## 3 Results and Discussions
The comparison of the left and right images in Figures 9 and 10 show that the wavelet transform can well filter out the high frequency interference components (burr phenomena) of the original radar signal and the useful signals have been reinforced. As the difference is small between the dielectric parameters of piles and soil, the reflected wave signal is weak, and not easy to distinguish at the interface. Sometimes the anomaly reflection of expansive soil is understood as the anomalous reflection of a pile, and the interference of the reflected signal is bigger. These factors finally affect the accurate interpretation. Time-varying automatic gain and wavelet processing methods enhance the effective reflection wave signal and improve the accuracy of GPR data interpretation. The water injection test was carried out by excavating the bottom of the foundation ditch to the elevation of the riverbed to evaluate the water injection effect of the sheet-pile wall. The results of the excavation can be used to further check the effect of the radar test before water injection. The surface of the U-shaped sheet pile after excavation is shown in Figure 11. The damages to the U-shaped sheet pile also verify the correctness of the radar test results. All the U-shaped sheet piles are cracked to varying degrees in Figure 11. The maximum length of the cracks is more than 70 cm in Pile#2, and the minimum length of the cracks is also more than 10 cm in Pile#4. The crack lengths of Pile#1, Pile#3, Pile#5 and Pile#6 are, respectively, about 30, 35, 30 and 13 cm. The excavation results also prove that the damage of Pile#4 is minimal. Most cracks are slant. In the process of hammering, the piles are subjected to significant vertical soil resistance, which results in the piles cracking and breaking. Through the expansive soil and hard texture, the pile is difficult to drive down. Figure 10: Comparison of images before and after wavelet transform processing of radar signals (Center frequency: 100 MHz, (**a**) is the preprocessing image, (**b**) is the processed image). Expansive soil generally has high strength and low compressibility in the natural state. In the process of driving the U-shaped sheet pile, it is subjected to great resistance and requires many hammerings (up to hundreds of times) to drive, which inevitably has a high impact on the pile structure. From the analysis of GPR images and cracks causes, some conclusions can be drawn:  
1. The 400 MHz antenna radar image within the range of the white lines shows the highly abnormal area, which may be crushed, isolated, cracked or broken. The distortion is serious at the depth range of 4.5 to 8 m.
2. The pile length can be judged from the reflection of the pile bottom, which can be seen in the 100 MHz antenna image. The image shows that the deep distortion of the pile is serious, and the response to the shallow part and middle part is weak. 3. The 400 MHz antenna image shows that the distortion at the middle position of pile #1, #2, #3, #5, and #6 is more serious compared with pile #4. 4. The accuracy of the 400 MHz antenna is higher than that of the 100 MHz antenna, but the non-destructive testing depth is relatively shallow, and there is basically no response to deep distortion, which is also related to the attenuation of energy. 5. The non-destructive testing results of the GPR are consistent with those of field excavations, and the case study proves GPR is suitable for detection of the U-shaped-sheet pile. Figure 8.0 Crack diagram of the surface of U-shaped sheet pile #1–6 (**a**) Pile#1, (**b**) Pile#2, (**c**) Pile#3, (**d**) Pile#4, (**e**) Pile#5, (**f**) Pile#6.