Ground vibration limits for blasting in India: the DGMS table and how to stay inside it

The permissible ground vibration from blasting in India is set by DGMS Technical Circular 7 of 1997. It is stated as peak particle velocity (PPV) in mm/s at the foundation level of the structure, and it changes with the type of structure, who owns it and the dominant frequency of the vibration.

For an ordinary house that does not belong to the mine owner, the limit is 5 mm/s below 8 Hz, 10 mm/s from 8 to 25 Hz and 15 mm/s above 25 Hz. Staying inside the table comes down to two quantities a blast designer controls or measures: the charge fired on any one delay, and the distance to the structure.

What PPV measures

When a blast wave passes, each point in the ground moves a little and comes back. Peak particle velocity is the highest speed that point reaches, in millimetres per second. It is not the speed at which the wave travels through the rock.

A blasting seismograph placed at the structure records the motion and reports two things: the peak, and the frequency at which most of the energy arrived. Both are needed. A PPV with no frequency beside it cannot be placed in the DGMS table.

Why frequency changes the limit

Buildings respond most to slow shaking. Low-rise buildings have low natural frequencies, so vibration below 8 Hz makes a house move with the ground and strains its walls, while the same particle velocity at a higher frequency passes with less response. The circular allows three times as much above 25 Hz as below 8 Hz for a house.

So one reading can pass or fail on its frequency alone. Nine mm/s at 30 Hz is inside the limit for a villager's house. Nine mm/s at 6 Hz is nearly double it.

The DGMS table

Three points on reading it. 'Owner' means the owner of the mine, so a villager's house falls in group A and the quarry's own site office in group B. The lowest figures protect historical and sensitive structures, at 2 mm/s when the frequency is low. And the figures are ceilings, not design targets: a contract or a permission can set a tighter number, and then the tighter number is your limit.

What decides the vibration at a structure

Charge per delay and distance. The total explosive in a shot matters much less than the largest quantity detonating at one instant, because delay detonators separate the holes in time and the ground receives them as separate events. Delay sequencing and the other controlled blasting methods cover how that is arranged.

The two are combined in a scaled distance: the distance from the shot to the structure, divided by the square root of the maximum charge per delay. Predicted PPV is a site constant multiplied by the scaled distance raised to a negative power, the form given in the Dyno Nobel Blasting and Explosives Quick Reference Guide. Both constants belong to the site. Handbook values differ tenfold between a shot with a free face and a heavily confined one, which is reason enough not to design near a house from a handbook.

How compliance is shown

1. Trial blasts. A few small shots, each monitored at measured distances, to find how this rock carries vibration. They produce the site's own constants.
2. A design limit. From those constants, the maximum charge per delay that keeps the predicted PPV under the table value at the nearest structure, with a margin.
3. A seismograph at the nearest structure for the shots that follow. Where houses and a sensitive structure are both in range, each is checked against its own row.
4. A record for each blast: date, time, position on the bench, maximum charge per delay, distance, PPV and frequency.

On our sites vibration is monitored on every shot, and nearby structures are assessed before the first one. Blasting near houses, roads and structures covers that survey and the separate charge limits the mine regulations impose close to buildings.

Air overpressure and noise

Air overpressure is the pressure wave that travels through the air from a shot. It rattles windows and doors, and it brings complaints on days when ground vibration is well inside the table.

We have not found a notified Indian limit for it, from DGMS or from the Central Pollution Control Board. That is not proof that none exists, only that our search did not produce a number. Contracts and permissions sometimes set one, usually borrowed from American or Australian practice. If yours does, that figure is what you work to. If it does not, agree one before the first shot and not after the first complaint.

When readings approach the limit

The response is to cut the charge per delay or to change how the energy leaves the shot. There are several ways to do each.

• Put fewer holes on each delay, down to one, so the maximum instantaneous charge falls while the shot stays the same size.
• Drill smaller-diameter holes on a closer burden and spacing, so each hole carries less explosive. It means more drilling for the same rock.
• Change the direction of initiation so that the shot progresses away from the structure and not towards it.
• Make sure every hole has a free face to break towards. A choked shot sends its energy into the ground instead of into moving rock.

All of them cost drilling metres or time. If a face still reads close to the limit, what remains is smaller shots, or mechanical breaking for the last strip of rock. Our blasting crews design each shot for the bench in front of them, and the readings from the last shot are part of that design.