Strain measurement distributed on a ground anchor bearing plate by fiber optic OFDR sensor

Abstract

The safety of soil slopes reinforced by ground anchors can be evaluated by monitoring the tensile force of the anchors. The tensile force of ground anchors can be determined by measuring the strain of the bearing plate that transfers the tensile force of the anchor to the ground. Therefore, in order to investigate the relation between the strain of the bearing plate and the tensile force of a ground anchor, the strain distributed on the bearing plate was measured by a fiber optic OFDR (optical frequency domain reflectometry) sensor, which was fabricated by a tunable laser source, auxiliary interferometer, and main interferometer. This OFDR sensor was operated through a sweep range of 500 GHz with a spatial resolution of 0.2 mm, and a strain accuracy of approximately 4 με, considering the system noise when operating in 5-cm segments. The sensing fiber was circularly bonded onto the bearing plate using epoxy, and the distributed strain was measured on the bearing plate while increasing the load up to 10 tons. From the experimental results, the difference between the strain near the anchor head and the strain at the far site is significant in the region where compression strain is dominant. However, such a tendency did not appear in areas where bending strain dominates. Therefore, in order to monitor an anchor tensile force, it is necessary to carefully study the calibration factor between the anchor tensile force and the strain of the bearing plate.