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Rock Mass Classification Guide: RQD, RMR89 and Q-System
Rock-mass classification turns selected measurements and engineering-geology observations into reproducible indices or classes. It is a disciplined way to organize evidence, compare internally consistent domains, communicate conditions, and test whether an investigation is complete enough for the next question. It is not a direct measurement of rock-mass strength, a complete failure model, or proof that an excavation, slope, foundation, dam abutment, or underground opening is stable. The three most common terms in this workflow answer different questions. Rock Quality Designation, or RQD, is a directional core-based index. It sums qualifying lengths of hard, sound core pieces across a defined run and divides by the full drilled run. Drilling direction, core diameter, recovery, run boundaries, mechanical-versus-natural break decisions, weathering, and the governing approximately 100 millimetre or 4 inch threshold all influence the result. RQD omits discontinuity orientation, persistence, aperture, roughness, infill, groundwater, stress, and block geometry. It is an input to RMR89 and Q, not a complete classification or design result. Rock Mass Rating after Bieniawski 1989 combines intact-rock strength, RQD, discontinuity spacing, discontinuity condition, and groundwater into basic RMR, then applies a separate orientation deduction for the intended tunnel or mine opening, foundation, or slope. The exact version must be stated because historical and modified RMR systems do not necessarily use identical tables. RMR89 supports consistent description and comparison only when the rock unit, structural domain, discontinuity set, observation scale, application, and evidence quality are explicit. The Q-System developed and maintained by the Norwegian Geotechnical Institute is an empirical classification for underground openings. Q equals RQD divided by Jn, multiplied by Jr divided by Ja, multiplied by Jw divided by SRF. The three quotients express relative block size, inter-block joint friction, and active stress from water and the selected stress regime. Jn, Jr, Ja, Jw, and SRF require mapped geometry, surface and filling character, water conditions, stress context, and behavioural evidence; they cannot be selected reliably from a rock name, photograph, RQD value, or another classification. The current NGI handbook also requires excavation span or height and Excavati