The sufficient knowledge of rock mass properties can ensure that excavation and construction of any geotechnical structure would be safer, time and cost effective. However, rock mass structure is often very complex, because it is a result of diverse geological evolution of a particular region. Therefore, prediction of the rock mass behaviour before the excavation, bound to be addressed with many difficulties. Many different approaches usually used in engineering practice have numerous limitations and its results can lead to incorrect decisions. Due to the rapid development of computer technology, the numerical methods and tools have shown a significant improvement for dealing with problems in rock mechanics and geotechnical engineering. The research presented in this thesis is using the ability of numerical methods for estimation of jointed and heterogeneous rock mass properties, such as flysch. For the first time the synthetic rock mass (SRM) methodology was used in the Universal Distinct Element Code, where intact blocks were simulated by using isotropic-elastic Voronoi elements, and constitutive behaviour of discontinuities was represented by Coulomb residual joint model. The numerical laboratory was developed to simulate the standard laboratory tests on intact rocks, discontinuities and SRM rock mass block. The parametric and sensitivity analysis which was done in numerical laboratory gives guidelines to intact rock calibration procedure. It shows that Voronoi model can give a good prediction of quantitative and qualitative properties of intact rock. An application of SRM methodology on a laboratory-sized and large-scaled SRM model of flysch can predict a non-linear failure envelope, residual and anisotropic behaviour of jointed and heterogeneous rock mass. These results confirm our hypothesis that SRM methodology used in UDEC based on Voronoi model can be used as an advanced tool for predicting the mechanical behaviour of rock masses comparing to approaches commonly used in engineering practice.