Heave of a railway bridge: modelling gypsum crystal growth

The heave of the central pillars of a railway bridge, founded on large-diameter bored piles, is a consequence of gypsum crystal growth in an anhydritic claystone stratum underlying the tip of the piles. The heave mechanism is explained by the presence of circulating water in the fractured anhydritic level. The model developed to reproduce the observed phenomena integrates a set of balance equations that includes two soluble species (gypsum and anhydrite) in addition to the inert minerals, kinetic equations for dissolution and precipitation, mechanical equilibrium, and the effect of precipitated mass on induced volumetric strains. Model calculations are compared with measured long-term field records of heave. Most of the model parameters are physico-chemical constants, a positive feature that limits the effort to calibrate material constants. The model is capable of accurate mid-term heave prediction, and it also correctly includes the effect of building a surface embankment with the purpose of reducing the heave rate. A sensitivity study is reported to achieve a deeper insight into the phenomena, and to investigate the relevance of a few controlling parameters.