18-Geol-A5 Rock Mechanics · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, December 2015 — 04-Geol-A5, Rock Mechanics. Open-book, 3-hour exam; 5 questions of 20 marks each; candidates were instructed to answer only 4 of the 5 — all 5 are answered below as a complete study resource.
Reference texts for this subject:
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Four empirical rock mass classification systems dominate practice. Each converts field-mappable parameters into a single index that a designer can carry straight into a support or slope recommendation.
Rock Quality Designation, RQD (Deere, 1964). RQD is the percentage of a diamond-drill core run recovered as sound pieces ≥10 cm long, measured along the core axis: RQD = 100 × (Σ lengths of pieces ≥ 0.1 m) / (total run length). It can also be estimated from an outcrop or scanline without core, via the volumetric joint count RQD = 115 − 3.3 Jv, where Jv is the summed number of joints per metre across all joint sets. RQD is the simplest input parameter and feeds directly into RMR, Q and GSI.
Rock Mass Rating, RMR (Bieniawski, 1973, updated 1989). RMR sums six weighted ratings — intact rock strength, RQD, discontinuity spacing, discontinuity condition (roughness/persistence/aperture/infilling/weathering), groundwater condition, and an orientation adjustment specific to the structure (tunnel, slope or foundation) — to a 0–100 index that is looked up directly against tabulated span/stand-up-time and support (bolt length/spacing, shotcrete thickness) charts, e.g. Table 1/Table 2 used throughout this paper.
Rock Tunnelling Quality Index, Q (Barton, Lien & Lunde, 1974, NGI). Q = (RQD/Jn)×(Jr/Ja)×(Jw/SRF), where the three ratios in turn describe block size (RQD over joint-set number Jn), inter-block shear strength (joint roughness Jr over joint alteration Ja), and active stress (joint water reduction Jw over the stress reduction factor SRF). Q ranges over several orders of magnitude (0.001 exceptionally poor to 1000 exceptionally good) and is used with an excavation support ratio to size permanent support and unsupported span directly.
Geological Strength Index, GSI (Hoek & Marinos, 2000). GSI is read from a qualitative chart of rock mass structure (blocky … disintegrated) against discontinuity surface condition (very good … very poor), without a numeric field survey. It was purpose-built to feed the generalized Hoek-Brown strength criterion (mb, s, a) for numerical stress analysis, rather than to select support directly the way RMR/Q do.
| System | Strengths | Limitations |
|---|---|---|
| RQD | Fast, objective, needs only a core log; universal input to the other three systems. | Ignores joint orientation, roughness, infilling and water entirely; two rock masses with identical RQD can behave completely differently; drilling-induced breaks inflate apparent fracturing. |
| RMR | Direct, well-validated look-up to span/stand-up-time and support (bolt/shotcrete) schedules; six parameters are all field-measurable; large worldwide case-history base. | Ratings change in coarse steps at class boundaries (a UCS of exactly 100 MPa or an RQD of exactly 90% is genuinely ambiguous between bands); the orientation adjustment is qualitative; was developed mainly from tunnelling case histories and needs caution outside that range. |
| Q-system | Explicitly separates block size, inter-block strength and active stress into independent, physically meaningful ratios; wide dynamic range suits both very poor and exceptional rock; strong empirical support-chart base (Grimstad & Barton). | Six input parameters demand more/rarer field data than RMR (Jn, Ja, SRF in particular can be highly judgment-dependent); does not include an explicit orientation term; multiplicative form means one badly mis-estimated parameter can swing Q by an order of magnitude. |
| GSI | Fast visual field estimate; purpose-built to convert cleanly into Hoek-Brown mb/s/a for numerical models; works well for very poor/weak rock masses where RQD is meaningless (RQD=0 for many such masses). | Chart-reading is subjective and shows significant inter-observer scatter; not intended for direct support selection the way RMR/Q are; loses resolution for good-quality, blocky rock where RMR/Q discriminate better. |
