Rock Quality Designation (RQD)

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Rock Quality Designation (RQD) in Core Logging

1. Introduction to RQD

Rock Quality Designation (RQD) is a widely used geotechnical index for evaluating the quality of rock masses based on drill core recovery. It was introduced by Don U. Deere in 1963 to assess rock quality in engineering and construction projects. RQD helps in determining the suitability of a rock mass for tunnels, foundations, slopes, and mining.

2. Measurement of RQD
RQD is measured from drill core samples obtained during exploratory drilling. The core samples are laid out in sequence, and the lengths of intact core pieces greater than or equal to 10 cm (4 inches) are summed.

Formula for RQD Calculation:


Example Calculation:

Total drill run: 1.5 m (150 cm)
Intact core pieces ≥ 10 cm: 120 cm

RQD (%) = (120 cm / 150 cm) × 100 = 80% (Good Rock Quality)




3. RQD Classification

RQD values are classified into different rock quality categories:

4. Importance of RQD in Engineering Geology

RQD is widely used in geotechnical and geological engineering because it provides insights into rock mass conditions. Some important applications include:

A. Tunnel Engineering

Helps in determining support requirements such as bolts, shotcrete, or lining.

RQD < 50% indicates a weak rock mass, requiring heavy reinforcement.


B. Foundation Design

Affects the bearing capacity of rock for structures.

High RQD (>75%) suggests stable rock, while low RQD may indicate potential settlements.


C. Slope Stability & Excavation

Low RQD (<50%) suggests a high risk of landslides.

Helps in deciding slope angles and stabilization measures.


D. Mining & Underground Works

Used in mine design to predict rock fragmentation and support needs.

Low RQD rocks may require grouting and reinforcement.



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5. Factors Affecting RQD Values

Several factors can influence RQD calculations, including:

Fracturing & Jointing: More fractures lead to lower RQD.

Drilling Method: Improper drilling can cause artificial fractures.

Weathering & Alteration: Highly weathered rocks may give lower RQD values.

Core Recovery Issues: Lost or broken core pieces affect the accuracy of RQD.


6. Limitations of RQD

Does not account for weak rock between fractures.

Neglects orientation of fractures, which is crucial for stability analysis.

Subject to drilling bias, as improper handling can break core samples.

Not applicable in soil-like or highly weathered formations, where core recovery is low.


7. Alternative Rock Mass Classification Systems

While RQD is widely used, other classification systems incorporate additional parameters:


8. Conclusion

RQD is a simple but effective method for evaluating rock quality, particularly for tunneling, foundations, and mining projects. However, it should be used alongside other classification systems for a more comprehensive understanding of rock mass behavior.



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