The geological strength index system (GSI), widely used for the design and practice of underground numerical process, is a rock mass classification system that considers to the rock mass strength and deformation parameters based on the generalized Hoek-Brown and Mohr-Coulomb failure criteria. The GSI can be estimated using standard chart and field observations of rock mass block size and discontinuity surface conditions. The GSI value gives a numerical representation of the overall geotechnical quality of the rock mass. This study proposes a method to determine the GSI quantitatively using photographic images of in situ stratified heterogeneous rock masses with image processing technology, fractal theory and the geomechanic analysis for joins. Using the GSI system to characterize the jointed rock mass around the Guillermo Gaviria Echeverri or El Toyo Road tunnel, Antioquia, Colombia.

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