## 2 Method for a Strain Field Measurement on the Bearing Plate  
If the tensile force applies to the tendon in Figure 1, the anchor head presses the bearing plate. The bearing plate transmits the compressive load to the anchored structure. In this load transfer process, the bearing plate experiences bending deformation due to the mismatch of the loading location, as shown in Figure 1b. We numerically analyze the strain field on the anchor plate for understanding its strain distribution. We make a numerical model that an anchor plate presses an anchored structure made of concrete, as shown in Figure 2a. The anchor plate has the dimension of \(200\times 200\times 25\) mm, and the hole diameter of 85 mm, and the material is general steel in which the elastic modulus is 200 GPa and Poisson's ratio of 0.26. The diameter of an anchor head is 110 mm. The anchored structure is \(800\times 800\times 800\) mm concrete, which has the elastic modulus of 32 GPa, and 0.1 Poisson's ratio. The bearing plate is divided as 8035 elements, and the anchored structure is modeled with 129,532 elements. The load is inflicted on the hole edge by an anchor head of a 110 mm diameter.Figure 2b represents the strain field on the top of the bearing plate in a circumferential direction. The bending deformation of the bearing plate makes a varying strain field on the top surface of the plate, i.e., high compressive strain appears near the hole at the center, and the strain decreases as it moves away from the center. We can identify that the strain variation can be minimized when the distributed optical fiber sensor is installed in a circumferential direction. In addition, the strain level is clearly distinguished depending on the radial distance from the center. Moreover, in the installation point view, this circumferential direction is the best way to install the distributed optical fiber without (or minimizing) discontinuity in a strain filled measurement. Figure 3 compares the strains in the circumferential direction at two different radial distance, 70 mm (o140 mm) and 90 mm (o180 mm) in a 90\({}^{\circ}\) interval. It shows a minimum compressive strain at the direction pointing corner of the square plate. This is a geometry effect appearing in the square plate and may change depending on the shape of the bearing plate. However, the strain difference between the two radial distance at each angle direction is almost the same. Thus, averaging the strain measured in the circumferential direction could be a representative strain value when the sensor is installed in a circumferential direction. Moreover, the averaging of the strain field helps to minimize an  
Figure 1: Conceptual diagram of the ground anchor system, (**a**) anchor system with the enlarged bearing plate, (**b**) force equilibrium of anchor system [12]. Figure 2: (**a**) Finite element model and (**b**) angular strain distribution on the bearing plate. experimental error due to variations in installing the optical fiber sensor. We numerically demonstrate that the strain difference between the two radial direction changes depending on the tensile force applied to the tendon. It gives an idea to measure the tendon force indirectly by measuring the strain fields at two different radial distances. A Principle of Anchor Force Determination from the Strain of a Bearing Plate with the Rejection of Temperature Effect  
The strain field on the bearing plate also changes depending on the temperature. Therefore, the strain can fluctuate depending on the monitoring time (e.g., day or night), weather conditions, and seasons. For the accurate measuring of the anchor force, we need to remove the temperature effect on the strain field. When we use a fiber optic distributed sensor, the signal output, \(\Delta\omega\), is related with strain, \(\Delta\varepsilon\), and temperature, \(\Delta T\), as in Equation (1).