Table 1: Guidelines for excavation and support of 10 m span rock tunnels in accordance with the RMR system (after Bieniawski, 1989).
|Rock mass class
||Rock bolts (20mm, fully grouted)
|I – Very good rock RMR: 100-81
||Full face, 3 m in advance.
|II – Good rock RMR: 80-61
||Full face 1.5 m in advance, Complete support 20 m from face.
||Locally bolts in crown, 4 m long, 2.5 m spaced with occasional mesh wire.
||5 mm in crown, in walls where required.
|III – Fair roock RMR: 60-41
||Top heading and bench, 3-1.5 m advance in top heading. Commence support after each blast. Complete support 10 m from face.
||Systematic bolts 4 m long, 1.5 m spaced in crown and walls, with mesh wire in crown.
||50-100 mm in crown, 30 mm in walls.
|IV – Poor rock RMR: 40-21
||Top heading and bench, 1.5 -1.0 m in advance in top heading. Install support concurrently with excavation. Complete support max. 10 from face.
||Systematic bolts 4-5 m long, 1.5-1.0 m spaced in crown and walls with mesh wire.
||100-150 mm in crown, 100 mm in walls.
||Light to medium rips, 1.5 m spaced where required.
|V – Very poor rock RMR: <20
||Multiple drifts, 0.5 – 1.5 m advance in top heading. Install support concurrently with excavation. Shotcrete as soon as possible after blasting.
||Systematic bolts, 5-6 m long, 1.5-1 m spaced in crown and walls with mesh wire, additional bolts in invert.
||150-200 mm in crown, 150 mm in walls and 50 mm in front.
||Medium to heavy ribs, 0.75 m spaced, with steel lagging and fore poling if required, close invert.