Rock Quality Designation (RQD) is a quantitative index of rock mass quality based on core recovery. Developed by Don Deere in 1967 at the University of Illinois, RQD measures the percentage of intact core pieces longer than 10 cm (4 inches) relative to the total core run length. It is one of the most widely used parameters in geotechnical engineering, mining, and tunneling for classifying rock mass quality and estimating rock mass behavior.
This guide covers the RQD measurement procedure, the Priest and Hudson theoretical relationship, how RQD integrates into rock mass classification systems (RMR, Q-system), and practical applications in geotechnical and mining engineering.
Measuring RQD from Core
RQD is defined as:
RQD (%) = (Σ Length of intact core pieces ≥ 10 cm) / Total core run length × 100
The measurement procedure follows these rules:
- 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 (Deere 1967):
- 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
Priest and Hudson (1976) derived a theoretical relationship between RQD and the average number of discontinuities 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 (also called the volumetric joint count or linear frequency). This formula allows RQD to be estimated from scanline surveys or borehole televiewer data without requiring intact core. It is particularly useful when core is not available or when comparing different rock masses.
The relationship shows that RQD is 100% when λ < ~6 discontinuities/meter and approaches 0% when λ exceeds ~40/meter. The most sensitive range is 8–30 discontinuities/meter, where small changes in fracture frequency produce significant RQD changes. Below 6/m and above 40/m, RQD is insensitive to fracture frequency, a known limitation of the index.
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%
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.
RQD in Rock Mass Classification Systems
RQD is a component of several major rock mass classification systems used in civil and mining engineering:
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.
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.