What is Rock Mass Rating (RMR)?

Rock Mass Rating (RMR) is a geomechanics classification system developed by Bieniawski (1973, revised 1989). It rates a rock mass on six parameters — UCS, RQD, discontinuity spacing, discontinuity condition, groundwater, and orientation adjustment — producing a score from 0 to 100. It is the most widely used rock classification system in tunneling, mining, and slope engineering worldwide.

What is a good RMR score?

Scores of 81–100 (Class I) indicate very good rock. Scores of 61–80 (Class II) are good rock. Scores of 41–60 (Class III) are fair and require systematic support. Scores below 40 (Classes IV and V) represent poor to very poor rock requiring heavy, immediate support.

What is the difference between RMR76 and RMR89?

RMR89 (Bieniawski 1989) is the current industry standard. It has revised ratings for UCS and RQD, expands discontinuity condition into five sub-parameters, and updates the groundwater rating scale. Always use the 1989 version unless a project specifically requires the earlier edition.

How does RMR relate to the Q-System?

Both are empirical rock classification tools. The common correlation is: RMR ≈ 9 × ln(Q) + 44. On important projects, calculate both independently and compare. Significant divergence signals unusual site conditions worth investigating further.

How do I calculate RQD without core drilling?

Use Palmstrom's formula: RQD = 115 − 3.3 × Jv, where Jv is the number of joints per cubic meter. Alternatively use Priest and Hudson (1976): RQD = 100 × e^(−0.1λ) × (0.1λ + 1), where λ is mean joint frequency per meter.

How is GSI related to RMR89?

For RMR89 greater than 23, GSI equals RMR89 minus 5. For RMR89 less than or equal to 23, use the GSI chart directly — the formula is unreliable for very poor rock.

What are Hoek-Brown parameters mb s and a?

mb = mi x exp((GSI-100)/(28-14D)). s = exp((GSI-100)/(9-3D)). a = 0.5 + 1/6 x (exp(-GSI/15) - exp(-20/3)). These define the Hoek-Brown failure envelope for rock mass strength estimation.

GSI Calculator – Free Geological Strength Index Tool | Hoek 1995

Geological Strength Index (GSI) Calculator

Free online GSI calculator based on Hoek 1995. Click your rock mass structure and surface condition on the chart — or enter RMR89 directly. Get instant GSI, Hoek-Brown parameters (mb, s, a), radar chart, and rock mass strength estimates.

✓ Hoek 1995 / Marinos 2000 ✓ Interactive Visual Chart ✓ Hoek-Brown mb, s, a ✓ Radar Chart ✓ RMR Correlation ✓ PDF Export ✓ 100% Free

→ Choose how to estimate GSI

1 GSI Rock Mass Structure & Surface Condition — Click Your Match

Rows = how blocky is the rock mass. Columns = quality of discontinuity surfaces. Click the cell that best describes your site — the intersection gives your GSI range and midpoint.

STRUCTURE ↓
SURFACE →

VERY GOOD
Fresh, rough

GOOD
Slightly altered

FAIR
Smooth, altered

POOR
Slickensided

VERY POOR
Clay / gouge

INTACT / MASSIVE
No or few joints

90–100
≈95

80–90
≈85

70–80
≈75

N/A

BLOCKY
Well interlocked, undisturbed

75–85
≈80

65–75
≈70

55–65
≈60

45–55
≈50

35–45
≈40

VERY BLOCKY
Interlocked, partially disturbed

65–75
≈70

55–65
≈60

45–55
≈50

35–45
≈40

25–35
≈30

BLOCKY / DISTURBED
Folded, faulted, many joints

50–60
≈55

40–50
≈45

30–40
≈35

20–30
≈25

10–20
≈15

DISINTEGRATED
Heavily broken, poor interlock

N/A

25–35
≈30

18–27
≈22

10–20
≈15

5–15
≈10

LAMINATED / SHEARED
Shear zones, slickensided

N/A

15–20
≈18

10–15
≈13

5–10
≈8

Fine-tune GSI manually (overrides chart selection)

Disturbance Factor (D)

0.0

D = 0 for undisturbed rock (TBM, careful blasting). D = 1.0 for heavily blasted slopes. Affects Hoek-Brown mb and s — does not change GSI itself.

Intact Rock Constant (mi) — for Hoek-Brown mb

Select rock type to auto-fill mi, or enter directly. Required to calculate mb = mi × exp((GSI−100)/(28−14D)).

Rock Type

mi Value (editable)

GSI Classification Table

Hoek 1995 — five rock mass quality classes and engineering significance.

GSI Range	Rock Mass Quality	Description	Engineering Significance
75–100	Very Good to Intact	Fresh massive or blocky rock, rough discontinuities	High strength. Tunnels self-supporting. Em > 30 GPa.
55–75	Good	Blocky, interlocked, good surface condition	Moderate support. Systematic bolting + shotcrete sufficient.
40–55	Fair	Very blocky to disturbed, fair surface condition	Significant support. Squeezing possible at depth.
25–40	Poor	Disturbed, heavily jointed, poor surface condition	Heavy support. Deformation monitoring essential.
10–25	Very Poor	Disintegrated or sheared zones	Very heavy support. Face support often required.

How to Use This Calculator

3 simple steps — takes about 2 minutes with basic field observations.

Choose method and select GSI: Use the Visual Chart if you have field observations of rock mass blockiness and joint surface condition. Click the cell matching your site. Or use the RMR89 method if you already have an RMR89 value above 23.
Set Disturbance Factor D: D = 0 for TBM tunnels or careful excavation. D = 0.5 for mechanical excavation. D = 0.7 for controlled blasting. D = 1.0 for heavy production blasting or large open-pit slopes. D only affects Hoek-Brown parameters, not GSI.
Enter mi: Select your rock type from the dropdown or enter mi directly. This calculates the Hoek-Brown mb parameter. GSI, s, and a are calculated even without mi.

What is GSI? Complete Guide

Everything engineers and students need to know about Geological Strength Index — Hoek 1995.

What is GSI?

GSI (Geological Strength Index) was developed by Evert Hoek in 1995. Unlike RMR or Q-System, it was designed specifically as geomechanical input for the Hoek-Brown failure criterion — to estimate rock mass strength from two field observations: degree of blockiness and quality of discontinuity surfaces.

The Hoek-Brown Formula

mb = mi × exp((GSI−100)/(28−14D))
s = exp((GSI−100)/(9−3D))
a = 0.5 + ⅙ × (e^−GSI/15 − e^−20/3)

Rock mass UCS: σcm = σci × s^a (σci = intact UCS)

GSI from RMR89

For RMR89 > 23: GSI = RMR89 − 5. Convenient but loses accuracy for very poor rock. For RMR89 ≤ 23, estimate GSI directly from the chart — it was specifically calibrated for weak and sheared rock masses where RMR gives unreliable results.

Disturbance Factor D

D = 0: TBM tunnels, careful blasting
D = 0.5: Mechanical excavation
D = 0.7: Controlled drill and blast
D = 1.0: Heavy blasting, large open-pit slopes

D reduces mb and s only — does NOT change GSI itself.

Applications of GSI

Hoek-Brown failure criterion input
Rock mass UCS: σcm = σci × s^a
Deformation modulus Em estimation
Tunnel support software (RocSupport, RS2)
Slope stability analysis in rock
Foundation bearing capacity in rock

Key mi Reference Values

Granite: 28–33
Basalt: 16–25
Limestone: 9–12
Sandstone: 17–19
Shale / Mudstone: 6–8
Quartzite: 20–23
Gneiss: 25–30
Schist / Phyllite: 7–12

Intact Rock Constant (mi) Reference Table

Hoek-Brown mi values for common rock types — Hoek and Brown 1997.

Rock Type	Category	mi Value	Typical UCS (MPa)
Granite	Igneous	28–33	100–250
Granodiorite / Diorite	Igneous	25–29	80–200
Gabbro / Diabase	Igneous	25–27	100–300
Basalt	Igneous	16–25	50–250
Sandstone (coarse)	Sedimentary	17–19	20–170
Limestone	Sedimentary	9–12	30–250
Dolostone	Sedimentary	9–11	30–250
Siltstone / Mudstone	Sedimentary	7–9	5–100
Shale	Sedimentary	6–8	5–100
Quartzite	Metamorphic	20–23	150–300
Marble	Metamorphic	9–25	50–200
Gneiss	Metamorphic	25–30	100–250
Schist / Phyllite	Metamorphic	7–12	10–100
Slate	Metamorphic	7–9	25–180

Frequently Asked Questions

Common questions from engineers and students about the GSI system.

What is the Geological Strength Index GSI? +

GSI is a rock mass classification system developed by Evert Hoek in 1995. It estimates rock mass quality from two visual observations: the degree of blockiness or structure of the rock mass, and the condition of the discontinuity surfaces. GSI ranges from about 10 for very poor heavily sheared rock to 100 for intact fresh rock. It was designed specifically as input for the Hoek-Brown failure criterion — not for tunnel support classification like RMR or Q-System.

How do I estimate GSI in the field? +

Assess two things: (1) How blocky is the rock mass — intact, blocky, very blocky, disturbed, disintegrated, or laminated and sheared? (2) What is the condition of joint surfaces — very good (fresh, rough, tight), good (slightly altered), fair (smooth, altered), poor (slickensided), or very poor (clay and gouge filling)? Find the intersection on the GSI chart. Always record a range rather than a single number to acknowledge field uncertainty.

When should I NOT derive GSI from RMR89? +

Do not use GSI = RMR89 minus 5 when RMR89 is 23 or below. In very poor to extremely poor rock, RMR ratings become unreliable and the linear relationship to GSI breaks down. Use the GSI chart directly — it was calibrated to give realistic estimates for heavily sheared, disintegrated, or laminated rock masses where RMR is not reliable.

What does disturbance factor D do to Hoek-Brown parameters? +

D accounts for blast damage and stress relaxation near the excavation surface, ranging from 0 for undisturbed rock (TBM or careful blasting) to 1.0 for severely blasted slopes. D reduces mb and s significantly — at GSI = 50, increasing D from 0 to 1 roughly halves mb. D does not change the GSI value itself, only the derived Hoek-Brown parameters.

How is rock mass UCS estimated from GSI? +

Rock mass UCS: σcm = σci × s^a, where σci is intact rock UCS, and s and a come from GSI and D. For example, GSI = 50, D = 0, σci = 50 MPa: s ≈ 0.0041, a ≈ 0.506, so σcm ≈ 3.2 MPa. This large reduction from intact to rock mass strength is typical for moderately jointed rock.

Is GSI the same as RMR or Q-System? +

No. RMR and Q-System were designed for tunnel support classification. GSI was designed for rock mass strength estimation via Hoek-Brown. GSI correlates with RMR89 for good to fair rock (GSI = RMR89 − 5) but diverges significantly for poor and very poor rock. GSI does not include the groundwater rating, orientation adjustment, or stress factor that RMR and Q-System use.

Is this calculator suitable for professional use? +

This calculator correctly implements Hoek 1995 GSI and Hoek-Brown 2002 parameters. It is suitable as a computation aid for professional work. All input parameters must be assessed by a qualified geotechnical or mining engineer from actual field investigations and lab tests before use in final design.

GSI Results

Updates in real-time

—

Click on chart to begin

10GSI Range100

Parameter Radar Chart

Hoek-Brown Parameters (D = 0.0)

—

reduced mi

—

cohesion

—

curvature

GSI Value	—
Rock Mass Quality	—
RMR89 estimate	—
Disturbance D	—
mi (intact rock)	—
mb (rock mass)	—
s	—
a	—
σcm / σci	—

Implements Hoek 1995 / Hoek-Brown 2002.
Verify with a qualified geotechnical engineer.

Geological Strength Index (GSI) Calculator

GSI Classification Table

How to Use This Calculator

What is GSI? Complete Guide

What is GSI?

The Hoek-Brown Formula

GSI from RMR89

Disturbance Factor D

Applications of GSI

Key mi Reference Values

Intact Rock Constant (mi) Reference Table

Frequently Asked Questions

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