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Concept
Rock mass rating
Résumé
The rock mass rating (RMR) is a geomechanical classification system for rocks, developed by Z. T. Bieniawski between 1972 and 1973.
Since then it has undergone multiple modifications out of which, RMR89 is commonly used. Recently RMR14 has been proposed to improve the RMR performance by incorporating new experiences from tunnel practices. Continuous functions and a software "QuickRMR" for RMR89 and RMR14 have also been proposed by Kundu. RMR 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
Uniaxial compressive strength of rock material
Rock quality designation (RQD)
Spacing of discontinuities
Condition of discontinuities.
Groundwater conditions
Orientation of discontinuities
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 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. The lump ratings for each parameter makes it difficult for less experienced personnel to determine the RMR accurately. There are also subjective parameters like roughness and weathering which may pose difficulty in rating assignment.
Source officielle
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