Built by a geologist
Rock Mass Rating (RMR89) Calculator
Rock Mass Rating is an empirical engineering-geology classification, not a measured material property or a complete design method. This calculator implements the Rock Mass Rating table published as Table 6-3 in the U.S. Federal Highway Administration Road Tunnel Manual, after Bieniawski 1989 . Labeling the version matters because historical RMR tables and later modifications do not always use identical ratings, descriptors, correlations, or application rules. Do not combine values from RMR76, RMR89, modified RMR, slope-specific variants, or another source without documenting the exact method. Basic RMR is the sum of five ratings: intact-rock strength, rock quality designation, spacing of discontinuities, condition of discontinuities, and groundwater. Orientation-adjusted RMR adds a sixth, application-specific deduction for the relationship between the relevant discontinuity set and a tunnel or mine opening, foundation, or slope. The arithmetic shown on the page is adjusted RMR equals strength rating plus RQD rating plus spacing rating plus condition rating plus groundwater rating plus orientation adjustment. Intact-rock strength uses a laboratory-supported uniaxial compressive strength input. The RMR89 ratings are 15 above 250 MPa, 12 from 100 through 250 MPa, 7 from 50 to below 100 MPa, 4 from 25 to below 50 MPa, 2 from 5 to below 25 MPa, 1 from 1 to below 5 MPa, and 0 below 1 MPa. GeoMiner accepts MPa or psi and converts psi to MPa before assigning a band. Exactly 250 MPa remains in the 100-250 MPa band because the top band is printed as greater than 250 MPa. Strength of a small intact specimen is not the strength of the fractured in-situ rock mass. Use representative testing, sample preparation, saturation state, loading direction, lithology, weathering, anisotropy, scale, and quality controls. RQD contributes 20 points from 90 through 100 percent, 17 from 75 to below 90, 13 from 50 to below 75, 8 from 25 to below 50, and 3 below 25. RQD must come from a defensible core-logging procedure. It is directional, threshold-sensitive, recovery- and drilling-dependent, and does not describe aperture, roughness, infill, persistence, orientation, water, stress, or block geometry. Never infer RQD from a photograph or visual rock name. Discontinuity spacing contributes 20 points only when spacing is greater than 2 m, 15 from 0.6 through 2 m, 10 from 0.2 to below 0.6