Fast jack on track
The last level crossing on the West Coat Mainline is being replaced this Easter with the help of a mighty box jack at Tipton in the west Midlands.

This weekend a 100m section of the West Coast Main Line just south of Birmingham will be ripped up, the embankment it runs on will be removed and a massive 6,500t concrete box shoved through the gap with the help of 30,000kN thrust from three jacks. With that, it will end a 50 year effort to get rid of the nearby level crossing on the main road into Tipton.

Sandwell Metropolitan Borough Council first started trying to get rid of the crossing in the 1950s. It is believed to be the last such crossing on the West Coast Main Line (WCML) and its road barriers are down for an incredible 45 minutes in every hour as a succession of high speed and local commuter trains whip by. This is hardly ideal for encouraging business into Tipton.

“Plans to put a tunnel under the railway were first drawn up in the 1950s, when the scheme would have cost just £500,000. It’s a bit more now,” says Sandwell major works structures manager Irfan Choudry. Quite a lot more, in fact.

Indeed, that the scheme is being built at all owes much to the persistence of the council and the inventiveness of the project team. The scheme was resurrected in 1999 as part of the West Coast Main Line modernisation programme, when ridding the WCML of level crossings was seen as a high priority. A preliminary design was put together and a box jack rapidly emerged as the best way to minimise disruption to the railway and minimise risks from poor ground conditions.

A price of £7.8M (including £3.3M land purchase costs) was estimated and a bid for funding was thrown together in time for the first Local Transport Plan process in 2000. From there, the scheme entered the Department for Transport’s programme.

So far, so good. Once the project was in the programme, detailed design work could start. It was here problems started to emerge. Detailed site investigations revealed shocking ground conditions. A fault cuts right across the site, the area used to be a canal basin and was filled in with poor material, there are old mine workings in the footprint of tunnel box and the whole area is contaminated from former industrial workings. “The ground was described as toothpaste,” says Steve Beech, project manager for D&B contractor BAM Nuttall. “It certainly was not exactly good materials to put a 6.5t structure on.”

Other complications included the need to rebuild pedestrian subways to comply with the Disability Discrimination Act, provide new access for a boatyard and rebuild a car park. These pushed costs to £13.5M. Nevertheless, the council pressed on and appointed BAM Nuttall in 2003, going for the ECI approach.

The next problem was Network Rail. “We negotiated a tender price in 2004 but it took until 2007 to get all the approvals,” explains Choudry. “The reason for the delay was Network Rail. Network Rail clearly benefits from the scheme, as the level crossing is being removed, but its works agreement is designed for third party works – and we wanted some balance as its original demands were very one sided.” “Network Rail wanted the contractor to put in place insurance for £130M, which we thought was unheard of,” says Choudry. “We couldn’t get more than £50M from the marketplace.” In the end the council ended up paying Network Rail an “undisclosed premium”.

There were also squabbles over construction methods, with Network Rail understandably edgy about works on its railways after the Gerrards Cross tunnel collapse (NCE 19 March). “It took two years to get agreement,” says Choudry. “And it’s all still in favour of Network Rail – but we got some concessions.” The rail operator is even footing some of the bill.

The delay pushed costs up to £17.7M and work finally began in September 2007. Since then it has been plain sailing. Sheet piles were driven on each side of the embankment and 16m long reinforced concrete augered piles driven to support a casting and jacking basin. The basin has been excavated, a steel wire anti-drag system laid, and finally the box itself has been cast – a beast of a structure 50m long, 14m wide, 8m deep with 950mm thick walls, floor and ceiling.

Now all the pressure is on the box jack itself. The final Network Rail hurdles have been overcome – the proposed methodology scored an impressive 95% in the rail operator’s new “probability of success test” which all rail schemes now have to satisfy after last Christmas’s Rugby resignalling shambles.

BAM Nuttall is well versed in the art of the box jack, with its recent success of jacking beneath a live M1 carriageway giving confidence. This one is different. A 101 hour possession starts tomorrow evening and the jack has to be finished when it ends on Easter Monday.

On the M1, the box inched forward at 2.5m to 3m a shift. Here, it will march forward at 2.5m to 3m an hour. As soon as the overhead power lines are turned off, workers will spring into action: track will be lifted, ballast removed, and excavators moved into position to start digging out ahead of the box. A full height channel will not be excavated and the box is fitted with a cutting head so it can create its own path. This will eliminate overdigging and minimise the amount of backfill required. Less backfill means less chance of settlement under the heavily skewed box.

The box will be driven along by three jacks, each connected top and bottom to its rear end. Pretressed Macalloy bars will hold the jacks in place at the top, each carrying 91t of load. By hour 27, more than 10,400m3 of spoil will need to have been excavated and by hour 35, the box should be in place – three shifts will carry out the work.

Despite the need for speed, care and precision are required as the overhead catenaries and cables will remain in place. So the excavator operators will have to exercise extreme caution. Lubrication is also allimportant. Steel ropes will be fed along the underside of the box to help it slide, “much like the caterpillar treads on a tank”, says Beech, with four “treads” each with 128 ropes.

In addition, bentonite will be injected along the sides of the box through nozzles between the box wall and the excavated face. After jacking is complete, the bentonite will be expelled with grout, which will also fix the box in place.

Once the box is in place, ballast must be replaced and track relaid – and there is only a 12 hour contingency for bad weather, breakdowns and unexpected obstructions. Best of luck – but then with a 95% probability of success, who needs it?

Article courtesy of New Civil Engineer (09/04/2009)


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