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 |
Excavation |
Rock bolts (20mm, fully grouted) |
Shotcrete |
Steel arch |
| I – Very good rock RMR: 100-81 |
Full face, 3 m in advance. |
Spot bolting. |
– |
– |
| 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. |
