Short note: Rock Quality Designation

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

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.