🎓 Lesson 6
D5
Backfill-Rock Mass Interaction Physics
Backfill-rock mass interaction is how the engineered backfill material pushes against, supports, and changes the stress and movement of the surrounding rock in an underground mine stope.
🎯 Learning Objectives
- ✓ Analyze stress redistribution patterns using numerical models (e.g., FLAC2D/FLAC3D) for varying backfill modulus and rock mass quality
- ✓ Design backfill compressive strength and stiffness to limit rock mass convergence to ≤10 mm/year in high-stress stopes
- ✓ Calculate interfacial shear capacity at the backfill–rock contact using Barton–Bandis joint law parameters
- ✓ Explain how backfill curing time affects load transfer efficiency and stope closure kinetics
- ✓ Apply empirical backfill confinement criteria (e.g., ratio of backfill modulus to rock mass modulus) to assess stability risk
📖 Why This Matters
When a stope is mined out, the void must be filled — but simply pouring in backfill isn’t enough. If the backfill doesn’t interact properly with the rock, walls can spall, pillars can fail, and ground control systems collapse. Real incidents — like the 2018 failure at the Driefontein mine — were traced directly to underestimated backfill–rock interaction, leading to uncontrolled convergence and fatal falls of ground. Understanding this interaction isn’t academic: it’s the difference between safe, productive stoping and catastrophic instability.
📘 Core Principles
Interaction begins at the interface: roughness, moisture, and chemical bonding govern shear transfer. As backfill cures, its modulus increases — from ~0.1 GPa (fresh paste) to >5 GPa (28-day cured cemented backfill) — changing how stress is shared with the rock. The rock mass responds based on its GSI (Geological Strength Index), RMR, and pre-existing fracture networks; low-GSI rock deforms more, requiring compliant backfill, while high-strength rock demands stiff backfill to prevent excessive dilation. Time-dependent effects — including backfill creep, rock relaxation, and pore pressure dissipation — make this a dynamic, not static, problem. Full interaction only emerges after 7–28 days of curing under confinement — meaning early-stage monitoring often misrepresents final behavior.
📐 Interfacial Shear Capacity
The peak shear strength along the backfill–rock interface controls whether slip occurs before rock or backfill fails. It’s modeled using a modified Barton–Bandis approach calibrated for backfill contacts, incorporating joint roughness and effective normal stress.
💡 Worked Example
Problem: Given: JRC = 8.5 (joint roughness coefficient measured on stope wall), JCS = 45 MPa (joint wall compressive strength), σₙ' = 1.8 MPa (effective normal stress at interface), φ_b = 22° (backfill internal friction angle). Calculate τ_peak.
1.
Step 1: Compute basic friction angle: φ_b = 22° → tan(φ_b) = 0.404
2.
Step 2: Apply Barton–Bandis: τ_peak = σₙ' × tan[φ_b + JRC × log₁₀(JCS / σₙ')] = 1.8 × tan[22° + 8.5 × log₁₀(45 / 1.8)]
3.
Step 3: log₁₀(45/1.8) = log₁₀(25) ≈ 1.398 → 8.5 × 1.398 ≈ 11.88° → total angle = 22° + 11.88° = 33.88° → tan(33.88°) ≈ 0.672
4.
Step 4: τ_peak = 1.8 MPa × 0.672 ≈ 1.21 MPa
Answer:
The interfacial shear capacity is 1.21 MPa, which exceeds typical design thresholds (0.8–1.0 MPa) for stable confinement in moderate-stress stopes.
🏗️ Real-World Application
At the Red Lake Mine (Ontario), a narrow-vein stope sequence used 28-day cured cemented rockfill (CRF) with 2.5 MPa UCS. Monitoring showed 3.2 mm/month convergence in the first month — dropping to 0.4 mm/month after 28 days. Numerical back-analysis revealed that initial low backfill modulus (0.3 GPa) allowed stress shedding into weak hangingwall quartzite (GSI = 35), triggering microseismicity. Post-cure, modulus rose to 4.1 GPa, shifting >70% of horizontal stress to the backfill and stabilizing convergence. This confirmed that interaction timing — not just strength — dictated success.
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