A civil engineering contractor was tasked with the fabrication of a road which had to be suspended from a cliff face through part of its route to enable access to a remote building sited on a remote coastal UK headland. The optimum road route meant that 160m portion of the new access road had to be structurally suspended from ground anchor piles, drilled deep into the rock face of the steep cliff. Cantilever steel I beams were bolted to the anchor pile flanges at regular intervals and these formed the road foundation which acted as structural support for the new road surface along the 160m section.
This construction was considered highly vulnerable to land slip and rock fall damage because of long term coastal erosion and the challenging topography of the headland.
Sensor Solutions Ltd was contacted to devise a suitable structural monitoring system to determine long term health monitoring of the structure and to determine vehicle weights from vehicles using the access road as it had a maximum design load that was not to be breached. Sensor solutions presented a range of structural monitoring sensing and data acquisition options to the civil contractor and a solution was agreed that met the various budget and implementation constraints.
Sensor Solutions was contracted to install the structural monitoring system and data acquisition, providing real time monitoring capability to the new road structure during the road building phase. Sensor Solutions accessed the site at various stages of the road construction to install the various senor arrays, serial data bus hubs, cabling infrastructure and data acquisition system. The sensor array consisted of strain gauges , seismic sensors and displacement sensors which were positioned at various points along the structure. Displacement sensors were supplied by LD Sensors and had the advantage of having a rugged construction, ability to detect micron deflections in the road structure and are very accurate.
These sensors were wired to serial data bus hubs located at local points along the 160m road which converted the analogue sensor outputs to digital data and transmitted the sensor data via the serial data bus to the data acquisition system house in a cabinet at the entrance to the headland. This method of converting the analogue sensor outputs to digital outputs and transmitting via serial data bus is a very efficient method of transmitting lots of data sets over long distances with the need for minimal cabling infrastructure.
Once the structural monitoring system was installed the system required commissioning to ensure the various sensor outputs were set to various engineering units that would be recorded and displayed by the DAQ. This meant a calibration exercise was required to set the strain gauge values to mV/Nm and the displacement sensor outputs to mV/mm. The strain gauge calibration was undertaken using a vehicle of known mass and axle weights being driven along the structure at various speeds whilst the sensor outputs were being recorded. This routine meant that the mV/Nm values could be accurately established, and a calibration factor be set within the DAQ software. The displacement sensors were calibrated using calibrated slip blocks to establish the mV/mm calibration factor of each sensor. The data collected during this calibration routine was also useful in confirming to the structural design team that the road structure was operating within its safe design parameters.
The structural monitoring system was designed to record data and display real time. Alarm points were set within the DAQ software for each sensor to notify the remote engineering team of any significant structural events that the road structure experienced. These alarm events would have been a result of rock fall, land slip, seismic activity or overloading of the cantilever supports.
A successful long term structural monitoring project was achieved.
