## 2 Tests
### Compaction Coefficient Tests  
When the pile construction was completed in the SP\(-\)PSC pile group tests, in order to obtain the true compaction coefficient of soil at different locations among the SP\(-\)PSC pile group, we conducted borehole sampling with the drilling machine, and the compaction coefficient tests were carried out (Figure 4). In the borehole sampling method, before sampling, the soil layers above the sampling depths were removed by an auger with as little disturbance as possible. Then, the compacted soil samples at the predetermined depths were \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{2}{c}{**A\({}_{1}\)**} & \multicolumn{2}{c}{**A\({}_{3}\)**} & \multicolumn{2}{c}{**A\({}_{5}\)**} \\ \hline Process & Test1 & Test2 & FEM & Test1 & Test2 & FEM & Test1 & Test2 & FEM \\ P\({}_{1}\) & 33.1 & 37.5 & 40.8 & 143.2 & 107.3 & 127.6 & 53.9 & 41.4 & 60.2 \\ P\({}_{2}\) & 48.3 & 40.1 & 58.3 & 156.3 & 131.7 & 178.4 & 56.2 & 69.3 & 80.2 \\ \hline  & \multicolumn{2}{c}{B} & \multicolumn{2}{c}{C} & & & & \\ \hline Process & Test1 & Test2 & FEM & Test1 & Test2 & FEM & & \\ P\({}_{1}\) & & / & & 296.7 & 315.2 & 297.3 & & \\ P\({}_{2}\) & 263.5 & 245.4 & 287.5 & / & & & & \\ \hline \hline \end{tabular} Note: For more direct comparison between the finite element and the test results (soil stresses), the finite element results are listed in advance in Table 3, and the finite element analysis is described in Section 3.  
\end{table} 
Table 3: Soil stress results of pile group tests (Unit kPa). taken out by the thin-walled steel barrel of the drilling machine (Figure 4c). As shown in Figure 4d, the sampling points were distributed along two lines. Points a, b, and c were located on the connecting line between Pile 1 and Pile 3, and Point c was located at the midpoint between Pile 1 and Pile 3; the distances between Points a, b, and c were \(\frac{2D}{3}\) and \(\frac{2D}{3}\), respectively. Point f was located at the centroid of the pile group. Points d and e were located on the connecting line between Pile 2 and Point f, and the distances between Points d, e, and f were \(\frac{4\sqrt{3}}{9}D\). The above six sampling points completely avoided the embedded positions of the soil pressure cells. At each sampling point, three groups of soil samples were collected from 1, 3, and 5 m underground. The compaction coefficient of soil samples can be derived from Equation (1). \[\eta_{c}=\frac{\rho_{\text{dc}}}{\rho_{\text{dmax}}} \tag{1}\]  
where \(\rho_{\text{dc}}\) is the dry density of the compacted soil samples, and is tested as Section 2.1.1; \(\rho_{\text{dmax}}\) is the maximum dry density of the soil samples, and is measured by indoor compaction tests. The compaction coefficient of the undisturbed soil was 0.74 in this study. As shown in Table 4, the compaction coefficient of the soil within the SP\(-\)PSC pile group was obtained based on the compaction test results and Equation (1). It can be found from Table 4 that with the increase in the distance from the pile core, the compaction coefficient of soil decreases gradually. Since the distance between the centroid (point f) of the pile group and the pile cores is the largest, the soil compaction coefficient at point f is the smallest. On the connecting line of two piles, the soil compaction coefficient at the midpoints is the minimum. At different depths of the same sampling point, it can be found that the soil compaction coefficient at the depth of 3 m is the largest. It can be seen from Table 4 that the variation law of the compaction coefficient of the soil was similar to that of the soil stress in both horizontal and vertical directions. Compression tests have validated that loess collapsibility is closely related to its compaction coefficient [19]. When the loess compaction coefficient is larger than 0.9, its collapsibility coefficient is less than 0.015, and it can be assumed that the loess collapsibility is basically eliminated.