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Valley landmark

For the northern crossing of the Sirhowy Valley, client Caerphilly Council insisted on a 'major landmark structure'.

This turned out to be a 227m long single pylon cable stayed bridge, climbing 87.2m from the valley floor. Design is slightly asymmetric, with spans of 105m and 122m.

Originally the plan was for an even more asymmetric structure, with the pylon further up the valley side. But this would have demanded a back span to balance the loads. So instead the pylon was moved to the valley floor, putting it into the flood plain.

The deck freely hangs from 18 sets of stays, integral with the western abutment and sitting on bearings on the eastern abutment. Lateral stability is provided by the cables leaning in to the top and by the western end, which is integral to the abutment.

The pylon is founded on boulder clay on one big pad foundation - 40m long, 13m wide and 2.5m deep. Piles down to the underlying sandstone were rejected because the rock is fractured and has high artesian pressures.

The two piers forming the central pylon taper in all directions from 3m by 6m at ground level to 2.5m by 4.25m at the top. The only constant is the 350mm wall thickness. Even the corners are curved on a soft radius.

'This is a significant part of the scheme. The client wanted a landmark because the bridge is very visible all over the town. So we haven't compromised on the architecture, ' says Randall.

'What we would have done in hindsight is to have got some features on the pylon to make the shuttering easier.' The piers were jump formed in 4.5m lifts and with the tapering the shuttering had to be remodelled at every lift in a seven day turnaround.

The deck was far more straightforward - the steel frame was lifted in sections and propped on temporary tresses.

Precast Omni planks were then laid, reinforcement fixed and the concrete deck cast insitu.

Erection of the pylon took place in summer 2004, with the back span deck completed in March 2005 and the front span deck scheduled to finish this month. Tensioning and tuning of the cable stays should be completed next month.

Who's who

Client: Caerphilly County Borough Council

Client's representative: Capita Symonds

DBFO concessionaire: Sirhowy Enterprise Way Limited, a 50/50 joint venture of Costain and Laing Roads

Design and build contractor: Costain

Contractor's designer: Arup

Ecology sub-consultant: Cresswell Associates

Earthworks subcontractor: Walters

Blacktop subcontractor: RMC

Bridge steelwork subcontractor: Fairfield Mabey

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