Rock Mass Rating (RMR) — The Complete Guide
Table of Contents
Introduction to Rock Mass Rating
The Rock Mass Rating (RMR) system is one of the most widely used geomechanical classification systems in the world. Developed by Professor Z. T. Bieniawski at the South African Council for Scientific and Industrial Research (CSIR), the system was first published in 1973 and has since become an essential tool for geotechnical engineers, mining engineers, and engineering geologists working on underground excavations, rock slopes, and foundations.
The fundamental premise of the RMR system is that the engineering behaviour of a rock mass can be quantified by summing ratings assigned to six measurable parameters: the uniaxial compressive strength of intact rock material, Rock Quality Designation (RQD), spacing of discontinuities, condition of discontinuities, groundwater conditions, and the orientation of discontinuities relative to the engineering structure. Each parameter receives a rating based on measured or estimated values, and the sum of these ratings produces a total RMR score between 0 and 100.
The original 1973 version of RMR was based on case histories from tunnelling projects in South Africa. Bieniawski subsequently revised the system in 1974, 1975, 1976, and 1979, incorporating data from hundreds of additional case studies worldwide. The most significant revision came in 1989, when Bieniawski published the definitive version of the system in his textbook "Engineering Rock Mass Classifications." This 1989 revision introduced five explicit sub-parameters for assessing discontinuity conditions (persistence, aperture, roughness, infilling, and weathering), refined the orientation adjustment tables, and provided clearer descriptions for each rating category. The 1989 version is the standard used in engineering practice today, and it is the version implemented in this guide and our free online RMR calculator.
The RMR system offers several key advantages that explain its enduring popularity. First, the input parameters are readily measurable from standard geotechnical investigations including core logging, borehole data, and outcrop mapping. Second, the additive scoring approach is intuitive and easy to understand, making it accessible to engineers and geologists at all experience levels. Third, the system has been correlated with a wide range of engineering properties and design parameters, including stand-up time for unsupported excavation spans, rock mass cohesion and friction angle, deformation modulus, and support requirements. Fourth, RMR can be related to other classification systems, most notably the Q-system through the empirical correlation RMR = 9 ln(Q) + 44, and the Geological Strength Index (GSI) through the approximation GSI = RMR - 5 (for RMR > 23).
Over the past five decades, the RMR system has been applied to thousands of projects on every continent. It is referenced in design standards and guidelines published by the International Society for Rock Mechanics (ISRM), the International Tunnelling Association (ITA), and numerous national codes. Whether you are designing a highway tunnel, evaluating slope stability for an open-pit mine, or assessing foundation conditions for a dam, understanding how to apply the RMR system correctly is a fundamental skill in geotechnical engineering.
The 6 RMR Parameters
The RMR system evaluates rock mass quality through six parameters. The first five produce the "basic RMR" score (maximum 100 points), and the sixth parameter applies an adjustment based on discontinuity orientation relative to the engineering application. For a detailed explanation of each parameter including field measurement techniques and common assessment errors, see our dedicated RMR parameters guide.
Parameter 1: Uniaxial Compressive Strength (UCS) — Maximum 15 Points
This parameter rates the intact rock material strength. It is best determined through laboratory uniaxial compression testing per ISRM standards, but can also be estimated from the Point Load Strength Index (Is50) multiplied by an appropriate conversion factor (typically 20–25). For very weak rocks below 25 MPa, the UCS should be measured directly rather than estimated from point load tests.
| UCS (MPa) | Point Load Index (MPa) | Rating |
|---|---|---|
| > 250 | > 10 | 15 |
| 100 – 250 | 4 – 10 | 12 |
| 50 – 100 | 2 – 4 | 7 |
| 25 – 50 | 1 – 2 | 4 |
| 5 – 25 | — | 2 |
| 1 – 5 | — | 1 |
| < 1 | — | 0 |
Parameter 2: Rock Quality Designation (RQD) — Maximum 20 Points
RQD measures the percentage of intact core pieces longer than 100 mm recovered from a borehole core run. It was originally developed by Don Deere in 1964 and provides a simple index of rock mass fracturing. Where core is unavailable, RQD can be estimated from discontinuity frequency using the relationship RQD = 115 - 3.3 Jv (where Jv is the volumetric joint count), capped at 100%.
| RQD (%) | Quality Description | Rating |
|---|---|---|
| 90 – 100 | Excellent | 20 |
| 75 – 90 | Good | 17 |
| 50 – 75 | Fair | 13 |
| 25 – 50 | Poor | 8 |
| < 25 | Very Poor | 3 |
Parameter 3: Spacing of Discontinuities — Maximum 20 Points
This parameter evaluates the average spacing between adjacent discontinuities in the rock mass. Spacing is measured perpendicular to the discontinuity surfaces along scan lines or from borehole core logs. Where multiple discontinuity sets are present, the rating should be based on the set with the smallest (most unfavourable) spacing, as this controls the block size and therefore the rock mass behaviour.
| Spacing | Description | Rating |
|---|---|---|
| > 2 m | Very wide | 20 |
| 0.6 – 2 m | Wide | 15 |
| 200 – 600 mm | Moderate | 10 |
| 60 – 200 mm | Close | 8 |
| < 60 mm | Very close | 5 |
Parameter 4: Condition of Discontinuities — Maximum 30 Points
This is the most influential parameter in the RMR system, with a maximum of 30 points. In the Bieniawski 1989 revision, it is assessed through five sub-parameters: persistence (continuity length), aperture (opening width), roughness, infilling (gouge material), and weathering of the discontinuity walls. Each sub-parameter receives an individual rating, and the sum gives the total condition rating.
| Sub-parameter | Condition | Rating |
|---|---|---|
| Persistence | < 1 m | 6 |
| 1 – 3 m | 4 | |
| 3 – 10 m | 2 | |
| 10 – 20 m | 1 | |
| > 20 m | 0 | |
| Aperture | None (closed) | 6 |
| < 0.1 mm | 5 | |
| 0.1 – 1 mm | 4 | |
| 1 – 5 mm | 1 | |
| > 5 mm | 0 | |
| Roughness | Very rough | 6 |
| Rough | 5 | |
| Slightly rough | 3 | |
| Smooth | 1 | |
| Slickensided | 0 | |
| Infilling | None | 6 |
| Hard filling < 5 mm | 4 | |
| Hard filling > 5 mm | 2 | |
| Soft filling < 5 mm | 2 | |
| Soft filling > 5 mm | 0 | |
| Weathering | Unweathered | 6 |
| Slightly weathered | 5 | |
| Moderately weathered | 3 | |
| Highly weathered | 1 | |
| Decomposed | 0 |
Parameter 5: Groundwater Conditions — Maximum 15 Points
Groundwater significantly influences rock mass behaviour by reducing effective normal stresses on discontinuities, softening infilling materials, and creating hydrostatic pressures. This parameter can be assessed using the inflow rate per 10 m tunnel length, the joint water pressure ratio (ratio of pore water pressure to major principal stress), or a general qualitative description of groundwater conditions.
| Inflow per 10 m | Pressure Ratio (pw/σ1) | General Condition | Rating |
|---|---|---|---|
| None | 0 | Completely dry | 15 |
| < 10 L/min | 0 – 0.1 | Damp | 10 |
| 10 – 25 L/min | 0.1 – 0.2 | Wet | 7 |
| 25 – 125 L/min | 0.2 – 0.5 | Dripping | 4 |
| > 125 L/min | > 0.5 | Flowing | 0 |
Parameter 6: Orientation Adjustment
The final parameter adjusts the basic RMR score to account for the relationship between discontinuity orientation and the direction of the engineering structure. The adjustment is always zero or negative, and its magnitude depends on whether the application is a tunnel, slope, or foundation. For tunnels, the maximum penalty is -12 points. For foundations, the maximum penalty is -25 points. For slopes, the orientation adjustment can be as severe as -60 points, reflecting the critical influence of adverse joint orientations on slope stability.
| Strike & Dip Orientation | Tunnels | Foundations | Slopes |
|---|---|---|---|
| Very favourable | 0 | 0 | 0 |
| Favourable | -2 | -2 | -5 |
| Fair | -5 | -7 | -25 |
| Unfavourable | -10 | -15 | -50 |
| Very unfavourable | -12 | -25 | -60 |
RMR Classification System — Rock Classes I to V
Based on the total adjusted RMR score, the rock mass is assigned to one of five classes. Each class has associated engineering properties including stand-up time (the time an unsupported excavation remains stable), cohesion, and internal friction angle. These values are empirical estimates based on Bieniawski's compilation of case histories and should be used for preliminary design only. For full details on each rock class including engineering behaviour and typical rock types, see our RMR classification table guide.
| 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 |
How to Calculate RMR
The RMR calculation follows a systematic procedure that ensures consistent and repeatable results. For a comprehensive step-by-step walkthrough with field data checklists, printable recording sheets, and common errors to avoid, visit our detailed how to calculate RMR guide.
- Determine Uniaxial Compressive Strength (UCS): Obtain UCS from laboratory testing or estimate from point load index. Assign the corresponding rating (0–15 points) from the UCS table.
- Determine Rock Quality Designation (RQD): Calculate RQD from borehole core logs as the percentage of intact pieces longer than 100 mm, or estimate from volumetric joint count. Assign the rating (3–20 points).
- Measure Discontinuity Spacing: Record the average spacing of the most closely spaced discontinuity set. Assign the rating (5–20 points).
- Assess Discontinuity Condition: Evaluate each of the five sub-parameters (persistence, aperture, roughness, infilling, weathering) independently and sum the sub-ratings (0–30 points total).
- Evaluate Groundwater Conditions: Assess using inflow rate, pressure ratio, or general description. Assign the rating (0–15 points).
- Sum Parameters 1–5: Add the five ratings to obtain the Basic RMR. This value ranges from 8 to 100.
- Apply Orientation Adjustment: Determine the relationship between dominant discontinuity orientation and the engineering structure. Apply the appropriate adjustment (0 to -60 depending on application type) to obtain the Adjusted RMR.
- Determine Rock Class: Use the adjusted RMR value to classify the rock mass into Class I through V using the classification table above.
Applications of RMR
Tunnel Design and Support
The RMR system was originally developed for tunnelling applications, and this remains its primary use. Bieniawski 1989 provides specific support recommendations for each rock class, including rock bolt patterns, shotcrete thickness, and steel set requirements. The stand-up time values are particularly useful for determining the sequence and timing of excavation and support installation. For example, in Class III rock (RMR 41–60), the stand-up time of one week for a 5-metre span means that support must be installed within this timeframe to prevent progressive failure. The New Austrian Tunnelling Method (NATM) and Norwegian Method of Tunnelling (NMT) both utilize RMR as a key input for support design.
Rock Slope Engineering
RMR is widely used in slope stability assessments for highway cuts, open-pit mines, and dam abutments. The slope-specific orientation adjustments (0 to -60) reflect the critical importance of joint orientation relative to the slope face. Engineers commonly extend the basic RMR into the Slope Mass Rating (SMR) system by applying correction factors for the parallelism between joints and slope strike, the joint dip angle relative to the slope angle, and the excavation method used. SMR values directly correlate with slope failure probability and recommended stabilization measures.
Foundation Engineering
For foundations on rock, particularly dam foundations, the RMR system provides estimates of rock mass deformation modulus and shear strength parameters. The empirical relationship between RMR and deformation modulus proposed by Bieniawski is Em = 2 RMR - 100 (in GPa, valid for RMR > 50). For lower RMR values, Serafim and Pereira proposed Em = 10^((RMR-10)/40) GPa. These relationships allow engineers to estimate settlement and bearing capacity from RMR values obtained during site investigation. The foundation-specific orientation adjustments (0 to -25) account for the influence of unfavourably oriented discontinuities on bearing capacity and sliding resistance.
Mining Applications
In mining, RMR is used extensively for designing underground openings, assessing pillar stability, and selecting appropriate mining methods. The Mining Rock Mass Rating (MRMR) system developed by Laubscher in 1990 modifies the basic RMR to account for mining-induced stress changes, blasting damage, and weathering effects. RMR values also feed into empirical pillar design methods and stope stability assessments using the modified stability graph approach. Open-pit mine design relies on RMR for bench design, inter-ramp slope angles, and overall pit slope optimization.
RMR vs Other Classification Systems
Several rock mass classification systems are used in geotechnical engineering practice. The following table compares RMR with the two other most common systems: the Q-system and the Geological Strength Index (GSI).
| Feature | RMR (Bieniawski 1989) | Q-System (Barton 1974) | GSI (Hoek & Brown 1997) |
|---|---|---|---|
| Developer | Z. T. Bieniawski | N. Barton, R. Lien, J. Lunde | E. Hoek, P. Marinos |
| Scale | 0 – 100 | 0.001 – 1000 | 5 – 100 |
| Scoring Method | Additive (sum of ratings) | Multiplicative (product of ratios) | Visual chart-based |
| Number of Parameters | 6 | 6 (combined into 3 ratios) | 2 (structure + surface condition) |
| Includes UCS | Yes (up to 15 points) | No (strength in SRF only) | No (separate Hoek-Brown input) |
| Orientation Adjustment | Yes (0 to -60) | No (implicit in ESR) | No |
| Support Design | Empirical table | Q-support chart | Not directly (feeds Hoek-Brown) |
| Primary Application | Tunnels, slopes, foundations | Tunnels (mainly) | Numerical modelling input |
| Correlation | — | RMR = 9 ln(Q) + 44 | GSI ≈ RMR - 5 (RMR > 23) |
Each system has strengths in different contexts. RMR is the most versatile system for general-purpose rock mass classification across tunnels, slopes, and foundations. The Q-system excels in tunnel support design, particularly for Norwegian-style projects. GSI is specifically designed to provide input for the Hoek-Brown failure criterion used in numerical rock mechanics modelling. In practice, many projects apply two or more systems in parallel to cross-check results and gain a more complete understanding of rock mass conditions.
Use Our Free Online RMR Calculator
Our free Rock Mass Rating calculator implements the complete Bieniawski 1989 classification system with all six parameters. Simply enter your field data for each parameter and the calculator instantly computes the basic RMR, applies your selected orientation adjustment, determines the rock class (I–V), and provides associated engineering properties including stand-up time, cohesion, and friction angle. You can also generate a PDF report of your results for inclusion in geotechnical reports. The calculator runs entirely in your browser with no data sent to any server, ensuring the confidentiality of your project data.
Explore RMR Topics
RMR Parameters Explained
Detailed guide to all six Bieniawski 1989 rating criteria with complete rating tables, field measurement techniques, and common assessment errors for each parameter.
RMR Classification Table
Full breakdown of rock classes I through V with score ranges, engineering properties, stand-up times, typical rock types, and support recommendations for each class.
How to Calculate RMR
Step-by-step calculation guide with field data checklists, a printable data recording sheet, and a list of common calculation errors to avoid.
RMR Worked Examples
Three fully solved RMR problems covering granite tunnel, shale slope, and limestone foundation scenarios with parameter-by-parameter justification and engineering interpretation.
Frequently Asked Questions
The original RMR system published by Bieniawski in 1973 used a simpler approach to rating discontinuity conditions and had different weighting for some parameters. The 1989 revision introduced five explicit sub-parameters for discontinuity condition assessment (persistence, aperture, roughness, infilling, and weathering), refined the orientation adjustment values for tunnels, foundations, and slopes, and provided clearer guidelines for groundwater assessment. The 1989 version produces more consistent results between different practitioners because the sub-parameter approach reduces subjective interpretation. It is the standard version referenced in engineering codes and used in practice worldwide today.
The maximum possible basic RMR score (before orientation adjustment) is 100 points. This would require a UCS greater than 250 MPa (15 points), an RQD of 90–100% (20 points), discontinuity spacing greater than 2 m (20 points), excellent discontinuity conditions across all five sub-parameters (30 points), and completely dry conditions (15 points). After applying the orientation adjustment, which is always zero or negative, the adjusted RMR score could range from the basic RMR value down to as low as basic RMR minus 60 (for very unfavourable slope orientations). In practice, achieving a basic RMR of 100 is extremely rare because it requires perfect conditions across all five parameters simultaneously.
Yes, the Bieniawski 1989 RMR system includes specific orientation adjustment values for slopes that range from 0 to -60, which are significantly more severe than the tunnel adjustments (0 to -12). This reflects the critical influence that adverse joint orientations have on slope stability. However, for dedicated slope stability analysis, many geotechnical engineers extend the basic RMR into the Slope Mass Rating (SMR) system developed by Romana in 1985. SMR applies correction factors based on the geometric relationship between joint strike and dip relative to the slope face, as well as the excavation method used. The basic RMR value calculated from the first five parameters serves as the starting point for SMR calculations.
The most commonly used correlation between RMR and the Q-system is the empirical relationship RMR = 9 ln(Q) + 44, proposed by Bieniawski in 1976. This correlation was developed from a database of case histories where both systems had been applied to the same rock mass. However, the scatter around this correlation is significant, with typical variations of plus or minus 10 to 15 RMR points. Other researchers have proposed alternative correlations such as RMR = 9 ln(Q) + 49 (Rutledge and Preston, 1978) and RMR = 13.5 log(Q) + 43 (Abad et al., 1984). These conversions should be used cautiously for preliminary estimates only, because the two systems measure different aspects of rock mass quality and are not directly equivalent.
Absolutely. Despite major advances in numerical modelling techniques such as finite element, finite difference, and discrete element methods, the RMR system remains one of the most widely used tools in geotechnical engineering. It serves several critical roles that numerical modelling cannot replace: it provides a standardized, universally understood language for communicating rock mass quality between engineers and geologists; it is required by many international and national design codes and standards; it produces empirical estimates of engineering properties (cohesion, friction angle, deformation modulus) that are commonly used as inputs to numerical models; and it provides a rapid preliminary assessment that guides the scope and complexity of subsequent analyses. Most major tunnelling, mining, and civil engineering projects worldwide continue to require RMR classification as a fundamental component of their geotechnical investigation and design process.