This paper presents a review of the estimation of rock mass strength properties through the use of GSI. The GSI classification system greatly respects the geological constraints that occur in nature and are reflected in the geological information. A discussion is given regarding the ranges of the Geological Strength Index for typical rock masses with specific emphasis to heterogeneous rock masses.
Reliable estimates of the strength and deformation characteristics of rock masses are required for almost any form of analysis used for the design of surface excavations. Hoek and Brown (1980a, 1980b) proposed a method for obtaining estimates of the strength of jointed rock masses, based upon an assessment of the interlocking of rock blocks and the condition of the surfaces between these blocks. This method was modified over the years in order to meet the needs of users who applied it to problems that were not considered when the original criterion was developed (Hoek 1983, Hoek and Brown 1988). The application of the method to poor quality rock masses required further changes (Hoek, Wood and Shah, 1992) and, eventually, the development of a new classification called the Geological Strength Index (Hoek 1994, Hoek, Kaiser and Bawden 1995, Hoek and Brown 1997, Hoek, Marinos and Benissi, 1998), extended recently for heterogeneous rock masses (Marinos and Hoek, 2000). A review of the development of the criterion and the equations proposed at various stages in this development is given in Hoek and Brown (1997).
The basic input consists of estimates or measurements of the uniaxial compressive strength (—ci) and a material constant (mi) that is related to the frictional properties of the rock. Note that both tables are updated from earlier versions (Marinos and Hoek, 2000).
