Anil Kumar FIStructE is a Technical Approvals Manager for TfL Engineering. Here he explains the intricate considerations structural engineers considered when replacing a railway bridge in a busy London location.
Making the right assessment
When replacing a bridge like Ardleigh Green Railway Bridge, structural engineers must first complete a feasibility study to consider all possible repair, refurbishment, and replacement options. This includes a Whole Life Cost Analysis (WLCA) to support the business case for their proposed solution - this compares options on factors like the costs of initial construction, ongoing maintenance and monitoring, potential interim measures like weight restrictions, and the impact on road safety and journey times.
For Ardleigh Green Railway Bridge, replacing the bridge provided significantly better value for money than refurbishing or strengthening it. TfL appointed contractor Hochtief (UK) to deliver the new structure, and Hochtief assigned a dedicated team of structural engineers to work on the project.
The structural engineering team planned the demolition and produced detailed designs for replacement works in liaison with TfL, Network Rail and other key stakeholders. TfL provided the technical approvals for the works, and both TfL and Network Rail provided approvals to close and/or restrict the road and rail networks where required.
Thinking about people
A key consideration when planning construction works is the impact they will have on people – those who use the transport network, and who live and work in the local area. We have to safely design temporary works to comply with road and rail access constraints and take into account other environmental considerations such as noise and working hour restrictions. There is also the need to balance the costs and disruption of an extended works programme - which could keep the road and railway open - versus a shorter programme with road and railway network closures.
Factoring in utilities
The design was made even more complex by the need to account for utilities which would need to be carried over the new bridge - gas, electricity, water, and telecommunication companies all had to be consulted and approve the proposed works. TfL employed Skanska Utilities to manage and co-ordinate the works, allowing them to carry out utility works in parallel with bridge demolition and construction, resulting in a shorter works programme. Major utility diversion works were delivered using a single lane closure (of the four available traffic lanes), earning TfL and Skanska the National Joint Utility Award for “Minimising Disruption” in 2016.
Demolition and deck laying
The eastern side of the old bridge was demolished within a 72 hour track possession during the 2017 May bank holiday. The first prefabricated steel half-through deck (eastern side) was successfully launched in July 2017, within a Rule of the Route possession, using Self Propelled Modular Transporter (SPMT) units.
The western side old bridge demolition and new bridge launch will be carried out within similar track possessions, which would mean the railway will be out of action for only five days in total, over two weekends, minimising disruption to road and rail users.
The final phase of the work is to demolish and remove the trestle piers that support the old bridge and the overhead line equipment. This is planned to be completed using a 120 hours rail possession, during Christmas 2018. The overall project remains on track to open all four lanes of traffic by spring 2019.
Innovative Design Solutions
Perhaps the most striking feature of the replacement structure is this solution of using two bridge decks with a common substructure and foundation. This solution was selected because it would facilitate pre-fabrication and easy transportation to the site and satisfy TfL’s requirement to keep at least one lane open to traffic in each direction throughout construction – which was hugely complex when considering the need to relocate utilities from the existing bridge to the new utility service bridge, make demolitions and divert traffic.
We also had to design a structure that will last, preventing the need for such major works for some time to come: the design life of the replacement structure is 120 years.
Read more blogs about bridges