The 1km-long Hitchin viaduct will enable trains using the Cambridge line to cross over the top of four high-speed tracks.
The Hitchin Grade Separation Project is being delivered for Network Rail by the Hitchin Alliance, a partnership between Network Rail and main contractor Hochtief. Spanning the East Coast Mainline, the construction of the curved composite steel and concrete viaduct section has been made possible thanks to a combination of expertise from Sian Formwork and formwork supplier RMD Kwikform.
With various challenges to overcome on the project, RMD Kwikform engineers designed numerous formwork and shoring solutions for Sian Formwork using a range of equipment. This included Minima Panels, Superslim and Alform beams, Alshor Plus and Rapidshor shoring systems, with the deck construction supported by a combination of Paraslim and Webtie composite deck formwork combined with Ultraguard edge protection.
In addition to the standard equipment used on the project, RMD Kwikform designed and fabricated special steel quarter and half round shutters to enable the casting of the crossheads and oval piers to the viaduct.
The whole programme had to be geared around a limited track possession period. In addition, all concrete works had to be completed in just six months.
Hitchin Alliance project manager Julian Spiller said: “When you are working to a dedicated date it really sharpens the focus on the project delivery. No matter what the weather or conditions we could not miss this rail possession, or the project would be put back by months. This is why we had to be 100% confident in the formwork and shoring design delivery.”
Sian Formwork project manager Ben Jenkins said: “One of the first challenges we were faced with was how to build the oval shaped piers. Each pier varies in height and has a curved ‘bullnose’ profile at each end. We therefore needed to standardise the design and come up with a workable, fast solution that met the geometrical requirements for the whole viaduct.”
To cast the oval piers, RMD Kwikform designed a modular ‘half round’ steel shutter system, with sections varying in height to make up the required height of each pier. By combining the half rounds with Superslim, Alform beam and plywood forms for the flat pier sides, the overall oval shape was created.
RMD Kwikform UK engineering director Ian Fryer said: “With a large number of piers needing to be cast, the design team recognised that the formwork solution needed to be strong, rigid and robust. The creation of a steel faced shutter provided the stiffness and consistent dimensional accuracy needed to achieve a high quality finish demanded by the client.”
Once the piers had been cast, crosshead beams were needed to support the viaduct deck. Here Alshor Plus shoring was used to create a combined falsework and access platform, while standard Superslim and Alform shutters were teamed with custom steel quarter round formwork to form the rounded crosshead corners.
Having cast the crosshead beams, the composite deck of the viaduct could be erected. The huge steel beams were craned into place over the East Coast Main Line, with Paraslim deck edge formwork already mounted in-situ, complete with integral debris loss prevention hoardings.
Mr Fryer explained: “Between the girders Webtie soffit formwork equipment was assembled and suspended from the girder shear connectors. Both Paraslim and Webtie systems are configured with the minimum quantity of equipment giving fast erect and strip times, while providing an excellent soffit finish. Most importantly though, the way both systems work acts to prevent grout loss at the interface between the soffit formwork and the steel beams. This in-turn minimises the cost associated with cleaning the girder paintwork and making good.”
In addition to the formwork and shoring design, Sian Formwork needed to keep workers safe whilst working at height and to prevent any debris falling from the deck of the viaduct. RMD Kwikform supplied more than 1km of Ultraguard mesh edge protection equipment for the general areas and designed full height plywood hoardings into the Paraslim sections that crossed the East Coast Main Line to prevent debris falling onto rail tracks below.
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