London’s St Pancras station is being lovingly restored and at the same time updated to accommodate Eurostar travel. Adrian Greeman details the logistical, engineering and safety challenges of the job
EUROSTAR passengers arriving through the new Channel Tunnel Rail Link next year will be the first members of the public for a century and a half to see St Pancras station as it was meant to be: an airy, light, cathedral-like space filled with sunlight and the blue sky above ? British weather permitting, of course.
When it was first constructed, designer William Barlow’s train shed was a phenomenal engineering achievement ? it was the widest wrought iron singlespan roof in the world. But in mid-Victorian times, when the industrial revolution was at its height, the glor ious roof space suffered soot deposits f rom steam locomot ive smoke. Later, diesel fumes gr imed the glass panes further and various boarding additions, protective measures during wartime and maintenance coverings altered and cluttered the original clear spaces.
Now all that has been stripped away and the massive wrought iron structure is almost completely renovated, strengthened and repainted in the sky-blue colour of the original design.
New tiling for the lower half of the roof curve is almost finished and on the upper section the ridgeand-furrow glazing has been completely replaced.
All of which sounds much easier to do than it has proved in practice. The roof, along with the station below and the famous towered hotel at the front, is one of London’s famous landmarks and a favourite with the public. It is, inevitably, a Grade I-listed structure, which means it must be retained exactly in its original condition as far as possible.
But it has decayed over the years since it was finished in 1876 and parts, especially the gable ends, have corroded and weakened almost to the point of failure. Some of the old ironwork is holed through and, in a few places, it is non-existent.
Now part of the north-end roof must also serve a new structural purpose, helping support the top of the new station extensions, which the sleek 400 m-long Eurostar trains require.
But even the original shed is huge, 212 m long and spanning a width of just under 74 m. At the cent re of the arch the roof is 32 m high. So getting access to the structure has been difficult and working on it even more so.
Safety is major concern, not simply for the workers on the arches but also for those below. The roof is only one of th ree levels of work that are simultaneously being carried out in the shed in addition to other works either side of the station, which also interact with the whole.
Train wells and the platforms below are set in a raised floor that carries trains at mid-level. Allie MacAdam, from engineer and supervisor Rail Link Engineering and overall contract manager for the roof works, explains that this design feature was originally needed to allow 19th-century locomotives a f lat enough gradient to pass above the Grand Union Canal just north of the station.
Below this busy raised slab, being converted into a sealed international area for six Eurostar tracks, is the undercroft, where freight trains formerly off loaded barrels, boxes and bales and London’s traders brought their wagons to collect them. It is about to become a Customs area, which will also accommodate building management sections and offices.
But new light wells created through these layers have added to the potential risk for workers: now even those on the lower level could have been at risk from items such as scaffolding poles falling from the working platforms above.
So the first requirement was to create a huge crash platform beneath the arches. The whole temporary structure had to be built to allow good access for the cleaning, structural, glazing and tiling crews to the roof space.
‘The scaffolding for this job is absolutely key, ’ says Mervyn Dunwoody, MacAdam’s delivery manager for the C105 roofing contract. Specialist suppliers Palmers needed around 12 months in total to erect the double-layer structure, which was designed by consultant Raygold. The two firms had previously worked on the renovat ion of St Paul’s Cathed ral. It cost ‘several million pounds’, is all RLE will reveal.
The crash layer, which also serves as weatherproofing once the roof is opened above, comprises aluminium structural elements and decking to keep the weight down. It is suspended from a number of steel runways through the building, which were installed at some stage to support a 60-tonne maintenance gantry. Some of these runways will be removed at the end of the job.
The work on the structure is done from a second scaffold layer, this time hung directly from the structure to allow access to the main wrought iron beams. Later, it is reconfigured and extra stepped layers added to access the top for glazing and tiling.
‘The main arch beams at 9 m intervals are some 4 m deep, ’ explains Mr Dunwoody. ‘But for safety reasons we did not want later work to require ladders, and so the whole was designed to be reconfigured into a high position.’ The deep main beams divide the roof into bays with two smaller intermediate arches. These formed convenient sections for the working cycle.
The first task for Corber, a joint venture between Costain, Laing O’Rourke, Bachy, and Emcor Rail, was an initial survey to ensure the integrity of the structure and to search for any asbestos. It was known there was some present, but there was the possibility that more existed elsewhere.
‘Most of the roof was OK, but some 10 per cent had quite a lot, ’ says Mr Dunwoody. It was only in one area and the only explanation the engineers can devise is that pigeons were using it for nesting where the warm diesel exhaust rose above parked locomotives.
The final restoration will include extensive spiked netting over the troughs and spaces in the roof to deter the nesting that has plagued the station since it was built. With the asbestos safely removed, power and light could be installed.
The next stage was to clean the steel, which involved high-pressure blasting, up to 220 N/sq mm to remove the mainly lead-based paints. Paint particles and wash water flushed onto the crash layer beneath, draining down to a collection and filtration system to remove contaminants for specialised disposal.
When this was done a full ‘as built’ survey could be carried out on the ironwork. Structural repairs had to follow, though many of the arches were in remarkably good condition. Repairs were primarily over-plating, adding thickness to the iron with new steel, and had to be done sensitively to avoid any visual change.
A ser ies of ‘generic repairs’ had been worked out with English Heritage beforehand, although it also consults on the work day-by-day and has its own people on site all the time. The generic repairs allowed modern tension-control bolts, for example, which have one end shaped very similarly to a rivet. When this faces downwards, it is indistinguishable from the original to observers below.
‘Then we began the redecoration, which uses a three-layer painting, finishing with a special light blue that it has been determ ined was Barlow’s preference, ’ says Mr Dunwoody. Much of this work was carried out with the installed glazing intact, which provided a controlled working environment. Only the last 10 per cent had to be finished in the open.
But before that, there was more asbestos to remove as the old glazing was set in asbestos rope fixings.
‘There was a hodge-podge of flat glazing, timber panelling and the like, which had been put in over the years, ’ says Mr Dunwoody.
New glazing is being installed by supplier FSEL to the original Victorian design of ridge and furrow.
Clamps onto the original arches support this aluminium framework, which in shape and feel restores the Barlow concept. It was made off-site in the UK and in Germany, and used more than 14,000 panes of glass.
The glazing lies above a longitudinal walkway about halfway down the roof curve, which was the first restored element to be constructed ? by McNealy Brown. ‘All the other work is referenced to this line, ’ says Mr Dunwoody. ‘So we had to get it accurate.’ Below this goes a slated area. On the east side, this was laid straight from one end to the other, but on the west, special intermediate vertical joints have been allowed. This means the 300,000 slates can be laid in shorter bays. As a result, they can be done in a northto-south direction. To go the other way would mean changing the overlap direction, which is difficult for craftsmen using their right hand.
‘Lef t-handed slaters are almost impossible to find , ’ says Paul Bridgeman, project manager for subcontractor T&P, which has also done the extensive leadwork and the acoustic soffits. The slates were obtained from the Penrhyn Quarry in Wales, which supplied the materials originally.
Materials for all this work have to be delivered on a just-in-t ime basis. As part of an extensive safety effort on site, the weight limits for scaffold bays are restricted. So too are lifts with materials.
The site is supplied by two Comedil tower cranes, one of them a CTT 331 with a 90 m tower and 75 m jib, reputed to be the highest freestanding crane in Europe. It needs to be, because of the high tower on the nearby St Pancras hotel. The two cranes are limited to 250 kg load maximum.
But loading space for materials below is also restricted and must be shared with two other work sites on other parts of the project ? the western buildings and western deck extension building ? which all run simultaneously.
‘It is a huge exercise co-ordinating crane lifts, ’ says Mr Dunwoody. Deliveries are made to an off-site holding area and then brought in for scheduled loading slots.
Organising the lifts is just one part of the massive logistical operation to co-ordinate work, he says.
‘There are nine projects running, essentially ? three levels in the main building and three in each of the side buildings ? and these all impact on each other.’ He draws two-dimensional ‘geographical’ charts of scheduled work for each layer on a weekly basis so that teams can see and discuss exactly what will affect them. Real progress can be tracked as well, and charts are updated with a traffic light system of red-ambergreen for fully, partially or yet-to-be achieved tasks.
‘Interface management takes half my time, ’ says Mr Dunwoody.
Suppliers also co-ordinate work and commit to task at regular early-morning daily and weekly meetings, where interfacing issues can be raised and the overlap of work, access problems and materials discussed.
They are deemed to ‘own’ the tasks they have committed to.
Respect for heritage and the integrity of the building inform all the refurbishment. One area that has given major trouble is the north-end gable. This hanging vertical screen across the arch, unlike its southern sister protected by the hotel, has taken 140 years of weather. Victorian engineering did not fully understand how the wrought iron sections would act as drains and so penetration of water into the arches at the side has created extensive corrosion.
A first consequence was that the planned suspended scaffold had to be replaced by a conventional platform on the train deck. It sits 5 m high to allow the lower contractors access.
But it also meant substantial work at the side of the arches. ‘The more we uncovered, the worse it got, ’ says Ms MacAdam. ‘In places there was no iron left when we chipped the paint away.’ The biggest problems were in the arch ‘boot’, which had been filled with concrete in the 1970s to ‘help’ support the structure. Hard work was required to break it out and do the re-propping.
Huge new concrete beams have been installed in a process which involved breaking out a substantial amount of br ickwork , although it should not be visible at the end.
The obligation to retain the original structure as much as possible is frustrating, the team says, but English Heritage was as helpful as possible.
‘English Heritage has the experience that allows it to suggest measures we thought would be ruled out or hadn’t thought of ourselves, ’ says Mr Dunwoody.
The main roofing work is now virtually complete.
But there is still plenty to do on the gable and throughout the station before it can open in 2007, restored to its Victorian splendour.