The powder factor (also called specific charge or specific consumption) is the amount of explosive used per unit volume or weight of rock to be blasted. It is the single most important parameter in blast design, directly affecting fragmentation, throw, vibration, air overpressure, and cost. A properly calibrated powder factor produces rock that is well-fragmented for loading and crushing, with controlled throw, minimal flyrock, and acceptable environmental impact.
This guide covers powder factor calculation, how rock properties affect explosive requirements, bench geometry considerations, and the safety standards and regulations that govern blasting operations.
Calculating Powder Factor
Powder factor is expressed in two common forms:
Metric: kg of explosive / m³ of rock (kg/m³)
Imperial: lb of explosive / yd³ of rock (lb/yd³) or lb/ton
The basic calculation:
PF = Total explosive weight / Volume of rock broken
Volume of rock per blast hole in a bench blast:
V = B × S × H
- B = burden (distance from free face to nearest row of holes)
- S = spacing (distance between holes in a row)
- H = bench height
For example, with burden = 3.5 m, spacing = 4.0 m, bench height = 12 m, and 140 kg of explosive per hole: V = 3.5 × 4.0 × 12 = 168 m³. PF = 140 / 168 = 0.83 kg/m³.
Typical powder factors range from 0.2 kg/m³ for soft, well-fractured rock to 1.5+ kg/m³ for massive, hard, abrasive rock. Most production quarry blasting falls in the 0.4–0.9 kg/m³ range.
PF = Explosive per hole / (B × S × H)
Example: B=3.5m, S=4.0m, H=12m, charge=140 kg
Volume = 3.5 × 4.0 × 12 = 168 m³
PF = 140 / 168 = 0.83 kg/m³
Imperial: B=12ft, S=14ft, H=40ft, charge=350 lb
Volume = 12 × 14 × 40 / 27 = 248.9 yd³
PF = 350 / 248.9 = 1.41 lb/yd³
Blasting Powder Factor Calculator
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Rock Properties and Powder Factor Selection
The required powder factor depends primarily on rock properties:
Rock hardness and strength: Harder, stronger rocks (granite, basalt, quartzite) require higher powder factors (0.6–1.2 kg/m³) because more energy is needed to fracture the intact rock. Softer rocks (limestone, sandstone, shale) need less (0.3–0.7 kg/m³).
Joint density and orientation: Heavily jointed rock is pre-broken and requires less explosive energy to fragment to the desired size. Massive, unjointed rock needs more energy. Joint orientation relative to the free face affects breakage: joints parallel to the face aid breakage; joints perpendicular to the face resist it.
Rock density: Denser rocks require more energy per cubic meter because there is more mass to accelerate. The conversion from volumetric PF to weight-based PF accounts for this: PF (kg/ton) = PF (kg/m³) / rock density (ton/m³).
Desired fragmentation: Finer fragmentation requires higher powder factors. If the crusher can accept 1 m top size, less explosive is needed than if the specification calls for 0.3 m top size. Fragmentation models (Kuz-Ram, Swebrec) predict the size distribution for a given powder factor and blast geometry.
Soft shale/clay: 0.2–0.4 kg/m³
Weathered sandstone: 0.3–0.5 kg/m³
Medium limestone: 0.4–0.7 kg/m³
Hard limestone/dolomite: 0.5–0.8 kg/m³
Granite/gneiss: 0.6–1.0 kg/m³
Massive basalt/quartzite: 0.8–1.5 kg/m³
These are starting points. Adjust based on
actual blast results and site-specific conditions.
Blasting Powder Factor Calculator
Calculate powder factor for surface and underground blasting. Enter burden, spacing, hole depth, and charge weight for blast pattern optimization.
Safety Factors and Regulatory Guidelines
Blasting operations are governed by strict safety regulations. In the United States, key standards include:
- MSHA 30 CFR Part 56/57: Surface and underground metal/nonmetal mine blasting regulations.
- ATF 27 CFR Part 555: Federal explosive storage, handling, and transportation regulations.
- ISEE Blaster's Handbook: Industry reference for blast design, safety practices, and field procedures published by the International Society of Explosives Engineers.
- NFPA 495: Explosive materials code covering storage magazines, transportation, and use.
Ground vibration limits: The most common regulatory constraint is peak particle velocity (PPV) at the nearest structure, typically limited to 0.5–2.0 inches/second depending on the structure type and frequency. The scaled distance formula (SD = D / √W, where D is distance and W is maximum charge weight per delay) is used to predict vibration levels. Higher powder factors and larger individual hole charges increase vibration.
Flyrock control: Stemming height (typically 0.7–1.0 times the burden), burden adequacy, and initiation timing are the primary controls against flyrock. Inadequate burden or stemming is the leading cause of flyrock incidents. Blast exclusion zones (typically 300–500 m minimum from the blast) must be cleared of personnel before firing.
• All blasters must hold valid certification
• Blast exclusion zone: minimum 300 m (1,000 ft)
• Stemming: minimum 0.7 × burden
• Pre-blast survey of nearby structures
• Seismograph monitoring at nearest receptor
• Written blast plan before every shot
• Two-way radio communication during loading/firing
• Misfire procedures for any hole that fails to detonate
Blasting Powder Factor Calculator
Calculate powder factor for surface and underground blasting. Enter burden, spacing, hole depth, and charge weight for blast pattern optimization.