This paper presents data analysis on engineering geological mapping and rock mass classification of the São Bento Gold Mine, Brazil, using Q and RMR systems (Barton et al.,1974 and Bieniawski, 1974). Although mapping development encompassed all sublevels of mining levels 21, 22 and 23, data presentation was restricted to the 4th Sublevel of Level 21. Geomechanical maps using both classification systems were produced for each mining gallery allowing for easy identification of each rock mass class occurring at both footwall and hanging wall.
Mining is a very important activity of Minas Gerais State economy. Galleries of underground mining may reach depths of several hundred meters, where severe confining pressures may exist. At such conditions, stability problems, usually occur resulting in high operational costs. The execution of geological and geomechanical studies of rock masses is necessary to provide previous and sufficient knowledge to allow anticipation of rock behavior under the designed excavation. Decision regarding work methodology, mining methods and reinforcement adequacy can be also defined based on such studies (Goodman, 1993). These classification systems are not to be taken as a substitute for engineering design. They should be applied intelligently and used in conjunction with observational methods and analytical studies to formulate an overall design rationale compatible with the design objectives and site geology (Bieniawski, 1989). Application of rock mechanics to underground excavations in Brazil started in the 60's. Among several practical examples, the work developed by Albuquerque et al (1990a, b) at Fazenda Brasileiro gold mine and the construction of Serra da Mesa Hydroelectric power plant must be pointed out. In recent years, several rock mass classifications at open pit mines have been applied to slope stability design projects (Souza, 1999). 1.1. Objectives The main purpose of this paper is to present and discuss geological data collected through an engineering geological mapping development to classify the Sao Bento Gold Mine rock masses, using Q and RMR systems (Barton et al., 1974 and Bieniawski, 1974).
The methodology applied is well known in geotechnical practice. It is based on analysis and resume of existing data, engineering geological mapping, geological description of rock cores, analysis of structural data, laboratory tests and, finally, geomechanical classification of galleries mining at Level 21 of the São Bento Gold Mine. This methodology has been applied to Sao Bento Gold Mine since 1992, generating a large collection of pre-existing data that was very useful in the development of the rock masses geomechanical classification.
São Bento Gold Mine region rock types form a lithological sequence of metamorphic rocks including: a lower mafic-ultramafic unit (Quebra Ossos Group), an intermediate chemicalpelitic unit (Nova Lima Group), and an upper clastic (sedimentary) unit (Maquine Group). Quebra Ossos Group is mainly composed by schists. Nova Lima Group is formed by chlorite schist, micaceous schist and Banded Iron Formation (BIF). Maquine Group is formed by clastic sedimentary rocks that change to schists to the top of the sequence (Santos, 1997).
