Fibre optic monitoring of pile load tests, Bilten Canton Glarus

In autumn 2020, Solexperts carried out three static pile pressure tests as part of the construction of a new waste water treatment stage for the elimination of micropollutants in Bilten in the canton of Glarus (CH). These tests form the basis for dimensioning the future piles, which will be installed in post-glacial fluvial, peaty and lacustrine deposits.

Each of the three piles with a length between 20 and 25 m was instrumented with about one hundred metres of fibre optic cable, so that distributed strain sensing could be performed along four 90° profiles. Before the piles were installed in the borehole and the cement was injected, the fibre optic cables were mounted on the reinforcement cages to achieve a good connection to the structure.

Fibre optic measurement technique

The measurements were carried out with a fibre optic readout device (interrogator) combining distributed measurements based on both Brillouin scattering (PPP-BOTDA) and Rayleigh scattering (TW-COTDR). With the high-quality Neubrex interrogator, deformation measurements were possible over the entire length of the piles with a local resolution of 5 cm, an accuracy of 0.5 μm/m and a data acquisition frequency of a few minutes.

Compared to the conventional method with point measurements, the fibre optic sensing provides a much more complete and detailed profile of the pile behaviour.