In production blasting, overbreak refers to the excavation of rock beyond the planned design limit, while underbreak is the failure to remove rock within that limit. Both represent significant technical failures: overbreak leads to dilution and increased support costs, while underbreak requires expensive secondary blasting or mechanical scaling. Let’s explore the causes and mitigation strategies.
Main Technical Causes
The causes of overbreak and underbreak generally fall into three categories: drilling errors, blast design, and geology.
Overbreak (excessive breakage)
- Excessive explosive energy: high powder factors (too much explosive per cubic meter) or using high-density, high-VOD (Velocity of Detonation) explosives in perimeter holes creates excessive radial cracking and gas pressure that shatters the surrounding rock.
- Improper drilling (look-out angle): to maintain the tunnel’s profile as the drill progresses, drillers often tilt the holes outward. If this “look-out angle” is too steep, it creates a “saw-tooth” profile, resulting in significant over-excavation.
- Poor delay sequencing: if perimeter holes fire too early or simultaneously with production holes without a free face to move into, the energy is forced back into the solid rock mass (backbreak).
- Geological intersections: high-pressure gas from the blast can penetrate existing joints, faults, or bedding planes, prying large blocks of rock loose far beyond the intended boundary.
Underbreak (insufficient breakage)
- Inadequate burden: if the distance between the perimeter holes and the last row of production holes (the burden) is too large, the explosive energy cannot effectively shear the rock, leaving “tights” or protrusions.
- Hole deviation: if drill holes converge (tilt toward each other) rather than remaining parallel, the area between them may not receive enough energy to break, leaving unexcavated rock.
- Low explosive energy: using too little charge or an explosive with insufficient gas pressure to overcome the rock’s tensile strength.
- Misfires or short rounds: technical failures in the initiation system (cut wires or faulty detonators) leave whole sections of the face unblasted.
Technical control measures
Controlling these phenomena requires a combination of high-precision engineering and “Perimeter Control” techniques.
Perimeter control techniques
| Technique | Description | Primary Goal |
| Smooth Blasting | Perimeter holes are loaded with light, decoupled charges and fired last in the sequence. | Minimize overbreak and leave a smooth wall. |
| Pre-splitting | A row of closely spaced holes is fired before the main production blast to create a fracture plane. | Protect the final wall from production blast vibrations. |
| Line Drilling | A row of very closely spaced, uncharged holes is drilled at the boundary. | Acts as a physical barrier to stop cracks from propagating. |
| Cushion Blasting | Large diameter holes with small diameter charges; the “air cushion” softens the shock wave. | Reduce borehole pressure and micro-fracturing. |
Operational controls
- Decoupled charges: using a cartridge with a smaller diameter than the borehole (e.g., a 25mm charge in a 45mm hole).5 The air gap acts as a buffer, significantly reducing the Peak Particle Velocity (PPV) and preventing radial cracks from extending too far.
- Drilling accuracy: using computerized drill jumbos with GPS or laser guidance to ensure holes are perfectly parallel and the look-out angle is minimized (typically kept under 3⁰ to 5⁰).
- Electronic detonators: these provide millisecond precision (±0.1ms accuracy). Precise timing ensures that each hole has a “free face” to break into, reducing the back-pressure that causes overbreak.
- Stemming management: proper use of stemming (inert material like crushed stone) ensures that explosive gases are confined long enough to break the rock effectively, reducing the “cratering” effect at the collar of the hole.
