Last reviewed 30 Sept 2026 · 8 min read
The method
Drill-and-blast is the traditional and still most widely used method for excavating hard-rock tunnels: holes are drilled into the face in a planned pattern, filled with explosives and blasted, and the broken rock (muck) is removed; the roof and walls are then supported, and the cycle is repeated. It is flexible — it can excavate any cross-section and cope with changes in the ground — and needs less capital than a tunnel-boring machine (TBM), but it is slower in long tunnels, causes vibration and overbreak, and demands strict safety procedures.
The excavation cycle
- Survey and marking — the centreline and grade are set out, and the drill pattern is marked on the face (or by a computer-controlled jumbo).
- Drilling — the holes are drilled by a drill jumbo (a carriage with two to three hydraulic booms) to the specified length and direction.
- Charging — the holes are loaded with explosive, primers and detonators, and stemmed.
- Blasting — the round is fired, after a safety clearance and warning.
- Ventilation — the fumes and dust are cleared (re-entry time of the order of 15–30 minutes or as required).
- Scaling — loose rock on the roof and walls is removed (mechanical scaler or manually with scaling bars).
- Mucking — the blasted rock is loaded by a wheel loader or excavator into dumpers, rail cars or a conveyor.
- Support — shotcrete, bolts, steel ribs or lattice girders are installed as required by the ground class.
- Survey — the profile and the position are checked; the next round is set out.
The cycle time for one round is typically 4–8 hours for a medium tunnel; the daily progress is the advance per round times the number of rounds (for example 3 m × 2–3 rounds per day).
Methods of excavating the section
| Method | Description | Suitable for |
|---|---|---|
| Full-face | The whole cross-section is drilled and blasted at once | Good rock and small to medium sections (up to about 100 m²); the fastest |
| Heading and bench | The upper part (heading) is excavated first and the lower part (bench) follows; the heading can be advanced ahead with the support installed | Larger or weaker sections; allows the roof to be supported early |
| Top heading, bench and invert | Three stages, with the invert closed to form a ring | Poor ground, squeezing rock, NATM |
| Multiple drifts (pilot drifts, side drifts) | The section is divided into small drifts, each excavated and supported, then linked | Very large caverns and very poor ground |
Drilling
- Equipment: hydraulic drill jumbos with rock drills that use percussion, rotation and flushing (water or air); computer-controlled ("jumbo with computerised positioning") for precise hole patterns and profile control.
- Hole diameter about 38–51 mm (up to 64 mm); hole length typically 3–5 m (longer in large tunnels); the advance per round is about 90–95 % of the hole length (the "pull").
- Bits: button bits of tungsten carbide, regrinded regularly.
- Accuracy: hole position and inclination determine the overbreak and the fragmentation; the look-out angle (drilling slightly outward) is needed to keep the profile from shrinking.
The blast pattern
Holes of a round are classed by function, and they fire in order:
| Group | Location | Function |
|---|---|---|
| Cut (burn cut, wedge cut, V-cut, cylinder cut) | The centre of the face | Creates the first free face: a cavity into which the following holes can break |
| Easers (stoping, enlargers) | Around the cut | Break the rock toward the cavity in successive layers |
| Lifters (floor holes) | Along the floor | Lift and break the floor rock; heavier charges to lift the muck |
| Contour (perimeter, roof and wall holes) | Around the edge | Shape the final profile; light charge in smooth blasting |
A burn cut uses parallel holes, some empty (uncharged), in the centre; the charged holes break into the empty ones. It is common in modern practice because it gives a consistent cut without directing the holes at an angle.
Number of holes: depends on the area of the section and the rock: roughly 1.5–3.5 holes per m² for hard rock, more in strong rock. The specific charge (kg of explosive per m³ of rock) is typically about 1–2.5 kg/m³ (higher in small sections and hard rock); the value is refined by trial rounds.
Explosives and initiation
- Explosives: emulsion and watergel cartridges, and bulk emulsion pumped into the holes; ANFO (ammonium nitrate/fuel oil) in dry holes; dynamite in small quantities for special cases. The choice depends on water (emulsions are water-resistant), the rock hardness and the fume characteristics.
- Detonators: non-electric (NONEL, shock-tube) and electronic detonators with programmable delay; electric detonators are avoided where stray currents or radio transmissions are possible.
- Delay timing: each hole or group fires with a millisecond (or half-second) delay so that the cut fires first and the others follow, each with a free face to break toward. Total round duration is typically a few seconds.
- Stemming with sand or clay confines the charge.
- Handling: explosives are stored in approved magazines and transported under licence, with strict inventory, a blast plan for each round, and a blaster with a licence.
Smooth blasting and overbreak control
In smooth (contour) blasting the perimeter holes are closely spaced, lightly charged (decoupled charges — cartridges much thinner than the hole, or string charges) and fired last, so that a clean crack forms between holes and the rock is left undamaged. Benefits: reduced overbreak (less shotcrete and concrete, less extra excavation), a more stable rock surface and less loosening of the rock mass. Overbreak (excavation beyond the design line) is measured by the profile scan and is a major factor in cost; underbreak (rock left inside the line) must be trimmed.