The stability of waste dumps of former open-pit mines must be guaranteed for its use on construction and other activities. In this work, a sandy waste dump subjected to a seismic event is carefully analysed. Its sudden settlement is simulated with a Boundary Value Problem (BVP) under dynamic conditions. The Intergranular Strain Anisotropy (ISA) constitutive model, able to simulate the behaviour of sands under monotonic and cyclic loading, is used for this purpose. The model revealed some unloaded zones due to the mining process, and showed that compacted dumps may experience significant settlement after a seismic event.