Underground Ground Support
Rate the rock mass, get the class, and read off the bolt length, bolt spacing
and shotcrete the published guidelines call for, plus how many bolts that is per meter of
heading and what it costs.
Underground development headings only: a drill-and-blast tunnel, drift,
crosscut or ramp in hard rock. It is not a slope design, not a coal roof-bolting design, not a
pillar design, and it is not a substitute for a ground control management plan. The support it
quotes is an EMPIRICAL STARTING POINT from a 10 m wide tunnel under 25 MPa. Your ground engineer
signs the pattern, not this page.
Step 1: The rock
Three numbers. They carry 55 of the 100 rating points between them.
Step 2: The joints
Five things about the joint surfaces, worth 6 points each. This is the condition parameter, scored in detail rather than picked off one prose description.
Step 3: Water and orientation
Water costs you up to 15 points. Orientation is a penalty, never a bonus.
Step 4: The heading and the cost
The pattern comes from the class. Override it and the working table says so.
| Step | Value | Where it comes from |
|---|
Where the rating comes from
The pattern, drawn
How each rating is scored
| Parameter | Range | Rating |
|---|---|---|
| 1. Strength of intact rock | ||
| >250 MPa | 15 | |
| 100-250 MPa | 12 | |
| 50-100 MPa | 7 | |
| 25-50 MPa | 4 | |
| 5-25 MPa | 2 | |
| 1-5 MPa | 1 | |
| <1 MPa | 0 | |
| 2. Drill core quality RQD | ||
| 90%-100% | 20 | |
| 75%-90% | 17 | |
| 50%-75% | 13 | |
| 25%-50% | 8 | |
| <25% | 3 | |
| 3. Spacing of discontinuities | ||
| >2 m | 20 | |
| 0.6-2 m | 15 | |
| 200-600 mm | 10 | |
| 60-200 mm | 8 | |
| <60 mm | 5 | |
| 4. Discontinuity length (persistence) | ||
| <1 m | 6 | |
| 1-3 m | 4 | |
| 3-10 m | 2 | |
| 10-20 m | 1 | |
| >20 m | 0 | |
| 4. Separation (aperture) | ||
| None | 6 | |
| <0.1 mm | 5 | |
| 0.1-1.0 mm | 4 | |
| 1-5 mm | 1 | |
| >5 mm | 0 | |
| 4. Roughness | ||
| Very rough | 6 | |
| Rough | 5 | |
| Slightly rough | 3 | |
| Smooth | 1 | |
| Slickensided | 0 | |
| 4. Infilling (gouge) | ||
| None | 6 | |
| Hard filling <5 mm | 4 | |
| Hard filling >5 mm | 2 | |
| Soft filling <5 mm | 2 | |
| Soft filling >5 mm | 0 | |
| 4. Weathering | ||
| Unweathered | 6 | |
| Slightly weathered | 5 | |
| Moderately weathered | 3 | |
| Highly weathered | 1 | |
| Decomposed | 0 | |
| 5. Groundwater | ||
| Completely dry: None, ratio 0 | 15 | |
| Damp: <10 L/min, ratio <0.1 | 10 | |
| Wet: 10-25 L/min, ratio 0.1-0.2 | 7 | |
| Dripping: 25-125 L/min, ratio 0.2-0.5 | 4 | |
| Flowing: >125 L/min, ratio >0.5 | 0 | |
| B. Orientation adjustment, tunnels and mines | ||
| Drive with dip, dip 45-90 degrees: Very favorable | 0 | |
| Drive with dip, dip 20-45 degrees: Favorable | -2 | |
| Strike parallel to axis, dip 45-90 degrees: Very unfavorable | -12 | |
| Strike parallel to axis, dip 20-45 degrees: Fair | -5 | |
| Drive against dip, dip 45-90 degrees: Fair | -5 | |
| Drive against dip, dip 20-45 degrees: Unfavorable | -10 | |
| Dip 0-20 degrees, irrespective of strike: Fair | -5 |
The rock mass rating system after Bieniawski 1989,
printed in three places on this shelf and cross-checked between them: SME Mining
Engineering Handbook, 3rd ed. Table 8.4-6, p. 536; SME Mining Reference Handbook,
2nd ed. Table 14.3, p. 333; and SME Mining Reference Handbook, 1st ed. Table 13.2,
p. 244. All three agree on every rating.
Three printings, not three authorities. Two of those are successive editions of
the same reference handbook, so this is a weaker claim than three unrelated books and it is
worth stating plainly rather than letting the count do the talking. It is still a real check:
the editions are separately typeset, they were separately transcribed here, and a disagreement
between them is exactly what found the error below.
One disagreement, resolved. Table 14.3 p. 333 prints the middle spacing band as
“2–600 mm”. It should read 200–600 mm, which is what
the other two books print, and the band sequence settles it on its own: >2 m, 0.6–2 m,
200–600 mm, 60–200 mm, <60 mm is descending and contiguous, while
“2–600 mm” would swallow the band below it. That is an error in the printed
page rather than in our reading of it, so this page uses 200–600 mm.
Estimating strength without a lab
| Term | UCS, MPa | What you can do to it |
|---|---|---|
| Extremely strong | >250 | Chipped with a geological hammer |
| Very strong | 100-250 | Many blows with geological hammer to break core |
| Strong | 50-100 | More than one blow with geological hammer to break core |
| Medium strong | 25-50 | Cannot be scraped or peeled with a pocket knife; core breaks with a single hammer blow |
| Weak | 5-25 | Can be peeled with a pocket knife; shallow indentation from a firm blow with the hammer point |
| Very weak | 1-5 | Crumbles under a firm blow with the hammer point; can be peeled with a pocket knife |
| Extremely weak | 0.25-1 | Indented by fingernail |
SME Mining Engineering Handbook Table 8.4-4, p. 532, adapted from Hoek and Brown 1997. The seven field terms map one for one onto the seven strength bands the rating uses, which is why picking one in Step 1 can fill the box rather than just hint at it.
| Class | MPa | psi | Typical rocks |
|---|---|---|---|
| Weak | ≤41.3 | ≤6,000 | Coal, weathered rock, alluvium |
| Moderate | 41.3-137.9 | 6,000-20,000 | Shale, sandstone, limestone, schist, evaporites, disseminated deposit |
| Strong | 137.9-206.8 | 20,000-30,000 | Metamorphic, igneous, veins, marble, slate |
| Very strong | ≥206.8 | ≥30,000 | Quartzite, basalt, diabase |
SME Mining Engineering Handbook Table 6.2-7, p. 351, as a sanity check on what the hammer told you. Published in both MPa and psi, and the two agree: 41.3 MPa against 6,000 psi is 41.37, 137.9 against 20,000 is 137.90, 206.8 against 30,000 is 206.84. Note these are broad rock-type bands and a weathered example of any of them sits lower.
What the guidelines call for, class by class
| Class | Excavation | Rock bolts, 20 mm fully grouted | Shotcrete | Steel sets |
|---|---|---|---|---|
| I: Very good rock RMR 81-100 | Full face; 3 m advance | Generally no support required except for occasional spot bolting | n/a | n/a |
| II: Good rock RMR 61-80 | Full face; 1.0-1.5 m advance; complete support 20 m from face | Locally bolts in crown 3 m long, spaced 2.5 m with occasional wire mesh | 50 mm in crown where required | None |
| III: Fair rock RMR 41-60 | Top heading and bench; 1.5-3 m advance in top heading; begin support after each blast; complete support 10 m from face | Systematic bolts 4 m long, spaced 1.5-2 m in crown and walls with wire mesh in crown | 50-100 mm in crown and 30 mm in sides | None |
| IV: Poor rock RMR 21-40 | Top heading and bench; 1.0-1.5 m advance in top heading; install support concurrently with excavation 10 m from face | Systematic bolts 4-5 m long, spaced 1-1.5 m in crown and walls with wire mesh | 100-150 mm in crown and 100 mm in sides | Light to medium ribs spaced 1.5 m apart where required |
| V: Very poor rock RMR 0-20 | Multiple drifts; 0.5-1.5 m advance in top heading; install support concurrently with excavation; shotcrete as soon as possible after blasting | Systematic bolts 5-6 m long, spaced 1-1.5 m in crown and walls with wire mesh; bolt invert | 150-200 mm in crown, 150 mm in sides, and 50 mm on face | Medium to heavy ribs spaced 0.75 m apart with steel lagging and forepoling if required; close invert |
Bieniawski 1979, printed identically in
the SME Mining Reference Handbook at 2nd ed. p. 335 and 1st ed. p. 245. Two separately
typeset editions, separately transcribed here, agreeing number for number. Not two independent
authorities, but a genuine check on the reading of both.
Read the basis before you use it. Both books print the same one: a
horseshoe profile, 10 m wide, vertical stress below 25 MPa, driven by drill and
blast, and both add that the table is a guide only. A 5 m heading is not a 10 m tunnel,
and this page tells you when your span, stress or profile has walked away from that basis
instead of quietly scaling the pattern as though it had not.
What each class means on the ground
| Parameter | Class I | Class II | Class III | Class IV | Class V |
|---|---|---|---|---|---|
| Average stand-up time | 20 years for 15 m span | 1 year for 10 m span | 1 week for 5 m span | 10 hours for 2.5 m span | 30 minutes for 1 m span |
| Cohesion of rock mass, kPa | >400 | 300-400 | 200-300 | 100-200 | <100 |
| Friction angle of rock mass, degrees | >45 | 35-45 | 25-35 | 15-25 | <15 |
SME Mining Reference Handbook, 2nd ed. Table 14.3 D, p. 334. Stand-up time is quoted against a specific span, and the spans differ by class, so it is not a number you can carry across to your own heading without checking the span it belongs to. This page compares the two for you rather than interpolating between them, because the source publishes five points and no curve.
Friction bolt properties
| Property | ||||
|---|---|---|---|---|
| Actual bolt diameter, mm | 34 | 35.5 | 39 | 46 |
| Weight, kg/m | 1.64 | 1.64 | 1.88 | 2.8 |
| Minimum ultimate strength, kN | 81.65 | 81.65 | 90.7 | 136.1 |
| Recommended initial load, kN | 27-54 | 27-54 | 27-54 | 54-90 |
| Recommended bit diameter, mm | 31-32 | 31-33 | 35-38 | 41-45 |
| Bolt length, m | 0.45-2.4 | 0.45-3 | 0.45-3 | 0.9-3.7 |
SME Mining Engineering Handbook, p. 617, typical Split
Set properties, courtesy of Mansour Mining. Shown because bolt length is one of the two numbers
this page hands you and the hardware has to exist in that length: the shorter sets stop at 2.4 m
and only the largest reaches 3.7 m, so a guideline calling for 5 to 6 m bolts in very poor rock
is calling for a different product entirely. The guideline table itself is written for 20 mm
fully grouted bar, not friction sets.
A coal-entry roof bolt capacity table also sits in the corpus and is deliberately NOT used here:
its capacity column carries no unit on the printed page, and coal entry support is a different
problem from a hard-rock tunnel.
Spending more time on spreadsheets than on the job?
We build the reporting and admin automation for mining contractors, so the numbers keep themselves.
Book a 20 min callWorked example
Worked with the values this page loads with, so the method can be checked against the cited source without entering anything. Change any input above and the calculator recomputes; the numbers below stay as a fixed reference case.
- 60
- III: Fair rock
- $985 / m
| 1. Strength of intact rock | 12 pts | 100-250 MPa, Table 14.3 A p. 333, cross-checked p. 536 and p. 244 |
| 2. Drill core quality RQD | 13 pts | 50%-75%, Table 14.3 A p. 333, cross-checked p. 536 and p. 244 |
| 3. Spacing of discontinuities | 10 pts | 200-600 mm, Table 14.3 A p. 333, cross-checked p. 536 and p. 244 (see the note on this band) |
| 4. Discontinuity length (persistence) | 4 pts | 1-3 m, Table 14.3 E p. 334 |
| 4. Separation (aperture) | 4 pts | 0.1-1.0 mm, Table 14.3 E p. 334 |
| 4. Roughness | 3 pts | Slightly rough, Table 14.3 E p. 334 |
| 4. Infilling (gouge) | 4 pts | Hard filling <5 mm, Table 14.3 E p. 334 |
| 4. Weathering | 5 pts | Slightly weathered, Table 14.3 E p. 334 |
| = condition of discontinuities | 20 pts | the five above, max 30, Table 14.3 E p. 334 |
| 5. Groundwater | 10 pts | Damp, <10 L/min, Table 14.3 A p. 333, cross-checked p. 536 and p. 244 |
| = basic rating | 65 pts | parameters 1 to 5, max 100 |
| B. Orientation adjustment | -5 pts | Fair, tunnels and mines, Table 14.3 B p. 333 |
| = rock mass rating | 60 | Table 14.3 C p. 333 |
| = rock class | III: Fair rock | RMR 41-60, Table 14.3 C p. 333 |
| Average stand-up time | 1 week at a 5 m span | Table 14.3 D p. 334, quoted at that span, not yours |
| Cohesion of the rock mass | 200-300 kPa | Table 14.3 D p. 334 |
| Friction angle of the rock mass | 25-35° | Table 14.3 D p. 334 |
| Excavation | Top heading and bench; 1.5-3 m advance in top heading; begin support after each blast; complete support 10 m from face | Bieniawski 1979, p. 335 and p. 245 |
| Published bolt length | 4 m | Bieniawski 1979, p. 335 and p. 245 |
| Published bolt spacing | 1.5-2 m | Bieniawski 1979, p. 335 and p. 245 |
| Bolt length in use | 4.00 m | middle of the published range |
| Bolt spacing in use | 1.75 m | middle of the published range |
| Bolted perimeter | 14.0 m | span + 2 × height; the floor is not bolted |
| = bolts per meter of heading | 4.6 | perimeter ÷ spacing ÷ spacing, square pattern |
| Shotcrete | 50-100 mm in crown and 30 mm in sides | Bieniawski 1979, p. 335 and p. 245 |
| = shotcrete per meter of heading | 0.65 m³ | 75 mm over the crown, 30 mm on the sides |
| Mesh | 14.0 m² per meter | Bieniawski 1979, p. 335 and p. 245 |
| Bolts | $206 / m | 4.6 × $45 |
| Shotcrete | $387 / m | 0.65 m³ × your rate |
| Mesh | $392 / m | 14.0 m² × your rate |
| = support cost per meter of heading | $985 / m | the three above |
Estimating aid only. This is not an engineering design and not a substitute for the judgment of a competent or qualified person. Verify every result against the cited source and your own site conditions. Full terms and disclaimer.