The short answer
Ground vibration from blasting is measured as peak particle velocity (PPV) in mm/s. DGMS Technical Circular 7 of 1997 limits it at houses not owned by the mine to 5, 10 or 15 mm/s depending on the dominant frequency, and lower still for historic or sensitive structures. The main control is the weight of explosive fired on any single delay: halve it and vibration at a given distance falls by roughly 40 per cent.
A blast does three things beyond breaking rock. It shakes the ground, it pushes a pressure wave through the air, and now and then it throws stone. Controlled blasting is the practice of designing a shot so that all three stay inside limits set by what is standing nearby. This article is about the first.
01What PPV measures
When a charge detonates, a wave travels outward through the ground. A seismograph placed at a building records how fast the ground particles move as that wave passes, in millimetres per second, along three axes. The highest reading is the peak particle velocity. It is used because damage to buildings tracks velocity more closely than it tracks displacement or acceleration.
Frequency matters as much as the peak. Houses have natural frequencies of a few hertz to perhaps fifteen. A low-frequency wave near that range makes the structure respond and amplify the motion. A high-frequency wave of the same velocity passes with far less effect. That is why the permitted velocity rises with frequency.
02The DGMS limits
| Type of structure | Below 8 Hz | 8 to 25 Hz | Above 25 Hz |
|---|---|---|---|
| Buildings not belonging to the owner | |||
| Domestic houses: kutcha, brick and cement | 5 | 10 | 15 |
| Industrial buildings: RCC and framed | 10 | 20 | 25 |
| Historic objects and sensitive structures | 2 | 5 | 10 |
| Buildings belonging to the owner, limited life | |||
| Domestic houses and structures | 10 | 15 | 25 |
| Industrial buildings | 15 | 25 | 50 |
DGMS (Tech)(S&T) Circular No. 7 of 1997. A contract, an environmental clearance or a district order may set a lower figure, and the lower one applies.
03Distance and charge per delay
Vibration at a point depends on two things the blaster can know in advance: how far away the point is, and how much explosive detonates at one instant. The two are combined into a scaled distance, which is the distance in metres divided by the square root of the charge in kilograms. Vibration falls steeply as scaled distance rises.
The exact relationship belongs to each site and has to be measured there. A few small trial blasts are monitored at several distances, and the readings give two constants for that ground. Before any site data exists, a widely used first estimate for a confined bench blast is PPV = 1,140 x (scaled distance) to the power of minus 1.6. The table below comes from that estimate.
| Distance to structure | For 10 mm/s | For 5 mm/s | For 2 mm/s |
|---|---|---|---|
| 50 m | 6.7 | 2.8 | 0.9 |
| 100 m | 27 | 11 | 3.6 |
| 200 m | 107 | 45 | 14 |
| 300 m | 240 | 100 | 32 |
| 500 m | 670 | 280 | 90 |
An estimate for planning, not a design. Replace it with constants from monitored trial blasts on the actual site before production blasting begins.
Read across the 100 m row. A single 110 mm production hole on an 8 m bench carries around 50 kg of explosive. At 100 m from a house, with a 10 mm/s limit, even one such hole per delay is too much. The design has to change.
04What the blaster changes
- Smaller holes. A 64 or 76 mm hole carries a fraction of the charge of a 110 mm hole, on a proportionally tighter pattern.
- Lower benches. Half the bench height is half the charge per hole.
- Decking. The column in one hole is split into two or three charges separated by inert stemming, each on its own delay.
- One charge per delay. Every hole or deck fires at its own instant, so the waves arrive separately instead of adding up.
- Relief. Each hole must have a free face to break to. A choked shot converts its energy to vibration.
- Direction. Firing so the shot progresses away from the structure lowers the reading there.
- A pre-split line. A row of closely spaced, lightly charged holes fired first creates a crack between the blast and what has to be protected.
Every one of these adds drill metres, detonators or shifts for the same volume of rock. That is the real meaning of the phrase controlled blasting on a bill of quantities, and why its rate is higher. The build-up is in drilling and blasting cost per cubic metre.
05Air blast and fly rock
Air overpressure rattles windows and frightens people at levels far below those that damage anything, and it produces more complaints than ground vibration does. It is controlled at the collar: adequate stemming of crushed stone, no exposed detonating cord, and no charges close to an open face. Still mornings with low cloud carry sound further, so the time of firing matters too.
Fly rock is the hazard that injures. Close to property the shot is covered with heavy mats or sand-filled bags over wire mesh. Its causes and controls have their own article.
06Before the first shot
- Survey every structure inside the likely zone of influence, with dated photographs of existing cracks, and give the owner a copy.
- Fix the limit that applies to each one, from the table and from any stricter condition in the permissions.
- Fire monitored trial blasts and work out the site constants.
- Set the maximum charge per delay for each distance band from those constants.
- Place a seismograph at the nearest structure for every production blast and keep the records.
- Tell the neighbours the firing times and keep to them.
The condition survey is the step most often skipped and the one that settles most disputes. A crack photographed in March cannot be blamed on a blast in May.