Part of the original Severn crossing, the Beachley viaduct connects the main suspension bridge over the River Severn to the bridge over the River Wye, across the Beachley peninsula. The viaduct is linked to the Wye bridge by a pair of rocker arms, which regulate the relative movements of the two bridges due to temperature. These rocker arms, which were contemporary to the bridge’s construction in the late 1960s, were due for replacement in early 2013.
This was a major job that entailed the closure of the bridge for an entire weekend – so it was essential that the work was able to be completed safely under time constraint. This required accurate monitoring of the thermal expansion and contraction of the steel structures over a 45-hour period.
Working with the bridge maintenance contractors, we developed a monitoring scheme based on the use of a high-accuracy total station. Ten retro-reflective targets were affixed to the bridge, five on the Wye side and five on the Beachley side. Survey control was based on five control reference objects.
Procedure for monitoring was designed to permit live feedback of residuals – ensuring that all data was within tolerance. Initial placement of the instrument was based on a resection to the five ROs. If this was within tolerance the surveyor would proceed to measure, to millimetre accuracy, the five targets on each bridge in order. The time, along with the ambient air temperature and the temperature of the steel was also recorded. The data were then presented to the on-site civil engineer. Our surveyors worked in shifts around-the-clock to provide this monitoring at hourly intervals, with additional monitoring rounds at the request of the engineers.
This monitoring ensured that the engineers were well-informed of the movements of the steel and allowed the work to be completed in a safe and timely manner.