Much has been written about the Thameslink Programme which will give an enhanced rail service through the centre of London. It is being delivered in three phases. Key Output 0(K00) allowed a consistent eight-car train service to run at 15 train paths per hour (tph) between St Pancras International and Blackfriars station. This work was completed in March 2009. KO1 provided for an improved train service of 12-car train formations from December 2011. Farringdon and Blackfriars Stations were reconstructed during KO1 with overall completion during 2012.

The final stage, KO2, will allow the introduction of the enhanced Thameslink service by the end of 2018. KO2 includes the reconstruction of London Bridge station and the construction of the infrastructure to allow up to 24 tph through the Thameslink core area. The programme will require major infrastructure works and significant adaptations and improvements to existing stations along the proposed enlarged route.

Diving under Bermondsey

It is critical to the successful delivery of Key Output 2 that the Thameslink lines (South Central Sussex Fast lines) are separated from the South Eastern and South London lines to minimise conflicting crossing moves on the eastern approach to London Bridge station. The separation of these lines will be achieved by the construction of a dive-under in the Bermondsey area of South London, known as Bermondsey Dive Under, (BDU).

The BDU is the re-alignment of the South Eastern, South Central (Sussex) and South London lines between Rotherhithe New Road to the East and the East London line to the West to provide grade separation between the South Central Fast lines and the South Eastern and South London lines. This will be achieved by mainly raising the vertical alignment of the South Central Fast lines whilst in turn lowering the vertical alignment of the four South Eastern, South Central Slow and South London lines to dive under the South Central Fast lines.

A key element in the provision of the BDU is the Structures Strengthening Programme (SSP). A total of 43 structures were identified between Bermondsey and Waterloo East station of which at least 34 require work ranging from minor strengthening due to track realignment to major renewal.

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Stobart Rail, working as a specialist contractor for Skanska UK and Network Rail, has spent the last year developing detailed design and option reports for the SSP. This early engagement by Stobart’s construction teams has enabled them to provide crucial information to assist in the design, planning and methodologies for the delivery of each structure.

A variety of structures

The existing structures include brick arch underbridges/viaducts, steel and timber deck underbridges. The scope of the metallic structures works at the bid stage were likely to include strengthening, deck replacement, abutment strengthening, painting, waterproofing and the conversion of timber to ballasted decks. The aim of the scheme is to achieve at least a Route Availability (RA) rating of at least RA08 on all the structures.

The main problem, as always, is the limited time available to carry out the works. The first two bridges on the scheme, namely Crucifix Lane and Whites Ground – structures deep in the throat of London Bridge station – were planned to be delivered over the spring and summer Bank Holiday weekends in May and August 2013. To minimise risk, the plan was to undertake as much preparation work midweek as possible and so reduce the works to be delivered in the allotted possession times utilising rules of the route possessions. To this end, Stobart Rail worked hand-in-hand with Skanska, Network Rail and Cass Hayward to reach a buildable conclusion.

Crucifix Lane was the first bridge to undergo the improvements. The original structure consisted of longitudinal timbers supporting the tracks over a steel bridge deck. In order to accept the new track alignment, the timbers needed to be replaced with standard ballasted track. The ageing bridge deck was unable to carry the additional loads as the deck plates were suffering from corrosion.

The design solution was to remove the track and longitudinal timbers from the bridge deck and replace them with new steel deck plates supported directly on the existing cross girders. Rivets were removed from the cross girders, new steel cheese plates fitted and the new deck plates placed. The whole assembly was fixed by Stobart Rail’s steel fabrication team drilling vertically through the holes of the previously removed rivets and bolting the new components in-situ.

Planning and preparation

Midweek, prior to the bank holiday, scaffold was erected to provide access for the installation teams and also provided a secondary catchment area for any potential falling debris. A Haki staircase was also positioned at the end of the structure enabling access from road to track level.

Detailed planning went into the positioning of the crane, not only to facilitate the lifts required to deliver the works, but also to minimise the disruption caused to the residents of Whites Ground estate. Balfour Beatty Rail Systems provided unhindered access in their worksite enabling the smooth delivery of the project, also providing some support by way of removing the conductor rails and carrying out disconnections.

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Susan Fitzpatrick, Skanska project director, said: “This is a technically complex project and it took a huge amount of planning to get it right. We worked closely with Network Rail to make sure that we could time our works to coincide with the possession order for the bank holiday weekend.

We also engaged closely with the community to prevent disruption to local residents.”

The first possession, on the Spring bank holiday, went without a problem, with the first half of Crucifix Lane receiving the overplating solution. Stobart Rail’s RRV plant placed the new components working in conjunction with the road mobile crane which was lifting the plates to bridge deck level. The second possession, on the Summer bank holiday weekend, will be a little more involved as both Whites Ground and Crucifix Lane undergo remedial works in tandem. The bridges are in close proximity, with less than thirty metres separating the two. This could cause accessibility concerns but Stobart is very experienced at all aspects of logistical delivery.

Network Rail programme manager Andy Jenkins said: “It was important from a team momentum perspective that we successfully delivered the first of the 32 rail bridges that need strengthening as part of Thameslink’s KO2 programme. The success was further compounded by being achieved with no accidents, incidents or environmental complaints.”

In preparation for the August bank holiday works on the Whites Ground bridge, the team is extending 24 cross-girders, accessed on underslung scaffold in sections encompassing a skate park, pedestrian footway and road. This will significantly reduce the work required to be delivered in the disruptive weekend.

Stobart Rail’s managing director Kirk Taylor commented after the May bank holiday work: “It’s a successful start to a hugely important project for us here at Stobart; we are happy to be associated again with Skanska following on our ten-year working relationship. We are fortunate to be working together with a fully integrated and collaborative bunch of people, on an interesting and diverse project that will enable us to use our experience to the full.”