Two access embankments to a railway bridge, having a maximum height of 18 m, experienced a continuous and severe heave shortly after construction. Vertical displacements reached 120 mm in a 2-year period. The embankments were designed, by including soil–cement-treated transition wedges, to provide an increasingly rigid support as the trains approach the stiff bridge abutments. A grid of 10 m deep jet-grouting columns was also built, with the purpose of stabilising the embankments. Instead, a sustained swelling deformation, which extended to depths of 8–10 m, was activated. The compacted soil was low-plasticity clayey material, with a variable percentage of gypsum. The embankments suffered a massive ettringite–thaumasite attack, which was triggered by the simultaneous presence of cement, clay, sulfates and an external supply of water (rain). The paper describes the field extensometer and inclinometer records, the long-term laboratory tests performed, some mineralogical observations and the reactions leading to the growth of expansive crystals, and presents a model that simulates the measured heave. Forces acting against the bridge, which was seriously damaged, were estimated. Remedial measures include removal of the active upper zone of the embankments, and the construction in stages of slabs supporting the rails and founded on piles built on both sides of the embankments. The case is considered unique because of the magnitude of the reaction developed in the embankments, and its damaging action on the nearby bridge structure.