Airlift-assisted vacuum preloading: application of airlift via PVDs in vacuum preloading

To improve consolidation pressure along the prefabricated vertical drain (PVD), the presented study introduced a novel technique, airlift-assisted vacuum preloading, which incorporated the concept of airlift into traditional vacuum preloading. The innovative approach involves intermittent injections of pressurized air at the bottom of the PVD. A laboratory test captured direct observation of the fluid state in the PVD, and confirmed that water accumulated in the PVD could be efficiently expelled in a two-phase flow manner by the force of air. When the airlift was stopped, the air remaining in the PVD was quickly expelled, allowing the vacuum pressure to be transmitted without loss. Consequently, the distribution pattern of consolidation pressure was improved to trapezoidal-shaped that increased with depth. The efficacy of this method was validated against conventional vacuum preloading through a 4 m model test, revealing that water discharge rate improvement peaked at 74.2% after airlift, total water discharge increased 32.2% and vane shear strength of soil in the airlift-assisted method improved to over 20 kPa. The results demonstrate the superior performance of airlift-assisted vacuum preloading over the traditional method. Additionally, installation of airlift systems in engineering practice was discussed and related equipment was designed, which provided feasibility of the presented method in real projects.