Assessment of rock mass quality by the use of the Rock Mass Index (RMi)

RMi system is a geomechanical classification obtained through the multiplication of the uniaxial compressive strength of intact rock by a reduction factor of strength, named JP, which represents the effects of discontinuities on the strength of rock masses. JP factor is a combination of the block size, measured in terms of volume (Vb), and the joint condition factor of the joint (jC). Therefore, the RMi index is a dimensional number and it has the same unit of the uniaxial compressive strength. The individual components of the input in RMi, the strength of the intact rock and the characteristics of the joints are measured and combined to deduce the rock mass strength. In this paper the RMi index was applied in five rock masses with different characteristics. Geological and geotechnical surveys were carried out in these rock masses. The application of the method was discussed pointing out its advantages and disadvantages in relation to the traditional methods of geomechanical classification, i.e. the RMR and Q. The studied rock masses were classified according to the RMi as follows: a gneiss that was divided into ten sectors had an average value of 19.2 MPa and was classified as very strong; a quartzite that was divided into five sectors had an average value of 19.8 MPa and was also classified as very strong; a schist with only one sector had an average value of 3.8 MPa and was classified as strong; another schist that presented two very distinct sectors, with values of 0.008 MPa and 0.9 MPa and was classified as very weak and moderately strong and a phyllite with only one sector had an average value of 0.13 MPa and was classified as moderately strong. The geomechanical classification system proposed by Palmström (1995) offers best practice field guidelines to obtain the parameters. This is a great advantage because it reduces the uncertainties arising from the geotechnical survey. Furthermore, the method does not use the Rock Quality Designation (RQD), which it is an index that is highly dependent on the direction of the discontinuity surveys. In addition, the method has the advantage of providing a dimensional number, which represents the uniaxial compressive strength of the rock mass. The method has not been used by the geotechnical community, which can be considered a disadvantage.