When constructing tunnels in the gypsum keuper formations of South Germany, design engineers are confronted with particular problems, which result from the swelling potential of the unleached gypsum keuper. Swelling phenomena have already led to severe damage here in the past on several occasions and the repair work has involved a high consumption of money and time.
During his employment as a consultant, the author has had the opportunity to gain experience in the design and the construction of tunnels in swelling rock and thus to derive some design criteria for tunnels in such rock.
Before going into the details of these criteria, a brief explanation of the particular characteristics of gypsum keuper shall be given. Fig. 1 illustrates the three different phases in which gypsum keuper occurs in nature in an idealized sketch. The layer encountered below all others consists of claystone and siltstone as well as calcium sulphate prevailingly in the form of anhydrite. Here the anhydrite appears in finely dispersed form within the claystone and sandstone and in concentrated form in compact anhydrite layers. Above the socalled anhydrite level the mineraloyical composition of the intact rock alters. Here the major portion of the anhydrite is transformed into gypsum as a result of accumulating water. Whereas anhydrite is pure calcium sulphate, gypsum has two molecules of cristalline water. Since anhydrite is stable only at temperatures below 30°C and in the absence of water, a transformation of anhydrite into gypsum takes place as soon as water is introduced. Characteristic of the rock mass beneath the gypsum level is its horizontal stratification and vertical jointing.
(Figure in full paper)
With a sufficient supply of water and the presence of seepage, a further transformation of the rock takes place during geologically significant periods of time. The gypsum dissolves in the water and is transported away by the ground water. As a consequence above the gypsum level the so-called leached gypsum keuper remains, which consists of an alternating sequence of differingly weakened, pliable siltstones and pure silts. Characteristic of the pliable siltstone is that not only the bedding but also the jointing are still clearly recognizable, although on the other hand only a slight squeezing of a piece of this rock leads to its complete disintegration into silt. While the leached gypsum keuper is thus practically to be regarded as a soil, both the modifications taking place beneath it result in a competent rock. So for example a 10 m diameter tunnel with an overburden of 150 m was excavated with practically no temporary support. Fig. 2 illustrates the stable working face of a tunnel in an alternating sequence of siltstone, claystone and anhydrite. In addition the rock below the gypsum level is almost impermeable and thus practically dry in its undisturbed condition. Problems do however arise if along with the construction of a tunnel water is added to the rock. The manifestations of swelling that were mentioned at the start then arise. This process will be more closely described in the following text.
