Use of fibre optics is now coming into its own with more than 10 years of data to draw on. Claire Smith looks at how this could lead to true performance-based designs from pile testing.
Validation of design using pile testing is nothing new but the technology for recording results has evolved with the arrival of smart sensors and is helping to produce economies in design and reducing risk. But the old adage that a little bit of information can be dangerous is true in this situation and those in the know are urging the industry to proceed with care.
While the fibre optics themselves have become relatively cheap and the installation processes more routine, the challenge in interpreting the data still remains something that is not straightforward. Nonetheless, recent results suggest that the data they provide could be the key to pile testing giving results that enable performance-based design.
The possibility has come as a result of recent use of fibre optics in addition to conventional strain gauges by Cementation Skanska for pile testing on three major projects that are using different piling techniques in different ground conditions.
Cementation has been working on fibre optic smart sensors for over 10 years and has been collaborating with Cambridge University’s Centre for Smart Infrastructure Construction (CSIC) since before it gained its current name. Cementation believes that this wealth of knowledge puts it in good stead to deliv er commercial benefits from the use of fibre optics.
“Fibre optic sensors were not specified in any of the projects but we chose to install them at our own cost as we could see benefits in the information we could gather using them,” says Cementation chief engineer Andy Bell.
Fibre optics offer the ability to measure strain every 100mm along the length of the pile, whereas conventional strain gauges only provide data for the position they are located at, so often pile testing only results in three or four discrete sets of data.
At the Croydon site the consultant involved wanted strain gauges installed to the toe of the CFP piles but Cementation geotechnical engineer Echo Ouyang says that was not feasible. “Without taking the cage to the base of the pile it was impossible to position the gauge at the toe and plunging a cage to that depth could not be guaranteed,” she says. “Instead we developed a single bar that would allow us to plunge the fibre optics much deeper than the cage.”
According to Ouyang, the approach managed to get the fibre optics close to the base but the concrete hardened quickly which presented challenges for installing the conventional gauges. “The analysis of the results at Croydon didn’t result in the length of the piles being shortened but it validated our design,” explains Bell. “The additional data provided by fibre optics allows us to be more informed and better assess the risks. Normally if the settlement observed is minimal then a decision might be made to shorten the piles but using conventional strain gauges there isn’t enough data along the length of the pile to be sure about the effect on pile performance.”
At the Manchester site, Cementation had concerns over the potential to damage strain gauges during installation due to the size of the cage being used for the bored piles. “The number of strain gauges specified meant that there wasn’t much redundancy,” says Bell. “Using fibre optics meant that we would have continuous data even if the conventional gauges failed.”
The piles at Manchester are relatively short and rock socketed into the Sherwood Sandstone. “Using the fibre optics gave a clear insight into the rock socket performance,” says Bell. “The pile performaned well so using standard data there could be a temptation to shorten the rock socket but the fibre optics showed that the bottom metre of the rock socket was key to pile performance as the upper 2m of the socket were in more weathered rock. The data gives us the chance to properly analyse the effect of shortening or reducing the diameter of the piles. This is true performance-based design.”
“Industry needs to work with academia to simplify the data analysis. It’s not a case of dumbing it down but making it easier.”
Andy Bell, Cementation Skanska chief engineer
The system has also been used on a site in the City of London and analysis on the data is still underway.
The benefits in terms of reducing risk seem obvious but what are the barriers to wider use of fibre optics for pile testing?
According to Ouyang, cost is not one of them. “The cost of fibre optics has come down significantly as the technology is driven by the telecoms industry,” she explains. “Basic fibre optic cables can now cost as little as 20 or 30 pence a metre and the more specialist ones only slightly more, but they are still more cost effective than a conventional strain gauge which costs around £100.
“The machines used to splice the cable onto the cages are relatively cheap too and make work on site safer as with conventional strain gauges the site team have to put their hands inside the cages to fix them.
“The expensive part is the analyser itself. There are only three in the UK currently and these are based on systems used by the oil industry rather than being bespoke to geotechnics, but CSIC is working on developing its own. If they were cheaper then it could be possible for an analyser to be positioned permanently on site to give continuous data in the future.”
However, Ouyang believes that there is another significant hurdle to enable wider use of fibre optics – the analysis. “Interpretation is where the smart bit is,” she explains. “You need to work hard to get meaningful data out of the results.”
Bell adds: “Industry needs to work with academia to simplify the data analysis. It’s not a case of dumbing it down but making it easier.
“The benefits could be huge but there is a risk that if the technology is misused, not analysed well and commoditised, resulting in failures then acceptance by clients could be impaired.”
Bell and Ouyang believe that standards and guidance, rigorous training and interpretation software will be key to fibre optic sensors benefitting not just the pile testing sector, but ground engineering in general.
According to Bell, the benefits are not far out of reach as he expects the challenges to be overcome in the next five years.