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Rock classes needing steel rib and shotcrete

Short answer

Steel ribs and thick shotcrete are required for Class IV (Poor) and Class V (Very poor). Class IV uses light to medium ribs at1.5 m spacing where necessary and 100–150 mm shotcrete; Class V demands medium to heavy ribs at0.75 m and150–200 mm shotcrete applied immediately.

According to the RMR system, which rock mass classes require steel rib and thick shotcrete for support of underground excavation?

According to the RMR system (Bieniawski, 1989) the classes needing steel ribs together with shotcrete are the two weakest:

ClassRMRSteel ribsShotcrete
IV (Poor)21 to 40Light to medium ribs at 1.5 m spacing, where required100 to 150 mm crown, 100 mm sides
V (Very poor)below 20Medium to heavy ribs at 0.75 m with steel lagging and forepoling if required; close the invert150 to 200 mm crown, 150 mm sides, 50 mm on the face

Class V definitely needs steel ribs and thick shotcrete, applied immediately after blasting. For Class IV ribs are used where required (weak zones, high water or squeezing), with thick shotcrete. Classes I to III do not need steel sets.

✓ Answer checked by Claude Opus 5.5 (high effort)

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