Assessment of rock mass quality by the use of the Rock Mass Index (RMi)
Tatiana Barreto dos Santos1; Cássio Ricardo de Ávila2; Milene Sabino Lana1; Allan Erlikhman Medeiros Santos1; Larissa Regina da Costa Silveira1
1 Universidade Federal de Ouro Preto; 2 IBGE/CREN-RJ
Resumo
Several authors, including Bieniawski (1989), Barton et al. (1974) and Hoek (1994), have proposed geomechanical classification systems for rock masses. These systems have been largely accepted by the geotechnical community. Despite that, Palmström (1995) proposed a classification system named Rock Mass Index (RMi), which presents some advantages in relation to the traditional classification systems. RMi is calculated by the product of the uniaxial compressive strength of intact rock and a reduction factor of strength (jointing parameter, JP) that represents the effects of discontinuities on the strength of rock masses. JP is a combination of block size, measured in terms of its volume (Vb), and the joint condition factor (Jc). JP is equal to 1 when the rock is intact and equal to 0 when the rock is completely fractured. Therefore, the RMI index is a dimensional number and it represents the uniaxial compressive strength of the rock mass. The individual quantities in RMi, the strength of the intact rock and the characteristics of the joints are measured and combined to deduce the rock mass strength. Large-scale tests and field measurements of rock mass strengths were used to develop an expression for the RMi as close as possible to reality. This paper proposes the application of RMi index in five slopes excavated in rock masses with different characteristics. Two hard rocks (gneiss and quartzite) and three soft rocks (two schists with different degrees of weathering and a phyllite) have been studied. Geological and geotechnical survey were carried out and the Rock Mass Index was applied. The results were discussed in order to evaluate the advantages and disadvantages of its use in relation to the traditional methods of geomechanical classification. The studied rock masses were classified according to the RMi. The gneiss was divided into ten sectors, it had an average value of RMi equal to 19.2 MPa and was classified as very strong. The quartzite was divided into five sectors; it had average value of RMi equal to
Palavras-chave: Geomechanical Classifications; Rock Mass Index; Geological and geotechnical survey; Rock masses