Extensive use of pre-cast arches, recycling of aggregates and clever re-purposing of Victorian structural elements are key features of the £45M Bermondsey Dive Under project.
When NCE visited the massive rebuilding project at London Bridge station in the latter part of last year, one of the most striking elements of the work was the way the station was essentially being flipped on its head to improve throughput of trains.
The overall objective of the project is to change the station from one that has nine terminating tracks and six through tracks to one that has six terminating tracks and nine through tracks.
The increased number of northbound tracks will ultimately allow 24 Thameslink trains to pass through the central London station, and on to further destinations, every hour.
Inevitably, the changes to the main line station have called for a reconfiguration of the tracks on an equal scale east of the station – not least in the notoriously congested Bermondsey area.
“The whole idea of our demolition and construction is to work from the middle out with two work fronts”
Charl de Kock, Skanska
At the moment, Thameslink track in Bermondsey runs on an earth embankment from the south. This converges at its north end with more lines running on two separate viaducts from east to west, carrying trains between London Bridge and Kent.
The Thameslink trains must cross two sets of east-west tracks via a series of points. They are often held at signals before being allowed to cross over onto the correct set of tracks in the middle of the viaduct.
The £45M Bermondsey Dive Under project, which forms part of Network Rail’s £6.5bn Thameslink programme, and which is being delivered by main contractor Skanska with consultant Ramboll, will realign and untangle the tracks at this pinch point to eliminate the need for Thameslink trains to disrupt London Bridge to Kent services and vice versa.
Discovering a “ghost station”
During the project the construction team has had to work on the remains of a long-lost South London railway station.
Southwark Park station, situated on a viaduct above Rotherhithe New Road at the western end of the site, only served passengers from 1902 to 1915 before it closed for good.
“We uncovered the footings for the former platforms while we were preparing the top of the viaduct for new track and we are now working up in the roof space of the former ticket hall to fill in the old sky lights, ready to carry the final track alignment,” says Network Rail project manager Greg Thornett.
“Much of the existing stretch of viaduct will be replaced by the ramps into and out of the new dive under, but the arch that used to house the old booking hall will remain.”
In addition, the same team, from Network Rail and contractor Skanska is rebuilding 20 bridges between New Cross and Waterloo East to increase their strength at a cost of £25M.
“There are two teams working on the two projects but because Skanska is our principal contractor to do both, there is a lot of crossover,” says Thornett.
“We’re creating a large concrete box structure that allows Kent lines to dive down and then come up the other side, and allows the Thameslink lines to fly over the top so they can get across to the through platforms at London Bridge station without having to stop and queue at the signals in the Bermondsey area,” says Network Rail project manager Greg Thornett.
When work is complete, two ramps carrying three Kent lines at the eastern end of the site will dive down and converge in the concrete box before they rejoin the east-west lines via an upwards running third ramp at the western exit to the box (see diagram).
The northernmost of these first two ramps, which has already been built, will carry just one line, while the ramp to the south will carry two.
Meanwhile, two Thameslink lines will “fly over” this structure while a third Thameslink line will split off to join the first two ramps passing through the box.
One of the key technical challenges of the project for consultant Ramboll was to develop designs that minimised the disruption to the live railway while integrating the new structures with the existing Victorian masonry arch viaducts on which the lines currently sit.
Sections of these masonry arches will be fully demolished to make way for the concrete box structure. Elsewhere, parts of the piers will be retained as a foundation for the ramps that plunge into the box and then out again on the westernmost side.
The first unavoidable disruption to the live railway occurred during a 100-hour possession in Easter 2014. During that weekend, the construction team took possession of two of the four Kent slow lines at the southern edge of the northernmost of the two converging viaducts.
This was to erect a three-span composite steel and concrete bridge across the East London Line which crosses the project site at its eastern edge. The first of the dive under ramps was then constructed west of this structure (to watch a time-lapse video of this part of the project, click here).
This ramp will ultimately carry one South Eastern slow line down into the concrete flyunder box when the project is finished.
This part of the project was completed early because it required the closure of two rail lines that will otherwise have had to remain live for the duration of the project.
“The new arches are similar to the existing masonry arches. The benefits of this are that the load distribution on the foundations is unchanged”
Will Duckett, Ramboll
Now that the steel-concrete composite structure and the first of the ramps has been built, the contracting team can focus on a controlled sequence of demolition and construction in those areas where the railway tracks can remain closed for a more extended period of time.
This includes the two fast South Eastern Kent lines on the southernmost of the two converging viaducts and the three Thameslink lines on the southernmost embankment which were all closed when NCE visited the site in April.
“The whole idea of our demolition and construction is to work from the middle out with two work fronts,” says Skanska delivery manager Charl de Kock.
“We will bring everything in the middle to ground level and then demolish out to the east and west and then the same goes for the piling: you start in the middle and pile outwards with two gangs.”
Ramboll lead designer and director of rail Will Duckett explains that a series of “bookend” abutments has been developed to stabilise the existing viaduct structures and avoid overstressing the existing pier foundations as sections are demolished.
“The bookends utilise spread footings founded on the river terrace gravels and are designed to act monolithically with the existing pier,” he says.
“We’re creating a large concrete box structure that allows Kent lines to dive down and then come up the other side, and allows the Thameslink lines to fly over the top”
Greg Thornett, Network Rail
“Post-tensioned bars are used to ensure composite action between both book ends and the existing pier to ensure a good spread of load on to the river terrace gravels.
“As well as providing the appropriate support to the viaduct in the final as-built condition they will also act as a temporary support to the existing viaduct during demolition.”
Once the central part of the site is cleared, work on the 150m long concrete box structure can commence.
“It’s not classified as a tunnel because of its length,” reveals Thornett. “And if it was classified as a tunnel, there would have to be several additional safety features in terms of access for emergency services and lighting that we would have had to incorporate.”
A large number of the roughly 900 continuous flight auger piles (CFA) that will be used for the job will support the reinforced concrete box structure.
“It’s a standard insitu wall and slab structure, and the walls are cast in full height lifts,” says Thornett.
The deck has been designed in three sections with joints at the three points where they meet. The end sections are designed to be cast in two pours and the central section in three pours, so a total of seven pours will be required for the deck.
“There is a central wall towards the east end where the track alignments below provide enough space to allow a wall which minimises the span of the deck slab,” says Duckett.
An added complication in the project lies in the fact that Bolina Road, a small pedestrian path, crosses the site from north to south at a point just before the different lines enter or fly over the box structure.
“The box is not classified as a tunnel because of its length.If it was classified as a tunnel, there would have to be several additional safety features”
Greg Thornett, Network Rail
To straddle the road, the site team will therefore construct five insitu bridge structures at different stages in the project. The northernmost two of these five bridges are simple reinforced concrete structures, the first of which has already been completed so plant can access the site from the north.
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.
There was a desire to minimise any adjustment to the Bolina Road alignment to reduce the requirements for service diversions, avoid drainage issues and provide a footway alignment that suited Lewisham Council’s requirements.
It was agreed that the minimum highway clearance under the Bolina Road bridges would be 2.7m. These two constraints give a high span to depth ratio of 23 that meant it would be difficult to get a reinforced concrete deck solution to work here.
The solution was to use filler beam decks - essentially concrete reinforced with steel I-beams for these two bridges.
The final even longer span bridge, which will take the Thameslink flyover lines from their reinforced embankment over Bolina Road and onto the concrete box, is not constrained by height limits. As a result, the most efficient solution is a 24m single span steel composite bridge.
Although the box and structures in the Bolina Road area require piling, Duckett explains that one of the key innovations on the project was to minimise the amount required elsewhere by using a precast arch solution for parts of the dive-under ramps and elements of the Thameslink “flyover” structure.
This was done by designing the new viaduct structures as precast concrete arches which could rest on the existing piers of the masonry arches which had to be demolished to make way for the new structures.
“In some respects [it’s as] novel as it is uncommon for new railway structures to be designed as arch structures,” says Duckett.
“The major feature is that the new arches are similar to the existing masonry arches. The benefits of this are that the load distribution on the foundations is unchanged, and many of the existing piers can be reused reducing the requirement for piled foundations.”
“They have concrete backing with a curved upper profile, as typical arches, and then the fill material is foamed concrete,” says Duckett. “The foamed concrete has a similar performance to a rubble fill with little tensile capacity and low density, to avoid overloading the existing foundations. This material was selected as it could be easily delivered to site and did not require compaction,”
Careful thought regarding the thermal performance of the fill material was required. The engineers wanted the arches to behave in a similar fashion to long lengths of masonry arch viaducts that breathe under thermal loading and do not distribute loads longitudinally along the viaduct.
“The use of the precast arches reduced the amount of heavy duty work and cost”
Charl de Kock, Skanska
To ensure this behaviour, nominal 10mm joints are post cut into the foamed concrete at the crowns of the arch. A concrete slab is then installed over the arch structures to ensure good transverse distribution of the loads across them.
This concrete slab is typically formed by robust precast kerbs at each edge and a reinforced concrete slab cast insitu to join them together.
Brick facing to the spandrels is then installed to give long term protection to the foamed concrete and provide an aesthetically pleasing finish.
“The use of the precast arches reduced the amount of heavy duty work and cost,” says De Kock. “It took a lot of the insitu work out of the project, [there were] no lorry movements and there was no site congestion, and we could delete the need for pilings [in this part of the project]. Precast is typically a little bit more expensive but if you weigh it against the savings on the pilings you’re okay.”
Both of the westernmost dive under ramps sit on these precast arches at their western ends. Further along, at their lower, easternmost ends where they meet the Bolina Road bridge structures, however, they sit on reinforced earth.
These sections consist of modular blockwork filled in with recycled aggregates and reinforced with polymers.
“Reinforced earth has been designed to give an opportunity to reuse the demolition material which has positive environmental benefits,” says Duckett.
The same precast arch and reinforced earth configuration applies to the western ramp which rises up on the west side of the box and allows the four tracks that have passed through it to rejoin the lines into London Bridge.
Similarly, the main structure on which the Thameslink track will join the east-west Kent Lines after “flying over” the concrete box will also consist of these precast arches.
Meanwhile, the earth embankment on which the Thameslink lines join the box from the south will be regraded, raised and have lightweight fill added to avoid settlement issues.
A reinforced concrete wall supported by CFA piles is also to be constructed to separate the Thameslink slow line which splits off from the two Thameslink fast lines that pass over the box to join the three Kent lines passing through the box.
“The use of the precast arches reduced the amount of heavy duty work and cost” Charl de Kock, Skanska
De Kock says most of the structural work should be complete by June 2016 at which stage the focus will be on completing the waterproofing, hand rails, steps access and drainage – including the installation of large attenuation tanks – in time for a 31 August handover date.
At that stage, Balfour Beatty Rail will begin reinstating the South Eastern and Thameslink tracks.
“We had a good run at the first ramp [the eastern ramp carrying the South Eastern slow line],” he says. “We learnt a lot from that and we’ve reprogrammed the whole project based on our outputs from that.”
But, regardless of the unforeseen, he is confident the project will be completed on time: “The biggest risk is the unknowns such as underground obstructions when you pile.
“Any delay we incorporate won’t affect our 31 August hand back date. We’ll have to mitigate that risk to make sure we deliver on time.”
NCE RAIL CONFERENCE
The Bermondsey dive under and the London Bridge upgrade are just two of many dynamic projects that are coming to fruition in the lucrative rail sector at the moment.
And now that Britain has a majority government that has stated its commitment to even larger rail projects such as High Speed 2 and Crossrail 2 it’s hard to remember a time when there has been a more promising outlook for the industry.
To help you pinpoint how future investment and project pipelines will be procured, designed and engineered, NCE has brought together all of the major UK rail clients for a two-day rail conference on 23-24 June.
At the NCE UK Rail Conference, which takes place at the Victoria Park Plaza Hotel, these clients will reveal their plans for short- to medium- term investment and share best practice in the delivery of challenging projects.
The speakers for the event include Network Rail digital transformation director Patrick Bossert, London Underground and London Rail managing director Mike Brown, Crossrail chief executive Andrew Wolstenholme and Transport for Greater Manchester Metrolink director, Peter Cushing.
To find out more and register for the event go to www.ncerail.co.uk