London takes stock of its underground asset Jul 1991

Lack of capacity on existing London Underground lines caused a major problem during the 1980s as ridership increased dramatically in efforts to escape chronic traffic congestion in the UK capital. In four years alone, ridership increased by 50% to 2.6 million rides/day. In order to ensure that the system continues to serve long into the future, London Underground embarked on an extensive programme of refurbishment and extension in the 1990s.

As with most public transport systems, the highest passenger demand is during the rush hours. Although occupying only 25% of the operating time, it is this demand that must be satisfied for the system to maintain its serive and popularity. Again, as with most existing underground mass transit systems, the rights of way and the underground spaces are its greatest assets of the system. While new lines and extensions are part of the strategy to keep pace with growing demand, increasing the capacity of the existing system is equally as important.

Enlargement is required to ensure dynamic clearance for the longer cars of the new trains

Mass transit systems, with very few exceptions, are subsidised by government. Under-funding of asset renewal on the London Underground over the last 30 years, and a practically non-existent budget for capital expenditure, has left the system in "an appalling shambles" said Wilfrid Newton, Chairman of London Underground after being Chairman of the Hong Kong Mass Transit Railway. Government revenue subsidy for the London Underground has been reduced from £188 million in 1981 to £51 million for 1989/90, although capital investment has increased to an all-time high and is set to increase greatly in the coming years.

Despite this, London Underground has embarked on an extensive programme of refurbishment and extension. With a 40% contribution from private industry, the Jubilee Line is to be extended some 19km south of the River Thames via Westminster, Waterloo, London Bridge, to London Docklands, Greenwich, and north to Kenningtown and Stratford, with the £1 billion scheme scheduled to come into operation by 1995. Again, with a 50% contribution from private industry, there is to be an Underground link with the London Docklands Light Rail system at Bank Station.

A station rebuild at the Angel on the Northern Line will meet an intense increase in ridership. Under a multi-million pound improvement scheme approved by the UK Government in October 1988 – £555 million at December 1987 prices and £750 million as the estimated out turn, including inflation, when completed at the end of 1994 – the Central Line is to become the most modern and efficient of all London Underground lines. As the principal east-west link, the Central Line carries up to 600,000 passengers/day and earns some 18% of the total London Underground revenue. It was the need to upgrade the life-expired signalling system that initiated a complete service overhaul. By optimising the very latest in rolling stock, signalling and power supply technology, journey times are to be reduced by 12% and capacity increased by 16%.

“Current rolling stock is of the 1962 generation and could have remained in service for some years yet,” said Tony Humphrey, Manager of the Central Line refurbishment project for London Underground, “but the total package approach maximises the advantages to be gained from optimum technical interface improvements with replacement also of the rolling stock.”

The 680 cars of the 85 new trains to replace completely all the rolling stock on the line will be the most advanced on the system, more modern even then those on the new Jubilee Line extension which must conform to the line's existing operating systems, designed some 20 years ago and built in the mid-1970s.

Fig 1. Modified diameters along the Central Line and design of current enlargement ring

Early Central Line history

As was the case with most of the early lines on the London Underground, the first part of what became known as the Central Line, from Shepherds Bush to Cornhill, was built by a private company following an Act of Parliament in 1891. The 9.1km of running tunnels and the 14 underground stations were built between 1896 and 1898, with the service officially inaugurated in 1900.

Failure of individual private companies to run at a profit, together with other operating and management problems, led to takeover of all lines into one public company under the direction of the Government Department of Transport in 1929. Introduction of new rolling stock over the years has led to modification of the tunnel lining on many early lines. To provide extra clearance without interfering with the track bed in the invert (a time-consuming and costly operation to be avoided if at all possible) the crown segments are taken out and replaced, the ground behind cut back and the larger tunnel relined with new segments and joint packers. The resulting modified tunnel takes on a very odd but nonetheless stable shape (Fig 1).

In 1937-1938, 10,000 segments along 40% of the Central Line tunnels were replaced to provide a larger structure gauge. Today, the process, on a much smaller scale, is being repeated.

Project management

The current Central Line project started in 1985. Due to the scope of the work, London Underground invited tenders and subsequently entered into a joint venture-type project management agreement with the John Brown firm. “London Underground did not have the staff nor the expertise to manage a project on such a large scale,” explained Humphrey. “The project management JV establishes a dedicated co-ordinating team which makes use of private enterprise expertise and provides the integrated project control required to ensure cost-efficient execution of such a complex undertaking.”

Following competitive tendering, Westinghouse won the £70 million order for the new signalling system, while the £330 million order for the new rolling stock was placed in August 1989 with BREL of Derby in the UK. The scheme also includes building 20 new power supply sub-stations and replacing all the cables from the 22kV AC feed cables to the 600V DC track cables in the tunnels.

Among many design improvements, the new 133m-long, eight car trains are 5m longer than existing stock. With longer cars travelling faster through the tunnels, a new survey had to be undertaken to ensure that the minimum 1in (25.5mm) dynamic clearance was maintained.

After gauging runs by the London Underground Permanent Way Department, a static survey was required to identify those specific rings that infringed the dynamic gauge. Following a competitive tender, independent consultant Howard Humphreys was engaged in early 1990 to undertake the static survey, design the necessary modifications and supervise the required civil work.

Fig 2. Plan of nine of the 11 zones in the Central Line that require enlargement

Fig 3. Longitudinal section of the vertical curves where tolerance in the crown is too tight for the new longer cars

Infringements of between 3mm and 30mm were identified at 61 locations in the running tunnels between Oxford Circus and Holland Park stations. To reduce cost and time, it was decided to rectify only those infringements which exceeded 12mm and across the top two segments and the key of each ring of lining. About half of 137 rings at 11 locations - five in the original 1890s lining and six of the rebuilt 1937 lining - have to be removed, the clay excavated back by 150mm, and the tunnel crown relined with new back-grouted cast iron segments with steel, cast iron and deal timber packers (Fig 3). Most of the locations are at the vertical curves in the line where the trains accelerate down hill away from stations and slow down on the uphill approach to the next station (Fig 4). The longest section requiring modification involves 24 x 510mm (20in) wide rings, the average length being about 14 rings. To minimise disruption of the services, all work has to be concluded during short track possession windows of 24h on Sundays with a bus service on the streets above providing substitute transportation.

Design and contract documentation was completed by Howard Humphreys in October 1990 and five prequalified contractors were invited to submit tenders. Of these, J Murphy & Sons won the contract, not on its contract price alone, but in combination with its recent experience and a highly flexible method, well suited to meet any contingency.

Murphy has established two independent works trains comprising two wagons each, provided by London Underground, one for each running tunnel. On these is carried a duplicate set of all vital equipment, including grout mixing and pumping stations, the pneumatic clay spades and torque wrenches, compressors, and diesel generators to provide their own electric power supply rather than relying on power supplies from London Underground. Station platforms are not needed for equipment set-up and London Underground staff are only needed to drive the trains and monitor the works for London Underground purposes. The trains, transported into and out of the two running tunnels from the White City depot at the start and finish of each possession period, also carry the new segments and packers, as well as provide room to take out the old segments and the excavated muck.

Work started in April 1991, with the first of eight consecutive 24h Sunday track possessions. These also included two public holiday Mondays in May, providing two long weekend possession periods of 48h in which to conclude the more difficult sections.

One such section is 1m in from the Oxford Circus Station west portal where eight of the 19 rings to be replaced were encased in concrete and accommodated a lateral opening to a ventilation shaft. In addition to the normal sequence, the concrete and segments had to be broken out, since the bolts could not be undone and it was only after the infill concrete had been removed that the original placing of the segments could be seen and the new ring placement devised. As it happened, the key in the rings to be replaced had been moved from the normal crown position to accommodate the ventilation opening. Fortunately, the segments and the packers of the new ring could be arranged to suit this situation and no time was lost designing and waiting for delivery of specifically designed cast iron segments.

Actual removal of the old cast iron segments revealed one other surprise. In the same section at Oxford Circus, the Victoria Line, built in the 1970s, runs directly overhead. The clearance between the two was expected to be at least 150mm. In fact, the Victoria Line tunnel is sitting directly on the Central Line tunnel. There is no room to re-excavate the crown area. “What we will have to do instead is to acquire the necessary clearance by trimming the flanges of the bolted segments by about 13mm,” explained David Garrett, Project Manager for Howard Humphreys. “This will be adequate since 13mm is the minimum increase in clearance required in this location. Raising the crown by 120mm elsewhere builds in a more than adequate safety margin,” he said.

The new segments are of the same grey cast iron material used in the original lining and were tendered by the London Underground project team in early 1991. Manufacture and supply was awarded to Ferry Capitan, the French foundry in Joinville.

Experienced miners removing old cast iron segments in the Central Line to excavate the clay behind and erect new larger diameter segments

Work in progress

Although only a very small part of the overall Central Line refurbishment project, this small job at about a total of £1 million is nonetheless vital. It requires well experienced tunnel miners who can work as efficient teams to ensure that the work for the allotted possession period is completed strictly to time and so avoid any costly or disruptive delay to the start of normal public services. Here Murphy also had a competitive edge. It is currently completing the traditional hand mining redevelopment of the Angel Station on the Northern Line and teams from this project are being employed on the Central Line job. Since they are experienced teams, there was no need to allow time for a learning curve nor to acclimatise the workers to the particular rigours of undertaking possession work on such a vital system as the London Underground. This has undoubtedly contributed to the smooth, and in many cases faster than expected, progress.

During a visit to the work sites over the public holiday in early May 1991, the teams were completing a replaced ring cycle in about 90 min, including taking down the old segments, excavating the clay behind and erecting the new segments with their 17 circle and 18 cross joint bolts. Replacement of the 510mm wide rings allows unsupported exposure of only two rings maximum. '”Thankfully, the ground is good quality dry and compact London Clay which will stand unsupported in the crown for up to 8h and more,” said Garrett.

At the end of the rebuilding cycle, about two and a half hours must be reserved in which to complete the grouting of the annulus and point the joints to complete the job ready to move to the next section to be modified or to leave the tunnel.

With current progress rates, and providing there are no delays, the current work will be completed as scheduled by the first weekend in June 1991. New trains will then begin to be introduced in 1992, running on the old operating system with the old rolling stock. Change over to the completely new operating system is expected by the end of 1994.

Refurbishment of the Central Line is designed for a life expectancy of at least 40 years. In recognition of early engineering, the principles of future mass transit development are expected to continue to be based on the steel wheel on rail system. “There are still major improvements that can be achieved in current technology, particularly in the signalling systems”, explained Humphrey, a civil engineer who worked for London Underground before taking up posts on the construction project management teams of the Hong Kong mass rapid transit system during the 1980s. “New transmission-based signalling technology will introduce time and speed elements into signal operations. It will dramatically reduce the headway between trains and increase capacity. But whatever developments the future may hold, the tunnels and the underground space occupied by systems all around the world will remain the most valuable capital asset.”