Rock Mass Rating

The Rock Mass Rating(RMR) System is a geomechanical classification system for rocks, developed by Z. T. Bieniawski [1] between 1972 and 1973. It combines the most significant geologic parameters of influence and represents them with one overall comprehensive index of rock mass quality, which is used for the design and construction of excavations in rock, such as tunnels, mines, slopes and foundations.

The following six parameters are used to classify a rock mass using the RMR system

Each of the six parameters is assigned a value corresponding to the characteristics of the rock. These values are derived from field surveys and laboratory tests. The sum of the six parameters is the "RMR value", which lies between 0 and 100.

Below is the classification table for the RMR system.

The details for calculating RMR are given elsewhere by edumine and provide a series of tables for RMR determination while the latest charts for the same purpose are given in the references and further reading. In particular, the charts enclosed here for the RMR parameters intact rock strength and the combined parameters RQD and discontinuity spacing (represented by the number of discontinuities per meter), show the advantage of using the charts for better accuracy, rather than relying on the tables which show the average ratings for the ranges of each RMR parameter.

Rock Mass Rating RMR has found wide applications in various types of engineering projects such as tunnels, slopes, foundations, and mines. It is also adaptable for knowledge-based expert systems. Engineers informally classify rock structure into two general classifications: continuous homogenous isotropic linear elastic (what most geotechnical engineers would like to see) and discontinuous inhomogenous anisotropic non-elastic (what most in-situ rock masses actually are). A rock mass rating system provides a method of incorporating some of the complex mechanics of actual rocks into engineering design.

Moreover, the system was the first to enable estimation of rock mass properties, such as the modulus of deformation, in addition to providing tunnel support guidelines and the stand-up time of underground excavations.

Recently, after over 40 years of use, renewed attention was paid to the RMR System because of its applications to the assessment of rock mass excavability (RME) and, especially, its direct correlation with the specific energy of excavation (SEE) for TBMs used effectively to detect changes in tunneling conditions, in real time, thus serving as a warning of adverse conditions as construction proceeds.

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