Anhydrite and rocks containing clay minerals increase in volume when they come into contact with water. In tunnelling this can lead to a strong heave of the floor of the tunnel and even to failure of the invert arch (Fig. 1). In a Keuper formation a chemical change of anhydrite to gypsum takes Place, in which the crystalized gypsum due to the intake of water is about 60 % bigger in volume than the anhydrite. In clays, marls and certain shales the swelling is due to the water adsorption by the flakey strucutre of the clay minerals. Of the different clay minerals those belonging to the montmorillonite group stand out due to their very high swelling capacity.
From laboratory tests and in-situ observations it is known that by preventing Swelling strains in some cases considerable compressive stresses, the so called swelling pressure, are manifested. Under laboratory conditions due to the direct supply of water and the relatively small specimen size the process occurs much more quickly than in-situ. Yet even a simple swelling test may take months to complete. In a tunnel the development of swelling deformations Or (by their prevention) swelling pressures may take decades to reach the final values.
(Figure in full paper)
Of great practical importance is the finding, that in a tunnel cross-section swelling is restricted almost completely to the bottom part. There are, however, known cases in which swelling caused the heave of the entire opening including the crown as well (Schaechterle 1926),(Wagenburgtunnel 1957). If there is no invert arch and the abutments of the lining are pushed inwards then these movements are definitely to be explained as a secondary consequence of the swelling in the floor(Golta 1967).
The swelling process in a tunnel is caused both by the relief of stresses acting upon the floor and the stress redistribution in the rock due to the excavation of the opening (Terzaghi 1925). The relief of the isotropic stress component results in the percolation of water into the affected zone from distant parts of the rock mass, so that floor heave is still to be expected if the entry of water from the tunnel itself is completely prevented. The water does not derive from the moisture in the air in the tunnel, but is drawn from the rock mass adjoining the tunnel tube. The permeability of the rock mass obviously plays an important role with respect to the transient swelling process. In this connection it is worth mentioning an observation made recently at various places in two road tunnels in marly rock. The boreholes drilled into the base of the tunnels for the purpose of obtaining rock cores(without any water jetting)showed the boreholes to be dry. However, in a matter of several days to weeks they filled with water to a certain depth. The water could only have Flowed from the surrounding rock into the boreholes Similar observations are reported elsewhere (Grob 1972). The average water content of the core samples amounted to w = 3.0.