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Tomorrow’s tunnels

Atkins | 03 Sep 2015 | Comments

Crossrail aims to set a new standard for tunnelling projects.

Crossrail is aiming to set new standards for the delivery of infrastructure. The mammoth task of excavating 42km of new railway tunnels beneath the UK capital is now complete. The first trains are expected to run through these tunnels in 2018.

According to Rob McCrae, Atkins’ framework director for Crossrail, success starts at the drawing board: “One of the decisive factors is that we had adequate time to get the designs right – and relatively few changes have been made since construction started.”

Atkins, working in a partnership with Arup, produced the detailed design of Crossrail’s twin-bore tunnels, the latest in its long history of work on a multitude of tunnel and underground works projects around the globe. The two firms are also working on the engineering and design of the new stations at Tottenham Court Road and Woolwich. In addition, Atkins is responsible for the architectural component contract for all the stations and associated structures.

Crossrail’s delivery has led the Commons Public Accounts Committee to describe the £15 billion scheme as “a textbook example of how to get things right.”

The experience of Crossrail has led to increased interest in tunnelling expertise. Earlier this year, the Government announced funding worth £1.1 million to develop tunnelling skills, with the construction industry providing an additional £1.7 million. The money will be used to create apprenticeships and train the next generation of tunnelling workers. Crossrail itself established the Tunnelling and Underground Construction Academy, a specialist training centre for the UK construction industry, to ensure the project had the right skills to undertake the project.

Getting it right

What has Crossrail learnt so far – and what lessons are there for other major projects such as High Speed 2 and the Thames Tideway super sewer?

One way to ensure projects run efficiently is to minimise and control the number of changes made during construction.

“Change can drive things over budget, so from the outset Crossrail focused on getting the design right and getting the programme right,” says McCrae. “It helps to have a mature client: Crossrail Ltd is run by experienced people who have done a lot of these projects before.”

Keeping local stakeholders on-side throughout the project also helped to ensure trouble-free delivery. Digging tunnels and shafts can cause ground movement if not controlled, so actively monitoring and compensating for any movement as well as ensuring that local residents and businesses were kept in the loop was vital.

“Explaining what would happen at an early stage and demonstrating what you are doing to minimise the effect makes a big difference,” emphasises McCrae.

Crossrail is one of the most complex infrastructure projects ever carried out in London, with more than 40 construction sites spread across its route. A project of this scale must deal with thousands of structural interfaces: existing pipe and cable networks have to be taken into account, along with the often highly complex relationships between tunnels and stations.

Smart technology is helping to avoid potential snarl-ups and ensure that work progresses smoothly. For example, BIM – Building Information Modelling – is a collaborative three-dimensional design tool that takes every aspect of a design or project into account.

“Traditionally, engineers worked in 2D and this meant it could be quite difficult to spot clashes in complicated structures,” explains McCrae. “BIM helped tremendously, particularly on the stations. Better visualisation of the project at an early stage means there’s less need to change designs once construction begins. Crossrail was one of the first major infrastructure projects where BIM was used effectively in the UK.”

Atkins will be applying the same successful approach to Britain’s new high-speed north-south rail link, having been appointed by HS2 Ltd to provide BIM support on the project, covering the planning and delivery of a range of activities including BIM educational and assessment tools for the supply chain.

Better machinery and materials are also helping to transform urban tunnelling. “Slurry shield” tunnel boring machines, for example, have made it possible to drive Crossrail’s tunnels through highly permeable rocks such as chalk, and weak, leaky strata such as Thanet Sands.

Mastering these geological challenges matters because most of the world’s major cities have grown up near water. Coasts and rivers bring life to settlements, but the weak and wet geology that underlies many cities has often proved a hindrance to tunnellers. With modern tunnelling equipment, though, geological formations that were once off-limits can now be economically and safely tunnelled.

Jessica Daughtry, principal tunnel engineer with Atkins, worked with the team that built the new tunnel between Plumstead and North Woolwich – the only point where the Crossrail route crosses the River Thames. This section of tunnel traverses sands, gravels and chalk.

“As soon as you see the chalk, you see water,” says Daughtry. “With the slurry tunnel-boring machine, though, you wouldn’t know you were in wet ground. Tunnelling of this sort is a challenge, but not if you have the right equipment.”

Advances in materials technology are also helping to reduce the cost and risk of tunnel construction. Tunnel linings are a case in point. Instead of using traditional bar reinforcement, the pre-cast concrete segments used in Crossrail’s tunnels incorporate fibre reinforcements – typically, high-tensile steel strips about the length of a paperclip. Using fibre reinforcement simplifies the manufacturing process and allows engineers a greater level of control over the concrete’s properties: adding polymer fibres, for example, improves fire resistance.

“Fibre reinforced concrete has a number of technical advantages and it also results in a reduction in costs,” says McCrae. “The way the lining for Crossrail was designed is going to benchmark what is used on HS2 and on the Thames Tideway.”

Bridging the gap

Turning designs into real tunnels and stations depends on a close working relationship between designers, contractors and the client. Traditionally, these relationships have been arm’s length at best. But that’s changing, with an emphasis on collaboration and partnerships to boost efficiency. Co-location is one way of achieving this.
“We share an office with the client and the contractor – I’m a big fan of that, because you can talk to each other directly, you develop better relationships and minimise the risk of miscommunication. The whole thing moves forward a lot more smoothly,” says Daughtry.

The role of designers is also changing. In the past, designers focused on drawings rather than delivery. Getting a design engineer back on site to sort out a problem could take days. On Crossrail, though, the designers are part of the delivery team and an engineer can be down in the tunnel sorting out problems in minutes. This is vital because of the often complex nature of the relationship between temporary and permanent works, and interactions between different contracts such as stations and tunnels.

“There’s a much more active role for designers continuing through into construction,” says Harriet Rutledge, principal engineer with Atkins. “On Crossrail, we were involved in the regular daily shift meetings. This meant we were able to monitor performance and also to check that contractors were aware of any specific design aspects that they may not have picked up from our drawings. We’re part of the delivery team rather than just doing the designs and going away.”

Every major project reveals new insights and a deeper understanding of what works and what doesn’t. Crossrail is no exception. Ensuring that best practice is systematically captured and shared is a vital part of the process. Daughtry and Rutledge are among the Atkins engineers who have written up their findings to share with other designers.

Rutledge’s paper, co-authored with the contractor, considers the interactions between close-proximity excavation works and how they can best be monitored: in this case, the simultaneous construction of new tunnels and the excavation of a new station directly above them at Paddington. Establishing that tasks of this sort can be carried out in parallel – and in safety – is important because it eliminates the need for a potentially long and expensive wait by contractors.

“As well as a safety case, it provided a lot of data you can take forward to another project to better understand how the ground and the tunnels will behave in similar situations,” says Rutledge.

Daughtry, who wrote a paper on the fire design aspects of segmental linings, agrees that the systematic recording and sharing of such findings has real value: “Fire design requirements, for example, are becoming more stringent in many of the markets in which we operate. Sharing our findings helps Atkins’ engineers from around the world in their own projects.”

From developments in tunnelling techniques to improvements in knowledge sharing, Crossrail has helped to drive a step-change in the way tunnelled infrastructure is designed and delivered. With the UK planning a series of major tunnelling projects over the next decade – including Thames Tideway, High Speed 2, new electricity cable tunnels and proposals for Crossrail 2 – the lessons learned beneath the streets of London look set to be put to good use.

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