The paper describes the 3D numerical modelling studies of a 280 m height slope cut in a rock mass having complex geological formations. The modelling was carried out to evaluate both the static and dynamic stability of the projected rock slope, which will be located in northern Mexico. The construction site presents complex geological features like faults and dikes as well as several geological formations from varying qualities and fracture conditions. Available field data made it possible to incorporate the most important geological and topographical features with high degree of detail. The geomechanical model was developed into the commercial three-dimensional distinct element code 3DEC. This code was chosen because it has been widely used for rock mechanics problems and has been evaluated in a number of instances. Also, the code has a built-in programming language (FISH) that allows the user to expand the code's usefulness by using it as a numerical-analysis platform. Data available from field explorations allowed applying the Hoek-Brown and Bieniawski's criteria to estimate mechanical and elastic properties for the rock mass. These properties were consistent with in situ geophysical explorations. Before simulating the construction, an analysis was carried out for the pre-construction conditions under both static and dynamic loading as to validate the model's ability to represent the initially-stable site conditions. During the static analyses of the construction process, the full 3D model was used to simulate this process in four stages. A FISH function was implemented into the code to compute the factor of safety against material failure using the Mohr-Coulomb criteria in each excavation stage. The effect of blast damage and stress relaxation in the factor of safety was also investigated. After the construction simulation was done, a seismic motion was input at the model's base in the slope's normal direction. To substantially reduce the computing time due to the size and degree of detail of the model, dynamic analyses were performed on selected 3D portions (three-dimensional slices) of the model. However, these dynamic analyses simulated the true three-dimensional behavior by applying velocity conditions at the slices' outer boundaries, thus establishing continuity conditions at slices' lateral boundaries. This initial assessment of the rock-slope stability was carried out purposely to anticipate problems that may arise during the construction, and thus take decisions on the possible support measures.