Bridge Replacement – Three Colts Lane

Scope of Work

On first inspection, the bridge looked like it would be a small, simple job; however, the site constraints made this a complex project.

The bridge featured an unusual construction with a ballasted deck and a combination of primary and secondary girders, with limited cover between the girders and sleepers, adding to the project’s complexity.

Site Constraints

The site presented several challenges,

• The overhead line equipment (OLE) not being lowered
• An adjacent bridge limiting access from one side
• Constrained site access and workspace
• Services at track level.

To address these challenges, our team explored various demolition options and ultimately selected a method that involved deconstructing the bridge in five major lifts. 

The following methods were considered as part of the demolition engineering process.

Lifting the bridge deck in one piece: This option required removing the track and ballast before lifting the entire structure. However, this option was deemed infeasible due to the overhead line equipment (OLE), limited crane capacity, and remaining services.

Mechanical demolition: Mechanical demolition would have required heavy machinery and large equipment, which were not suitable for the constrained site access and working conditions. Additionally, the bridge structure did not lend itself to mechanical demolition from the accessible side.

Demolition Methodology

After evaluating these options, the project team decided on a deconstruction method that involved dismantling the bridge in three major lifts, five when including the girders.  

The selected method involved protecting the road beneath the bridge using standard heavy-duty mats and then undertaking the deconstruction process in stages.

Each stage required precise planning and execution to avoid clashes with the OLE and other site constraints.

Precise 3D modelling: To ensure safe and efficient lifting operations, the project team employed accurate 3D modelling to visualise the lifting gear arrangements and maintain clearances with the OLE and other site constraints.

Multi-stage lifting process:  The bridge was deconstructed in five distinct lifts, with the first involving the removal of the central section of the bridge. This was followed by the removal of triangular sections and edge girders in subsequent lifts, ensuring minimal disruption to the surrounding infrastructure and services.

Temporary works and bracing: To maintain the stability of the bridge during the deconstruction process, temporary works, including RMDs and scaffolding, were installed to secure and brace the edge girders. This approach minimised the risk of structural failure during the demolition process.

Use of a saddle weight: To manage the centre of gravity during the final edge girder removal, a saddle weight of approximately 1,400 kg was employed. This innovative solution allowed for the safe and controlled lowering of the girder while avoiding clashes with the OLE.

Outcome

Clarke Demolition successfully deconstructed the bridge at Christmas 2022.

Murphy’s, the contractor responsible for installing the new bridge, faced a tight program for completing the works. Despite these constraints, the team managed to execute the project efficiently, ensuring that the rail line was reopened on time.