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RMR class IV support and excavation

Short answer

Poor rock (Class IV, RMR 21–40): top heading and bench, 1.0–1.5 m advance, immediate concurrent support. Systematic bolts 4–5 m at 1–1.5 m spacing, wire mesh in crown and walls, shotcrete 100–150 mm crown, 100 mm sides, and light to medium steel ribs at 1.5 m spacing if required.

What are the support system and excavation methods of rock class IV according to the RMR system?

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

Excavation method

  • Top heading and bench.
  • Advance of 1.0 to 1.5 m in the top heading.
  • Install support concurrently with excavation, and complete it within 10 m of the face.

Support system (10 m span tunnel)

ElementRecommendation
Rock bolts (20 mm, fully grouted)Systematic, 4 to 5 m long, spaced 1 to 1.5 m in crown and walls
Wire meshIn crown and walls
Shotcrete100 to 150 mm in the crown and 100 mm in the sides
Steel setsLight to medium ribs spaced 1.5 m where required
        ___________________
      /  shotcrete 100-150  \
     |  bolts 4-5 m @1-1.5 m |
     |   light-medium ribs   |
     |_______________________|

Because the rock stands only a few hours, short rounds and immediate shotcrete are needed, and the invert may need support if the floor is weak.

✓ Answer checked by Claude Opus 5.5 (high effort)

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