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Fracture Frequency Calculator

Linear fracture frequency λ is simply the number of natural discontinuities intersected divided by the length over which they were counted: λ = N ÷ L (fractures per metre).

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How to use this calculator

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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Count only the natural fractures (open joints, bedding partings, healed defects that separate under handling) along the core run or scanline — exclude fresh mechanical breaks caused by drilling or handling.
  2. Enter that count and the measured length of the core run or scanline in metres.
  3. Read the fracture frequency (λ), mean spacing, and the Priest–Hudson estimated RQD; use the mm spacing to compare against ISRM spacing descriptors.

How it works

Linear fracture frequency λ is simply the number of natural discontinuities intersected divided by the length over which they were counted: λ = N ÷ L (fractures per metre). The mean fracture spacing is its reciprocal, x̄ = L ÷ N, reported here in metres and millimetres so it can be read against ISRM discontinuity-spacing descriptors (e.g. widely, moderately or closely spaced).

When RQD has not been measured directly, this tool also reports the Priest & Hudson (1976) theoretical correlation, RQD ≈ 100 · e^(−0.1λ) · (0.1λ + 1), which assumes discontinuity spacings follow a negative-exponential distribution and uses the standard 100 mm intact-length threshold. It is an estimate for planning and cross-checking only — RQD logged directly from the core (Σ intact pieces ≥ 100 mm ÷ total run × 100) always governs.

Worked example

5 m core run with 18 natural fractures. A geologist logs 18 natural discontinuities over a 5 m core run. Fracture frequency λ = 18 ÷ 5 = 3.6 fractures/m, mean spacing = 5 ÷ 18 = 0.278 m (277.8 mm), and the Priest–Hudson estimated RQD = 100 · e^(−0.36) · (0.36 + 1) = 94.9 %, indicating good-quality rock.

Common mistakes

  • Including drilling- or handling-induced breaks in the fracture count — only natural discontinuities should be counted, or λ is overstated and estimated RQD understated.
  • Mixing units: entering the run length in centimetres or millimetres instead of metres, which scales the frequency by 100 or 1000.
  • Treating the Priest–Hudson estimated RQD as measured RQD — the correlation assumes a specific spacing distribution and is not a substitute for RQD read directly off the core.

Frequently asked questions

What is fracture (discontinuity) frequency?

It is the number of natural discontinuities per unit length along a line, λ = N ÷ L, usually expressed in fractures per metre. It is the reciprocal of mean fracture spacing and is a basic index of how broken the rock mass is.

How is the estimated RQD related to fracture frequency?

When RQD is not logged directly, Priest & Hudson (1976) give RQD ≈ 100 · e^(−0.1λ) · (0.1λ + 1) for a 100 mm threshold, assuming a negative-exponential spacing distribution. Higher λ (more fractures per metre) gives lower RQD. This is a correlation for estimating and cross-checking, not a replacement for measured RQD.

Which breaks should I count?

Only natural discontinuities — open joints, bedding and foliation partings, faults and healed defects that separate the core. Exclude fresh, clean breaks caused by drilling, extraction or handling, as these are not part of the in-situ rock mass structure.

Does this replace core logging to a standard?

No. This is a guidance/estimate tool. Fracture logging, RQD and rock-mass classification must be carried out and verified against the relevant standard (ISRM suggested methods, AS or ASTM) by a competent geotechnical professional.

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