Common Blast Design Mistakes That Hurt Fragmentation

One of the most common mistakes is changing burden and spacing without reviewing the entire blast design. Stemming length, charge distribution, confinement, and geological conditions must all be considered together.

When burden or spacing changes, stemming may become too short. This can result in stemming ejection and gas escape through the collar, causing explosive energy to be lost instead of breaking rock.

Another common mistake is increasing powder factor by extending subdrilling instead of optimizing the blast pattern. This often damages the bench floor and creates problems for the next mining level.

When switching from a square pattern to a staggered pattern, burden-to-spacing ratios must also be adjusted. Otherwise, the staggered pattern may perform worse than the original design.

Blast designs often specify inclined holes, but field crews may drill vertically for convenience. Without proper supervision, execution no longer matches the design, reducing blast effectiveness.

Modern blasting practice relies on trial benches. Multiple powder factor variants are tested to determine which design provides the best fragmentation and overall performance.

The stiffness ratio, defined as bench height divided by burden, should generally remain above 2.5. Lower values reduce fragmentation efficiency because more energy is spent on rock displacement.

Powder factor only measures the amount of explosive energy used. Charge design determines where and how that energy is applied within the hole.

The bottom charge provides concentrated energy at the toe and typically has higher density. Its purpose is to ensure complete toe breakage and a clean bench floor.

The main column breaks the majority of the rock mass. Dry holes often use ANFO, while wet holes require heavy ANFO or emulsion products.

Boosters provide reliable initiation and are usually placed in the lower third of the explosive column. Long holes may require multiple boosters.

Air decks or inert decks reduce explosive concentration in specific intervals and help adapt the blast design to geological conditions such as weak interbeds.

ANFO is economical but not water resistant. Heavy ANFO offers greater flexibility, while emulsion explosives provide reliable performance in wet conditions.

Water can reduce detonation velocity and increase the risk of misfires. Explosive selection must match actual hole conditions.

In partially wet holes, mines often use pumped emulsion in wet sections, heavy ANFO above the water level, and standard ANFO in dry intervals.

Weak clay layers and geological discontinuities require customized charge designs. Air decking can reduce excessive energy concentration and improve overall blast performance.

Contour holes often use decoupled charges to reduce wall pressure, minimize backbreak, and protect final pit walls.

Overcharging the toe, using ANFO in wet holes, and applying the same booster size to all holes can significantly reduce blast performance.

Modern blasting requires detailed hole logging, individualized charge designs, proper booster placement, and separate wet and dry loading strategies.

Successful blasting is not about using more explosive energy. It is about applying the right amount of energy in the right geometry, with the right products, and validating the results through field measurements.