The objective of this paper is to develop a methodology for calibration of a discrete element grain-based model (GBM) to replicate the hydro-mechanical properties of a brittle rock measured in the laboratory, and to apply the calibrated model to simulating the formation of excavation damage zone (EDZ) around underground excavations. Firstly, a new cohesive crack model is implemented into the universal distinct element code (UDEC) to control the fracturing behaviour of materials under various loading modes. Next, a methodology for calibration of the components of the UDEC-Voronoi model is discussed. The role of connectivity of induced microcracks on increasing the permeability of laboratory-scale samples is investigated. The calibrated samples are used to investigate the influence of pore fluid pressure on weakening the drained strength of the laboratory-scale rock. The validity of the Terzaghi's effective stress law for the drained peak strength of low-porosity rock is tested by performing a series of biaxial compression test simulations. Finally, the evolution of damage and pore pressure around two unsup-ported circular tunnels in crystalline granitic rock is studied.