Strain development measured with DFOS in experimental passive anchors used for landslide stabilization

Distributed optical fiber sensors (DFOSs) as detector of strain and temperature variations are an innovative and very attractive monitoring system because they allow measures distributed along large distances and with medium-high resolution. This paper deals with the monitoring of a new type of self-drilling reinforcements, namely composite anchors, specifically developed for the slope stabilization. They consist in self-drilled bars equipped with some tendons inserted and cemented in inner hole of the bar after installation. Compared to the passive reinforcements with the same external diameters, composite anchors offer a higher tensile strength with small increments of cost. Similar to soil-nailing bars, they are passive reinforcements and the knowledge of the bond strength at the soil-anchor interface is crucial for their design and the evaluation of the long-term stabilization effects; however, the in-situ bond strength measurements performed in the past with traditional sensors have not always produced satisfactory outcomes. The paper presents the preliminary measurements obtained along seven composite anchors installed in a test site on an active translational landslide moving at a displacement rate of 40-100 cm/year. The monitoring system is composed by DFOSs exploiting the optical frequency domain analysis (OFDA) which allows the determination of strain and temperature variation profiles with a spatial resolution of 20 cm. The measures showed that, due to the high displacement rate of the landslide, after only 14 days the anchor traction was about 40% of the maximum strength of bars and arrived at 65% after 1 month. The strain profile clearly showed the localization of the maximum traction close to the sliding surface and that the most external anchor portion is quite unloaded.