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Monitoring Bored Tunnels on HS2 Using the Osprey Measurement Systems In-Place Extensometer

As part of Mega-Project, HS2, linking London and the Midlands with High Speed Rail, a 16km twin bored tunnel is being bored through the Buckinghamshire Chilterns to the North-West of London. The initial stage of the tunnel involved passing beneath a stretch of the M25 – the UK’s busiest motorway. Osprey partner, SOCOTEC UK, were responsible for monitoring any surface and subsurface deformation along the tunnel alignment, with emphasis on the M25 crossing.

HS2 C1 Southern Portal

Osprey Measurement Systems In-Place Extensometers (IPX) were installed on the approach, within the carriageway and beyond the motorway to monitor tunneling impacts on the chalk above and beside the twin tunnels.

The IPX was selected in part thanks to its capacity for multiple measurement points within a single borehole – up to 12 on this section and as many as 44 in an installation elsewhere on this project. This significantly reduced installation time on the carriageway, reducing cost, road delays and risk to site operatives.

Top support for subsidence applications
Simplified IPX configuration. Between 12 and 44 sensors were installed within each borehole.

The devices were monitored via small, battery powered radio data loggers and the low-profile design was well suited to installation within carriageway protective covers, making this the ideal solution for our project.

Laser patch scans of the road surface above the extensometers were used to bring external control to the subsurface monitoring points, providing absolute position of the targets installed beneath the carriageway.

Use of the In-Place Extensometer allowed high frequency, high density (sensors every 1m), sub-millimetric precision measurement of subsidence above the tunnel alignment. Get in touch if you would like to learn more about how Osprey Measurement Systems can help characterise ground behaviour on your tunneling project.

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