Construction materials commonly used in civil engineering are mostly characterized by their strength properties. In rock engineering, however, no strength characterization of the rock mass is in common use. Most engineering is carried out using various descriptions, classifications and unspecified experience.
Hoek and Brown (1980), Bieniawski (1984), Nieto (1983) and several other authors have, therefore, indicated the need for a strength characterization of rock masses. The RMi system has been developed to meet this need.
The development of the RMi system
The concept of RMi was first presented at the Norwegian National annual conference on Rock mechanics (Bergmekanikkdagen) in 1986. This was partly a continuation of the ideas of the volumetric joint count (Jv), which had been presented in several papers since 1975.
The practical development of RMi was performed from 1991 to 1995 in a PhD. thesis named RMi – a rock mass characterization system for rock engineering purposes, at Oslo University, Norway.
Since 1995 the RMi has been applied in several projects; several papers on have been published. Some developments and improvements of the RMi rock support method were presented in 2000, in the latest published paper on the RMi rock support method Recent developments in rock support estimates by the RMi, with some simplifications. In addition, a few minor errors have been corrected since the RMi system was published in 1995.
Description of the RMi system
The Rock Mass index (RMi) combines numerical values of relevant parameters in the rock mass to express the RMi value. Most of these parameters, including the rock material and the joints intersecting it, can be found from common observations or measurements in the field.
The RMi-value is an approximate measure of the uniaxial strength of the rockmass. It can be used in several calculation methods in rock engineering and rock mechanics, such as for rock support estimates in underground excavations, input parameters to the Hoek-Brown failure criterion for rock masses, and for estimating penetration rate of TBMs (tunnel boring machines). In addition, RMi can be useful in estimation of some input data used in numerical modelling.
The Ph.D. thesis on RMi gives the whole development and background on this characterization/classification system and how it can be applied in rock engineering.