## 3 Effect of Different Influencing Factors
In addition, more evident plastic strain areas (\(f_{2}\)) and (\(f_{3}\)) at the interface of the two soil layers and the plastic strain areas (\(f_{4}\)) under the wheel are captured when \(T_{\text{s}}/L\) = 0.1 and \(\varphi\) = 20\({}^{\circ}\) as compared to that of \(\varphi\) = 40\({}^{\circ}\). When \(T_{\text{s}}/L\) increases to 0.8, a significant plastic strain area (\(f_{4}\)) can be seen on the lower left region of the wheel when \(\varphi\) = 20\({}^{\circ}\). Generally, increasing \(s_{\text{um}}\) can significantly enhance the bearing capacity of the composite foundation when \(T_{\text{s}}/L\) is relatively small, while soil strength gradient \(k\) has limited influence on the bearing capacity, which can be concluded by the \(H_{\text{ult}}/(2\pi R^{3}\gamma^{\prime}s)\)-\(T_{\text{s}}/L\) curves and the failure mechanisms. Figure 5: Effect of the pre-vertical loading on the lateral capacity and the failure mechanism with different loading height. Figure 7 demonstrates the lateral earth pressure distributions along the right side of the pile body under different \(T_{\rm s}/L\) and \(\varphi\) by plotting normalized lateral earth pressure \(P_{\rm p}/(2\pi R\gamma^{\prime}_{s})\) versus depth below the mudline \(z/D_{\rm p}\). In terms of \(T_{\rm s}/L\) = 0.1, there are no obvious differences between the earth pressure distributions when \(\varphi\) = 20\({}^{\circ}\) and 40\({}^{\circ}\), with the same depth of maximum earth pressure \(z_{\rm max}\) = 0.4 \(L\), which is in accordance with the case when the composite foundation is in pure clay [32]. Besides that, no abrupt change is captured at \(T_{\rm s}/L\) = 0.1, and it indicates that the bearing capacity is mainly controlled by the combined wheel rather than the soil resistance provided by the pile. For \(T_{\rm s}/L\) = 0.4, the overall earth pressure distribution is significantly different from that of \(T_{\rm s}/L\) = 0.1, with its \(z_{\rm max}\) elevate to above 0.3 \(L\). Apparently, a sharp decrease of soil resistance can be found at \(T_{\rm s}/L\) = 0.4 as the depth descends. In addition, the depth of the maximum earth pressure raises with larger \(\varphi\) (\(\varphi\) = 20\({}^{\circ}\): \(z_{\rm max}/L\) = 0.28; \(\varphi\) = 40\({}^{\circ}\): \(z_{\rm max}/L\) = 0.19). As \(T_{\rm s}/L\) increases from 0.4 to 0.8, for \(\varphi\) = 20\({}^{\circ}\), the earth pressure along pile barely changes, while it experiences an obvious increase within the range of \(z/L\) = 0-0.55 when \(\varphi\) = 40\({}^{\circ}\), which is consistent with the phenomenon shown in Figure 6.  
Figure 6: Effect of wheel diameter on the lateral capacity and the failure mechanism. Figure 7: Lateral soil resistance along the pile under different sand layer thicknesses. Effects of Wheel Thickness and Height of Collar (\(T_{\rm w}/D_{\rm p}\) and \(T_{\rm collar}/D_{\rm p}\))  
To explore the effect of wheel thickness \(T_{\rm w}\) and the height of collar \(T_{\rm collar}\), a group of analyses was conducted (Group V, Table 1). Figure 8 illustrates the relationship between ultimate lateral bearing capacities and normalized wheel thickness \(T_{\rm w}/D_{\rm p}\) with different \(T_{\rm collar}/D_{\rm p}\). It shows that \(T_{\rm collar}\) has a negligible influence on the capacity of composite system. As \(T_{\rm w}/D_{\rm p}\) changes from 0.5 to 1.25, the bearing capacity increases linearly by about 17%. This is because that additional vertical pressure is applied to the soil underneath the wheel when \(T_{\rm w}\) is greater, and it can be validated by the plastic strain distribution displayed in Figure 8, which is similar to the situation in Figure 5 when vertical load is applied to the composite system. In addition, with the aim of cost-efficiency in the manufacture and installation of the composite foundation, a friction wheel with a greater thickness would not be suitable for the application of this type of foundation.