"There are individual big civil engineering projects that you work on which are fascinating and fun – the sort of thing that engineers get out of bed for – and then there's building lots of really difficult things on a 100-year-old structure, over a heavily used, fast-flowing river while still running a busy railway through the centre of it," exclaims Andrew Mitchell when asked what is on his mind.
He is the programme director for the £5.5bn Thameslink project and is describing the complexity of the job at Blackfriars Bridge station in London.
Currently, up to half of commuters travelling from Brighton, Bedford and Sutton into London are unable to use the service due to overcrowding. The project involves increasing capacity on the line by introducing trains which are 50% longer, extending platforms, building new stretches of track, some of which will be on a new viaduct, and completely rebuilding some of the central London stations which are at full capacity. Work to extend platforms at Luton Airport Parkway began last month and work will continue somewhere on the route until 2015.
Calling the project "an upgrade", Mitchell points out, is a gross understatement. "It's a major transformation," he says and a huge step change in the quality of rail service which has come to be accepted in the capital.
Work on Blackfriars Bridge station will involve widening the existing station over the River Thames and extending it so that it spans the entire width of the river. Clearly, at the heart of the project's complexity will be sequencing the construction, perched on a bridge around a live railway. The 19th century bridge itself will need to be partially rebuilt to support the extra load of longer trains and a longer station.
"Strengthening the bridge will be the first priority. At the moment, it's covered in strain gauges and movement sensors to understand how it behaves. The iron lattice work will have to be replaced [to support the new loads]." The extended station will also make use of three "ghost" piers beside the bridge which are remnants of an older bridge which was too weak to support high-speed trains. Extending the station to the south side of the river and providing a new entrance for commuters is also hoped to help regenerate this area, which is currently dominated by the Tate Modern art gallery.
So what else is on Mitchell's mind? "It's a well thought out scheme, and you could argue that it should be, given that it's been around since the 1990s. But the challenge is confirming that those solutions work in today's climate."
Ultimately, Mayor Ken Livingstone's aspiration for London's population growing by one million by 2025 is the main driver to projects like Thameslink, as well as increasing house prices pushing people away from the Tube network and on to the commuter railway. This certainty of growth was absent in the 1990s designs, so they have had to be checked now to make sure the current scheme has enough capacity.
"Only last year, we decided an extra concourse was needed at Farringdon," adds Mitchell.
Competing for contractors and keeping costs down when work for the London 2012 Olympics is under way is another headache. Work on Blackfriars and Farringdon stations is on a similar completion deadline to the Olympic projects – 2011. "But we have an arrangement with the Department for Transport and the rail regulator should there be problems," he says confidently.
Fortunately, the colossal work at London Bridge is due to start after the Olympics and can take advantage of a possible downturn in the construction market. This was decided after finding out London's Olympic bid had been successful. "It was the only way of making sure the project was launchable from a risk point of view," admits Mitchell.
London Bridge station, says Mitchell, has not fundamentally changed since the early 1900s. All that has happened is that more track has been weaved onto the site, creating bottlenecks and blockages on the system. "So we have to start again. This means demolishing the building to the ground and building it back up and completely relaying the track."
London Bridge has nine platforms where trains terminate and six where they stop and continue through. The main work at London Bridge will be to swap these numbers around so that it becomes predominantly a stopping station and not a terminus.
During the London Bridge reconstruction, trains will have to be diverted to other stations such as Waterloo and Victoria to free up space.
Mitchell is a bit of an old hand when it comes to being part of a mega project having worked on Hong Kong's Chep Lap Kok airport and KCRC West Rail railway. He holds a degree in civil engineering from Imperial College, but years of experience in the petrochemical industry, coupled with an MBA gives him a unique perspective on how to deliver this tangled web of projects. "Network Rail is an informed and involved client and the only way of managing this sort of programme of work efficiently is to keep close control and make sure you're right slap bag in the middle of where everything is happening."
THAMESLINK IN A NUTSHELL
£5.5bn of work required on Thameslink
50% longer trains will be able operate
Currently up to half of regular Thameslink users are unable to use the service due to overcrowding. The biggest change to the service will be to operate trains which are 50% longer. To facilitate this involves Ł5.5bn of work building new stations, track, tunnels and viaducts and extending platforms.
The detailed design contract for Blackfriars Bridge has been awarded to consultant Jacobs and contractors currently in talks with Network Rail on an Early Contractor Involvement basis are Laing O'Rourke, Morgan Est and Skanska. In the next few weeks contracts will be awarded on the Borough viaduct and Farringdon Station projects and the outer London extents of the project.
Invitations to tender on Farringdon, Blackfriars Bridge and Borough Viaduct are about to be announced. These contracts will be awarded in March. ICE Target Cost contracts will be used for most of the work, where appropriate.