## 4 Results  
The axial strain of PCC model pile at different depths under a vertical load was measured using a OBR4600 instrument. The strain curves were obtained using OFDR technology. With the obtained data, the deformation characteristics of the PCC pile could be analyzed. The strain data measured by  
Figure 6: Numerical model of soil and pile. the U-shape part of the optical fiber was affected by optical loss. The effect of optical loss should get rid of from the measure data. Additionally, traditional electronic strain gauges were bonded to the surface of model pile near the optical fiber cable, as shown in Figure 4; however, the strain gauges broke. This proves the high survival rate of the optical fiber sensor. Figure 7 shows the strain curves of the left and right sides of the model pile at 16 kN and 24 kN. The measurement results from the optical fiber by OBR can be analyzed. The strain of the model pile at depths from 0.1 m to 1.3 m was measured without any breaks. This illuminates the effectiveness of the layout, packaging, and protection of the optical fiber. The strain data of the model pile at depths from 0.0 m to 0.1 m was not recorded, because of the influence of the outer environment. Every datum is 1 \(\upmu\varepsilon\), it shows the advantage of OFDR in measuring strain along the optical fiber with high spatial resolution and accuracy. The strain curves of the left side of the pile at 16 kN and 24 kN have similar variation trends. The strain gets smaller from the top to the bottom of the pile, which corresponds with the deformation characteristic of the pile at a vertical load in general. These results confirm the stability and accuracy of OFDR technology indirectly. The strain of both sides on the same load, which should be the same in an ideal situation, had obvious differences. This may have been caused by the small eccentricity of the load. There are many fluctuations on curves, which are bigger than those on the strain curves of metal and the composite; this was because the concrete had a rougher surface than that of the metal or the composite. In addition, the pile was subjected to the friction from surrounding soil during loading. The fluctuations of both sides are different in position and range, which is due to the local heterogeneity of the concrete material. Figure 8 shows the average strain curves of model pile at 16 kN and 24 kN. The strain of the pile gets larger with load increasing. The two curves have the same variation trend. There is an abnormal fluctuation below the position of 1.05 m, which gets larger with load increasing; this is because the model pile was given the small load twice for testing the feasibility of loading and the monitoring devices. The loading caused movement of the model pile and disturbance of the surrounding soil. Due to the properties of the equipment, the features of the signal, and effects of the environment, the obtained data contained much signal noise; this is a barrier for analyzing the deformation characteristics. There are many methods to deal with the optical fiber data with a noisy signal, e.g., wavelet transform. The wavelet transform is a signal analysis method of time-frequencies. It has the characteristics of a multi-resolution analysis (MRA), and the ability to describe the local features of a signal both in time and the frequency domain. Therefore, this method is suitable for resolving the data obtained by OBR to eliminate the signal noise. Figure 9 shows the process of using wavelet transform to deal with the amplitude data obtained by OBR on 3 levels. From Figure 9, the signal noise reduces notably, i.e., from level 1 to 3. Figure 7: Strain curves of two sides of model pile. (**a**) Strain curves at 16 kN; (**b**) Strain curves at 24 kN. The spatial resolution of OFDR technology is high. The data passing through the wavelet transform is very fluctuant, especially for material with tough surfaces; as such, it was difficult to find a variation trend. The polynomial fitted method is a good way to match the curve with many data.