Rock Mass Classification: RMR
Rock mass classification systems — RMR (Rock Mass Rating), Q-system (Barton), and GSI (Geological Strength Index) — with rating ranges, rock quality descriptions, support recommendations, and estimated strength/deformation parameters
| classification system | parameter or class | cohesion kPa max (kPa) | cohesion kPa min (kPa) | estimated modulus gpa max (GPa) | estimated modulus gpa min (GPa) | friction angle deg max (deg) | friction angle deg min (deg) | joint spacing mm typical | notes | rating max (dimensionless) | rating min (dimensionless) | rock mass description | rqd pct range | support recommendation |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| RMR | Class_I | 700 | 400 | 80 | 45 | 55 | 45 | >600 mm | RMR per Bieniawski 1989; 5 parameters: UCS + RQD + joint spacing + joint condition + groundwater; orientation adjustment up to -12 per Bieniawski | 100 | 81 | Very Good Rock | 90-100 | Generally no support required; local bolting only if needed |
| RMR | Class_II | 400 | 300 | 45 | 30 | 45 | 35 | 200-600 mm | RMR 61-80; good rock; stand-up time 1 week for 10m span; Bieniawski (1989) Table 8 support guidelines | 80 | 61 | Good Rock | 75-90 | Systematic bolting 1.5-2.0m long at 2.5m c/c; 50mm shotcrete in crown |
| RMR | Class_III | 300 | 200 | 30 | 10 | 35 | 25 | 60-200 mm | RMR 41-60; fair rock; stand-up time 1 week for 5m span; possible swelling or squeezing | 60 | 41 | Fair Rock | 50-75 | Systematic bolting 3-4m long at 2.5m c/c with wire mesh; 50-100mm shotcrete |
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