On the south bank of the Thames, opposite Victoria station, sits the sprawling, disused site that houses the well-known landmark Battersea Power Station. Alongside is the smaller but just as well known Battersea Dogs & Cats Home.
Between the two are the Up and Down Fast and Reversible lines to Chatham from Victoria that pass over the Battersea reversible line, the Up Stewarts Lane line and two sidings.
The structure that carries the Chatham lines over the others is known as the Brighton Goods Bridge No. 6. The bridge is an impressive and complex five span intersection bridge with skews that vary from span to span.
The bridge was constructed in 1913, just before the First World War. The construction of the deck for each span consists of wrought iron longitudinal girders, cross girders and rail bearers. The up and Down Chatham lines are both supported on cross-sleepered track on longitudinal timbers, whereas the reversible line runs on wheel timbers.
The longitudinal girders span between padstones supported by brick piers and masonry abutments. The main girders are discontinuous over the intermediate piers allowing each span to behave independently.
The structure is hemmed in by the Dogs’ Home and the power station and although there have been many false starts with schemes to rejuvenate the power station site, there is now a growing certainty that a new £5.5billion scheme will transform the location.
The underlying concern for Network Rail is that if the scheme progresses, access to their Brighton Goods Bridge No. 6 is likely to become even more difficult so now is the ideal time to carry out any remedial work that may be necessary.
For many years, structures with timber decking and wheel timbers have created significant challenges for the maintenance teams involved. Previous assessments undertaken by Atkins Rail in 2004 concluded that the structure was generally sound but that the cross girders limited the route availability of the structure.
Network Rail had decided that at such an important location, any new structure would have to comply with full RU loading. Therefore, knowing that train disruption had to be kept to an absolute minimum, Tony Gee & Partners, design consultants, prepared various preliminary schemes for Network Rail to achieve this objective.
Subsequently, these schemes have been developed into more detailed options by the current designer, Mott MacDonald, and BAM Nuttall has been invited to undertake this project as part of their 5 year Framework Contract with Network Rail. The work is valued at approximately £7.6m.
The innovative design that was chosen included the need for regular weekend possessions and sequential blockades of the rail filled Spans (3, 2 and 1) leading up to Christmas 2011 and then, from Christmas Eve, a 9 day blockade, closing all lines for 3 days and then the top three Chatham lines for a further 6 days.
Work started on site in the spring of this year and there are currently about 50 operatives working day shifts, 7 days a week. From the start of August the workforce will double and the site will become operational round the clock, increasing the momentum of activity that will continue right up to the Christmas blockade.
However, before any real engineering work could start, a significant number of dog kennels, exercise areas and recognised dog walking routes had to be relocated. Two weekly liaison meetings were organised with the Dogs’ Home to ensure that the walking routes for the dogs were realigned well in advance.
A cattery had to be relocated as well and buildings demolished, and a number of health & safety issues had to be addressed relating to the proximity of the animals.
Contaminated ground conditions, created by man as well as animals, had to be managed, a “super highway” constructed for a cluster of important and potentially dangerous cables, and an ordnance survey undertaken to ensure that there were no nasty surprises still lurking around from WW2 .
Concrete Box design
The design chosen is interesting and demands a significant level of site ingenuity. An in-situ reinforced concrete box will be constructed in each span with the top at the level of the soffit of the existing superstructure.
These concrete box structures will be different for each of the five unique and individual spans and must be completed before the Christmas blockade.
The new reinforced concrete boxes are designed to carry the required rail loading so when the existing superstructure is removed, no additional strengthening is required. This removal will take place during the Christmas blockade using a large mobile crane.
The newly exposed top face of the concrete box girder that this will reveal will then be made waterproof. A suitable drainage system will need to be installed and 3,000 tonnes of new ballast imported by an end discharging ballast train provided by Network Rail.
New track will be laid across the five spans for all 3 lines, top ballast placed and the track aligned, stressed and tamped ready for the demanding post-Christmas timetable.
That all sounds quite straight forward, but the tricky bit is how do you create a structure from concrete in-situ, within a bridge, whilst avoiding too much disruption? This is where the on-site skills of the site team have been tested.
The project has identified five key milestones leading up to the Christmas blockade, one for each span. Span 4 cuts across part of the Dogs’ Home so, as there is no railway under the bridge, this span was used to try out the proposed method of working and became the first milestone.
This was followed by Span 5 which was in progress when this article was written. Spans 3, 2 then 1, are programmed for September to December.
Method of working
Concrete was poured to form the base slab to span 4 in March this year, followed by the construction of the side walls. A polythene membrane was inserted between the existing abutments and piers and the new concrete forming the walls to enable them to act independently.
Reinforcement couplers were cast into the walls approximately a metre below the soffit of the existing superstructure. These were designed to receive the connecting reinforcement bars that would extend down vertically from the top slab of the box girder.
So far, it was all relatively straight forward. The real challenge was how to construct the top section of the concrete box to enable it to fit directly below the soffit of the existing superstructure whilst minimising disruption to trains.
Bob Snow, Project Manager BAM Nuttall, explained to me how they addressed this challenge.
With the invaluable assistance of their sub-contractor Kilnbridge Construction Services, and especially their Pre Construction & Engineering Manager, Plamen Petkov, three tables were constructed onto which a substantial 32mm diameter reinforcement framework was fixed to form the top section of the concrete box.
The tables were supported on trestling so that they were at the correct required height for installation. Wheels were fixed to the base of the supporting trestling and angle rails bolted to the base of the new concrete box.
Then each table in turn was moved into its final position using Turfers and the reinforcement coupled together to form the roof of the concrete box. Intermediate staging enabled the vertical reinforcement to be coupled to the side walls.
The devil is in the detail
We have to remember that no two spans are the same so the challenge for the Kilnbridge steel fixing team is substantial and the devil is definitely in the detail.
So far the technique is working well and during a 24 hr possession of the Chatham lines, the timber walkway decking was removed exposing the reinforcement, 2 concrete pumps were installed alongside the structure and over an 8 hour period a 300 cubic metre concrete pour was successfully completed. It is proving to be an effective method requiring substantial on-site skills. Only four more spans to go!
In times gone by, the area around the power station was used as a water treatment plant so the ground conditions are a little suspect, especially around spans 1 and 2. Four piling rigs have been brought to site to be operated by Keller Geotechnique.
They will install over 150 reinforced concrete piles over 12 weeks during weekend possessions to strengthen the existing ground foundation within the spans. The piles will vary from 300 to 450mm dia. and will be founded into London clay ranging from 18-22m below ground level.
The work so far is going well. The technique they are using to cast a bridge within a bridge, five times, is proving sound. The plan is to ramp up to round the clock working so that everything will be ready for Christmas when the old five span superstructure is removed.
The train operating companies involved understand and support the overall plan. The dogs and cats appear to be taking everything in their stride and there is no evidence of stray paw marks in newly cast concrete.
It is estimated that a total of 1,100 tonnes of reinforcement and 4,500 cubic meters of concrete will be used. There will be ballasted track in place which will be much easier to maintain. The structure will have adequate clearances and walkways to provide a safer environment.
By the New Year, trains will be running over the reconstructed Brighton Goods Bridge No 6 and Battersea’s famous animal residents will no longer have their walks interrupted.