Bench Blast Design

Designing a production blast: burden, spacing, stemming, subdrill, hole length, charge per hole, rock per hole and powder factor. Learn Konya's burden formula, standard ratios, drilling patterns (square, staggered), delay timing and the blast design sheet.

Blast design converts hole diameter, explosive and rock properties into a drilling and charging pattern. Burden — the distance from the hole to the free face — is the most important dimension: too large and the rock is not broken; too small and flyrock and airblast result.

An analogy

Burden is like the thickness of a chocolate bar you're trying to snap: too thick and it won't break; too thin and pieces fly everywhere.

Pattern geometry

Holes are drilled in rows parallel to the free face. Burden is the distance between rows (and from the face), spacing is the distance between holes in a row. Staggered patterns distribute energy better than square ones.

Stemming and subdrill

Stemming (crushed rock, not drill cuttings) confines gases — too short causes flyrock and airblast, too long leaves boulders in the collar. Subdrill ensures the floor breaks to grade without toes.

Timing

Delays between holes (e.g. 17–42 ms) and rows (65–150 ms) let each hole break towards relief created by the previous one, control throw and limit charge per delay for vibration.

Mine-to-mill

Higher powder factors increase explosive cost but produce finer fragmentation, raising loading productivity and mill throughput — often a net gain of several percent in overall margin. Measure-while-drilling data adjusts charging hole by hole.

Formulas

• Konya burden: B = 0.012 (2ρ_e/ρ_r + 1.5) d (d in mm, B in m)
• Ratios: S ≈ 1.15 B, T ≈ 0.7 B, J ≈ 0.3 B
• Hole length: L = H + J
• Charge per hole: Q = (L − T) × q_l
• Powder factor: PF = Q / (B × S × H)

Worked example: Pattern design

Given: d = 115 mm, ANFO 0.85, rock 2.7 t/m³; Bench 10 m

Find: B, S, T, J, charge and PF.

• B = 0.012 × (0.63 + 1.5) × 115 = 2.94 m; S = 3.38; T = 2.06; J = 0.88
• L = 10.88 m; charge length 8.82 m × 8.83 kg/m = 77.9 kg
• Rock/hole = 2.94 × 3.38 × 10 = 99.3 m³ (268 t) → PF = 0.78 kg/m³

Answer: ≈ 2.9 × 3.4 m pattern, 78 kg/hole, PF ≈ 0.78 kg/m³

Worked example: Holes for a blast

Given: Blast volume 50 000 m³; 99.3 m³ per hole

Find: Number of holes and drilling.

• N = 50 000/99.3 ≈ 504 holes
• Drilling = 504 × 10.88 ≈ 5480 m

Answer: ≈ 500 holes, ≈ 5.5 km of drilling

Common mistakes

• Wrong: Using drill cuttings as stemming. — Right: Use angular crushed aggregate ≈ d/10 in size.
• Wrong: Ignoring face burden variations. — Right: Laser-profile the face; adjust front-row charges to avoid flyrock.

Applications

• Open-pit production: Daily blast patterns for ore and waste.
• Quarries: Designs tuned for aggregate size and vibration.
• Underground rings: Similar principles with fan-drilled holes.

Key takeaways

• B = 0.012(2ρ_e/ρ_r + 1.5)d.
• S ≈ 1.15B, T ≈ 0.7B, J ≈ 0.3B.
• PF = Q/(BSH).
• Timing and stemming control results.

Going further

• Measure-while-drilling: Rock hardness from drill data adjusts charges.
• Blast simulation: Monte Carlo timing models (e.g. Blo-Up, JKSimBlast).
• Face profiling: Laser/drone scanning for front-row burden.