Modifications to the GSI for granite in drilling

The Geological Strength Index (GSI) is widely used to estimate mechanical parameters of rock mass, but estimation of GSI is relatively subjective because of the lack of quantitative parameters. Hence, the existing method for estimating GSI is not suitable for rock samples from drilled cores. Inspired by the GSI estimation method of flysch (Marinos and Hoke, Proceedings of the GeoEng2000 at the international conference on geotechnical and geological engineering, Melbourne, Technomic publishers, Lancaster, pp 1422–1446, 2000), this paper proposes two parameters, the rock mineral condition and rock core length, for use in a modified, more quantitative method for estimating GSI. This modified method can be used for granite specimens from drilled cores. It uses images of typical core samples instead of the sketches given by Hoek’s GSI estimation method. A modified chart of GSI for granite, which is suitable for estimating mechanical parameters of rock mass from drilled core samples, is then developed. The E (elastic modulus) and Q _ult (ultimate bearing capacity) values obtained using our modified method were compared with those from experimental data; the results indicate that the modified method can produce reasonable GSI values and can be used to estimate the mechanical parameters of rock mass from drilled cores.