Heave of a railway bridge induced by gypsum crystal growth: field observations

The central pillars of a bridge belonging to a recently built high-speed railway line experienced an unexpected and continuous heave after the end of construction. Pillars were founded on 3 × 3 pile groups capped by a rigid slab. The tips of piles supporting the central pillars reached a hard Tertiary anhydritic claystone. Deep extensometers allowed the identification of an active layer, 12–15 m thick, located below the pile tips. Observations in recovered cores suggest that the heave is induced by the growth of gypsum crystals in discontinuities of the anhydritic claystone. No heave was observed in gypsum-rich claystones located above the anhydritic layer. Gypsum crystal growth is associated with dissolution of anhydrite and subsequent precipitation from a super-saturated aqueous solution. Boreholes and pile construction allowed a hydraulic connection of upper aquifers and the lower anhydritic formation. This hypothesis explains the role of the bridge construction in the triggering of a dormant heave phenomenon. The heave rate has been reduced to a small value by the weight added by an embankment 33 m high, which partially fills the original valley.