Covered Ways 12 and 58 are two 110m parallel structures dating back to 1868. Covered Way 12 is on the District Line between High Street Kensington and Earls Court and Covered Way 58 is on the Circle line between High Street Kensington and Gloucester Road. Both are in the course of being strengthened to comply with the 1968 Transport Act. This statutory obligation requires London Underground Limited (LUL)to maintain, improve or strengthen the structures to carry the overhead load from Kelso Place above, which has expensive residential properties.
The present structures, comprising cast iron girders spanning the tunnels with brick jack-arches between adjacent girders, are weakening and are being underpinned with a relieving structure of curved-top fabricated beams. Each beam weighs about 6.6 tonnes and there are 44 in each covered way. The proposed project will cost approximately $24.6m.
Technical solution
There were only three possibilities: to replace the roof; strengthen it; or make it redundant. The first was totally impractical because of the properties above. Strengthening would be far from straightforward, and LUL’s policy is to make cast iron elements redundant wherever possible. The good news was that long-term monitoring by strain gauges and bi-weekly inspections showed no apparent deterioration of the structure, apart from some crudely repaired World World II bomb damage. On the other hand, head-room between the tracks and the roof was already below modern optima, especially at the eastern end close to High Street Kensington Station.
To improve existing vertical clearance for trains, therefore, some of the track will be lowered. During this time, approximately 480m of track and track drainage will be replaced and a ballast mat installed below the track ballast to reduce noise and vibration experienced by residents of properties directly above the covered ways. This is the first time a ballast mat has been used by LUL.
The engineering also has to take account of two sewers running beneath the track.
As part of the feasibility and design processes, it has been discovered that the existing foundations are weak as a result of the low quality of concrete used in the Victorian structure. Another worry is the long-term stability of the brick retaining walls that formed the outer supports for the roof structure.
The first solution considered made the cast iron sections redundant by filling in the recesses between the outer wall piers with structural concrete. This would support a grillage of steel beams below the existing roof structure, reducing headroom and requiring the track bed to be lowered significantly to compensate.
However, a problem with this solution would be the undermining of the retaining walls, which would involve underpinning them at least. The discovery of the major sewers below the trackbeds effectively ruled out that solution.
An innovative approach was demanded and this was achieved as a product of value management. Specially fabricated steel beams of ‘aerofoil’ cross section would sit tight up under the jack arches, supported as before on the infilled recesses. Headroom would still have to be restored to modern standards, but maximum excavation to achieve this would be no more than 800mm, keeping the trackbed above the sewers. Mini-piling beneath both outer walls and the central dividing wall would ensure stability and increase load bearing capacity.
However, a significant engineering problem was the fact that the arches were far from uniform. An 8000 point survey and 3-D modelling enabled five different beam types to be fabricated. But few of the 45 new beams over each covered way will be either level or parallel.
Handling and positioning the aerofoil beams – which weigh up to 6.5 tonnes each – is another problem to be overcome. A custom built mechanical arm mounted on an engineer’s train will erect the beams. A similarly pragmatic low-tech approach has been adopted to bring concrete to the site. The contractor will simply drill down from Kelso Place above and install a concrete pump line to place the 1300m of concrete needed in each Covered Way. Permanent glass-fibre cement formwork is another rationalisation introduced by the team.
Having the contractor on board early has enabled the team to scrutinise the plans from top to bottom. The team has been very successful in reducing the number of processes and components involved.
Enabling work
Enabling work for both Covered Ways began in January 1999. Much of this work involved moving and protecting cables and equipment. This was particularly true for the 30 year old high voltage, gas-filled cables that supply power to five different Tube lines, as they will be live throughout the contract. More than 50 tonnes of 8mm steel plate is currently being fixed to protect them during the main works.
Although carbon fibre composites were considered and rejected as ‘much too expensive’ for the permanent aerofoil design, carbon fibre plates were bonded to the underside of the cast iron beams to mitigate against a failure during construction. This was an important risk mitigator as trains would be running in service during the project in the adjacent Covered Way, only 3m away.
While these issues pose little risk at present, track lowering may reduce the integrity of the structure. So the foundations will be strengthened using 650 piles and the recesses at the side of the covered ways will be filled with concrete to support the steel beams.
Considerable costs and time savings will be made on the project thanks to the application of 3-D virtual reality modelling techniques. While the technique is frequently used as a visualisation tool for architectural schemes, it is believed that this is the first engineering application of the method within London Underground.
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“Having the contractor on board early has enabled the team to scrutinise the plans from top to bottom” |
Because each arch has a different profile and level, the ‘fit’ of the new beams is crucial to the success of the solution. Graphical modelling of the arches, based on data obtained from electronic surveying techniques, has enabled the available space between beams and brickwork, together with resulting clearances to the track, to be quickly assessed, saving many hours of laborious manual calculations. The method could be further developed to model the proposed beam installation methods.
Safety
The project constitutes a Category 1 safety case change, which means that the Railway Inspectorate has to be satisfied across a range of issues concerning the design and implementation of the project.
Project management issues
The project to strengthen Covered Ways 12 and 58 on the Circle and District lines impacts upon almost every aspect of LUL’s business. It was necessary to form a large, well integrated team immediately which represented all the functions necessary both to confirm the feasibility of the project and to deliver it. This involved, among other things, transport planning; timetables and duty schedules; engineering; procurement; public relations; and logistics.
An early decision was taken not only to integrate all LUL disciplines and directorates but also to integrate the civil engineering contractor at an early stage in the project cycle. This would help assist with the complex issues involved in planning the work and developing the design. Developing integrated project teams in this way and securing the early involvement of the contractor is an example of one of the strongest tenets of the recent government-sponsored report by Sir John Egan.
The report, entitled ‘Rethinking construction’, asks leading clients in the construction industry to nominate certain innovative projects which demonstrate this working partnership. Its objective is to encourage teamwork across a variety of disciplines and functions in a manner which seeks to improve the overall performance of the whole team.
A London Underground Limited first
Covered Ways 12 and 58 is one such project and is among the first within LUL to develop and promote alternative project management practices.
Throughout the project’s life, risk and value management exercises are being conducted to ensure that an excellent working partnership is maintained which remains focused on achieving all principal objectives. Various workshops will be held periodically that involve all the key stakeholders to resolve any issues and problems which arise.
The project team is clearly focused on getting the content and structure of the job right long before taking possession. It is essential to the project’s strategy that a viable schedule is in place which is maintained in order that LUL has a chance to cope with the disruption which will inevitably come about.
Train service patterns
Undertaking complex engineering within an operating rail environment is not easy and, therefore, the design has been influenced by its buildability and its relationship with the means by which materials will be delivered to site. To accommodate the closures, new timetables have to be published.
The biggest issue has been ensuring that LUL provide the best possible train service to customers using the line. This will require that it has sufficient numbers of train operators and station staff to meet the demands of the new service patterns. Planning staff deployment for such a project is a complicated process which requires co-operation across the network. However, this is not a problem because LUL has an experienced operational team to manage it.
Due to the complexity of the Covered Ways project and the changes to service patterns, there is a strong emphasis on external relations. LUL’s primary focus must be to provide effective and relevant information to its customers and neighbours, and it is critical that it has in place a robust communications strategy.
Conclusion
Covered Way 58 was undertaken during a ten week closure of the Circle Line this summer; Covered Way 12 will be closed for a similar period from around May 2000. By late summer 2000, Kelso Place should have its weight restriction removed.
Related Files
Plan of Covered Way 12
Lateral section of Covered Ways 12 and 58
Lateral section of Covered Ways 12 and 58 showing aerofoil beam
Location map
