# Strain Measurement Distributed on a Ground Anchor Bearing Plate by Fiber Optic OFDR Sensor  
[PERSON]  
1 Center for Safety Measurement, Korea Research Institute of Standards and Science (KRISS), Yuseong-gu, Daejeon 34114, Korea; [EMAIL_ADDRESS] (Y.-S.K. ); [EMAIL_ADDRESS] (D.-C.S.) [PERSON]  
1 Center for Safety Measurement, Korea Research Institute of Standards and Science (KRISS), Yuseong-gu, Daejeon 34114, Korea; [EMAIL_ADDRESS] (Y.-S.K. ); [EMAIL_ADDRESS] (D.-C.S.) [PERSON]  
3 Department of Materials Physics, Dong-A University, Saha-gu, Busan 49315, Korea; [EMAIL_ADDRESS]  
[PERSON]  
22221  
body, (2) the anchor head, and (3) other parts related to the anchor head [9]. The ground anchor body is further divided into free anchor and fixed anchor sections. The free anchor, in which the strand or rod is covered by the sheath, transfers the tensile force from the anchor head to the fixed anchor section, where the tendon is grouted. The fixed anchor section transfers the tensile force to the ground by friction and compression. The ground anchor system is operated by various related accessories (e.g., wedges of anchor heads, nuts, and saddles) for easy operation. Slopes are influenced by problems with the ground anchor components (e.g., secondary quality of the anti-corrosive material and poor quality of the anchor) and changes within the slope (e.g., freezing, thawing, and ground expansion). In the worst case, deformation of the slope causes damage to the anchor system as well as to the slope. Therefore, it is very important to detect a change in tensile force because damage to the anchor system necessarily involves a change in tensile force. If the change in tensile force is abnormal, the administrator must initiate a follow-up to identify the exact cause and ensure timely maintenance. In addition, an emergency warning can be issued if monitoring is expected to result in rapid slope failure. An electrical strain gauge can be applied to detect the tensile force change of the anchor system. An anchor tensile force measurement study has been performed using a fiber optic FBG (fiber Bragg grating) sensor [10]. This sensor used in this study has difficulties in fabrication because the optical fiber is embedded in the tensile strand in order to measure tensile force. When the tensile force of the anchor is changed, the bearing plate is deformed, as shown in Figure 1b, so that the strain of the bearing plate can be measured and converted into the tensile force of the anchor. Ideally, the tensile force of the anchor acts on the center portion of the bearing plate, so that the strain at the surface of the bearing plate will be distributed symmetrically with respect to the center. The strain on the surface of the bearing plate consists of the circumferential strain and the radial strain. The axial strain (\(\epsilon_{axial}\)) of the anchor is related to the circumferential strain (\(\epsilon_{circ}\)) and radial strain (\(\epsilon_{md}\)) as follows:  
\[\epsilon_{axial}\approx-\
u\epsilon_{circ}\approx-\
u\epsilon_{rad} \tag{1}\]  
where \(\
u\) is Poisson's ratio. However, in reality, the bearing plate can undergo bending deformation, which should also be considered when calculating the anchor's tensile force. Since it is easy to install the optical fiber on the surface of the bearing plate in a circular shape, it is preferable to examine the correlation with the anchor tensile force after measuring the circumferential strain distribution of the bearing plate. Therefore, in this study, to investigate the feasibility of measuring anchor tensile force from bearing plate strain, which is measured by a sensing optical fiber installed in a circumferential direction, we performed basic research to measure the strain distributed on a bearing plate surface. A fiber optic OFDR sensor was fabricated to acquire the strain distribution. The sensing fiber was glued onto the bearing plate with an epoxy adhesive, and four electrical strain gauges were also applied for comparison.