Excavation damage under high in situ stress depends largely upon the potential block size associated with any violent ejection. The size and shape of the dynamic instability are largely controlled by the location, orientation and extent of the pre-existing geological discontinuities. A new methodology is presented in which the rock mass demand can be expressed in terms of the mass in tonnes of unstable rock that is ejected per unit area of the excavation surface where failure occurs. A probabilistic approach has been implemented to estimate the potential rock mass instabilities and their associated static and dynamic demands. The new methodology considers that the strain energy released by the rock mass during violent stress-driven failure is largely converted into kinetic energy of ejection for blocks. The estimated dynamic demand has been favourably compared with observations of rock mass damage in a number of underground excavations.