Good Vibrations
Measuring blast-generated vibration
by Andy Maslin of Accudata Ltd
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 threepenny bit on the window sill 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 in order to obtain the most accurate readings.
The sensing unit consists of three geophones arranged so that two are set in a horizontal position at right angles to each other and one is set 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 such that only the vibrations of the structure are recorded. This may be easier said than done and in some cases, especially if the co-operation of the complainant is not forthcoming, it is thought better to monitor clandestinely so that unnecessary worry is not caused to residents.
If it is only possible to couple the sensor to the ground rather than the structure, certain precautions should be observed in order to minimize 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 orientated but firmly held in place. It is important that the sensing elements are not able to move independently of the structure being monitored. A paving slab adjacent to a property or a thin layer of asphalt are poor surfaces on which to place 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-1, ground coupling may be achieved by merely placing the sensing unit on a firm surface. With higher levels of vibration, if ground coupling is not good, the entire pack may be jolted and ‘jump’ upwards. The resulting fall back into place, although perhaps only a millimetre or less, is likely to cause additional and 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% of being level, so 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 or, if they are set too high, the geophone sensor may be unstable and cause added vibration.
Monitoring personnel skilled in the reading of vibration traces are often able to spot the telltale ‘vibration signature’ arising from 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 millimetres) of the signal is the actual movement. The formula for sine waves is PPV=2?fa, where f is the frequency in Hz and a is the amplitude in mm. For a PPV of 10mm/s-1, at a frequency from a typical UK quarry blast, the actual movement of the ground at the surface is very likely to be less than 1mm.
British Standards measure PPV with the lowest frequency being 4Hz. It is these low frequencies below 50Hz that are the most likely to give rise to damage, if levels of PPV are high.
When the relevant British Standards were written it was not feasible to measure the vector sum (resultant) of the PPV, so the Standards ask for ‘maximum plane’ readings, the largest reading from any of the three axes.
While most seismographs are now able to measure the resultant PPV, and there are those who use it, it is not required when monitoring in compliance with British Standards. It will normally be up to 10% higher than the maximum plane reading and its use could mean that levels are exceeded that would not have been had maximum plane readings been used.
An often misunderstood feature of a blast event is the air overpressure caused by the explosive. There are two components to consider, the acoustic component, the characteristic booming sound of the explosion that is carried by the second component, the non-audible pressure wave. It is the increased air pressure that has the potential to be damaging.
The air overpressure is of low frequency and usually measured 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 20kHz) measured in decibels with an ‘A’ weighted adjustment and written as dB(A). That is 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 the occupants of a building is of a longer period of ground vibration.
Air overpressure is monitored by a seismograph’s fourth channel by way of 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 the monitoring point is, 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 UK quarry blasting, where relatively small charge weights are used and blast design is among the best in the world, is highly unlikely to cause damage but, through its perception, 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 very much 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. Minimizing air overpressure, however, is always beneficial for as well as greatly reducing the likelihood of complaints, the less energy that escapes into the atmosphere, the more energy will be concentrated on rock fragmentation.
Regular use of a seismograph greatly helps public relations as well as being necessary in the fulfilment of obligations imposed by conditions.
by Andy Maslin of Accudata Ltd
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 threepenny bit on the window sill 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 in order to obtain the most accurate readings.
The sensing unit consists of three geophones arranged so that two are set in a horizontal position at right angles to each other and one is set 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 such that only the vibrations of the structure are recorded. This may be easier said than done and in some cases, especially if the co-operation of the complainant is not forthcoming, it is thought better to monitor clandestinely so that unnecessary worry is not caused to residents.
If it is only possible to couple the sensor to the ground rather than the structure, certain precautions should be observed in order to minimize 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 orientated but firmly held in place. It is important that the sensing elements are not able to move independently of the structure being monitored. A paving slab adjacent to a property or a thin layer of asphalt are poor surfaces on which to place 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-1, ground coupling may be achieved by merely placing the sensing unit on a firm surface. With higher levels of vibration, if ground coupling is not good, the entire pack may be jolted and ‘jump’ upwards. The resulting fall back into place, although perhaps only a millimetre or less, is likely to cause additional and 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% of being level, so 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 or, if they are set too high, the geophone sensor may be unstable and cause added vibration.
Monitoring personnel skilled in the reading of vibration traces are often able to spot the telltale ‘vibration signature’ arising from 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 millimetres) of the signal is the actual movement. The formula for sine waves is PPV=2?fa, where f is the frequency in Hz and a is the amplitude in mm. For a PPV of 10mm/s-1, at a frequency from a typical UK quarry blast, the actual movement of the ground at the surface is very likely to be less than 1mm.
British Standards measure PPV with the lowest frequency being 4Hz. It is these low frequencies below 50Hz that are the most likely to give rise to damage, if levels of PPV are high.
When the relevant British Standards were written it was not feasible to measure the vector sum (resultant) of the PPV, so the Standards ask for ‘maximum plane’ readings, the largest reading from any of the three axes.
While most seismographs are now able to measure the resultant PPV, and there are those who use it, it is not required when monitoring in compliance with British Standards. It will normally be up to 10% higher than the maximum plane reading and its use could mean that levels are exceeded that would not have been had maximum plane readings been used.
An often misunderstood feature of a blast event is the air overpressure caused by the explosive. There are two components to consider, the acoustic component, the characteristic booming sound of the explosion that is carried by the second component, the non-audible pressure wave. It is the increased air pressure that has the potential to be damaging.
The air overpressure is of low frequency and usually measured 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 20kHz) measured in decibels with an ‘A’ weighted adjustment and written as dB(A). That is 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 the occupants of a building is of a longer period of ground vibration.
Air overpressure is monitored by a seismograph’s fourth channel by way of 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 the monitoring point is, 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 UK quarry blasting, where relatively small charge weights are used and blast design is among the best in the world, is highly unlikely to cause damage but, through its perception, 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 very much 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. Minimizing air overpressure, however, is always beneficial for as well as greatly reducing the likelihood of complaints, the less energy that escapes into the atmosphere, the more energy will be concentrated on rock fragmentation.
Regular use of a seismograph greatly helps public relations as well as being necessary in the fulfilment of obligations imposed by conditions.
