🎓 Lesson 16
D2
Field RMR Data Collection & Scoring
RMR (Rock Mass Rating) is a number that tells engineers how strong and stable a rock mass is, based on simple field observations like cracks, water, and rock hardness.
🎯 Learning Objectives
- ✓ Explain the six RMR parameters and justify their weighting in field scoring
- ✓ Calculate total RMR score from field-collected data using standardized rating tables
- ✓ Analyze RMR results to classify rock mass quality (e.g., 'very poor' to 'very good') and recommend appropriate ground support
- ✓ Apply correction factors for discontinuity orientation to adjust raw RMR for specific mining geometries
- ✓ Compare RMR-derived support recommendations against industry guidelines (e.g., Q-system or empirical charts)
📖 Why This Matters
In underground mines and open-pit highwalls, misjudging rock mass strength can lead to catastrophic failures—roof collapses, wall slumps, or unplanned ground support costs. RMR is the frontline tool used by every rock engineer during site visits: it transforms subjective field notes into objective, actionable numbers. Mastering RMR collection ensures your blast design, stope layout, and support plans are grounded—not just in theory—but in what the rock actually *does* at the face.
📘 Core Principles
RMR is built on six independent parameters, each scored on a defined scale (0–20 or 0–30). The first five—UCS, RQD, discontinuity spacing, discontinuity condition, and groundwater—are intrinsic to the rock mass. The sixth—discontinuity orientation—is geometric and requires context (e.g., tunnel axis vs. stope wall). Scoring follows strict field protocols: RQD must be measured on core with ≥10 cm intact pieces; discontinuity condition uses roughness, aperture, infilling, and weathering criteria; groundwater is rated by inflow per 10 m of tunnel length. Orientation correction is applied *only after* summing the first five scores—and only when unfavorably oriented joints threaten stability (e.g., bedding planes parallel to a stope wall).
📐 RMR Total Score Calculation
The total RMR is the arithmetic sum of the six component scores. Orientation correction is subtracted *from* the sum of the first five scores (RMR₅), not the full six. This reflects its role as a destabilizing modifier—not an inherent property.
RMR Total
RMR = UCSₛc + RQDₛc + Jₛpacingₛc + J_conditionₛc + GWₛc − ORIENT_corrSum of six component scores, where orientation correction is subtracted from the sum of the first five.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| UCSₛc | Uniaxial Compressive Strength Score | points | Score (0–15 or 0–25) derived from lab or point-load test of intact rock |
| RQDₛc | Rock Quality Designation Score | points | Score (0–20) based on % RQD measured in drill core |
| J_spacingₛc | Discontinuity Spacing Score | points | Score (0–20) reflecting average spacing between dominant joint sets |
| J_conditionₛc | Discontinuity Condition Score | points | Score (0–30) evaluating roughness, aperture, infilling, and weathering |
| GWₛc | Groundwater Score | points | Score (0–15) based on observed inflow rate per 10 m of tunnel or stope height |
| ORIENT_corr | Orientation Correction | points | Subtracted penalty (0–12) when joint dip/dip direction threatens stability |
Typical Ranges:
Hard massive granite: 75 – 90
Sheared schist in underground drift: 20 – 45
Weathered limestone bench: 35 – 60
💡 Worked Example
Problem: During a stope mapping exercise, students record: UCS = 85 MPa → 15 pts; RQD = 72% → 17 pts; Discontinuity spacing = 0.4 m → 12 pts; Discontinuity condition (rough, tight, unfilled) → 20 pts; Groundwater (damp, no flow) → 7 pts; Discontinuity orientation: bedding planes dip 15° into stope → moderate adverse effect → −12 pts correction.
1.
Step 1: Sum first five scores: 15 + 17 + 12 + 20 + 7 = 71
2.
Step 2: Apply orientation correction: RMR = 71 − 12 = 59
3.
Step 3: Classify result: RMR = 59 falls within 'Fair' rock mass (51–70), indicating moderate support needed (e.g., 2.0 m rockbolts @ 1.5 × 1.5 m pattern).
Answer:
The final RMR is 59, classifying the rock mass as 'Fair', which aligns with typical support guidance for sublevel stopes in porphyry copper deposits.
🏗️ Real-World Application
At the Red Lake Mine (Ontario, Canada), geotechnical teams use RMR during daily stope inspections. In Zone 4B, initial RMR surveys yielded RMR = 43 ('Poor'), prompting installation of 3.6 m fully grouted bolts. After re-mapping post-blast (noting improved joint wall contact due to stress relief), RMR rose to 58 ('Fair'), allowing reduction to 2.4 m bolts—cutting support cost by 32% without compromising safety. This iterative RMR-based decision-making is now codified in the mine’s Ground Control Management System (GCMS) per CSA Z621-22.