Figure 1: Denham Viaduct (above)
The A40 Denham Viaduct is a reinforced concrete viaduct built in two phases. The south side was built in 1940, and the northern portion was built in 1960, providing a two-lane elevated dual carriageway. The steel elements of the bearings are heavily corroded, leading to the bearings not exhibiting the movement expected. A monitoring regime had to be carried out to assess the articulation of the bridge during seasonal use by traffic.
The first step in facing the issue is to monitor the structure under a range of temperatures to establish the current movement mechanisms. Murphy Geospatial was asked to measure the relative movements between the deck piers and foundations to establish the presence of element flexure, rotation, and sliding.
The project presented a few challenges due to the nature of the site and the locations in which the prisms were to be installed and observed. For example, a majority of the prisms needed to be positioned at height and required a Mobile Elevating Work Platform (MEWP) or podium for access. Before each survey, the temperature measurement had to be taken at each 3D movement monitoring location as parts of the viaduct would be influenced differently by temperature variations in shady and sunny areas. However, all challenges were overcome with advanced techniques and the expertise and experience of the teams on site.
Murphy Geospatial was tasked by CET Infrastructure with overseeing the installation of 90 glass prisms strategically placed around the bridge sections. The prisms were installed across either side of the bridge and joints to better understand of the joint’s movements. Further still, Murphy operatives were instructed to carry out multiple readings from a permanent baseline. An initial inspection and record of the installation locations had been carried out, and then the prisms were fixed using expandable M10 through bolts – a 50mm deep hole was pre-drilled prior to installing the bolt and fixing of the prism. Before fixing the screw to the prism, an operative ensured the bolt was stable and secure.
An installation report and initial baseline report was produced. The survey works were carried out using a Leica TM50 to achieve the required accuracy of the articulation study. The baseline survey was set as the commissioning monitoring survey at the “Hot temperature” level, two monitoring readings then followed to compare to the first readings.
Our site team consisted of an experienced monitoring surveyor, a remote sensing specialist, and an installation/work at height team.
Each monitoring visit included three sets of readings to each prism with two face measurements for each observation, minimising collimation and tilt error. The results were then averaged to form one set of 3D location values for the recorded temperature. The survey instrument had been set in the same position for every visit, increasing consistency and repeatability.
The reports show the difference in movement whilst the area was subject to hot, ambient, and cold weather conditions. The images to the right show the results, both north and south sides of the bridge section were monitored.
For more information on this project, please contact Matthias Gropp at email@example.com or +44 203 598 3775