Tao Hu

The current engineering practice has shown that post-grouting improves the vertical bearing capacity of bored piles. However, some engineers are concerned about the effectiveness of post-grouting in the context of extra-long bored piles. At present, the effect of pile tip post-grouting on the side resistance of these extra-long bored piles remains uncertain. This paper investigated the effects of post-grouting on the compressive and uplift responses of extra-long piles, drawing on case studies from seven field static load tests. It proposed a mechanism for pile tip post-grouting across various bearing layers to enhance the side resistance of extra-long piles by reinforcing the soil beneath the pile tip. Additionally, it outlined the effectiveness and limitations of post-grouting in improving the vertical response of extra-long piles situated in different bearing layers. The analytical results indicated that post-grouting at the pile tip improved the strength of the soil beneath the pile tip. This improvement not only decreased the settlement at the pile tip but also enhanced the side resistance of the pile, especially the side resistance near the pile tip. The effectiveness of pile-tip post grouting in improving pile side resistance is significantly and positively correlated with the reinforcing effect on the soil beneath the pile tip. The effectiveness of post-grouting on the vertical response of extra-long piles can be inconsistent under working loads. Therefore, it is crucial to improve the quality of post-grouting construction in these piles to enhance the overall effectiveness of post-grouting improvements.

This paper investigates the influence of grouting pipe types and hole-forming methods on the bearing characteristics of post-grouted drilled shafts (PGDSs) using field test data. It compares the percussion, auger, and rotary drilling methods, as well as straight and U-tube pipe types, analyzing their respective advantages. The bearing capacity enhancement coefficients of PGDSs with U-tube pipes are derived by comparing calculated and measured values, with the interaction mechanism of these factors are explored. The results show that auger drilling provides superior hole formation quality compared to rotary and percussion drilling, greatly improving shaft quality. The U-tube pipe outperforms the straight pipe in cement spreading and effectively controlling shaft tip settlement, thereby maximizing side resistance. Additionally, rotary-drilled shafts with U-tube pipes exceed the designed bearing capacity by 10%. Post-grouting technology is recommended for rotary drilling, with the economic benefits of rotary-drilled shafts requiring careful consideration. This study provides valuable insights into selecting the appropriate hole-forming method and grouting pipe type for engineering practice and offers guidance for improving design methods in existing specifications.

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.

Bearing characteristics and interactions of a pile group are complex. and there is almost no on-site full-scale test data for cemented-soil composite pile group. In this research, vertical static load tests were conducted on two isolated pile, two-pile group and six-pile group for long-core SDCM pile. The bearing behavior of long-core SDCM pile and the interaction between pile groups have been studied. The results indicate that the long-core SDCM single pile is a typical friction type pile, and cemented soil provides most of the lateral friction resistance for the bearing capacity. In pile groups, pile spacing is the main factor affecting the pile group effect. The model calculation results based on the equivalent pier method are verified using on-site test data and the key parameters in the pile group for long-core SDCM pile including pile spacing S_ij, cemented soil Elastic modulus E_c and PHC pipe pile Elastic modulus E_p are analyzed using a mathematical model. Parameter analysis results indicate that the pile group interaction for long-core SDCM pile is mainly reflected in settlement. Group effect is inapparent at pile spacing greater than 4 D. The appropriate value of E_c and E_p are taken in the range of 0.8-1.0 GPa and 30-40 GPa.

In recent years, Chinese offshore wind farms have experienced varying degrees of foundation scour since their completion. The maximum scour depth of pile foundations has far exceeded the design scour depth, which seriously threatens the safety of wind turbines. Among the current scour depth prediction formulas, the values calculated by the Chinese specification 65-1, 65-2 formula are small and the prediction results are on the dangerous side. The calculated value of the American Hec-18 formula is safe but conservative. The prediction formula of other specifications has a large deviation from the actual situation. Based on the available test data, the main factors influencing the local scour depth of pile foundations, the gauge analysis method, and the group method of data handling (GMDH), this paper proposes a prediction formula for the local scour depth of monopile foundation under the action of wave–current. In addition, monopile scour flume experiments were conducted. Combining the experimental data of the flume test and the scour monitoring data of the Rudong wind farm in Jiangsu, the calculated values of the depth prediction equation in this paper and the Chinese code equation, DNV code equation, HEC-18 equation, Rudolph equation, and Raaijmakers equation were compared and analyzed. The results show that the relative error, mean relative error, variance, and normalized variance between the predicted and measured values of this paper’s formula are smaller than those of other prediction formulas. The formula in this paper has a high calculation accuracy and practical application value.