writer Chris Parker
As a regular user of the Midland Main Line, I have seen Network Rail’s capacity improvement works on the Thameslink route at first hand, with platform extensions for 12-car trains much in evidence south of Bedford. Major works at Blackfriars, London Bridge and elsewhere in London have been well covered in recent editions of the rail engineer. All this is evidence of Network Rail’s determined effort to improve the capacity and reliability of its infrastructure to better serve the capital’s needs.
One of many tight bottlenecks constraining this mission has been that at Metropolitan Junction where the Thameslink route meets key arteries from south of the river. Stations including London Bridge, Blackfriars, Cannon Street and Charing Cross have been affected by the layout limitations in this area. It has long been an aspiration to double the number of tracks through the section from London Bridge, known locally as ‘the critical’, from two to four. Plans have been proposed and debated for many years but a major stumbling block has always been the need to provide a bridge over Borough High Street – part of Borough Viaduct – capable of carrying the additional tracks.
This is one of the most important parts of London for tourists – a conservation area that includes the famous Borough Market and a unique local streetscape. So the new viaduct through it attracted considerable concern and opposition thereabouts. There were worries about what the final result would look like and fears that the structures would consume land that the market could not afford to lose. Equally important were the apprehensions about the effects of the construction process, it being thought that there would be considerable disruption to the business of the market with lots of noise, dirt and traffic whilst the work proceeded.
The railway’s view was that there was no alternative given the importance of the routes involved and the seriousness of the congestion. Thameslink services were jam-packed and the works already in hand to lengthen trains were only a partial solution. With the current levels of overcrowding and further predicted increases in patronage, a significantly higher frequency service was regarded as essential. Other routes affected by the bottleneck faired little better. The viaduct, with its new bridge over Borough High Street, just had to be built!
Local engagement
Network Rail is now well on the way to completing this contentious task, having inched the bridge into position over the recent May bank holiday. But more than that, with the help of principal contractor Skanska and the rest of the project team, it did so to some acclaim from many of the local people and businesses who were so worried by the proposal in the past.
So how was this achieved given the previously strong opposition? It resulted from the way the company and its suppliers set about the project right from the outset, when detailed planning began about two years ago. Key was the decision to work in collaboration with local stakeholders from the earliest stages, requiring significant effort to be invested. This was not just about communicating the proposals; high priority was also given to listening to concerns and doing something positive to address them.
As a consequence the project team did many things to ensure that locals were kept happy. One unusual example concerned Southwark Cathedral which is very near the site. Because of the importance to the cathedral of its Sunday services, it was agreed that work would not take place on Sundays unless there was absolutely no alternative – clearly a highly unusual commitment for a rail project of this nature. The acting Dean has recently expressed great satisfaction that the job has been done with minimal disturbance to the Cathedral’s services.
Allaying fears
Another example of what was done concerns the Borough Market. Finding space for a major civil engineering project in a city is always a challenge and it was no different here. The old Victorian building at 3 Crown Square played an important part in the market’s operation and the conservation area within which it sits. By negotiation with the Borough Market trustees and their traders, it was possible to agree the temporary relocation of the traders from 3 Crown Square, on condition that the project provided suitable accommodation nearby and that the area was appropriately restored for market use after the viaduct works were finished.
That accommodation was found in the old wholesale market area which was roofed over and converted to an agreed high standard. There has been favourable feedback from the trustees and traders as a result of this provision. It permitted the demolition of certain buildings, freeing up the necessary space for the project works. The old Victorian roof has been retained and refurbished, and will be incorporated in the rebuilding of the market around the viaduct.
A further measure to deal with local fears was the consultation and information process that was maintained throughout the planning and construction stages. Regular presentations were given, permitting the airing of concerns and ensuring the team could understand and deal with them in the appropriate manner. During the bank holiday works, a converted Routemaster double-decker bus was placed on site to act as a viewing platform. It proved incredibly popular.
Finally, the design and construction of the new bridge were deliberately arranged so as to mitigate peoples’ worries about them. The look of the superstructure was chosen to create a modern, iconic gateway into London over Borough High Street. The original concept, produced by Tony Meadows Architects some years ago, was taken on, adapted and worked up into a fully detailed design by Atkins, the design consultants for the Borough project. This appears to have been a great success – the project team telling me that locals are definitely impressed with the bridge. Indeed, there has apparently been an internet discussion about giving it an appropriately complementary title.
Drawing the crowds
The construction process was adapted to minimise disruption in the locality. An original proposal was to prefabricate the structure off site and assemble it nearby in two halves at ground level. It was then to have been transported along local streets to site and lifted into place on the viaduct. This approach was abandoned because of concerns about potential difficulties. It was considered that the abundant underground services beneath the roads presented too great a risk given the heavy loads involved; the necessary road closures and track possessions were also deemed undesirable. A number of London Underground lines lay below the site, giving rise to worries about the tunnels, and the Jubilee Line’s Operations Control Room is close by.
Instead fabricators Watson Steel proposed that the bridge be assembled at high level on an adjacent part of the newly constructed viaduct, slid into its final position across Borough High Street and lowered onto its bearings. This reduced the impact on local roads and the need for possessions. But it meant that the viaduct’s capacity to carry the resultant load first had to be assessed. Atkins passed it fit for the job. Tony Gee & Partners was also involved as temporary works engineers employed by Skanska.
Given that the structure is some 72m long and 6m high, weighing in at about 1,200 tonnes, the task of moving it into place was interesting to say the least, drawing crowds to watch its progress. Fulfilling this role was a highly specialist job, with Watson’s subcontracting it to Mammoet, using SPMTs (self-propelled modular trailers) brought in specially from the Netherlands. These are multi-axle pneumatically-tyred bogies of the sort we are increasingly used to seeing on heavy shifting operations. Mammoet’s part of the job went smoothly and the bridge was soon in its final position.
The plan was to begin work on this stage of the project at 02:00hrs on Saturday 30th April and hand the railway back to traffic at 05:00hrs the following Tuesday. In fact things went so well that the job was completed nearly five hours early. A quirk of this schedule is worth mentioning. The start time was going to be several hours earlier but those organising the royal wedding were concerned that the work might be getting underway too soon after the festivities, disrupting the return home of the expected crowds. The team therefore agreed to a slight delay. That the job finished early despite this is a tribute to the skill and foresight of all concerned.
Impressive performance
The adoption of suitable designs, construction methods and choices of plant was important in other areas too. The site was so close to underground services and existing buildings – in one case only 90mm away – that it was inappropriate to use large-bore piles for all the foundations. So from the outset plans had to be drawn up around the use of some multiple small piles instead, allowing smaller plant to be exploited and the piles to be threaded around potential obstructions.
Congratulations go to client Network Rail, its in-house project team and principal contractor Skanska. The latter was employed by the client under a target cost contract and Graeme Campbell, Network Rail’s Senior Programme Manager, believes that the target of £60 million has been beaten. In fact, Network Rail has been sufficiently impressed by the performance of Skanska that its contract has been extended by around £7 million to encompass the reconstruction of the demolished market buildings. These works are due to be finished in time for the 2012 Olympics, giving the firm another demanding target to beat.
The new bridge means that the doubling of the tracks on the viaduct from two to four can now happen. This will tie in with the major layout changes and capacity improvements already underway at London Bridge and eliminate the pinch-point at Metropolitan Junction. A huge increase in the frequency of Thameslink services will be delivered, allowing up to 24 12-car trains per hour from Blackfriars to St Pancras in the future. Of course, Network Rail and its customers are not waiting for the full completion of the current works; instead incremental improvements are being introduced as and when each stage permits. So today passengers are already benefitting from some of these but they will no doubt be glad when the whole venture comes to fruition.
Borough Viaduct
Several listed buildings have made way for the new 507m viaduct spanning Borough Market. Atkins undertook its detailed design and further developed it from the Transport and Works Act submissions. The firm also created a 3D CAD model to identify areas of tight clearance through the market area.
The structure is founded on four pairs of 2.4m-diameter monopiles extending to 28m and lined to prevent water ingress. The first deck section was installed over the weekend of 10th October 2010 although the completed structure will not see its first train until 2018.
Borough High Street bridge is wider than the viaduct to accommodate a curvature of the line as it crosses the road. It comprises a reinforced concrete deck sitting between a truss and a plate girder. The latter is alongside the existing lines and was assembled above the viaduct from six sections, then moved outwards with cherry pickers – this approach precluded the need for extensive line blockages. The tubular outer truss was welded together on site having been brought there in modular form. Its design involved complex finite element analysis with a critical aspect being to satisfy BS5400 fatigue requirements.
The slide took place at an average speed of 7mm per second. Skid shoes were added at the bridge’s four corners with integral jacks to lift it clear of the temporary works whilst the top flanges, suitably braced, provided an ideal support to the bridge launch slide track. The angle and height of the launch allowed the structure to clear nearby buildings but it then had to be rotated and lowered by 5m into its final position. Its clearance from one building is just 16cm.
