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Piling on board

A guided busway in Cambridgeshire will be the world’s longest when finished. Piling has played a vital part in ensuring passengers get a smooth ride.

More from: Piling on board

The Cambridgeshire Guided Busway (CGB) will run for 25.1km and is expected to be the world’s longest on completion.

Designed and built for Cambridgeshire County Council by BAM Nuttall, the £116M busway will run along the route of the old St Ives to Cambridge railway track. It will connect St Ives to Cambridge’s northern fringe and then run from Cambridge railway station to Trumpington, with a link to Addenbrooke’s Hospital.

The contractor and its specialist piling subcontractor Aarsleff Piling developed an alternative, cost-effective strategy to reduce the risk of delay in installing 2,150 precast concrete piles to support the busway over 4km of a 4.5km western section between St Ives and Swavesey.

Instead it was decided to install the vast majority of the piles with a half joint on their tops. If piles needed to be extended and driven further, a similar half-jointed overpile with a simple single-pinned connection could be quickly added without delay.

A dedicated twintrack for buses

CGB will be a dedicated twintrack just for buses. Each track is made up of two precast concrete longitudinal beams with kerbs each side. Any bus will be able to use the guided busway channel as long as it has two small extra guide wheels attached in front of its normal steering front wheels. These engage and connect with the kerbs and steer the bus along the track.

“We weren’t able to probe ahead and determine the ground as we would normally do.”

Steve Gilbank, Aarsleff contracts manager

Aarsleff originally designed the piles and layout to accommodate the site investigation and track loadings supplied by BAM Nuttall’s consultant, CGB Design, a joint venture between Arup and Parsons Brinckerhoff.

Piles are installed in transverse rows spanning the twin tracks and spaced at 7.5m intervals between rows. They have been arranged in a standard pattern of a row of six, 350mm² section area piles, followed by three rows of four, 250mm² section piles, followed by another line of the larger piles for the sequence to be continued.

All piles are supplied and made by subcontractor subsidiary Centrum Pile. The larger sections have to accommodate a working compressive load of 167kN and 80kN shear to cater for the breaking forces generated by the buses, while the smaller 250mm² section piles have a working load of 274kN.

Unpredictable gravels

The piles have to be driven through made ground, soft clay overlying variable and unpredictable bands of sands and gravels over stiff clay. It was the intention for all piles to be driven to a predetermined depth and founded as end-bearing piles in the gravels, but the thickness, depth and location of the gravels proved to be very unpredictable. If the design set was not achieved in the gravel, piles would need to be driven to a designed penetration into the underlying clays.

“We sat down with BAM Nuttall to highlight the risks and worked together to devise a viable alternative.”

Steve Gilbank, Aarsleff contracts manager

“The ground would vary even between the rows of piles and we weren’t able to probe ahead and determine the ground as we would normally do on a conventional site, as there wasn’t the room or access along the narrow route,” says Aarsleff contracts manager Steve Gilbank.

“We needed to know the length of the piles required before ordering as it would have been unacceptable to stand the rigs for several days waiting for longer piles to be made. So we sat down with BAM Nuttall to highlight the risks and worked together to devise a viable alternative.”

It was agreed that the smaller section piles, about 1,400 of the total, would be supplied with a half-pin joint. If a pile could not be founded in the gravel it was then just a simple task of pinning on an overpile section and driving to completion into the clay.

Cambridgeshire Guided Busway route

 

Record breaking route: The £116M guided busway will be the world’s longest when it is complete. It will run along the route of the old St Ives to Cambridge railway.

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