About this tool Estimate the Geological Strength Index (GSI) from two visual assessments: rock mass structure (blockiness) and discontinuity surface condition. Based on the quantified GSI chart by Hoek & Marinos (2000).
Optionally enter intact UCS (σci), the intact rock constant (mi), and disturbance factor (D) to calculate the Hoek-Brown rock mass parameters mb, s, and a. These feed directly into rock mass strength and deformation analysis.
GSI is the standard input to the Hoek-Brown failure criterion and is used globally for rock slope stability, tunnel design, and foundation analysis. Per Hoek & Marinos (2000) and Hoek et al (2002).
How to use this tool 1. Select the rock mass structure — from Intact/Massive to Laminated/Sheared, based on the blockiness observed in the exposure or core.
2. Select the surface condition — from Very Good (rough, fresh) to Very Poor (slickensided with clay).
3. (Optional) Enter σci, mi, and D — to get Hoek-Brown parameters mb, s, and a for strength analysis.
Technical information mb = mi × exp((GSI - 100) / (28 - 14D))
s = exp((GSI - 100) / (9 - 3D))
a = 0.5 + (1/6)(exp(-GSI/15) - exp(-20/3))
GSI is estimated from the intersection of structure and surface condition on the quantified chart. The ±5 range reflects the inherent uncertainty in visual assessment. Equations per Hoek et al (2002).
Limitations GSI should not be used for rock masses with clearly defined structural control (e.g. one dominant joint set causing planar or wedge failure). In those cases, analyse the specific failure mechanism directly.
GSI was developed for jointed rock masses and is unreliable for very high quality (GSI > 85) or very low quality (GSI < 15) rock. For intact rock, use UCS directly; for soil-like materials, use soil mechanics.
The disturbance factor D is one of the most debated parameters. Use D = 0 for undisturbed rock (TBM, careful blasting) and D = 0.7–1.0 for poor blasting. Avoid D = 1 for all cases — it is overly conservative for most applications.
Revision history 2 June 2026: Initial release
Disclaimer This tool is provided for educational and general information purposes only. It is not a substitute for professional engineering advice, design or verification.
Diggy and its contributors are not licensed engineering consultants and no results generated by this tool should be used directly for construction, design or safety-critical decisions.
All values and outputs are based on published empirical correlations and should be independently checked and confirmed by a qualified geotechnical engineer before use.
By using this tool, you accept full responsibility for how you interpret and apply the information provided.
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