## 5 Analysis of the Combined Load-Bearing Characteristics of Piles and Soil in Composite Foundations
Starting from a depth of 1 m from the pile bottom, the additional vertical force on the soil between the piles increased rapidly. 3. Within a depth range of 1 m from both the top and bottom of the pile, the proportion of additional axial force on the central pile exhibited the most noticeable variation. Subsequently, it remained relatively constant within the depth range from 0.1 \(L\) to 0.9 \(L\). Figure 11: Variation in the additional axial force on the central pile along the depth direction with a pile length of 30 m: (**a**) the additional axial force of the central pile; (**b**) the ratio of the additional axial force on the central pile. Figure 14: Proportion of vertical forces between central pile and inter-pile soil: (**a**) soil elastic modulus of 50 MPa; (**b**) soil elastic modulus of 100 MPa. Figure 12: Variation in the additional axial forces along the depth direction for the central pile and inter-pile soil with a soil elastic modulus of 50 MPa: (**a**) the additional axial force of the central pile; (**b**) the additional vertical force of the soil around the central pile. Figure 13: Variation in the additional axial forces along the depth direction for the central pile and inter-pile soil with a soil elastic modulus of 100 MPa: (**a**) the additional axial force of the central pile; (**b**) the additional vertical force of the soil around the central pile. The Variation in the Additional Internal Forces on the Central Pile and Inter-Pile Soil along the Depth Direction  
Figure 15 illustrates the variation in the additional vertical forces along the depth direction for all the piles and inter-pile soil when the pile length was 30 m. Figure 16 depicts the variation in the additional vertical forces along the depth direction for all the piles and inter-pile soil when the pile length was 20 m. The calculation results revealed the following:  
1. The additional vertical forces of the piles and inter-pile soil gradually decreased with an increasing soil depth, exhibiting an approximately linear relationship. This indicates that the load borne by the piles and inter-pile soil diminishes as a result of stress diffusion within the soil. Additionally, the additional vertical forces of the piles and inter-pile soil increased with an increase in the pile's elastic modulus, but decreased with an increase in the soil's elastic modulus. The additional vertical forces of the end-bearing friction piles were consistently greater than those of the friction piles, suggesting that end-bearing friction piles have a significant advantage in bearing top loads. 2. The ratio of the pile-soil additional vertical forces to the top load reflects the proportion of the top load borne by the piles and inter-pile soil. With an increasing soil depth, this ratio gradually decreased. For the friction piles, it decreased from 0.9-1.0 at the top to 0.20-0.35 at the bottom, while for the end-bearing friction piles, it decreased from 0.9-1.0 at the top to 0.7-0.85 at the bottom. This indicates that end-bearing piles experience more uniform loading, which is beneficial for enhancing the overall bearing capacity of the composite foundation. 3. Based on the slope of the curves, the additional vertical forces on all the piles and soil can be divided into three stages. Among these, the most significant changes occurred within a depth range of 1 m downwards from the pile's top and 1 m upwards from the pile's bottom. In this interval, the additional vertical force decreased linearly with an increasing depth. In comparison to the friction piles, the end-bearing piles exhibited a more gradual reduction trend. Increasing the pile stiffness effectively delayed the decrease in the additional vertical force, particularly when the pile stiffness increased to 25 GPa, where the additional vertical force remained essentially constant.