Novel Applications of Distributed Fiber-optic Sensing in Geotechnical

In the last two decades, Brillouin distributed fiber-optic sensing has became a widely accepted, mature technology. On the other hand, geotechnical monitoring applications of this technology are still rare, as the fragile fiberoptic and the harsh soil environment are a difficult combination. Additionally, due to high uncertainties in soil behavior, deeper understanding of geomechanical principles is necessary in order to achieve meaningful results when using these sensors.

In this study, novel applications of distributed fiber-optic sensing in geotechnical engineering were identified, developed, implemented and evaluated. Firstly, one-dimensional structures were considered:

Strain distribution along a soil-embedded cable during pullout;
Strain distribution along a monitoring ground anchor during pullout.

As a result, new insight into the progressive failure phenomenon was achieved by documenting the phenomenon of residual shear stress increase with increasing pullout load. This phenomenon is explained in a conceptual analytical model.

The successful implementation of the technology to one-dimensional structures inspired an attempt to apply the sensors in two- and threedimensional problems:

Road-embedded sensor for landslide boundary evaluation;
Soil-embedded sensor for landslide boundary evaluation;
Borehole-embedded sensor for landslide boundary evaluation.

For the ongoing landslide research and monitoring in St. Moritz, Switzerland, new understanding of the landslide mechanisms in the Brattas and Laret areas was achieved. The road-embedded sensor at the Brattas site detected an additional shear zone, which was later confirmed by a water pipe breakage that occurred at exactly the same location. The soil-embedded sensor at the Laret site confirmed seasonal patterns of the surface displacement in a moving soil mass independently observed in inclinometer measurements. To facilitate fiber-optic sensing for the above applications, significant advances in the technology, the sensors and the data interpretation were necessary:

Spatial resolution of the Brillouin sensing technology had to be improved significantly. This was achieved by facilitating and testing the development of Brillouin Echo Distributed Sensing;
Elaborate laboratory testing of the sensors and the sensing system led to the development and improvement of new commercial strain sensing cables. In addition, sensor integration techniques were developed and successfully applied;
Options of improving the data interpretation had to be evaluated and applied.

The present study describes in detail the development and progress of these novel geotechnical monitoring applications at the IGT of ETH Zurich during the last 5 years.