This note is for anyone who needs to measure levels of ground-borne vibration and air over-pressure arising during quarry blasting. You will need to use a ground vibration monitor (seismograph) and these may be rented or purchased from Accudata Ltd.
The Accudata GVM-10 is specifically designed for the job and will meet in full the requirements outlined in the British Standard BS 7385-Part 2:1993.
The GVM-10 is as accurate if not more accurate than any other seismograph currently available. However, as with all monitoring instrumentation, accuracy may be impaired by incorrect use*.
The GVM-10 is designed for extreme ease of use by someone who has never seen a seismograph before. The unit has a single range and the user guide is less than a single sided A4 page.
To start monitoring, place the tri-axial sensor and the air over-pressure sensor (low frequency microphone) in position (more of this later*) and switch on. A self-test is performed with each of the sensors being checked for accuracy. After a few seconds the main screen appears. Pressing the waveform button will put the GVM-10 into monitoring mode.
The GVM-10 will use the previous settings for this mode, typically a threshold level of 0.5mm/s with a 3 second scan time after each triggered event. When triggered, the vibration level in each of the three axes and the air over-pressure level is shown on the screen together with their frequencies.
To stop monitoring, press the waveform button. The results are stored on an SD card. Remove the SD card and plug it into a laptop or pc and use the supplied software to view the results.
*Correct placement, levelling and siting of the tri-axial geophone sensor unit is crucial to ensure that accurate measurements are recorded. The sensor unit must be level to within 10o and well connected (coupled) to the ground or structure. Three 60mm long ground spikes are provided with the GVM-10. These may be used in areas where the ground is compacted clay or mud. They are screwed into the base of the tri-axial geophone’s housing and the spikes are then pushed into the ground to ensure good coupling. The sensor has a built-in spirit level to assist levelling. The air over-pressure microphone senses low frequency unweighted Peak pressure levels in terms of dB between 100dB and 148dB with a frequency range from 2Hz to 250Hz.
There are various methods for fixing the tri-axial sensor in place dependent upon the site’s conditions. Direct connection of the sensor to the structure being monitored, is the ideal solution but it is not always possible. BS:5228 recommends placing the sensor unit outside at foundation (ground) level. If ground vibration levels are below 10mm/s then placing the sensor unit on solid ground is a sufficient method of use. A paving slab is not a good place to site the sensor unit as the slab may move far more than the structure that is the subject of the monitoring.
It is very important to make sure that the tri-axial sensor is rigidly attached to the ground or structure as a loose connection will amplify the vibration and give higher false readings.
Accudata will be pleased to suggest ways of working to ensure the best possible accuracy is achieved.
