In short
- Controlled blasting is any blast designed to keep ground vibration, flyrock, overbreak and noise inside a stated limit. It is a family of methods, not one.
- Delay sequencing does most of the work on vibration, because what reaches a structure depends on the charge fired on any one delay and the distance.
- Pre-splitting, smooth blasting and line drilling protect the final wall. Muffling contains flyrock.
- It costs more per cubic metre than an open bench, and the client should receive a blast design for each bench and a vibration record for each shot.
Controlled blasting is blasting designed to keep ground vibration, flyrock, overbreak and noise inside limits set by whatever stands near the shot. It is not a single technique, and it is not a special explosive.
The term covers delay sequencing, pre-splitting, smooth blasting, line drilling and muffling. A house wants low vibration. A carriageway wants no flying rock. The wall of a highway cutting wants to be left sound, because it stands beside traffic for as long as the road does.
Four things a controlled blast limits
A production shot on an open bench has one target: rock the crusher can take, at the lowest cost per tonne. Control adds a second target, sometimes a third, and each is paid for in drilling metres or in time.
- Ground vibration, measured as peak particle velocity in mm/s at the structure and limited by the DGMS vibration table.
- Flyrock: fragments thrown beyond the area cleared for the shot.
- Overbreak: rock shattered or loosened behind the line where the excavation was meant to stop.
- Noise and air overpressure: the pressure wave that travels through the air and rattles windows.
The methods at a glance
| Method | What it does | Where it is used |
|---|---|---|
| Delay sequencing | Fires the holes in a timed order, so only part of the explosive in the shot detonates at any instant | Almost every bench blast, and the first tool near structures |
| Pre-splitting | A row of closely spaced, lightly charged holes along the final line, fired ahead of the production holes to form a crack the main blast breaks to | Final walls of highway cuttings and permanent quarry faces |
| Smooth blasting | A row of closely spaced, lightly charged and stemmed holes along the final line that trims the rock back to the wall | Final faces and trimming where a clean wall matters |
| Line drilling | A row of very closely spaced holes left uncharged, giving the blast a plane of weakness to break to | Close to structures where even a light perimeter charge is unwelcome |
| Muffling | Covers the blast area with heavy material so that fragments are held down | Small shots close to buildings, roads and services |
Pre-splitting, smooth blasting and muffling follow the Dyno Nobel Blasting and Explosives Quick Reference Guide (2010) and DGMS Circular 2 of 2003. Line drilling is given in its general textbook sense.
Delays and the maximum charge per delay
Delay detonators fire the holes of a shot a fraction of a second apart, in a planned order. To the ground, each hole arrives as a separate event.
That is why the number that matters near a structure is the maximum charge per delay: the largest quantity of explosive detonating at one instant. The total in the shot matters far less. Vibration rises with the charge per delay and falls with distance, and the two are combined into a scaled distance: the distance divided by the square root of the charge per delay. The constants that turn it into a predicted vibration belong to the site and come from trial blasts.
Delays do a second job. Each hole needs a free face to break towards, and the sequence gives it one. Fire a back row before the front row has moved and the rock can only go up. DGMS Circular 2 of 2003 lists millisecond delay detonators and shock tubes among the means of a controlled blast.
Protecting the final wall
Pre-splitting and smooth blasting are aimed at overbreak, not vibration. Both use a row of closely spaced holes along the final line, charged far more lightly than production holes, with the charge narrower than the hole so that it does not bear hard on the rock that is to remain. Blasters call that a decoupled charge.
Textbook first estimates put pre-split holes about 12 hole diameters apart, and smooth-blasting holes at 15 diameters in hard rock to 20 in soft. Those are starting points from the Dyno Nobel guide. The rock and a trial length of wall decide.
The cost is drilling: many more holes than a production row over the same length, which is why a perimeter row is specified for final walls and not for every bench. Burden, spacing and hole depth explains the production pattern. On a highway cutting the return comes later, in a slope that needs less scaling and less protection work.
Muffling and flyrock
Muffling means covering the blast area with heavy material so that fragments are held down. Old conveyor belting weighted with sandbags is the usual example. DGMS Circular 2 of 2003, 'Dangers due to blasting projectiles', lists adequately muffled holes among the accepted controls.
It has limits. Covering is practical for small shots and slow on large ones, and it contains rock without curing the reason the rock wanted to fly. What causes flyrock goes through those reasons one by one.
When controlled blasting is called for
- Houses, a village or any permanent structure stand within range of vibration or thrown rock.
- A public road or a railway runs close enough to need guards and a traffic stop for each shot.
- The face being cut is a final one: the side slope of a highway cutting or a permanent quarry wall.
- The contract or the permission states a vibration limit at a named structure.
And when it is not: an open bench with nothing inside the danger zone. Pre-splitting every production bench, or muffling a shot nobody is near, spends money on a limit that is not there.
What it costs against an open bench
More per cubic metre, always. Near a boundary the design is a smaller charge per delay, tighter sequencing and more shots of less size. That means more drilled metres for each cubic metre of rock, more visits by the shotfirer, labour for muffling, guards on the road, and excavators that wait longer between shots.
Set that against a complaint, a stop-work notice and a district officer on site. Controlled blasting costs more than an open bench and a great deal less than a stop-work order. The rules that apply close in are covered in blasting near houses, roads and structures.
What the client should receive
Two documents, at the least. The first is a blast design for each bench: the pattern, hole depth and diameter, the delay sequence, the maximum charge per delay and the structures it was designed around. The second is a vibration record for each shot, giving peak particle velocity and frequency at the nearest structure.
Where structures are close, add a survey of their condition made before the first shot. Our controlled blasting crews work to a design for each bench with vibration monitored on every shot, and the drilling is done by the same firm, so the hole and the charge have one owner.
Standards and sources
- DGMS (SOMA)/(Tech) Circular No. 2 of 2003, 31 January 2003, 'Dangers due to blasting projectiles': controlled blasting code of practice and muffling
- DGMS (Tech)(S&T) Circular No. 7 of 1997, 'Damage of structures due to blast induced ground vibrations in the mining areas'
- Dyno Nobel, Blasting and Explosives Quick Reference Guide (2010): pre-split and smooth-blasting first estimates, scaled-distance vibration predictor
- Blast design and vibration monitoring practice: SM Infra's own operating record
Your contract and its technical schedules override anything written here. Check the clause before you build to it.