Geotechnical Engineering and Ground Control
Rock mass characterisation
RMR (Bieniawski), Q (Barton) and GSI (Hoek) describe the same rock mass from different angles. Q = (RQD/Jn)(Jr/Ja)(Jw/SRF). GSI feeds the Hoek–Brown failure criterion, the standard for jointed rock; Mohr–Coulomb with c and φ suits soil-like or heavily broken ground. In-situ stress magnitude and orientation control underground behaviour: high stress relative to rock strength produces spalling, rockburst and squeezing; low stress with persistent structures produces wedge and block falls.
Design tools
- Slopes: kinematic analysis for planar, wedge and toppling failure; limit-equilibrium or numerical methods for circular failure. Permanent slopes are commonly designed to a factor of safety of about 1.3–1.5, supported by probabilistic acceptance criteria.
- Underground: the Mathews stability graph (stability number N' versus hydraulic radius) sizes stopes; the NGI/Q support chart sizes tunnel support.
- Support: bolts and mesh, shotcrete, cable bolts, and backfill (rock fill, cemented rock fill, paste) for pillar and stope stability.
Traps
- Treating FS = 1.0 as acceptable. It is not.
- Using RMR without blast-damage or wet-weather adjustment.
- Blaming the number of joint sets rather than the orientation of the critical set; a set parallel to the slope face drives planar failure.
- Ignoring scale: large slopes mobilise far less apparent cohesion than laboratory samples.
Worked example
Planar sliding on a cohesionless joint: FS = tanφ/tanψ. With φ = 35° (tan = 0.700) and joint dip ψ = 30° (tan = 0.577), FS = 1.21 - below a 1.3 target, so drainage or a buttress is needed.