## 5 Discussion and Interpretation  
Analysis of Tables 6 and 7 reveals that the main factors influencing the characteristics of the grouting cement returned during the process of the pile tip post-grouting, under both loading and unloading conditions in the soil, are grouting pressure and grout volume, with the grouting mode transition primarily affected by the grouting pressure. The lower grout volume and grouting pressure corresponding to the grout diffusion shape is mainly half-ellipsoidal or half-droplet-shaped, and the grouting mode is compaction grouting. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Grout Volume (mL)** & **Grouting Pressure** & **Digital Photos** & **Horizontal** & **Vertical Diffusion** & **Main Features** \\  & **(kPa)** & **Digital Photos** & **Distance (cm)** & **Grout Distance (cm)** & \\ \hline  & & & & & Compaction and fracture; Half-spherical shape & \\
300 & 700 & 8 & 5 & closing to the grouting hole and & \\  & & & & & \\  & & & & & \\  & & & & & \\
300 & 700 & 8 & 4.8 & Compaction dominant; Grout & \\  & & & & bulb gathered on the & \\  & & & & & \\
500 & 8 & 4.8 & & grouting hole like & \\  & & & & & \\  & & & & & \\
400 & 600 & 9 & 5.4 & Compaction and fracture; Broad and & \\  & & & & long of grout bulb & \\  & & & & with short and thick & \\  & & & & veins and grout bulb & \\  & & & & & \\  & & & & & \\
700 & 8 & 7.2 & Fracture dominant; & \\  & & & & Similar to the “ball & \\  & & & & snout” at the bottom & \\  & & & & of the ship, an & \\  & & & & obvious grout & \\  & & & & bulb shell and & \\  & & & & cracked surface & \\  & & & & morphology existed \\ \hline \hline \end{tabular}
\end{table} 
Table 7: _Cont._With the increase of grout volume and grouting pressure, other irregular shapes of the grout bulb and grout veins and shells will appear, and the grouting mode will change to fracture grouting. However, there is no sharp distinction between the compaction and fracture grouting phases. In other words, for the process that is governed by the compaction effect, some fracturing also exists [14]. For example, the grouting pressures at 600 kPa and 700 kPa and the grout volume at 300 mL and 400 mL in Table 6, where thick and short grout veins in the horizontal and lateral directions were also formed. The fracture grouting mode is generally produced by continuing grouting cement after the compaction grouting mode is maintained for some time. The soil/grout interactions during pressure injection have been noted in previous studies [22], and a simplified model is given to describe the process of compaction grouting and fracture grouting during pressure injection [23; 24]. This paper is based on the grouting cement returned along the pile body during the process of pile tip post-grouting as the background to study the grout diffusion characteristics on the shaft of the pile. Combined with test results in this paper, a simplified model to describe the grouting cement diffusion pattern and the corresponding grouting mode of the grouting cement returned along the pile body during the process of pile tip post-grouting was proposed, as shown in Figure 11a-d, they are modes of compaction grouting, compaction grouting, compaction grouting dominant and some fracture grouting and some fracture grouting also exists, and fracture compaction grouting dominant and some compaction grouting also exists, respectively. Through the four types of grout diffusion patterns and grouting modes in Figure 11, it is possible to generalize all test results and explain the aforementioned test variation laws. The pile/soil interface is weaker than the soil particles interface generally [25], and the grouting cement is easier to return along the weak surface. Hence, the returned height is greater than the radial expansion distance of the grouting cement returned along the pile body under the same work conditions (Figures 5, 6 and 9). When the grout volume is low, the grouting mode is generally compact according to the model proposed above in this paper.