Short answer: successful tunnel perimeter hole drilling depends on reproducing the approved contour at the actual face—not merely drilling a row of closely spaced holes. Collar position, initial angle, look-out direction, depth, straightness, flushing, and geology must stay controlled from the first hole to the last. A smooth-blasting design can still produce overbreak, underbreak, rough walls, or poor advance when the drilled holes do not match the plan.
For small underground headings that use air-leg pneumatic rock drills, the practical challenge is repeatability. The operator must work from a surveyed and updated face profile, stabilize the drill and air leg, collar without wandering, maintain the specified axis, and record exceptions before the face is handed to the licensed blasting team. The blast designer—not this guide—controls hole spacing, burden, charging, initiation, exclusion zones, and regulatory compliance.
What are tunnel perimeter holes?
Perimeter holes, also called contour or smooth-wall holes, form the outer row around a tunnel or mine drift profile. Their drilling position and direction help define the final excavation boundary. The production holes inside the profile are intended to break the main rock volume; the perimeter row is designed and charged separately by qualified blasting personnel to limit damage beyond the required line.
NIOSH describes perimeter control as a way to reduce damage to the remaining rock, loose material that must be scaled, and ground-fall exposure. Its DRIFT design work also coordinates the perimeter row with the adjacent buffer row. For the drill crew, this means the perimeter cannot be treated as an isolated cosmetic line: errors in either row can change the final contour.
A smooth result is therefore a system outcome:
- the face is surveyed and marked correctly;
- the drill plan reflects the actual—not assumed—cross-section;
- holes are collared, angled, and drilled to the specified depth;
- geological changes are recorded;
- drilling results are checked before charging; and
- post-blast contour results are fed back into the next round.
Why drilling accuracy controls overbreak and underbreak
| Drilling variable | If it moves outside the plan | Likely effect to investigate |
|---|---|---|
| Collar position | The hole starts inside or outside the marked contour. | Local underbreak, overbreak, uneven wall profile, or insufficient room for the planned row. |
| Initial angle | The bit enters at the wrong orientation during collaring. | The error grows with depth even if the operator later corrects the drill body. |
| Look-out direction | The hole diverges too much, too little, or in the wrong plane. | Profile enlargement, loss of tunnel line, a narrowing heading, or poor round-to-round continuity. |
| Hole straightness | The drill steel deflects through joints, weak bands, uneven feed, or poor support. | The toe misses its intended position and spacing differs at depth. |
| Hole depth | Holes finish short, long, or with large variation. | Uneven pull, toes or brows, irregular contour, and difficult next-round setup. |
| Hole cleaning | Cuttings remain or the hole collapses locally. | Uncertain usable depth and an unsafe or unreliable handoff to the blast crew. |
The collar and toe should be considered as one line in three-dimensional space. Measuring collar spacing alone cannot reveal two holes that converge, fan apart, or cross behind the face. Epiroc's tunneling guidance emphasizes fitting the drill plan to the actual section, while NIOSH research identifies precision drilling as a key control against unintended perimeter damage.
Understand look-out without guessing the angle
“Look-out” is the planned divergence of a hole from the tunnel centerline or theoretical profile. A tunnel round advances the face; if perimeter holes were simply drilled parallel without accounting for equipment geometry and the next face position, the practical opening could become smaller or discontinuous from round to round. Too much divergence, however, can enlarge the excavation and increase damage or support demand.
There is no universal look-out angle for every heading. The value depends on the approved drill-and-blast design, round length, profile, drill geometry, collar location, rock structure, and required final contour. The operator should receive a clear directional reference for the wall, shoulder, crown, corner, and lifter zones. “Aim slightly outward” is not a controlled instruction.
Where an air-leg drill cannot physically achieve the planned collar or axis because of wall clearance, floor condition, services, or operator position, stop and report the constraint. Moving the collar or improvising a new angle can transfer the problem to the toe and to the following round.
How geology changes the drilling response
A correctly set drill can still deviate when the bit meets an inclined joint, bedding plane, fault gouge, hard inclusion, open crack, or weathered band. The operator may feel a sudden change in penetration, rotation, feed resistance, vibration, flushing return, or cuttings. These observations are useful quality data, not merely production interruptions.
- Joints crossing the face: may deflect the bit, leak flushing water, or allow the collar edge to break away.
- Weak seams: can cause rapid penetration, poor hole-wall stability, or loss of the original axis.
- Hard stringers: may slow penetration and encourage the bit to track along the contact.
- Open fractures: can interrupt cuttings return and make the apparent depth unreliable.
- Changed crown or shoulder geology: may require the engineer to review the perimeter and buffer design by zone rather than apply one assumption around the full profile.
Do not compensate for a geological change by forcing feed, bending the drill steel, or changing the hole direction without authorization. Mark the affected collar and record the depth or drilling behavior at which the change occurred. The published guide to joints, bedding planes, and faults in rock drilling explains the discontinuity terms used in this handoff.
Air-leg setup for repeatable perimeter drilling
An air-leg pneumatic rock drill can be effective in restricted headings, but accuracy depends on a stable reaction path. The drill, air leg, floor or wall contact, and operator form one positioning system. If the air-leg foot slips or the thrust line sits away from the drill axis, the operator may fight the tool and introduce collar movement or rod bending.
Before the first perimeter hole:
- Scale, support, ventilate, illuminate, and declare the face safe under the site's procedure.
- Confirm the current survey line, finished profile, and approved drill plan.
- Remove loose muck that prevents stable footing or air-leg contact.
- Route air and water hoses so they cannot pull the drill sideways or create a trip hazard.
- Check the drill, air leg, rod, bit, retainer, hose restraints, line oiler, and water system.
- Verify that the selected rod length can reach the planned depth without an unsafe or unstable setup.
- Provide a rigid directional reference or approved alignment aid for each profile zone.
The current PerfoMax YT28 air-leg rock drill page lists a reference 34–42 mm hole range and H22 × 108 shank. Those figures are a preliminary system-matching basis, not permission to substitute the project's required hole diameter, rod, bit, air leg, or operating inputs. The final quotation and attached supplier data sheet must govern the configuration.
A practical perimeter-hole drilling sequence
1. Mark the actual face
Use the current survey and approved plan after scaling. Do not transfer yesterday's profile without checking the face advance, line, grade, overbreak, underbreak, and obstruction changes. Make perimeter, buffer, production, and special holes distinguishable under the site's marking system.
2. Establish the collar before full feed
Place the bit precisely, support the drill, and begin with controlled thrust so the bit forms a stable seat. Excess feed at first contact can skate the bit, spall the collar, or bend a light rod. Once the collar holds the intended axis, increase feed only within the drill and bit instructions.
3. Maintain the planned axis
Watch the drill body, air-leg line, and reference—not only the bit. Correct small setup movement early. Do not use body weight, a side-loaded air leg, or intentional rod bending to force an angle. Review the YT28 feed-force and collaring guide for the operating checks behind this step.
4. Keep flushing effective
Maintain the approved water or air-flushing arrangement so cuttings leave the hole and dust controls remain effective. A weakening return can indicate blocked passages, leakage into fractures, insufficient supply, or local hole collapse. Stop and investigate rather than drilling blind to an assumed depth.
5. Control and record depth
Use a consistent physical or instrumented depth reference. Account for collar breakout and any section that is not a usable hole. If the rod sticks, the hole loses return, or the axis changes, mark it as an exception instead of recording only the nominal rod length.
6. Inspect before handoff
After drilling and before any charging activity, verify collar positions, visible angles, usable depth, blockage, water condition, and exceptions. Keep drilling tools, hoses, and personnel clear under the site's explosives-control procedure. Only trained and authorized personnel should accept the face for the next stage.
Post-blast signs that should change the next drilling round
| Observed result | Drilling questions for the review | Do not assume |
|---|---|---|
| Localized overbreak | Were collars outside the line? Did holes diverge? Did joints redirect the holes? Was the adjacent buffer row drilled as planned? | That the explosive alone caused the damage. |
| Localized underbreak or toe | Were holes short, convergent, blocked, or inside the profile? Was usable depth overstated? | That more energy is the first answer. |
| Irregular crown or shoulders | Did operator access, air-leg footing, or profile-zone geometry change the hole axis? | That one look-out instruction suits the entire arch. |
| Poor round continuity | Did collar references and toe positions connect correctly to the previous and next face? | That the same plan still fits the actual section. |
| Excess scaling and loose perimeter rock | Were perimeter and buffer holes positioned accurately, and were geological exceptions recorded? | That a visually neat collar row proves precision at depth. |
Use survey measurements, drilling records, face photographs, visible half-casts where meaningful, and scaling observations together. Change the drilling or blast plan only through the responsible engineer and licensed blasting process.
Common mistakes to prevent
- Marking collars before the face is fully scaled and surveyed
- Checking collar spacing but not hole direction or usable depth
- Using an old drill pattern on a cross-section that has changed
- Letting hoses pull the drill away from the intended plane
- Applying excessive feed during collaring
- Allowing the air-leg foot to creep on muck or a wet floor
- Forcing a deflected rod back toward the line while drilling
- Failing to flag water loss, open joints, blocked holes, or weak seams
- Moving an inaccessible collar without engineering approval
- Beginning charging before the drilling result and exceptions are accepted
RFQ checklist for a pneumatic tunnel-drilling system
- Tunnel or drift profile and available working space
- Rock type, abrasiveness, jointing, water, and weak-zone observations
- Approved hole diameter range and typical round depth
- Rock-drill model and actual supplier data-sheet revision
- Air-leg model and suffix, extension, mounting, and working orientation
- Drill-steel shank, rod lengths, taper or thread system, and required straightness
- Bit diameter, face, flushing path, and regrinding plan
- Compressor pressure and delivered airflow at simultaneous demand
- Hose lengths, internal diameters, restraints, line oiler, and water supply
- Alignment aids, depth controls, inspection gauges, and spare-tool quantities
Share photographs or drawings of the current drill, air leg, chuck, shank, hose connections, and available face clearance. Do not order from a model name alone when the exact air-leg pairing or drill revision is uncertain.
Frequently asked questions
Are perimeter holes the same as production holes?
No. They occupy different positions and have different jobs in the approved drill-and-blast design. The perimeter row defines the excavation boundary; its spacing, drilling accuracy, and licensed charging design are controlled separately.
What is the most important drilling check for smooth blasting?
No single measurement is enough. Verify collar position, hole direction, look-out, usable depth, and straightness together. A correct collar with a deviated toe is still a wrong hole.
Can the operator choose a look-out angle in the field?
Not by guesswork. The responsible engineer's plan should define the direction for each profile zone. If access or tool geometry prevents that axis, report the constraint for review.
Can an air-leg rock drill produce an accurate tunnel contour?
It can support controlled drilling in suitable headings when the drill, air leg, rod, bit, survey references, operating inputs, and operator method are matched. Repeatability becomes difficult when footing, clearance, hose routing, or alignment references are poor.
Why does one section overbreak while the rest looks acceptable?
Possible contributors include local collar or angle error, drill deviation, a changed joint set, weak seams, the adjacent buffer row, or the blast design. Review drilling and geological records before assigning one cause.
Match the drill system to the heading
For small-heading air-leg applications, review the active YT28 pneumatic rock drill and the published air-leg drilling guide for narrow tunnels. To prepare a configuration review, send PerfoMax a rock-drill RFQ with the tunnel profile, hole requirements, air supply, drill-steel system, water conditions, and current equipment references.
Technical references
- FHWA controlled-blasting guidance — definitions and design variables for smooth blasting.
- NIOSH DRIFT perimeter-control software — coordinated perimeter and buffer design for underground drifting.
- NIOSH research on controlled blasts and excavation damage — the role of precision drilling and perimeter-damage control.
- Epiroc Dynamic Tunneling Package — fitting drill plans to the actual tunnel section and improving contour quality.