Half-Pin Solution
A Guided Busway in Cambridgeshire when finished will be the world's longest, but it has not been all smooth running...

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

Designed and built for Cambridge shire County Council by BAM Nuttall, the 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.

When complete, the £116.2M project — funded by infrastructure developers and £92M of govern ment grants — will provide alterna tive public transport to driving along the busy section of the A14 between St Ives and Cambridge.

The contractor and its specialist piling subcontractor Aarsleff Pil ing 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 sec tion between St Ives and Swavesey.
The initial piling scheme favoured installing predetermined lengths of piles in the variable and unpredict able ground. However, it became apparent that the probability of hav ing to drive longer piles than expect ed, and the considerable and costly delays while waiting for them to be made, was too great.

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 over-pile with a simple single-pinned connection could be quickly added without delay.

The CGB will be a dedicated twin-track 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 steer ing front wheels. These engage and connect with the kerbs and steer the bus along the track. The bus driver stays in control of the bus when it is on the busway, but does not need to steer it.

The vehicles will also drive on the normal roads, where they form part of the busway route, and will use bus priority measures, includ ing new bus lanes and bus priority traffic signals. The CGB will also include two new park and ride sites. A new bridleway for pedestrians, cyclists and horse riders will also be built alongside it from St Ives to Cambridge Science Park.

Layout and accommodate
Aarsleff designed the piles and lay out to accommodate the site inves tigation and track loadings supplied by BAM Nuttall's consultants, 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 sec tion piles, followed by another line of the larger piles for the sequence to be continued.

All piles are supplied and made by subcontractor subsidiary Cen trum Pile. The larger sections have to accommodate a working com pressive 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.

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 under lying clays.

"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 wait ing for longer piles to be made. So we sat down with BAM Nuttall to high light the risks and worked together to devise a viable alternative."

Half-pin joint
It was agreed that the smaller sec tion 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 sec tion and driving to completion in to the clay.

Less than 10% of the piles have been extended in this way. There has been an added cost for the joints, but they have proved cost-effective since they have enabled the companies to balance the risk of costly delays and provided the flexibility needed to compensate for the unpredictability of the ground. "Aarsleff were very amenable to working with us through the design, which was based on piles end bear ing on the gravel," says BAM Nut-tall site agent John Ely.

"Other contractors couldn't pro vide the size of pile required as the loadings are quite high, particularly the lateral loads. "But the problem is that we don't know how thick the gravel is."
Aarsleff originally designed the piles for a particular depth and if the design didn't work for the depth of gravel encountered it would come up with an alternative.

"We couldn't wait for that, since if piles popped through the gravel we needed to have an immediate alternative solution. So we agreed that we would pay for a half joint on all the 250mm2 piles.

"As the factor of safety on the larger 350mm2 piles was greater, we didn't expect them to fail to stop at the gravel layer." This alternative gave the companies the ability and the flexibility, if a pile failed to stop in the gravel, to drive it on instead of waiting for new piles to be cast.

More piles have been extended than expected, but the cost of extending them is probably less than the delay costs in waiting for those problem piles to be redesigned, says Ely. "It was the right thing to do, as we have been able to manage the risk."

"We also did our own pile quality audit at Aarsleff's Centrum factory, which went very well."

Aarsleff started on site in Octo ber and had originally planned to complete its £630,000 contract in one visit. However, unexpected flooding along one part of the site forced the piling to be carried out over two visits. Using a combination of two or three of its Banut 700 piling rigs with 4t drop hammers, Aarsleff com pleted the first visit in eight weeks, four weeks ahead of its programme. Aarsleff is expected to return in Feb ruary to finish the piling.

After piling, BAM Nuttall will follow on cutting piles to length and linking them in each row with a concrete pile cap beam cast insitu at ground level. The pile caps support the pre cast concrete bus guideway beams, which are placed by a launching gantry running on the previously placed beams. Precast concrete pad founda tions support the twin tracks for the remainder of the guided route.

BAM Nuttall started on its £87M design and build contract in January 2007 and is on schedule for the guided busway service to start operating in spring 2009.

Article courtesy of Ground Engineering - march 2009


Further Info
Peter Bishop - Head of Public Relations & Corporate Communication
BAM Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax: 01276 66060
peter.bishop@bamnuttall.co.uk