Rock quality in simple numbers
Drilling a rock core gives a quick record of how broken the rock is. Two simple numbers come straight from the core box.
Core recovery is the total length of core recovered divided by the length drilled. Low recovery may mean weak, crushed or soft zones that washed away during drilling.
Rock Quality Designation, RQD, introduced by Deere, counts only the sound core pieces that are at least 100 mm long, adds their lengths, and divides by the length drilled. A high RQD suggests massive rock; a low RQD suggests closely jointed or sheared rock. Rough descriptive bands are widely used, for example below 25 percent as very poor and above 90 percent as excellent.
RQD is easy and cheap but limited: it ignores joint orientation, roughness, infilling and water. It therefore feeds into fuller rock mass classification systems such as RMR and Q, which are used to choose tunnel support and estimate rock mass strength.
Breaks caused by drilling or handling should not be counted as natural joints, so the logger must judge each break.
Core recovery (%) = total core length / drilled length × 100RQD (%) = Σ(lengths of sound pieces ≥ 100 mm) / drilled length × 100
Problem. A 1.5 m run recovers pieces of 250, 80, 140, 60, 300, 90 and 400 mm. Find recovery and RQD.
- Total recovered = 1 320 mm, so recovery = 1 320 / 1 500 = 88 %.
- Pieces ≥ 100 mm: 250 + 140 + 300 + 400 = 1 090 mm.
- RQD = 1 090 / 1 500 = 72.7 %.
Answer. Recovery 88 %, RQD ≈ 73 %
Quick quiz
1Minimum piece length counted in RQD?
100 mm.
2Name one thing RQD ignores.
Joint orientation, roughness, infill or water.
3Should drilling breaks count?
No.
Related videos
Road tunnel through a mountain: drill, blast, NATM
Excavate in rounds and let the rock help support itself.
References and further reading
Standard texts, listed in no order of merit. We do not copy text, figures, tables or solved problems from any book; our explanations and examples are original.
- R. E. Goodman, Introduction to Rock Mechanics, Wiley, 2nd edition.
- E. Hoek, E. T. Brown, Underground Excavations in Rock, Institution of Mining and Metallurgy, 1980.
Free and official sources
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