On the south bank of the Thames, opposite Victoria station, on the disused site is Battersea Power Station. Alongside the power station there is another smaller but just as well known location, the Battersea Dogs & Cats home.
Between these two locations are the Up and Down Fast and Reversible lines to Chatham 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 a complex five span intersection bridge with skews that vary from span to span.
During the late summer of last year, the rail engineer wrote about Bridge No.6 at Battersea and the complex preliminary work that was underway in preparation for the final stages planned for Christmas 2011 (Issue 83, September 2011). This work has now been completed and the following is a recap of those preparatory stages and the completion work that took place during a nine day blockade over the Christmas period.
Headache for track maintenance
The rail-over-rail bridge was constructed just before the First World War, in 1913. 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 a cross-sleepered track on longitudinal timbers, whereas the reversible line runs on wheel timbers. As you could imagine, it has been a headache for track maintainers for many years.
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 which is ripe for development. The underlying concern for Network Rail is that if and when a development scheme is approved, access to their Brighton Goods Bridge No. 6 is likely to become even more difficult, hence the urgency to carry out any remedial work now that may be necessary.
The need for full RU loading
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 and Partners, design consultants, prepared various preliminary schemes for Network Rail to achieve this objective. Subsequently, these schemes were developed into more detailed options by the current designer, Mott MacDonald working for BAM Nuttall, which was appointed as Principal Contractor by Network Rail. The value of the completed work is approximately £12m.
The chosen design requirement was to construct an in-situ reinforced concrete box in each of the five individual spans. The top of the box was designed to be at the level of the soffit of the existing superstructure, thereby allowing the new structure to be constructed whilst the existing bridge was still in operation.
Network Rail’s Project Manager Phil Avery explained to me that the innovative design was chosen to allow minimum rail disruption and to maintain rail pathways into London Victoria from Kent. It utilised a series of weekend possessions and blockades of individual lines leading up to Christmas 2011 and then, from Christmas Eve, a 9 day blockade, closing all lines for 3 days and the top three Chatham lines for a further 6 days. Work started last February with 24 hour working from August when the individual lines were blocked. The main blockade started on Christmas Eve. This allowed valuable time to prepare the site for this critical and final phase of the work.
Local Liaison
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, plus a number of health and safety issues had to be addressed, relating to the proximity of the animals.
A “super highway” was constructed for a cluster of important and potentially dangerous cables and an ordinance survey undertaken to ensure that there were no nasty surprises still lurking around from WW2.
Ground Conditions
The design demanded a significant level of site ingenuity. Contaminated ground conditions were created by man as well as the local animals. In the past, the area around the power station was used as a water treatment plant so the ground conditions were rather suspect, especially around spans 1 and 2. To stabilise this area, four piling rigs were brought to site, operated by Keller Piling. They installed more than 100 reinforced concrete piles over 12 weeks during weekend possessions to underpin the existing piers and abutment. The piles varied from 300 to 450mm dia. and were founded into London clay ranging from 13 to 18m below ground level.
Minimising disruption to trains
Concrete was poured to form the base slab, followed by the construction of the side walls. A polythene membrane was inserted between the existing abutments, 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 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.
Complex reinforcement installation
Three tables were constructed on which the substantial 32mm diameter reinforcement framework was fixed to form the top section of the concrete box. The tables were supported on trestles to ensure they were at the required height for installation. Wheels were fixed to the base of the supporting trestles and angle rails bolted to the soffit 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 process worked very well. There was a 24 hr possession for each span to remove the timber decking and expose the reinforcement. Two concrete pumps were then installed alongside the structure and approximately 400 cubic metres of concrete was poured in an eight hour period. It proved to be an effective method requiring substantial on-site skills and this phase of work was completed two weeks before the Christmas blockade. In total 1,100 tonnes of reinforcement and 4500 cubic meters of concrete was used. The new reinforced concrete boxes are designed to carry the required rail loading and they were ready to act as the base for the bottom ballast for the new bridge deck when it was planned to be exposed over the Christmas period.
Removing the old superstructure
The Network Rail Scheme Project Manager, Alfie Chimedza, and Site Construction Manager Paul Adams, worked alongside BAM Nuttal’s Site Project Managers, Rick Kopek, Emily Short and Tony Russell to ensure that everything went according to plan.
Two road mobile cranes, of 350 and 500 tonnes capacity, were used to remove the existing steel deck. They lifted road-rail plant onto the bridge superstructure to remove the three existing tracks. Then two 35t tracked excavators armed with large hydraulic cutters were used to cut the steel superstructure into sections which could be removed from site by the cranes, exposing the top face of the concrete box and parapets. After making good the pier tops, sections of waterproofing were welded together to form a continuous waterproof membrane for the new wider bridge deck.
The drainage system was then installed and 3,000 tonnes of new bottom ballast were transported to site by an end-discharging ballast train provided by Network Rail, using 80 self discharging wagons. Discharge wagons can only operate on track with up to 50mm of cant, hence the restricted length of each train. These trains had to arrive in maximum units of ten wagons in length to accommodate for the canted track off the bridge deck which increased to 75mm away from the bridge. This work was completed after three days. New sleepers were then brought to site and positioned with road-rail machines, the rail was dragged onto site, clipped, welded and distressed, and the track tamped to line and level.
Success!
This challenging undertaking was to replace a five span rail over rail bridge. The technique used to construct a bridge within a bridge, to five different skews and spans, proved very effective. The published 9 day Christmas blockade was to be handed back at 4am on the 3 January 2012. As a result of innovative engineering methodologies and collaborative working, the possession was actually handed back at 16.00hrs on Saturday 31 December 2011, 3 days early with all three lines operating at full line speed.
The new structure will require minimal maintenance and the ballasted track will improve ride quality. The new superstructure will have adequate clearances and walkways to provide a safer working environment. The new Route Availability will offer improved flexibility for operators. Brighton Goods Bridge 6 was a successful project and a credit to all concerned.
