnel floor, which can impair the serviceability of the structure. If one attempts to prevent the heave by means of an invert arch a so-called swelling pressure results. This can damage the lining or cause the tunnel to be lifted as a whole. The sulphatic claystones of the late Triassic Gypsum Keuper are particularly demanding, as they con- tain anhydrite and exhibit a very high swelling potential. In the past, a number of older railway tunnels crossing sulphatic claystones have had to undergo extensive repair work in the north western part of Switzerland, in south Germany and recently also in Spain (1, 2). Recent Swiss projects suffering damage as a result of sulphatic swelling rock are the Chienberg motorway tun- nel close to Basle (3) and the Adler tunnel of the Swiss Federal Railways (4). A detailed descrip- tion of the state of the art as regards tunnelling in swelling rock can be found elsewhere (1). The present paper focuses on the causes and the engineering significance of uncertainties with regard to the behaviour of rock formations con- taining anhydrite. The swelling of purely argillaceous rocks, i.e. rocks not containing anhydrite, such as the marls of the upper freshwater molasse or opali- nus clay, is for all practical purposes sufficiently well understood. It is discussed here only inso- Wir benötigen noch folgende Unterla- gen: Deutsche Kurzfasssung, deutsche Bildunterschriften. Die Redaktion