Rock Mass Rating (RMR) Calculator

The Bieniawski Rock Mass Rating (RMR) system is a widely used geotechnical classification for scoring how competent a rock mass is for tunneling, mining, and excavation design, first published by Z.T. Bieniawski and refined into the RMR89 version used here in his 1989 book Engineering Rock Mass Classifications (John Wiley & Sons). It combines five separately-scored parameters -- uniaxial compressive strength (UCS) of the intact rock, Rock Quality Designation (RQD, a measure of fracturing from drill core), joint spacing, joint condition, and groundwater condition -- into a single additive total (RMR = sum of all five sub-ratings). This is the BASIC RMR89 score; the complete Bieniawski system also subtracts a discontinuity-orientation adjustment (how favorably the joints are oriented relative to the excavation, ranging from 0 to -12 for tunnels, 0 to -25 for foundations, and 0 to -60 for slopes) that this calculator does not include, so treat the reported score as an upper bound on the final adjusted RMR.

Because the basic total is a simple sum with no interaction terms among the five scored parameters, every point earned on any one of them contributes exactly one point to the overall score, which is part of what makes RMR straightforward to apply consistently across different sites and engineers. Each sub-rating is selected from its own published band of discrete values rather than typed in freehand, since RMR's sub-ratings are themselves looked up from standard tables, not measured directly. The resulting score sorts the rock mass into one of five classes, from Class I ("Very Good Rock," RMR 81-100) down to Class V ("Very Poor Rock," RMR below 21), each with an associated qualitative guide to unsupported stand-up time, cohesion, and friction angle used for preliminary support design. RMR is a widely used starting point for excavation planning, but it is a classification tool based on field observations and engineering judgment in each sub-rating, not a substitute for detailed geotechnical analysis or site-specific engineering review before final support design.