The swelling ability of anhydrite can be a major problem during tunnel construction. It’s a fact that under humid atmospheric conditions every natural anhydrite (CaSO4) sooner or later dissolves, or alters to gypsum (CaSO4*2H2O), which coincides with a calculated volume increase of 61 %. The swelling capability depends greatly on the type of anhydrite itself. In order to explain and verify the differences in the swelling behaviour of pure anhydrite rocks, different anhydrite samples were investigated. The results show that the “maturity” of the anhydrite rock provides the decisive difference. The maturity is herein reflected in a larger former rock cover which led to high temperature and stress conditions. That again produces a massive rock with large crystal grains and a relatively low specific surface area. The result is a material with a low reactive surface area and thus more or less inert (“sluggish”) to swelling.
1.1 Many tunnel projects of the past had to deal with the swelling capability of anhydrite. The prediction of the anhydrite swelling potential was thereby often difficult. In hindsight we now know that some anhydrites showed less swelling as expected (Table 1). Then again some anhydrites showed strong swelling pressure and swelling strain that resulted in floor heave and in damaging of the invert (Table 1). The anhydrite bearing rocks encountered in tunnel projects are usually investigated more or less intensive on anhydrite swelling. However to date comparisons between anhydrites from different formations have rarely been conducted. The question arose whether and why pure anhydrites differ in their swelling capacity and how these differences could be investigated. An additional aim of this study is to develop tools for a faster and more reliable prediction of the swelling behavior of pure anhydrites.