Design, Testing, and Realization of a Distributed Fiber Optic Monitoring System to Assess Bending Characteristics Along Grouted Anchors

Grouted steel anchors combined with shotcrete layers at the surface are a common method to stabilize slopes and construction pits. Their design is based on the equilibrium consideration of a rigid body motion, and therefore, only tensile stresses are considered and accompanying forces, such as bending, are neglected. Modern approaches substitute the steel with wooden anchors, which show a sufficient tensile, but poor shear strength capacity. Distributed bending curves along the embedded anchors are required to design the optimal anchoring system, which, however, may not be measured using conventional sensing techniques. This paper reports about a distributed fiber optic sensing approach that enables the assessment of curvature and bending characteristics along grouted anchors. The designed system was tested within comprehensive laboratory investigations, in which various loading scenarios were applied to an instrumented, real-scale steel anchor. The suitability of the fiber optic sensing system was proven using image-based and geodetic total station measurements at the surface as well as by comparisons to theoretical beam models. In addition, three instrumented anchors were installed at a highway construction site and autonomously monitored over several weeks. The presented studies demonstrate the high potential of distributed fiber optic monitoring systems and their capability to extend traditional measurement methods in foundation engineering.