The Elizabeth line (formerly Crossrail) which opened in 2022, provides a vital new rail service from Reading and Heathrow to the west of the London, before reaching Shenfield and Abbey Wood in the east. The line has increased central London’s rail capacity by 10% and links Heathrow Airport, the West End, the City of London, and Canary Wharf. 

In joint venture with Atkins, Arup designed the 42km of new tunnels under central London that were constructed in just over three years. Our work included the detailed design of the tunnels as well as an assessment of the impact that such significant tunnelling and excavation works could have on surrounding structures. The JV also designed the Tottenham Court Road, Custom House and Woolwich stations, and Plumstead sidings for Crossrail Ltd. 

Crossrail tunnelling map
Crossrail tunnelling map
Map of the tunnelling route for Crossrail. Click to zoom

Tunnelling

Our involvement in Crossrail extends from the early 1990’s, when we undertook the geotechnical site investigation for the entire route. After a lull in the project, from 2006 to 2009, we updated the proposal for the section between Royal Oak and Farringdon, where the existing National Rail services will enter the tunnels on the western section of the route. Working with Atkins, we achieved significant cost reductions by simplifying the underground connections and refining the scope of Paddington, Bond Street and Tottenham Court Road stations. 

Crossrail tunnel between Royal Oak and Farringdon
Learning from our work on HS1, we helped develop the tunnelling strategy, reducing the number of tunnel boring machines required. We avoided the need for an underground junction at Stepney Green by changing the boring direction along the route. By eliminating this junction, we helped save £20 million and reduced the health and safety risks for various construction teams needing to work in a confined space. 

Geotechnical

Creating 42km of giant bored tunnels underneath the busy streets of London, navigating through its congested subterranean world of building foundations, London Underground tunnels and many other structures and utilities, presented huge risks. When we began looking at the best alignment for the tunnels, it was a stark reminder of the questions that needed to be answered. What will we find underground? How will the tunnelling effect structures and people above ground? 

Early on, we identified the need for a comprehensive digital GIS platform to manage the vast amount of geotechnical, land and site data that would be involved. Using this data, we navigated major infrastructure and utilities below ground, identifying possible obstructions during tunnelling, while flagging buildings that were at greater risk above to refine the tunnel alignment. 

This then novel approach is now common practice and proved its value on this immensely complex scheme. Crossrail built on these foundations and developed the platform further into a BIM model of the entire line, becoming in effect, a ‘digital railway’. 

Arup's Alison Nourish speaks to BBC Radio 5

Alison Norrish discusses the challenges of delivering the Elizabeth line's tunnels. Discover this and more via the BBC Sounds App

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Tunnelling, noise and vibration

In its entirety, we undertook impact assessments on over 17,000 buildings, railways, major utilities, and other assets including listed buildings across the new line. Where the potential for movement had been identified, we designed the instrumentation and monitoring systems that ensured the construction and associated impact did not exceed expected levels, safeguarding critical infrastructure, businesses, homes and livelihoods throughout construction.

At Whitechapel, we proved that the 150m long tunnel boring machines could excavate large platform tunnels below a school and residential building without the need for destruction and expensive mitigation measures that could impact pupils, teachers and residents. For other buildings on the route, we designed measures including adjustments to tunnelling construction methods and the use of compensation grouting and underpinning to minimise ground movement.

We also designed various railway track-forms within the 7.1m diameter tunnels. The impact of noise and vibration from at some locations above the route had to be alleviated for the benefit of the local community. We designed special floating slabs – a specialised technique that ‘floats’ the entire track structure on springs to minimise the transmission of noise and vibration into the surrounding ground – a necessity for the tunnels passing under the Barbican estate and along the 3km section beneath Soho’s many recording studios.

Floating track preparations in a Crossrail tunnel
Floating track slab preparation below Soho © Crossrail Ltd
Floating track slab
Floating track slab beneath Soho prior to rail installation © Crossrail Ltd

Keeping the city moving

At some stations, the only option was for the tunnels to almost touch critical infrastructure and buildings. At Tottenham Court Road, we threaded the tunnels over the Northern line and below the escalators for the London Underground station. This created only 700mm between the Elizabeth line and Northern line tunnels.

We developed an approach to existing foundations so that any obstructions below ground could be removed safely and without impacting to the structures above. As a result of this work, 42km of tunnelling through London’s dense urban environment was completed without any unknown obstructions or damage to water, gas and electricity arteries, all whilst managing the impact on many different buildings along the route.