Rock Quality Designation (RQD) is a core-logging index that screens the percentage of sound, intact core pieces at least 100 mm (about 4 inches) long relative to the total core-run length. It is useful for communicating rock-core condition, but it is only one input to broader rock-mass characterization.
This guide summarizes measured RQD arithmetic, the Priest-Hudson theoretical relationship, RMR RQD-component context, and the source and review limits that should travel with any RQD number. Use current ASTM D6032/D6032M, project logging procedures, and qualified geotechnical review for project decisions.
Measuring RQD from Core
RQD is defined as:
RQD (%) = (Σ Length of intact core pieces ≥ 10 cm) / Total core run length × 100
The screening logic follows these rules, subject to current project logging procedure:
- Only count pieces of intact, sound rock that are 10 cm (4 inches) or longer, measured along the core centerline axis.
- Mechanical breaks caused by the drilling process are fitted together and counted as intact pieces. Only natural fractures and discontinuities are treated as breaks.
- Highly weathered or soft zones where core is fragmented or washed away contribute zero to the RQD numerator but are included in the total run length (denominator).
- The total core run length is the drilled interval, not the recovered core length. Low core recovery automatically reduces RQD.
RQD quality classification bands:
- 0–25%: Very poor
- 25–50%: Poor
- 50–75%: Fair
- 75–90%: Good
- 90–100%: Excellent
Core run length: 1.5 m (150 cm)
Recovered core pieces:
• 35 cm (intact) ✓
• 8 cm (too short) ×
• 22 cm (intact) ✓
• 5 cm (too short) ×
• 45 cm (intact) ✓
• 12 cm (intact) ✓
• 3 cm (rubble) ×
• 15 cm (intact) ✓
Sum of pieces ≥ 10 cm = 35+22+45+12+15 = 129 cm
RQD = 129 / 150 × 100 = 86% (Good)
Rock Quality Designation Calculator
Calculate RQD from core logging data. Enter core piece lengths to get rock quality percentage with classification (excellent/good/fair/poor) per Deere 1967.
The Priest and Hudson Theoretical RQD
The Priest-Hudson theoretical relationship estimates RQD from linear discontinuity frequency per meter (λ) by modeling discontinuity spacing as a negative exponential distribution:
RQD = 100 × e−0.1λ × (0.1λ + 1)
Where λ is the mean number of discontinuities per meter. Treat this as a model comparison only: the fracture-frequency source, scanline or televiewer orientation, spacing distribution, censoring, and anisotropy can strongly affect the result.
The relationship can be insensitive at very low or very high discontinuity frequencies, and it does not replace measured core logging, ASTM RQD procedure, or project-specific rock-mass review.
RQD = 100 × e−0.1λ × (0.1λ + 1)
λ = 5/m: RQD = 100 × 0.607 × 1.5 = 91%
λ = 10/m: RQD = 100 × 0.368 × 2.0 = 74%
λ = 15/m: RQD = 100 × 0.223 × 2.5 = 56%
λ = 20/m: RQD = 100 × 0.135 × 3.0 = 41%
λ = 30/m: RQD = 100 × 0.050 × 4.0 = 20%
RQD in Rock Mass Classification Systems
RQD is one component of several rock mass classification systems. It should not be treated as a complete classification by itself:
Rock Mass Rating (RMR, Bieniawski 1989): RQD contributes up to 20 points out of a total possible score of 100. It is one of six parameters: uniaxial compressive strength (0–15), RQD (3–20), joint spacing (5–20), joint condition (0–30), groundwater (0–15), and orientation adjustment (−12 to 0). An RQD of 90–100% scores 20 points; 25–50% scores 8 points.
Q-system (Barton 1974): RQD appears directly in the numerator: Q = (RQD/Jn) × (Jr/Ja) × (Jw/SRF). The ratio RQD/Jn represents the relative block size. A high RQD with few joint sets (low Jn) indicates large blocks and favorable conditions.
Geological Strength Index (GSI, Hoek 1994): GSI is estimated from rock structure (which correlates with RQD) and surface conditions of discontinuities. While RQD is not directly input to GSI, the block interlocking assessment used in GSI charts correlates strongly with RQD ranges.
RQD 90–100%: 20 points
RQD 75–90%: 17 points
RQD 50–75%: 13 points
RQD 25–50%: 8 points
RQD <25%: 3 points
RQD is necessary but not sufficient for rock mass
classification. Always combine with joint condition,
spacing, groundwater, and strength data.