Open-Pit Copper Mine Bench Optimization in Northern Chile

Scenario

A Tier-1 copper mining operation in the Atacama Desert (Chile) faced excessive toe formation and oversized boulders in its primary ore zone — a moderately weathered andesite with interbedded tuff. The site operates under strict water conservation mandates (no dewatering), high ambient temperatures (>35°C), and logistical constraints limiting explosive types to ANFO with booster cartridges. Bench height was fixed at 10 m due to fleet compatibility (rigid-body 930E haul trucks), but fragmentation inefficiency was increasing secondary crushing costs by 18% YoY.

Given Data

• Rock density: 2.45 t/m³
• Specific charge: 0.18 kg/m³ (adjusted for lower ANFO VOD in arid, low-humidity conditions)
• Bench height: 10.0 m
• Coefficient (k): 0.26 (reduced from default due to moderate rock fracturing and presence of tuff layers)
• Spacing to burden ratio (n): 1.1 (increased slightly to improve inter-hole stress wave interaction in heterogeneous rock)

Calculation

Using Blast Design Software’s embedded empirical formulas:

1. Burden (B) = k × √(ρ × H / q)
   Where ρ = rock density (t/m³), H = bench height (m), q = specific charge (kg/m³)
   → B = 0.26 × √(2.45 × 10.0 / 0.18) = 0.26 × √(136.11) = 0.26 × 11.67 ≈ 3.03 m

2. Spacing (S) = n × B = 1.1 × 3.03 = 3.33 m

3. Subdrill (SD) = 0.3 × B = 0.3 × 3.03 = 0.91 m
   (Note: Tool uses fixed 0.3×B heuristic for subdrill; validated against local geotechnical logs showing 0.8–1.0 m competent floor zone)

Result and Decision

The software output: Burden = 3.03 m, Spacing = 3.33 m, Subdrill = 0.91 m. Field validation via photogrammetric fragment analysis (after test blast with 12 holes) confirmed 82% passing 300 mm — meeting target (≥80%). The design was rolled out across the 120-m-wide production front, reducing secondary blasting frequency by 35% and eliminating toe rework passes. Drill pattern was updated from 3.5 m × 3.5 m square to 3.0 m × 3.3 m staggered rows.

Lesson

In heterogeneous volcanic rock, calibrating coefficient (k) using geological logging—not just rock type charts—is essential. Default k=0.3 overestimated burden by 12%, causing poor toe breakage; reducing k to 0.26 aligned with observed joint spacing (0.4–0.6 m) and fracture density.