🎓 Lesson 14
D5
Rock Quality Designation (RQD) Field Measurement and Pitfalls
RQD is a simple way to measure how broken or intact a rock core is by counting how much of it is in solid pieces longer than 10 cm.
🎯 Learning Objectives
- ✓ Calculate RQD from field core logs using standardized measurement protocols
- ✓ Explain how RQD influences rock mass classification (e.g., RMR, Q-system) and support design decisions
- ✓ Analyze limitations of RQD in highly fractured, foliated, or weathered rock and propose complementary indices
- ✓ Apply ASTM D6032 and ISRM guidelines to assess RQD measurement validity
📖 Why This Matters
In open-pit mines and tunneling projects, misjudging rock mass quality can lead to slope instability, excessive ground support costs, or unsafe blasting outcomes. RQD is one of the first and most cost-effective indicators engineers obtain directly from drill cores — often before lab testing or geophysical surveys. A single RQD value may trigger major design changes: e.g., shifting from bolt-only support to shotcrete + mesh in tunnels, or adjusting blast burden spacing in hard rock benches. Yet, it’s routinely misapplied — making accurate field measurement and interpretation critical.
📘 Core Principles
RQD is not a substitute for full rock mass characterization but serves as a foundational, field-quantifiable proxy for discontinuity density. It assumes core recovery is representative and that fractures >10 cm apart reflect significant structural breaks. The index works best in massive, isotropic rocks (e.g., granite, basalt); its reliability drops in schists, heavily weathered zones, or where drilling-induced breakage dominates. Modern practice treats RQD as one input among many — combined with joint set orientation (via scanline surveys), RMR, or GSI — because high RQD does not guarantee high strength (e.g., intact shale may have RQD >90% but low shear strength). Understanding sampling bias — such as preferential core loss in soft seams or over-rotation causing artificial breakage — is essential to avoid systematic underestimation.
📐 RQD Calculation
RQD is computed as a percentage ratio of the sum of lengths of core pieces ≥10 cm to the total length of the core run. Only intact, cylindrical core segments are included — crushed, splintered, or fragmented material <10 cm is excluded. The formula applies strictly to NX-size (54 mm diameter) or larger core; smaller diameters increase breakage noise and reduce reliability.
💡 Worked Example
Problem: A 1.85 m core run from an NX-size borehole yields the following intact pieces: 0.32 m, 0.18 m, 0.45 m, 0.21 m, 0.11 m, and 0.09 m (excluded). All other fragments are <10 cm.
1.
Step 1: Identify pieces ≥0.10 m: 0.32, 0.18, 0.45, 0.21, 0.11 → five pieces
2.
Step 2: Sum their lengths: 0.32 + 0.18 + 0.45 + 0.21 + 0.11 = 1.27 m
3.
Step 3: Divide by total core run length: 1.27 / 1.85 = 0.6865 → RQD = 68.7%
Answer:
The result is 68.7%, which falls within the 'Fair' RQD range (60–80%) per Deere’s classification, indicating moderately fractured rock suitable for moderate support in tunnels or stable benches at ≤45° slopes.
🏗️ Real-World Application
At the Cadia East underground mine (NSW, Australia), initial RQD values from diamond core ranged from 45–75% across the porphyry ore zone. However, detailed core logging revealed that ~30% of low-RQD intervals coincided with clay-altered shear zones invisible in standard televiewer logs. Engineers cross-validated RQD with point load index (Is(50)) and GSI estimates, leading to revised bolting patterns — increasing pattern density from 1.5 m × 1.5 m to 1.2 m × 1.2 m in RQD <55% zones. This adjustment reduced unplanned roof falls by 62% during development drive advance.
📚 References
/books/engineering-rock-mass-classification/deere/978-0-444-41876-1" target="_blank" class="text-blue-600 hover:underline">Engineering Rock Mass Classification