RMR Classification Table: Rock Classes I to V
The Rock Mass Rating (RMR) classification system assigns every rock mass to one of five quality classes based on the total adjusted RMR score. Each class carries empirical estimates of engineering properties that are widely used for preliminary design of tunnels, slopes, and foundations. This page presents the complete classification table, explains what each class means in practical engineering terms, and provides guidance on interpreting your RMR results.
Complete RMR Classification Table
The following table summarises the five rock classes defined by Bieniawski (1989), including the RMR score ranges, qualitative descriptions, estimated stand-up times for unsupported excavation spans, and rock mass strength parameters (cohesion and friction angle).
| Class | RMR Score | Description | Stand-up Time | Cohesion (kPa) | Friction Angle (°) |
|---|---|---|---|---|---|
| I | 81 – 100 | Very Good Rock | 20 years for 15 m span | > 400 | > 45 |
| II | 61 – 80 | Good Rock | 1 year for 10 m span | 300 – 400 | 35 – 45 |
| III | 41 – 60 | Fair Rock | 1 week for 5 m span | 200 – 300 | 25 – 35 |
| IV | 21 – 40 | Poor Rock | 10 hours for 2.5 m span | 100 – 200 | 15 – 25 |
| V | < 21 | Very Poor Rock | 30 min for 1 m span | < 100 | < 15 |
Detailed Class Descriptions
Class I: Very Good Rock (RMR 81–100)
Score range meaning: An RMR of 81 to 100 indicates a rock mass of exceptional quality with strong intact rock, high RQD, wide discontinuity spacing, tight and rough joints with no infilling, and dry conditions. The rock mass behaves almost as a continuum, with discontinuities exerting minimal influence on overall strength and deformability.
Engineering behaviour: Class I rock masses are essentially self-supporting for typical civil engineering excavation spans. Tunnels up to 15 metres wide can remain stable without support for periods measured in decades. Deformation around excavations is negligible, and convergence is typically less than a few millimetres. The rock mass deformation modulus approaches the intact rock modulus, typically exceeding 50 GPa. Overbreak during excavation is minimal and controlled.
Typical rock types: Fresh, massive granite and granodiorite with widely spaced joints; unweathered, thick-bedded quartzite; massive basalt flows with few cooling joints; fresh gneiss with widely spaced foliation partings. These conditions are most commonly encountered at moderate depths in tectonically stable regions where weathering has not penetrated and groundwater levels are below the excavation.
Support implications: Generally, no systematic support is required for tunnels. Spot bolting may be installed locally where individual wedges are identified by structural analysis. In some jurisdictions, a thin layer of fibre-reinforced shotcrete (25–50 mm) may be applied as a safety measure regardless of rock quality, but this is a precautionary measure rather than a structural requirement. For slopes, steep angles (70–80 degrees or steeper) are achievable in Class I rock. Foundations on Class I rock provide excellent bearing capacity, typically exceeding 10 MPa.
Class II: Good Rock (RMR 61–80)
Score range meaning: An RMR of 61 to 80 represents good quality rock with moderate to high intact strength, good RQD, moderate to wide discontinuity spacing, and generally favourable discontinuity conditions. Some of the parameters may rate below ideal, but no single parameter is critically deficient.
Engineering behaviour: Class II rock masses are stable for moderate spans and durations. A 10-metre-wide tunnel excavation can remain unsupported for approximately one year, providing ample time for support installation during normal construction operations. Rock mass deformation modulus is typically in the range of 20 to 50 GPa. Minor block falls may occur from the crown, particularly where two or three discontinuity sets intersect to form removable wedges. Convergence is generally less than 10–20 mm for typical tunnel sizes.
Typical rock types: Slightly weathered granite with two to three joint sets at moderate spacing; well-cemented sandstone with bedding partings at 0.6 to 2 m intervals; fresh to slightly weathered dolerite or diabase sills; moderately jointed limestone with clean, rough joint surfaces. Class II conditions are common in the upper portions of competent rock formations where some weathering has occurred but the rock mass retains good structural integrity.
Support implications: Tunnels typically require locally placed rock bolts (3 m long, spaced 2–3 m) in the crown, primarily targeting identified wedge geometries. A 50 mm layer of shotcrete is commonly applied to the crown for block containment. Wire mesh may be used in isolated areas where small block ravelling is anticipated. For slopes, bench face angles of 60–75 degrees are generally achievable depending on discontinuity orientations. Foundations provide bearing capacities of 5–10 MPa and excellent resistance to settlement.
Class III: Fair Rock (RMR 41–60)
Score range meaning: An RMR of 41 to 60 indicates fair quality rock where several parameters may rate in the moderate to low range. Typical characteristics include moderate intact strength, RQD between 50% and 75%, discontinuity spacings of 200–600 mm, some joint opening or soft infilling, and possible damp to wet conditions.
Engineering behaviour: Class III rock masses require prompt support installation after excavation. The stand-up time of one week for a 5-metre span means that delays in support installation can lead to progressive loosening and potential collapse. Rock mass deformation modulus is typically 5 to 20 GPa. Block falls from the crown and sidewalls are common, and progressive ravelling can occur if support is delayed. Convergence may reach 20–50 mm or more in larger tunnels, and monitoring is essential to verify that support is adequate.
Typical rock types: Moderately weathered granite or diorite with closely spaced joints; thinly bedded siltstone or shale with moderate joint surface conditions; jointed limestone with some clay-filled discontinuities; moderately foliated and fractured schist or phyllite. Class III conditions are frequently encountered in transition zones between weathered and fresh rock, and in moderately tectonized rock.
Support implications: Tunnels require systematic rock bolting (4 m long bolts on a 1.5–2 m grid) combined with 50–100 mm of shotcrete in the crown and walls. Wire mesh reinforcement is typically required. Excavation should proceed using a top heading and bench method with heading advances of 1.5 to 3 m before support installation. Steel sets may be required in localised weak zones. For slopes, overall slope angles of 45–55 degrees are typical, and drainage measures may be needed. Foundation bearing capacity is typically 2–5 MPa, and settlement analysis may be required for heavy structures.
Class IV: Poor Rock (RMR 21–40)
Score range meaning: An RMR of 21 to 40 represents poor quality rock with low intact strength, low RQD, closely spaced discontinuities, unfavourable discontinuity conditions (smooth surfaces, clay infilling, or significant weathering), and often wet to dripping groundwater conditions. Multiple parameters typically rate poorly.
Engineering behaviour: Class IV rock masses are inherently unstable for all but the smallest excavation spans. The stand-up time of 10 hours for a 2.5-metre span demands immediate support installation as part of the excavation cycle. Progressive ravelling and squeezing are common failure modes. Rock mass deformation modulus is typically 1 to 5 GPa. Large convergences of 50–150 mm or more are expected, and closure of the excavation profile is possible if support is inadequate. Groundwater inflows may be significant and require active dewatering.
Typical rock types: Highly weathered granite or gneiss; closely jointed and clay-altered volcanic rocks; fault zones in otherwise competent rock; weak mudstone or marlstone with slickensided bedding surfaces; coal measure sediments with multiple weak horizons. Class IV conditions are common in fault zones, shear zones, weathered zones beneath topographic lows, and in inherently weak lithologies.
Support implications: Tunnels require heavy support consisting of systematic rock bolts (4–5 m long on a 1–1.5 m grid), 100–150 mm of fibre-reinforced shotcrete, and possibly steel ribs or lattice girders at 0.5–1.5 m spacing. Excavation must use multiple drift or top heading and bench method with very short advances (1–1.5 m). Invert closure (a full ring of support) is often required to prevent floor heave. For slopes, overall slope angles are limited to 30–45 degrees, and extensive drainage and reinforcement may be needed. Foundation bearing capacity is typically 1–2 MPa, and pile foundations may need to be considered for heavy structures.
Class V: Very Poor Rock (RMR < 21)
Score range meaning: An RMR below 21 indicates very poor rock mass quality with extremely weak intact material, very low or zero RQD, very closely spaced or crushed discontinuities, severely weathered or decomposed joint surfaces with thick soft infilling, and flowing groundwater. The rock mass may behave more like a soil than a rock.
Engineering behaviour: Class V rock masses provide essentially no self-supporting capacity. The stand-up time of 30 minutes for a 1-metre span means that unsupported excavation is not feasible even for small openings. The material may squeeze, swell, or flow into the excavation under its own weight and the action of groundwater. Rock mass deformation modulus is typically less than 1 GPa. Very large deformations (more than 150 mm and potentially exceeding 5–10% of the tunnel diameter) are expected, and tunnel convergence may continue for weeks or months after excavation.
Typical rock types: Completely weathered or decomposed rock (saprolite); major fault gouge zones consisting of clay and crushed rock fragments; severely squeezing rock under high overburden; swelling clay-bearing rock such as montmorillonite-rich mudstone or shale; highly fractured and altered rock adjacent to hydrothermal zones. Class V conditions represent the most challenging ground conditions for underground construction.
Support implications: Tunnels in Class V rock require immediate and heavy support, typically including steel ribs or lattice girders at 0.5–0.75 m spacing, 150–200 mm of shotcrete applied in multiple layers, full invert closure, and possibly forepoling or pipe roof canopy ahead of the face. Excavation is by multiple small drifts with advances of 0.5–1 m. Ground improvement techniques such as grouting, ground freezing, or jet grouting may be required ahead of excavation. For slopes, angles are limited to less than 30 degrees, and soil mechanics approaches may be more appropriate than rock mechanics methods. Deep foundations (piles or caissons) extending through Class V material to competent rock are typically required for structural foundations.
How to Interpret Your RMR Result
When you obtain an RMR value from your field data (either by manual calculation or using our free online RMR calculator), the classification table provides a first-pass engineering assessment. However, the following considerations are important for proper interpretation:
- Boundary values require judgement: A rock mass scoring exactly 60 falls at the boundary between Class III (Fair) and Class II (Good). In such cases, examine which parameters are driving the score and consider whether conditions are more representative of the higher or lower class. The practical difference in support requirements between adjacent classes can be significant.
- The same RMR does not mean the same behaviour: Two rock masses with the same total RMR but different individual parameter scores may behave very differently. A rock mass with strong intact rock but poor discontinuity conditions will form large, potentially unstable blocks, while a weak rock mass with tight joints may squeeze rather than form block failures. Always examine the individual parameter ratings alongside the total score.
- Stand-up time is span-dependent: The stand-up time values in the table correspond to specific spans. For different spans, the stand-up time will change. Bieniawski published a stand-up time chart relating unsupported span, stand-up time, and RMR that should be consulted for spans other than those listed in the classification table.
- Properties are empirical estimates: The cohesion and friction angle values are empirical estimates based on back-analyses of case histories. They represent rock mass properties (not intact rock or discontinuity properties) and should be used for preliminary design only. Site-specific testing and analysis should supplement these estimates for detailed design.
RMR Class vs Excavation Method Selection
The RMR rock class directly influences the selection of excavation methods, particularly for tunnelling. The following guidelines relate rock class to excavation approach:
| RMR Class | Drill & Blast | TBM Suitability | Excavation Sequence | Advance Rate Guidance |
|---|---|---|---|---|
| I (81–100) | Full-face blasting; controlled perimeter blasting recommended | Excellent; high utilisation rates | Full face | Full round advance (3–5 m) |
| II (61–80) | Full-face blasting with perimeter control | Good; occasional support delays | Full face or heading and bench | Full round advance (2.5–4 m) |
| III (41–60) | Heading and bench; controlled blasting essential | Fair; support installation limits advance | Top heading and bench | Heading advance 1.5–3 m |
| IV (21–40) | Multiple drifts; careful blasting or mechanical excavation | Difficult; risk of cutter head entrapment | Top heading and bench or multiple drifts | Heading advance 1–1.5 m |
| V (< 21) | Small drifts; mechanical excavation preferred; forepoling required | Very difficult; squeezing and face instability | Multiple small drifts with pre-support | Advance 0.5–1 m with pre-support |
These are general guidelines for conventional tunnelling. Actual excavation methods depend on many additional factors including tunnel size, shape, depth, in-situ stress conditions, contractual requirements, and available equipment. The RMR class provides a starting point for method selection that is refined through detailed analysis and engineering judgement.
Frequently Asked Questions
An RMR Class III rating (score 41–60) indicates fair rock conditions with a stand-up time of approximately one week for a 5-metre unsupported span. For tunnel design, this typically requires systematic rock bolting on a 1.5 to 2 metre grid pattern using 4-metre-long bolts, combined with 50 to 100 mm of shotcrete applied to the crown and upper walls. Wire mesh reinforcement is usually needed in areas where small block fallout is anticipated. The excavation should proceed using a top heading and bench approach, with the heading advance limited to 1.5 to 3 metres before support is installed. Continuous convergence monitoring is essential to confirm that the support system is performing adequately and to detect any progressive deterioration.
Yes, rock mass quality can deteriorate over time due to several processes. Excavation removes confinement and allows stress relaxation, which can open discontinuities and reduce the effective ratings for aperture, spacing, and groundwater. Prolonged exposure to water or atmospheric conditions causes progressive weathering of joint surfaces and softening of clay infilling materials, reducing the discontinuity condition rating. Blasting damage during excavation creates new fractures that reduce RQD and increase fracture frequency. Seasonal or long-term groundwater changes can alter the groundwater rating. For these reasons, engineers should assess RMR for both the initial conditions encountered during excavation and the anticipated long-term conditions, and design support systems for the more adverse scenario.
The stand-up time values are empirical estimates derived from Bieniawski's compilation of case histories from tunnelling projects, primarily in sedimentary and metamorphic rocks at moderate depths. They provide useful order-of-magnitude guidance for preliminary design but should not be treated as precise predictions. Actual stand-up times depend on many factors not fully captured by RMR alone, including the three-dimensional excavation geometry, the magnitude and orientation of in-situ stresses, the excavation method and quality of blasting, time-dependent rock behaviour such as creep, squeezing, or swelling, and the specific structural failure mechanism that governs stability. Stand-up time is most useful as a comparative index for assessing relative risk between different rock mass conditions rather than as an absolute design parameter.