## 4 Peak Penetration Resistance in Sand Overlying Clay  
A parametric study was performed to investigate the influence of various factors on the bearing capacity of spudcan. A total of 120 FE simulations were performed, the matrix of which is summarized in Table 2. The unit weights of sand and clay were set to 11 kN/m\({}^{3}\) and 7.5 kN/m\({}^{3}\), respectively. For the sand layer, \(E\) and \(\
u\) were set to 25 MPa and 0.3, respectively. For the clay layer, \(\
u=0.49\) and \(E\) was set to 500 \(s_{u}\)[16]. \(\phi^{\prime}\) of sand assigned were = 31\({}^{\circ}\) and 35\({}^{\circ}\). The dilation angle was calculated from the following empirical formulation [9; 27; 28]:  
\[\psi^{\prime}\ =\phi^{\prime}-30^{\circ} \tag{11}\]  
The undrained strength of clay at the interface of sand-clay were set to 10, 20, 30, 40, and 60 kPa. The strength gradient \(k\) was set to 2 kPa/m. Four sizes were used for the spudcan foundation, where \(D\) was varied from 6 to 14 m. The protruded spigot and conical angles of spudcan were fixed to 76\({}^{\circ}\) and 13\({}^{\circ}\), respectively. The sand layer height was varied from 4 to 14 m. \(H_{\text{g}}\)/\(D\) ranges from 1.0 to 0.28. \(q_{peak}\) were defined as the peak penetration resistance calculated within the sand layer regardless of  
Figure 5: Comparison of penetration resistance profiles measured from centrifuge tests (sand over stiff clay) and calculated from FE analyses. penetration depth, because the results demonstrated that in stiff clays, the peak resistance does not always develop at a depth of 0.12 \(H_{s}\) as reported in [PERSON] et al. (2012). Figure 6 displays the plastic shear strain increment contours calculated at \(q_{peak}\) for \(H_{s}\)/D = 0.7 and various combinations of \(\phi^{\prime}\) and \(s_{u}\). Comparisons show that an increase in \(s_{u}\) produces corresponding change in the slope of the failure surface for both \(\phi^{\prime}\) = 31\({}^{\circ}\) and 35\({}^{\circ}\) cases. Whereas the failure surface are tilted outwards with respect to the centerline for \(s_{u}\) = 10 kPa, they are inclined inwards for the case of \(s_{u}\) = 60 kPa. This shift produces the area of influence for the clay to be smaller for stiff clay relative to the soft clay. For \(s_{u}\) = 10 kPa, the lateral spreading of the soft clay can also be observed. It is shown to influence the trace of the plastic strain within the sand layer. Profiles with larger \(\phi^{\prime}\) produce higher levels of shear strain in the sand layer, displaying that the mobilized resistance in the sand layer increases with an increase its shear strength. Figure 7 plots the variation of plastic shear strain traces with change in \(H_{s}\)/D. For \(H_{s}\)/D = 1, limited shear strain develops at the clay layer. The plastic shear strain and also the area of influence of the clay layer increase with a decrease in \(H_{s}\)/D ratio. Figures 6 and 7 demonstrate that \(q_{peak}\) is influenced  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Case No. ** & **D (m)** & \(H_{s}\) **(m)** & \(H_{s}\)**/D** & \(s_{u}\) **(kPa)** & \(k\) **(kPa/m)** & \(\phi^{\prime}\)-\(\psi\) & \(\gamma^{\prime}\)s (kN/m\({}^{3}\))** & **\(I_{0}\) (\%)** \\ \hline
1 & 14 & \(14,10,7,4\) & \(1,071,0.5,0.28\) & \(10,20,30,40,60\) & 2 & 35-5 & 9.96 & 60 \\
2 & 14 & \(14,10,7,4\) & \(1,071,0.5,0.28\) & \(10,20,30,40,60\) & 2 & 31-1 & 9.96 & 44 \\
3 & 10 & \(10,7,4\) & \(1,07,0.4\) & \(10,20,30,40,60\) & 2 & 35-5 & 9.96 & 60 \\
4 & 10 & \(10,7,4\) & \(1,07,0.4\) & \(10,20,30,40,60\) & 2 & 31-1 & 9.96 & 44 \\
5 & 8 & \(8,6,4\) & \(1,075,0.  
by \(H_{s}\)/D, \(\phi^{\prime}\), and \(s_{u}\). As for \(\phi^{\prime}\), the influence is not as significant compared with the parameters \(H_{s}\)/D and \(s_{u}\), because only loose to medium dense sand with \(\phi^{\prime}\) ranging from 31 to 35\({}^{\circ}\) is considered in this study. Figure 8 shows the results of all calculated \(q_{peak}\). \(q_{peak}\) is revealed to be positively correlated with \(H_{s}\)/D, \(D\), \(\phi^{\prime}\), and \(s_{u}\).