## 3 Effect of Different Influencing Factors
Figure 3(b) shows the vertical passive earth pressure distribution under the wheel through presenting the normalized passive pressure (\(P_{\mathrm{v}}/(\gamma^{\prime}sD_{\mathrm{p}})\)) versus normalized distance to the wheel center (\(d_{\mathrm{w}}/D_{\mathrm{p}}\)) relationship. It is evident that the overall distribution of the vertical passive earth pressure is similar for the foundation with different wheel diameters, which indicates an increasing trend as the distance develops towards the wheel edge with the maximum value at about 0.75 \(D_{\mathrm{w}}\). It can be explained that the right intersection soil area between the pile and the wheel experiences soil densification due to the compression of the pile and the wheel, which generates a higher soil resistance vertically. The total vertical passive resistance generated under the wheel increases as the wheel diameter increases, which provides a larger bearing area. Effects of Pre-Vertical Load and Lateral Loading Height (V/V\({}_{\mathrm{ult}}\) and e/D\({}_{\mathrm{p}}\))  
Numerical modeling is carried out to examine the effect of the pre-vertical load and the lateral loading height, where the foundation geometry and the soil properties are listed in Group III of Table 1. The relationship of the normalized lateral bearing capacity and the pre-vertical load is plotted in Figure 5. It can be found that, with \(D_{\mathrm{w}}/L=0.6\), the maximum lateral bearing capacity of the composite foundation occurs when \(V/V_{\mathrm{ult}}=60\%\)  
Figure 4: Effect of wheel diameter on the lateral capacity and the failure mechanism. for different lateral loading heights. It can be concluded that the lateral loading height has little influence on the shape of the failure envelopes but only alter asymptotes. Figure 5 also illustrates the failure mechanism of the composite foundation under the combined \(V\)-\(H\)-\(M\) loadings. The plastic strain area \(f_{1}\) is found to expand downwards as the pre-vertical load increases, which means more soils under the wheel are mobilized to provide the bearing capacity. In addition, with the elevation of the lateral loading height, plastic strain area \(f_{1}\) and \(f_{3}\) increase and show greater values. This behavior can be attributed to the increasing dominance of the rotational movement of the composite foundation as the lateral loading height increases, which results in a lower initial stiffness and lateral bearing capacity of the composite foundation. Effects of Sand Layer Thickness and Soil Properties (\(s_{\text{um}}\), \(k\) and \(\varphi\))  
To study the effect of the soil layer thickness and the soil properties, a group of numerical analyses is conducted with \(T_{\text{s}}/L\) = 0.1-0.8, \(s_{\text{um}}\) = 20 kPa and 40 kPa and \(k\) = 1.75 and 2 kPa/m (Group IV, Table 1). The influences of the internal friction angle on sand (\(\varphi\)) and \(T_{\text{s}}/L\) are presented in Figure 6, where the normalized lateral bearing capacity is plotted against \(T_{\text{s}}/L\) with different \(\varphi\) and \(s_{\text{um}}\). It can be found that the lateral bearing capacity of the composite foundation increases nonlinearly as \(T_{\text{s}}/L\) increases. The increasing trend becomes more evident with a greater value of \(\varphi\), which is partly due to the \(P\)-\(\Delta\) effect. From the failure mechanism of the composite foundation depicted in Figure 6a,b, it can be seen that, when \(T_{\text{s}}/L\) = 0.1 and \(\varphi\) = 20\({}^{\circ}\), a \"punching-through\" failure pattern is developed in region (\(f_{1}\)), which includes a rigid block near the pile beneath the footing, a passive wedge at lower right of the wheel in the sand layer, and a fan in the lower clay layer. Similar conclusions are also reported by [PERSON] et al. (2018) [PERSON] et al. (2018) in their investigation of the failure mechanism of the strip footing in sand-over-clay soil deposit.