Ground movements monitored on HS2 site with fibre optic instrumented geogrid

A system for detecting subsurface ground movements, combining geosynthetics and fibre optic sensing cables, is in use on a High Speed 2 cutting.

Early ground movement detection to help prevent damage caused by geohazards has become increasingly important as climate change causes more extreme weather events.

However, conventional sensing technology capable of monitoring large areas with sufficiently large spatial and temporal resolution to provide an early warning of subsurface ground movement is limited.

A new fibre optic instrumented geogrid, known as Sensorgrid, could offer real time monitoring of movements within geotechnical assets.

The solution was developed by Cambridge Centre for Smart Infrastructure & Construction (CSIC), CSIC spinoff and fibre optic sensing specialist Epsimon, and geogrid manufacturer Huesker.

Sensorgrid has already been trialled and installed on a part of the High Speed 2 (HS2) scheme.

It has been implemented at the Tilehouse Lane cutting, a 710m long and up to 13m deep excavation in chalk along the HS2 route. It connects the Colne Valley Viaduct with the south portal of the Chiltern tunnels, close to the M25 motorway.

The main contractor for this section is Align JV, comprising Bouygues Travaux Publics, Sir Robert McAlpine and Volker Fitzpatrick.

During ground investigation and excavation work, dissolution features were revealed at the cutting site.

As part of the design solution, a geogrid reinforced mattress is being installed at the base of the cutting to mitigate against potential voids under the track slab.

It also became clear that being able to detect movement beneath the mattress would be crucial during construction when the site would operate as a haul road, and during the operation of the rail line itself.

Distributed fibre optic sensing (DFOS) is nothing new for the industry and has previously been used to measure strain along fibre optic cables covering many kilometres on various types of infrastructure. However, ground movement measurements rely on the mechanical coupling between these cables and the soil, which cannot always be guaranteed.

CSIC operations manager Cedric Kechavarzi says: “Our field of work is around fibre optic sensing and for this we use small cables a few millimetres in diameter. Of course, if you put them in concrete, they will bond with it, and if the concrete expands or contracts, the cable will strain accordingly. But in soils the cable is much more likely to slip through.

“This will depend on soil type, water content and overburden load, but when the cable slips, only partial ground movement and strain will be transferred to the cable.”

To overcome this challenge, CSIC, Epsimon and Huesker developed a solution in which the fibre optic strain sensing cables are integrated in a geogrid during production.

“Geogrids are designed to have a really good interaction with the soil and hold the soil in place. In collaboration with Huesker, a German manufacturer of geogrids, trials were carried out to incorporate various fibre optic cables into geogrids during manufacture," Kechavarzi explains.

“The geogrids are made of yarns stitched together to form a mesh, and we were thus able to simply replace given yarns with fibre optic cables of similar stiffness.

“By doing so, we developed strain sensing geogrids of various strengths for which the measurement density can be adjusted during manufacture by changing the number of yarns replaced by sensing cables."

CSIC research associate Xiaomin Xu adds: “In the past, we tried to manually glue the cable onto extruded geogrids, but that is not scalable. There have also been trials involving putting cables into nonwoven geotextile, which would not provide the same level of bonding as stitching.

“Incorporating cables during the knitting process followed by polymeric coating of the Sensorgrid ensures exceptional strain transfer and sensing performances.”

Sensorgrid went through initial small-scale trials in 2021

Since the initial discussions with Huesker in 2017, several prototypes have been manufactured and extensively tested.

“The strength of the bond between the various cables and geogrid types had to be tested to validate the concept and optimise the configuration,” Xu notes.

Kechavarzi says: “We did extensive tests in the lab, tensile tests in particular, but also vertical load tests, to investigate strain transfer characteristics and sensitivity of these prototypes.

“CSIC, Epsimon and Huesker then partnered with Jacobs and Align JV to carry out a small scale field trial funded by HS2 Ltd, prior to the full installation at Tilehouse Lane cutting.”

As part of this trial, near the Chiltern south portal, sections of Sensorgrid were sandwiched between earth above and heavy duty water-filled bags below.

“We had two 3m by 3m pits with water bags that were deflated to simulate a sinkhole, which created a void under the soil resulting in subsidence and stretching of the geogrid. This displacement was measured with the fibre optic cable and other instrumentation,” Xu says.

This small-scale trial, which took place in 2021, demonstrated the sensitivity of the Sensorgrid system to small vertical settlement as well as its robustness.

Subsequently, CSIC, Epsimon, Huesker, Jacobs, Align JV and HS2 Ltd worked together to design, install and operate the monitoring system at Tilehouse Lane cutting.

The system was deployed in May 2022. The Sensorgrid, with sensing cables spaced every 500mm, was installed over a 100m by 10m area in the chalk cutting, providing 10,000 sensing points and enabling the localisation of movement with a spatial resolution of 250mm.

“The Sensorgird came in a 5m roll that was uncoiled into two parallel lines. The 20 sensing cable ends were fusion spliced together to form two fibre optic circuits, which were then routed with a fibre optic extension cable a kilometre away to a compound where we could have the monitoring equipment in a safe place within the right environment,” Kechavarzi adds.

Measurements are taken every 20 minutes and the resulting data is displayed on an Epsimon-designed online dashboard in real time, allowing the project partners to monitor any movement that occurs beneath the base of the cutting.

This deployment has showcased the potential of spatially distributed subsurface ground movement monitoring over a large area in real time.

The lessons from this project have led to Sensorgrid being developed into a commercial offering by Huesker, and it is being supplied by Epsimon for projects in the UK.

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