RQD

The “Rock Quality Designation” Index, RQD, was introduced by Deere (father) in 1963 [1].

The RQD is a quantitative index based on a core recovery procedure in which the core recovery is determined incorporating only those pieces of hard, sound core that are 100 mm or greater in length. Shorter lengths of core are ignored.

The RQD is calculated as:

\[\text{RQD (%)} = 100 \cdot \frac{\text{Length of core in pieces 100 mm or longer}}{\text{Length of borehole}}\]

Figure 1: Conventional method for evaluating RQD from drill core

RQD Categories of Rockmass Quality

Rock Quality Designation, RQD	Rock Mass Description
0 - 25	Very Poor
25 - 50	Poor
50 - 75	Fair
75 - 90	Good
90 - 100	Excellent

ISRM (1981) [2] and Deere (1963) [1]:

“RQD must be determined over intervals not exceeding 2.0 [m], ideally 1.0 [m]” — longer intervals reduce sensitivity to local fracturing.

⚠️ What happens when RQD is calculated over 3 [m] or more?

• Fault zones or fractured bands may be diluted (a 0.3 [m] fractured band in 3.0 [m] looks insignificant).

• Misclassification in systems like RMR, Q-system, or GSI.

• Support designs based on misleadingly “good” rock may fail.

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References

[1] (1,2)

D. U. Deere. Technical description of rock cores for engineering purposes. Rock Mechanics and Engineering Geology, 1(1):17–22, 1963.

[2]

ISRM Commission on Standardization of Laboratory and Field Tests. Suggested methods for the quantitative description of discontinuities in rock masses. 1981. doi:10.1016/0148-9062(81)90221-8.