Free MRMR calculator based on Laubscher 1990. Calculate in-situ RMR from four base parameters, then apply adjustment factors for weathering, induced stress, joint orientation, and blasting to get the final MRMR — used for mine support design, cavability, and fragmentation prediction.
IRS is the unconfined compressive strength of intact rock. In MRMR this is rated out of 20 (vs 15 in Bieniawski RMR). Use lab UCS, point load, or Schmidt hammer data.
In modern MRMR (Laubscher & Jakubec 2001) joint frequency is preferred over RQD. However RQD remains acceptable for early-stage studies. Rated 0–20 in MRMR vs 0–20 in Bieniawski.
Laubscher uses a graph-based method considering up to three joint sets to determine block size and form. The rating reflects the combined effect of the dominant sets. Rated 0–20 points.
In MRMR, groundwater is incorporated into the Joint Condition parameter (unlike Bieniawski RMR where it is separate). Rate the most adverse joint set. Rated 0–40 points.
Accounts for susceptibility of rock mass to weathering over the mine life. Fresh, hard rocks = 1.0. Highly susceptible to slaking and swelling = 0.30.
Based on ratio of unconfined compressive strength to major principal stress (UCS/σ1). High stress relative to strength reduces MRMR. Stress relief (tension) also reduces rating.
Relationship between dominant joint sets and the excavation. Applied to the most critical joint set for the specific excavation geometry.
Accounts for damage induced by excavation method. Mechanical excavation or controlled blasting causes least damage. Production blasting causes most damage to remaining rock mass.
Laubscher 1990 — five classes used for mine design, support selection, and cavability assessment.
| Class | MRMR Range | Description | Cavability | Typical Support |
|---|---|---|---|---|
| I | 81–100 | Very Good Rock | Non-caving — requires inducement | Spot bolts only |
| II | 61–80 | Good Rock | Caving possible with large hydraulic radius | Systematic bolts + light mesh |
| III | 41–60 | Fair Rock | Caving with moderate hydraulic radius | Pattern bolts + shotcrete |
| IV | 21–40 | Poor Rock | Readily caving — small hydraulic radius | Heavy support + steel ribs |
| V | 0–20 | Very Poor Rock | Spontaneous caving — unstable without support | Immediate, heavy support + grouting |
The two-stage nature of Laubscher's system: in-situ quality vs mining-adjusted quality.
| Aspect | IRMR (In-Situ RMR) | MRMR (Mining RMR) |
|---|---|---|
| Definition | Rock mass quality before any mining disturbance | Adjusted quality accounting for mining environment |
| Parameters | IRS + RQD/Frequency + Spacing + Joint Condition (incl. GW) | IRMR × W × S × O × B |
| Maximum value | 100 | 100 (can only decrease from IRMR) |
| Use | Resource characterisation, geological model input | Mine design, support selection, cavability |
| Reinforcement potential | High IRMR = high reinforcement potential | Low MRMR may still have reinforcement potential if IRMR is high |
| When assessed | During exploration / pre-feasibility | At design and excavation stage |
The four adjustment factors that convert IRMR to MRMR. Each is a multiplier between 0 and 1 (except induced stress which can go up to 1.2 in favourable low-stress conditions).
| Factor | Symbol | Range | Description | Key Considerations |
|---|---|---|---|---|
| Weathering | W | 0.30–1.00 | Susceptibility of rock mass to deterioration over mine life | Slaking tests, mineralogy, exposure time, water contact |
| Induced Stress | S | 0.60–1.20 | Ratio of intact rock strength to major principal stress | In-situ stress measurements, depth, mining-induced stress changes |
| Joint Orientation | O | 0.63–1.00 | Relationship of dominant joints to excavation orientation | Strike and dip of joint sets vs tunnel/stope axis direction |
| Blasting | B | 0.80–1.00 | Damage induced to remaining rock mass by excavation method | Overbreak measurements, PPV monitoring, method selection |
Everything engineers and geologists need to know about the Mining Rock Mass Rating system.
MRMR (Mining Rock Mass Rating) is a geomechanical classification system developed by D.H. Laubscher starting in 1975 and refined through 2001 (Laubscher and Jakubec). It modifies the basic RMR system specifically for underground mining by adding adjustment factors that account for the mining environment — particularly critical for block and panel caving operations.
MRMR is the primary tool for predicting whether a rock mass will cave under block caving mining. Low MRMR values indicate readily caving rock. The critical hydraulic radius (CHR) — the minimum footprint area required to initiate caving — increases with higher MRMR. Laubscher's stability graph plots MRMR against hydraulic radius to predict caving behaviour.
A key concept unique to MRMR: even if MRMR is low (indicating poor post-mining quality), a high IRMR means the rock can still be effectively reinforced with rock bolts. Conversely, low IRMR rock cannot be reinforced even if MRMR appears adequate, because the rock between the bolts is already too weak to form a stable arch. This guides support strategy selection.
MRMR is used to predict fragmentation in caving operations. The number of joint sets, their spacing, and the joint condition all influence the natural block size. Low MRMR generally corresponds to smaller fragment sizes, while high MRMR rock tends to produce larger blocks and poorer draw. Fragmentation affects mucking efficiency, crusher sizing, and draw control strategy.
Evolution of Laubscher's Mining Rock Mass Rating from 1975 to present.
When to use each classification system in geotechnical mining practice.
Standard procedure for MRMR assessment in underground mining projects.
Common questions from mining engineers, geologists, and students about MRMR.
| IRS rating | — |
| RQD rating | — |
| Spacing rating | — |
| Joint Condition | — |
| IRMR total | — |
| Adj. combined | — |
| MRMR final | — |
| Rock class | — |
| Cavability | — |