The Geological Strength Index (GSI) was introduced as an empirical tool to scale the intact rock properties, from samples to a rock mass scale. Since its conception, GSI was estimated empirically according to a visual estimation of degree of fracturing and joint condition. Over time, GSI has been modified for a better rock mass characterisation and different methods have been proposed to quantify GSI in jointed rock masses.
At the present, mine operations are facing the challenge to mine ore deposits at deeper conditions, with higher stress, and in rock masses characterised by cemented and sealed veins.
The Los Sulfatos orebody, owned by Anglo American Sur, represents a porphyry copper deposit in a hypogene environment and is characterised by a stockwork of cemented veins (Figure 1). The geotechnical assessment of the different geotechnical units, according to the traditional classification systems, shows a high degree of uniformity between them, requiring a better characterisation of the primary rock mass. Because of this, SRK Chile developed a new method to quantify the GSI.
The proposed intact GSI (IGSI) is quantified based on the spacing between cemented veins and the Mohs hardness of the mineral infill. The potential degree of fracturing of the rock mass is evaluated according to the spacing between cemented veins that have a Mohs hardness up to 5, representing the weaker mineral infill that could be open during drilling or mining activities, such as blasting and caving. The range of spacing that defines different categories of rock mass is assigned according to spacing.
Cemented veins, in a geotechnical interval, are counted according to the mineral association and the Mohs hardness scale. The veins with a Mohs hardness of 2, then 3, etc. up to >5 are counted and then a representative Mohs hardness of the geotechnical interval is calculated by applying a weighted average as a function of the vein spacing. Once the spacing and the Mohs hardness are calculated, it is possible to assign their ratings and calculate the final IGSI.
This method to quantify the proposed IGSI has been tested by characterising almost 1,000 geotechnical intervals from several drill cores of the exploration campaign. The calculated IGSI has been compared with the conventional GSI and with the main classification systems. Results indicated a better geotechnical quality differentiation among geotechnical units, due to the different mineralogical infill strength and different alteration types, compared with the traditional geotechnical assessment.
The proposed method has been developed for primary rock masses and should not be used in a jointed rock mass characterised by open and weathered joints.
