The traditional design method for drilled shafts rarely considers the influence of construction effect and tip resistance on side resistance, and lacks attempts to improve the performance of drilled shafts without increasing drilled shafts sizing, economic costs and carbon emissions. In this paper, the influences of construction-effect and shaft tip resistance on the side resistance of large-diameter drilled shafts before and after combined grouting were studied by full-scale field tests. The reinforcing effect of combined grouting on bearing capacity was confirmed, and the potential of applying combined grouting to realize sustainable development, the economy, and environmental protection was introduced. The mechanisms of softening-effect and reinforcing-effect on the side resistance of large-diameter drilled shaft are discussed in depth. The field test results showed that the construction-effect and shaft tip sediments caused the side resistance to soften, and the lower the shaft tip resistance, the more serious the side resistance softening-effect. The mobilization of side resistance was affected by the soil strength underneath the shaft tip. The shaft tip bearing capacity was improved after sediment compression, which helped to reinforce the side resistance near the shaft tip. Combined grouting improved the side resistance reinforcing-effect even further and broadened the range of soil layers that can benefit from it. The fundamental reason influencing the side resistance softening-effect and reinforcing-effect of the drilled shaft was the change in soil radial effective stress. This research provided certain references for improving the performance of drilled shafts without increasing drilled shafts sizing, economic costs and carbon emissions.

Constructing offshore wind turbines on artificial islands is considered a viable option, but negative skin friction (NSF) is a significant adverse factor that cannot be ignored. The NSF adversely affects the bearing capacity of pile foundations. Currently, design methods for studying the impact of NSF group effects mainly rely on empirical approaches. Moreover, existing experimental studies do not simulate the NSF experienced by offshore wind turbine pile groups on artificial islands. In order to further explore the impact of pile group effects on NSF experienced by offshore wind turbine pile foundations on artificial islands, this study conducted indoor model tests on single piles and 3 × 3 rectangular pile groups in sandy soil under uniformly distributed loading on surrounding soil. The experiment measured the settlement of piles at various positions within single piles and rectangular pile groups, as well as the settlement of the soil surrounding the piles and the NSF. Through calculations, the experiment determined the neutral points and NSF group effect coefficients for each pile. The results indicate that densely spaced pile groups are advantageous in reducing settlement of the surrounding soil, thereby mitigating the adverse effects of NSF. Due to the influence of pile group effects, different positions within the group experience varying degrees of NSF. Consequently, in practical engineering applications, settlement of both the pile groups and the surrounding soil should be calculated separately. Furthermore, design considerations for the uplift forces and neutral points of piles at different positions within the pile group should adhere to distinct standards.