GSI Helper
Estimate the Geological Strength Index (GSI) from an RMR89 value using Hoek's correlation. GSI feeds the Hoek–Brown failure criterion, so engineering geologists and geotechnical engineers use it for rock-mass strength estimation.
Enter Values
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How to use this calculator
- Enter your RMR89 value (0–100); the relation is only valid for RMR89 above 18.
- Read the estimated GSI (clamped 5–100) and the alternative estimate.
- Cross-check against the Hoek GSI chart using rock-mass structure and discontinuity surface condition before using it in design.
How it works
The Geological Strength Index describes a rock mass by its blockiness (structure) and the condition of its discontinuity surfaces. Hoek, Kaiser and Bawden (1995) related it to RMR89 by GSI = 1.5·RMR89 − 10 for RMR89 > 18, with GSI ≈ RMR89 − 5 as an alternative.
GSI is really meant to be read directly off the Hoek chart, not converted from RMR. Once you have GSI it combines with the intact-rock constant mᵢ and the disturbance factor D (0 for undisturbed, up to 1 for heavily blast-damaged rock) in the generalised Hoek–Brown criterion to give rock-mass strength and modulus.
Worked example
Good rock mass. An RMR89 of 65 gives GSI = 1.5 × 65 − 10 = 87.5, so about 88, while the alternative GSI ≈ RMR89 − 5 gives 60. Read the Hoek chart to settle between them, then pair GSI with mᵢ and D for Hoek–Brown.
Common mistakes
- Applying the formula below RMR89 18, where the linear relation breaks down — read GSI off the chart instead.
- Treating the RMR-derived GSI as exact; it is a proxy and can differ from the chart by 5–10 points.
- Forgetting the disturbance factor D — blast damage and stress relief can drop rock-mass strength sharply.
Frequently asked questions
How do you calculate GSI from RMR?
For RMR89 above 18, GSI = 1.5·RMR89 − 10. An alternative is GSI ≈ RMR89 − 5. Both are approximations of the value you would read off the Hoek chart.
What is GSI used for?
GSI is the rock-mass input to the generalised Hoek–Brown criterion, together with the intact constant mᵢ and disturbance factor D, to estimate rock-mass strength and deformation modulus.
Why is GSI clamped between 5 and 100?
The Hoek–Brown framework defines GSI on a 0–100 scale; in practice values below about 5 (extremely poor, soil-like rock) and above 100 are not meaningful, so the estimate is clamped.
Is GSI the same as RMR?
No. They are correlated but GSI is based on visual structure and surface condition and is intended to be read directly from the Hoek chart, whereas RMR sums measured parameter ratings.
What is the disturbance factor D?
D reflects blast and stress-relief damage to the rock mass: 0 for undisturbed in-situ or careful excavation, rising toward 1 for heavily blast-damaged slopes. It is applied in Hoek–Brown, not in the GSI value itself.
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