RMR Parameters Explained: All 6 Bieniawski 1989 Rating Criteria
The Rock Mass Rating (RMR) classification system evaluates rock mass quality through six parameters that together capture the most important factors controlling the engineering behaviour of jointed rock. The first five parameters produce the basic RMR score (maximum 100 points), and the sixth parameter applies an adjustment for the orientation of discontinuities relative to the engineering structure. This page provides a detailed explanation of each parameter including what it measures, why it matters, the complete rating table, how to measure or estimate it in the field, and the most common mistakes practitioners make during assessment.
Parameter 1: Uniaxial Compressive Strength (UCS) — Max 15 Points
What It Measures
The uniaxial compressive strength (UCS) quantifies the strength of the intact rock material when loaded in unconfined compression. It represents the maximum stress that a cylindrical rock specimen can withstand before failure when compressed along its axis without lateral confinement. UCS values range from less than 1 MPa for extremely weak rocks like weathered mudstone to over 250 MPa for strong igneous rocks like fresh granite or basalt.
Why It Matters
Intact rock strength determines the capacity of the rock material between discontinuities to resist crushing, particularly around excavation openings where stress concentrations develop. In massive rock with few discontinuities, UCS directly controls the rock mass behaviour. Even in well-jointed rock, the intact strength provides an upper bound on the strength of individual blocks and influences the shear resistance along rough discontinuity surfaces through asperity damage.
Rating Table
| UCS (MPa) | Point Load Index Is(50) (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 |
How to Measure in the Field
The most reliable method is laboratory uniaxial compression testing on prepared cylindrical specimens following ISRM suggested methods (length-to-diameter ratio of 2.5:1, diameter at least 54 mm). Where laboratory testing is impractical, UCS can be estimated from the Point Load Strength Index (Is50) by multiplying by a conversion factor, typically 20 to 25 depending on rock type. For preliminary assessments, the Schmidt hammer rebound number can provide a field estimate using published correlations. For very weak rocks (UCS below 25 MPa), the point load test is unreliable and UCS should be determined by laboratory testing or estimated from descriptions using the ISRM simple field identification chart (such as indentation by thumbnail, knife, or geological hammer).
Common Mistakes
- Using an inappropriate point load to UCS conversion factor. The factor varies significantly with rock type (15 for some shales, up to 25 for granites).
- Testing weathered specimens that are not representative of the intact rock material between discontinuities.
- Failing to account for anisotropy in foliated or bedded rocks, where UCS can vary by a factor of 2 or more with loading direction.
- Not correcting point load index values to the standard 50 mm diameter equivalent (Is50).
Parameter 2: Rock Quality Designation (RQD) — Max 20 Points
What It Measures
Rock Quality Designation (RQD) is a modified core recovery index that measures the percentage of intact core pieces longer than 100 mm (10 cm) in a core run. It was developed by Don Deere at the University of Illinois in 1964 as a quantitative index of rock mass fracturing observed in drill core. RQD ranges from 0% (all pieces shorter than 100 mm, indicating intensely fractured rock) to 100% (all pieces longer than 100 mm, indicating massive or sparsely jointed rock).
Why It Matters
RQD provides a direct measure of the degree of fracturing in a rock mass, which is one of the primary factors controlling deformability, permeability, and potential for block instability. Rock masses with low RQD values tend to have more degrees of freedom for block movement and higher water conductivity. RQD is also a component of other classification systems including the Q-system (where it appears as the numerator of the first quotient RQD/Jn).
Rating Table
| RQD (%) | Quality | Rating |
|---|---|---|
| 90 – 100 | Excellent | 20 |
| 75 – 90 | Good | 17 |
| 50 – 75 | Fair | 13 |
| 25 – 50 | Poor | 8 |
| < 25 | Very Poor | 3 |
How to Measure in the Field
RQD is measured directly from diamond drill core by summing the lengths of all intact pieces equal to or greater than 100 mm and dividing by the total core run length. Only natural fractures count as breaks; mechanical breaks caused by the drilling process (identified by fresh, rough surfaces with no staining or infilling) should be fitted back together and counted as intact core. The core run length should be between 1.0 and 1.5 m for meaningful results. Where borehole data is unavailable, RQD can be estimated from surface mapping using the volumetric joint count (Jv) with the relationship RQD = 115 - 3.3 Jv, where Jv is the total number of discontinuities per cubic metre. The result is capped between 0% and 100%.
Common Mistakes
- Counting mechanical breaks caused by drilling, handling, or the core splitting process as natural fractures, which artificially reduces RQD.
- Measuring along the core axis instead of along the centreline when core is broken at angles oblique to the axis.
- Using a single short core run as representative of the entire zone; RQD should be averaged over representative intervals.
- Not recognizing that RQD is insensitive to spacing changes when all spacings are above 100 mm (RQD = 100% whether spacing is 110 mm or 2 m).
Parameter 3: Spacing of Discontinuities — Max 20 Points
What It Measures
This parameter quantifies the average distance between adjacent discontinuities within the rock mass. Discontinuities include joints, bedding planes, foliations, faults, and any other planar features along which the rock has negligible tensile strength. The spacing directly determines the block size in the rock mass, which in turn controls deformability, permeability, and the potential for kinematic instability.
Why It Matters
Closely spaced discontinuities produce small blocks that can ravel or fall from excavation surfaces, increase rock mass deformability, and create multiple flow paths for groundwater. Widely spaced discontinuities produce large, interlocking blocks that tend to be self-supporting. The spacing parameter complements RQD by providing additional resolution in describing the fracture intensity: while RQD becomes insensitive when all spacings exceed 100 mm, the spacing parameter distinguishes between 200 mm and 2 m spacings.
Rating Table
| 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 |
How to Measure in the Field
Discontinuity spacing is measured along scan lines set up on exposed rock faces or from borehole core logs. For surface mapping, stretch a measuring tape along the rock face perpendicular to the dominant discontinuity set and record the distance between successive discontinuity intersections. When multiple sets are present, measure each set separately. The rating should be based on the set with the smallest average spacing, as this controls the minimum block dimension and therefore the most unfavourable rock mass behaviour. In core, measure the distance between natural fractures along the core axis. Take care to distinguish between true spacing (perpendicular to the discontinuity set) and apparent spacing (measured along the scan line or borehole), applying a correction factor if necessary.
Common Mistakes
- Averaging spacings across multiple discontinuity sets rather than identifying and rating the most closely spaced individual set.
- Not accounting for measurement bias due to the orientation of the scan line relative to the discontinuity sets (sets parallel to the scan line will be underrepresented).
- Confusing true spacing with apparent spacing, especially in inclined boreholes intersecting steeply dipping discontinuities.
Parameter 4: Condition of Discontinuities — Max 30 Points
What It Measures
This parameter assesses the surface characteristics and condition of the discontinuities, which directly control the shear strength available along these planes of weakness. In the Bieniawski 1989 system, condition is evaluated through five sub-parameters: persistence (trace length or continuity), aperture (opening width), roughness of the discontinuity surface, type and thickness of infilling material, and degree of weathering of the discontinuity walls. Each sub-parameter receives a rating from 0 to 6, and the five sub-ratings are summed to produce the total condition rating (maximum 30).
Why It Matters
Discontinuity condition is the most heavily weighted parameter in the RMR system (30 out of 100 points) because it has the greatest influence on rock mass shear strength. A rough, tight, unweathered joint with no infilling has much higher shear resistance than a smooth, open, clay-filled discontinuity. The shear strength along discontinuities controls the stability of blocks and wedges in excavation roofs and walls, the sliding resistance of rock slopes, and the bearing capacity of foundations on jointed rock.
Sub-parameter Rating Tables
4a. Persistence (Trace Length)
| Persistence | Rating |
|---|---|
| < 1 m | 6 |
| 1 – 3 m | 4 |
| 3 – 10 m | 2 |
| 10 – 20 m | 1 |
| > 20 m | 0 |
4b. Aperture
| Aperture | Rating |
|---|---|
| None (closed) | 6 |
| < 0.1 mm | 5 |
| 0.1 – 1 mm | 4 |
| 1 – 5 mm | 1 |
| > 5 mm | 0 |
4c. Roughness
| Surface Roughness | Rating |
|---|---|
| Very rough | 6 |
| Rough | 5 |
| Slightly rough | 3 |
| Smooth | 1 |
| Slickensided | 0 |
4d. Infilling
| Infilling Type & Thickness | Rating |
|---|---|
| None | 6 |
| Hard filling < 5 mm | 4 |
| Hard filling > 5 mm | 2 |
| Soft filling < 5 mm | 2 |
| Soft filling > 5 mm | 0 |
4e. Weathering
| Weathering Grade | Rating |
|---|---|
| Unweathered | 6 |
| Slightly weathered | 5 |
| Moderately weathered | 3 |
| Highly weathered | 1 |
| Decomposed | 0 |
How to Measure in the Field
Each sub-parameter should be assessed on the most representative and accessible discontinuity surfaces. Persistence is estimated by measuring the visible trace length on exposed faces. Aperture is measured using feeler gauges or calipers at multiple locations along a discontinuity. Roughness is assessed visually and by touch, comparing against ISRM reference profiles or using a Barton comb to measure the Joint Roughness Coefficient (JRC). Infilling is identified by examining the material between discontinuity walls and measuring its thickness. Weathering is assessed by examining the discolouration, weakening, and decomposition of rock adjacent to the discontinuity surface, following ISRM weathering grade descriptions (W1 through W5).
Common Mistakes
- Assessing roughness only at the small scale (millimetre-scale asperities) without considering large-scale waviness that also contributes to shear resistance.
- Confusing soft infilling with highly weathered wall rock; the distinction matters because infilling shear strength may be different from weathered rock strength.
- Underestimating persistence because only partial trace lengths are visible on the exposure. True persistence is typically greater than the observed trace length.
- Assigning aperture ratings based on surface observations where stress relief has opened joints beyond their in-situ condition.
Parameter 5: Groundwater Conditions — Max 15 Points
What It Measures
This parameter evaluates the presence and influence of groundwater within the rock mass. Water in discontinuities reduces effective normal stresses through pore pressure, can soften clay infillings, promotes weathering and erosion of joint surfaces, and generates hydrostatic forces that act on excavation surfaces. The groundwater parameter can be assessed using one of three approaches: the measured inflow rate per 10 m of tunnel length, the ratio of joint water pressure to major principal stress (pw/sigma1), or a general qualitative description of the moisture condition.
Why It Matters
Groundwater is one of the most significant factors affecting rock mass behaviour in underground excavations, slopes, and foundations. Even moderate water pressures can dramatically reduce the effective shear strength along discontinuities, triggering block falls, wedge failures, or progressive ravelling. In clay-bearing rock masses, water ingress can cause swelling and softening of infilling materials, further degrading rock mass quality over time. In cold climates, freeze-thaw cycles in water-bearing discontinuities accelerate weathering and increase apertures.
Rating Table
| Inflow per 10 m Tunnel | 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 |
How to Measure in the Field
During tunnelling, inflow rates can be measured directly by collecting water from defined sections of the tunnel in containers over a known time period. For pre-construction assessment, groundwater conditions are typically estimated from piezometer data, water level observations in boreholes, or pump-in/out tests. The joint water pressure ratio requires measurement of both pore water pressure (from piezometers) and the in-situ major principal stress. For surface exposures and preliminary assessments, the general qualitative description (dry, damp, wet, dripping, or flowing) is the most practical approach. Describe the condition at the time of mapping, but consider seasonal variations and the worst-case scenario for design.
Common Mistakes
- Assessing groundwater conditions during dry season site visits and not accounting for higher water tables and increased pressures during wet seasons.
- Assuming dry conditions at depth based on dry surface exposures; groundwater conditions underground may be very different from surface observations.
- Not considering the effect of construction on groundwater, such as the intersection of aquifers during tunnelling or the creation of new flow paths from blasting.
Parameter 6: Orientation Adjustment
What It Measures
The orientation adjustment accounts for the relationship between the strike and dip of the dominant discontinuity set and the orientation of the engineering structure (tunnel axis, slope face, or foundation surface). Certain geometric relationships between joints and structures are favourable (where discontinuities dip away from the excavation or slope face), while others are highly unfavourable (where discontinuities dip into the excavation or daylight on the slope face at angles conducive to sliding).
Why It Matters
The orientation of discontinuities relative to an excavation can transform an otherwise competent rock mass into an unstable one. A rock mass with high basic RMR may still experience large-scale wedge failures or plane failures if the discontinuity orientations are critically adverse. The orientation adjustment can reduce the basic RMR by up to 60 points for slopes, reflecting the reality that kinematically controlled failures in slopes are far more sensitive to joint orientation than tunnel instabilities.
Adjustment Table
| 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 |
How to Assess
For tunnels, Bieniawski provided qualitative guidelines based on the strike direction relative to the tunnel axis and the dip angle and direction. Driving perpendicular to the strike with dip 45–90 degrees is very favourable when dipping with the drive direction, while driving parallel to the strike with dip 45–90 degrees is very unfavourable. For slopes, engineers typically assess the orientation by comparing the dip direction and angle of the discontinuity set with the dip direction and angle of the slope face using kinematic analysis (stereographic projection). For foundations, the assessment considers whether discontinuities dip towards the loaded area in orientations that could facilitate sliding under applied loads.
Common Mistakes
- Using tunnel orientation adjustments when assessing a slope or foundation, and vice versa. The adjustment tables differ significantly between application types.
- Considering only one discontinuity set when multiple sets may produce different kinematic failure modes.
- Not performing proper kinematic analysis with stereographic projection to determine the actual favourability of orientations.
How the Parameters Combine
The RMR system uses a straightforward additive approach. Ratings for the first five parameters are summed to produce the Basic RMR (ranging from a minimum of approximately 8 to a maximum of 100). The orientation adjustment is then subtracted to produce the Adjusted RMR, which is used for rock class determination and engineering property estimation. The additive approach means that a very low score on one parameter can be partially compensated by high scores on others, which is appropriate because a rock mass with strong intact rock but poor discontinuity conditions behaves differently from one with weak rock but tight joints. However, practitioners should recognize that the single RMR number is a simplification: two rock masses with the same total RMR but very different individual parameter scores may behave differently in practice, particularly if one has a single dominant weakness (such as high groundwater) while the other has uniformly moderate conditions.
Tips for Consistent Parameter Assessment in the Field
- Define geotechnical domains first: Before assigning ratings, divide the site into zones of similar geological character (same lithology, similar fracture intensity, same structural domain). Classify each domain separately rather than trying to apply a single RMR to the entire site.
- Use multiple data sources: Combine information from borehole core logs, surface exposures, geophysical surveys, and excavation face mapping to develop a complete picture. No single data source provides all the information needed for reliable classification.
- Photograph and document everything: Take photographs of core boxes with scale bars, outcrop exposures with measuring tapes, and discontinuity surfaces with reference objects. This allows subsequent review and reduces the risk of misclassification.
- Apply the rating tables strictly: Resist the temptation to interpolate between rating categories. Bieniawski designed the system with discrete intervals, and interpolation introduces inconsistency between practitioners. If a measured value falls on a boundary, assign the lower (more conservative) rating.
- Have multiple people classify independently: When possible, have two or more geotechnical professionals classify the same rock mass independently and compare results. Differences highlight areas where the descriptions or measurements are ambiguous and need clarification.
- Consider seasonal and temporal variation: Groundwater conditions and weathering states can change over time. Document the conditions at the time of assessment and note whether the observations represent typical, dry-season, or wet-season conditions.
Frequently Asked Questions
The condition of discontinuities parameter has the highest maximum rating at 30 points out of the 100-point basic RMR total. This reflects the dominant influence that discontinuity surface characteristics have on rock mass shear strength and deformability. The five sub-parameters (persistence, aperture, roughness, infilling, and weathering) each contribute up to 6 points. By comparison, RQD and discontinuity spacing each contribute up to 20 points, while UCS and groundwater each contribute up to 15 points. This weighting recognises that the mechanical behaviour of a jointed rock mass is controlled more by the nature of the discontinuities than by the intact rock properties.
Yes, RQD can be estimated from surface exposures using the volumetric joint count (Jv), which represents the total number of discontinuities per cubic metre of rock mass. The correlation proposed by Palmstrom in 1982 is RQD = 115 - 3.3 Jv, where the result is capped at a maximum of 100% and a minimum of 0%. To measure Jv, count the number of discontinuities intersecting three mutually perpendicular scan lines of known length, sum the linear frequencies for all sets, and calculate the volumetric count. This method provides reasonable estimates for most rock types but tends to underestimate RQD in rocks with highly variable spacing.
In highly fractured or intensely jointed rock masses, individual discontinuity surfaces may be difficult to isolate and measure. Focus on the most prominent or continuous discontinuity set for the condition assessment. Where surfaces are accessible, measure persistence along the most visible traces, estimate aperture from exposed joints, use a straight edge or Barton comb to assess roughness profiles, identify any infilling material in accessible openings, and evaluate the degree of weathering on joint walls using ISRM grade descriptors. If the rock mass is so heavily fractured that individual joints cannot be distinguished, assign conservative ratings reflecting the generally poor condition implied by the intense fracturing.
Aperture refers to the perpendicular distance between the adjacent rock walls of a discontinuity in the absence of filling material. It describes how "open" the joint is. Infilling refers to material deposited or formed between the discontinuity walls, such as clay gouge, calcite, quartz veins, silt, or crushed rock fragments. A discontinuity may be open with no infilling (rated by aperture only), completely filled (rated by infilling type and thickness), or partially filled (rated by both). In the RMR system these are assessed as separate sub-parameters because they have distinct engineering effects: aperture governs permeability and the potential for block displacement, while infilling type and thickness control the available shear strength along the discontinuity plane.
The Bieniawski 1989 system is designed to classify the general quality of a rock mass domain, not isolated worst-case features. Use average or representative values for the domain being classified. For discontinuity spacing, use the average spacing of the most closely spaced set within the domain. For RQD, use the average over representative core runs, not the single worst run. For discontinuity condition, assess the typical conditions of the most prominent set. However, if a specific critical feature (such as a fault zone, shear zone, or exceptionally weak layer) could control engineering behaviour, classify that feature as a separate domain with its own RMR. This approach ensures that both the general rock mass quality and specific critical features are properly characterised for design.