Strain Response of Tunnel Anchor Using Rayleigh Scattering-based Optical Fiber Sensing Technology in the Field Test

Tunnel anchor is a relatively new type of anchor for suspension bridges. Loading tests on a scale model in site are generally considered to be the most direct way to evaluate the bearing capacity of the tunnel anchor system. The response of the anchor body has received minimal attention and involves only a few or no measuring points, because the anchor plug does not undergo elastoplastic failure in a model test. This article intends to explore the relationship between the strain response of the anchor and the state of the surrounding rock. Optical fiber strain sensing technology based on adjustable wavelength optical time domain reflectometry (i.e., TW–COTDR) was applied to the scale model test of Baotaping Bridge tunnel anchor. The strain distribution and evolution of the entire anchor body were determined by optical measurements during overload. In the elastic and plastic stages of the anchor system, the strain around the anchor body linearly decreased from back to front, except for the top arch and the bottom plate at the end. The anchor strain–load curve was nonlinear, similar to the displacement–load curve, but unrelated to concrete damage, which signaled that plasticity began to appear in the anchor system. Therefore, the strain response of the anchor body can be used as an alternative to determine bearing capacity of the anchor system, especially when the displacement is too small to observe.