# Performance Analysis of Pile Group Installation in Saturated Clay
## 3 Two-Pile Group Analyses
### Vertical Bearing Capacity of Adjacent Pile  
After pile installation, excess pore pressure is generated within soil. Positive EPP is mainly concentrated near the pile tip, while negative EPP is mainly distributed around the upper part and lower part of the pile, as shown in Figure 14(a). Notably, the EPP at the tip of the subsequent pile is significantly higher than that of the adjacent pile. As the soil undergoes drainage consolidation, the EPP gradually dissipates. According to the principle of effective stress in saturated soils, the effective stress in the soil changes with the pore pressure. Figure 16 shows the variation in radial effective stress at different depths. The results indicate that, after 100 days of soil consolidation, the radial effective stress at the middle of the pile (\(z=0.5\,L\)) is significantly lower than immediately after pile installation, primarily due to the recovery of negative pore pressure. Additionally, due to the dissipation of EPP, the radial effective stress at the pile tip (\(z=L\)) is significantly higher than immediately after pile installation. It is noteworthy that, after 100 days of consolidation, the radial effective stress around the subsequent pile is higher than that around the adjacent pile. In this study, the vertical bearing capacity of the pile is defined as the endpoint of the elastic stage in the load-displacement curve. The load-displacement curve is obtained using the displacement control method, where vertical displacement is first applied to the pile, and the corresponding reaction force at the pile head is then extracted. As shown in Figure 17, after 10 days of soil consolidation, the vertical bearing capacity of the subsequent pile is significantly higher than that of the adjacent pile. Figure 18 compares the variation in vertical bearing capacity (\(Q(t)\)) between the adjacent pile and the subsequent pile over time. Figure 14: Variation in peak axial force and peak bending moment with \(L/D_{\text{e}}\). Figure 13: Variation in peak axial force and peak bending moment with \(S\). The results show that the vertical bearing capacity initially increases and then decreases, with the peak capacity for the adjacent pile occurring around day 20 and the peak capacity for the subsequent pile around day 50. This is primarily due to the linear distribution of pore pressure in the direction of gravity after soil consolidation is completed. In the later stages of soil consolidation, pore water continuously flows into the soil at the top of the pile, gradually reducing the effective stress in this region and consequently decreasing the vertical bearing capacity of the pile. Figure 15: Pore pressure distribution of soil at different times: (**a**) after pile installation, (**b**) after 5 days, (**c**) after 10 days, (**d**) after 20 days, (**e**) after 50 days, and (**f**) after 100 days. Additionally, at various time intervals, the vertical bearing capacity of the subsequent pile is consistently higher than that of the adjacent pile. This phenomenon can be attributed to two main factors: first, the additional stress generated within the soil during the installation of the subsequent pile accumulates on top of the stress caused by the adjacent pile, resulting in a greater additional stress around the subsequent pile; second, the excess pore pressure generated after the installation of the subsequent pile is significantly higher than that of the adjacent pile, and as the soil consolidates, the increase in effective stress around the subsequent pile is more pronounced. To facilitate the discussion on the impact of \(S\) and \(L/D_{\rm e}\) on the vertical bearing capacity of the adjacent pile, normalization was performed using \(Q_{\rm a}(t)/Q_{\rm s}(t)\) (adjacent pile vertical bearing capacity/subsequent pile vertical bearing capacity). As shown in Figure 19, increasing both \(S\) and \(L/D_{\rm e}\) reduces the impact of the subsequent pile installation on the bearing capacity of the adjacent pile.