Bridge monitoring at the Spaghetti Junction, Birmingham

Mabey Hire Services (MHS) in partnership with JF Strainstall, is employing a sophisticated computer-controlled monitoring system to ensure safety at a bridge repair project on the Gravelly Hill Interchange (better known as “Spaghetti Junction”) in Birmingham.

Principal contractor Amey is overseeing repairs to the half-joints on the elevated concrete structure at Junction 6 of the M6 where it joins the A38(M). The main contractor Geoffrey Osborne Limited is building a new permanent concrete pier to support the effected half joint and allow the temporary propping to be removed. 

Gravelly Hill Interchange comprises of 559 concrete columns up to 24.4m high. The half joint being supported is located at the slip road from the M6 northbound onto the A38 Aston Expressway. The support propping being used is Mabey universal propping with prop capacities of 500 tonnes and 20 hymat hydraulic jacks for pre-deflection of the spanning beam.

These temporary works support the bridge deck while the new pier is built to support the half joint and it is essential that any movement is kept to a minimum during the works. Different monitoring proposals were developed for the each phase of the works, an initial proposal, a proposal for monitoring the temporary works during the construction of the new pier and then a proposal for monitoring the new pier after the load transfer had taken place from the temporary propping to the new permanent pier.

MHS and JF Strainstall are monitoring the temporary works and new permanent pier using four separate methods: 

1. Pressure monitoring of the 20 Hymat jacks and 9 new bearings is via inline pressure sensors in the hydraulic hoses
2. Optical monitoring is done using two robotic Leica total stations fixed to columns away from the works, utilising prisms on the both the temporary and new foundations together with the new permanent piers
3. Tilt sensors on each of the foundations, the new permanent columns and cross-head, are used to back-up the prop monitoring and compensate for the loss of any of the optical prisms
4. Thermistors allow trends and associated correction factors to be implemented due to temperature fluctuations

All four monitoring systems are fed into a logging system which captures data in real-time and sends it to a central computer. This information is then loaded onto a specially developed web page accessible by authorised members of the construction team.

Any data that exceeds pre-set tolerances will trigger an alarm which alerts all key personnel via SMS text message. At the same time, an email is automatically sent to these individuals giving full details of the condition.