In Italy the average age of existing concrete dams is about 65 years and there are few possibilities to design and build new dams. This means there is a strong need to keep existing dams in safe service conditions, considering particularly the effects of seismic loadings, not always foreseen during the design.

The modelling of the cracking behaviour of concrete dams during earthquakes is a challenging issue that can be tackled with models based either on the discrete crack or the smeared crack approach. Otherwise, the non-linear response of dams can be accounted for by means of geometric discontinuities, but in this case it is mandatory to previously define their position into the mesh; furthermore, it is not possible to reconstruct the trigger point and the crack propagation, but only to analyse the opening and the sliding between the crack faces, evaluating their influence on the global stability of the structure.

In this paper, the eXtended Finite Element Method (XFEM), a numerical technique based on the discrete crack approach, was adopted to analyse the propagation of cracks. This method, innovative in the world of dam engineering, is available in the commercial FEM code Abaqus: either a cohesive approach or an energetic approach based on the linear elastic fracture mechanics are implemented to describe the crack initiation and propagation. The XFEM can be very effective to simulate cracks, since the crack propagation is not required to follow the element boundaries.

The method allows to assign the position from which a crack will start, but it can also generate a crack without the trigger point, only by defining the region in which the crack will initiate and propagate.

To verify the validity of the method, a simply supported plain concrete beam subject to imposed displacements was considered; then, the cracking process of Koyna dam during the 1967 earthquake was numerically simulated.

The results of the XFEM approach were compared with those obtained with the Concrete Damage Plasticity model, based on the smeared crack approach