  Recently, the rock mechanical and rock engineering designs and calculations
are frequently based on Geological Strength Index (GSI) method, because it is
the only system that provides a complete set of mechanical properties for
design purpose. Both the failure criteria and the deformation moduli of the
rock mass can be calculated with GSI based equations, which consists of the
disturbance factor, as well. The aim of this paper is the sensitivity analysis
of GSI and disturbance factor dependent equations that characterize the
mechanical properties of rock masses. The survey of the GSI system is not our
purpose. The results show that the rock mass strength calculated by the
Hoek-Brown failure criteria and both the Hoek-Diederichs and modified
Hoek-Diederichs deformation moduli are highly sensitive to changes of both the
GSI and the D factor, hence their exact determination is important for the rock
engineering design.
