Field validation of fibre Bragg grating sensors for measuring strain on driven steel piles

Abstract

In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

Distributed fibre-optic monitoring of an Osterberg-cell pile test in London

“…Recent advances have made use of the interaction of laser pulses in fibre optics (FOs), through techniques such as fibre Bragg gratings (Kersey & Morey, 1993;Lee et al, 2004;Liu & Zhang, 2012;Doherty et al, 2015) and distributed Brillouin optical time-domain reflectometry (BOTDR) (Cheung et al, 2010;Mohamad et al, 2011;Hauswirth et al, 2014;Klar et al, 2014;Kechavarzi et al, 2016;Acikgoz et al, 2017) to obtain larger datasets. The latter, due to their continuous nature, has a promising advantage over conventional, discrete instrumentation in providing detailed information about the pile performance (Klar et al, 2006;Ouyang et al, 2015).…”

PISA: New Design Methods for Offshore Wind Turbine Monopiles

“…The background to this approach is discussed in Doherty et al (2015). The excellent resolution, stability and sensitivity of these instruments allow for the depth-wise variation of the embedded pile response to be accurately captured, and either integrated or differentiated obtaining other relevant quantities.…”

PISA: Recent Developments in Offshore Wind Turbine Monopile Design

“…Had the p-y method been used for test design then it is likely that the field testing program would have failed, as the loading system would have been under-designed. Of interest, the PISA Project is the first known instance of successful instrumentation of impact driven piles with bonded fibre optic strain gauges [7] this provided high resolution measurement of strain through both monotonic and cyclic testing. It is probable that this type of instrumentation will find further use for field monitoring of full scale offshore wind turbine structures.…”