## 3 Analysis of the Load-Bearing Characteristics of a Single Pile
### The Axial Force of Cement Soil and the Axial Force of Concrete  
As shown in Figure 6, soil pressure cells were installed at depths of 1 m, 2 m, and 5 m below the surface of the cement soil outside the pile to measure the internal soil pressure of the cement soil under the action of the upper load. The axial force of the cement soil was calculated using Equation (1). The axial force conditions of the cement soil at different depths during the test are shown in Figure 8. Also, as shown in Figure 6, rebar strain gauges were embedded in the center of the pile to monitor the variation of axial stress in the concrete with depth. The embedding depths of the rebar strain gauges were the same as those of the strain sensors. A frequency meter was used to collect the frequency data of the rebar strain gauges. The axial force conditions of the concrete at different depths during the experiment are shown in Figure 9.  
\[N=K\left(F^{2}-F_{0}^{2}\right)A, \tag{1}\]  
In the formula, \(N\) represents the axial force of the cement soil or the axial force of the concrete; \(K\) is the calibration coefficient of the soil pressure cell or the rebar strain gauge; \(F\) is the measured frequency; \(F_{0}\) is the initial frequency; \(A\) is the cross-sectional area.  
As shown in Figure 8, the axial force of the cement soil at different depths of the test piles increased with the increment of the load. Among them, the axial force at 1 m below the cement soil mixing pile was minimal, while the axial forces at 2 m and 5 m below the pile were similar in magnitude. Therefore, the axial force at the 5 m position was selected for the analysis of the load-sharing condition of the pile body.  
For Pile No. 2, the readings of the rebar strain gauge at a depth of 39.5 m from the pile head were unstable and varied significantly, rendering the collected data invalid. As shown in Figure 9, the axial force of the concrete decreased with depth. Under the action of vertical load, the soil around the pile exerted an upward resistance to prevent the pile from sinking, and the side frictional resistance of the pile was mobilized, resulting in a continuous decrease in the axial force of the concrete along the pile body. As the upper load continued to increase and the pile body sank further, the side frictional resistance was further mobilized, and the axial force of the concrete also increased. Through the pile body as a load transfer channel, more load was transferred to the pile end, and the end-bearing capacity of the pile was also mobilized.  
Based on the monitoring data of the axial force of the concrete in Pile No. 1, the axial force experienced by the concrete showed a significant peak in the section from the pile head to half the pile length. Accordingly, when designing the variable diameter and adjusting the steel tube thickness of the steel tube concrete mixing composite pile, the focus of design optimization was placed on the part below half the pile length. In this way, the load-bearing capacity of the steel tube concrete could be fully utilized in the pile structure, redundant material consumption could be reduced, and the material utilization efficiency of the composite pile could be improved.