# Study on the Influence of File Installation Disturbance on the Settlement of Composite Foundation
## 4 Results  
### CPTU Parameters Before and After Pile Installation  
The CPTU results before and after pile installation are shown in Figure 8, where \(q_{c}\) is cone tip resistance, \(u_{2}\) is pore water pressure, and \(q_{t}\) is corrected cone tip resistance. The three parameters have the following relationship:  
\[q_{t}=q_{c}+u_{2}(1-a) \tag{6}\]  
where \(a\) is the net area ratio, taken as 0.8, and is only related to the shape of the cone tip. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Soil Layer**} & \(\mathbf{G_{s}}\) & \(\mathbf{e}\) & \(\mathbf{c}\) & \(\mathbf{\varphi}\) & \(\mathbf{E_{s}}\) & \(\mathbf{\lambda}\) \\ \cline{3-8}  & & \(\mathbf{k}\mathbf{N}\cdot\mathbf{m^{3}}\) & **-** & \(\mathbf{k}\mathbf{Pa}\) & \(\circ\) & **MPa** & **-** \\ \hline
1 & Embankment soil & 18.6 & 0.78 & 10.5 & 21.4 & 8.1 & 0.30 \\
2 & Plain soil & 17.6 & 0.80 & 7.0 & 15.0 & 4.1 & 0.30 \\
3 & Muddy soil & 16.3 & 1.20 & 4.8 & 3.8 & 2.1 & 0.32 \\
4 & Moderately weathered rock & 18.0 & 0.50 & 21.0 & 36.0 & 20.5 & 0.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Physical and mechanical parameters of soil layers. Figure 8: CPTU on original soil layer and soil layer disturbed by CMP and PHC pile installation. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Composite Foundation**} & \(\mathbf{E_{p}}\) & \(\mathbf{G_{s}}\) & \\ \cline{2-4}  & \(\mathbf{Mpa}\) & \(\mathbf{k}\mathbf{N}\cdot\mathbf{m^{3}}\) & \(\mathbf{\lambda}\) \\ \hline CMP & 150 & 21 & 0.3 \\ PHC & 1000 & 29 & 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Physical and mechanical parameters of CMP and PHC. To quantify the degree of disturbance in the soil layer before and after pile installation, a disturbance index was established based on corrected cone tip resistance from first and second test, which is:  
\[SD=\frac{q_{t}-q_{t}{{}^{\prime}}}{q_{t}}\times 100\% \tag{7}\]  
The test results of \(SD\) are shown in Figure 9. \(SD(h)=ah^{-b}\) is applied to fit the test results. For CMP and PHC pile, respectively, \(a_{1}=0.497\), \(b_{1}=-0.757\) and \(a_{2}=0.645\), \(b_{2}=-0.374\). From the test data, it can be observed that in shallow strata (0 to 3 m), the disturbance generated during the piling process of both types of piles is relatively large, exceeding 50%. However, as the treatment depth increases, the disturbance within the CMP length remains at around 10% to 15%, and there is essentially no disturbance below a depth of 2 m from the pile bottom. The reasons for this pattern in the impact behavior of the surrounding soil during the piling process can be attributed to three factors:  
Firstly, in addition to being influenced by cutting in shallow soil layers, the lifting force of the mixing shaft also affects them. Due to the small self-weight stress of shallow soil layers, the lifting force causes significant disturbance, resulting in a disturbance exceeding 60%. Secondly, as the drilling depth increases, the surrounding soil is increasingly affected by the cutting action of the mixing shaft, but this impact on the disturbance of the surrounding soil is approximately 5% to 10%. Thirdly, the disturbance experienced by the soil below the pile bottom is mainly caused by the requirement for the drilling shaft to hover for a period and inject slurry after reaching the design depth. According to the test results, the depth of this disturbance is approximately 2 m, with an average disturbance of 5%. Compared to CMP, the hammer-driven piling process of PHC pile results in greater disturbance to the surrounding soil, with an average disturbance exceeding 30%. This is due to several reasons:  
Firstly, during on-site hammer-driven piling, noticeable undulations in the surface soil within a range of 1 m to 2 m around the pile are observed, indicating significant disturbance  
Figure 9: Composite foundation of CMP and PHC pile. of the soil.