Article - Issue 7, February 2001
Crossing the rails at Brixton: replacing the UK's busiest rail junction
Derek M. Clark, Paul J. Pragnell and Jon M. Wirth
London Underground Limited recently created three new companies to maintain and develop its network infrastructure. They are due to be transferred in 2001 into the private sector under a Public–Private Partnership (PPP). Infraco BCV Limited, the company responsible for maintaining the infrastructure of the Bakerloo, Central and Victoria Lines, was the first of these companies to carry out a major track replacement within this new working environment.
The 21 km long Victoria Line was the first new underground line to be built after the Second World War. It was completed in two parts, from Walthamstow to Victoria by 1969 and then on south to Brixton in 1971. Innovation at this time was focused on tunnelling and signalling. The line was London Underground’s first automated train operated service. The tracks were constructed in the conventional form of 95 lb bullhead rail on timber sleepers, all held in place by concrete.
The crossover to the north of Brixton station was designed to allow trains to enter either of the station’s platforms and subsequently to leave from either platform and proceed north at a full line speed of 72 kmph. The crossover, a vital asset in the operation of the Victoria Line, currently experiences some 250,000 train movements per year (687 train movements per day), making it the most heavily used junction in the UK.
The existing rails, timber sleepers and concrete had deteriorated over the past 30 years to such an extent that they reached the end of their useful life. This meant that rather than perpetuating the continuous repair regime (which, although keeping the crossover in a safe condition, had become an expensive and timeconsuming exercise), it was agreed that a total replacement of the crossover had become the only viable solution.
London Underground and Infraco BCV agreed to replace the life-expired crossing between 5 and 26 August 2000. During these three weeks both the northbound and southbound lines south of Victoria had to be closed. To facilitate this London Underground provided an alternative bus service calling at intermediate stations between Victoria and Brixton. They employed additional staff to manage the revised service; the staff were based at strategic locations throughout the network. The general public, through their numerous letters of appreciation, later offered praise for the smooth running of this replacement service.
For many years (with the obvious exception of the Jubilee Line Extension), there has been little innovation on the underground and renewals have traditionally been on a like-for-like basis. Design input has therefore been focused on the geometrical issues of the track with the aim of improving the overall ride quality. However, the tide has recently turned and modern engineering solutions, such as those adopted by the Jubilee Line Extension, are now being sought. Infraco BCV identified the Brixton Crossover Replacement Project as the ideal vehicle with which to carry forward this type of policy.
London Underground are required to be self-certificated to Her Majesty’s Railway Inspectorate (HMRI). This means that unless proposed installations meet currently approved London Underground standards, a waiver process has to be followed. The original brief for the replacement works was compiled using a number of innovative solutions previously introduced under separate waivers from similar replacement works. This plan proposed 56 kg/m flatbottom rail supported on baseplates with a 30 mm thick resilient grout. Holding-down bolts for the baseplates would be directly fixed into a mass concrete slab. The crossings themselves were to be manufactured from cast manganese, a more robust and work-hardening material than a conventional built-up steel rail crossing. The crossover’s UIC54B shallow depth switches were to sit on roller baseplates powered by airoperated point motors with mechanical supplementary drives (necessary because of the length of the switches). During the finalisation of the design the project adopted an integrated system approach that considered track, signal and rolling stock issues. This was necessary because any change in one discipline was likely to have a knock-on effect in the other areas.
We measured actual loads exerted on the existing crossover to ensure that the proposed design not only continued to meet London Underground standards but that it also improved the existing installation. We subjected the proposed holding-down bolts and support arrangements to finite element analysis; we tested the resulting recommendations using internationally recognised fatigue tests. These test results were used to improve the original design. At the same time we reviewed buildability and maintenance issues. As a result we increased the bolt size and changed the resilient grout to a pre-formed pad. This permitted the pre-casting of the bolts into concrete blocks and enabled the pre-assembly of the pad and baseplate, a process considered essential in order to eliminate the potential signal leakage from the rail into the concrete. We found that the lifts generated by the roller baseplates on the switches imposed too great a stress on the point motor linkages. This was unacceptable, as the motors were a safety-critical item. Therefore we compared the time and cost required to change the point motor design with the benefits of the roller baseplates and decided to revert to a tried and tested self-lubricating slide baseplate system.
A standard approach to this type of work is to pre-fabricate the ironwork prior to installation. This provided an ideal opportunity to test the switches, point motors and supplementary drives. The results showed that at a low air pressure, the point motors were not powerful enough to throw through to completion both their own drive and the mechanical supplementary drive. An air supplementary drive motor, linked to the primary point motor to avoid independent working, was subsequently designed and successfully tested.
All passenger trains on the Victoria Line are driven between stations automatically. This is achieved by using a signalling system referred to as Automatic Train Protection (ATP). We identified a potential problem in the signal-code loop used to carry the ATP code. The risk was that the increase in solid mass within the cast manganese crossings could have an adverse effect on the electromagnetic field generated by the signal-code loop cable. This in turn could affect the train’s ability to receive the code, without which the service would be restricted to slow manual (10 mph) mode – obviously not an ideal situation on a stretch of track that carries 68,000 people per day! Doubling the cable loops through the cast manganese crossings considerably reduced this risk. This was proven to work by simulating the passage of a train over the crossings whilst the layout was being pre-fabricated in the Lillie Bridge Depot at Fulham.
The crossover is located 60 m to the north of Brixton station in tunnels 25 m below the ground and stretches over a length of 120 m. The old crossover parts had first to be removed: 58 tonnes of running rail, 21 tonnes of conductor or power rail, 24 tonnes of timber sleepers, 420 tonnes of concrete and 23 tonnes of chairs (the bullhead equivalent to flatbottom baseplates). A similar amount of materials had then to be brought in, including some 34 tonnes of plant and temporary materials.
There is no direct access to the crossover from the surface. The only practicable access was by engineer’s trains running from both Lillie Bridge and Ruislip Depots (some 50 trackkilometres away). These would run in engineering hours (outside passenger service) under London Underground operating procedures. We prepared a programme of works and subjected it to a risk management exercise. The train formation and loading requirements were then agreed. A specially constructed borehole supplied electrical and compressed air power and provided a facility to pump concrete from the surface. The borehole was sunk into a cross passage 100 m to the north of the works. We also used it to provide a direct communication line down to the site, which was essential for the control of the placement of concrete.
Risk assessments had identified that the availability of engineer’s trains was of critical importance. All trains were booked to arrive on site before they were required and, although we did not need to use them, additional train paths had been reserved in case of problems with the planned arrival dates. The loading and unloading of the engineer’s trains at site was also identified as critical. To facilitate this, and for handling the cast crossings and rails, we installed an overhead gantry crane in the main crossover cavern prior to the closure of the line.
To ensure safe working practices and reduce manual handling, the existing track bed was broken out down to a natural plane of separation in the existing concrete base. The concrete was removed using electrically driven remote controlled concrete breakers. Rubble was loaded onto the engineer’s trains using either a dieseloperated loader or in 1 tonne bags using the overhead gantry crane.
We monitored the air throughout the work site continuously for dust, diesel fumes and movement to ensure that a safe working environment existed at all times. The Victoria Line’s existing ventilation system, augmented by portable fans positioned within the work area, provided air movement.
The works naturally fell into two halves: breaking out the northbound road and crossings while using the southbound road for engineering train access, then vice versa. Trains delivered new track support blocks, rails and cast crossings and removed the existing rails, timber sleepers and concrete rubble. The new track was then connected up to the blocks with the whole layout temporarily supported on purpose-designed alignment jigs and cross-braced using Acrow props.
This effected a top-down construction. We checked the final alignment using sophisticated surveying equipment (tied into a control network survey) before the pumping of any concrete took place. Once the newly placed concrete had achieved initial strength, trains were brought on to the new northbound track and the breakout of the southbound road began. When all the running rails were in place, the installation of the conductor rails and associated cabling commenced, closely followed by the re-instatement of the signalling system and final testing.
Because of obvious safety implications related to the signalling, we conducted rigorous testing over a fourday period. This culminated in the successful running of test trains prior to the opening of the line to traffic.
The project was successfully completed to time (21 days) and cost (£4.9 million). As planned, the whole of the Victoria Line resumed a normal operating service on Saturday 26 August 2000, ensuring that tens of thousands of people from South London could travel by tube to the Notting Hill Carnival. Initial noise and vibration readings for the crossover, both at tube and street level, are promising. The ride quality has also been dramatically improved. The crossover is functioning well, and three months into service (the time of writing this paper) no lost-time incidents have been attributable to these works. We have established that the new processes being developed within Infraco BCV work in practice and it is our belief that these newly adopted methods will enable Infraco BCV to successfully deliver their future track programme. This can only bode well for the future of the underground, especially as it moves towards a Public–Private Partnership.
Quality and safety
Engineering functions for London Underground are split between three Infraco Companies, each responsible for a number of lines. Each Infraco must assure London Underground of the quality and safety of the work that it undertakes. Infraco BCV achieved this by ensuring that their principal contractor, TrackForce, was ISO 9002 accredited and that they were fully compliant with Construction, Design and Management (CDM) regulations.
Infraco BCV appointed an Assurance Manager with Asset Engineers to oversee the works. TrackForce undertook the complete scope of work and prepared the assurance plans for acceptance. The design and implementation programme provided the main milestones for this process.
Taking into consideration the numerous site activities and the closely confined work interfaces, the safety record was very good. The Railway Inspectorate together with the contractor, their sub-contractors and London Underground Systems Assurance Section all contributed towards the project’s safety review arrangements. This pro-active approach kept the number of minor site incidents to a minimum. The quality control process was also constantly kept under review and the lessons that we learnt from the first half of the shutdown were successfully taken through into the second half of the works.