Bridge slide methods and large scale prefabrication have solved tight site constraint challenges on the massive Reading station expansion project, as Adrian Greeman discovers.
The Reading station remodelling project would be a complex exercise under any conditions. The upgrade is part of an £850M multi-contract scheme that ties together several major projects for new platforms, widened road bridges, a futuristic station building, a new rail depot, new signalling, new track and rail viaducts. It would be a difficult planning and logistics exercise under any circumstances, but it is made more challenging when sited in a congested city centre.
In addition, the rebuilding and extension contract must be carried out alongside, and over, one of the busiest rail lines in the country. Local and high speed trains arrive and leave every minute at peak hours on the important route between London the West Country and Wales. This makes construction a potential nightmare of scheduling, planning and safety restrictions.
On top of that, the station and its daily flow of 45,000 passengers must keep operating throughout the works, on pain of penalties payable to train and freight operating companies. And the schedule is tight. The new station has to open in 2014.
This has been a significant challenge for the client, Network Rail, and joint venture contractor Costain/Hochtief, which is
carrying out the £90M contract to remodel and expand the Victorian station. The work will double passenger capacity to cope with increases expected by 2035. It will simultaneously remove major rail bottlenecks on the track corridor.
“There is a cross-line connection here to the Basingstoke line and at present main line trains can be held up for five or six minutes outside the station waiting for points,” says Network Rail civils project manager Matt Jackson.
To deal with that, the station will add five new platforms to the existing 11, while several others will be lengthened. The number of tracks through the station will correspondingly increase.
Demolition of old structures on the north side of the station, including the signal box, now replaced by electronic control from Swindon, was an early part of Costain/Hochtief’s contract, which began in 2010.
It makes room for the new platforms and their buildings, which are now almost complete.
“Those are straightforward enough,” says the jv’s project director Brian Fisher. Work involved clearing and building up the ground, including the installation of a reinforced earth retaining wall and a new northern station entrance structure.
Platforms themselves are relatively simple piled concrete structures, says Fisher. The only difficulty is an even tighter schedule for those than for the other work, and they must be handed over a year ahead of the 2014 project completion deadline. Track and signal contractors have to get in next Easter if work is to stay on schedule.
“Working here means nights, weekends and Christmas mainly, and usually bookings have to be made one or even up to two years ahead”
Matt Jackson, Network Rail
But the greater part of the rebuild is for the mainline station itself. Essentially a completely new station is being created in the form of a gigantic bridge across the tracks with escalators down to the platforms. Two, four-storey buildings will bookend the bridge, while the 1980s southern entrance building and retail area will remain.
From the central transfer deck, stairs and escalators will descend 4m to each of the platforms on either side. The deck and new single storey buildings along the platforms will be covered by a long metallic roof, swooping up over the escalators and the bridge and down the other side. It is designed by architect Grimshaw and structural engineer Tata Steel Projects.
The deck is complex to build, at least in logistical terms, because it spans the busy railway lines. Unlike the northern section, this requires working under full railway safety conditions.
Lookouts are required and operatives on site must also be suitably trained. Working above the lines is also fraught with risk and only possible within track possessions – not easy to programme on such a busy line.
“It means nights, weekends and Christmas mainly, and usually bookings have to be made one or even up to two years ahead,” says Jackson.
To solve the problem, the jv has treated the great transfer deck as a bridge construction project using a launching system from the side.
“It is a huge structure, virtually a football pitch size, 100m long by 30m wide,” says Fisher. It comprises a large vierendeel truss with some substantially large steel sections and a concrete deck poured later.
“The largest steel section are 12m long, 14.5t jumbo sections fabricated in Japan,” says Jackson.
The bridge was built in the newly cleared space on the north side, and a first 28.4m long section, weighing 744t, was jacked over the lines followed by a second 1,100t section later. The third and final component was built in place over the new platforms.
The jacked sections included the steel frame and some parts of the roof.
Offsite prefabrication also helped limit the amount of work above the tracks. “There were also a lot of services pre-installed underneath and a vitreous enamel soffit. We added as much as we could, though the curtain walling had to wait until after it was in place,” says Fisher.
The concrete slab was poured onto permanent formwork after the jacking operation.
“Coordinating with the yard was one of the critical aspects of the whole project”
Brian Fisher, Costain/Hochtief
The truss sits on relatively conventional reinforced concrete columns, which had been installed previously. Some are on the platforms and some between the lines, many of them requiring possessions. So too did the bored piled supports for the piers, which penetrate 4m of fill, river terrace gravels and then into chalk 20m down.
Cleverer thinking was required for the next stage, which is the construction of the long canopies forming the roof. These are separate for each platform and range from 13m to 17m wide with small gaps between them for ventilation.
“Originally, this was to be built in relatively small pieces,” says Fisher. “But we re-thought it completely and devised a system of prefabricated cassettes, 3m long and 500mm high, across the full width.”
These would contain electrical components, steel frame and cladding, with pre-made holes for lighting and as much else as could be fitted. By using them, the number of canopy elements to be lifted in was reduced from 2,400 to 700, greatly reducing the number of crane lifts required for the project and importantly, therefore, the number of possessions and attendant plant movements.
The units were made in Poole, at supplier Bourne Steel’s yard. Subcontractor Lakesmere fitted the aluminium cladding in the Poole yard before the units were delivered.
“Coordinating with the yard was one of the critical aspects of the whole project,” says Fisher. The difficulty was to make sure that the units arrived on time and on schedule to fit with the booked night possessions.
Mostly, these allowed for around four hours of working time once the site had been taken and set up safely.
“We could do about four units in that time but they had to be the right ones; the platforms curve and each unit is different,” says Fisher.
The concrete structures for the escalators and stairs, originally to be done insitu, were also swapped for precast units manufactured in Ireland. The elements came as twin walled pieces with insitu stitching.
Other work for the project includes the two buildings at either end of the bridge, the northern entrance building and the western gateline building.
This will add to and complement the existing southern entrance building dating from the 1980s, which remains. There is also a pub alongside that was probably the original Victorian ticket hall.
Construction is also underway beneath the platforms where an existing underpass is being widened and extended further north to become a public right of way.”
A central wall in the underpass, dividing off a services corridor, has been moved over.