Understanding Rock Quality Designation (RQD) in Underground Mining

 

What is RQD?

Rock Quality Designation (RQD) is a geotechnical measurement developed by Deere et al. to quantify the quality of a rock mass. It is widely used in mining and civil engineering to assess rock structure during core drilling and exploration.

RQD is calculated as the percentage of core pieces longer than 10 cm in a drill run, divided by the total length of core recovered. It helps indicate how fractured or intact the rock mass is.

RQD Ratings and Rock Behavior

• RQD < 50%: Very fractured rock, poor quality

• RQD 50-75%: Fair quality rock

• RQD 75–90%: Good rock quality

• RQD > 90%: Excellent rock mass, close to lab conditions

A low RQD indicates a high number of joints or cracks, which can compromise structural stability. A high RQD suggests stronger, more continuous rock with fewer discontinuities.

How is RQD Measured?

• Drill cores are inspected and measured immediately after recovery

• Core segments shorter than 10 cm are excluded

• Breaks due to mechanical handling are ignored

• RQD is often recorded in 2–10 m windows, depending on site complexity

Care must be taken not to count cracks caused by core handling or blasting damage.

Alternative Estimation Methods

When core recovery isn’t possible, Palmström (1982) proposed estimating RQD based on joint counts from exposed rock faces. This method, called RQDW, evaluates post-excavation rock conditions and is useful for tunnel or wall support design.

Other alternatives include using scanlines (Priest & Hudson, 1976, 1981) to estimate joint frequency and orientation.

References:

• Deere & Miller (1966); Deere & Deere (1988)

• Bieniawski (1979); Coon & Merritt (1970)

• Palmström (1982, 1995); Priest & Hudson (1981)

• Løset (1992); Cording & Deere (1972)

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