## 3 Application
### Analysis of Test Results
The propagation velocity of elastic wave in the layer gradually decreased from 1400 m/s to 1200 m/s in the range of depth above 14 m, from 1900 m/s to 1650 m/s in the range of depth 14-33 m, and from 2950 m/s to 2800 m/s in the range of depth below 33 m as the distance from the pile became closer. Similar to the C1-C3 profile, the propagation velocity of elastic wave in the layer tended to decrease gradually as the distance from the pile became closer to the depth direction. According to the engineering experience, the above law is related to the construction characteristics of bored piles. During the construction of bored piles, the soil in stress equilibrium is removed and the soil around the borehole is displaced into the hole due to unloading, resulting in the soil around the borehole becoming relaxed. The closer to the borehole, the more unconsolidated the soil is. The propagation velocity of elastic waves and the compactness of the layer are closely related. The more compact and harder the layer is, the greater the propagation velocity of elastic wave is. Therefore, the closer the soil is to the pile, the smaller the elastic wave propagation velocity is. One month after the pile grouting, the profiles were detected again. The inversion results of the three pile-side profiles C3-C6, C6-C1, and C6-C5 after grouting are shown in Figure 7. The variation of velocity before and after slurry injection in the pile side profile is shown in Table 3. By comparing the three clouds, five common stratigraphic interfaces can still be distinguished. The propagation velocity of elastic wave above 33 m in depth was basically unchanged compared with that before grouting; however, the propagation velocity of elastic wave in the range of 33-51 m increased from 1200 m/s before grouting to about 1300 m/s; the propagation velocity of elastic wave below 51 m increased by about 50 m/s. This indicates that post-grouting has an effect on the strength of the soil in a certain depth range around the pile. Combined with relevant research results, considering the slurry up-flow phenomenon in the grouting process, it was presumed that this was the result of the action of the cement paste upward return on the soil around the pile during the grouting. In the range of below 33 m, the silty clay was mainly interspersed with a medium dense silt. The silty clay and silt were extruded or osmosed and reinforced under the high pressure of the grouting, and the strength and compactness of the soil increased; thus, the propagation velocity of the elastic wave increased. Based on the inversion results of the pile profiles after grouting, in order to more intuitively compare the changes of cross pile profile before and after grouting, each profile was compared before and after grouting, as shown in Figures 8 and 9. \begin{table}
\begin{tabular}{c c c c} \hline
**Profile** & **Depth/m** & **Pre-Pressing Speed/m/s** & **Post-Pressing Speed/m/s** \\ \hline  & 0–14 & \textless{}900 & \textless{}900 \\  & 20–27 & 1000 & 1000 \\ C6-C1 & 27–33 & 1100 & 1100 \\  & 33–51 & 1200 & 1300 \\  & 51–56 & \textgreater{}1300 & \textgreater{}1350 \\ \hline \end{tabular}
\end{table} 
Table 3: Variation of pile lateral profile velocity before and after grouting. Figure 8: C1-C3 profile before and after grouting. Figure 7: Side profile of pile (after grouting). Figure 8 shows the before and after grouting of pile profile C1-C3 (before grouting is in the left panel and after grouting is in the right panel). From the figure, it can be seen that the velocity field distribution of the profile changed obviously after grouting, and the characteristic of the stratigraphic interfaces at 14 m and 33 m depths could still be discerned. Ignoring the two inversion artifacts in the upper and lower left corners of the right figure, the propagation velocity of elastic wave in the layer in the range above 14 m depth, from far to near to the pile, still showed a decreasing trend, from 1800 m/s to about 1500 m/s gradually.