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RMR fair rock support and excavation

Short answer

For fair rock (Class III, RMR 41–60), excavate by top heading and bench, 1.5–3 m advance. Systematic 4 m fully grouted bolts at 1.5–2 m spacing, wire mesh in crown, shotcrete 50–100 mm crown and 30 mm sides; no steel sets. Complete support within 10 m of face.

Discuss the excavation and support system in fair rock class according to RMR.

Fair rock is Class III with RMR 41 to 60 (cohesion 200 to 300 kPa, friction angle 25° to 35°, stand-up time about 1 week for a 5 m span). The guideline (Bieniawski, 1989, for a 10 m span horse-shoe tunnel, drill and blast) is:

Excavation

  • Top heading and bench method.
  • Advance of 1.5 to 3 m in the top heading.
  • Support is started after each blast, and the support is completed within 10 m of the face.

Support

ElementRecommendation
Rock bolts (20 mm, fully grouted)Systematic, 4 m long, spaced 1.5 to 2 m in crown and walls
Wire meshIn the crown
Shotcrete50 to 100 mm in the crown and 30 mm in the sides
Steel setsNot required
   Top heading   ____________
                /  bolts 4 m \
   Bench       |  mesh+shotcr |
               |______________|

The bolts and shotcrete hold the loosened blocks together and let the rock mass carry itself. Local support is increased where water or weak seams occur.

✓ Answer checked by Claude Opus 5.5 (high effort)

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