Controlled Blasting

Controlled blasting protects final walls and tunnel profiles. Learn presplitting, trim (buffer) blasting, smooth-wall and cushion blasting, decoupling and light charges, presplit spacing and charge rules, perimeter damage (half-barrel factor), and blast-induced overbreak underground.

Production blasts crack rock several metres behind the last row. For final pit walls and permanent tunnels this damage reduces stability and increases support costs. Controlled blasting uses light, decoupled charges in closely spaced perimeter holes to create a clean fracture plane.

An analogy

Presplitting is like perforating paper before tearing it: a line of closely spaced weak points makes the rock split exactly where you want.

Techniques

Presplit: perimeter holes fired before the main blast to form a crack. Trim/buffer rows: reduced charges next to the wall. Smooth-wall (underground): perimeter holes fired last with light charges. Line drilling: very close unloaded holes for sensitive structures.

Decoupling

Using explosives smaller than the hole (e.g. 32 mm cartridges in 89 mm holes) or low-density products reduces borehole pressure below rock compressive strength while exceeding its tensile strength between holes.

Quality measurement

Half-barrels visible on the wall show success; HCF > 80 % in good rock indicates excellent control. Underground, overbreak/underbreak from laser scans against design profile is tracked.

Wall control programmes

Combining presplits, buffer rows, electronic timing and careful face scaling allows steeper bench face angles — one of the cheapest ways to reduce stripping in open pits.

Formulas

• Konya presplit charge: q ≈ d² / 12 140 (kg/m, d in mm)
• Presplit spacing: S_p ≈ 10–12 d
• Half-cast factor: HCF = visible half-barrel length / perimeter hole length × 100

Worked example: Presplit design

Given: 89 mm perimeter holes

Find: Charge and spacing.

• q = 89²/12 140 = 0.65 kg/m
• S = 10–12 × 0.089 = 0.9–1.07 m

Answer: ≈ 0.65 kg/m at ≈ 0.9–1.0 m spacing

Worked example: Half-cast factor

Given: Perimeter holes total 120 m; Visible half-barrels 96 m

Find: HCF.

• HCF = 96/120 = 80 %

Answer: 80 % — good wall control

Common mistakes

• Wrong: Full production charges in perimeter holes. — Right: Use light, decoupled charges only.
• Wrong: Poor drilling alignment. — Right: Deviated holes ruin the split plane — use guides and measure.

Applications

• Final pit walls: Presplit + buffer rows for steep faces.
• Tunnels & drives: Smooth-wall blasting reduces overbreak and support.
• Civil excavations: Near foundations and sensitive structures.

Key takeaways

• Presplit, trim, smooth-wall, line drilling.
• q ≈ d²/12 140; S ≈ 10–12 d.
• Decoupled light charges.
• Measure HCF and overbreak.

Going further

• Damage zone modelling: Near-field PPV thresholds for rock damage.
• Electronic presplits: Precise timing in groups.
• Overbreak analytics: Scan-based reconciliation of drive profiles.