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  • Writer's pictureSafer Highways

How HS2 monitored the M25 while tunnelling beneath it

Tunnelling beneath Britain's busiest motorway was always going to be a major construction milestone for the country's most expensive railway project.

So ensuring that the work beneath the M25 went off without a hitch was integral for project momentum.

Last week, HS2 celebrated the completion of construction on the first mile of the northward Chiltern tunnelby its tunnel boring machine (TBM) Florence. This segment of what will become the C1 route included going beneath the M25 at one of its busiest points, between junctions 16 and 17.

Working on behalf of Align – a joint venture between Bouygues Travaux Publics SAS, Sir Robert McAlpine and VolkerFitzpatrick – SOCOTEC Monitoring was tasked with supplying and installing instrumentation along the C1 route to monitor ground movement as Florence got to work.

This was particularly useful for the segment under the busy motorway, especially as a second TBM called Cecilia started shortly after Florence, boring the southward tunnel under the M25.

To support the monitoring of ground movement under the motorway, SOCOTEC used monitoring prisms, wireless tilt sensors and boreholes containing automated extensometers (IPX). It also used automated patch scanning to monitor the movement of highways assets before, during and after the TBMs went to work under the M25.

With these tools in place, SOCOTEC Monitoring was able to record and specify the exact location of the TBMs. With the organisation’s online data visualisation system Calyx OMS, hourly monitoring data at every stage of the route was made available. The team watched over the tunnel portals – approaches and escape/ventilation shafts along the tunnel passage – to ensure properties along the route were protected, as well as maintaining safety on site and structural integrity.

These monitoring methods will be used along the entirety of the 21km route across the Colne Valley and through the Chiltern Hills, ensuring the protection of key assets while allowing the tunnelling team to safely proceed with drilling operations.

SOCOTEC Monitoring UK managing director Jon Scott said: “We are delighted that the tunnel monitoring for the HS2 C1 Main Works Package has been so successful.

"Thanks to our level of expertise in terms of asset management and our implementation of state-of-the-art technologies, we are proud to have delivered the project to the best of our ability, and look forward to continuing to continuing to contribute to the HS2 C1 Main Works as the project develops.”

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