RQD Calculator – Rock Quality Designation
Compute Rock Quality Designation (RQD) from core run lengths or discontinuity spacing. Get RQD %, rock mass quality class, and quick interpretation for geotechnical and tunneling design.
RQD from Core Run Pieces
Enter the total core run length and the lengths of all sound core pieces longer than 10 cm (4 in) in that run.
Typical core run length is 1.5 m or 5 ft.
Pieces shorter than this are excluded from RQD.
Enter each acceptable piece length on a separate line. You can paste from Excel/CSV (numbers separated by spaces, commas, or line breaks).
RQD from Discontinuity Spacing
When core is not available, RQD can be estimated from discontinuity spacing along a scanline or borehole using empirical relationships.
Length over which discontinuities are counted.
Both are empirical; Palmström’s exponential form is more widely cited.
Enter spacing between joints/fractures along the line (same unit as length). One value per line or separated by spaces/commas.
Results
Run a calculation to see RQD and rock quality classification.
RQD
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Rock quality class
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Method
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What is Rock Quality Designation (RQD)?
Rock Quality Designation (RQD) is a widely used index that quantifies the degree of jointing and fracturing in a rock mass based on drill core. It was introduced by Deere (1963) and is now incorporated into many rock mass classification systems (e.g., RMR, Q-system).
RQD definition (core-based):
RQD is the percentage of the total core run length that consists of sound core pieces longer than 10 cm (4 in), measured along the core axis.
Formula:
\( \text{RQD} = \dfrac{\sum L_i}{L_{\text{total}}} \times 100 \)
- \( L_i \) = length of each acceptable core piece (> 10 cm / 4 in)
- \( L_{\text{total}} \) = total length of the core run
Typical RQD rock quality classes
| RQD (%) | Rock quality | General description |
|---|---|---|
| 0 – 25 | Very poor | Highly fractured, crushed, or weathered rock; heavy support usually required. |
| 25 – 50 | Poor | Very blocky, numerous discontinuities; significant support needed. |
| 50 – 75 | Fair | Moderately jointed rock; moderate support for tunnels and slopes. |
| 75 – 90 | Good | Slightly jointed, relatively intact rock; limited support required. |
| 90 – 100 | Excellent | Massive, intact rock with few joints; often self-supporting. |
How this RQD calculator works
1. Core run mode (recommended when core is available)
- Enter the core run length (e.g., 1.5 m or 5 ft).
- Measure each sound core piece longer than 10 cm (4 in) along the core axis and enter its length.
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The calculator:
- Sums all acceptable piece lengths \( \sum L_i \)
- Computes RQD = \( (\sum L_i / L_{\text{total}}) \times 100 \)
- Clips the result to 0–100% and assigns a rock quality class.
2. Discontinuity spacing mode (approximate)
When core is not available, RQD can be estimated from discontinuity spacing along a scanline or borehole using empirical relationships. This tool implements two commonly cited forms:
Palmström-type exponential approximation:
Let \( \lambda \) be the number of discontinuities per meter (or per foot, if using imperial).
\( \lambda = \dfrac{N}{L} \)
\( \text{RQD} \approx 100 \times e^{-0.1 \lambda} \)
This captures the rapid decrease in RQD as joint frequency increases. Coefficients are empirical and should be used with engineering judgment.
Simple linear approximation:
\( \text{RQD} \approx \max\left(0,\; \min\left(100,\; 115 - 3.3 \lambda \right)\right) \)
This linear form is sometimes used for quick estimates but is less realistic at very low or very high joint frequencies.
The calculator computes \( \lambda \) from your spacing data, applies the selected formula, and then classifies the resulting RQD.
Best practices and limitations
- Always record drilling and logging methods; RQD is sensitive to core handling and breakage.
- Exclude mechanically broken pieces shorter than 10 cm that are clearly due to drilling.
- Use multiple runs and boreholes; report both individual and average RQD values.
- Do not use RQD alone for design—combine it with RMR, Q-system, and detailed structural logging.
- Spacing-based RQD is an estimate; whenever possible, prefer core-based RQD.
Worked example (core-based)
A 1.5 m core run yields sound pieces longer than 10 cm with lengths: 0.35 m, 0.42 m, 0.18 m, 0.27 m.
Sum of acceptable lengths: \( \sum L_i = 0.35 + 0.42 + 0.18 + 0.27 = 1.22 \,\text{m} \)
RQD: \( \text{RQD} = (1.22 / 1.5) \times 100 \approx 81.3\% \)
According to the table above, this corresponds to good rock quality.
Frequently asked questions about RQD
Is RQD the same as percent core recovery?
No. Percent core recovery is the ratio of total recovered core length to the core run length, regardless of piece size. RQD only counts sound pieces longer than 10 cm (4 in). A run can have 100% recovery but low RQD if the core is highly fractured into short pieces.
Should I measure RQD in metric or imperial units?
Either is acceptable as long as you are consistent. The threshold is 10 cm (metric) or 4 in (imperial). This calculator lets you choose meters or feet; the logic is unit-agnostic as long as all inputs use the same unit.
How many core runs are needed for a reliable RQD?
There is no universal number, but good practice is to compute RQD for each run and then summarize by depth intervals, lithology, or structural domains. Longer intervals smooth variability but may hide weak zones; shorter intervals show detail but increase scatter.
Can RQD exceed 100%?
In theory, no. In practice, rounding and measurement errors can give slightly more than 100%. This calculator automatically clips RQD to the range 0–100%.
How does RQD relate to RMR and Q-system?
RQD is one of several input parameters in many rock mass classification systems. For example, in the RMR system, RQD contributes to the basic rock mass rating. In the Q-system, RQD appears in the numerator of the Q index. However, RQD alone does not capture joint orientation, aperture, infilling, or groundwater, so it should not be used as the sole design criterion.