Time behaviour of new passive anchors on unstable slope monitored via optical fiber

Composite anchors are a new type of self-drilling reinforcements recently developed for the slope stabilization. They consist in hollow bars equipped with some tendons, inserted and cemented in their inner hole after installation. Compared to traditional self-drilling anchors, they offer a higher tensile strength with small increments of cost. This aspect, combined with a high execution flexibility, has favoured their rapid diffusion in the last years. Similar to soil-nailing bars, they are passive reinforcements, but thanks to strand presence, they can be also used as active anchors. The bond strength at the soil-anchor interface is crucial for their design, because it controls the pull-out resistance and 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 some preliminary results obtained using distributed optical fibre sensors (DFOS) exploiting the optical frequency domain reflectometry (OFDR) to measure the strain of a fibre embedded with the tendons inside the bar. This allows to reconstruct the interface action profile of three anchors installed on an active landslide. Upon an extensive validation, the system may become a standard practice to evaluate the effectiveness of these anchors with time.