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Q System Worksheet Calculator

Combine the six Barton Q-system parameters into the rock-mass quality Q and read its quality band. Widely used for tunnelling and underground support design as an alternative or complement to RMR.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

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. Read each parameter off Barton's standard tables: RQD (%), Jn joint set number, Jr roughness, Ja alteration, Jw water reduction and SRF stress reduction.
  2. Enter all six values (use RQD 10 if the logged RQD is below 10).
  3. Read Q, its quality band and the three quotient groups (block size, inter-block shear, active stress).

How it works

The NGI Q-system multiplies three quotients: Q = (RQD/Jn)·(Jr/Ja)·(Jw/SRF). RQD/Jn represents the relative block size, Jr/Ja the shear strength between blocks, and Jw/SRF the active stress state (water and in-situ stress).

Q spans many orders of magnitude, so it is grouped into nine quality bands from exceptionally poor (<0.01) through poor, fair and good up to exceptionally good (>400). Higher Q means a more competent, self-supporting rock mass.

Worked example

Blocky, dry rock. RQD 80, Jn 9, Jr 2, Ja 1, Jw 1, SRF 1 gives block size 80/9 = 8.9, shear 2/1 = 2 and stress 1/1 = 1, so Q = 8.9 × 2 × 1 ≈ 17.8 — a 'good' rock mass.

Common mistakes

  • Using RQD below 10 — Barton sets a floor of 10 for the RQD/Jn quotient.
  • Swapping Jr and Ja, or Jw and SRF; roughness and water go on top, alteration and stress on the bottom.
  • Reading Jn for the wrong number of joint sets, which shifts Q by a whole band.

Frequently asked questions

What does the Q value mean?

Q is the NGI rock-mass quality index. It runs from below 0.01 (exceptionally poor) to over 400 (exceptionally good); higher Q means more competent, self-supporting rock.

What are the three quotients?

RQD/Jn is relative block size, Jr/Ja is the inter-block shear strength, and Jw/SRF is the active stress (water pressure and in-situ stress).

How does Q relate to RMR?

Barton's approximate correlation is RMR ≈ 9·ln(Q) + 44. It is only indicative — assess both systems from the field data rather than converting one into the other.

Why is the minimum RQD 10?

For very poor rock the Q-system uses a nominal RQD of 10 in the RQD/Jn quotient, even where the measured RQD is 0–10, to keep the ratio meaningful.

Is Q the same as Q-bar (Qc)?

No. Q is the basic index. Qc = Q·(σc/100) is the strength-normalised value used in some support charts; this worksheet computes the basic Q.

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