The paper presents an analysis of the mechanical behaviour of rockfill using the discrete element method. A background reference is the set of results of a few large scale triaxial tests performed at the UPC geotechnical laboratory. In those tests Relative Humidity (RH) was controlled by means of a vapour equilibrium technique. RH is a fundamental aspect in the behaviour of rockfill which has received limited attention in the past. The Relative Humidity within the large pores between rock fragments controls the velocity of crack propagation within the particles. An increase in RH means faster crack propagation and eventually breakage of some particles and subsequent re-arrangement of the granular structure. The basic tool used to simulate some of the tests is the computer code PFC3D. Rockfill particles (they have the size of gravels, typically ranging from 1 cm to 4 cm) were simulated as breakable clusters of 3D balls. Particle breakage occurs in time according to fracture mechanic’s laws. In a parallel study, the development of cracks in time has been examined. This information was taken into account in the numerical analysis to derive criteria for particle breakage. The paper describes the preliminary results of the work in progress. The actual shape of rock gravels has been approximated by means of clusters of spherical particles. Several arrangements, comprising a different number of particles, have been numerically tested. The results of the modelling exercise are encouraging and test results are reasonably well reproduced. The model is fairly general and it has a number of interesting capabilities.