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RMR poor rock support and excavation

Short answer

Poor rock (Class IV) needs systematic bolts 4–5 m long at1–1.5 m spacing with mesh, 100–150 mm crown shotcrete, 100 mm side shotcrete, and light ribs at1.5 m if needed. Excavation uses top heading and bench with1–1.5 m advance, and support is completed within10 m of the face.

What are the support system and excavation method of poor rock class according to the RMR system? Mention.

Poor rock is Class IV (RMR 21 to 40, cohesion 100 to 200 kPa, friction angle 15° to 25°, stand-up time about 10 hours for 2.5 m span).

Support system (10 m span tunnel)

  • Rock bolts: systematic, 4 to 5 m long, spaced 1 to 1.5 m in crown and walls, with wire mesh.
  • Shotcrete: 100 to 150 mm in the crown and 100 mm in the sides.
  • Steel ribs: light to medium ribs at 1.5 m spacing where required.

Excavation method

  • Top heading and bench.
  • Advance of 1.0 to 1.5 m in the top heading.
  • Support installed together with excavation and completed within 10 m of the face.

Short rounds and quick support are needed because the rock deteriorates fast after excavation.

✓ Answer checked by Claude Opus 5.5 (high effort)

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