Project outline 

The Sharnbrook Viaducts are two side by side viaducts, crossing the Great River Ouse seven miles beyond Bedford. They were built by the engineer John Underwood in 1879 and 1881 using brick piers and decks on wrought iron girders. One viaduct is now used for the fast trains and the second for the slower commuter trains on the Midland services.

Network Rail commissioned Murphy to repaint and strengthen the existing steel viaduct, in addition to installing a new walkway. The project was designed to secure safe use of the viaduct for the next 125 years and allow for the safe passing of trains at increased speeds of up to 95mph.

The 10 span viaduct was strengthened by lifting approximately 40 tons of steel per span into place under the existing bridge. The project required bespoke fabrication of permanent and temporary steel works with an innovative method of construction that was detailed in 3D animations. The walkway was constructed with glass reinforced plastic (GRP) and structural steel.

Key challenges

There was a tight programme of works, with challenging key dates required by Network Rail. This was tackled with the use of efficient design, mobilisation and weekend working, with Saturday night possessions taking place over the course of 7 weeks.

To mitigate the risks of working at height, we alleviated harness work through scaffold design. Scaffolding on the site had to serve the dual purpose of facilitating the installation of the new strengthening steel and also repairing the existing steel. It was installed to the underside of the viaduct and, as it was supported by the existing brick piers, it could not hold the weight of the new beams. As such, they were suspended from the new walkway and existing steelwork during the innovative installation process.

Working next to a live railway brought a number of challenges. Safe distances were maintained at all times and our engineers undertook monitoring of the tracks. As it was unfeasible to use a crane (given the length of time it would take to obtain permits along with the issue of restricted access) a bespoke hoist was introduced and operatives were trained in its use. Due to the live railway, all jacking works had to be carried out during possessions.  

Project delivery and innovations

Murphy adopted an innovative technique of using detailed 3D animations to demonstrate the construction methodology. This allowed the client to visualise and understand each stage of the process. It was also invaluable in demonstrating the methodology to the operatives in advance of works. As such, it was a fantastic tool in helping to achieve our SHESQ aims and meet the key dates required.

The steel fabrication workshop in Ireland allowed Murphy to deliver significant cost and time savings to the project. The preliminary strategy involved trials being carried out when the steel arrived on site at Bedfordshire. However, we were able to carry out these trials in Ireland, making improvements and refinements to the initial design before transportation to the UK. Having the capacity to perfect the materials and method in advance, allowed us to arrive on site ready to deliver installation straight away. As welding was not needed on site, it also hugely minimised health and safety risks.

The scope of work involved cleaning all the existing steelwork by shot blasting back to bare metal, carrying out repairs, repainting and strengthening the existing steel by installing new support steelwork to the underside.

Having a large team of experienced operatives on the site, using innovative working techniques, meant there were no reported health and safety incidents for the duration of the project.  

Key facts

The following works were undertaken by Murphy at Sharnbrook:

  • Brick repairs
  • Design
  • Temporary works design
  • Installation of 440 ton of steel and the 30,000 TCB bolts
  • Fabrication, transportation and erection of temporary and permanent steel
  • GRP walkway installation
  • Scaffolding
  • Paintwork
  • Monitoring

Related projects

Sharnbrook Viaduct fixing steel in place.

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