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Partners in time

Sian Formwork used a diverse range of equipment to support the construction of complex formwork across the East Coast Main Line in Hertfordshire. NCE reports.

Hitchen_Viaduct_with_Paraslim2

Fast response: The Sian team had to come up with a standardised design for the project

The new Hitchin viaduct is a strategic part Network Rail’s East Coast Main Line upgrade programme. Part of it will enable trains using the Cambridge line to cross four high speed tracks on a viaduct rather than via a series of at grade points. Completion of the viaduct will remove a pinch point on the route between London and the North.

Commissioned by Network Rail, the curved composite steel and concrete viaduct is being delivered by the Hitchin Alliance, a partnership between Network Rail and main contractor Hochtief with specialist formwork erector Sian Formwork and formwork manufacturer RMD Kwikform.

RMD Kwikform engineers designed numerous formwork and shoring solutions for Sian Formwork, utilising a full range of equipment. This included Minima panels, Superslim and Alform beams and Alshor Plus and Rapidshor shoring systems. Deck construction was 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, to enable the casting of the crossheads and oval piers, RMD Kwikform designed and fabricated special steel quarter and half round shutters.

The section of viaduct which spans the East Coast Main Line had to be lifted into place during a possession in July last year, so the whole programme had to be geared around this. This meant that Sian Formwork had to achieve all of the preparation works in a very tight programme time. In addition, all concrete works had to be completed in an overall timeframe of just six months.

Hitchin_Viaduct_Alshor

Pier solution: A combination of modular steel shutters and plywood was used to form the bullnosed piers

“We could not miss this rail possession, or the project would be put back by months”

Julian Spiller, Hitchin Alliance

“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,” says Hitchin Alliance project manager Julian Spiller.

To meet the timescales for the project, engineers from the Hitchin Alliance, Sian Formwork and RMD Kwikform worked together to overcome some of the design challenges. “One of the first challenges we were faced with was how to build the piers, which are oval in plan. Each pier varies in height and has a curved “bullnose” profile at each end. We needed to standardise the design and come up with a workable, fast solution that met the geometrical requirements for the whole viaduct,” says Sian Formwork project manager Ben Jenkins.

With the need to have a robust and reusable solution to cast the oval piers, RMD Kwikform engineers designed a modular “half round” steel shutter system. Sections varied in size so that they could be put together to form piers of different heights. By combining the half rounds with Superslim, Alform beam and plywood forms for the flat pier sides, the overall oval shape was created.

“With a large number of piers needing to be cast, the design team recognised that the formwork had 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,” says RMD Kwikform UK engineering director Ian Fryer.

Hitchen_Viaduct_with_Paraslim1

Once the piers had been cast, lateral crosshead beams were required to support the viaduct deck. Here Alshor Plus shoring was used to create a safe 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.

The huge steel beams were craned into place with Paraslim deck edge formwork already mounted insitu

“Having cast the crosshead beams, the composite deck of the viaduct could be erected,” says Fryer.

The huge steel beams were craned into place over the East Coast Main Line, with Paraslim deck edge formwork already mounted insitu, complete with integral debris loss prevention hoardings. Between the girders, Webtie soffit formwork equipment was assembled and suspended from the girder shear connectors.

The Paraslim and Webtie systems are configured with the minimum quantity of equipment, giving fast erect and strip times. 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 had to keep workers safe while working at height and had to prevent debris falling from the deck.

“We asked RMD Kwikform to provide a safety package that would support all areas of the project.

“They brought in over 1km of their Ultraguard mesh edge protection equipment for the general areas, while designing in full height plywood hoardings into the sections that crossed the East Coast Main Line. This was critical, as trains pass beneath the new deck at 125 miles per hour, so any possibility of debris falling had to be eliminated,” says Jenkins.

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