Swelling laws for clay-sulfate rocks revisited

The swelling of clay-sulfate rocks is a major threat in tunnel engineering and in the installation of shallow geothermal systems. It can cause serious damage to tunnels and buildings; and produce high additional costs during tunnel construction and operation. The swelling may result in a heave of the tunnel invert, destruction of the lining or uplift of the entire tunnel section. Heave-pressure-time relations are therefore needed in order to predict the mechanical behavior of swelling rock as a basis for an optimal tunnel design. The present study revisits different stress-strain relations ("swelling laws") for swelling clay-sulfate rocks proposed by various authors. Published laboratory data from oedometric swelling tests are presented that may confirm the proposed stress-strain relationships. These data are re-examined by testing each of the different data sets with the different proposed relations. One main outcome of this study is that different interpretations of stress-strain data are possible and none of the swelling laws proposed in the literature could be generally confirmed or rejected. We conclude that a generally valid swelling law in the form of a stress-strain relation does not yet exist. A promising approach to describe the swelling behavior of clay-sulfate rocks, however, is process-based numerical modeling, which is also briefly introduced in this study.