---
title: "Short note: Rock Quality Designation"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:short-note-rock-quality-designation"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-rock-quality-designation
site: BCT Engineering
---

# Short note: Rock Quality Designation

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2066 Jestha (old course), 4 marks
- Video: [Ultimate Guide to RQD in Geology | Core Recovery & Rock Quality Interpretation](https://www.youtube.com/watch?v=pbwGNFvL9QI) — Geotourism and Modern Geotechniques
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/short-note-rock-quality-designation
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Write a short note on Rock Quality Designation (RQD).

## Short answer

RQD is the percentage of core pieces longer than 10 cm in a drill run, introduced by Deere. It classifies rock: <25% very poor to 90100% excellent. RQD = (sum of lengths ≥10 cm/total run)×100%; used in RMR and Q, but ignores joint condition and orientation.

## Answer

**Rock Quality Designation (RQD)** was introduced by **Deere (1964)** as a simple index of rock quality from drill core. It is the percentage of the total core run made up of sound core pieces longer than 10 cm (4 inches).

$$
RQD = \frac{\sum \text{length of core pieces} \ge 10\ \text{cm}}{\text{total length of core run}} \times 100\ \%
$$

- Only natural fractures are counted; breaks caused by drilling or handling are ignored, and the core must be at least NX size (54 mm).
- If cores are not available, RQD can be estimated from the number of joints per cubic metre ($J_v$): $RQD = 115 - 3.3 J_v$.

**Classification**
| RQD (%) | Rock quality |
|---|---|
| < 25 | Very poor |
| 25 - 50 | Poor |
| 50 - 75 | Fair |
| 75 - 90 | Good |
| 90 - 100 | Excellent |

**Example**: in a 150 cm run, pieces longer than 10 cm total 105 cm, so RQD = 105/150 × 100 = 70 % (fair rock).

**Use**: RQD is a parameter in the RMR and Q systems, and it helps in judging foundations and tunnel support. Its limitation is that it ignores joint orientation and condition, and does not distinguish between pieces of different lengths above 10 cm.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is RQD calculated?
   A. Sum of core pieces >= 10 cm divided by total run length, times 100
   B. Number of pieces times 10
   C. Total length of all pieces divided by 10
   D. Total run length divided by piece length
   Answer: A. Only sound pieces of at least 10 cm count. (Key point: RQD formula)

2. In a 150 cm run, pieces longer than 10 cm total 105 cm. What is RQD and quality?
   A. 105 %, excellent
   B. 70 %, fair
   C. 30 %, poor
   D. 70 %, good
   Answer: B. 105/150 = 70 %, which is in the 50 - 75 fair range. (Key point: Example)

3. Which breaks are counted?
   A. All breaks
   B. Only drilling breaks
   C. Only natural fractures, not drilling or handling breaks
   D. None
   Answer: C. Mechanical breaks are ignored. (Key point: Core counting)

4. Which estimates RQD from joints per cubic metre?
   A. RQD = 3.3 Jv
   B. RQD = 100 + Jv
   C. RQD = Jv/115
   D. RQD = 115 - 3.3 Jv
   Answer: D. More joints means lower RQD. (Key point: Jv relation)

## Related in other subjects

- [RQD and its calculation](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arqd-calculation) — Engineering Geology II, chapter 6, asked 8×
- [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×