How Many Air-Leg Rock Drills Does an Underground Heading Need?

One underground heading does not automatically need one air-leg rock drill, and buying four drills does not mean four can work at the face together. The correct count depends on the approved hole program, real cycle time per hole, available drilling time, physical space for crews and hoses, delivered air and water, and the production schedule across all headings.

This guide is a procurement planning worksheet. It does not design blast holes or prescribe staffing. The mine's qualified engineering and safety team must approve the hole program, work sequence, operator positions and exposure controls. First estimate the active tools the schedule requires; then cap the count by the workface and utilities that can support them. The remaining gap is a planning issue, not a reason to force more equipment into a confined heading.

Five different counts that buyers often confuse

Keep these numbers on separate lines. A request that only says “four jacklegs” leaves the supplier unable to verify whether the air system, leg, hose and drill steel package are complete.

Step 1: freeze the approved work program

Record the number of holes by category, planned depths, diameters and angles, plus the number of headings expected to drill during the same shift. Use the site's approved design rather than an example hole pattern found online. Note the shift length, but separately state the actual time available for face drilling after access, inspection, services, scaling, setup and other planned activities.

For each hole category, the operations team should provide measured or trial-based cycle time. Cycle time includes positioning, collaring, active drilling, steel/bit handling, flushing and moving to the next hole within the agreed boundary. A net ROP figure measures only part of that cycle; use the ROP definition guide before comparing quoted rates.

Step 2: calculate total planned tool time

For each hole group, multiply approved hole count by measured average cycle time under a documented condition. Add the groups:

Planned tool-minutes = Σ (hole count by group × cycle minutes per hole).

Then add separately justified changeover, inspection and service time that is not already inside the measured cycle. Do not add the same delay twice. If the rock varies across the face, use more than one cycle-time group rather than a single optimistic average.

Example *only to demonstrate arithmetic*: suppose an approved program has 24 holes in one group at 6 minutes of measured cycle time and 12 holes in another at 9 minutes. Planned tool time is 24×6 + 12×9 = 252 tool-minutes. That number is not a productivity claim for any PerfoMax model or mine. The site must replace the illustrative inputs with its own measured values.

Step 3: estimate a theoretical active-tool count

Divide total planned tool-minutes by the usable drilling window per tool, then round up:

Theoretical active tools = ceiling(planned tool-minutes ÷ usable drilling minutes per tool).

If the available drilling window in the illustration were 150 minutes, ceiling(252÷150) = 2 tools before shared delays and constraints. This is only a first-pass scheduling calculation. Two drills can make the plan only if both can work safely at the face for their assumed minutes, and air/water can support simultaneous use.

Do not convert an eight-hour shift into eight hours of bit-on-rock time. Stantec's Hard Rock Miner's Handbook treats development productivity as a whole operation that includes much more than drilling; the same discipline applies here without importing its rule-of-thumb rates into a different site.

Step 4: apply the workface constraint

Check the heading width/height, collar locations, leg placement, operator clearance, hose routing, escape/access path, visibility, dust-control arrangement and whether one tool's operation interferes with another. The safe simultaneous count can be lower than the theoretical count.

The narrow-tunnel air-leg guide covers physical envelope questions. A second drill that has no approved operator position, leg position or hose route is not usable capacity. It may be more effective to re-sequence work, use another heading or evaluate a different system than to crowd one face.

Step 5: apply the air and water constraint

Each active drill needs model-specific delivered air at its required pressure and suitable water flushing. The compressor panel reading with the tools idle cannot confirm this. Count every simultaneous pneumatic user, use compressor FAD at the site condition, and verify dynamic pressure near each tool while the planned number operate.

Use the mixed-fleet compressor sizing guide for total-demand inputs and the air-hose sizing guide for branch delivery. If the existing supply can run only two drills under load, purchasing a third active drill without an approved utility upgrade will not produce a third drill's output.

Water supply and dust control also matter. If the site's approved method requires wet drilling, count only tools that can receive stable flushing through compatible connections. Do not treat a missing water line as a reason to switch silently to dry drilling.

Step 6: distinguish active tools from owned reserve

The owned fleet may exceed the simultaneously active count because tools are in inspection, repair, changeover or transit. Build reserve from evidence, not a universal spare percentage:

1. how many headings use the same model or compatible package;
2. how often tools are unavailable and for how long;
3. whether a spare drill has a compatible air leg and steel/bit system;
4. how quickly a repaired or replacement machine can reach the site;
5. whether the compressor and crews can use the spare if a new heading opens.

The Air-Leg Rock Drill selection guide helps confirm the machine family. A spare with the wrong shank, leg interface or water/air connection is not a functional backup, even if its nameplate is similar.

Planning worksheet

If the approved active count is lower than the theoretical count, change the schedule, utilities or equipment approach. Do not hide the shortfall by claiming a higher rated impact rate will automatically shorten every hole cycle.

RFQ inputs

Send heading dimensions, approved hole count/depth/diameter categories, measured cycle-time basis, usable drilling window, simultaneous headings, site altitude, available compressor FAD and pressure basis, water layout, tool models, crew/maintenance capacity and current usable fleet. Review the Pneumatic Rock Drills collection, then request a fleet-package review. A supplier can then quote the drill, air leg, hose/oiler, steel/bit and spare scope against a defined task rather than a guessed machine count.

FAQ

Why can't I divide hole count by a brochure drilling rate?

A brochure net ROP is not a complete hole cycle or shift. Positioning, collaring, flushing, steel handling, rock variation, access and delays must be included on a consistent measurement basis.

If the calculation says three tools, must three drill at once?

No. The heading, crews, air and water may safely support fewer. The gap means the schedule or infrastructure must be revised; it does not authorize an unsafe workface arrangement.

Does a spare drill require its own compressor capacity?

Not while it remains a spare, but if the plan uses it as another simultaneous active tool, the source and distribution system must support that additional demand at the required condition.