The challenge
First built in 1965, Kennington Bridge is a major transport link in Oxfordshire, carrying the A423 over four railway lines and used daily by around 50,000 road users. However, a regular inspection survey revealed various structural issues, including deteriorating bearings on the bridge’s western pier and a concerning angle of inclination detected. Whilst not an immediate risk, it was determined that plans should be drawn up to replace the entire bridge, with temporary propping installed to support the structure while these plans were finalised.
Chris Sadler, Principal Engineer at Milestone Infrastructure (now M Group Highways), explained further: “We have a contract with Oxfordshire County Council to perform all road and bridge works in the catchment area. As a result, when it was flagged that Kennington Bridge was in need of major works, we immediately set about arranging temporary support to the west pier bearings and beams.
“Mabey Hire became involved in Phase 2 of this work and were subcontracted for propping works to the bridge’s east pier. During this Phase 2 scheme Mabey Hire also supplied additional jacks to the Phase 1 west pier propping and re-energised the existing jacks that were already in place.
Our solution
“Mabey Hire’s propping scheme was installed in between the in-situ columns, with seven props used to support the bridge deck. We used a mixture of Mabey Hire’s Mat 125 propping and Hymat hydraulic jacks, with its in-house design team drawing up a concept design based on our requirements.
“This design was then developed in AutoCAD and an analysis model in CADS A3D MAX for the anticipated load combinations. Due to the long-term nature of the equipment installation, each prop was designed to a 65 tonnes loading capacity, with Mabey Hire’s jacking experience integral to ensuring the equal load distribution of the bridge deck.”
Due to the structural concerns that had triggered the works and the fact that the bridge was remaining open to traffic throughout, it was essential that Milestone had continuous insight into the bridge’s condition and behaviour. Chris explained: “As well as the propping and jacking scheme delivered by Mabey Hire, we also utilised their monitoring solutions and expertise. This provided us with invaluable insight, ensuring we had an understanding of how the structure was behaving, and gave us forewarning of future damage, should loads exceed pre-approved limits.”
To gain more information about the structure, we installed a range of our wireless monitoring sensors onsite, programmed to measure the bridge bearings in two axis, as well as tilt sensors to measure the change of movement on the pier crossheads and abutments. The entire system was installed by our site engineers over the course of planned rail possessions spanning multiple weeks. This was then connected to our unique web portal, with automatic alerts sent to key stakeholders at Milestone and Oxfordshire County Council should limits be breached.
Speaking about working with us, Chris said: “Mabey Hire have been really helpful throughout this whole process. Its live portal, in particular, is a great tool from an asset management perspective, ensuring that both the Council and ourselves are up to date with the bridge’s condition. We also receive monthly data reports from Mabey Hire’s monitoring team, which we use for benchmarking purposes and for presenting to other stakeholders.
“Mabey Hire are really easy to work with and the team have undergone multiple site visits to check on the equipment and its performance, as well as assist with other elements of the temporary works scheme (not delivered by themselves) in the early stages.”
