Road Transport

Measuring blast-generated vibration

It is said that in the 1960s quarry managers who received complaints about vibration from blasting would visit the complainant immediately before a blast, place a coin on the windowsill and if it did not fall over during the blast, the event would be considered to have been acceptable.

Of course, this was at a time before vibration monitors were commonplace.

The first mechanical instruments were very expensive and, therefore, rare and tricky to set up and use.

The output acetate ribbon had to be blackened with carbon, placed on a light box and the maximum amplitude of the trace had to be physically measured with a ruler. A look-up table was then used to convert the readings into peak particle velocity (PPV).

How easy things have become. The latest generation of seismographs have a menu-driven set-up and are intuitive to use, making a user manual almost unnecessary.

They may be left unattended (security permitting) for days, if not weeks, before being downloaded.

Now, the main consideration is the placement of the sensor to obtain the most accurate readings.

The sensing unit consists of three geophones that are arranged so two are set in a horizontal position at right angles to each other and one is in a vertical alignment.

The three axes x, y and z are often named, radial, transverse and vertical (R, T and V).
Each geophone consists of a coil of fine copper wire around a cylindrically-shaped, powerful, permanent magnet held centrally within the coil by delicate springs. Any small movement of the coil around the magnet creates a voltage proportional to the movement. This is the same principle as a loudspeaker but in reverse; in a loudspeaker, voltage is applied to a coil to produce proportional movements.

The geophone sensor must be connected (or coupled) to the structure being monitored in such a way that only the vibrations of the structure are recorded. This may be easier said than done and, in some cases, especially if the cooperation of the complainant is not forthcoming, it is thought better to monitor clandestinely so that residents
do not worry unnecessarily.

If the sensor can only be coupled to the ground rather than the structure, certain precautions should occur to minimise spurious readings. On soil, a ground spike may be used. Some geophone packs have provision for spikes to be screwed into the base so that the pack may be spiked into the ground. Alternatively, the geophone pack may be buried or have a loosely filled sandbag placed on top of it so that the pack remains correctly oriented, but firmly held in place. It is important that the sensing elements cannot move independently of the structure being monitored. A paving slab next to a property or a thin layer of asphalt are poor surfaces for placing a geophone pack because a paving slab is likely to move more than the structure and a thin layer of asphalt could amplify the vibration.

If levels of vibration are below 10mm/s, ground coupling may be achieved by merely placing the sensing unit on a firm surface. With higher levels of vibration, if ground coupling is poor, the entire pack may be jolted and ‘jump’ upwards. The resulting fall back into place, although perhaps only a millemetre or less, is likely to cause spurious vibration to be generated.

Some manufacturers use levelling legs, with locking nuts and a spirit level bubble, so that the geophone pack may be set dead level. Modern geophones, however, are accurate when they are within 12 per cent of being level, therefore, the requirement for a bubble or levelling legs is minimal. Incorrectly used, levelling legs may lead to inaccuracies if they are not locked in place. Similarly, if they are set too high, the geophone sensor may be unstable and cause added vibration.

Monitoring personnel who are skilled in the reading of vibration traces are often able to spot the telltale ‘vibration signature’ from an incorrect coupling of a sensor unit.

It is important to note that peak particle velocity (PPV) is the velocity of molecular particles within the ground and not surface movement. The amplitude (in millimeters) of the signal is the actual movement. The formula for sine waves is PPV=2pfa, where ‘f’ is the frequency in Hz and ‘a’ is the amplitude in mm. For a PPV of 10mm/s, at a frequency from a typical quarry blast, the actual ground movement at the surface is very likely to be less than 1mm.

PPV with the lowest frequency is around 4Hz. The low frequencies below 50Hz are the most likely to give rise to damage, if levels of PPV are high.

A frequently misunderstood feature of a blast is the air overpressure caused by the explosive. There are two components to consider – the acoustic component or the characteristic booming sound of the explosion that is carried by the second component, namely, the non-audible pressure wave. The increased air pressure has the potential to be damaging.

The air overpressure is of low frequency and usually measures between 2Hz and 250Hz. It is normally measured in Pascals and stated in terms of decibels. It is the peak pressure level that is measured.

Air overpressure, where peak levels are used, should not be confused with full spectrum noise (16Hz to 20Hz) measured in decibels with an ‘A’ weighted adjustment and written as db(A). This means noise, as detected by the ear, where root mean square (rms) values are used rather than peak levels.

Vibration travels through the ground faster than through air so during a blast, ground-borne vibration arrives at the monitoring point first, followed by the air overpressure moments later. A building already affected by ground vibration may then be additionally moved by the air overpressure. The overall effect to a building’s occupants is a longer period of ground vibration.

Air overpressure is monitored by a seismograph’s fourth channel, by a pressure sensor (microphone), with the necessary characteristics to measure the very limited low-frequency range at the relatively high levels of amplitude. As soon as the seismograph has been triggered by ground vibration, the pressure sensor is activated to record the blast wave that will always arrive behind the ground-borne vibration. The further away from the blast of the monitoring point, the longer the lag between the vibration and the air overpressure.

Certain seismographs may be set to trigger on the air overpressure. This may mean that ground vibration is not recorded, as it may already have fully dissipated by the time the air overpressure arrives at the monitoring location. Gusts of wind are also likely to trigger a seismograph set to trigger on air overpressure, so there may be very few circumstances when it can be used sensibly.

In practice, air overpressure from quarry blasting, where relatively small charge weights are used, is highly unlikely to cause damage. However, the perception is that air overpressure is highly likely to increase awareness of ground vibration.

Although most of the air overpressure generated by a blast will tend to move away from the front of the quarry face, the actual direction taken and distance travelled are highly unpredictable and intensely affected by the prevailing weather conditions. Low cloud, wind direction and temperature inversions may, on occasion, channel air overpressure into areas from where complaints are not normally heard. However, minimising air overpressure is always beneficial for greatly reducing the likelihood of complaints, and recognising that the less energy that escapes into the atmosphere, the more energy will be concentrated on rock fragmentation.

Regularly using a seismograph greatly helps public relations and is necessary in fulfilling obligations imposed by conditions.

Andy Maslin is a technical expert with vibration monitoring company, Accudata Ltd.

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