A sensitive railway embankment and a tightly hemmed in site in Battersea, south London, presented Rock & Alluvium with a tough challenge. GE reports.
A new housing development squeezed into land close to a live railway embankment in Battersea, south London, has created a challenge for foundations contractor Rock & Alluvium.
To make best use of the valuable urban space “a creative approach to groundworks was paramount”, says Lou Apcevski, building manager for developer Linden Homes.
Piled walls close to the railway allow the construction of reinforced concrete frame housing blocks using as much of the space as possible.
Within the site, nearly 400 load-bearing piles will support structures.
The project also uses a below ground automated “car stacker” to remove the need for ramps and turning areas while still providing parking for more than 20 vehicles.
The tightly constrained site is surrounded by residential streets, and, to the north, by the overground railway line from Willesden to busy Clapham Junction station.
Until recently the space was occupied by warehouses, but it is now being redeveloped by Linden Homes into housing and office space in this prime inner city district, just across the Thames from upmarket Chelsea.
Four five-storey blocks will house 100 residential units and 1,325m2 office space and roof gardens. Linden Homes is part of the Galliford Try Group, which owns Rock & Alluvium.
One of the pile walls runs parallel to the railway line.
It comprises a contiguous wall, made up of 139 piles each 600mm in diameter and 11.5m long.
These were drilled at 750mm centres and reinforced to full depth with six H25 steel bars. At each end of the wall are piled boxes of 4m by 7m and 3m by 3m.
“No one is quite sure what it is made of,”
For these, the piling interlocks to create secant walls around what will later be pumping facilities. A right-angled piled wing wall is also included at one end.
The extremities of the main wall are situated just within the boundaries of the site, but strips of land about 2m wide will be left vacant at both ends of the site to allow access to a station which may be built in the future.
Protecting the railway embankment was the most critical issue.
The site perimeter comes to within 7m of the railway line at the top of the 12m high Victorian rail embankment. “No one is quite sure what it is made of,” says Rock & Alluvium head of operations Nick Dewey.
As a result, he says, Network Rail engineering staff were quite sensitive about it, so the embankment was constantly monitored to ensure movement was kept within acceptable limits.
Working near the line meant a series of discussions with Network Rail.
“Their concern was firstly for the radius of the rigs we used, which is to say, the radius a mast would have if it toppled over,” says Dewey. “It had to be short enough to avoid the tracks.”
The constraints meant using Soilmec CM50 piling rigs which can handle a 19.5m CFA auger with 5m extension poles on the mast.
That is fine, says Dewey, but for two small sections of secant wall in particular, he thinks a bigger rig might have been useful.
The female piles can require a fairly high torque to cut through once they reach three-day concrete strength, he says.
Some 45 female piles and 45 reinforced male piles were needed.
The males were 11.5m long using eight H25s wrapped in H12 helical bars for reinforcement. The female piles are 8.5m long.
In front of the wall and the secant boxes, two temporary sheet piled cofferdams are also being constructed under an independent contract with another firm, Dew Piling.
These will facilitate excavation for the automated car stacker.
The eastern cofferdam will be incorporated into the secant wing wall.
Dew employed a Tosa silent piler for the sheet piles to minimise vibration and any consequent effect on the embankment.
The 395 load-bearing piles being bored on site average 22.6m long with diameters of 750mm, 600mm and 450mm.
These are quite difficult to install because the ground conditions comprise made ground over ballast and then 7m of clay.
A high water table just 3m below ground level requires constant monitoring with piezometers. Dewatering is expected to be needed during basement excavation.
“Preliminary test piles constructed in mid-September interfered with the sheet pile line so the sheet piles have had to be relocated.”
The load-bearing piles inside the cofferdams were bored by Rock & Alluvium before basement excavation, with cut-off levels inside each cofferdam up to 5m below ground level.
Each cofferdam will be excavated to a depth of 1m and then the cavity braced and propped before the excavation is completed.
Dewey says that Rock & Alluvium has also had to act as consultant designer to a certain extent.
“The co-ordinates of the plot were still being finalised when we started on site in mid-October.
As a result, preliminary test piles that were constructed in mid-September interfered with the sheet pile line so the sheet piles have had to be relocated around them,” he says. “We’ve tried to add value in the design process as far as possible.”
An added constraint is constantly used road parking alongside the 0.25ha space, making site deliveries occasionally “very interesting to sort out to say the least”, says Dewey.
Rock & Alluvium’s £529,000 contract for main contractor O’Halloran & O’Brien was due to end in December but ran into the new year due to snow conditions.