# Strain Measurement Distributed on a Ground Anchor Bearing Plate by Fiber Optic OFDR Sensor
## 4 Anchor Tension Test with the Bearing Plate  
A universal testing machine (UTM) was used for loading the bearing plate with an anchor head as shown in Figure 6. Anchor tension testing was carried out with a load of up to 10 t while measuring the strain at 1 t intervals. Testing was performed by installing the load cell, fixed jig, anchor head, and bearing plate as shown in Figure 6a. In addition, strain gauges were connected to the bridge box and data collected from a computer via a signal conditioner. The anchor head on the bearing plate was connected to the bottom actuator of the universal testing machine by one bolt. In addition, the fixing jig connected to the base plate supporting the lower surface of the bearing plate was connected to the upper part of the universal testing machine shown in Figure 6b. The fixing jig was constructed with carbon steel of 23 mm thickness and dimensions of 270 mm \(\times\) 270 mm \(\times\) 250 mm. The base plate was reinforced by two bars in the same fashion as for the upper part of the fixing jig in order to prevent bending deformation. Therefore, when the universal testing machine is operated under the load control, the bearing plate is compressed between the anchor head and the base plate of the fixing jig. Figure 5: Strain signal variation at some portion of the sensing optical fiber. Figure 6: Anchor loading by universal testing machine (UTM) (**a**) UTM (**b**) the beating plate with a sensing fiber. Figure 10 shows the distributed strain measured by OFDR while increasing the anchor load by 1 t increments in a range of 1 t to 10 t. The horizontal axis represents the position of the sensing fiber, with 5250 mm near the anchor head, and 8450 mm far from the anchor head. However, the strain distribution measured by the OFDR shows a tendency to periodically increase or decrease in the longitudinal direction of the sensing optical fiber, unlike the strain distribution behavior due to compression of the anchor head, which can be expected to be distributed in a monotonically decreasing pattern from the anchor head. Figure 11 maps the distributed strain shown in Figure 10 to each position on the plane of the bearing plate. The larger the compressive strain, the brighter the representation in Figure 11; portions that are in measurement positions are shaded black. If the effect of compressive load due to the anchor head only affects the bearing plate, a point symmetric strain distribution should be shown with respect to the center of the bearing plate. However, contrary to this, the experimental results are symmetrical with respect to the horizontal position of 10 cm as shown in Figure 11. These results suggest that not only the compressive load of the anchor head but also the bending load due to the fixing jig influenced the deformation behavior of the bearing plate. Accordingly, the bearing plate is subjected to bending deformation, and the largest compression deformation occurs at a horizontal position of 10 cm. It can be seen that there is little difference in strain between near and far positions from the anchor head. On the other hand, it can be seen that there is a difference in the magnitude of  
Figure 10: Strains from the sensing fiber of OFDR sensor bonded on the bearing plate due to loads. Figure 9: Strains from four strain gauges bonded on the bearing plate installed with 0 degrees with varying loads. the strain near and far from the anchor head at the vertical position of 10 cm, so that the compressive strain due to the anchor head dominates at that position. Figure 12 shows the variation of strain according to the load at two specific positions taken from Figure 11. That is, the strains labelled (b) in Figure 12 are those selected from each point of five lines of the sensing fiber at the 10 cm horizontal position, where the bending load has an effect.