A new facility to help London Underground keep its new fleet of trains on the tracks is close to completion. Paul Thompson reports.
On a sliver of land alongside Neasden Underground station in north west London, contractor Bam Nuttall is closing in on an ambitious project that will help support the maintenance of London Underground’s latest additions to its fleet of trains.
As part of the massive upgrade of London Underground’s rail network and facilities some 58 new Metropolitan Line S8-stock trains have been delivered as part of the overall fleet of 191 S-stock class trains. These feature one single walkway along their entire length with no interconnecting doors and most importantly for summer Tube passengers, state of the art air conditioning systems.
The Neasden depot - the largest depot on the London Underground network - will be the location for the train maintenance facility.
This will include cleaning sheds where London Underground’s staff will be able to clean S-stock trains inside and out; and lifting sheds where whole trains will be raised allowing mechanics and engineers to work beneath them.
The two steel portal-framed sheds sit alongside one another and share a party wall along their 170m length. The maintenance shed is slightly wider at 22m to the 19m wide lifting shed but both feature a main steel framework of 610mm x 305mm universal beams and are supported on a raft of 516 CFA piles of both 450mm and 600mm diameters. Installed by ground engineering specialist Keller these penetrate as deep as 17m into the London clay strata.
Deed helps draw line to delivery
It is fair to say the Neasden scheme has evolved quite considerably since Bam Nuttall first became involved back in 2007. At that time the client was Metronet before it ran into financial difficulties and London Underground brought its contracts back in-house in late 2008.
The uncertainty of those times has been reflected in the project delivery. London Underground took time to reassess what it needed, what it could afford and just who it wanted to deliver those requirements.
“Once it had been decided at a senior level that Bam Nuttall would continue to be responsible for the delivery of the project we all had to reassess how that would be done,” says Neil Gaskin, senior project manager at London Underground. That reassessment culminated in the signing of a ‘deed of variation’ that settled and pinpointed exactly the requirements from the site team.
“There had been some drift in the project - on both sides - and the deed of variation helped draw a line underneath that period and allowed everyone to move on afresh. Everyone knew exactly what was required and we have been able to focus on the scheme’s delivery,” says Bam Nuttall contracts manager Steve Horton.
“The Bam Nuttall and LU teams are working very closely together to such a point that we are on track to deliver the main works by the end of 2012,” adds Gaskin.
Getting the sheds up and clad was key to the delivery of the whole project. It allowed the site team to continue with the installation of the complicated reinforced concrete slabs within the sheds free from the impact of any adverse weather conditions and importantly shielded against any risk associated with those conditions.
“Originally we had planned to install the slabs then erect the frame but by resequencing that work we have protected ourselves against the risk of the weather affecting the project,” explains Bam Nuttall contracts manager Steve Horton.
The lifting shed features two tracks and a specially designed in-floor lifting system which jacks the entire 250t eight car trains up from the track-bed, enabling London Underground’s engineers to access the underside.
“It was a project specific concrete recipe because of the aggressive environment”
Steve Horton, Bam Nuttall
The system is installed in lifting pits, nine in total along the length of the shed, seven of which are double pits and measure approximately 10m long by 5m wide and a depth of 5m down to the base. The two end pits are smaller in size at 5m long with a similar width and depth to the others.
As is to be expected, thanks to the weight involved with lifting the trains, these pits are of an extremely robust construction. The 1m thick slab has been poured using a specially designed C32/40 concrete mix with waterproofing admixture, with steel reinforcement diameters of up to 32mm at 150mm centres.
The walls of the pits have been cast at 450mm thicknesses with a 50mm cover to the reinforcement. There is a tolerance of 5mm in the final constructed vertical and lateral position of the 5m deep pits. To help achieve this strict tolerance and maintain the speed of construction the Bam Nuttall site team used double-sided steel shutters to cast the pits within temporary sheet piled cofferdams.
“It was a project specific concrete recipe because of the aggressive environment and also the fact that the pits are below the water table. There are huge amounts of complex mechanical, electrical and data systems in use, we don’t want those affected by groundwater seepage,” says Horton.
All clear sounds after Neasden air raids
The quality of the ground across the Neasden Depot site is very good, a metre or so of made ground over stiff London clay.
Part of the reason for that quality is its continual history of work as a depot. Nothing much has changed since it underwent a major refurbishment in the mid 1930s, although its strategic importance didn’t go unnoticed by the German Luftwaffe during the Second World War. It tried to carry out a refurbishment of a different kind, leaving the site with a well documented legacy of areas that could be affected by unexploded ordnance.
Although areas of concern were well recorded the site team brought in unexploded ordnance survey experts to make sure.
Probing to depths of more than 20m the team investigated areas that had been highlighted in war-time impact maps made after air raids. Nothing was found and therefore piling works could continue unhindered.
In the maintenance shed the reinforced concrete slabs are more complicated. With 1,800 cast-in items to be accurately located and awkward steps throughout the slab the team cast as much in one pour as possible, hanging kickers were used to cast around the intricate stepping details.
Track support challenge
But it was the track support columns that created the main headache for the Bam Nuttall setting out engineers. Rows of 254 x 254 universal columns support the track and train above the inspection pit when inside the facility. These are set at 1,200mm centres running down the three roads of the maintenance shed and each require four cast-in bolted connections to the column base plate.
Accurate setting out is essential.
Much of the depot’s land is already taken up with various crew quarters, offices and training facilities making access to the live construction site difficult. Conventional access is down a security controlled narrow concrete road running between the depot buildings and the Jubilee line which services Neasden station and the Metropolitan line which also runs through but does not stop.
Transporting construction materials along this busy track would have hampered the depot’s ongoing operations and so the team installed a temporary Bailey bridge across some of the track sidings and laid a haul road around the back of the site, exiting at its western end.
That separation of construction work and day-to-day running was vital to the delivery of the scheme. Without it mitigating the impact on the depot might have had an adverse effect on the delivery schedule.
But with barely six months left before the team hand over the completed maintenance facility the Bailey bridge and temporary haul road seems to have done its job.