ABSTRACT: During the design process in rock engineering Hoek-Brown failure envelope is used for the determination of rock mass failure envelope mainly in brittle rocks. An important input parameter of the Hoek-Brown failure envelope is the Geological Strength Index (GSI), which varies between 0 and 100, and concentrates on the description of rock structure and block surface conditions. There are several methods which define GSI but a general international standard has not been specified yet. Our aim is to analyze different methods of GSI determination on the basis of observations during the construction phase of the Bátaapáti radioactive waste repository. Examinations of the values determined on-site gave significantly different results. Different correlations were determined between the calculated GSI values.
The Geological Strength Index (GSI) represents today the most widely used engineering index for the categorization of rock mass quality for obtaining input data into the continuum numerical analysis codes and closed form solutions based on the Hoek-Brown failure criterion (e.g. Marinos & Hoek 2000, Marinos et al. 2007). The exact determination of this value is very important for the exact calculation of the failure envelope or the deformation moduli of the rock mass.
Ván & Vásárhelyi (2014) determined the sensitivity of the GSI based on mechanical properties, such as the Hoek-Brown equation (failure envelope of the rock mass) and the Hoek-Diederichs equation (deformation moduli of the rock mass). It was shown that sophisticated empirical equations can be highly sensitive to the uncertainties in the GSI values - even if the error of the GSI is only 5%, the relative sensitivity can reach 100%!
Recently, Morelli (2015) analyzed the different calculation methods of GSI. Using Monte-Carlo simulations, his simulation results indicate that the diverse relationships may predict dissimilar values of the GSI for the same rock mass. He obtained the highest GSI value from the equations which apply the conventional RMR1989 values, and the lowest results were obtained by using the RMi method for GSI calculation.