Logger keeps watch over ground vibration data

AV Technology has successfully completed a five month solar powered ground vibration monitoring project for Oakmayne Properties at a brownfield development site in central London.

Stockport based AV Technology has successfully completed a five month solar powered ground vibration monitoring project for Oakmayne Properties at a brownfield development site in the Elephant and Castle area of central London.

The 10 storey development, on a former HMSO printing site in Steedman Street, involves the building of 88 residential flats, ten live-work units on the ground floor and an adjacent social housing block of 25 units.

The project is being managed by John Sisk and Sons, and following the demolition of the print works, the work required areas of concrete to be broken up and piling operations to be carried out near a boundary wall.

Unfortunately this was in close proximity to the underground holding tanks and associated pipe work of an adjacent Shell petrol station.

The petrol holding tanks are 15 years old and there was concern that excessive vibration could cause a rupture with serious environmental pollution consequences.

To ensure no problems were encountered, Oakmayne contracted AVT to provide a ground vibration measurement system with a local alarm unit to indicate if peak acceleration limits were being exceeded at any time during the work.

The solution provided by AVT involved the installation of a solar powered system to measure vibration in three planes between +/-1g, within the frequency range 4-250Hz.

The system was based around three Monitran MTN 1100 accelerometers arranged to measure in the x-y-z planes with a minimum measurement resolution of 5mg.

These were housed in a waterproof enclosure mounted to a steel spike that had been driven into the ground such that the enclosure was at least 600mm below ground level and 1m from the boundary wall (to comply with ISO4866:1990(E) sect 7.2.2.

- Guidelines for the measurement of vibrations and evaluation of their effects on buildings).

A six core screened twisted pair cable connected the accelerometers back to a bespoke three channel peak hold device built by AVT and mounted in an enclosure fitted to the boundary wall.

This was designed to hold the maximum reading which was sampled and reset at 1s intervals by a Campbell CR10X data logger housed within the same enclosure.

The data logger then applied integration and scaling factors to produce a velocity value, which was subsequently stored and used to produce trend data graphs.

The data logger was configured to detect an alarm condition if any of the three peak velocity signals exceeded the pre-set threshold (adjustable between zero and full scale).

If this occurred, a remote alarm signal was produced to activate the remote visual flashing strobe beacon and/single tone siren rated at 116dB audible alarm unit mounted on the boundary wall.

The system was completely self-contained and powered from a single 500 x 300mm solar panel, charging a 12V lead acid battery housed within the wall-mounted enclosure.

The battery voltage was monitored and remained between 12.5 and 14V for the duration of the five month monitoring period.

Whenever triggered, the audible alarm was set to operate for a maximum of 30s or until manually reset to minimise undue noise pollution.

The visual alarm continued until manually reset by pressing the manual alarm accept button on the unit.

A test button was also provided to check the function of both alarms.

The monitoring system included a GSM modem to enable data to be retrieved remotely by AVT at its offices in Stockport.

This also allowed the alarm limits to be adjusted as the client required.

Initial vibration limits were set at 50 mm/sec2.

However part way into the monitoring programme, at the request of the Petroleum Officer from Shell UK Oil Products, AVT reset the vibration limits to 25mm/s2 via the modem link.

For the duration of the project, the vibration data were routinely monitored by AVT engineers, and periodically supplied to Oakmayne personnel in Excel graphic format.

The alarm level was exceeded six times but in each instance, it was exceeded only once, and for a maximum duration of 2s seconds.

In five of the six cases, the alarm initiation was followed by a slightly elevated and constant vibration level of typically 6mm/s2 for a subsequent period of 30s, before returning to the normal background vibration levels of typically 0-2mm/s2.

The project was completed with the minimum of disruption to the site work and, as Keith Yearsley Project Manager for Oakmayne Properties concludes: "The AVT equipment gave our contractors the confidence to carry out the work as planned, secure in the knowledge that any unwanted vibration levels would be highlighted, thereby allowing them to temporarily halt the work well in advance of any potential problems".

"Any damage to the underground tanks or pipework could have caused serious environmental issues not to mention major delays to the construction of the ten storey building".

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