## 2 Site Geological Conditions and Test Pile Overview
### Pile Construction for the Test Piles  
The construction process of the test pile is shown in Figure 1.  
The pile driver for the steel-concrete composite mixing pile included a frame, a lifting mechanism, a rotating mechanism, a steel pipe, a flexible conduit, a drill bit, and a grout pipe. The lifting mechanism was mounted on the frame. The rotating mechanism was connected to the lifting mechanism, which could drive the rotating mechanism to move vertically up and down. The first end of the steel pipe was connected to the rotating mechanism, which could drive the steel pipe to rotate around its axis. The second end of the steel pipe was detachably connected to the drill bit through a locking device. One end of the flexible conduit was connected to the drill bit, and the other end was connected to the pouring opening of the steel pipe. One end of the grout pipe was connected to a slurry pump through a spray head and a grout inlet pipe, and the o \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & \(\gamma\) [**kN/m**\({}^{-3}\)] & \(E_{s}\) [**GPa**] & \(\mu\) \\ \hline Cement soil & 20.65 & 0.1 & 0.30 \\ Concrete & 25.00 & 30.0 & 0.18 \\ Steel tube & 80.00 & 200.0 & 0.30 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: File parameter indicators. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Pile Number** & \begin{tabular}{c} **Diameter of Cement Soil** \\ **Mixing File [mm]** \\ \end{tabular} & \
\begin{tabular}{c} **Steel Tube Dimensions** \\ **(Length [m] \(\times\) Diameter [mm]** \\ \(\times\) Thickness [mm])** \\ \end{tabular} &
\begin{tabular}{c} **Design Ultimate Vertical** \\ **Compressive Load Capacity [kN]** \\ \end{tabular} \\ \hline
1 & 1000 & 40 \(\times\) 273 \(\times\) 16 & 40 & 7000 \\  & & first section (20 \(\times\) 273 \(\times\) 16) & 40 & 6000 \\
2 & 800 & second section: (10 \(\times\) 243 \(\times\) 12) & 40 & 6000 \\  & third section: (10 \(\times\) 249 \(\times\) 12) & 22 \(\times\) 129 \(\times\) 4 & 22 \(\times\) 129 \(\times\) 4 \\
3 & 700 & first section: (12 \(\times\) 219 \(\times\) 4) & 22 & - \\
4 & 700 & second section: (10 \(\times\) 188 \(\times\) 4) & 22 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Test pile parameters. connected to the drill bit. The locking device included an L-shaped groove and a strip-shaped plate, which could be engaged or disengaged. This ultimately enabled the recovery of the steel pipe and grout pipe, improving the concentricity of the new type of cast-in-place concrete composite pile. The pile driver is shown in Figure 2[14]. Construction of Steel Tube Concrete Mixing Composite Piles  
1. The first section of the steel tube was welded to the pile head, and grout pipes were installed. The grout pipes were connected to the grout pump via flexible hoses. 2. The steel tube was securely clamped to the pile machine's clamping block. The pile machine was then moved to align the pile head with the designated pile position. 3. The pile machine was started to drill the steel tube mixing pile to the predetermined depth. During the sinking of the steel tube mixing pile, the grout pump was activated to inject cement mortar into the foundation through the grout pipes. 4. While grouting, the steel tube was rotated and lifted simultaneously. The lifting speed of the mixing machine was strictly controlled according to the design specifications. 5. The steel tube mixing pile was then rotated and sunk back into the foundation to the predetermined depth. This process was repeated three to five times (multiple mixing processes ensured thorough mixing of the soft soil and additives). The next section of the steel tube was welded, and steps (2) to (5) were repeated until the construction of the steel tube mixing pile was completed. After removing the grout pipes, concrete was poured into the hollow steel tube to form the pile, completing the construction of the steel tube concrete mixing composite pile. The construction process flowchart is shown in Figure 3.  
Figure 1: Construction process flowchart.