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Rock Mass Classification Systems
- • Rock Quality Designation (RQD)
- • Geological Strength Index (GSI)
- • Bienawaski's Rock Mass Rating (RMR)
- • Q-System
- The percentage of intact core pieces longer than 100mm (4 inches) in the total length of core.
- Very Poor 0-25%
- Poor 25-50%
- Fair 50-75%
- Good 75-90%
- Excellent 90-100%
- Determines strength of a jointed rock mass according to properties of intact rock pieces and the freedom of those pieces to slide and rotate under different stress conditions.
- See p4 Week 6 lecture notes.
- • Intact Rock Strength UCS (15 pts)
- • RQD (20 pts)
- • Mean Joint Spacing (20 pts)
- • Joint Condition (30 pts)
- • Groundwater conditions (15 pts)
- Sum all conditions and then adjust for orientation of joints with respect to the tunnel.
- See p7 Week 6 lecture notes.
- RQD Rock Quality Designation
- Jn joint set number
- Jr joint roughness number
- Ja joint alteration number
- Jw joint water reduction factor
- SRF stress reduction factor
Q system, ESR
- Excavation support ratio
- A Temporary mine openings. 3-5
- B Permanent mine openings, water tunnels for hydro power (excluding high pressure penstocks), pilot tunnels, drifts and headings for large excavations. 1.6
- C Storage rooms, water treatment plants, minor road and railway tunnels, surgechambers, access tunnels. 1.3
- D Power stations, major road and railway tunnels, civil defence chambers,portal intersections. 1.0
- E Underground nuclear power stations, railway stations, sports and public facilities, factories. 0.8
Tunnel Support Design
Most widely used classifications
- Q System Barton et al. (1974)
- Rock Mass Rating (RMR)
Excavation in rock. Methods
- Blasting (bulk, pre-split, drill & blast)
- Mechanical excavation (TBM, partial-TBM, roadheader)
- Rock Reinforcement (insert bars, roads or cables to stiffen rock mass)
- Continuous medium; behaves as reinforced composite material. Discontinuous medium; behaves as stiffer & stronger continuous medium.
- Rock support (use of structural elements to inhibit rock mass displacement)
- Continuous medium; inhibited displacement of continuum. Discontinuous medium; inhibited displacement of individual blocks.
Rock Reinforcement / Rock Support
Installation of systematic rockbolts helps to create a zone of compression on
the roof, forming a self-supporting arch.