Post Office Railway tunnels, underground services and old basements necessitated a flexible approach to the Rathbone Square development in central London.
Redevelopment of the former Royal Mail Group’s West End Delivery and Sorting Office behind Oxford Street in central London may not attract as many headlines as Crossrail’s Tottenham Court Road Station nearby, but both teams have similar challenges.
“The biggest challenge is the logistics of working on a relatively small site in central London,” says Careys contracts manager Pierre de Villiers. “At one stage we were demolishing a seven-storey building, while installing temporary props through its two basement levels and a new secant pile wall around the site perimeter.
“We also had hundreds of lorry movements every day – all a few metres from one of London’s busiest shopping areas.”
Developer Great Portland Estates bought the 100m by 90m Rathbone Square site from Royal Mail Group in September 2011, with the sorting office finally closing in August 2013. Planning consent was given for the redevelopment in February 2014, with principal contractor Careys starting construction in May.
The scheme, designed by London architect Make, with AKTII as engineer, involves replacing the Royal Mail building and its car park with two L-shaped buildings either side of a new public square. The buildings will be between five and seven storeys high and each will have two basement levels. The northern building will include a mix of private and affordable housing, while the southern one, closer to Oxford Street, will have offices and retail space.
The site is tight, bounded by buildings to the north and south, Newman Street to the west and Rathbone Place to the east.
De Villiers explains that while ground conditions are fairly standard for this part of London – Terrace Gravels over London Clay – the team had to deal with a number of man-made obstacles.
“Two London Post Office Railway tunnels run east-west across the site at a depth of around 23m, and BT has a communications tunnel running along Newman Street,” he says. “This used to link to the Royal Mail building via a tunnel in the northwest corner of the site.”
The London Post Office Railway ran from Paddington Head District Sorting Office in the west to the Eastern Head District Sorting Office at Whitechapel in the east. It had eight stations, including the sorting office at Rathbone Place. While the line closed in 2003, it has to be protected during works with a 4m exclusion zone in place.
The new buildings will be supported on a 1.5m thick slab at the base of the two-level basement. The basement’s perimeter wall will be formed in part by the original basement wall of the Royal Mail building (in the northeastern corner and along the eastern side) and a new secant pile wall around the rest of the site.
Demolition contractor Erith Group began demolishing the Royal Mail building, which sat on the eastern part of the site, in May. At the same time, Careys began installing two levels of temporary concrete props in the basement below. These 1m square raking props were cast insitu, through slots cut in the basement floors, to support the section of the original basement wall being retained.
De Villiers explains that concrete props were chosen to save time. “Using steel props would have involved moving steel sections into the basement and then bolting them together. Using concrete shuttering was much simpler and faster.”
While work in this area of the site was down to base slab level and two of the lift cores had started to rise out of the ground by the end of 2014, progress on the western side of the excavation, beneath what was once the car park, was not so advanced.
“The BT cables caused some delays with installation of the secant pile wall,” de Villiers explains. “The cabling had to be moved further into Newman Street before we could install the wall along the western edge and we had to backfill part of the now redundant tunnel with foam concrete to allow plant access. So we had to reorganise the foundation work.”
Balfour Beatty Ground Engineering (BBGE) arrived on site last August to begin work on the secant pile wall.
“The original plan was to use two rigs and, starting from each end of the wall, work towards one another,” explains BBGE contracts engineer Luke Pierce. “However, because we couldn’t get access to the wall along Newman Street, it was decided to use both rigs on the southern section.”
The secant pile wall comprises 390 piles. There are 195, 850mm diameter rotary bored male piles installed to a maximum depth of 20m and 195, 750mm diameter CFA female piles. The female piles reach about 1m into the underlying London Clay, which is about 9m below ground level.
“We changed the original design of the female piles from rotary bored to CFA,” Pierce says. “These ‘hanging’ piles are also shorter than originally planned but the wall is still able to perform as intended. And, as CFA piles are faster to install, there were both cost and time benefits.”
Safe delivery of materials
The southern wall was built along a narrow corridor just 8m wide, which presented challenges, not least safe working and delivery of materials to the piling positions, with two rigs and a crane having to operate in such a tight space, Pierce says. “We had a building on one side and demolition and excavation on the other, so we had to plan work carefully.”
Wall construction was timed so that it would reach the western edge of the site as it became clear of the BT cables. However, further delays meant plans had to be changed again.
Instead, BBGE moved to the north-western corner of the site to install this section of the wall and also 1,000mm diameter temporary bored piles to support a ground slab allowing access to this part of the site.
Four temporary plunge column piles were also put down in this area for one of the tower cranes. These 27m deep piles, with 12m long steel plunge columns, were installed with a positional accuracy of +/-10mm and 1:400 verticality, Pierce says.
Finally, the way was clear for the western section of the wall to be tackled, with BBGE finishing its work at the end of November.
When GE visited site in early January, installation of the 1,500mm by 1,650mm capping beam was nearly complete and basement excavations had begun. With the exception of the northwest corner (which will be excavated top down beneath the ground slab), this will be an open excavation, using temporary steel props to support the new walls.
Construction of the aboveground structures is due to start at the end of 2016. Careys will hand over its principal contractor role to Lend Lease in Spring 2015, while continuing to work on the scheme.
Rathbone Square is due to open in early 2017 and will play a key role in transforming the Oxford Street and Tottenham Court Road area.