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Open plan surgery

Foundations - Opening up the basement of St Pancras Station's famous Midland Grand Hotel has called for radical re-engineering of its foundations, discovers Andrew Mylius.

Getting miners, their excavation equipment and construction materials into the tight spaces beneath the Midland Grand Hotel fronting London's St Pancras Station was like playing sardines, says Claire Carr.

She is overseeing a surgical operation to remove walls in the old hotel's basement in order to create a direct link between King's Cross St Pancras Underground station just in front of the Midland Grand Hotel, and St Pancras International railway terminus, which is immediately behind it.

'Eurostar will start using the station in November. We're creating easy through-access for passengers moving between the London Underground and high speed trains, ' says Carr.

She is section manager for CORBER, a joint venture between Costain, Laing O'Rourke, Bachy Soletanche and Emcor Rail, which is carrying out the rejuvenation of St Pancras station. It is doing the work for London & Continental Railways, owner and operator of High Speed 1, formerly known as the Channel Tunnel Rail Link.

Carr says that to support the hotel's seven storeys of neogothic brickwork, walls in the basement chambers were up to 1.5m thick, carrying point loads of 500kN.

The space was divided into four rooms, roughly 7m square, two either side of a 3m wide corridor. Doors giving access to the corridor, and from the corridor into each of the rooms, were between 800mm and 900mm wide. 'Space within the chambers was limited, and the doorways formed extremely tight bottlenecks on movements of people and materials, ' Carr says.

Opening the basement up to create space for free-flowing passenger movement follows a 60-point method statement.

'We've arrived at the point where we've got a large open plan area dotted with columns ? we've come a long way, ' Carr summarises.

Alongside working in conned spaces, one of CORBER's key challenges was to limit settlement. 'Our work strategy, have been governed by the requirement to keep settlement to under 5mm, ' says Carr.

Instrumentation has been installed on the upper floors of the hotel to keep tabs on the building's response to changes being carried out to its footings.

Work started 17 months ago with the excavation of 3m by 2m pits to locate the hotel's corbelled brick foundations.

These were found 6m down, bearing onto London Clay.

A team of miners employed by Costain carried out the excavation work, using timber props and shoring to support the sides of the holes.

'Because of the conditions in which we're working, we've gone back to very traditional methods and materials, ' Carr notes.

'Timber's far easier to use than steel in tight spaces like this.' With footing levels established, ground was taken down to the same level throughout the basement area. Powered wheelbarrows and a small conveyor were used to remove spoil as two mini-diggers toiled away.

Next, 1m wide, 4.5m deep reinforced concrete strip foundations were cast either side of the walls to take temporary works loading. 'We needed very substantial foundations to take propping forces when it came to opening up the walls, ' Carr explains.

Opening up the walls involved taking cores at high level, where they met the edges of vaults making up the basement's jack arch ceiling. Subcontractor Shepley inserted I-section needles through these holes supporting them on propped I-beams running flush with and either side of the walls.

'We were strictly prohibited from opening up more than 25% of the wall at once, so we had to install the needles using a hit one, miss three, hit one pattern. Once we'd been around all the walls once, we went back and did the same again, and again.' Grout was used to ll cavities in the brickwork of the topmost section of wall, sandwiched between the longitudinal I-beams.

The grout also flooded the void between thewall and the web and inner anges of the I-beams, creating a composite steelmasonry-steel sandwich.

Only when the grout had achieved full design strength were props supporting the I-beams jacked imperceptibly, relieving the walls of load. This enabled slots to be cut in the walls. Reinforced concrete saddles were cast, bridging between the strip foundations, on which new cast iron columns were positioned.

With all of the columns in place it was nally possible to cut out the remaining brickwork.

Floor level between the strip foundations was raised to the same height by placing mass concrete.

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