Bench blasting terms describe different parts of the hole pattern and rock geometry; they are not interchangeable. Burden is the rock distance a hole must break toward a free face. Spacing is the distance between adjacent holes in a row. Stemming is the inert material placed in the collar portion of a charged hole. Subdrill is the part of a hole that extends below the intended floor elevation. The toe is the rock at the base of the face that must be broken to achieve the intended floor.
These definitions sound simple, but field teams often use a planned dimension, a drilled dimension, and an effective blasting dimension as though they were the same. They are not. Face shape, collar position, hole angle, borehole deviation, geological discontinuities, and hole condition can change the geometry that the competent blasting team must evaluate.
Scope and safety boundary: This guide explains terminology, measurement context, drilling records, and buyer communication. It does not provide explosive quantities, charge layouts, stemming lengths, delay timing, firing procedures, or a blast-design formula. Every blast requires a site-specific specification prepared and approved by authorized, competent personnel under local law and site rules.
Bench blasting terms at a glance
| Term | Plain-language meaning | Common source of confusion |
|---|---|---|
| Free face | An exposed rock surface toward which the rock can move during the blast. | The visible crest line is treated as if it represents the whole face at every depth. |
| Burden | The rock distance from a hole or charge toward the relevant free face. | Collar burden is assumed to equal effective burden down the full hole. |
| Spacing | The distance between adjacent blast holes in the same row. | It is confused with the distance between rows, which relates to burden. |
| Stemming | Inert material placed in the upper hole to confine gases and separate the charge from the collar. | The material, length, placement, and actual loaded condition are treated as one simple value. |
| Subdrill | Hole depth below the intended final floor or grade. | Planned subdrill is assumed to remain correct when the floor elevation or actual hole path changes. |
| Toe | Rock at the base of a bench face, especially material that remains insufficiently broken at floor level. | Every high spot is blamed on inadequate depth, although geology, burden, deviation, loading, and timing can also matter. |
| Bench height | The vertical difference between the upper bench level and the intended lower floor. | It is confused with total drilled depth, which may include subdrill and vary with inclination. |
| Hole inclination and azimuth | The hole angle from vertical and its horizontal direction. | An inclination reading alone is treated as a complete direction. |
What is burden in bench blasting?
Burden is the rock thickness between a blast hole and the free face that provides relief. For an interior row, it is commonly discussed as the distance toward the previously relieved row. For a front-row hole, it relates to the quarry face. The important idea is direction: burden is measured toward the surface or space into which the broken rock is expected to move, not simply toward the nearest collar mark.
Three burden values may appear in the same job:
- Designed burden is stated in the approved blast specification and assumes a defined face and hole geometry.
- Drilled or collar burden is based on the marked or surveyed position of the hole at the bench surface.
- Effective burden reflects the actual hole path, face shape, geology, and firing conditions at the location being evaluated.
A front-row hole can have an acceptable collar position while moving closer to the face at depth because the face leans inward or the hole deviates. The opposite can also occur. That is why face profiling, accurate collar control, drill-hole logging, and borehole tracking can be more useful than a single tape measurement.
What is spacing in a blast-hole pattern?
Spacing is the distance between adjacent holes in the same row. It describes how the holes interact along the row, while burden describes the relationship across rows or toward the free face. A drilling pattern therefore needs both dimensions and a clear row orientation.
Spacing on a drawing is not automatically the spacing achieved in the rock. Survey layout error, rig positioning, an incorrect azimuth, and borehole deviation can make holes converge or diverge. A row may look uniform at the collars while the bottom-hole positions are uneven. The drilling record should distinguish the planned pattern from measured exceptions.
Square, rectangular, and staggered layouts also change how spacing is interpreted. A staggered row shifts the holes relative to the row ahead; it does not remove the need to define the burden direction. Field drawings should show the free face, row sequence, hole identifiers, north or another reliable directional reference, and the unit of measurement.
What is stemming—and why is it not just “material on top”?
Stemming is inert material used in the collar portion of a loaded blast hole to help contain high-pressure gases long enough for the intended rock-breaking process. The term can describe the material, the filled collar section, or the act of placing it, so context matters.
Stemming is not explosive and must not be confused with drill cuttings that happen to fall into an open hole before loading. The approved blast specification should define the intended material and placement. The shotfiring team must verify the actual loaded condition because cavities, water, collapsed ground, product movement, or an incorrect charge position can change the uncharged collar length.
NIOSH research on small borehole tests found that stemming length and material affected the timing of stemming ejection and gas venting. That finding supports the principle that stemming is a functional part of blast control—not disposable backfill—but it does not establish one universal design value. Hole diameter, material quality, rock condition, explosive system, and local rules still govern the competent person’s decision.
What is subdrill or subgrade drilling?
Subdrill is the portion of the drilled hole below the intended final bench floor elevation. Its purpose is associated with breaking rock at the base of the bench so the working floor can be achieved. It is also called subgrade drilling or subdrilling.
Subdrill should be discussed as an elevation relationship, not only as “extra metres.” The intended floor elevation must be known, and the actual collar elevation, hole direction, and measured depth must use the same reference. On an inclined hole, total hole length and vertical depth are different quantities.
Too little effective subdrill can contribute to toe and an uneven floor. Excessive subdrill can add drilling and explosive cost and may damage the floor or the bench below. Those consequences are why field teams should report actual depth and conditions rather than changing depth informally.
What does “toe” mean at a quarry face?
The toe is the rock at the base of a bench face. In operational discussions, “a toe” often means a ridge or mass of insufficiently broken rock left at the floor after a blast. It can obstruct loading, leave an uneven floor, and create extra drilling or secondary breakage work.
A toe is a result, not a diagnosis. Inadequate effective subdrill is one possible cause, but the investigation may also need to consider excessive burden at floor level, hole deviation, incorrect depth reference, hard or massive geology, adverse bedding, loss of charge in a cavity, water conditions, loading differences, or initiation performance. A responsible review uses drill logs, measured hole paths, face geometry, loading records, and blast results together.
How the terms connect in one bench cross-section
Picture a row of holes drilled from the upper bench toward a vertical free face. Bench height runs from the upper surface to the intended lower floor. The hole extends farther than that vertical height because it may be inclined and may include subdrill. Burden connects the hole to the free face; spacing connects it to the next hole along the row. After loading, stemming occupies the upper collar zone. The toe is the floor-level rock whose breakage is influenced by the bottom-hole geometry and the complete blast design.
Changing one element changes the context for the others. Moving a collar alters burden. Changing inclination alters the hole path and bottom location. A shorter-than-planned hole reduces effective subdrill. A cavity may change the loaded column and collar condition. A collapsed hole may make the recorded depth different from the usable depth. These are not automatic instructions to change the blast; they are reasons to stop, document, and escalate.
Designed, drilled, and as-loaded values must stay separate
| Record stage | Typical information | Owner of the decision |
|---|---|---|
| Designed | Hole IDs, collar positions, burden, spacing, diameter, inclination, azimuth, depth, and intended subdrill. | Authorized blast designer or explosives supervisor under site rules. |
| Drilled | Actual collar position, bit diameter, measured depth, observed ground changes, water, voids, blockage, deviation, and completion status. | Driller records facts; competent blasting personnel assess exceptions. |
| As loaded | Actual loading and stemming records, hole condition, approved deviations, and initiation details. | Authorized shotfiring and explosives personnel. |
| Post-blast | Floor condition, toe, fragmentation, backbreak, muckpile, vibration or airblast records, and exceptions. | Site review team uses results to inform future approved designs. |
Do not overwrite a planned value with an actual value in the same field. A traceable record preserves both, identifies units, and records who approved a change. The HSA’s quarry guidance states that a blast specification should be specific to each blast and updated when drilling or loading information affects it.
Common terminology mistakes that create risk or cost
- Calling row-to-row distance “spacing” without defining the free-face direction.
- Using crest burden as a proxy for burden down the entire front-row hole.
- Recording hole length without collar elevation, inclination, or floor reference.
- Treating subdrill as a universal percentage instead of a site-specific design value.
- Assuming every toe proves the driller stopped short.
- Using “stemming” for loose contamination in an open hole before loading.
- Failing to distinguish the specified hole diameter from the diameter produced by a worn bit.
- Changing a collar, angle, or depth in the field without preserving the exception and approval trail.
What drillers should hand over before loading
A practical drilling handoff should identify the blast and hole, then compare specified and actual conditions. Include:
- hole ID and actual collar position;
- specified and actual inclination and azimuth;
- bit type, nominal diameter, and relevant wear observations;
- measured depth and the reference used for the intended floor;
- water, cavities, clay bands, joints, loss of return, collapse, or blockage by depth;
- any abandoned, redrilled, short, or unidentified hole;
- borehole survey results where required;
- the time, recipient, and approval status of each reported exception.
For front-row geometry and borehole deviation checks, see PerfoMax’s published guide to quarry drilling near a free face. The quarry and blast-hole drilling solutions page also summarizes PerfoMax’s relevant application scope.
Information to prepare for a drilling-tool inquiry
A tool supplier can review drilling-system fit, but it cannot design or approve the blast. To discuss drill bits, rods, pipes, hammers, or rock drills, prepare:
- rig, drifter, or DTH hammer make and model;
- current drill-string components, connections, shanks, and dimensions;
- planned hole diameter, depth range, inclination, and bench access constraints;
- rock type, strength information, abrasiveness, joints, cavities, and water;
- compressor pressure and airflow for DTH, or applicable drifter data for top hammer;
- photos of normal wear and representative failed parts;
- drilling performance and deviation records with units and conditions;
- the exact buyer problem: compatibility, service life, hole cleaning, straightness, or supply continuity.
Send the verified drilling-system and ground-condition details to PerfoMax for a focused tooling review. Blast geometry, explosive selection, loading, and firing remain with the site’s authorized blasting professionals.
Frequently asked questions
Is burden the distance between two blast holes?
Not usually. Spacing is the distance between neighboring holes in the same row. Burden is measured toward the free face or across to the relevant preceding row. A drawing should identify the row direction and free face so the terms cannot be swapped.
Is subdrill the same as total hole depth?
No. Subdrill is only the part of the hole below the intended floor elevation. Total drilled length also includes the portion through the bench and changes with collar elevation and hole inclination.
Does more subdrill always remove toe?
No. Toe can have several causes, and excessive subdrill can create other damage and cost. The competent blast team should review actual depth, burden at floor level, hole path, geology, loading, and blast performance before changing a design.
Are burden and spacing measured only at the collar?
Collar measurements are important, but they may not represent the geometry at depth. Irregular faces and deviated holes can change effective burden and bottom-hole spacing. Profiling and borehole tracking may be required by the specification or risk assessment.
Who may change the blast specification after drilling?
Only the authorized competent person defined by local law and site procedure. The driller reports actual conditions and exceptions; the responsible explosives supervisor or equivalent evaluates and formally approves any change.