Construction activities such as hammering, drilling and piling can cause transient levels of shock and vibration to neighbouring property. These transient vibrations must be monitored and minimised in accordance with British Standards for safe levels of ground vibration during construction operation to ensure that no long-term structural damage is caused.
Monitran was asked to develop a system for continuously monitoring the vibrations caused during the construction of an extension to a girls’ school in London. The construction work, close to a row of old houses and their boundary walls, required some very deep piling.
This type of measurement, though within the scope of Monitran’s sensors, was different from most standard applications in that it required the acquisition of transient vibration data at low frequency and with unpredictable timing. The sensors needed to be robust enough for installation on a construction site, yet the signal acquisition capabilities had to be sensitive enough to capture low-frequency and unpredictable vibration episodes and raise an alarm if levels approached maximum safely limits.
The specific requirements on this site were:
- To log the vibrations (in 3 axes) at 7 locations during site working hours and in all weather conditions
- To activate a visual alarm when vibrations exceeded safe levels (audible alarms were unsuitable due to high levels of ambient site noise)
- To log vibration events to provide access to data in case of complaints from residents
Monitran was required to install, assemble and commission all elements of the system on site.
Standard MTN1100 accelerometers were used for vibration detection, mounted tri-axially on blocks, which were then bolted to the building walls considered to be at risk.
Signal acquisition was achieved via a custom-built microcontroller capable of scanning the outputs of all sensors to capture the transient vibrations. The microcontroller incorporated an industrial touch screen for command input and output display, while continuously logging out-of-range vibrations with time and magnitude data. In real time any sustained high-level vibrations would trigger xenon alarm beacons to warn the site operator to suspend or modify operations until safe vibration levels were restored.
In summary, Monitran’s development team was able to combine state-of-the-art microcontroller technology with well-established field-tested sensors to provide a tailored solution to a challenging brief.