What Is Rock Mass Rating (RMR)?
The rock mass rating (RMR) system is a geomechanical classification method developed by Z.T. Bieniawski between 1972 and 1973, with the most widely used version published in his 1989 book "Engineering Rock Mass Classifications." RMR combines six critical geological and geomechanical parameters into a single composite score from 0 to 100 — where higher values indicate better rock quality requiring less engineering support.
The RMR system is used globally by geotechnical engineers, engineering geologists, and rock mechanics specialists for the design and construction of tunnels, mines, rock slopes, and foundations. It provides a systematic, quantitative approach to what was previously a subjective process of rock mass characterization.
The 6 RMR Parameters — Complete Guide
Parameter 1 — Uniaxial Compressive Strength (UCS): Max rating 15UCS measures the strength of intact rock material — not the rock mass. It is determined by laboratory testing of core samples (point load test, Brazilian test, or direct compression). Very strong rocks (granite, quartzite, gneiss) typically have UCS > 150 MPa. Weak rocks (chalk, weathered shale) may be under 5 MPa. Rating ranges from 0 (UCS < 1 MPa) to 15 (UCS > 250 MPa).
Parameter 2 — Rock Quality Designation (RQD %): Max rating 20RQD quantifies the degree of fracturing in the rock mass. It is calculated as the percentage of intact core pieces longer than 100mm (10 cm) in a total core run. Formula: RQD = (Sum of pieces ≥ 100mm) / Total core length × 100%. Excellent rock (90–100% RQD) earns 20 points; very poor rock (0–25% RQD) earns only 3 points.
Parameter 3 — Spacing of Discontinuities: Max rating 20Discontinuities include joints, faults, bedding planes, cleavage, and any other planar weakness features. Spacing is measured as the average distance between adjacent discontinuities perpendicular to their orientation. Wide spacing (>2m) earns 20 points; very close spacing (<60mm) earns only 5 points.
Parameter 4 — Condition of Discontinuities: Max rating 30This is the highest-weighted parameter (30 points maximum) and the most influential on rock mass behavior. It evaluates: roughness of discontinuity surfaces, degree of weathering of wall rock, presence and thickness of gouge (infilling), aperture (gap width), and persistence (continuity). Very rough, unweathered surfaces with no infill earn 30 points; soft gouge >5mm or continuous open gaps earn 0 points.
Parameter 5 — Groundwater Conditions: Max rating 15Groundwater significantly affects rock mass stability. Dry conditions earn 15 points. Flowing water with high pressure earns 0 points. This parameter is assessed from drill hole observations, tunnel mapping, or piezometric data. For slopes, adjust the groundwater assessment to reflect seasonal variation and surface infiltration.
Parameter 6 — Orientation of Discontinuities: Adjustment (-60 to 0)This parameter adjusts the RMR based on how the orientation of discontinuities (their strike and dip) relates to the planned excavation direction. For tunnels: discontinuities dipping 45–90° and striking perpendicular to the tunnel axis are "very favorable" (0 adjustment). Discontinuities striking parallel to the tunnel axis and dipping at 45–90° are "very unfavorable" (-12 for tunnels, -25 for foundations, -60 for slopes).
RMR Rock Mass Classes — What Your Score Means
| RMR Range | Class | Quality | Stand-Up Time | Cohesion | Friction Angle |
|---|---|---|---|---|---|
| 81–100 | I | Very Good Rock | 10 yrs at 15m span | >400 kPa | >45° |
| 61–80 | II | Good Rock | 6 months at 8m span | 300–400 kPa | 35–45° |
| 41–60 | III | Fair Rock | 1 week at 5m span | 200–300 kPa | 25–35° |
| 21–40 | IV | Poor Rock | 10 hours at 2.5m span | 100–200 kPa | 15–25° |
| 0–20 | V | Very Poor Rock | 30 minutes at 1m span | <100 kPa | <15° |
Tunnel Support Guidelines Based on RMR (10m Span)
| Class | Excavation Method | Rock Bolts | Shotcrete | Steel Ribs |
|---|---|---|---|---|
| I (RMR 81–100) | Full face, 3m advance | Spot bolts only | None required | None |
| II (RMR 61–80) | Full face, 1–1.5m advance | Systematic 3m bolts @ 2.5m pattern | 50mm crown | None |
| III (RMR 41–60) | Top heading and bench, 1.5–3m advance | Systematic 4m bolts @ 1.5–2m pattern | 50–100mm crown & 30mm walls | Light ribs where required |
| IV (RMR 21–40) | Top heading and bench, 1.0–1.5m advance | Systematic 4–5m bolts @ 1–1.5m pattern with mesh | 100–150mm crown & 100mm walls | Medium ribs @ 1.5m spacing |
| V (RMR 0–20) | Multiple drifts, ≤0.5m advance, shotcrete immediately | 5m bolts @ 1–1.5m with mesh, face bolts | 150–200mm crown, walls & face | Heavy ribs @ 0.75m spacing with lagging |
RMR vs Q-System — Key Differences
| Feature | RMR (Bieniawski 1989) | Q-System (Barton 1974) |
|---|---|---|
| Score range | 0–100 (linear) | 0.001–1,000 (logarithmic) |
| Number of parameters | 6 | 6 |
| Includes rock strength | Yes (UCS) | No (separate consideration) |
| Includes in-situ stress | No (indirect via UCS) | Yes (Stress Reduction Factor) |
| Ease of field application | Simpler | More complex |
| Best application | Initial design, all excavation types | Detailed tunnel support design |
| Approximate correlation | RMR ≈ 9 ln(Q) + 44 (Bieniawski 1976) | |
| Geological Strength Index (GSI) | GSI ≈ RMR − 5 (for RMR > 18) | GSI from Q: GSI = 9 log(Q') + 44 |
Derived Geotechnical Parameters from RMR
Once the RMR value is determined, several important geotechnical parameters can be estimated for preliminary design:
| Parameter | Formula | Notes |
|---|---|---|
| Geological Strength Index (GSI) | GSI = RMR − 5 | Valid for RMR > 18 only |
| Deformation Modulus Em (GPa) | Em = 10^((RMR-10)/40) | Serafim & Pereira (1983) |
| Deformation Modulus (RMR>50) | Em = 2·RMR − 100 (GPa) | Bieniawski (1978) |
| Friction Angle (degrees) | φ = 5·RMR/12 + 8.5° | Approximate — verify with testing |
| Cohesion (kPa) | c = 5·RMR (kPa) | Approximate — use range from class table |
| Q-system equivalent | Q = e^((RMR-44)/9) | Bieniawski 1976 correlation |
Engineering Applications of Rock Mass Rating
Tunnel Design and Support SelectionRMR is most widely used for tunnel design. The RMR value directly determines the recommended excavation method (full face vs top heading and bench vs multiple drifts), rock bolt length and spacing, shotcrete thickness, and steel rib requirements. The tunnel support table from Bieniawski (1989) is reproduced above and forms the basis for preliminary support design in most geotechnical projects worldwide.
Rock Slope Stability (with SMR)For rock slopes, the Slope Mass Rating (SMR) system builds on RMR by adding correction factors specific to slope geometry. SMR adjusts the RMR based on the relationship between discontinuity strike/dip and slope face orientation. SMR = RMR + (F1 × F2 × F3) + F4, where F1–F4 are correction factors for planar, toppling, and wedge failure modes.
Foundation Bearing CapacityRMR can be used to estimate allowable bearing pressure for rock foundations. Based on plate load test data from over 60 sites, the correlation between RMR and net allowable bearing pressure (Mehrotra 1992) provides preliminary foundation design values. Very good rock (Class I, RMR 81–100) can typically support pressures exceeding 2,000 kPa; poor rock (Class IV, RMR 21–40) may only support 200–400 kPa.
Mining and Cavern DesignIn mining engineering, RMR guides the design of stope dimensions, pillar sizing, and support requirements for underground openings. The stand-up time concept from RMR helps miners determine safe advance rates and support installation timing. RMR is also used in cavern design for hydroelectric projects, LNG storage, and civil defense facilities.
Rock Mass Rating Calculator — Frequently Asked Questions
What is a good RMR value?
RMR values above 60 (Class I or II) indicate good to very good rock requiring minimal tunnel support. RMR 40–60 (Class III, "Fair Rock") is typical for many engineering projects and requires moderate support. RMR below 40 (Class IV or V) indicates poor quality rock requiring extensive support. For context, fresh massive granite typically scores 75–85, weathered granite 40–55, and heavily fractured rock with clay-filled joints can score below 20.
What is the RMR89 formula?
RMR89 = R1 + R2 + R3 + R4 + R5 + R6, where: R1 = UCS rating (0–15), R2 = RQD rating (3–20), R3 = Discontinuity spacing rating (5–20), R4 = Discontinuity condition rating (0–30), R5 = Groundwater rating (0–15), R6 = Orientation adjustment (-60 to 0). Maximum unadjusted score = 100. The orientation adjustment reduces the score for unfavorable joint orientations relative to the excavation.
How is RQD measured in the field?
RQD is measured from drill core samples. Record the total length of all intact core pieces 100mm or longer in a given core run length. RQD = (Total length of pieces ≥ 100mm / Total core length) × 100%. For example, in a 1.5m core run with 3 pieces measuring 120mm, 200mm, and 350mm respectively, RQD = (120+200+350)/1500 × 100 = 44.7%. Core runs should be approximately 1.5–3m long for representative results.
Can RMR be used for slopes?
The standard RMR89 was designed primarily for tunnels. For slopes, use the Slope Mass Rating (SMR) system which extends RMR with slope-specific correction factors. SMR accounts for the geometric relationship between slope face direction and discontinuity orientation in more detail than the standard orientation adjustment in RMR89. For rock slopes, SMR values are typically interpreted as: SMR 81–100 = completely stable, 61–80 = stable, 41–60 = partially stable, 21–40 = unstable, 0–20 = completely unstable.