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Geo-mechanics classification of rock mass
Short answer
The Geomechanics Classification (RMR) by Bieniawski sums ratings of six parameters: intact rock strength, RQD, joint spacing, joint condition, groundwater, and orientation adjustment, yielding classes I–V. Other classification systems include RQD, Q-system, GSI, RSR, and RMi.
Describe the geo-mechanics classification of rock mass (mention the different rock mass classification systems).
The Geomechanics Classification is the Rock Mass Rating (RMR) system of Bieniawski (1973, updated 1989). It classifies a rock mass by adding the ratings of six parameters, so that engineers can estimate stand-up time, shear strength and the support needed.
Parameters
- Uniaxial compressive strength of intact rock (0 to 15)
- RQD (3 to 20)
- Spacing of discontinuities (5 to 20)
- Condition of discontinuities: persistence, aperture, roughness, infilling, weathering (0 to 30)
- Groundwater (0 to 15)
- Adjustment for orientation of discontinuities (0 to -12)
ratings (0 to 100).
Classes
| Class | RMR | Rock mass | Stand-up time |
|---|---|---|---|
| I | 81 to 100 | Very good | 20 years (15 m span) |
| II | 61 to 80 | Good | 1 year (10 m span) |
| III | 41 to 60 | Fair | 1 week (5 m span) |
| IV | 21 to 40 | Poor | 10 hours (2.5 m span) |
| V | below 21 | Very poor | 30 minutes (1 m span) |
Each class has a cohesion (from above 400 down to below 100 kPa), a friction angle (from above 45° down to below 15°) and a recommended excavation and support method (bolts, shotcrete, steel ribs).
Other rock mass classification systems
| System | Author and year | Basis |
|---|---|---|
| Rock load | Terzaghi, 1946 | Rock load on steel arches |
| Stand-up time | Lauffer, 1958 | Span and unsupported time |
| RQD | Deere, 1967 | Modified core recovery (pieces of 10 cm or more) |
| RSR | Wickham et al., 1972 | Rock structure rating |
| RMR | Bieniawski, 1973 | Six parameters |
| Q-system | Barton et al., 1974 | Six parameters (NGI) |
| GSI | Hoek, 1995 | Structure and surface condition |
| RMi | Palmstrom, 1996 | Rock mass index |
✓ Answer checked by Claude Opus 5.5 (high effort)
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