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18-Geol-A5 Rock Mechanics · May 2016

Question 3 of 5: Rock mass classification systems — systems, strengths and limitations

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Notes on this paper

National Exams, May 2016 — 04-Geol-A5, Rock Mechanics. Closed-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:

It does not affect the solutions below, which are worked from the real printed question text on pages 3–5.

Question 3: Rock mass classification systems — systems, strengths and limitations (20 marks)

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 discontinuity and strength parameters into a single index that a designer can carry directly into a support, span or slope-angle recommendation, sparing a full numerical stress analysis for routine ground.

(a) The major systems

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). Where no core exists it can be estimated from an outcrop or scanline volumetric joint count, RQD = 115 − 3.3 Jv, where Jv is the summed joint frequency (joints per metre, summed across all joint sets). RQD is the simplest system and is also an input parameter to the other three below.

Rock Mass Rating, RMR (Bieniawski, 1973, updated 1989). RMR sums six weighted ratings — intact rock strength, RQD, discontinuity spacing, discontinuity condition (persistence/aperture/roughness/infilling/weathering), groundwater condition, and a structure-specific orientation adjustment — into a 0–100 index that is looked up directly against tabulated unsupported span/stand-up-time and support (bolt length and spacing, shotcrete thickness) charts, exactly as used elsewhere in this exam's own Tables 1 and 2.

Rock Tunnelling Quality Index, Q (Barton, Lien & Lunde, 1974, NGI). $$Q=\frac{RQD}{J_n}\times\frac{J_r}{J_a}\times\frac{J_w}{SRF}$$ where the three ratios describe, in turn, block size (RQD over the 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 spans several orders of magnitude (0.001 exceptionally poor to 1000 exceptionally good) and, together with an excavation support ratio, sizes permanent support and unsupported span directly — the same tunnelling-quality index reproduced in this exam's own Figure 6 support chart.

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, and was purpose-built to feed the generalized Hoek-Brown strength criterion (mb=miexp[(GSI−100)/28], s=exp[(GSI−100)/9]) for numerical stress analysis, rather than to select support directly the way RMR/Q do.

(b) Strengths and limitations

Strengths and limitations of the four systems
SystemStrengthsLimitations
RQDFast, objective, needs only a core log; universal input to the other three systems.Ignores discontinuity orientation, roughness, infilling and water entirely; two rock masses with identical RQD can behave very differently; drilling-induced mechanical breaks inflate apparent fracturing.
RMRDirect, well-validated look-up to span/stand-up-time and support schedules; six parameters are all field-measurable; large worldwide case-history base. Ratings step discretely at class boundaries (a UCS of exactly 100 MPa is genuinely ambiguous between bands); the orientation adjustment is qualitative; developed mainly from tunnelling case histories, so needs caution outside that range.
Q-systemExplicitly separates block size, inter-block strength and active stress into independent, physically meaningful ratios; very wide dynamic range suits both very poor and exceptional rock; strong empirical support-chart base. Six input parameters demand more/rarer field data than RMR (Jn, Ja, SRF in particular are judgment-dependent); no explicit orientation term; multiplicative form means one badly mis-estimated parameter can swing Q by an order of magnitude.
GSIFast visual field estimate; purpose-built to convert cleanly into Hoek-Brown mb/s/a for numerical models; works for very poor/weak rock masses where RQD is meaningless (RQD=0 for many such masses).Chart-reading is subjective with real 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.