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RMR class II support and excavation

Short answer

For Class II good rock, full-face excavation with 1–1.5 m advance is used, and support is completed 20 m from the face. Local rock bolts (3 m long at 2.5 m spacing) with occasional mesh and 50 mm shotcrete in the crown suffice; steel sets are not needed.

Discuss the support system and excavation method in class II rock type according to RMR.

Class II is good rock with RMR 61 to 80 (cohesion 300 to 400 kPa, friction angle 35° to 45°, stand-up time about 1 year for a 10 m span). Guidelines for a 10 m span tunnel:

  • Excavation: full face, advance of 1.0 to 1.5 m; complete support about 20 m from the face.
  • Rock bolts: locally in the crown, 3 m long, spaced 2.5 m, with occasional wire mesh.
  • Shotcrete: 50 mm in the crown where required.
  • Steel sets: not required.

The rock is mostly self-supporting, so only local support for loose blocks is needed.

✓ Answer checked by Claude Opus 5.5 (high effort)

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