Quarry Expansion for Highway Aggregate Supply in Ontario, Canada

Scenario

A limestone quarry supplying Ontario Ministry of Transportation (MTO) Class 504 base material required expansion into a newly permitted zone characterized by tight bedding (0.15–0.25 m spacing) and localized dolomitic bands. Environmental restrictions prohibited vibration >12 mm/s at nearby residential properties (350 m away), mandating precise energy control. Existing drill rig (Atlas Copco Pit Viper 271) limited hole diameter to 102 mm, constraining maximum burden. Project timeline demanded rapid ramp-up — no time for multi-phase calibration.

Given Data

• Rock density: 2.72 t/m³ (measured on core samples; dolomite-rich zones elevated average)
• Specific charge: 0.12 kg/m³ (reduced to meet vibration limits and avoid oversize from bedding parting)
• Bench height: 8.5 m (lowered from 10 m to reduce throw and peak particle velocity)
• Coefficient (k): 0.33 (increased above default to account for favorable bedding planes aiding lateral fracture propagation)
• Spacing to burden ratio (n): 0.95 (tightened to enhance confinement and reduce airblast near residences)

Calculation

Using Blast Design Software’s standardized formulas:

1. Burden (B) = k × √(ρ × H / q)
   → B = 0.33 × √(2.72 × 8.5 / 0.12) = 0.33 × √(192.67) = 0.33 × 13.88 ≈ 4.58 m But constrained by rig max burden: 102 mm holes → practical max B = 4.2 m per OEM guidelines. → Adopted B = 4.20 m (software flagged constraint; user override applied)

2. Spacing (S) = n × B = 0.95 × 4.20 = 3.99 m

3. Subdrill (SD) = 0.3 × B = 0.3 × 4.20 = 1.26 m
   (Increased from typical 0.9 m due to observed 1.1–1.3 m weathered floor zone in probe holes)

Result and Decision

Software recommended B=4.58 m, but engineering judgment capped burden at 4.20 m. Final design: Burden = 4.20 m, Spacing = 3.99 m, Subdrill = 1.26 m. Seismic monitoring during first production blast recorded 10.3 mm/s at nearest residence — within limit. Fragmentation met MTO gradation specs (D80 < 250 mm) without sorting. Pattern adopted as standard for Phase 2 expansion.

Lesson

Software outputs must be reviewed against mechanical and regulatory hard constraints — not just geotechnical logic. When burden exceeds equipment or vibration limits, use the tool’s sensitivity analysis (e.g., varying q or k) to find the highest viable burden that satisfies all boundaries.