From Burden to P80: How Blast Geometry Controls Fragmentation

When the goal is to improve fragmentation, the best place to start is usually the geometry of the blast pattern. Geometry is the most economical optimization lever and often the fastest one to test. In many cases, improving fragmentation only requires recalculating the blast design and communicating the changes to the drilling crew.

Even small adjustments can produce measurable results. For example, changing burden by only 0.3 meters can shift P80 by 15–25%. Results are typically visible within two or three blast rounds, making geometry optimization one of the quickest ways to improve fragmentation.

Burden is usually the first parameter evaluated during blast optimization. A common guideline is that burden should be approximately 25–40 hole diameters.

For a 152 mm hole diameter, burden typically ranges from 4.5 to 5.5 meters. For a 200 mm hole diameter, burden generally increases to 5.5–7.0 meters.

Spacing is usually defined as a ratio of burden. In staggered patterns, spacing-to-burden ratios commonly range from 1.15 to 1.30. Square patterns typically use a ratio of 1.0.

A staggered pattern distributes explosive energy more uniformly through the rock mass and can reduce P80 by 10–15% without increasing explosive consumption.

Stemming is the inert material placed above the explosive column to contain energy within the rock mass. Typical stemming length ranges from 20–30 hole diameters and is often approximately equal to the burden.

If stemming is too short, explosive gases escape through the collar, reducing breakage efficiency. If stemming is too long, the upper bench receives insufficient energy. Both situations result in poor fragmentation near the top of the bench.

Subdrilling typically ranges from 8–12 hole diameters.

Insufficient subdrilling can leave an unbroken toe beneath the hole. Excessive subdrilling may damage the bench floor. Effective blast design requires balancing both risks.

Vertical holes are easier to drill and control. However, inclined holes, typically between 10° and 20°, can better follow bench geometry.

This helps maintain a more consistent burden along the entire bench height and can reduce backbreak behind design contours by 20–30%.

Powder factor represents the amount of explosive energy applied to a given volume of rock.

Typical values include:

• Soft rock: 0.25–0.40 kg/m³
• Medium rock: 0.55–0.75 kg/m³
• Hard rock: 0.80–1.20 kg/m³

As a first approximation, increasing powder factor generally reduces P80. However, the relationship must be calibrated using site-specific fragmentation measurements because every rock mass behaves differently.

Consider a bench with a hole diameter of 152 mm and rock strength of approximately 180 MPa.

Base design:

• Burden: 4.5 m
• Spacing: 5.2 m
• Staggered pattern
• Stemming length: 4 m
• Emulsion density: 1.15 g/cm³

This configuration produces:

• Charge weight: approximately 250 kg
• Powder factor: 0.72 kg/m³
• Predicted P80: 320 mm

After reducing burden to 4.2 m and spacing to 4.8 m:

• Pattern volume decreases from approximately 350 m³ to 300 m³
• Charge weight remains unchanged
• Powder factor increases from 0.72 to 0.83 kg/m³
• P80 decreases from 320 mm to 245 mm

The tighter blast pattern improves fragmentation but increases drilling and blasting costs by approximately 12% because more holes are required.

At the same time:

• Primary crusher throughput increases by 6%
• SAG mill throughput increases by 4%

Although drill and blast costs rise initially, downstream processing benefits offset the additional expense. In this example, the integrated cost curve reaches a positive crossover point after approximately 7–10 days of plant operation.

Blast design should never be evaluated solely by drilling and blasting costs. The real measure of success is the impact on the entire mining value chain, from blasting and loading to crushing, grinding, and overall mine-to-mill performance.