An analytical framework to analyze the progressive failure behavior of axially loaded single pile embedded in unsaturated soils is presented by means of the load transfer method (LTM) coupling with shear displacement method (SDM). In this study, the proposed DSC-based interface constitutive model and modified small-strain stiffness model undertake the role of load-transfer mechanism for the pile shaft and pile end, respectively, and the shear displacement method is adopted to take the soil deformation surrounding the pile shaft in consideration. This study adopts a stress loading approach in the solution process, differing from traditional strain loading methods, which involves assuming a segment of displacement at the pile end. By successively applying loading increments, the entire load-displacement relationship of the pile is accurately determined, effectively reproducing the authentic stress process of pile foundation, and analyzing the gradual failure processes of friction-dominated piles and end-bearing friction piles under varying suction conditions in unsaturated soil. Customized model piles with smooth, rough and ribbed surfaces and a stress-controlled servo system were developed to conduct static load tests on pile foundations in unsaturated sand-clay mixture and grey clay. Model parameters were calibrated through suction-controlled unsaturated ring shear tests. Finally, the validity of the solutions proposed in this study was verified by comparing the results of static load tests on smooth, rough and ribbed model piles. Subsequently, the effects of suction, interface dilation, and environmental factors on the load-displacement response were analyzed. The research findings of this study can provide a theoretical basis for the design of pile foundations with displacement control in unsaturated soil.

To accelerate the dissipation of excess pore water pressure, enhance the bearing capacity of piles, and mitigate long-term settlement in soft ground, a novel green and lowcarbon pile foundation technology, termed the precast drainage pile (PDP) technology, is proposed. This innovative approach integrated precast pipe piles with prefabricated vertical drains (PVDs) attached to their sides. The piles were installed using static pile pressing and were subsequently subjected to vacuum-induced negative pressure to facilitate soil consolidation, which enhances the resource utilization rate of pile foundations and promotes the sustainable utilization of soft soil foundations. To investigate the bearing characteristics of the PDP, this study combined the shear displacement method for piles with the consolidation theory of soft soil foundations. A calculation model for the load-settlement behavior of precast piles, accounting for the influence of vacuum-induced soil consolidation, was derived, establishing a method for analyzing the load transfer mechanism of PDPs. The reliability of the theoretical model was validated through comparisons with engineering test results. Building on this foundation, the influence of factors such as consolidation period and pile length on the bearing characteristics of PDPs was analyzed. The results demonstrated that, compared to a 10 m precast pile without drainage, the ultimate bearing capacity of single piles with drainage durations of 3, 7, 14, and 28 days increased by 7.3%, 12.7%, 20.3%, and 29.6%, respectively. Furthermore, under a 7-day drainage condition, the bearing capacity of piles with lengths of 10 m, 20 m, and 30 m increased by 12.7%, 12.8%, and 13.1%, respectively. Overall, the findings of this study provide a theoretical basis for the research, development, and design calculations of this new sustainable pile technology.