## 2 Materials and Methods
### The FEM Parameters  
The 3D calculation model of HSCM-FT is shown in Figure 2 with 1/2 modeling due to the symmetry of the structure. The soil layers were divided into six layers according to the site exploration. The test pile model was divided into four parts: the helical pile, the cement in the steel pipe, the soil-cement column, and the interface. The interface was divided into the steel pipe and soil-cement interface, pile and soil interface, and the end-bearing interface depending on the material. The ontological model of the material uses Moore-Coulomb. The drainage type of the soil was chosen as the undrained (B) with undrained shear strength, and the soil-cement was set as a non-porous material with linear elasticity. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Soil Layer** & \begin{tabular}{c} **Weight** \\ **\(\gamma\) kN/m3** \\ \end{tabular} & \
\begin{tabular}{c} **Compression Modulus** \\ **\(E_{\delta}\) (kN/m2)** \\ \end{tabular} &
\begin{tabular}{c} **Undrained Shear Strength** \\ **\(s_{u}\) (kN/m2)** \\ \end{tabular} \\ \hline Clay-1 & 18.40 & 3450 & 26.17 \\ Clay-2 & 19.47 & 5320 & 27.16 \\ Silt & 16.57 & 1790 & 5.68 \\ Stiff clay-1 & 20.47 & 9860 & 102.00 \\ Stiff clay-2 & 20.65 & 10,000 & 114.36 \\ Stiff clay-3 & 20.08 & 9360 & 227.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Marine soft clay physical properties. Figure 1: The installation process of HSCM-FT. The helical pile was buried at a depth of 13.5 m with seven helix plates located 1.15 m, 2.65 m, 5.65 m, 8.65 m, 11.75 m, 12.55 m, and 13.35 m below the ground surface. The helix plates and the steel pipe had an elastic modulus of 200 GPa and Poisson's ratio of 0.3. The soil-cement column was divided into three parts according to the strength of the soil to improve the calculation accuracy. The upper layer of soil-cement-1, the silt layer of soil-cement-2, the holding layer of soil-cement-3, and the corresponding steel pipe and soil-cement interface were established. The material parameters are shown in Table 2. The interface coefficient was related to the material properties and strength. According to [PERSON], based on shear tests of precast piles and soil-cement, the interfacial friction coefficient was 0.116 to 0.141 when the soil-cement strength ranged from 65 to 1500 kPa [22]. This study used the strength reduction factor \(R_{inter}\) = 0.13 for the interface of the steel pipe, soil, and soil-cement. In addition, for the interface of the pile and soil in cohesive soil, \(R_{inter}\) was usually taken as 0.4. The model dimensions were evaluated by varying the boundary positions on the x and y axes to reduce the influence of boundary conditions on the pile and soil interaction until the calculations converged and reached relative stability. The model dimensions of 2 m \(\times\) 1 m, 2 m \(\times\) 2 m, 4 m \(\times\) 2 m, and 4 m \(\times\) 4 m were taken for the simulations, and the calculated results are shown in Figure 3. The variation in the ultimate bearing capacity and displacement showed that the calculated results were close when the model size was 4 m \(\times\) 2 m and 4 m \(\times\) 4 m. The model of 4 m \(\times\) 2 m was chosen for the simulation. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Interface** & **Elastic Modulus** & **Compressive Strength of** & **Tension Strength of** \\  & **(kN/m\({}^{2}\))** & **Soil–Cement \(C_{ref}\) (kN/m\({}^{2}\))** & **Soil–Cement \(C_{inc}\) (kN/m\({}^{2}\))** \\ \hline Pile and soil-1 & \(84.8\times 10^{3}\) & 424 & 42.4 \\ Pile and soil-2 & \(19.2\times 10^{3}\) & 96 & 9.6 \\ Pile and soil-3 & \(34.6\times 10^{4}\) & 1728 & 172.8 \\ Steel pipe and soil-cement-1 & \(84.8\times 10^{3}\) & 424 & 42.4 \\ Steel pipe and soil-cement-2 & \(19.2\times 10^{3}\) & 96 & 9.6 \\ Steel pipe and soil–cement-3 & \(34.6\times 10^{4}\) & 1728 & 172.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Material parameters of FEM. Figure 2: Diagram of the FEM.