--- title: "Q-system of rock mass classification" subject: "Engineering Geology II" chapter: 6 key: "semester_4/engineering_geology_ii:q-system-classification" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/q-system-classification site: BCT Engineering --- # Q-system of rock mass classification Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data - Asked in: 2081 Chaitra, 3 marks; 2079 Chaitra, 2 marks; 2078 Baisakh, 3 marks; 2076 Baisakh, 2 marks; 2073 Bhadra, 2 marks; 2070 Bhadra, 4 marks; 2071 Magh, 2 marks (asked 7 times) - Video: [ENGINEERING GEOLOGY: ROCK TUNNELLING QUALITY INDEX (Q SYSTEM)](https://www.youtube.com/watch?v=PVi966NJw0A) — Aleeya Erdyna Binti Mohd Noor Azmi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/q-system-classification - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6 ## Question Describe the Q-system (rock tunnelling quality index) of rock mass classification, its parameters and rock mass classes. How is Q calculated? ## Short answer The Q-system rates tunnel rock mass quality by the index Q = (RQD/J_n)×( J_r / J_a )×( J_w / SRF). Q ranges from 0.001 to 1000, with classes from exceptionally poor to exceptionally good, and support is determined using an equivalent dimension chart. ## Answer The **Q-system** was developed by Barton, Lien and Lunde (1974) at the Norwegian Geotechnical Institute from many tunnel case histories. It gives the **rock tunnelling quality index Q**, used to classify rock mass and to design tunnel support (shotcrete, rock bolts, spans). #### Calculation of Q $$ Q = \frac{RQD}{J_n} \times \frac{J_r}{J_a} \times \frac{J_w}{SRF} $$ | Term | Meaning | Represents | Range | |---|---|---|---| | $RQD$ | Rock Quality Designation | Degree of jointing | 0 to 100 | | $J_n$ | Joint set number | Block size | 0.5 to 20 | | $J_r$ | Joint roughness number | Shear strength between blocks | 0.5 to 4 | | $J_a$ | Joint alteration number | Shear strength between blocks | 0.75 to 20 | | $J_w$ | Joint water reduction factor | Water pressure effect | 0.05 to 1 | | $SRF$ | Stress reduction factor | Active stress | 0.5 to 20 | The three quotients mean: $RQD/J_n$ = relative block size, $J_r/J_a$ = inter-block shear strength, $J_w/SRF$ = active stress. Q varies from 0.001 to 1000. **Example:** $RQD = 75,\ J_n = 9,\ J_r = 3,\ J_a = 1,\ J_w = 1,\ SRF = 1$ gives $Q = (75/9)(3/1)(1/1) = 25$, i.e. good rock. #### Rock mass classes | Q value | Class | |---|---| | 400 to 1000 | Exceptionally good | | 100 to 400 | Extremely good | | 40 to 100 | Very good | | 10 to 40 | Good | | 4 to 10 | Fair | | 1 to 4 | Poor | | 0.1 to 1 | Very poor | | 0.01 to 0.1 | Extremely poor | | 0.001 to 0.01 | Exceptionally poor | The support is then chosen from the Q chart using the **equivalent dimension** $D_e = \text{span or height} / ESR$, where ESR is the excavation support ratio. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which expression gives Q? A. (RQD/Jn)(Jr/Ja)(Jw/SRF) B. (RQD/Jn)(Ja/Jr)(SRF/Jw) C. RQD x Jn x Jr x Ja D. (RQD/SRF)(Jr/Jn)(Jw/Ja) Answer: A. Q = block size x inter-block shear strength x active stress. (Key point: Q formula) 2. What does Jr/Ja represent? A. Relative block size B. Active stress C. Water pressure effect D. Shear strength between blocks Answer: D. Jr/Ja is inter-block shear strength; RQD/Jn is block size; Jw/SRF is active stress. (Key point: Meaning of quotients) 3. RQD = 75, Jn = 9, Jr = 3, Ja = 1, Jw = 1, SRF = 1. What is Q? A. 8.3 B. 25 C. 75 D. 2.8 Answer: B. (75/9)(3/1)(1/1) = 25, which is good rock (10-40). (Key point: Worked example) 4. How is support chosen from the Q chart? A. Using Q alone with no dimensions B. Using equivalent dimension De = span or height divided by ESR C. Using RQD only D. Using the RMR class Answer: B. ESR is the excavation support ratio; De with Q picks the support category. (Key point: Support design) ## Related in Engineering Geology II - [RMR rock mass classification](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-classification) — Engineering Geology II, chapter 6, asked 8× - [Geo-mechanics classification of rock mass](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Ageomechanics-classification) — Engineering Geology II, chapter 6, asked 5× - [Rock classes needing steel rib and shotcrete](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-steel-rib-classes) — Engineering Geology II, chapter 6 - [Properties of rock mass and discontinuities](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-properties) — Engineering Geology II, chapter 6, asked 3× - [Rock mass versus intact rock](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-vs-intact-rock) — Engineering Geology II, chapter 6, asked 7× - [Meaning of rock class and shear parameter](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-class-shear-meaning) — Engineering Geology II, chapter 6 ## Related in other subjects - [Rock mass classification](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification) — Engineering Geology I, chapter 5 - [Rock mass classification and discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification-and-discontinuities) — Engineering Geology I, chapter 5 - [Tunnelling and rock drilling equipment](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atunnel-rock-drilling-equipment) — Construction Management, chapter 4, asked 3× - [Petrology; engineering significance of rock classes](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Apetrology-engineering-significance-of-rock) — Engineering Geology I, chapter 4