Cofferdams, TBMs and a Roman horseshoe

Posted by Derek Mason

5th July 2022

Photo credit: Joshua Ng on Unsplash

Many major infrastructure projects are plagued by stories of overspending and delay. The Elizabeth Line has been no different. The project that began as “Crossrail” was delivered three and a half years late and £4 billion overbudget, costing £18.9 billion.

Despite this, the project has certainly done what it set out to do, and the excitement generated was made clear as people queued overnight to be the first to take a journey on 24th May this year. By 10am, there had been 130,000 passenger journeys on the new line.

Although only partially open, we now have a line stretching from Abbey Wood and Shenfield to Heathrow and Reading, that will increase central London’s transport capacity by 10 per cent. That’s the biggest single increase in more than 70 years, and it puts an extra 1.5 million people within 45 minutes of London.

This is an impressive achievement that will have a real impact on quality of life for commuters, but what I find most fascinating is the structural engineering behind the project. It’s no easy undertaking to create new tunnels and underground train stations in a busy capital city. Therefore, I thought it would be interesting to look at some of the feats of engineering and the construction challenges of the project.

Firstly, and perhaps most obviously, there’s the tunnelling. Construction workers reconstructed, enlarged and refurbished a Victorian tunnel – the Connaught Tunnel – to an immovable deadline. Cofferdams were used at the Royal Docks, and the tunnel was completed, and the cofferdams removed in time for boats to access the ExCeL Centre for the defence and security exhibition in 2013.

For the new tunnels required for the line, eight Herrenknecht tunnel boring machines (TBMs) were brought in. Six of the TBMs were earth pressure balance machines, cutting through 42km of clay, sand and gravel beneath London. Two mixed-shield slurry machines, nicknamed Mary and Sophia, tunnelled through wet chalk and flint below the Thames.

Next, the tunnels needed to be lined. There are now 220,000 concrete segments lining the tunnels. I think that helps to give some idea of the scale of the work – it has been a huge undertaking.

After the tunnelling, the track was laid, with a concreting train pouring over 13,500m3 of concrete – enough to fill several Olympic-sized swimming pools. The statistics for the track itself give an even clearer idea of scale, with 63,000 sleepers, 51,419m of rail and over 800 sections of Long Welded Rail (LWR).

Then there are the train stations themselves. One immense challenge was the construction of Liverpool Street station for the Elizabeth Line. It’s the deepest of the new stations, with platforms at 34m below ground level, 15 escalators and seven lifts.

The ground below London in this area is a maze of sewers, pre-existing Tube tunnels and the Post Office railway. Plus, it was necessary to excavate the Bedlam burial site to create the new station’s eastern ticket hall. During the work, archaeologists uncovered more than 3,300 skeletons of people who’d been buried at the infamous psychiatric hospital, as well as Roman artefacts including horseshoes and cremation urns.

Understandably there were many challenges with this part of the project, leading to tunnelling delays, but Laing O’Rourke clawed this time back by using a design for manufacture and assembly (DfMA) approach. One example is the angled ceilings in the ticket halls, which were formed from pre-cast panels and manufactured off site. This approach meant the team could complete 17 months of activity in 11 months, bringing this part of the project back on schedule.

The examples I’ve mentioned are just a snapshot of some of the structural and logistical challenges that have been met by the contractors. Whenever you work below ground, budgets and timescales can change, as there are always unknowns.

It is not on the same scale but, when we work on a basement project, we will arrange a soil investigation report, so we have an idea of what we’re working with. The contractors will have done this with boreholes along the route of the Elizabeth Line, but there could easily be a change 5m from the borehole so you can never be completely sure of what you’re working with.

Getting a project of this size over the line is an impressive achievement, and it will be interesting to see if the contractors hit their autumn 2022 milestone. This is when passengers from Reading and Heathrow should be able to use the full Elizabeth Line service through central London, without having to change at Paddington or Liverpool Street. Fingers crossed they can keep the project “on track”.

Meanwhile, if you need assistance with the structural elements of an upcoming project – underground or otherwise – please do get in touch.

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