🎓 Lesson 1
D5
Support Sizing Using RMR-Based Empirical Charts
Support sizing using RMR-based empirical charts means choosing the right type and size of rock bolts, shotcrete, or other support by looking up values on simple charts that match a rock mass rating (RMR) number.
🎯 Learning Objectives
- ✓ Calculate the Rock Mass Rating (RMR) from field data using Bieniawski’s six-parameter system
- ✓ Select appropriate support type (e.g., fully grouted bolts, pattern bolting, shotcrete) and quantify key dimensions (bolt length, spacing, shotcrete thickness) using published RMR-based empirical charts
- ✓ Analyze discrepancies between chart-recommended support and site-specific conditions (e.g., water inflow, stress regime, blast damage) to justify design modifications
- ✓ Explain the limitations of RMR-based charts—including their applicability domain, assumptions about in-situ stress, and absence of dynamic loading considerations
📖 Why This Matters
In underground mines, inadequate ground support causes 70% of serious incidents—including falls of ground, bolt failures, and wall collapses. Yet over-design wastes capital and delays production. RMR-based empirical charts bridge this gap: they translate quick, field-measurable rock properties into actionable support specifications—often within minutes—making them indispensable for mine planners, shift supervisors, and junior engineers during rapid development drive. Their use is mandated in many jurisdictional safety codes (e.g., MSHA Part 46 compliance audits) and forms the baseline for all major mining contractors’ support protocols.
📘 Core Principles
The RMR system (Bieniawski, 1989) scores six parameters: uniaxial compressive strength (UCS) of intact rock, RQD (Rock Quality Designation), spacing of discontinuities, condition of discontinuities, groundwater conditions, and orientation of discontinuities relative to excavation. Total RMR ranges from 0–100; higher values indicate better rock mass. Empirical support charts—such as those in the ISRM Suggested Methods (2014), Hoek’s ‘Practical Rock Engineering’, and the SME Ground Control Handbook—map RMR ranges to recommended support systems. Critically, these charts assume static, low-to-moderate stress environments (<5 MPa horizontal stress), dry to damp conditions, and typical mining geometries (e.g., 4–6 m wide drifts). They do not account for seismic events, creep, or blasting-induced damage zones—so calibration with convergence monitoring or numerical modeling is required for high-stress or complex geometries.
📐 RMR Calculation
RMR is the arithmetic sum of six weighted parameters. Each parameter has defined scoring criteria and maximum points. The total score determines the rock mass class and drives support selection from standardized charts.
💡 Worked Example
Problem: A quartzite development drift has: UCS = 120 MPa; RQD = 85%; joint spacing = 0.8 m; joint condition = slightly weathered, rough, tight (no infilling); groundwater = damp (12 L/min per 10 m); joint orientation = unfavourable (dip 45°, strike parallel to drift). Calculate RMR and identify recommended support.
1.
Step 1: Assign scores — UCS (120 MPa → 15 pts), RQD (85% → 20 pts), spacing (0.8 m → 15 pts), condition (rough/tight → 25 pts), groundwater (damp → 10 pts), orientation (unfavourable → −12 pts).
2.
Step 2: Sum: 15 + 20 + 15 + 25 + 10 − 12 = 73.
3.
Step 3: Refer to Hoek & Marinos (2000) RMR–support chart: RMR 71–80 → 'Good rock' → recommend 2.4 m long, 22 mm diameter fully grouted rebar bolts at 1.5 m × 1.5 m spacing; optional 30 mm fibre-reinforced shotcrete if convergence >2 mm/week.
Answer:
RMR = 73 → Recommended support: 2.4 m × 22 mm fully grouted bolts @ 1.5 m spacing; shotcrete optional. This falls within the 'Good rock' band (RMR 71–80), consistent with typical quartzite performance in stable, low-stress horizons.
🏗️ Real-World Application
At Newmont’s Boddington Mine (Western Australia), RMR-based charts guided initial support design for the 5.5 m × 5.5 m ventilation raise (depth: 850 m). Field RMR averaged 68 (moderately jointed granodiorite, RQD 72%, minor seepage). Chart recommendations specified 2.7 m long, 25 mm resin-grouted bolts at 1.6 m spacing. Post-installation convergence monitoring (Leica GeoMoS) showed average wall displacement of 1.8 mm/month—well below the 5 mm/month threshold—validating the chart selection. When RMR dropped to 52 in a shear zone (RQD 35%, flowing water), the team escalated to 3.6 m bolts + 50 mm shotcrete, demonstrating how charts serve as both baseline and trigger for escalation.
📋 Case Connection
📋 Coal Mine Longwall Gate Road Support Upgrade
Excessive roof sag and rib spalling compromising ventilation and haulage