## 4 Analysis of Test Results
### Tip Resistance-Tip Displacement Curve  
Pile tip resistance is a crucial factor in determining the bearing capacity of pile, particularly under heavy loads. Its role becomes even more prominent as it directly influences the pile's overall load-bearing capacity. Notably, the displacement of the pile tip, which is necessary for pile tip resistance to develop, is significantly impacted by growth compression. Figure 5 presents a comparison of the pile tip resistance (\(q_{\text{b}}\)) versus pile tip displacement (\(\text{s}_{\text{b}}\)) curves for test piles, both before and after grouting. Initially, when the pile top load is minimal, whether before or after grouting, the pile tip resistance for both TP1 and TP2 is relatively low. At this phase, the load is predominantly borne by the lateral resistance of the pile, leading to minimal displacement at the pile's termination point. As the load applied to the pile's apex progressively intensifies, the resistance offered by the pile tip starts to become evident, which is accompanied by a corresponding augmentation in the displacement at the pile's termination. However, once the top load exceeds a certain threshold, the growth rate of the tip resistance for the test pile before grouting slows down and stabilizes, while the pile tip displacement continues to rise. This indicates that the pile top load has surpassed the ultimate bearing capacity of the piles, leading to soil damage.  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline \multirow{2}{*}{**Soil Name**} & \multirow{2}{*}{**Elevation of Soil Layer**} & \multicolumn{2}{c}{**Average Side Resistance of TP1 (kPa)**} & \multicolumn{2}{c}{**Average Side Resistance of TP2 (kPa)**} \\ \cline{3-6}  & & **Before Grouting** & **After Grouting** & **Before Grouting** & **After Grouting** \\ \hline
2-1 Silty clay & 8.572–7.572 m & 77 & 92 & 72 & 92 \\
2-1 Silty clay & 7.572–3.84 m & 79 & 94 & 74 & 94 \\
2-2 Silty clay & 3.84–2.14 m & 69 & 81 & 68 & 85 \\
2-3 Silty clay & 2.14–2.36 m & 61 & 74 & 59 & 78 \\
2-4 Silty clay & \(-\)2.36–9.16 m & 94 & 107 & 85 & 104 \\
2-5 Silty clay & \(-\)9.16–19.26 m & 96 & 109 & 87 & 108 \\
3-1 Clay & \(-\)19.26–19.428 m & 98 & 115 & 94 & 120 \\
3-1 Clay & \(-\)19.428–28.16 m & 96 & 112 & 92 & 117 \\
3-2 Clay & \(-\)28.16–38.46 m & 95 & 109 & 85 & 106 \\
3-3 Clay & \(-\)38.46–43.26 m & 90 & 103 & 82 & 100 \\
3-3 Clay & \(-\)43.26–44.928 m & 88 & 98 & 78 & 99 \\
3-4 Clay & \(-\)44.928–46.928 m & 94 & 168 & 96 & 173 \\
3-4 Clay & \(-\)46.928–47.928 m & 92 & 163 & 94 & 168 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Pile side resistance statistics.  
Figure 4: Distribution curves of side resistance along the depth of test piles under ultimate load: (a) TP1; (b) TP2.  
Conversely, after combined grouting, the pile tip resistance of TP1 and TP2 does not exhibit any noticeable slowdown in the growth rate. The grout injected at the pile end consolidates the pile end slag, reinforces the soil, and forms a consolidated grout body at the pile end. This results in a significant improvement in tip resistance. When providing the same pile tip resistance, the pile tip displacement of the test piles after combined grouting is reduced, leading to decreased settlement of the pile foundation under the same pile top loading condition. Furthermore, Figure 5 reveals that after combined grouting, the pile tip resistance responds more swiftly, with a faster growth rate. When the test pile reaches its ultimate bearing capacity, the proportion of pile tip resistance increases. This is attributed to the pre-pressurization effect of the grout injected into the soil at the pile end after grouting, which allows for the tip resistance to develop earlier. This not only enhances the bearing capacity of the single pile but also significantly reduces the settlement of the pile foundation. In summary, combined grouting effectively improves pile tip resistance, reduces pile tip displacement, and enhances the overall bearing capacity and stability of the pile foundation.