RMR Parameters: Field Assessment Guide
The Bieniawski 1989 Rock Mass Rating system evaluates rock mass quality through six parameters, each measured or estimated from field observations, borehole data, or laboratory testing. The accuracy of any RMR classification depends directly on the quality of these measurements. This guide explains how each parameter is assessed in the field, what tools and techniques are used, and how measured values translate to RMR ratings. For the underlying theory behind the RMR system, see our guide on what Rock Mass Rating is, and for more reference material, visit our geotechnical guides hub.
Overview of the Six Parameters
The six RMR parameters are weighted differently to reflect their relative importance in controlling rock mass behavior. The table below summarizes each parameter, its maximum rating, and the type of measurement required.
| Parameter | Max Rating | Measurement Method |
|---|---|---|
| 1. Uniaxial Compressive Strength (UCS) | 15 | Lab UCS test, point load test, or Schmidt hammer |
| 2. Rock Quality Designation (RQD) | 20 | Core logging or volumetric joint count |
| 3. Spacing of Discontinuities | 20 | Tape measurement on face or core |
| 4. Condition of Discontinuities | 30 | Visual inspection and measurement of 5 sub-parameters |
| 5. Groundwater Conditions | 15 | Observation, piezometer data, or inflow measurement |
| 6. Orientation Adjustment | 0 to -60 | Compass measurement of strike and dip vs. excavation geometry |
The basic RMR (before orientation adjustment) is the sum of Parameters 1 through 5, with a maximum possible score of 100. The orientation adjustment in Parameter 6 is always zero or negative, reducing the basic RMR to produce the adjusted RMR that determines the final rock class. For tunnelling applications, the maximum reduction from orientation is 12 points. For slopes, it can be as much as 60 points, reflecting the much greater sensitivity of slope stability to adverse joint orientations.
Measuring Uniaxial Compressive Strength in the Field
Uniaxial compressive strength (UCS) represents the strength of the intact rock material between discontinuities. In the RMR system, UCS is rated from 0 points (less than 1 MPa) to 15 points (greater than 250 MPa). There are three common methods for determining UCS, each appropriate for different project stages and accuracy requirements.
Schmidt Hammer (Rebound Hammer)
The Schmidt hammer is the most widely used field tool for estimating UCS. The L-type hammer is standard for rock testing. To obtain a reliable measurement, prepare a smooth, flat rock surface by removing weathered material with a geological hammer. Apply at least 20 hammer impacts to the prepared surface, each separated by at least one plunger diameter. Record all rebound values, discard the lowest 50%, and calculate the mean of the upper 50%. Convert the mean rebound number to UCS using the published correlation chart for the specific hammer type, accounting for the hammer orientation (horizontal, upward, or downward) and the rock density.
Schmidt hammer results are approximate, with typical scatter of plus or minus 20-30% compared to laboratory UCS values. They are most reliable for homogeneous, medium to high strength rocks (UCS 25-200 MPa) and less reliable for very weak, very strong, or highly anisotropic rocks. For RMR purposes, this level of accuracy is usually sufficient because the UCS rating ranges are broad (for example, 50-100 MPa is a single category worth 7 points).
Point Load Test
The point load test (ASTM D5731) can be performed in the field using a portable point load testing machine. Irregular rock lumps or core pieces are loaded between two conical platens until failure. The point load strength index Is(50) is calculated from the failure load and specimen dimensions, corrected to a standard diameter of 50 mm. UCS is estimated using the empirical correlation UCS = k x Is(50), where k typically ranges from 20 to 25 for most rock types, with a value of 22 to 24 commonly adopted in practice.
The point load test is more accurate than the Schmidt hammer and can be applied to irregular specimens that cannot be used for standard UCS testing. It requires a minimum of 10 valid tests per geological unit, with the two highest and two lowest results discarded and the mean of the remaining values reported. The test is particularly useful for weak to medium strength rocks where Schmidt hammer results are less reliable.
Laboratory UCS Testing
Laboratory UCS testing (ASTM D7012 or ISRM Suggested Method) provides the most accurate measurement of intact rock strength. Cylindrical specimens with a height-to-diameter ratio of 2.0 to 2.5 are loaded axially at a controlled rate until failure. The UCS is calculated as the peak load divided by the cross-sectional area. A minimum of five specimens per geological unit is recommended, with the mean value used for RMR classification.
Laboratory testing requires prepared specimens transported to a testing facility, which introduces a time delay that may not be acceptable during tunnel construction. For this reason, field methods (Schmidt hammer or point load test) are typically used for real-time RMR classification at the tunnel face, with laboratory results used to calibrate and verify the field estimates.
Determining RQD from Core
Rock Quality Designation (RQD) is measured from diamond drill core of NX size (54.7 mm diameter) or larger. The procedure involves laying out the core from a single run on a logging table, identifying all natural breaks (excluding mechanical breaks caused by drilling), and measuring the length of each intact piece along the core centerline. RQD is then calculated as the sum of all piece lengths greater than or equal to 100 mm divided by the total core run length, expressed as a percentage.
The critical skill in RQD measurement is correctly distinguishing natural fractures from mechanical breaks. Natural fractures typically show staining, mineral coatings, slickensides, or weathered surfaces. Mechanical breaks have fresh, unweathered surfaces that fit together cleanly. When in doubt, the piece should be treated as naturally broken, which is the conservative approach.
RQD ratings in the Bieniawski 1989 system range from 3 points (RQD less than 25%) to 20 points (RQD 90-100%). When borehole core is not available, RQD can be estimated from the volumetric joint count Jv using Palmstrom's correlation RQD = 115 - 3.3 Jv, or from scanline mapping on exposed faces. Our RQD calculator implements both the core-based and volumetric methods.
Measuring Discontinuity Spacing
Discontinuity spacing is the perpendicular distance between adjacent discontinuities of the same set, measured along a line perpendicular to the discontinuity planes. In practice, spacing is often measured along a scanline (a measuring tape stretched across an exposed rock face) and then corrected for the angle between the scanline and the normal to the discontinuity set using the Terzaghi correction.
For RMR classification, the parameter refers to the spacing of the most significant (most closely spaced or most continuous) discontinuity set. Ratings range from 5 points (spacing less than 60 mm) to 20 points (spacing greater than 2 meters). When multiple discontinuity sets are present with different spacings, the spacing of the set that is most adverse to stability should be used.
In borehole core, discontinuity spacing can be estimated from the frequency of natural fractures per meter of core, corrected for the angle between the borehole axis and the discontinuity normals. This correction is essential because a borehole drilled parallel to a joint set will intersect few joints regardless of their actual spacing.
Assessing Discontinuity Condition
Condition of discontinuities is the most heavily weighted parameter in the RMR system, contributing up to 30 points to the total rating. In the 1989 version, it is assessed through five sub-parameters, each rated from 0 to 6 points. This breakdown provides a more detailed and reproducible assessment than the single-value approach used in earlier RMR versions.
Persistence (Continuity)
Persistence describes the trace length of the discontinuity as observed on exposed surfaces. Very low persistence (less than 1 meter trace length) receives 6 points, while very high persistence (greater than 20 meters) receives 0 points. Highly persistent discontinuities are more adverse because they provide continuous planes of weakness through the rock mass. In practice, persistence is measured by tracing the visible extent of the discontinuity on the tunnel face or rock outcrop.
Aperture (Opening Width)
Aperture is the perpendicular distance between the rock walls of a discontinuity. Tight joints with no visible opening receive 6 points, while wide apertures greater than 5 mm receive 0 points. Aperture is measured using feeler gauges for tight joints or a ruler for wider openings. The aperture rating reflects the fact that open discontinuities have lower shear strength, higher permeability, and greater deformability than closed joints.
Roughness
Roughness describes the surface texture of the discontinuity walls at both small scale (roughness) and large scale (waviness). Very rough surfaces receive 6 points because asperities provide frictional resistance and dilatancy during shearing. Smooth or slickensided surfaces receive 0 to 1 points. Roughness is assessed visually and by running a finger or straight edge across the joint surface. ISRM provides standard roughness profiles (JRC profiles) that can be used for comparison, ranging from JRC 0-2 (smooth planar) to JRC 18-20 (rough undulating).
Infilling
Infilling refers to any material between the discontinuity walls, including clay gouge, silt, sand, breccia, or mineral precipitates such as calcite or quartz. No infilling (clean joint) receives 6 points. Hard infilling less than 5 mm thick receives 4 points, while soft infilling (clay) greater than 5 mm thick receives 0 points. The infilling material and thickness are assessed by opening the discontinuity where possible and examining the material with a hand lens. Clay infilling is particularly adverse because it reduces shear strength and may swell when exposed to water.
Weathering
Weathering describes the degree of alteration of the rock material on the discontinuity walls. Unweathered walls receive 6 points, while decomposed walls receive 0 points. Weathering is assessed by examining the color, hardness, and mineralogy of the joint wall material compared to fresh intact rock. The ISRM weathering grades (W1 fresh through W5 completely decomposed) provide a standardized reference scale. Weathered joint walls have reduced frictional strength because the original mineral grains have been altered to weaker products, often including clay minerals.
Evaluating Groundwater Conditions
Groundwater conditions are rated from 0 to 15 points in the Bieniawski 1989 system. The rating can be based on one of three measures: the inflow rate per 10-meter tunnel length, the joint water pressure expressed as a ratio to the major principal stress, or a general qualitative description of conditions.
For tunnel applications, the most practical approach during construction is to estimate the inflow rate at the tunnel face. A completely dry excavation receives 15 points. Damp conditions (minor seepage with no measurable flow) receive 10 points. Wet conditions with steady-state flow of 10 to 25 liters per minute per 10-meter length receive 7 points. Dripping with flow of 25 to 125 liters per minute receives 4 points. Flowing conditions with inflow greater than 125 liters per minute receive 0 points.
During the design stage when borehole data is available, the piezometric head measured in standpipe piezometers or vibrating wire piezometers can be used to calculate the joint water pressure ratio. A ratio of zero (dry) corresponds to 15 points, while a ratio greater than 0.5 corresponds to 0 points. In practice, groundwater conditions often change seasonally and during construction (due to drainage), so the assessment should reflect the anticipated worst-case conditions during the critical period of excavation and support installation.
Applying the Orientation Adjustment
The orientation adjustment accounts for the geometric relationship between the dominant discontinuity set and the excavation geometry. Unlike the first five parameters which always add positive ratings, Parameter 6 provides a zero or negative adjustment that reduces the basic RMR to reflect unfavorable orientations.
For tunnel applications, the assessment considers two factors: the strike of the critical discontinuity relative to the tunnel axis (perpendicular or parallel), and the dip angle and direction (dipping with or against the drive direction). The most favorable orientation for tunnels is when the dominant joint set strikes perpendicular to the tunnel axis and dips at 45 to 90 degrees in the direction of the drive, receiving an adjustment of 0 points. The most unfavorable orientation is when joints strike parallel to the tunnel axis with a dip of 45 to 90 degrees, receiving -12 points, because this geometry creates wedge failures in the roof.
For slopes, the orientation adjustment is much more severe because the geometric relationship between joints and the slope face directly controls the failure mode. Joints dipping out of a slope face at angles similar to the slope angle create conditions for planar or wedge failure, and the adjustment can reach -60 points. This large adjustment reflects the field observation that slope failures are overwhelmingly controlled by adverse discontinuity orientations.
For foundations, the adjustment ranges from 0 to -25 points and considers whether discontinuities dip into the foundation in a way that could cause bearing capacity failure or differential settlement.
The strike and dip of all significant discontinuity sets should be measured using a geological compass (Brunton or Clar type) with at least 25 to 30 measurements per set to establish a reliable mean orientation. Stereonet analysis is then used to identify the critical set and evaluate its geometric relationship to the excavation.
Field Data Recording Best Practices
Consistent, systematic field data recording is essential for reliable RMR classification. The following practices are recommended based on established geotechnical mapping standards from ISRM and common industry practice.
Use standardized data recording sheets that list all six parameters with their rating options. Pre-printed field sheets reduce the chance of omitting a parameter and ensure that all observers record data in the same format. Many organizations now use tablet-based digital logging systems that enforce data completeness and calculate the RMR automatically.
Record the location of each classification station precisely, using chainage (distance along the tunnel axis), coordinates, or borehole depth intervals. In tunnels, RMR classification is typically performed every 5 to 10 meters of advance, or whenever ground conditions change noticeably. At each station, photograph the face before and after marking up discontinuities to create a permanent visual record that can be reviewed later.
Document the measurement method used for each parameter. For UCS, record whether the rating is based on Schmidt hammer, point load test, or laboratory testing, along with the raw measurements (rebound numbers, Is(50) values, or laboratory test results). For RQD, record whether it came from core logging, scanline mapping, or Jv estimation. This documentation allows the classification to be audited and revised if better data becomes available.
When conditions are variable across the face, characterize the range rather than reporting a single average. For example, a tunnel face might expose two geological domains with different RMR values. Document both and use the lower value for support design, or design different support for different zones of the face if the contract allows it.
Compare your classifications with those of other team members periodically to ensure consistency. Inter-rater variability is a known issue in rock mass classification, and regular calibration exercises help maintain consistency across a project. Studies have shown that experienced observers typically agree within 5 to 10 RMR points, while less experienced observers may vary by 15 points or more for the same face.
Frequently Asked Questions
For field estimation of UCS, the Schmidt hammer (L-type rebound hammer) is the most common tool. Prepare a smooth rock surface, apply at least 20 impacts, discard the lowest 50% of rebound readings, and average the upper 50%. Convert the mean rebound number to UCS using published correlation charts. Alternatively, the portable point load test (ASTM D5731) can be used on irregular rock lumps, converting the Is(50) index to UCS using the factor UCS = 22-25 x Is(50). Laboratory UCS testing (ASTM D7012) is the most accurate but requires prepared cylindrical specimens and a testing facility.
The five sub-parameters in the Bieniawski 1989 system are: (1) Persistence (trace length), rated 0 to 6 points; (2) Aperture (opening width), rated 0 to 6 points; (3) Roughness (surface texture), rated 0 to 6 points; (4) Infilling (material between joint walls), rated 0 to 6 points; and (5) Weathering (degree of wall alteration), rated 0 to 6 points. Together these five sub-parameters contribute up to 30 points, making discontinuity condition the single most influential parameter in the RMR system.
The orientation adjustment is determined by measuring the strike and dip of the critical discontinuity set relative to the excavation direction. For tunnels, the adjustment ranges from 0 (very favorable) to -12 (very unfavorable), based on whether the strike is perpendicular or parallel to the tunnel axis and whether the dip direction is with or against the drive direction. For foundations, adjustments range from 0 to -25. For slopes, adjustments range from 0 to -60. The assessment requires compass measurements and ideally stereonet analysis to identify the most critical joint set orientation.
A basic field kit for RMR assessment includes: a geological compass (Brunton or Silva type) for strike and dip measurements, a measuring tape for discontinuity spacing and persistence, a Schmidt hammer (L-type) for UCS estimation, a geological hammer for assessing weathering, a hand lens for examining joint surfaces and infilling, feeler gauges for measuring aperture, a ruler or calipers for wider openings, standardized field data recording sheets or a digital logging tablet, and a camera for photographic documentation. For core-based assessment, add a core logging table and measuring equipment.