--- title: "RQD and its calculation" subject: "Engineering Geology II" chapter: 6 key: "semester_4/engineering_geology_ii:rqd-calculation" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rqd-calculation site: BCT Engineering --- # RQD and its calculation Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data - Asked in: 2080 Chaitra, 2 marks; 2078 Poush, 1.5 marks; 2076 Baisakh, 2 marks; 2075 Bhadra, 1 marks; 2073 Bhadra, 2 marks; 2072 Asoj, 1+1 marks; 2070 Magh, 2 marks; 2071 Bhadra, 3 marks (asked 8 times) - Video: [How to calculate Rock Quality Designation (RQD) and Core Recovery (CR)](https://www.youtube.com/watch?v=HtRcs2-NqtU) — civil engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rqd-calculation - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6 ## Question What is Rock Quality Designation (RQD)? How is it calculated from drill core samples and from rock outcrops (in the absence of core)? Explain its importance in rock mass classification. ## Short answer RQD is the percentage of sound core pieces ≥10 cm per total core run: RQD = (sum lengths ≥10 cm / total length)×100. Where no core exists, RQD = 115 - 3.3 J_v (volumetric joint count) or from scanline frequency. It is a key input to RMR and Q-system classifications. ## Answer **Rock Quality Designation (RQD)**, proposed by Deere (1967), is the percentage of sound core pieces 10 cm or longer in the total length of a core run. It is an index of the degree of jointing and fracturing of the rock mass. #### From drill core $$ RQD = \frac{\sum \text{length of core pieces} \geq 10\ \text{cm}}{\text{total length of core run}} \times 100\ \% $$ Only natural, hard and sound pieces are counted; breaks caused by drilling or handling are ignored. Example: in a 150 cm run with pieces of 12, 25, 8, 30, 15 and 20 cm, the pieces counted are 12 + 25 + 30 + 15 + 20 = 102 cm, so RQD = 102/150 × 100 = 68 % (fair). #### From outcrops (no core) 1. **Volumetric joint count (Palmstrom):** $$ RQD = 115 - 3.3\,J_v \quad (J_v \geq 4.5),\qquad RQD = 100\ \text{for}\ J_v < 4.5 $$ where $J_v$ is the number of joints per cubic metre (the sum of the joint frequencies per metre of all sets). 2. **Scanline (Priest and Hudson):** count the discontinuities per metre, $\lambda$, along a scanline; then $RQD = 100\,e^{-0.1\lambda}(0.1\lambda + 1)$. #### RQD classes | RQD (%) | Quality | |---|---| | 90 to 100 | Excellent | | 75 to 90 | Good | | 50 to 75 | Fair | | 25 to 50 | Poor | | below 25 | Very poor | #### Importance - It is a quick, cheap index of rock quality from boreholes. - It is one of the parameters of the RMR system (rating 3 to 20) and of the Q-system ($RQD/J_n$ gives block size). - It helps in choosing tunnel support, estimating rock mass strength and deformability, and deciding grouting and excavation methods. - Limitation: it ignores joint orientation, condition and filling. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is RQD defined? A. Percentage of sound core pieces longer than 10 cm in the total core run length B. Percentage of core pieces shorter than 10 cm C. Core recovery length divided by hole depth D. Number of joints per metre Answer: A. Deere's RQD sums sound pieces of 10 cm or more over total run length. (Key point: RQD definition) 2. A 150 cm run has pieces 12, 25, 8, 30, 15 and 20 cm. What is RQD? A. 68% B. 80% C. 72% D. 53% Answer: A. Pieces of at least 10 cm: 12+25+30+15+20 = 102 cm; 102/150 = 68%. (Key point: Core calculation) 3. Which breaks are ignored when computing RQD? A. Natural joints B. Breaks caused by drilling or handling C. Pieces of 30 cm D. Weathered zones in the rock Answer: B. Only natural, hard and sound pieces count; mechanical breaks are ignored. (Key point: Counting rule) 4. For RQD = 115 - 3.3 Jv, what is Jv? A. Joint roughness value B. Joint water value C. Joints per cubic metre (sum of joint set frequencies per metre) D. Joint spacing in metres Answer: C. Palmstrom's relation uses volumetric joint count; RQD = 100 when Jv is below 4.5. (Key point: Outcrop method) ## Related in Engineering Geology II - [Q-system of rock mass classification](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aq-system-classification) — Engineering Geology II, chapter 6, asked 7× - [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× - [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× ## Related in other subjects - [Short note: Rock Quality Designation](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ashort-note-rock-quality-designation) — Engineering Geology I, chapter 5 - [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 - [Rock mass, rock material, discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-rock-material-discontinuities) — Engineering Geology I, chapter 5