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Station waiting room

BUILDING STRUCTURES

Building over live rail lines has always been a challenge. Dave Parker reports from one London site where an alternative approach has been adopted. Pictures by David Jones.

When the second wave of office development around and over Liverpool Street station ended in 1991, one awkward corner was left untouched. Designated Broadgate Phases 12 and 13, it lay to the north of the main development area, hemmed in on two sides by a major railway signal box and the rusting Victorian trusses of the Worship Street overbridge. Most of the footprint consisted of live railway lines and signal gantries. Yet its location, close to the City of London, was superb. The challenge was to develop it in a way that made commercial sense.

Eight years later that challenge has been taken up, not to erect a building but to create a site on which an office could be built when a suitable client heaves into view.

'What we're actually doing now is converting the area into something that will be simple to construct a bespoke headquarters office building on,' explains construction manager Bovis project executive Andy Batehup. 'The site is so expensive to develop that a speculative office complex would be a very risky venture - but clients for bespoke offices don't like restrictions on what they can have in their buildings.'

So the developers, a joint venture between British Land and Railtrack Properties, opted for construction of a massive 8,000m2 raft over the site, which offers maximum flexibility to the designers of any later building. Steel beams 2.4m deep and spanning up to 38m make up the main structure of the raft, with precast slabs sitting on top and bottom flanges forming a 2m deep interstitial space over the whole area (see box).

Once complete, Batehup points out, the raft will enable development to proceed 'with no need to go back to the rail environment'; all the problems of working over busy main lines will have been concentrated into the £18M raft phase. Since work began last September operations have been dominated by the rhythm of possessions agreed with Railtrack, Batehup says (see box page 20).

'Eighteen rail lines enter the site from the south and merge into six by the time they reach the northern boundary. We can only close all the lines at once for two hours each night or on Christmas Day and Boxing Day. Otherwise the best access we can get is during the weekends, when half the lines can be closed for 25 hours.'

Luckily, before the lines merge, there is a tight triangular fenced- off 'green zone, where some work can go on during normal working hours. But with 25,000V live overhead lines on both sides, daytime work in the green zone is mainly confined to excavating the central under-reamed piles which will support the raft.

'Basically these have to go wherever there's a space between the lines,' says Batehup. 'There are 40 along each side of the site and 39 between the rails, all going down 16m below the rail lines into hard London Clay.

'All of them are 2m diameter at ground level. Those at the edges are machine-dug and under-reamed out to 16m diameter, while in the centre hand-digging is the norm and the under-ream is 7.5m in diameter.'

Precast concrete segmental linings are used to stabilise the hand-dug piles through the 'rubbishy' overburden. Man-access unsupported under- reams are no longer allowed, Batehup says, so a three stage 'Xmas tree' excavation pattern has been adopted, with each stage stabilised by ground anchors and a sprayed concrete lining.

Demolition and hand-dug foundation contractor Edmund Nuttall is due to finish all the central piles within three months. 'Getting the muck out of the eight piles in what we call the 'fingers', narrow gaps between the tracks, was particularly difficult,' Batehup reports.

'In the green zone we could erect an A-frame over the pile, winch up the bucket and tip it on to a miner's railway which took the muck to a gantry which lifted it up to street level. To get the muck out of the finger piles we had to drive headings under the rail lines to the central piles and install miners' railways to bring the muck across.'

Bachy Soletanche has already completed the perimeter piles, which took up to 160m3 of concrete each to fill. Conventional pilecaps will support 600mm square columns - with webs up to 100mm thick - encased in precast circular concrete jackets. These weigh up to 7t - a mere bagatelle when compared to the main beams. These typically weigh 90t when the two halves have been ground-spliced together on site with up to 240 bolts, although a one-off beam at the northern end of the site will top 140t.

Until last weekend the task of placing the beams fell to a 500t crane, which was set up on the Primrose Street overbridge to the south during each 25 hour possession. Two weeks ago it reached its maximum radius. So on Sunday it lifted its successors - two 200t crawler cranes - on to the completed section of raft.

'This will allow us to take delivery of the steel sections during normal working hours,' Batehup points out. 'The cranes will lift the beam sections onto the raft where they can be spliced together in preparation for the final lift the following weekend.'

More than 60,000 bolts will be needed to connect the 4,300t of raft steelwork. Special safety 'pannier cages' will sit over insitu splices and allow two workers on each side of the beam to bolt the splice during normal working hours. Steel erection has been facilitated by 'new temporary' gantries supporting the overhead lines, as Batehup explains.

'There were 14 overhead structures when we started. Most were supported at one end on the perimeter retaining walls we were due to demolish, and many had their inner supports right where we wanted to put the piles. The last two to the north were right across all the lines, and could only be worked on over the Christmas 1998 holiday period.'

In addition, nearly all were skew to the raft's gridlines. Redesigning and rationalising the lines onto the temporary gantries makes it possible to transfer them to new mountings on the underside of the raft as it advances 'within hours', Batehup says.

Other complications included cables running through the site from the large modern signal box to the east of the site, which controls signals as far as Colchester. Luckily, a preliminary archaeological dig in the same area uncovered nothing significant in key areas, although six Roman burials were found.

Close co-operation of all parties to the project has been crucial. Access for maintenance work is said to be better than normal and regular meetings are held with the four train operating companies affected to keep them abreast of progress.

Nevertheless, the iron straitjacket of the possessions timetable has kept the raft project on schedule for completion in March next year. The main raft structure should be finished off during the next Christmas holiday period.

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