# Study on the Bearing Characteristics of Composite Foundations with Permeable Concrete Piles
## 4 Results and Discussions
### Influence of the Parameters of Cushion  
According to the \"Technical Specification for Composite Foundation\" (GB/T 50783-2012 [26]), the cushion thickness for rigid pile composite foundations should be set between 150 and 300 mm. In this study, the influence of cushion thickness was analyzed for values of 0.1 m, 0.15 m, 0.2 m, 0.25 m, and 0.3 m, with other parameters kept constant. The simulation results are shown in Figures 20 and 21. Figure 20 shows the influence of load on the settlement of composite foundations with different cushion thicknesses. The figure indicates that the settlement of composite foundations increases with increasing load for all cushion thicknesses. Additionally, under the same load conditions, the settlement increases with increasing cushion thickness, especially when the load exceeds 150 kPa. Further analysis of the settlement at a load of 300 kPa shows that as the cushion thickness increases from 0.1 m to 0.3 m, the settlement increases from 9.58 mm to 16.83 mm, a rise of 75.68%. Figure 20: Influence of loads on settlement under different cushion thicknesses. Figure 21 shows the influence of load on the pile-soil stress ratio for different cushion thicknesses. The figure indicates that the pile-soil stress ratio decreases with increasing cushion thickness. More load is transferred to the inter-pile soil as the cushion thickness increases. The increase in cushion thickness helps to regulate the load distribution between the pile and soil, leading to a decrease in the pile-soil stress ratio. Therefore, when designing composite foundations, selecting an appropriate cushion thickness is necessary to minimize foundation settlement while reducing the load borne by the pile. When designing the cushion layer for composite foundations, well-graded medium-dense sand or gravel is recommended [24]. This study analyzed the influence of cushion modulus for values of 50 MPa, 100 MPa, 150 MPa, 200 MPa, and 250 MPa, with other parameters kept constant. The simulation results are shown in Figures 22 and 23. Figure 22 shows the influence of load on the settlement of composite foundations with different cushion moduli. The figure indicates that the settlement of composite foundations increases with increasing load for all cushion moduli. However, under the same load, the settlement decreases as the cushion modulus increases. When the load exceeds 150 kPa, the differences in settlement among the various cushion moduli become more significant. Further analysis of the settlement at a load of 300 kPa shows that as the cushion modulus increases from 50 MPa to 250 MPa, the settlement decreases from 15.6 mm to 10.43 mm,  
Figure 21: Influence of loads on pile–soil stress ratio under different cushion thicknesses. Figure 22: Influence of loads on settlement under different cushion moduli. a reduction of 33.14%. Therefore, increasing the cushion modulus can reduce the settlement of composite foundations; however, the effect on lowering settlement is limited. Figure 23 shows the influence of load on the pile-soil stress ratio for different cushion moduli. The figure indicates that when the cushion modulus is 50 MPa, the pile-soil stress ratio increases with increasing load. In contrast, for cushion moduli of 100 MPa, 150 MPa, 200 MPa, and 250 MPa, the pile-soil stress ratio initially increases and then decreases. Further analysis shows that the pile-soil stress ratio rises as the cushion modulus increases. A higher cushion modulus makes the cushion layer more rigid, causing the pile to bear more load and reducing the stress in the inter-pile soil. This leads to an increase in the pile-soil stress ratio. Therefore, when designing composite foundations, both settlement control and pile-soil stress ratio optimization should be considered when selecting a reasonable cushion modulus. The inter-pile soil modulus was adjusted to 20 MPa, 25 MPa, 30 MPa, 35 MPa, and 40 MPa, with other parameters remaining constant. The simulation results are shown in Figures 24 and 25.