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Rock mass classification

Short answer

Rock mass classification uses intact rock and discontinuity properties to rate quality for tunnels, slopes, and foundations. Key systems: RQD (core recovery ≥10 cm), RMR (sum of five parameter ratings), and the Q index Q = (RQD/Jn)×(Jr/Ja)×(Jw/SRF).

Describe the engineering classification of rock mass.

Rock mass classification is the grouping of a rock mass into classes according to its quality and engineering behaviour, based on the properties of the intact rock and of the discontinuities (joints, bedding, faults). It is used to estimate rock strength, stand-up time and support requirements in tunnels, slopes and foundations.

Common systems:

  • RQD (Deere) - percentage of core pieces longer than 10 cm; classes from very poor (<25 %) to excellent (90 - 100 %).
  • RMR (Bieniawski) - sum of ratings for strength, RQD, joint spacing, joint condition, ground water and orientation; five classes, 100 to below 21.
  • Q system (Barton) - Q=RQDJnJrJaJwSRFQ = \frac{RQD}{J_n}\frac{J_r}{J_a}\frac{J_w}{SRF}, used in tunnel support design.
  • Terzaghi's rock load classification and the GSI (Geological Strength Index).

✓ Answer checked by Claude Opus 5.5 (high effort)

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