In training - Crossrail

Materials World magazine
8 May 2013
Connaught tunnel (August 2012)

As Crossrail, Europe’s largest civil engineering project, reaches the
halfway point of construction, Simon Pugh, Lead Engineer in the Route
Control Centre for Crossrail, talks to Michael Forrest about how it is
built and how it will be run.

The Crossrail project is truly large scale. It provides vital capacity for East–West links and accessibility into central London and core business districts, links with the London Underground, and improves connectivity of Great Western, Great Eastern and South East sections of Network Rail. The link is 118 kilometres long from west to east and will have a capacity of more than 200 million passenger journeys a year. The central section will run 24 trains an hour (average every 2.5 minutes) in each direction and for the most part will be underground. More project statistics include the construction of 42 kilometres of tunnels, 37 new stations including eight sub-surface, more than 60 lengthened platforms, the removal of eight million cubic metres of spoil and 140 main works contracts. The total infrastructure budget is £14.8 billion.

To accommodate the planned passenger capacity in the central section, the trains will be larger than those on today’s underground system, with each extending more than 200 metres and requiring tunnels with a diameter of six metres. In comparison, the Victoria line tube tunnels are 3.81 metres in diameter and the Channel Tunnel some 7.6 metres. Eight tunnel boring machines (TBMs) are constructing tunnels up to depths of 40 metres below surface. The majority of the geology of the central section is London Clay, a stiff blue-brown clay that underlies most of central London. Below this is the Lambeth Group – up to 20 metres of sands and clays, the Thanet formation – up to 16 metres of fine sand, and the underlying chalk. In the west of the central section, London Clay is dominant from the portal at Royal Oak through Bond Street, Tottenham Court Road and Southampton Row (Fisher St), beyond which an anticline brings the Lambeth Group nearer the surface and where the tunnels will follow the approximate boundary between the London Clay and this Group. Further east, the Millwall anticline brings the Thanet Sands of Palaeocene age (54.8–57 million years old) near to the surface, while at the Victoria Dock portal the tunnels are back into London Clay. On the south side of the Thames, the North Woolwich portal takes the tunnels into the chalk to Plumstead and into the Thanet Sands at Woolwich Station.

When tunneling in an urban environment, the prime consideration is to ensure zero ground settlement, which can affect the structure of the buildings above. Tunnel boring machine (TBM) development has led to shielded machines that can extract and transport spoil at the same rate as tunnel advancement – called earth pressure balance machines. Six of these will be used in the central section, while two machines used within the chalk will be slurry shield TBMs to cope with soft ground and high water flows in the section between Plumstead and North Woolwich. The method of lining the tunnels is partly determined by the geology, resulting in a mix of sprayed concrete lining and bolted pre-cast segments, the latter being cast at tunnel portals with the first plant built at Old Oak Common near Acton in west London.

The tunnels are not the only underground constructions of the project. Bigger stations are required to accommodate the larger trains that will run on Crossrail, and to meet the projected capacity a fleet of 65 new trains is required, each with a capacity of 1,500 passengers. Power for the trains will be via 25kV overhead cables and will support faster accelerating and braking. A braking energy recovery system will be inbuilt that will reduce heat generation in the tunnels. With increased passenger loads over current underground trains, the design engineering has to maximise the passenger flow in and out of the carriages.

The construction of larger stations has been an equal challenge. In the central section, the historical legacy of London’s sewer system, water and electricity supply, large building foundations and the Tube lines have created difficult civil engineering environments for Crossrail. This is compounded by the established interconnects with existing underground and surface stations that are often in crowded parts of the city. An example of the challenges can be seen at Tottenham Court Road where the existing station, built as two separate tube platforms more than 100 years ago now hosts 150,000 passengers’ journeys every day. Crossrail will add to this with a new station with 260-metre underground platforms at 25 metres depth, a new eastern ticket hall on Dean Street, escalators to the existing Northern line platforms and a new ventilation shaft. The ‘box’ containing the new Crossrail station and interchange facilities all have to be built on a much expanded footprint confined by Centre Point and Charing Cross road. On site are a huge amount of facilities buried just below the surface, making the job extraordinarily difficult and requiring diversions at street level in and around Charing Cross Road and Oxford Street. The ‘box’ is constructed of reinforced concrete base and perimeter walls.

Simon Pugh, Lead Engineer, Route Control Centre (RCC), explains, ‘The role of RCC is to control the trains and infrastructure of the central section and also to ensure seamless integration of Crossrail services with the Great Western, Great Eastern and South East parts of Network Rail, and the London Underground network.’ This integration of new and existing systems is an enormous challenge. All facets of service operation are governed through a suite of Crossrail-specific Operational Concepts, which direct operations, safety and maintainability. Moreover, these ultimately determine how the performance, capacity, the command and control, and the operational interface with other infrastructure managers are provided. One of the major challenges within the project is combining existing network services with the high-frequency mass transit systems that Crossrail will provide. This includes the traffic management challenges of delivering a railway that will have to interface to existing timetabled, governed routes that at the same time ensure, where possible, even headways through the central operating section.

In late 2012, Crossrail let a contract to a consortium comprising Siemens and Invensys Rail to install a communications-based train control (CBTC) signalling system for the central operating section, which controls the section linking Portobello Road in the west, Pudding Mill Lane in the east and Abbey Wood in the south east. This system will allow the control of trains that will take into account the possible delays and gaps between trains by automatically adjusting their speed through a series of interlocking control systems, fixed position and on-board train position reporting, and movement authorities for each train, determined from central controllers that have interlocking interfaces. The driver has in-cab signalling equipment and displays, and features such as automatic train protection, automatic train operation and automatic train regulation will be provided, including trackside reporting of train position. As the number of trains per hour in the central section will be far greater than those on the Great Western and Great Eastern Network Rail sections, it is planned that there will be an automatic train reversal system of up to 14 trains per hour beyond Paddington at Westbourne Park.

Three control centres (Great Western, Central and Great Eastern) will manage Crossrail services and integrate with London Underground and Network Rail. Pugh says, ‘We have already made a model for control centres that will allow functions of supervisory management, signaling and regulation, engineering management (of fixed equipment, such as power and ventilation), train management and customer services control.’ While the new RCC control centres provide new integrated control and operational management, Pugh adds, ‘It is important that the centres do not risk unwarranted distraction between safety and support roles, and that the need for a backup control is also addressed.’ Pugh emphasises that integrating Crossrail is a complex problem, ‘but when the central operating section is operational in 2018, it will provide 10% additional capacity for east–west travel in central London, offering vital relief for the Central, District and Circle, Piccadilly, and Waterloo & City lines’.