## 1 Introduction
### Previous Work  
A number of studies have been carried out on the bearing capacity of the composite foundation through centrifuge model tests, detailed three-dimensional finite element analyses and theoretical derivations. Most of these studies are only limited to the behaviors of these foundations in pure sand or clay [10; 11; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36]. For the composite system, the interaction effect between the monopile and the wheel has been mostly conducted in two cases: (i) the composite foundation with a wheel diameter (\(D_{\rm w}\)) exceeds the pile embedment depth (\(L\)) (\(D_{\rm w}/L>1\)); and (ii) with a ratio of \(D_{\rm w}/L=0.5\)-\(1\). When the wheel diameter is larger than the pile embedment depth as in case (i), [PERSON] et al. (2018) [10] and [PERSON] et al. (2019) [11] conducted centrifuge tests, in which it was found that the lateral bearing capacity of the composite system was actually smaller than the summation of the individual capacities of monopile and wheel. To quantify the reduction, [PERSON] et al. (2018) [10] and [PERSON] et al. (2020) [35] proposed an add-up method with a reduction factor to estimate the lateral bearing capacity of the composite foundation. On the contrary, with a smaller wheel as in case (ii), a significant positive interaction effect was found by [PERSON] et al. (2014) [27] from their centrifuge tests, in which the bearing capacity of the composite foundation is greater than the sum of that provided by the individual monopile and wheel. In addition, [PERSON] and [PERSON] (2016) [28] proposed that the increase of pile length \(L\) had a more pronounced effect on the bearing capacity of the composite foundation than the improvement through increasing wheel diameter \(D_{\rm w}\). For the failure mechanism of the composite foundation, reported by [PERSON] and [PERSON] (2016) [28], substantial soil yielding was captured at the edge of the wheel in front of the lateral loading direction. The increase of vertical load (\(V\)) on the foundation led to the mobilization of the wheel and the soil mass beneath. The load transfer mechanism was also found to be significantly different between two configurations, that is, coupled and decoupled composite systems, respectively. As suggested by [PERSON] et al. (2018) [36], for a coupled system, the wheel rotated as a rigid body with the center on the pile axis; however, it was found that the uniform settlement of the wheel triggered by the vertical load played a dominant role in the failure mechanism for the decoupled arrangement. In order to assess the bearing capacity of the composite foundation, an empirical design procedure was proposed by adopting the concept of the soil wedge under the wheel in front of the composite foundation [34]. To investigate the bearing capacities of the composite foundation under \(V\)-\(H\)-\(M\) combined loadings, centrifuge tests have been carried out by [PERSON] (2011) [25] and [PERSON] et al. (2019) [11]. It was concluded that the lateral load bearing capacity and the moment resistant capacity were improved by increasing the vertical load \(V\) on the composite foundation, and the maximum lateral load resistance occurred at \(V/V_{\rm ult}=0.4\)-\(0.5\) (\(V_{\rm ult}\) is the ultimate vertical loading capacity). Unlike the previous studies, which mainly focus on the bearing behavior of the composite foundation in pure sand or clay, in this study detailed finite element analyses are carried out to study the bearing capacities and failure mechanisms of the composite foundation in sand-overlying-clay deposits under \(V\)-\(H\)-\(M\) combined loadings.