CIVILS, TUNNELS & EMBANKMENTS
Sensors to determine embankment stability
The University of Southampton is working with Network Rail on a project to develop low-cost sensors for use in embankments to take soil readings that can predict earthwork failures. RTM heard more from lecturer in geotechnical engineering Dr Joel Smethurst.
M
any of the earthworks our rail network depends on were constructed long ago
with sometimes shoddy methods, and many have aged badly. Weather – when it’s excessively wet or dry – can cause embankments and slopes to fail, as we’ve seen plenty of times during the past wet winter.
Network Rail would like more early warning of problematic embankments, and is funding University of Southampton researchers to develop sensors that can be pushed into the soil, measure factors like water content, and ideally alert the right people if certain thresholds are triggered so remedial works can be prioritised.
The sensors themselves have been developed as part of another project – FUSE, the Floodplain Underground Sensor Network, funded by the Natural Environmental Research Council.
But the technology is extendable to take other kinds of readings, and Dr John Atkinson is involved with both projects.
His colleague Dr Joel Smethurst has been involved with the embankment stability work and told RTM: “It’s a fairly new technology, but we thought it would be a good idea if we could also try to apply it to other problems, including the problem that Network Rail has with some of its earthworks.
“It’s when embankments get very wet or very dry that there tends to be a particular problem for Network Rail. Dry conditions normally causes the clay earthworks, as commonly found here in the south-east of England, to shrink, and you get a very uneven track level and poor track quality, requiring speed restrictions.
“When the soil gets very wet, in terms of failing the embankment, it’s the water pressure that matters. That pressure, essentially, gives the soil a buoyancy, and causes it to become lighter, and that’s when it fails.
“We want to use the sensors to measure the water content of the soil, which can act as an indicator that there is a higher risk of failure, as a proxy for water pressure.”
Traditional methods of measuring water pressure involve drilling deep holes, which is costly and time-consuming. The sensors, by contrast, could be much closer to the surface and remain there.
The sensors themselves are “not widely used at present” but have a lot of potential. Dr Smethurst described them by saying: “Different metals and other substrates are printed onto a small, thin glass plate, and you layer different materials onto the plate to create the sensor. It’s a bit like a printed circuit board, in some respects. The advantage is that you can ‘screenprint’ the various layers on, so you have a series of ‘templates’ – and you then print the thin fi lm layer of a particular pattern onto the plate. It’s pretty cheap, and you can print quite a large number of different sensors onto the same plate.”
Each sensor, about 10cm long and 5cm wide, would be connected to a wireless data logger.
“You could plant a number of these sensors across an earthwork or a fl ood plain, and the sensors would relay the information. The intention is to get the cost of each sensor down, so you can install quite large numbers of them at a relatively high distribution.”
In lab experiments, they’ve been placed into cylindrical columns of soil that water and chemicals are pushed through, to test how the sensor responds to changes. For the FUSE project, that has included things like the pH value of the soil, but for embankment stability it’s all about water content.
Dr Smethurst said: “At this stage, it’s about understanding how the sensor functions and
performs in practice. It would be desirable, once we’ve fi nished the work in the lab, to have a trial site fairly soon and have a go at installing some and see how that goes.
“There are issues about how they respond and how moisture content acts as a suitable proxy for measuring pressure. We’re thinking about how we might try to develop a similar fi lm- printed sensor to measure pressure [directly], but it’s not easy.”
He envisions an alert system based on threshold changes within water content measured by a group of sensors, which would then fl ag a higher risk of slope failure or shrinkage problems.
Network Rail can then take appropriate action, whether by re-grading the slope angle, or installing better drainage, for example. This often entails a trench drain from the top to the bottom, fi lled with a more free-draining material than the clay, which tends to be the problem substance where earthworks are concerned.
The size of the sensor also depends on the material. The moisture content of coarse- grained soils can be diffi cult to measure using a small sensor, whereas for fi ne-grained clay soils, the sensor can be “reasonably small”, he said.
Dr Smethurst said: “Many embankments were never built very well in the fi rst place, and they’ve progressively aged.”
More extremes of weather as the climate changes will cause a “signifi cant problem” and more embankment failures, he added.
FOR MORE INFORMATION
W:
www.tinyurl.com/RTM-embankments/ research/news/the-making-of-a-smart-tunnel
rail technology magazine Jun/Jul 14 | 59
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