How long does it take to calculate RMR for a tunnel section?

For an experienced geotechnical engineer or engineering geologist, a single RMR assessment for a defined rock mass domain typically takes 15 to 30 minutes of field observation and measurement, plus an additional 10 to 15 minutes for data compilation and rating assignment. However, the total time depends on the accessibility of the rock exposure, the complexity of the geological conditions, and whether laboratory test data is already available for UCS. During tunnel construction, face mapping and RMR classification is typically performed for each blast round advance, which means the process must be efficient enough to keep pace with excavation rates.

What should I do if my field data falls between two rating categories?

The Bieniawski 1989 RMR system uses discrete rating categories rather than continuous interpolation. If your measured value falls exactly on the boundary between two categories, the standard practice is to assign the lower (more conservative) rating. For example, if your measured discontinuity spacing is exactly 200 mm, which falls on the boundary between the 60–200 mm range (rating 8) and the 200–600 mm range (rating 10), you should assign a rating of 8. Some practitioners record both the measured value and the assigned rating so that sensitivity analyses can be performed later. Avoid interpolating between categories, as this introduces inconsistency when comparing classifications made by different people.

Can I calculate RMR from borehole data alone without surface exposures?

Yes, RMR can be estimated from borehole data, although with some limitations. UCS can be determined from laboratory testing of core samples or estimated from point load tests on core. RQD is measured directly from core. Discontinuity spacing can be assessed from the core log fracture frequency. Discontinuity condition can be partially assessed from core, although roughness and persistence are difficult to evaluate from borehole data alone and may need to be estimated conservatively. Groundwater conditions can be assessed from piezometer data and water loss tests. The orientation adjustment requires knowledge of discontinuity orientations, which can be obtained from oriented core, televiewer logs, or acoustic/optical borehole imaging. In general, borehole-only RMR tends to be slightly conservative compared to assessments that include surface mapping data.

How many RMR measurements should I take for a project?

The number of RMR assessments depends on the geological variability of the site and the project requirements. As a general guideline, at least one RMR classification should be performed for each distinct geotechnical domain identified during the site investigation. A geotechnical domain is a zone with reasonably uniform rock mass characteristics including the same lithology, similar fracture intensity, and comparable weathering grade. For tunnel projects, RMR is typically assessed at each face mapping station, which may be every blast round (3–5 m advance) or at defined intervals along the alignment. For slope assessments, classify each geological unit and structural domain separately. A minimum of three independent assessments per domain is recommended to evaluate variability and establish confidence in the assigned rock class.

How to Calculate Rock Mass Rating: Complete Step-by-Step Guide

Calculating the Rock Mass Rating (RMR) using the Bieniawski 1989 system is a systematic process that produces consistent, repeatable results when followed carefully. This guide walks you through every step of the calculation, from gathering field data to determining the final rock class. Whether you are performing face mapping during tunnel construction, logging borehole core in the office, or assessing a rock slope exposure, this procedure will ensure that your RMR classification is thorough and defensible.

What You Need Before You Start

Before beginning an RMR assessment, ensure that you have the following data, equipment, and reference materials available. Missing any of these items will result in incomplete or inaccurate classifications.

Field Data Checklist

Item	Source	Required For
UCS test results or point load index data	Laboratory testing or field point load apparatus	Parameter 1
Borehole core logs or surface scan line data	Core logging or outcrop mapping	Parameter 2 (RQD)
Discontinuity spacing measurements	Scan line survey or core log	Parameter 3
Discontinuity condition descriptions	Direct observation of joint surfaces	Parameter 4 (5 sub-params)
Groundwater observations or piezometer data	Field observation, piezometers, or pump tests	Parameter 5
Discontinuity orientation data (strike/dip)	Compass/clinometer measurements or oriented core	Parameter 6 (adjustment)
Engineering structure orientation	Design drawings (tunnel axis, slope face, foundation)	Parameter 6 (adjustment)

Equipment Needed

Geological compass with clinometer for measuring strike and dip
Measuring tape (minimum 30 m) for scan line surveys and spacing measurements
Feeler gauges or calipers for measuring aperture
Schmidt hammer for field UCS estimation (optional but recommended)
Barton comb or straight edge for roughness profiling
Camera with scale bar for documenting conditions
Field notebook or data recording forms
RMR rating tables (printed reference or this page)

Step 1: Determine Uniaxial Compressive Strength (UCS)

The first parameter rates the intact rock material strength. Follow this procedure to determine the appropriate rating:

Check for laboratory data: If UCS test results from the same lithological unit are available, use the average value from at least three valid tests. Discard any results where the failure mode was clearly influenced by pre-existing discontinuities within the specimen.
Use point load index if no UCS data exists: Conduct point load strength tests on core or irregular rock pieces following ISRM procedures. Apply the size correction to obtain Is(50), then multiply by the appropriate conversion factor (typically 20–25, varying by rock type) to estimate UCS.
Use field estimation as a last resort: If neither laboratory testing nor point load data is available, estimate UCS using Schmidt hammer rebound values with published correlations, or use the ISRM simple field identification table (rock can be scratched by thumbnail = R0, < 1 MPa; rock crumbles under hammer blow = R1, 1–5 MPa; requires several hammer blows to break = R3, 25–50 MPa; and so on).
Assign the rating: Match your UCS value to the appropriate range in the rating table and record the rating (0 to 15 points).

UCS (MPa)	Rating
> 250	15
100 – 250	12
50 – 100	7
25 – 50	4
5 – 25	2
1 – 5	1
< 1	0

Step 2: Determine Rock Quality Designation (RQD)

RQD quantifies the degree of fracturing in the rock mass:

From borehole core: Measure the total length of all intact core pieces 100 mm or longer within the core run. Divide by the total core run length and multiply by 100 to obtain RQD as a percentage. Only count natural fractures as breaks; reassemble any mechanical breaks caused by drilling or handling.
From surface exposure (no core): Count the total number of discontinuities per cubic metre (volumetric joint count, Jv) using three mutually perpendicular scan lines. Calculate RQD = 115 − 3.3 Jv. Cap the result at 100% maximum and 0% minimum.
Assign the rating: Match your RQD percentage to the table and record the rating (3 to 20 points).

RQD (%)	Rating
90 – 100	20
75 – 90	17
50 – 75	13
25 – 50	8
< 25	3

Step 3: Measure Discontinuity Spacing

Discontinuity spacing determines block size in the rock mass:

Set up scan lines: Place a measuring tape along the rock face or use borehole core. Orient the scan line perpendicular to the dominant discontinuity set for accurate true spacing measurement.
Record individual spacings: Measure the distance between each consecutive pair of discontinuities along the scan line. Identify which discontinuity set each fracture belongs to.
Calculate average spacing: For each discontinuity set, calculate the mean spacing. If multiple sets are present, use the set with the smallest average spacing for the RMR rating, as this controls the minimum block dimension.
Correct for orientation bias: If the scan line is not perpendicular to the discontinuity set, apply the Terzaghi correction: true spacing = apparent spacing multiplied by sin(angle between scan line and discontinuity plane).
Assign the rating: Match the average true spacing of the most closely spaced set to the table (5 to 20 points).

Spacing	Rating
> 2 m	20
0.6 – 2 m	15
200 – 600 mm	10
60 – 200 mm	8
< 60 mm	5

Step 4: Assess Condition of Discontinuities

This is the most detailed step, requiring assessment of five separate sub-parameters. Examine representative discontinuity surfaces and rate each characteristic independently:

4a. Persistence: Estimate the visible trace length of discontinuities on the exposed face. Where the full extent is not visible, record the observed length as a minimum value and note that the actual persistence may be greater. Rating: < 1 m = 6, 1–3 m = 4, 3–10 m = 2, 10–20 m = 1, > 20 m = 0.
4b. Aperture: Measure the perpendicular opening width of the discontinuity at several points using feeler gauges for small apertures and calipers for larger ones. Use the typical value for the joint set, not the extreme maximum. Rating: closed = 6, < 0.1 mm = 5, 0.1–1 mm = 4, 1–5 mm = 1, > 5 mm = 0.
4c. Roughness: Run your hand along the discontinuity surface and compare against ISRM standard profiles. Use a Barton comb to trace the roughness profile over a 100 mm length if precise assessment is needed. Rating: very rough = 6, rough = 5, slightly rough = 3, smooth = 1, slickensided = 0.
4d. Infilling: Identify any material between the discontinuity walls. Determine whether it is hard (calcite, quartz, epidote) or soft (clay, silt, fault gouge) and measure its thickness. Rating: none = 6, hard < 5 mm = 4, hard > 5 mm = 2, soft < 5 mm = 2, soft > 5 mm = 0.
4e. Weathering: Assess the degree of decomposition of the rock immediately adjacent to the discontinuity surface using ISRM weathering grade descriptors (W1 fresh to W5 completely decomposed). Rating: unweathered = 6, slightly weathered = 5, moderately weathered = 3, highly weathered = 1, decomposed = 0.
Sum the sub-ratings: Add the five individual sub-parameter ratings to obtain the total discontinuity condition rating (0 to 30 points).

Step 5: Evaluate Groundwater Conditions

Groundwater is assessed using the most appropriate of three methods, depending on available data:

Method A — Inflow rate: If the excavation or tunnel face is accessible, estimate the water inflow per 10 m of tunnel length by measuring flow rates from individual seeps and summing them for the section.
Method B — Pressure ratio: If piezometer data is available, calculate the ratio of pore water pressure (pw) to the major principal stress (sigma1) at the depth of interest. This ratio ranges from 0 (dry) to greater than 0.5 (high water pressure).
Method C — General description: For preliminary assessments or when quantitative data is unavailable, use the qualitative scale: completely dry, damp (moisture visible but no free water), wet (free water present), dripping (water dripping from discontinuities), or flowing (continuous water flow from discontinuities).
Assign the rating: Use whichever method provides the most reliable assessment. Ratings: dry = 15, damp = 10, wet = 7, dripping = 4, flowing = 0.

Inflow per 10 m	pw/σ1	Description	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

Step 6: Sum Parameters 1–5 to Obtain Basic RMR

Add the ratings from the first five parameters:

Basic RMR = Rating(UCS) + Rating(RQD) + Rating(Spacing) + Rating(Condition) + Rating(Groundwater)

The Basic RMR ranges from a theoretical minimum of approximately 8 (all parameters at their lowest possible ratings: 0 + 3 + 5 + 0 + 0 = 8) to a maximum of 100 (all at maximum: 15 + 20 + 20 + 30 + 15 = 100). The Basic RMR is useful for reporting because it describes the inherent rock mass quality independent of the specific engineering application.

Step 7: Apply Orientation Adjustment and Classify

The final step accounts for the relationship between the dominant discontinuity orientation and the engineering structure:

Determine the application type: Select whether you are assessing a tunnel, slope, or foundation, as the adjustment values differ significantly between these applications.
Assess the orientation favourability: For tunnels, compare the strike and dip of the dominant joint set to the tunnel axis direction using Bieniawski's qualitative guidelines. For slopes, perform kinematic analysis comparing joint dip direction and angle to the slope face geometry. For foundations, assess whether discontinuities dip towards the loaded area in orientations conducive to sliding.
Apply the adjustment: Subtract the appropriate penalty from the Basic RMR.

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

Adjusted RMR = Basic RMR + Orientation Adjustment (negative value)

Use the Adjusted RMR to determine the rock class:

81–100: Class I — Very Good Rock
61–80: Class II — Good Rock
41–60: Class III — Fair Rock
21–40: Class IV — Poor Rock
< 21: Class V — Very Poor Rock

You can verify your manual calculation using our free online RMR calculator, which performs all steps automatically and generates a downloadable PDF report.

Printable Field Data Recording Sheet

The following table can be printed and taken into the field for recording RMR data. Use the browser print function (Ctrl+P or Cmd+P) to generate a clean printout.

RMR Field Data Recording Sheet — Bieniawski 1989
Project:
Location / Chainage:
Date:	Assessed by:	Lithology:	Domain:
Parameter 1: Uniaxial Compressive Strength
UCS value (MPa):		Data source:
Rating (0–15):
Parameter 2: Rock Quality Designation
RQD (%):		Data source:
Rating (3–20):
Parameter 3: Spacing of Discontinuities
Spacing (mm or m):		Set measured:
Rating (5–20):
Parameter 4: Condition of Discontinuities
4a. Persistence:		Sub-rating (0–6):
4b. Aperture:		Sub-rating (0–6):
4c. Roughness:		Sub-rating (0–6):
4d. Infilling:		Sub-rating (0–6):
4e. Weathering:		Sub-rating (0–6):
Total Condition Rating (0–30):
Parameter 5: Groundwater Conditions
Observation:		Assessment method:
Rating (0–15):
Basic RMR Calculation
Basic RMR = P1 + P2 + P3 + P4 + P5 =
Parameter 6: Orientation Adjustment
Dominant joint set orientation:		Strike/Dip:
Application type:		Favourability:
Adjustment value:
Final Result
Adjusted RMR = Basic RMR + Adjustment =
Rock Class (I–V):		Description:
Notes & Sketches

Common Calculation Errors to Avoid

The following mistakes are frequently observed when practitioners calculate RMR. Being aware of these pitfalls will improve the accuracy and consistency of your classifications.

Mixing RMR versions: The 1973 and 1989 versions of RMR use different rating tables and produce different scores. Always use the 1989 tables consistently. Do not combine rating tables from different versions or from secondary references that may have introduced modifications. Use the original Bieniawski 1989 tables as published in "Engineering Rock Mass Classifications."
Using tunnel adjustment values for slopes: The orientation adjustment differs dramatically between application types. The tunnel adjustment ranges from 0 to -12, while the slope adjustment ranges from 0 to -60. Using tunnel values for a slope assessment will produce dangerously unconservative results. Always verify which application type you have selected before applying the adjustment.
Double-counting discontinuity condition: In the 1989 system, discontinuity condition is assessed through five separate sub-parameters that are summed. Some practitioners mistakenly apply both the sub-parameter method and an overall condition rating, effectively double-counting this parameter. Use only the five sub-parameters (persistence, aperture, roughness, infilling, weathering) and sum them for the total condition rating.
Ignoring the most unfavourable discontinuity set for spacing: When multiple joint sets are present, the spacing rating must be based on the set with the smallest average spacing, not on the average of all sets or on the most prominently visible set. The smallest spacing controls the minimum block dimension and therefore the most critical rock mass behaviour.
Confusing basic RMR with adjusted RMR: When reporting RMR values, always specify whether you are reporting the Basic RMR (sum of parameters 1–5) or the Adjusted RMR (after orientation adjustment). Failure to distinguish between these values creates confusion and can lead to incorrect design decisions. The rock class should be determined from the Adjusted RMR.
Assessing groundwater during unrepresentative conditions: A site visit during a drought may show dry conditions that are not representative of normal or worst-case groundwater conditions. Always consider seasonal variations and use the most adverse credible groundwater condition for design purposes. Review piezometer records spanning at least one full seasonal cycle if available.
Not defining geotechnical domains: Applying a single RMR value to an entire tunnel alignment or slope that traverses multiple rock types and structural domains is a common and serious error. Divide the site into zones of similar geological character and classify each zone independently. A single tunnel project may have five or more distinct RMR domains.
Exceeding maximum sub-parameter ratings: Each of the five discontinuity condition sub-parameters has a maximum rating of 6. Ensure that no individual sub-parameter exceeds 6, and that the total condition rating does not exceed 30. Similarly, verify that the total Basic RMR does not exceed 100.

Frequently Asked Questions

For an experienced geotechnical engineer or engineering geologist, a single RMR assessment for a defined rock mass domain typically takes 15 to 30 minutes of field observation and measurement, plus an additional 10 to 15 minutes for data compilation and rating assignment. The total time depends on the accessibility and quality of the rock exposure, the complexity of the geological conditions, and whether laboratory test data (particularly UCS) is already available. During active tunnel construction, face mapping and RMR classification must keep pace with excavation. For a typical drill-and-blast tunnel advancing 3 to 5 metres per round, the entire mapping and classification process is usually completed within 20 to 30 minutes while the next round is being drilled.

The Bieniawski 1989 system uses discrete rating categories rather than continuous interpolation. If your measured value falls exactly on a boundary between two categories, the standard practice is to assign the lower, more conservative rating. For example, a measured discontinuity spacing of exactly 600 mm falls on the boundary between the 200–600 mm range (rating 10) and the 0.6–2 m range (rating 15). In this case, assign a rating of 10. Record both the measured value and the assigned rating in your field notes so that sensitivity analyses can be performed during the design phase. Avoid interpolating between categories, as this introduces subjective variation and reduces the reproducibility of the classification between different practitioners.

Yes, RMR can be estimated from borehole data, although some parameters are more difficult to assess without direct observation of exposed rock surfaces. UCS is readily determined from laboratory testing of core specimens. RQD is measured directly from core. Discontinuity spacing is assessed from core fracture frequency logs. However, discontinuity condition sub-parameters such as roughness and persistence are difficult to evaluate from borehole core alone and may need to be estimated conservatively or supplemented with televiewer or acoustic imaging data. Groundwater conditions can be assessed from piezometer installations and water pressure tests. Joint orientations for the adjustment parameter can be obtained from oriented core, televiewer logs, or optical borehole imaging. In general, borehole-only RMR assessments tend to be slightly conservative compared to those that include surface mapping data.

The number of assessments depends on geological variability and project requirements. At a minimum, perform one RMR classification for each distinct geotechnical domain, defined as a zone with reasonably uniform rock mass characteristics (same lithology, similar fracture intensity, comparable weathering grade). For tunnelling projects, RMR is typically assessed at each face mapping station, which may be every blast round (3–5 m advance) or at defined intervals. For slope assessments, classify each geological unit and structural domain separately. A minimum of three independent assessments per domain is recommended to evaluate variability and establish statistical confidence. For large projects, plotting RMR values along the alignment or across the slope face reveals spatial trends and helps identify transitions between geotechnical domains.