This note is for EHO’s who need to measure ground-borne vibration arising from civil engineering works, mining, quarrying, railways, road traffic or demolition. You will need to use a ground vibration monitor (seismograph) which may be rented or purchased from Accudata Ltd.
The Accudata GVM-10 is specifically designed for all of the situations listed above and will meet in full the requirements outlined in the British Standards BS 5228-Part 2:2009+A1:2014 and
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 and has a single range measuring Peak Particle Velocity from 0.03mm/s up to 200mm/s and air over-pressure from 100 to 148dB unweighted linear peak. So easy is it to use the GVM-10 that the user guide is less than a single sided A4 page but a full manual is contained on the supplied memory stick.
The Accudata GVM-10 is a very well made instrument, contained in a durable weatherproof alloy housing. It is supplied in a quality carrying case with customised foam interior housing the all the accessories, including: tri-axial geophone sensor with 2m cable, three ground spikes, air over-pressure microphone with 2m cable and a tripod, battery charger, user guide, USB memory stick with analysis software, full manual and calibration certificate from an independent European laboratory.
To start monitoring, place the sensors 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. The press of a button will start a monitoring session.
The vibration levels in each of the three axes are shown on the screen together with their frequencies and the air over-pressure is also shown. The levels displayed are updated every 2 seconds.
To stop monitoring, press a single button and the results are stored on an SD card. To view the results, remove the SD card and plug it into a laptop or pc and use the supplied software to download them.
*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.
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.
The air over-pressure microphone’s placement is less critical because the pressure wave being measured has a low frequency range from 2Hz to 250Hz. The microphone should be on its tripod in an unobscured position.