Rebuilding a vital section of the Thameslink rail line at Bermondsey Dive Under in London included track and infrastructure work to increase traffic, with sustainability and urban regeneration aspects. Value engineering and detailed design by Ramboll, in collaboration with main contractor, Skanska, and Network Rail, delivered significant cost benefits.
Bermondsey Dive Under comprises a new section of railway track and infrastructure that allows Thameslink lines to cross Kent lines unimpeded. It forms part of the £6 billion Thameslink Programme and seeks to untangle the tracks approaching London Bridge Station.
Rebuilding for the Thameslink programme
First opened in the 1980s, Thameslink delivers a north-south main-line rail link through the heart of London. Upgrading it was a massive undertaking. The £6bn engineering project is a third of the cost of Crossrail’s east-west rail-link, but it transforms the existing cross-capital route into an equally significant part of the southeast of England's railway infrastructure.
The existing railway formation at the Bermondsey site comprised 11 tracks on elevated Victorian masonry arch viaduct structures. Rebuilding work was crucial to enable Network Rail to achieve its objective of allowing 24 trains to pass per hour through the Thameslink core. Putting it simply, the dive under project involved untangling the old track on the approaches to London Bridge.
To support the new Thameslink lines, the project scope also included crucial strengthening of numerous metallic bridges on the approaches to London Bridge.
The dive-under project involved the design of a new grade separated junction at Bermondsey to minimise conflicting crossings on the eastern approach to London Bridge Station. This is formed by a series of new structures constructed along the line of existing operational railway viaducts. Wherever possible, existing structures were reused to reduce disruption and waste.
The new structures carry two fast lines over four other lines lowered underneath. All these lines pass over a new underpass that improves pedestrian access to Millwall Football Club.
Ramboll and Skanska were appointed to undertake the outline design, detailed design and implementation of the strengthening works to existing structures and to design the Bermondsey Dive Under. The new structures include earth embankments, reinforced earth, pre-cast arch viaducts, steel composite bridge structures, concrete box structures and reinforced concrete integral bridges.
Within the Structural Strengthening part of the project a total of 52 structures were identified and 21 were strengthened.
“The key technical challenges for this project were to develop designs that minimise disruption to the live railway, de-risk the blockades and coordinate with the existing structures.” Will Duckett, Ramboll project manager.
Ramboll’s key innovation on this project was to propose a pre-cast arch solution as an alternative to the solution developed during the outline design stage, which was for reinforced in-situ concrete portals. This proposal was in some respects novel as it is uncommon for new railway structures to be designed as arch structures. On the other hand it is using a structural form that has been used often since Roman times.
Reduced number of piles
One of the major features of the arch structure is that the new arches have a similar form compared to the existing masonry arches. This ensured the load distribution on the foundations remained relatively unchanged, allowing for many of the existing piers to be reused, this contributed significantly to reducing the number of piles. Optimised designs also meant that the same piling rigs could be used throughout the project.
The pre-cast arches
The pre-cast arch design was developed with a constant arch radius to maximise repeatability and to allow off site manufacture, thereby de-risking material supply during the blockade, reducing site safety risks and allowing key elements to be prefabricated. This solution also reduces the long term maintenance requirements as expansion joints are eliminated and the concrete arch structures are in compression.
The concrete arch segments were designed to have a similar stiffness to conventional masonry arch rings. They have concrete backing with a curved upper profile, as typical arches, and then filled with modern foamed concrete material. Careful thought was required regarding the thermal performance of the fill material so that the arches behave in a similar fashion to long lengths of masonry arch viaducts that breathe under thermal loading.
The brick facing gives long term protection to the foamed concrete and provide an aesthetically pleasing finish.
- Increased capacity - Network Rail said the redevelopment work at Bermondsey will make London Bridge station more accessible, reduce congestion and increase passenger capacity by 50%. Over the whole route, peak train frequency will increase from the original 7-8 minutes, to four minutes currently, and 2-3 minutes by the end of the decade.
- Additional destinations - Multiple regional destinations on either side of the capital will be connected for the first time with a direct rail service through London with Tube-like service frequency. The current Brighton-to-Bedford route will add Cambridge, King's Lynn, Ashford and Eastbourne as destinations.
- Urban regeneration - The line upgrade has also acted as a catalyst for urban regeneration, linking many major commercial redevelopments in the capital, such as at Elephant and Castle in the south, and Brent Cross to the north.
- Improved biodiversity - In May 2017 a CEEQUAL Excellent Whole Team Award of 96.6% was presented to recognise that the project increased biodiversity in the area by 113%. At time of presentation, this score was the highest achieved by a completed project on the Thameslink Programme. Prior to construction, which commenced in 2012, the site had low conservation value and limited botanical diversity. The soil was heavily contaminated with hydrocarbons and asbestos, as well as the invasive species Japanese Knotweed. The project saw the eradication of the Japanese Knotweed and the removal of over 21,900 tonnes of contaminated material. Furthermore, wildflower planting and the installation of 765 square metres of green walls increased the site’s biodiversity and created green corridors and stepping stones to the wider area.
- Sustainability - In July 2017, Ground Engineering's Sustainability award judges were impressed by a detailed and quantified demonstration of sustainability benefits that were achieved with significant innovations, including: cost reduction from reduced piling and pile depth, reduced material movement and impact. They said it was clear that the team took great pride in meeting and exceeding the sustainability objectives set, and that sustainability was at the core of the project throughout, from inception to completion.
- ICE London - Infrastructure award, 2017. The ICE London awards celebrate the most innovative, creative and sustainable contributions to the physical and social environment in the Capital
- CEEQUAL Excellent Whole Team Award of 96.6%
- Ground Engineering - Sustainability award 2017.