What is Rock Mass Rating (RMR)?
Rock Mass Rating (RMR) is a geomechanical classification system that assigns a numerical score to a rock mass based on measurable geological and engineering properties. The system provides a standardized method for assessing rock quality and linking that assessment directly to engineering design recommendations for tunnels, slopes, and foundations. This guide explains the fundamentals of RMR for practicing engineers and students. For hands-on calculation, use our free RMR calculator, and for more geotechnical reference material, visit our geotechnical guides hub.
Definition and Purpose
The Rock Mass Rating system evaluates the quality of a rock mass by measuring six parameters: the uniaxial compressive strength (UCS) of intact rock, Rock Quality Designation (RQD), spacing of discontinuities, condition of discontinuities, groundwater conditions, and the orientation of discontinuities relative to the excavation. Each parameter is assigned a rating based on measured values, and the sum of all ratings produces a total RMR score ranging from 0 (extremely poor rock) to 100 (exceptionally good rock).
The total RMR score is then mapped to one of five rock classes, each associated with specific engineering properties and design recommendations. Class I (Very Good Rock, RMR 81-100) requires minimal or no support for typical excavation spans, while Class V (Very Poor Rock, RMR below 20) requires heavy support including steel sets, thick shotcrete, and closely spaced rock bolts. The system provides estimated values for cohesion, friction angle, and stand-up time for each class, allowing engineers to make preliminary design decisions based on the classification result.
The purpose of RMR is to convert complex, three-dimensional geological conditions into a single numerical index that communicates rock quality in a way that is meaningful for engineering design. This enables standardized communication between geologists, geotechnical engineers, designers, and contractors, and provides a documented, auditable basis for support design decisions.
History of the RMR System
The Rock Mass Rating system was developed by Professor Zbigniew T. Bieniawski at the South African Council for Scientific and Industrial Research (CSIR) and first published in 1973. The original system was based on case history data from coal mining and civil tunnelling projects in South Africa. Bieniawski's innovation was to combine multiple rock mass properties into a single additive rating scheme with direct links to support design, something that had not been done systematically before.
The 1973 version included the same basic parameters used today but differed in some rating details and classification boundaries. Bieniawski continued to refine the system based on additional case history data collected from tunnel projects worldwide. An updated version was published in 1979 with revised rating tables and clearer guidelines for assessing discontinuity conditions.
The definitive version was published in 1989 in Bieniawski's book "Engineering Rock Mass Classifications: A Complete Manual for Engineers and Geologists in Mining, Civil, and Petroleum Engineering." This version introduced the five sub-parameters for discontinuity condition assessment (persistence, aperture, roughness, infilling, and weathering), refined the groundwater rating, and provided separate orientation adjustment tables for tunnels, foundations, and slopes. The 1989 version remains the standard reference in current practice and is the version implemented in most commercial and online RMR calculators, including ours.
Since 1989, several extensions and modifications have been proposed by various researchers. Notable developments include the Geological Strength Index (GSI) introduced by Hoek in 1994, which adapted RMR concepts for use with the Hoek-Brown failure criterion, and the Slope Mass Rating (SMR) developed by Romana in 1985 for slope stability applications. However, the basic RMR89 system remains the foundation that these extensions build upon.
Why Rock Mass Classification is Needed
Rock engineering differs fundamentally from structural engineering in materials like steel and concrete because the engineer cannot specify or control the material properties. A structural engineer designs a steel beam by selecting a grade with known yield strength, elastic modulus, and ductility. A geotechnical engineer must work with whatever rock mass exists at the site, characterize its properties through investigation, and design accordingly.
The rock mass at any given site is a complex, heterogeneous material formed by millions of years of geological processes. It contains discontinuities at multiple scales, from microscopic grain boundaries to regional faults spanning kilometers. Its properties vary spatially in ways that cannot be fully characterized even with extensive investigation. Classification systems provide a practical framework for organizing the available information and extracting the engineering-relevant parameters needed for design.
Without classification, engineers would need to rely entirely on numerical modeling or empirical experience for every design decision. While numerical methods are powerful tools for detailed analysis, they require input parameters that must ultimately come from site characterization. Classification systems bridge the gap between raw geological data and engineering design parameters, providing a first-order estimate of rock mass quality that can be used for preliminary design, contractor communication, and construction monitoring.
Intact Rock vs Rock Mass
A fundamental concept in rock engineering is the distinction between intact rock and rock mass. Intact rock refers to the material between discontinuities, tested in the laboratory as a cylindrical specimen without visible fractures. Its properties, including uniaxial compressive strength (UCS), elastic modulus, Poisson's ratio, and tensile strength, can be measured through standardized tests to high precision.
The rock mass, however, includes the intact rock blocks plus all the discontinuities that separate them. Because discontinuities represent planes of weakness with lower shear strength and higher deformability than intact rock, the rock mass is always weaker and more deformable than the intact rock it is composed of. The ratio of rock mass strength to intact rock strength typically ranges from about 0.1 for heavily jointed rock to 0.5 for sparsely jointed rock, and depends strongly on the scale of the engineering problem relative to the discontinuity spacing.
RMR captures this scale effect by including parameters that describe both the intact rock (UCS) and the discontinuities (RQD, spacing, condition, and orientation). The relative weighting of these parameters in the RMR system reflects the engineering community's understanding that discontinuity properties are usually more important than intact rock strength in controlling rock mass behavior. The maximum possible rating for intact rock strength is 15 points, while discontinuity-related parameters account for up to 70 points of the basic rating, plus the orientation adjustment.
Who Uses RMR
RMR is used by a broad range of professionals across the geotechnical and mining industries. Geotechnical engineers use RMR to characterize rock masses for tunnel design, slope stability analysis, and foundation assessment. Mining engineers apply RMR to determine support requirements in underground openings, evaluate stope stability, and plan extraction sequences. Engineering geologists use RMR to communicate rock mass conditions from mapping and core logging to the design team.
The system is referenced in design guidelines and standards from numerous international organizations. The International Tunnelling and Underground Space Association (ITA) includes RMR in its guidelines for tunnel design. National codes in countries including South Africa, India, China, Turkey, and many South American nations reference or require RMR classification for underground construction projects. In many tunnelling contracts, RMR classification at each excavation face is a contractual requirement that determines support installation and may trigger payment adjustments based on rock class.
RMR is also widely used in education, serving as the primary rock mass classification system taught in geotechnical engineering courses at universities worldwide. Its additive structure makes it intuitive to learn and apply, which has contributed to its enduring popularity alongside the more complex but equally established Q-System.
Limitations of RMR
Despite its widespread adoption, the RMR system has recognized limitations that users should understand. First, RMR was developed primarily from case histories of tunnels with spans in the range of 5 to 20 meters. Its support recommendations may not be directly applicable to very large caverns (spans greater than 25 meters) or very small openings (such as raise-bored shafts) without additional analysis.
Second, the 1989 version was calibrated predominantly against data from civil tunnelling projects. While it can be applied to mining openings, the support recommendations were not specifically developed for mining conditions where excavation life, allowable deformation, and economic considerations differ from civil infrastructure. Mining-specific modifications such as the Mining Rock Mass Rating (MRMR) developed by Laubscher address some of these differences.
Third, RMR does not explicitly account for the time-dependent behavior of rock masses, including creep, stress relaxation, and deterioration of discontinuity surfaces due to moisture or chemical processes. In weak rock or rock with swelling minerals, time-dependent effects can significantly influence long-term stability and support requirements beyond what the RMR classification indicates.
Fourth, the orientation adjustment in Parameter 6 has been criticized for being overly simplified, as it attempts to capture the complex three-dimensional relationship between joint orientations and excavation geometry with a single correction factor. For this reason, many engineers supplement RMR with stereographic analysis of discontinuity orientations and kinematic assessments of potential failure modes.
Finally, the quality of any RMR classification depends entirely on the quality of the input data. Inaccurate field measurements, incorrect identification of discontinuity types, or inappropriate selection of parameter values will produce misleading results regardless of the mathematical accuracy of the calculation. RMR is a tool that assists engineering judgment; it does not replace the need for experienced professionals to evaluate site conditions and exercise sound engineering practice.
Frequently Asked Questions
The Rock Mass Rating (RMR) system was developed by Professor Z.T. Bieniawski at the South African Council for Scientific and Industrial Research (CSIR). He first published the system in 1973 based on case histories from South African tunnels and mines. Bieniawski continued to refine the system through the 1970s and 1980s, producing the definitive 1989 version in his book "Engineering Rock Mass Classifications." This 1989 version is the most widely used revision in current geotechnical practice worldwide.
The five RMR rock classes are: Class I (Very Good Rock, RMR 81-100) with cohesion greater than 400 kPa and friction angle greater than 45 degrees; Class II (Good Rock, RMR 61-80) with cohesion 300-400 kPa and friction angle 35-45 degrees; Class III (Fair Rock, RMR 41-60) with cohesion 200-300 kPa and friction angle 25-35 degrees; Class IV (Poor Rock, RMR 21-40) with cohesion 100-200 kPa and friction angle 15-25 degrees; and Class V (Very Poor Rock, RMR less than 20) with cohesion less than 100 kPa and friction angle less than 15 degrees.
Yes, RMR remains one of the two most widely used rock mass classification systems in geotechnical engineering, alongside the Q-System. It is referenced in international guidelines from the International Tunnelling Association, incorporated into design codes across many countries, and required by most tunnelling and mining specifications. The 1989 version continues to be the standard reference. Extensions such as GSI (Geological Strength Index) by Hoek and SMR (Slope Mass Rating) by Romana have been built on the RMR framework for specialized applications.
Intact rock strength is the strength of a laboratory specimen without visible fractures, measured through tests like uniaxial compressive strength (UCS). Rock mass strength is the strength of the entire in-situ rock body including all discontinuities. Rock mass strength is always lower than intact rock strength because joints, bedding planes, and faults create planes of weakness. The ratio between the two typically ranges from 0.1 in heavily jointed rock to 0.5 in sparsely jointed rock, depending on the degree of fracturing and the scale of the engineering problem.