Inventive site logistics and complex temporary works are vital for deep basement work in central London.
As property prices in central London soar, and space is limited, developers are looking at innovative ways to create space below the capital’s surface, by building ever deeper and more space efficient basements, to match the great heights of our developing skyline. Constructing these complex sub-structures below London’s congested streets requires a great deal of skill and engineering expertise, but that is only half the challenge.
Space is not a luxury afforded to London developers, and building within seemingly impossible spaces involves thinking outside the box.
Land Securities is developing one such scheme on Victoria Street, the main thoroughfare between Victoria Station and Parliament Square. Once complete, the aptly named Zig-Zag commercial and retail building will stand 13 storeys tall and incorporate a four storey basement, which will house service rooms and one level of retail. Its residential neighbour, Kings Gate, is due to be 14 storeys high and incorporate a five storey basement, providing very valuable car parking space for residents.
The two buildings sit together within a 160m long and 75m wide site, surrounded by busy roads, and close to Westminster City Hall and London Undeground’s District & Circle Line.
It imposes great discipline on organising the site during the day to find space for materials to go exactly where they are needed next
“Our basements are within 1m of the Tube for much of their length and at one point touch the tunnel structures” says Ian Ronchetti, project director for Lend Lease, which is managing the £170M design and construct project.
“On top of that, the site is split into three, with a major UK Power Networks (UKPN) substation being constructed to power the whole of Victoria bisecting the site at basement level,” he says.
“Both the basements for the Zig-Zag building and the three storey deep UKPN substation were constructed using conventional strutted excavation. A different strategy was devised for Kings Gate. To optimise the construction programme on the residential works, we proposed to construct the five story basement utilising top-down construction. This will allow the superstructure to rise while we are simultaneously excavating the basement, shaving several months off the programme, therefore allowing the client to take possession of the high-end apartments at the earliest time.”
Top-down construction means added complications for PJ Carey, the Wembley based civil engineering contractor which has been assigned the £30M basement and superstructure contract.
The first challenge was finding space to manage its work.
“There is virtually nowhere for laydown storage” says PJ Carey contract manager Bradley Barham. Two small entries to the site are available, from a side street and one running underneath the city hall where there is limited headroom. From here, a small route can be found alongside the structure, but the other entrance opens only onto an area sitting over the District & Circle Line.
“The access route runs on top of a Victorian brick arch, which required approval from London Underground (LU) to allow wagons to pass above with normal axle loadings” says Barham.
Further along, the arch gives way to a brick sided box tunnel, and here the contractor requested space to store materials. After discussion with LU engineers, a system was developed and agreed for laying steel beams across the tunnel roof to take loads to the side walls. The space found is “invaluable” says Barham.
Even that is insufficient for major deliveries. The team therefore needed to negotiate the possession of one lane of Victoria Street between 6.30pm and 9:30pm, to allow articulated wagons to be unloaded by the three tower cranes on site.
“The tight delivery window imposes great discipline on the management team to organise the site during the day, and to find space for materials to go exactly where they are needed next,” says Barham.
Our basement has to come within 1m of the Tube and at one point touches it
Positioning and erecting the tower cranes within the excavation was a challenge in itself. Two sit at the edge of the site on two plunge piles and the capping beam of the secant pile wall, while the third stands on four plunge column legs.
The secant wall was carried out by Expanded Group and was an early part of the work while demolition was being carried out on the old 1960s concrete and glass offices, which used to run like a cliff wall along the full length of the frontage.
Working as a subcontractor to Lend Lease, Expanded began work in October 2012 and overlapped with the demolition which continued until Christmas 2013.
Since then, 556 secant piles have been driven, up to 20m into the London clay, which sits beneath approximately 5m of gravel and 2m of centuries old fill, which combined make up London’s surface.
Expanded’s works also included just fewer than 50 bored piles for the apartments, each between 1.5m and 2m in diameter and 40m long, and fitted with a steel plunge column.
Drilling the piles meant breaking through the old basement slab and removing the support for the existing basement wall. In total 7,000m3 of old concrete basement slab and foundations were removed.
Support was provided for the existing wall by a system of complicated steel king posts strutted obliquely from the existing slab.
This system was devised jointly by Carey and Swantons Consultants, with cantilevered projections to hold the wall vertically and horizontally.
“The old slab is above the level of the first new basement slab,” says Barham, “and so we would have needed to underpin the old wall without this support.”
The first excavation task was for the central UKPN substation section which was separated with sheet piles, and then excavated with heavy tubular steel strutting. This is also used for the main office basement.
Carey likes to use bespoke strutting, rather than a proprietary system, to provide more flexibility to fit struts and allow for rising internal slabs and structures, once the base slab is installed.
The UKPN substation space splits the site awkwardly, and requires heavy steel shielding to contain the magnetism from the two large transformers. Laminated layers of connected plates have been used and penetrate the final concrete structures.
Current work is in the main office section, where the raft slab is in place, after excavation from a plunge column-mounted steel platform. In reality, the top down construction of the basement is the most complex element of works, mainly due to the space limitations of the car park design.
An unusual “spiral slab” has been used in the design. This slab provides parking space on each level and also acts as the ramp between the car park levels.
“To allow excavation at this point, a system of K-bracing struts is needed to hold the ramp in place until the slab is cast right around to the next connection point,” says Barham.
Together with a rearranged grid for columns inside the car park, the Lend Lease design created a significant number of additional spaces.
But to achieve the spiralling design, some very large transfer beams were required at the top of the spiral for the ground floor slab to sit on.
“Excavation is in progress using a special Hitachi 23t excavator mounted with a telescopic box boom. This allows a 21.5m reach for the clamshell bucket, which pulls out the clay and is much faster than a conventional methodology,” says Barham.
Work is now proceeding apace and the basement and the reinforced concrete frame will be completed in autumn 2014. The the project is due to open in spring 2015.