Tunneling Ground Classification: NATM_class
Rock mass classification systems for tunnel design — RMR (Bieniawski 1989), Q-system (Barton 1974), GSI (Hoek 2002), NATM classes, and squeezing potential (Hoek & Marinos 2000) with support requirements, TBM performance parameters, and convergence estimates
| classification system | rock class | GSI max (dimensionless) | GSI min (dimensionless) | Q max (dimensionless) | Q min (dimensionless) | RMR max (dimensionless) | RMR min (dimensionless) | bolt length m (m) | bolt spacing m (m) | class number | max unsupported span m (m) | notes | recommended support | rock class variant | shotcrete thickness mm (mm) | stand up time hr (hr) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| NATM_class | B_slightly_incompetent | 75 | 55 | 40 | 10 | 80 | 61 | 3 | 2.5 | 2 | 10 | Slightly incompetent: face stable short-term. Shotcrete 50 mm and spot bolts adequate. | spot_bolts | B_slightly_incompetent | 50 | 24 |
| NATM_class | C_moderately_incompetent | 55 | 35 | 10 | 1 | 60 | 41 | 4 | 1.5 | 3 | 7 | Moderately incompetent: face stable with immediate support. Systematic bolts plus shotcrete 100 mm required. | systematic_bolts | C_moderately_incompetent | 100 | 8 |
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