The positive history of an error. Modelling the heave of a nuclear power station

Abstract

The paper describes two different procedures to explain and to model the observed heave of a nuclear power plant founded on an expansive unsaturated claystone. In the initial model developed, the claystone was represented by a double structure (micro-macro) expansive clay. Field swelling records were accurately reproduced and the model was used to predict the long-term (40 yr) expected vertical displacements. Years after, it was realized that a more consistent explanation for the observed heave was the precipitation of gypsum crystals in discontinuities. The alternative physical phenomenon led to a re-formulation of an appropriate model, which is described in the paper. The key aspect of the long-term heave was the presence of anhydrite in the claystone mineralogical composition. The paper explains the physics of crystal growth that was triggered by the construction of the power station, the large excavation performed to locate the plant buildings and by the presence of a phreatic level, that was a consequence of the geometry of excavations. The new model, substantially more complex than the first one, is also able to reproduce available laboratory long-term swelling tests and field observations. It is hopefully a more reliable model to predict the long-term behaviour. The positive part of this history is that the initial model inspired the development of successful elastoplastic constitutive models for expansive clays.