The Thameslink Programme is now reaching its final stages. Despite this, demands on the engineers involved are certainly not getting any easier. Network Rail is now working through the process of untangling the complex track layouts on the approaches to London Bridge station to enable train operations to run more efficiently.
A major objective of the Thameslink Programme is to provide additional services through central London and reduce delays outside London Bridge station. To do this, two pieces of infrastructure are absolutely vital. One is the new viaduct at Borough Market, which provides for an additional two tracks at this otherwise-critical pinch point. The other is a grade-separated junction between New Cross/New Cross Gate and London Bridge station.
This is an essential requirement that will enable trains travelling from the South East to reach Charing Cross without conflicting with trains from the Brighton line going to Blackfriars and beyond.
This grade-separated junction is enabled by a structure known as the Bermondsey Dive Under and, in May 2012, Network Rail awarded a £75 million contract to Skanska to design and construct it.
The site for the Dive Under is located about a mile away from London Bridge station on its eastern approach. It will allow the Thameslink lines to cross over the Kent lines, unimpeded on their approach to London Bridge station. This will help to ensure that Network Rail is able to provide infrastructure for up to 24 trains per hour to run through London Bridge and travel north on the Thameslink route.
Throughout the work, one of the main challenges for Skanska has been the ability to carry out this complex programme of work whilst minimising disruption to the dense train service travelling through this location. It is not a job for the faint hearted.
The main construction works were started in 2013 and the plan is to complete the work by spring 2017. There are, in fact, two parts to the contract. In addition to the construction of the Dive Under itself, and following Network Rail completing the assessment of more than 50 structures in the vicinity, 20 bridges were identified as needing strengthening and Skanska is undertaking that programme as well.
This work is primarily associated with wheel timbered decks that require strengthening and converting to ballasted track. Each bridge is different and the strengthening work quite challenging.
Skanska has also reconstructed a three-span steel bridge in an interesting way. The two outer spans have been filled with foam concrete to just below the soffit of the existing decks. Then the decks have been removed and a concrete capping slab installed, waterproofed ready to receive ballasted track.
The central span has been removed and a concrete U shaped structure constructed. A rare 50-tonne forklift truck was used to install the 24-tonne precast units forming the deck of the new bridge. Skanska’s engineering manager, Julian See, explained that the team are very proud of this work which has been designed and built in 18 months and was a precursor to diverting services and obtaining the BDU site.
Returning to the Dive Under
The Dive Under consists of two new ramps carrying the Kent lines travelling from east to west. These lines will drop down and travel through a concrete box. A new embankment carries the Thameslink lines from the south east over the box and down a ramp on the west side. Similarly, the Kent lines that travel through the box then return along another ramp up to normal ground level, thus enabling Thameslink trains and Kent trains to swap over without incurring any delays. It’s all quite simple really, once enough space has been created, and that has been one of the key challenges.
In order to clear the space for the concrete ramps, reinforced earth embankments and concrete box, contractor Armac demolished more than 100 brick arches. Concrete ‘bookends’ were constructed around specified piers to ensure that the demolition of one arch didn’t create a ‘pack of cards’ failure.
A box not a tunnel
More than 500 metres of precast arches – 71 new arch spans – are being constructed to replace old ones and to form the new alignment (below). The material from the demolished structures is being recycled and used in the new earth embankments using two crushers on site.
The concrete Dive Under box itself is 155 metres long. Kevin Sullivan, the programme manager for Network Rail, pointed out that, because of its length and design, the box is not classified as a tunnel. If it had been, then there would have to be several additional key interoperability features included within the design.
During the early stages of the project, Skanska installed three large steel span sections onto four reinforced concrete piers which had been previously constructed over the East London Line. Once these beams had been lifted in using 500 tonne and 250 tonne cranes, 30 precast concrete L-shaped units were fixed onto the steel structures, secured by 1,000 shear studs that were welded on site.
This work, which took place alongside the brick-arched viaducts that carry six main lines, formed the start of the transitional structure that now spans from the existing brick viaduct to the Bermondsey Dive Under. This was followed by the demolition of 35 arches and track slewing by Balfour Beatty so that work could start on the construction of the Dive Under box. Around 400 of the 760 continuous flight auger (CFA) piles that have been installed on the site are for supporting the reinforced concrete box structure.
An innovative part of the design developed by Ramboll was to use a precast arch solution in place of the one developed during the outline design stage, which was for reinforced in-situ concrete portals.
One of the benefits of the arch structure proposal is that the form of the new arches is in sympathy with the existing masonry arches, ensuring that the load distribution on the foundations remains relatively unchanged. It has allowed many of the existing piers to be reused and, as a consequence, it has contributed significantly to reducing the number of piles and eliminating the requirement for low-headroom piling, thus ensuring that the same piling rigs could be used throughout the project.
The precast arch design also uses a constant arch radius, meaning that the arches could be cast and constructed off-site. This, in turn, ensured that quality standards were maintained and that work on site was kept to a minimum – a significant issue in a blockade and one which significantly reduced site safety risks.
The designers claimed that this solution will also reduce long-term maintenance requirements since expansion joints are eliminated as the arch structures are in compression.
The concrete arch segments were designed to have a similar stiffness to conventional masonry arch rings. They have a structural concrete backing with a curved upper profile, as typical arches, and were then filled with foam concrete in two layers. Careful thought was required regarding the performance of the fill material so that the arches behave in a similar fashion to long lengths of masonry arch viaducts that ‘breathe’ under loading.
Stobart Rail has been busy on several elements of the project. These have included removal and reinstatement of track, excavating fill from the arches, the placement of steel cages and framework, constructing concrete saddles over arches to strengthen the structure, forming robust concrete kerb and various waterproofing and drainage works. Many have been undertaken with the adjacent line open and much of it was accomplished in only 28 days, working 24/7.
Neil Bishop, Network Rail’s construction manager, was obviously impressed with the work carried out. “I was out on site frequently,” he commented, “during which time I was able to observe the machine controllers, drivers and COSS’s carrying out the works and it was clear from their actions and subsequent conversations with them how serious and professional they were in relation to this challenging element of working adjacent to the open lines.”
An added complication in the project lies in the fact that Bolina Road, a small single lane roadway, crosses the site from north to south at a point just before the different lines enter or fly over the Dive Under box structure at the east end. The existing steel bridge offers limited head room for vehicles and proudly displayed a series of bullet holes from the Second World War. It was ready for retirement!
To bridge the gap, the site team constructed five in-situ bridge structures at different stages in the project. Four of the five bridges are simple reinforced concrete structures and are well advanced, which has helped with plant movement on the site. A steel-decked span is being constructed over the roadway which is being converted into a walkway and cycle path.
A major constraint on construction resulted from the need for the centremost two bridges to straddle the road, while giving adequate clearance for the Bolina Road path. Track on the bridges also had to be aligned to meet the horizontal constraints imposed by the alignment of the box with the existing track.
It is a complex and difficult undertaking and one that has been sidestepped over the years. As Julian stated: “Everything is interrelated and it all has to be done at the same time”. The location is also extremely challenging and any delays caused by completing this work have the potential to create major concerns.
Having said that, walking round the site there appeared to be a very calming atmosphere, order was evident alongside a sense that everything is going to plan. There have not been any reportable accidents in three years. This all suggests that another critical pinch point will soon be removed, moving Network Rail even closer to a Thameslink route fit for the twenty- first century.
Written by Collin Carr