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3D Crossrail tunnel scan unwrapped into 2D for first time

A survey of Crossrail’s tunnels ahead of drilling required a new technique to analyse the data quickly and efficiently.

Laser scanning is a technology that’s been around for many years now and has become much more common on construction sites of all shapes and sizes.

But when the joint venture tasked with installing Crossrail’s railway systems needed a precise survey of its tunnels before it could drill holes, a new survey technique was required.

ATC, comprising Alstom, TSO and Costain, is currently working on the transformation of more than 42 km of tunnels into a live railway. A major part of this involves the drilling of around 250,000 holes into the tunnel walls to enable the installation of an emergency escape walkway, a cable management system, overhead power lines, and all the necessary signalling and lighting.

The JV is using an automated machine to drill the holes, developed by Swiss company Rowa, which meant that an as-built survey was required to ensure the holes were drilled in exactly the correct places.

It was crucial that the holes avoided the corners and ring joints of the concrete segments that make up the lining of the tunnel, so the ATC team needed a 3D CAD wireframe model and co-ordinate schedule to programme the machine properly.

Accurate data

“Scanning and surveying companies were invited to tender for the job, but because the drilling machine was new and no existing spatial data solution was in place, the challenges were myriad,” says Tom Wren, rail team operations manager at Plowman Craven, the surveying firm contracted by ATC to carry out the work.

Plowman Craven laser scanning Crossrail ATC 2

Plowman Craven laser scanning Crossrail ATC 2

The drilling machine will create around 250,000 holes

“When the tunnels are constructed the actual alignment of those [concrete] pieces isn’t recorded in any way – they’re put in place to fit where they need to.

“It was important we knew the co-ordinates of all of those so the drilling machine didn’t drill too close to the edges, and so ATC could do all of the design work of how the infrastructure fits in there, without doing it on site.

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Plowman Craven used a laser scanner to capture the necessary data, scanning around 500 m of the tunnel a day.

“Crossrail is a unique sort of construction site, as everyone wants access to these small tunnels at the same time, so being able to get in and out and leave the site clear for the actual construction work was very important,” Mr Wren says. “It was then a case of getting the data that the client needed from that.”

Tunnel slice

With 840,000 points to model over 42 km of tunnels, this was no simple task.

Ordinarily, the team would have had to work in 3D, extracting the co-ordinates manually in a time-consuming process. Instead, in what Mr Wren describes as “the innovative bit”, Plowman Craven unwrapped the 3D point cloud into a 2D surface.

Plowman Craven laser scanning Crossrail ATC Point cloud data with segments

Plowman Craven laser scanning Crossrail ATC Point cloud data with segments

Point cloud data showing the concrete segments

“Once we had that, it enabled the 2D CAD work to be undertaken very quickly, as the tunnels were on a standard template that could be copied around,” he says.

The firm worked closely with Atlas Computers to add a number of bespoke routines to their Survey Control Centre software that would enable the wireframe and co-ordinate schedule to be produced more efficiently.

“It was a reasonably simple mathematical calculation to transform the co-ordinates from one system to another – giving us our 2D point cloud”

Tom Wren, Plowman Craven

“We basically take slices through the point cloud every 1.6 m, and from those slices we can then work out where the centre of the tunnel is,” Mr Wren says. “That way we can work out the alignment of the centre of the tunnel and how it moves around in 3D space – a bit like a big snake in the ground.

“We can then use that alignment to unwrap the point cloud. It was a reasonably simple mathematical calculation to transform the co-ordinates from one system to another – giving us our 2D point cloud.”

Saving time

Mr Wren estimates that the 2D drafting process has been done in around a quarter of the time that it would have taken to work in 3D – no small saving.

This is believed to be a world first for a tunnel-scanning project – most similar jobs do not require this level of data, as they haven’t used the automated drilling machine that Crossrail has deployed. The machine has been drilling holes in the tunnel at a rate of around 300-400 m a day, meaning that the speed of Plowman Craven’s data analysis was especially important.

It has also had health and safety benefits: traditionally, the point cloud data would be used to physically mark the position of every hole in the tunnel lining before they were then manually drilled. But with the automated machine, workers are no longer in physical contact with the drilling tools, reducing the risk associated with hand arm vibration syndrome.

Plowman Craven laser scanning Crossrail ATC section points

Plowman Craven laser scanning Crossrail ATC section points

Visualisation of the section points

Plowman Craven is already eyeing up the solution for use on other upcoming jobs including the Thames Tideway Tunnel and High Speed 2, and has learned to explore new methods of working to achieve greater efficiencies.

“The main thing we’ve taken forward is to not do things as we’ve always done, but to take a step back and look at different techniques we can use to solve problems,” Mr Wren says.

“It’s been a very successful project for us, and it’s all because we’ve looked at things a bit differently and not done things in the same way.

“It will pay dividends if we take that approach in the future.”

Tech Sprint – book your place

This year’s Construction News Summit on 11-12 October plays host to its first ever Tech Sprint.

The Tech Sprint will take the form of a 24-hour hackathon seeking new ideas and solutions which could change the construction industry over the next five years.

These technology-based solutions should be centred on meeting the government’s Construction 2025 vision for reducing the cost of construction by 33 per cent.

Among the judges are Crossrail’s head of technical information Malcolm Taylor.

Visit the Summit website for more information and to book your place today.

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